Mulubrhan Haile Horticulture 2004

8/12/2019 Mulubrhan Haile Horticulture 2004

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THE EFFECTS OF NITROGEN, PHOSPHORUS, AND

POTASSIUM FERTILIZATION ON THE YIELD AND

YIELD COMPONENTS OF POTATO (Solanum tuberosumL.)

GROWN ON VERTISOLS OF MEKELLE AREA, ETHIOPIA

A Thesis

Submitted to the School o f Graduate Studies of the

Alemaya University

In Partial Fulfillment of the Requirements for the Degree of

Master of Science in Agriculture (Horticulture)


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Alemaya UniversitySchool of Graduate Studies

THE EFFECTS OF NITROGEN, PHOSPHORUS, AND POTASSIUM

FERTILIZATION ON THE YIELD AND YIELD COMPONENTS OF

POTATO (Solanum tuberosumL.) GROWN ON VERTISOLS OF

MEKELLE AREA, ETHIOPIA

By

Mulubrhan Haile G/Selassie

Approved by Board of Examiners:


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DEDICATION

This work is dedicated to the memory of Haleka Haile G/Selassie

(1906-1987 E.C.)


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STATEMENT OF AUTHOR

First, I declare that this thesis is my bonafidework and that all sources of materials used for

this thesis have been duly acknowledged. This thesis has been submitted in partial fulfillment

of the requirements for a M.Sc. Degree at the Alemaya University. I solemnly declare that this

thesis is not submitted to any other institution anywhere for the award of any academic degree,

diploma, or certificate.

Brief quotations from this thesis are allowable without special permission provided that

accurate acknowledgement of source is made. Requests for permission for extended quotation

from or reproduction of this manuscript in whole or in part may be granted by the head of the

Plant Science department or the Dean of the School of Graduate Studies when in his or her

judgment the proposed use of the material is in the interests of scholarship. In all otherinstances, however, permission must be obtained from the author.

Name: Mulubrhan Haile Signature: -------------------

Place: Alemaya University, Alemaya

Date of Submission: --------------------


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BIOGRAPHICAL SKETCH

The author was born in 1972 in Shire-Endaslelassie, North Western Zone, Tigray Regional

State, Ethiopia. He attended elementary and junior secondary school at Endaslelassie

Elementary and Junior School from 1980 to 1988. He then attended high school at Bole Senior

Secondary School from 1989 to 1992. In 1993, he joined Mekelle University and graduated in

August 1997 with B.Sc. degree in Dryland Crop Science. After graduattion he was employed

by Mekelle Agricultural Research Center in December 1997. He worked in the Horticulture

Research Division at Mekelle Agricultural Research Center from 1997 to 2003 in different

positions, until he joined the School of Graduate Studies at Alemaya University. He joined the

School of Graduate Studies at Alemaya University to do his postgraduate study in the field of

Horticulture.


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ACKNOWLEDGMENT

The author would like to express his heartfelt appreciation and special gratitude to all persons

who, in one way or the other contributed to the accomplishment of this study. Special

appreciation and deepest thanks go to the thesis research advisors Dr. Nigussie Dechassa

(Alemaya University) and Professor Tekalign Mamo (Ministry of Agriculture and Rural

Development) for their continued guidance, inspiration, encouragement and support

throughout the study period which made the completion of this study smooth and successful.

The visit made by Dr. Nigussie Dechassa to the study field is highly appreciable and never to

be forgotten. The author would also like to express his heartfelt and special gratitude to

Professor Tekalign Mamo for his persistent support in the course of the study period.

The welcome and kind-hearted treatment offered from the staff of the Plant Science

Department especially the Horticulture Section of Alemaya University is sincerely

acknowledged. The author would like to thank Mekelle Agricultural Research Center staff

members for their dedicated help in mobilizing and organizing all the necessary facilities that

enabled him to accomplish this work successfully. He would like to extend his thanks to

Tigray Agricultural Research Institute (TARI) for providing the training opportunity and

Ethiopian Agricultural Research Organization (EARO), Agricultural Research training project

(ARTP) for support of the research project His deepest thanks are extended to G/Hiwot


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Last, but not least, the author remains sincere, grateful, and indebted to his beloved wife,

Regbe Hagos, his daughters, Delina Mulubrhan and Eyoba Mulubrhan, and his mother, W/o

Hiwot G/Hiwot whose words of encouragement, affection and prayer served me as a source of

strength, inspiration and impetus throughout the study.


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TABLE OF CONTENTSPage

DEDICATION iii

STATEMENT OF AUTHOR iv

BIOGRAPHICAL SKETCH v

ACKNOWLEDGMENT vi

LIST OF TABLES x

LIST OF APPENDICES xi

ABSTRACT xii

1. INTRODUCTION 1

2. LITERATURE REVIEW 4

2.1. Nitrogen in Soils and Plants 4

2.2. Phosphorus in Soils and Plants 5

2.3. Potassium in Soils and Plants 6

2.4. Yield Components of Potato 7

2 4 1 St N b 7


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TABLE OF CONTENTS (Continued)

Page

3.2.4. Method and Time of Fertilizer Application 18

3.2.5. Soil Sampling 19

3.3. Data Collection 193.3.1. Data collected 19

3.3.2. Soil Analysis 20

3.4. Data Analysis 21

4. RESULTS AND DISCUSSION 22

4.1. Effect of N, P and K on Tuber Yields 224.1.1. Total Tuber Yield 22

4.1.2. Marketable Tuber Yield 244.1.3. Unmarketable Tuber Yield 24

4.2. Major Yield Components in Potato 284.2.1. Tuber Number 284.2.2. Average Tuber Weight 33

4.2.3. Stem Number 34

4.3. Potato Tuber Size Categories 37

4.4. Specific Gravity 43

4.5. Dry Matter Content of Potato Cubers 44

4.6. Potato Plant Growth Parameters 45

4.6.1. Plant Height 454 6 2 D Fl i d D M i 48


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LIST OF TABLES

Page

Table 1. Potato tuber yield (t ha -1) as influenced by N, P and K application 25

Table 2. Total tuber yield (t ha -1) as influenced by N x K and P x K interactions 26

Table 3. Total tuber yield (t ha -1) as influenced by N x P interactions 26

Table 4. Potato tuber number per hill as influenced by N, P and K applications 31

Table 5. Total tuber number per hill as influenced by N x K and P x K interactions 32

Table 6. Total tuber number per hill as influenced by N x P interactions 32

Table 7. Average tuber weight and stem number per hill as influenced by N, P and K

application 35

Table 8. Average tuber weight (g) as influenced by N x K and P x K interactions 36

Table 9. Stem number per hill as influenced by N x P interactions 36

Table 10. Potato tuber size categories (t ha -1) as influenced by N, P and K application 39

Table 11. Small sized potato tuber yield (t ha-1) as influenced by N x P interactions 40

Table 13. Large sized tuber yield (t ha -1) as influenced by N x K and P x K interactions 41

Table 14. Large sized potato tuber yield (t ha-1) as influenced by N x P interactions 42

Table 15. Potato specific gravity, percent dry matter, harvest index and plant height as

influenced by N, P and K application 46

Table 16 Potato specific gravity as influenced by N x P interactions 47


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LIST OF APPENDICES

PageAppendix table 1. Soil chemical properties of the study area prior to fertilization 68

Appendix table 2. Simple correlation coefficient of different parameters with K, N and P

treatments 69

Appendix table 3. Simple correlation coefficients among different parameters 70


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THE EFFECTS OF NITROGEN, PHOSPHORUS, AND POTASSIUM

FERTILIZATION ON THE YIELD AND YIELD COMPONENTS OF POTATO

(Solanum tuberosumL.) GROWN ON THE VERTISOLS OF MEKELLE AREA,

ETHIOPIA

ABSTRACT

By

Mulubrhan Haile G/Selassie (B.Sc. Mekelle University)

Advisors

Dr. Nigussie Dechassa (Alemaya University, P.O. Box 138, Diredawa)

Professor Tekalign Mamo (Ministry of Agriculture and Rural Development, Addis

Ababa)

A study was conducted to determine the effects of N, P and K application on yield and yield

components of potato (Solanum tuberosum L.) using two levels of K (0 and 93.5 kg K ha -1) and

four levels of N (0, 55, 110 and 165 kg N ha-1) and four levels of P (0, 13.2, 26.4 and 39.6 kg P

ha-1). A split plot design with three replications was employed. The two levels of K and the N

and P treatment combinations were assigned as the main plot and sub plot treatments


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tuber yield (14.93 t ha-1), on the other hand, was recorded at the treatment combination with

no fertilizer application (control treatment). Tuber number per hill (total and marketable)

significantly responded to the application of N and P, but not to applied K. With respect to the

distribution of potato size categories, N increased the yield of medium and large sized potato

tubers, while P increased the yield of large, medium and small sized tubers. K did not

influence either of the size categories. Analysis of post-harvest soil samples collected from 0 to

30 cm depth revealed that applied N significantly decreased soil pH, while P and K did not

significantly affect it. Post harvest level of soil available P was significantly increased in

response to applied P. The levels of soil NH4-N and NO3-N after harvest were increased

significantly due to application of N. Applied P on the other hand, reduced the post harvest

level of soil NH4-N and NO3-N. The three-way interaction was non-significant in influencing

most of the parameters. The limited response of yield and yield components to applied K in

this study should not preclude further research especially dealing with rapidly available soil K

and plant tissue K analysis on major soil types.


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1. INTRODUCTION

Potato (Solanum tuberosumL) is one of the most important food crops in the world. In volume

of world crop production, potato ranks fourth following wheat, maize, and rice (FAO, 1995).

Among the root and tuber crops potato ranks top followed by cassava, sweet potato and yams

in that order (Hawkes, 1990). As a crop in the developing world, potato also ranks fifth in

money value (Horton, 1987).

Potato has been identified as a cheap source of human diet, since it produces more food value

per unit time, land and water than any other major crops. The nutritional value of the potato

crop has been well appreciated and documented (FAO, 1980; Horton, 1987). The tuber

supplies carbohydrates, quality protein (lysine), minerals, nutrient salts, and several vitamins

from group B and large amount of vitamin C. Due to these merits, potato ranks first in the

expansion area of production in the developing world (Horton, 1987). Given that agriculture

is the mainstay of the Ethiopian economy, horticultural crops production is one of the

components of the Ethiopian agriculture. It includes different types of fruit and vegetable crop

husbandry, of which potato crop production is a major activity.

The potato crop was introduced to Ethiopia around 1858 by Schimper, a German botanist

(Berga et al 1994b) Ethiopia is endowed with suitable climatic and edaphic conditions for


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productivity of the potato crop (Berga et al., 1994a). Traditionally, farmers maintain or

improve farmland soil fertility using different management practices such as fallowing, use of

farm yard manure, intercropping and crop rotation. The use of some of these cultural practices

as a means of maintaining or improving soil fertility is limited to a great extent due to small

land holding of farmers. Available statistical data indicate that the average household land

holdings in the country in general and in Tigray region in particular are about 1.09 and 0.98

hectares respectively (CSA, 1995). Farmers with small plots of land are unable to maintain the

farmland soil fertility through cultural practices, as they are using their land exhaustively.

Under such situations, therefore, the use of inorganic fertilizers to optimize productivity

becomes indisputable in crop production, and hence potato cannot be an exception (Reijnties

et al., 1992).

Plants require a variety of elements for growth and development. Nitrogen, phosphorus, and

potassium are the most important among the elements that are essential to plants. Plants utilize

these nutrients in large quantities. The deficiency of these elements is manifested in the

detrimental effects on the growth and development of the plants (Tisdale et al., 1995).

Furthermore, high mobility of N and high affinity of P and K for chemical reactions and

fixation in the soils put these plant nutrients on the priority list in soil fertility management

studies.


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The requirement of crops for nitrogen, phosphorus, and potassium is known to increase with

the introduction and adoption of improved varieties, better cultivation, irrigation facilities and

better control of pests and diseases (Raheja, 1966). This situation would become more critical

in potato production in view of the fact that the potato crop is known to be a heavy feeder of

plant nutrients (Harris, 1978; Sikka, 1982; Horton, 1987).

A lot is known about soil potassium in different parts of the world. However, little is known

about the status of this nutrient in Ethiopian soils (Tekalign Mamo and Haque, 1988). Early

indications of favorable potassium supply except in a few acutely deficient soils have led

researchers and farmers to ignore needs for potassium in many parts of East Africa (Anderson,

1973). Experiments in the past few years have indicated potassium deficiency to be much

more widespread than hitherto known and the need for potassium application increases in

proportion to the intensity of cropping even in semi-arid areas where potassium applications

traditionally have given least response (Anderson, 1973; Tekalign Mamo, personal

communication).

Hence, considering that Ethiopian soils are deficient in fertility, and realizing the importance

of fertilizers in potato production, the use of inorganic fertilizers in potato production isindisputable. However, available information regarding soil fertility studies with regard to

potato production in Tigray region is limited. Fertilizer practices in the region have been


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2.2. Phosphorus in Soils and Plants

Phosphorus is claimed to be the second most often limiting plant nutrient (Tisdale et al.,

1995). Plants absorb phosphorus in the from of HPO4-2and H2PO4

- (Tisdale et al., 1995). The

physical and chemical properties of soils were reported to influence the solubility of

phosphorus and it adsorption reactions in soils. These include the nature and amount of soil

minerals, soil pH, cation effect, anion effect, extent of phosphorus saturation, reaction time

and temperature, flooding and fertilizer management (Tisdale et al., 1995). Moreover,

availability of phosphorus from fertilizers may be affected by the soil reaction, the degree of

soil phosphorus deficiency, rate and method of application, needs of the specific crops, certain

soil differences. The maximum availability of phosphorus for plant utilization is known to

occur at soil pH between 6.5 and 7.5 (Mengel and Kirkby, 1987).

According to Miller and Donanue (1995), the original source of soil phosphorus is the mineral

apatite. They indicated that soil microorganisms and organic matter including plant residue,

animal excretion and remains are known to contribute to the phosphorus pool upon

mineralization.

The use of phosphorus fertilizers becomes imperative because the concentration of phosphorus


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2.3. Potassium in Soils and Plants

Potassium occupies a very important position among those nutrients essential for plant growth

and crop quality. Potassium is present in relatively large quantities in most soils (Tisdale et al.,

1995).

The water soluble and exchangeable forms of potassium present in the soil are the sources of

easily available potassium to plants (Aqcuye et al., 1967). The potassium in exchangeable

from is only a small part is the soils total supply (Tisdale et al., 1995). The unavailable form

accounts for 90-98 per cent of the total soil potassium, the slowly available from 1-10 per cent,

and readily available form 1-2 per cent (Tisdale et al.,1975).

The amount of available potassium must always be high enough to satisfy peak requirements,

if maximum crop yields are aimed at. However, maximum uses of the potassium reserves, the

low flux rate of potassium from these reserves could sometimes limit yield (Beringer et al.,

1990). Moreover, intensive cultivation is reported to hasten the loss of potassium through crop

removal, leaching, and erosion (Westermann et al., 1994a).

Potassium acts as an osmoticum in plants and is important for the translocation of sugars and

synthesis of starches in potatoes (Westermann et al., 1994a). Potassium may also influence


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As yields increase and cropping of potatoes is continued the application of proper rates of

potassium might be needed to compensate for the relatively high amount of potassium

removed by the potato crop.

2.4. Yield Components of Potato

Yield development in potato is known to be the result of three physiological processes leading

to the formation of yield components (Lynch and Tai, 1989). These are stem numbers per

plant or per unit area, tuber numbers per plant or per unit area, and average tuber weight. The

yield components in potato have been reported to develop sequentially. The sequential system

of yield development of the potato involves interactions among individual yield components,

in which later developing components are found to be dependent upon earlier developing ones

(De la Morena et al., 1994).

2.4.1. Stem Number

The potato crop is usually propagated by using underground storage organs known as tubers.

Potato tubers show a wide range of variation and possess a variable number of growing points

(buds) arranged in groups (eyes) over their surface (Allen, 1978). According to Allen (1978)


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differ significantly from intact tubers in number of stems, suggesting that few or indeed only

one eye develop into stems in the intact tubers.

The number of stems per plant is reported to be under the influence of variety, seed (tuber)

size, physiological age of the seed, storage condition, number of viable sprouts at planting,

sprout damage at the time of planting and growing conditions (Iritani, 1968; Allen, 1978;

Horton, 1987; Peter and Hruska, 1988; Lynch and Tai, 1989; De la Morena et al., 1994). Allen

(1978) reported that the number of sprouts, which develop per seed tuber, is principally

determined by the temperature and duration of storage.

Allen (1978) reported the importance of increasing the stem number per plant for increased

graded and total tuber yield. Similarly, Gray and Hughes (1978) observed close relationships

between the number of main stems or aboveground stems and total yields and graded tuber

yields. These investigators claimed that high stem number per plant favored high tuber yield

through effect on haulm growth and tuber number per plant.

Many investigators reported the absence of close relationship between mineral nutrition and

the number of stems per plant (Lynch and Rowberry, 1997; Lynch and Tai, 1989; De la

Morena et al., 1994). Lynch and Rowberry (1997) and De la Morena et al.(1994) from theirstudies on yield development of potato as influenced by nitrogen fertilizer, observed that the

yield difference due to nitrogen treatment was not attributed to its effect on stem density as the


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2.4.2. Number of Tubers

According to Allen (1978) number of tubers set per potato plant (hill) largely governs the total

tuber yield as well as the size categories of potato tubers. He showed that the number of tubers

set by plants was determined by stem density, spatial arrangement, variety, season and crop

management. He further indicated any increase in the stem density over the economical range

(which varies with the soil type, climate, management etc.) resulted in a reduction in the

number of tubers set per stem. He also noted that increasing the stem density by planting

larger seed tubers would result in increased tuber number per plant despite the reduction in the

number of tubers per stem. Increasing stem density over a wide range either by planting larger

seed tubers or more seed tubers for most varieties resulted in increased number of tubers per

unit area (Allen, 1972; Gray and Hughes, 1978). According to Allen (1972) spatialarrangement affected the number of tubers in a similar manner to that of density, since

increasing rectangularity reduced number of tubers set per stem, while increasing tuber

number per plant.

Contradicting results have also been reported by different investigators regarding the effect of

mineral nutrition on the number of tubers set per plant. Thus, Sharma and Arora (1987)

reported no significant difference in the total number of tubers per square meter of land area as


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2.4.3. Average Tuber Weight

Average tuber weight has been reported to be the third most important yield component

contributing to the total tuber yield (Lynch and Tai, 1989; De la Morena et al., 1994). The

growth of tuber tissue is reported to occur both by cell division as well as expansion (Plaisted,

1957; Reeve et al., 1973). Plaisted (1957) showed that as tubers increased in weight from 37

mg to 200 g , the number of cells increased 500 folds, whereas the mean cell volume increased

only by 10 folds. From this, he concluded that cell division is more important than cell

expansion for tuber growth. Howeve r, Reeve et al. (1973) were able to show that tuber

growth, after the tubers had reached 30-40g, was by cell enlargement while cell division had

more contribution in earlier stages.

Tuber weight is reported to be affected by variety and growth conditions. Environmental

factors that favor cell division and cell expansion such as mineral nutrition, optimum water

supply, etc. were reported to enhance tuber size (Reeve et al., 1973). The result of a study

conducted by De la Morena et al. (1994) showed that variation in tuber yield due to nitrogen

treatments were related to the tuber weight increment. Similarly other studies indicated that

the potato yield component most affected by nitrogen and potassium application was the mean

tuber weight (Harries, 1978; Giardini, 1992). Sharma and Arora (1987) from their


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2.5. Effect of N, P and K on Potato Tuber Yields

Several factors limiting crop yields have been reported by many workers. According to Downs

and Hellmers (1975) and Tisdale et al. (1995), factors limiting crop yield (both in quantity as

well as quality) can be categorized into four major headings: the soil upon which the crop

grows, the genetic make-up of the crop, the climatic conditions during the growth of the crop,

and the management practices, mainly soil fertility. Maintaining adequate levels of soil

fertility has been recognized as one of the management practices that affect growth,

development and yield of plants (Tisdale et al., 1995).

Potato plants have been reported to have high requirement for mineral nutrition (Harris, 1978).

Depending on conditions, a normal potato crop has been found to remove 90 to 190 kg

nitrogen and 30 to 50 kg P2O5 ha-1 (Sikka, 1982). The potassium requirement of the potato

crop has been found to be much higher than that of other irrigated crops (Lin, 1966; Winston,

1966; Westermann et al.,1994a). According to Panique et al.(1997), a 13.6 Mg crop of potato

removes 113 kg of K2O for vines and tubers, with about 66% removed by tubers and 34% by

vines.

The yield increment of potato due to nitrogen fertilizers was found to be positive up to a

certain level beyond which yield reduction was observed (Wilcox and Hoff, 1970; Robert and


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response of six potato cultivars to different levels of nitrogen fertilizer and observed

significant yield difference among the different cultivars.

Potato tuber yield is also known to be influenced by phosphorus fertilizers through its effect

on the number of tubers produced, the size of the tubers and the time at which maximum yield

is obtained (Sommerfeld and Knutson, 1965; Sharma and Arora, 1987). They showed that

yield response to increasing levels of phosphorus fertilizer was generally positive up to a

particular level, above which the response became negative. According to these investigators,

excess use of phosphorus fertilizers is usually associated with reduced tuber weight by

hastening the maturation period and reducing tuber size. Fertilizer recommendations for

potassium in potato production are high. The amount of potassium needed by the potato crop

is observed to be directly proportional to tuber yield (Panique et al.,1997).

Many investigators reported that on potassium responsive soils there was a significant tuber

yield response to potassium fertilization (Downs and Hellmers, 1975; McDole et. al., 1978;

Sharma and Arora, 1987; Westermann et al., 1994a). Conversely, other workers found no

response to K fertilization (Berga et al., 1994). This is usually true on soils testing high for

available potassium.

According to Hanley et al. (1965), there seemed to exist a positive interaction between


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Similarly, Maier et al. (1994a) noted a significant N x K and P x K interaction on potato tuber

yields in some sites, while there was no interaction in other sites.

2.6. Effect of N, P, and K on Specific Gravity of Potato Tubers

According to Lujan and Smith (1964), specific gravity has been found to be accurate index of

mealiness in potatoes. It is useful in predicting suitability of potatoes for cooking, canning or

dehydrating in addition to its use to predict the yield of potato chips. Many studies conducted

on the texture of cooked potato tubers have shown the relationship, which exists between

texture and specific gravity of row tubers. Tubers with high specific gravity were noted to

have high starch contents and they tend to be mealy in texture and to slough when cooked

(Nelson and Shaw, 1976).

Timm and Flockner (1966) reported a reduced specific gravity of tubers when nitrogen level

was increased above 136 kg ha -1. Kleikopf et al. (1981) found the specific gravity of tubers

decreased with increasing rates of nitrogen. Contrary to this Robert and Cheng (1988) noted

non-significant difference in specific gravity of tubers due to nitrogen treatment.

Westermann et al. (1994a) reported that the effect of potassium on specific gravity depended

on the potassium source. They showed that potassium at 224 kg ha -1 as potassium chloride


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potassium rate from 0 to 480 kg K ha-1did not significantly affect specific gravity; in contrast

at 100 kg P ha-1specific gravity increased from 1.078 to 1.086. In a 12 year experiment Black

and White (1973) observed significant N x K interactions on percentage starch of potato

tubers.

These relationships show the significant effects that interactions between nutrients can have on

specific gravity and therefore on the recommendations that may come from fertilizer

experiments (Maier et al., 1994a).

Conflicting results have been reported regarding the effect of phosphorus on the specific

gravity of potato tubers. Zandstra et al. (1969) and Dubetz (1975) reported a reduction in

specific gravity as the rate of phosphorus fertilizer increased. However, Human (1961)

observed increased specific gravity with increased phosphorus application. As opposed to the

above findings, Lujan and Smith (1964) reported non-significant effect of phosphorus on the

specific gravity of tubers.

2.7. Effect of N, P, and K on Dry Matter Content of tubers

It is often necessary to know the dry matter content of potato tubers since this largely governs

the weight of processed products, which can be obtained from a given weight of raw tubers. It


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tuber dry matter and specific gravity of potato tubers. Regarding phosphorus Sparrow et al.

(1992) reported non-significant difference in dry matter contents due to increased phosphorus

application.

2.8. Potato Tuber Size Categories

Tuber size is reported to be an important aspect of potato production (Mass, 1963; Gray and

Hughes, 1978). The production of potato tuber of a requisite size may be of much economic

value both for seed and human consumption. The market demand for shapes and sizes of

tubers varies. The size of tubers required by consumers depends upon the ease of handling for

household purposes and also upon the acceptable level of peeling loss (Gray and Hughes,

1978).

The application of mineral nutrients has been found to affect the size of potato tubers by

affecting the plant establishment, number of tubers produced, growth rate of tubers and

duration of bulking (Harrison, 1982; Sommerfeld and Knut son, 1965; Kleinkopf et al., 1981;

Sharma and Arora, 1987).

Nitrogen and potassium application were been frequently reported to increase the proportion

of medium and large sized tubers (Reddy and Rao, 1968; Sharma and Arora, 1987). Sharma


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Several researchers have indicated that phosphorus also affects the size categories of potato

tubers (Birch et al., 1967; Hanley et al., 1965; Sharma and Arora, 1987). Sharma and Arora

(1987) observed an increase in medium (25-75 g) and small (less than 25 g) grades and

decrease in large (above 75 g) with an increase in applied phosphorus. Excessively high rates

of phosphorus fertilizer resulted in reduced yields of U.S. Grade 1 tubers over 283 g, and

increased the yields of undersized potatoes (Sommerfeld and Knutson, 1965).


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3. MATERIALS AND METHODS

3.1. Description of the Study Site

The experiment was conducted at Mekelle Research Center of the Tigray Agricultural

Research Institute, which is located at an elevation of 1970 meters above sea level. The

experimental site lies at 13o39iN latitude and 39o43iE longitude. The soil type is fine textured

Pellic Vertisol with a pH of 7.5. The average annual rainfall of the study area is 500 mm per

annuum which is essentially unimodal with about 80% of the precipitation falling in a two and

half months period. The average daily temperature ranges from 9 28oC. Although the

temperature falls during the cool season, the radiation and wind speed remain at a relatively

high level, which result in high evapotranspiration. (Mekelle Research Center, 1994).

3.2. Field Experiment

3.2.1. Experimental Materials

Plant material

A standard potato variety Tolcha was used as planting material The planting material was


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Fertilizer treatments

Urea (46% N), Triple Super Phosphate, TSP (46% P2O5), and Potassium Chloride (KCl) (60%

K2O) were used as fertilizer sources for nitrogen, phosphorus, and potassium, respectively.

The fertilizer treatments consisted of four levels of nitrogen, four levels of phosphorus and two

levels of potassium. The levels were 0, 55, 110 and 165 kg N ha -1; 0, 13.2, 26.4, and 39.6 kg P

ha-1; and 0 and 93.5 kg K ha-1.

3.2.2. Experimental Design

The experimental plots were arranged in a split plot design with three replications. The

potassium fertilizer levels were assigned to the main plots and the nitrogen and phosphorus

factorial combinations to the sub-plots. The size of the main plots was 186 m2 and sub-plots 9

m2. A distance of 1 meter was maintained between the main plots and within the sub-plot and

2 meters between replications. There were four rows in each plot each with 10 plants.

3.2.3. Cultural Practices

Land was prepared in accordance with a standard practice locally used. Medium size and well-


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3.2.5. Soil Sampling

Soil samples were randomly taken from the experimental field to a depth of 0-30 cm using an

auger and ten composite soil samples prepared from the collected samples. Soil samples were

collected from each plot after harvest in the way similar to collecting them before planting.

The collected samples were air -dried and ground to pass through 2 mm sieve for chemical

analysis.

3.3. Data Collection

3.3.1. Data collected

Days to emergence: recordedwhen 50% of the plants in each plot sprouted and emerged.Days to flowering: noted when 50% of the plant population in each plot produced flowers.

Plant height (cm): measured by harvesting plants after about 6 weeks from pollination

(Sikka, 1982). At this stage, the vines were still green but had practically ceased growth.

Days to maturity: wasrecorded when the haulms (vines) of 50% of the plant population have

yellowed or in each plot they showed senescence

Shoot biomass (g): fresh biomass of the haulm was recorded; and dry weight was noted after

air drying the samples and further oven-drying at 650C for 72 hours


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Harvest index: wasdeterminedasthe ratio of fresh weight of tubers to the total biomass fresh

weight. This was taken at harvest.

Dry weight of tubers: Five fresh tubers were selected from each plot and weighed, then

sliced, and dried in oven at 650C for 72 hours to a constant weight. Their dry weight was

recorded. Dry matter percentage was calculated from this value.

Size categories of tubers: Size categories of tubers (based on weight of tubers) were set after

undertaking a survey at the Mekelle market (small = 75 g).

Tuber nitrogen content: Three sample tubers were peeled and cut to small pieces. These

were first air dried and then oven dried at 650C for 24 hours. Then, tuber N concentration

was determined using Kjeldal digestion method (Dewis and Freitas, 1970).

3.3.2. Soil analysis

Soil pH: was determined in 1:2.5 soil to water ratio using a glass electrode attached to a

digital pH meter (Page, 1982).

Organic matter: was recordedbased on the oxidation of organic carbon with acid potassium

di-Chromate (K2C2O7-2) medium using the Walkley and Black method as described by

Dewis and Freitas (1970).

Total nitrogen: was determined using Kjeldal method (Dewis and Freitas, 1970)


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3.4. Data analysis

Analysis of Variance and correlation analyses were performed on computers using M- STAT

software (M-STAT, 1990). The correlation analysis was performed to determine simple

correlation coefficient between yield and yield components as affected by N, P, and K

application. Whenever the treatment differences were found significant, mean differences

were tested using Least Significant Difference (LSD) test procedure.


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4. RESULTS AND DISCUSSION

The present study was aimed at understanding the response of potato yield and yield components

to different rates of N, P and K fertilization. A factorial combination of four rates of N and P

were tested using two levels of K fertilizer. The plots were arranged in split plot design with

three replications. The two K levels and the N and P treatment combinations were assigned to

the main plot and the sub-plot, respectively. Yield parameters, yield components, growth

parameters and some soil characteristics were recorded during the course of the study. The

results of the investigation are discussed as follows.

4.1. Effect of N, P and K on Tuber Yields

4.1.1. Total Tuber Yield

Application of N and P highly significantly increased total tuber yield (Table 1). Increasing the

level of applied N from 0 to 165 kg N ha -1, increased total tuber yield by 94.13%. Similarly,

increasing P application from 0 to 39.6 kg P ha-1 highly significantly increased total tuber yield

by 24.27%. This indicates the existence of a room for further increases in tuber yield through

application of more N and P fertilizers beyond 165 kg N ha -1 and 39.6 kg Pha-1, respectively.

Application of K did not result in statistically significant increase in total tuber yield, though


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The N x P interaction effect was also highly significant in increasing total tuber yield (Table

3). This may be due to the fact that these two important plant nutrients have complementary

physiological functions in plants. Moreover, they are the major constituents of physiologically

active organic compounds in the plant system, leading to a combined increase in tuber yield.

The highest total tuber yield (40.86 t ha -1) was recorded in the treatment combination of 93.5

kg K, 165 kg N and 39.6 kg P ha-1. The lowest total tuber yield (14.93 t ha -1) on the other hand

was recorded in the treatment combination with no fertilizer application (control treatment).

The observed non-significant increase in total tuber yield due to the application of K fertilizer

seemed to prove that the soil of the present experimental site is rich in this nutrient. Various

investigators reported contradictory results with regard to the effect of K on tuber yield. Berga

et al. (1994a) observed the absence of a significant yield response to K fertilization.. On the

other hand, other workers (Downs and Hellmers., 1975; McDole et al., 1978; Westermann et

al., 1994a) reported a significant increment in yield due to K application only on K responsive

soils.

The positive effect of N and P applications on total tuber yield was further indicated by theobserved positive and highly significant correlation values between applied N and P, and total

tuber yield (r = 0.86** and r = 0.30** respectively) (Appendix Table 2).


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4.1.2. Marketable Tuber Yield

Highly significant difference in marketable tuber yield was observed due to increase in the

application rates of N and P (Table 1). However, the main effect of K on marketable tuber

yield was observed to be non-significant. Similarly, the K x N, K x P, N x P and K x N x P

interaction effects were statistically non-significant, which might probably indicate that the

mineral nutrients were probably acting independently in affecting marketable tuber yield.

Increasing N application from 0 to 165 kg N ha -1 highly significantly increased marketable

tuber yield from 11.84 to 25.67 t ha -1 (Table 1). Similarly, increasing P application from 0 to

39.6 kg P ha-1 increased marketable tuber yield from 16.44 to 21.55 t ha -1. The yield increment

in marketable tubers due to N and P application was found to be consistently significant up to

the highest rate (165 kg N ha -1 and 39.6 kg Pha-1) showing that these mineral nutrients can

contribute much to obtain healthy and marketable size tubers. Similarly, the correlation values

between marketable tuber yield and applied N and P were positive and significant (r = 0.84*

and r = 0.33**).

4.1.3. Unmarketable Tuber Yield


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Table 1. Potato tuber yield (t ha-1) as influenced by N, P and K application

Characters consideredSource of

variation

TTY (t ha-1) MTY(t ha-1) UTY(t ha-1)

K (K kg ha-1) NS NS NS

0

93.5

28.17

29.81

18.59

18.73

9.57

10.97

N (N kg ha-1) ** ** NS

0

55

110

165

19.19d

26.47c

33.07b

37.28a

11.84d

16.51c

20.63b

25.67a

7.31

9.96

12.44

11.38

P (P kg ha-1) ** ** NS

0

13.2

26.4

39.6

25.76d

27.92c

30.27b

32.00a

16.44d

17.11c

19.55b

21.55a

9.32

10.82

10.72

10.65


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Table 2. Total tuber yield (t ha-1) as influenced by N x K and P x K interaction

TTY TTY

K (Kkg ha-1) K (K kg ha-1)

N

(N kg ha-1)

0 93.5 D/f P (P kg

ha-1)

0 93.5 D/f

0 25.03d 26.48d 1.45NS 0 18.73 19.56 0.83NS

55 27.50c 28.34c 0.84NS 13.2 27.26 28.48 1.22NS

110 29.27b 31.28b 2.01* 26.4 32.52 33.61 1.09NS

165 30.88a 33.13a 2.25* 39.6 36.96 37.58 0.62NS

N x K interaction* P x K interactionNS

Means followed by the same letter within the same column are not significantly different at 5% level of

significance. LSD (0.05) = 1. 74 t ha-1

to compare N x K interaction. * = indicates significant difference at 5%level of significance. NS = non- significant. TTY= total tuber yield. D/f = difference

Table 3. Total tuber yield (t ha-1) as influenced by N x P interaction

P (P kg ha-1)N

(N kg ha-1) 0 13.2 26.4 39.6 Mean

0 14.93i 17.39h 20.52g 23.75f 19.19d


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4.2. Major Yield Components in Potato

4.2.1. Tuber Number

The main effects of N, P and K on total, marketable and unmarketable tuber numbers per hill

are shown in Table 4. Highly significant differences in total tuber number were observed due

to the increased application of N and P. Increasing the application of N increased highly

significantly total tuber number per hill from 8.44 to 9.84 (Table 4). Similarly, increasing thelevel of applied P highly significantly increased total tuber number per hill (Table 4). The

increase in total tuber number per hill due to applied P was statistically significant only up to

the rate of 26.4 kg P kg ha -1, beyond which non-significant difference was observed. The mean

total tuber number per hill for K averaged over all treatment combinations and replications

showed that total tuber number tended to increase due to potassium application, even though

the increment was statistically non-significant.

The N x K interaction effect was found to be significant for total tuber number (Table 5).

Nitrogen application at the rate of 0 and 110 kg N ha-1 with Kproduced a statistically

significant difference in total tuber number. The interaction effect between P and K in

affecting total tuber number was non-significant. Significant N x P interaction effect was

observed for total tuber number (Table 6). As shown in Table 6, the effect of N on total tuber


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As shown in Table 4, the main effects of N and P on marketable tuber number were observed

to be highly significant. However the main effect of K and all interaction effects were non-

significant.

Increasing the level of applied N from 0 to 165 N kg ha-1 highly significantly increased

marketable tuber number (Table 4). In general, N treatment increased the marketable tuber

number per hill by 23% over the control. Similarly, increasing the application of P from 0 to

39.6 P kg ha-1increasedmarketable tuber number by 24%.

The main effects of N, P and K on the number of unmarketable tubers per hill were non-

significant (Table 4). Similarly, all interaction effects were found to be statistically non-

significant. The observed non-significant and inconsistent effect of the three nutrients on

unmarketable tuber number might probably indicate the difficulties of manipulating this plant

parameter by the use of mineral nutrients.

The positive effects of N and P on tuber number was further indicated by the observed positive

and highly significant correlation values between total tube number per hill and applied N (r =

0.31

**

), and between total tuber number and P (r = 0.54

**

). Positive and highly significantcorrelation values were also found between marketable tuber number and applied N (r =

0.31**) and P (r = 0.30**), and non-significant but positive correlation value (r =0.02) between


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the number of tubers. The observed conflicting results may be explained by the fact that

season, inherent nutrient status of a soil and location could have exerted their effects in

determining the number of tubers that could be set by the potato plant. Despite these

conflicting results, however, Lynch and Rowberry (1997), Sharma and Arora (1987) and De

La Morena et al. (1994) confirmed that tuber number is not an important yield limiting

component while studying mineral nutrition. This could be due to the inverse association

between tuber number and average tuber weight (De La Morena et al., 1994). Thus, for

different tuber number, similar yield plateaus might be obtained through a corresponding

change in average tuber weight.


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Table 4. Potato tuber number per hill as influenced by N, P and K application


variation

TTN (per hill) MTN (per hill) UTN (per hill)


0

93.5

8.42

9.69

4.71

4.77

3.71

4.92

N (N kg ha-1) ** ** NS

0

55

110

165

8.44b

8.59b

9.36ab

9.84a

4.26c

4.57bc

4.88ab

5.24a

4.18

4.01

4.38

4.56

P (P kg ha-1) ** ** NS

0

13.2

26.4

39.6

7.89b

8.38b

9.58a

10.38a

4.29b

4.60b

4.76ab

5.31a

3.60

3.78

4.52

5.07

Means followed by the same letter within a column are not significantly different at 5% level of significance.

**= indicates significant difference at 1% level of significance respectively NS= non-significant TTN= total


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Table 5. Total tuber number per hill as influenced by N x K and P x K interaction

TTN TTN

K (Kkg ha-1) K (Kkg ha

-1)

N

(N kg ha-1)0 93.5 D/f P (P kg

ha-1)

0 93.5 D/f

0 7.69d 9.19abc 1.5* 0 6.75 9.02 2.27ns

55 8.29cd 8.89bcd 0.6ns 13.2 8.02 8.74 0.72ns

110 8.18cd 10.54a 2.36* 26.4 9.20 9.95 0.75ns

165 9.52abc 10.15ab 0.63ns 39.6 9.71 11.05 1.34ns

N x K interaction* P x K interactionNS

Means followed by the same letter within the same column are not significantly different at 5% level of

significance. LSD (0.05) = 1.43 tubers per hill to compare N x K interaction effect on TTN. * = indicates

significant difference at 5% level of significance. TTN= total tuber number per hill. D/f = difference

Table 6. Total tuber number per hill as influenced by N x P interaction

P (P kg ha-1)N

(N kg ha-1) 0 13.2 26.4 39.6 Mean


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4.2.2. Average Tuber Weight

The main effects of N, P and K on the average tuber weight of potato plants are shown on

Table 7. Application of N significantly increased average tuber weight. Average tuber weight

increased by 62% as N application increased from 0 to 165 kg N ha -1. Similarly applied P

highly increased average tuber weight except at the rate of 26.4 kg P ha -1. For instance

increasing the application of P from 0 to 39.6 kg P ha -1 increased average tuber weight of the

potato plants by 70% over the control.

The interaction effect of the nutrients on average tuber weight was statistically non-significant

except for N x K (Table 7). There was no increment in mean tuber weight at the two levels of

K application at the same level of N, except for N rate of 165 kg N ha -1. Maximum (123.78 g)

and minimum (41.11 g) average tuber weights were observed in the treatment combinations of

93.5 kg K ha-1, 165 kg N ha-1, 39.6 kg P ha-1 and the control treatment, respectively. According

to the present finding K alone did not increase average tuber weight, but it was effective when

applied in combination with N. The results of the present finding also indicated that

appropriate levels of N, P and K are required to improve average tuber weight of the potato

plant and hence to increase tuber yield.


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In the same manner, Harris (1978), Giardini.(1992) and De La Morena et al.(1994) reported

that yield increment due to mineral nutrition was attributed to its effect on average tuber

weight. The increase in average tuber weight of tubers with the supply of fertilizer nutrients

could be due to more luxuriant growth, more foliage and leaf area and higher supply of

photosynthates which helped in producing bigger tubers, hence resulting in higher yields

(Patricia and Bansal, 1999). In other words, the increased size and duration of the haulm

stemming from improved supply of nutrients favored the tuber weight (Peter and Hruska,

1988).

4.2.3. Stem Number

The main effects of N, P and K did not result in any significant difference in stem number

(Table 7). Similarly, all interaction effects except N x P (Table 8) were statistically non-

significant. Although stem density is one of the most important yield components in potato,

the results of the present study showed that the influences of N, P and K on stem number were

non-significant. This result is consistent with the findings of different authors (Lynch and

Rowberry, 1997; De La Morena et al., 1994). This could be due to the fact that stem number is

determined very early in the ontogeny of yield (Lynch and Tai, 1989) and as a result, at least


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4.3. Potato Tuber Size categories

The main effects of N, P and K on yields of various tuber size categories are presented in

Table 9. Nitrogen highly significantly decreased the yield of small sized tubers (Table 10).

However, highly significant increment in the yield of medium and large sized tubers were

observed in response to N application. Increase in applied N from 0 to 165 kg N ha -1 decreased

the yield of small sized tubers from 6.22 t ha -1 to 5.33 t ha-1 and it increased the yield of

medium and large sized tubers from 12.92 to 16.81 t ha -1 and from 4.56 to 6.26 t ha -1,

respectively. The reduction in small sized tuber yield due to N treatment was significant up to

the rate of 110 kg N ha -1. At 165 kg N ha-1 nitrogen highly significantly increased the yield of

small sized tubers. The increase in the medium and large sized tuber yield was significant only

at the rates of 110 kg N ha -1 and 165 kg N ha -1. These results are in agreement with those of

Sharma and Arora (1987), who found a significant increase in the yield of medium and largesized tubers due to N application. These workers observed a significant reduction in the yield

of small sized tubers with N application.

Contrary to the N effect, increase in P application highly significantly increased the yield of

small sized tubers (Table 10). Similarly, in contrast to the findings of different workers

(Herlihy, 1965; Sommerfeld and Knutson, 1968), who reported a significant decrease in the

yield of large sized tubers, P application in the present study tended to increase the yield of


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The reduction effect of N on the small sized tuber yield was decreased due to the presence of

P. This could be attributed to the counter (increasing) effect of P on the yield of small sized

tubers. Similarly, the increase in the small sized tuber yield due to P effect was reduced due to

the counter (reduction) effect of N.

The highest yield (9.24 t ha -1) in small sized tubers was recorded in the 93.5 kg K ha -1, 165 kg

N ha-1and 0 kg P ha-1 treatment combination. In the case of medium sized tuber yield the

highest yield (24.30 t ha-1) was obtained in the treatment combination of 93.5 kg K ha -1, 165

kg N ha-1and 39.6 kg P ha -1.

Significant and highly significant differences in the yield of large sized tubers were observed for

N x K (Table 13) and N x P (Table 14) interaction effects, respectively. In the K fertilized

treatments, N fertilization increased large sized tuber yield only at the level of 55 and 110 kg Nha-1; non- significant difference in the yield of large sized tubers was observed at the rate of 165

kg N ha-1 in the K fertilized plots. The increase in the yield of large sized tubers due to N x P

interaction effect was consistent at 165 kg N ha -1 and 39.6 kg P ha-1 for N and P, respectively.

The highest large sized tuber yield (6.85 t ha -1) was obtained for the 93.5 kg K ha -1, 165 kg N ha -

1and 39.6 kg P ha-1 treatment combination.

The results of this investigation clearly indicated that the levels of N and P application largely


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Table 10. Potato tuber size categories (t ha -1) as influenced by N, P and K application


Variation

STY METY LTY


0

93.5

5.05

6.03

14.87

15.10

5.01

5.56

N (N kg ha-1) ** ** **

0

55

110

165

6.22a

5.82a

4.77b

5.33ab

12.92d

14.46c

15.75b

16.81a

4.56c

5.14b

5.20b

6.26a

P (P kg ha-1) ** ** **

0

13.2

26.4

39.6

3.97d

4.92c

5.86b

7.39a

9.31c

15.46b

15.38b

19.79a

4.51c

5.04b

5.62a

5.99a

Means followed by the same letter within a column are not significantly different at 5% level of significance;

***= indicate significant difference at 5% and 1% level of significance respectively; NS= non-significant STY=


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Table 11. Small sized potato tuber yield (t ha-1) as influenced by N x P interaction

P (P kg ha-1)N

(N kg ha-1) 0 13.2 26.4 39.6 Mean

0 4.77efg 5.37cdef 6.10bcde 8.62a 6.22a

55 4.74efg 5.27def 6.14bcde 7.14abc 5.82a

110 3.11g 3.80fg 5.58bcde 6.60bcd 4.77b

165 3.24g 5.23def 5.63bcde 7.22ab 5.33ab

Mean 3.97d 4.92c 5.86b 7.39a

N x P interaction**

Means followed by the same letter within a column or row are not significantly different at 5% level of

significance; LSD(0.05) = 1.14 t ha-1

to compare N x P interaction; LSD (0.05) = 0.57 t ha-1

to compare N and P

main effects; **= indicates significant difference at 1% level of significance; STY= yield of small sized tubers (t

ha-1

).


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Table 12. Medium sized potato tuber yield (t ha -1) as influenced by N x P interaction

P (P kg ha-1)N

(N kg ha-1) 0 13.2 26.4 39.6 Mean

0 6.14j 14.92efgh 14.32fgh 16.28cdef 12.92c

55 8.47i 16.09def 14.76fgh 18.54bc 14.46b

110 9.96i 17.19cde 15.78defg 20.09b 15.75b

165 12.67h 13.65gh 16.64cd 24.26a 16.81a

Mean 9.31d 15.46c 15.34b 19.79a

N x P interaction**


significance; LSD (0.05)= 2.08 t ha-1to compare N x P interaction; LSD(0.05)= 1.04 t ha

-1to compare N and P main

effects; **= indicates significant difference at 1% level of significance.

Table 13. Large sized tuber yield (t ha-1) as influenced by N x K and P x K interaction

LTY LTY

K (K kg ha-1) K (K kg ha-1)N

(N kg ha-1

) 0 93.5 D/f

P (P kg

ha-1

) 0 93.5 D/f

0 4.60b 4.61b 0.01NS 0 4.02 4.99 0.97NS


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Table 14. Large sized potato tuber yield (t ha-1) as influenced by N x P interaction

P (P kg ha-1)N

(N kg ha-1) 0 13.2 26.4 39.6 Mean

0 4.18f 4.53def 5.37cd 4.14f 4.56c

55 4.92cdef 5.04cdef 5.22cde 5.39cd 5.14b

110 4.27ef 4.86cdef 5.15cdef 6.50b 5.20b

165 4.63def 5.71bc 6.73b 7.95a 6.26a

Mean 4.51c 5.04b 5.62a 5.99a

N x P interaction**


significance; LSD (0.05)= 1.01t ha-1

to compare N x P interaction; LSD (0.05)= 0.51 t ha-1

to compare N and P main

effects; **= indicates significant difference at 1% level of significance; LTY = Large sized tuber yield.


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4.4. Specific Gravity

The main effects of N, P and K on the specific gravity of potato tubers are shown in Table 15.Application of nitrogen highly significantly reduced the specific gravity of potato tubers (Table

15). Increasing the application of N from 0 to 165 kg N ha -1 reduced specific gravity from 1.076

to 1.069 (Table 15). These results are in agreement with those of Painter and Augustin (1976)

and Kleinkopf et al. (1981), where N application was reported to be associated with reduced

specific gravity of potato tubers.

The main effect of P on the specific gravity of tubers was found to be highly significant, though

the results were inconsistent (Table 15). The highest specific gravity due to applied P was

associated with the P rate of 26.4 kg P ha -1. In previous studies, conflicting conclusions were

reported with respect to the effect of applied P on the specific gravity of potato tubers. Human

(1961) noted an increase in specific gravity in response to an increase in applied P. Results by

Zandstra et al. (1969) and Dubetz (1975), however, showed the absence of strong relationship

between P application and specific gravity.

There was a significant decrease in the specific gravity due to the application of potassium.

Increase in the rate of applied K from 0 to 93.5 kg K ha-1 signifiicantly decreased the specific

gravity of tubers from 1.074 to 1.072. Similarly, Wetermann et al. (1994a) observed that K


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4.6. Potato Plant Growth Parameters

4.6.1. Plant Height

Application of N and P highly significantly increased the height of potato plant. However, the

difference in mean plant height between the two K levels was not statistically significant (Table

15).

Increasing application of N from 0 to 165 kg N ha-1 increased plant height from 48.19 cm to

61.88 cm. Similarly, increasing the rate of P application from 0 to 39.6 kg Pha-1 increased plant

height from 54.02 cm to 57.48 cm. However, the effect of P was significant only at the rate of

39.6 kg Pha-1, below which no significant difference in plant height was observed. Yohannes

(1994), working on enset, also observed a significant increment in the height of enset plants as

the rates of N and P applications were increased. The positive and highly significant correlation

value (r = 0.77**) between applied N and plant height further corroborates the present finding.

Similarly, a significant and positive correlation (r = 0.21*) was observed between applied P and

plant height (Appendix Table 2).

The positive and highly significant correlation value between plant height and total tuber yield (r

= 0.79**) might suggest the existence of positive association between these two parameters


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4.6.2. Days to Flowering and Days to Maturity

The main effects of N, P and K on days to flowering and days to maturity are shown in Table

17. The main effects of N and P on days to flowering were highly significant, while all the

interaction effects were statistically non-significant. Contrary to the effects of N and P, there

was no significant difference in days to 50 % flowering between the two K levels.

Increasing N application from 0 to 165 kg N ha -1prolonged the time required by the potato

plants to attain 50% flowering from 54.25 to 60.25 days (Table 17). Similar to the effect of

increased N application, increasing P application from 0 to 39.6 kg Pha-1prolonged the days

to 50% flowering stage from 56.5 to 58 days (Table 17).

Regarding the time required by the potato crop to reach physiological maturity, nitrogen

treatment highly significantly prolonged the time required by the potato crop to reach

physiological maturity from 106.38 to 110.92 days (Table 17). The increased yield obtained

due to N fertilizer may be attributed to the prolonged canopy life of the potato plant in

response to N treatment, which enabled the potato plant to maintain physiological activity for

an extended period, thereby continuing photosynthesis.


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4.6.3. Plant Biomass Yield

The main effects of N, P and K on the underground and aboveground dry matter yield are

shown on Table 17. The main effects of N and P on underground and aboveground dry matter

yield were highly significant. Increasing the application of N from 0 to 165 kg N ha -1 highly

significant increased the underground dry matter yield (0.19 to 0.31 t ha -1). Application of P

increased the underground dry matter yield by 23% over the control (Table 17). Although the

difference in underground dry matter yield between the K levels was statistically non-

significant, a slight increment (from 0.24 to 0.25 t ha -1) in underground dry matter yield was

recorded due to applied K. Similarly, the main effect of K on the aboveground dry matter yield

was non-significant (Table 17).

Regarding the interaction effect of the three nutrients, only the N x P interaction was found to

be significant in increasing the underground dry matter yield (Table 18). The effect of N on

the underground dry matter yield appeared to improve due to the presence of P (the same

holds for P).

The main effects of N and P on the aboveground dry matter yield were highly significant(Table 17). In general, N and P treatment increased the aboveground dry matter yield from

1 26 to 2 90 t ha-1 and from 1 82 to 2 20 t ha -1 respectively


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These results appeared to support the general view that yield is a function of the amount of

carbon assimilates produced (Millard and Marshall, 1986). The concomitant increment in yield

with the increase in dry matter yield of a plant may be explained by the fact that as more dry

matter is produced by the plant more assimilates would be partitioned to harvestable parts,

hence increasing yield.


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4.6.4. Harvest Index

The main effects of N and P were highly significant. However, the main effect of applied K on

harvest index was found to be non-significant (Table 15).

Increasing the application of N from 0 to 165 kg N ha -1 decreased harvest index from 0.75 to

0.68. Similarly, application of P highly significantly reduced harvest index from 0.74 to 0.72.

The reduction in harvest index due to N and P did not appear to be associated with a decrease

in total tuber yield. This is because the total biomass increased more than the harvestable

portion in response to the application of N and P. Therefore, the yield advantage obtained

through the use of N and P fertilizers might not be attributed to its effect on increment of

harvest index; rather a parallel increase in both harvestable and non-harvestable parts was

apparent. In general although harvest index is commonly used as a key plant parameter it may

not necessarily correlate with high yield (Gawronska et al., 1984). This is possible where the

application of mineral nutrients enables a potato crop to exhibit a high rate of assimilate

production (high total biomass) and maintain active growth later in the season (Gawronska et

al., 1984).


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Table 18. Underground dry matter yield (t ha-1) as influenced by N x P interaction

P (P kg ha-1)N

(N kg ha-1) 0 13.2 26.4 39.6 Mean

0 0.13g 0.19f 0.20f 0.22ef 0.19d

55 0.22ef 0.24de 0.24de 0.25de 0.24c

110 0.25cde 0.25cde 0.26bcd 0.27bcd 0.26b

165 0.28bcd 0.29bc 0.29b 0.38a 0.31a

Mean 0.22d 0.24c 0.25b 0.28a

N x P interaction**


significance; LSD (0.05)= 0.036 t ha-1

to compare N x P interaction; LSD (0.05) = 0.018 t ha-1

to compare N and P

main effects on UGDM; **= indicates significant difference at 1% level of significance.


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4.7. Potato Tuber Nitrogen Content

The effects of applied N, P and K on tuber N concentration are shown in Table 15. Increasing

the application of N from 0 to 165 kg N ha -1 increased the tuber N concentration from 0.43 to

0.64 (%). Increase in applied P also highly significantly increased the N concentration of

potato tubers from 0.51 to 0.57 (%). In agreement with the results of the present study

different workers (Widdowson and Penny, 1975; Millard and Marshall, 1986; Sharma and

Arora, 1987) have observed a rise in the N concentration of potato tubers as the N and P

applications were increased.

The difference in tuber N concentration between the two K levels and all interaction effects

were also statistically non-significant. Similar to the findings of the present investigation

Kanzikwera et al. (2001) reported that K application as KCl had no significant effect on tuber

N concentration.

4.8. Selected Post-Harvest Soil Chemical Properties

Analysis of the post harvest soil samples collected at the depth of 0 30 cm revealed that the


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Table 19. Selected Post-Harvest Soil Chemical Properties as Influenced by N, P and K

Application


Variation Soil pH NH4-N (mg

kg-1 soil)

N03-N (mg

kg-1 soil)

SAP

(mg kg ha-1)

SOM (%)

K (K kg ha-1) NS ** NS NS NS

0

93.5

7.835

7.835

0.190

0.226

0.178

0.223

1.280

1.322

0.663

0.802

N (N kg ha-1) ** ** ** ** **

0

55

110

165

7.883a

7.848b

7.821ab

7.787ab

0.072c

0.094c

0.249b

0.417a

0.048d

0.123c

0.252b

0.379a

1.574a

1.382b

1.200c

1.046d

0.815b

0.882a

0.662c

0.569d

P (P kg ha-1) NS ** ** ** **

0

13.2

26.4

39.6

7.813

7.864

7.841

7.821

0.274a

0.155c

0.193b

0.211b

0.282a

0.149d

0.191b

0.180c

1.224d

1.266c

1.290b

1.422a

0.939a

0.638b

0.700b

0.652b


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5. SUMMARY AND CONCLUSION

Documentation of information on fertility status of soils and crop response to different soil

fertility amendments is one of the most important factors for profitable crop production. To

this effect a study was conducted to investigate the effect of N, P and K application on the

yield and yield components of potato (Solanum tuberosumL.). The study was carried out at

Mekelle Agricultural Research Center, North Ethiopia, which lies at an elevation of 1970

meters above sea level. A split plot design was employed with three replications, which

constituted two levels of K (0 and 93.5 kg K ha-1) as main plot and factorial combinations of

four levels of N (0, 55, 110 and 165 kg N ha -1) and four levels of P (0, 13.2, 26.4 and 39.4 kg

P ha-1) as sub-plot treatments.

Generally, it was observed that the main effects of applied N and P and their interaction effectswere found to be highly significant on average tuber weight (g), total and marketable tuber

yields (t ha-1) and tuber numbers (per hill). Stem number per hill and unmarketable tuber

weight (t ha-1) were, however, affected neither by N nor P. It was also noted in this study that,

among the yield components, increases in both tuber number and average tuber weight were

responsible for the observed yield advantage.

On the other hand the application of K did not produce substantial effect on most of the


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The application of N, P and K were found responsible for the significant reduction in specific

gravity and dry matter content of potato tubers. Increases in applied N and P affected both

growth parameters significantly. Applied K did not, however, have a marked influence on

most of the potato growth parameters.

Results from the correlation analysis showed that N and P associated significantly positively

with all of the yield and yield components of potato. The observed non-significant correlation of

K and total tuber yield is not a common phenomenon in other areas, because most studies

showed significant and positive correlation between yield and applied K.

Although significant (due to N and P) and non-significant (due to K) responses in yield were

observed, it is too early to reach a conclusive recommendation since the experiment was

conducted only in one location for one season. Hence, studies involving more levels of Kunder various levels of soil fertility should be conducted. Moreover, studies on the effect of

different fertilizer sources of K should be conducted, as the alkalinity/acidity of soils is

important in determining the effect of K.


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6. REFERENCES

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Anderson, G.D. 1973. Potassium response of various crops in East Africa. In: Potassium in

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Dubetz, S. 1975. Effect of two depth of seed bed preparation and fertilizer on netted gem

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by four potato (Solanum tuberosum L.) clones. Crop Science. 24: 1031 1036.

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7. APPENDICES


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Appendix table 1. Soil chemical properties of the study area prior to fertilization

Exchangeable Cations (cmols/kgsoil)pH OM (%) Total N (%) Available P

(ppm) K+ Ca++ Mg++ CEC

7.5 1.80 0.15 8.60 2.34 24.19 4.17 25.6


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69

Appendix table 2. Simple correlation coefficient of different parameters with K, N and P treatments

Plant Characters Considered

TTY MTY UTY TTN MTN UTN SN PH DTF DTM ADM UDM PDM HI SG ATW TNC ST MT LT

K 0.10 0.02 0.06 0.15 0.02 0.19 0.06 0.08 0.13 0.57** 0.01 0.08 0.07 0.01 -0.24* 0.04 0.07 -0.20 -0.03 0.20

N 0.86** 0.84** 0.21 0.31** 0.31** 0.20 0.02 0.77** 0.84** 0.36** 0.96** 0.89** -0.92** -0.84** -0.70** 0.69** 0.91** 0.65** 0.75** 0.41**

P 0.30** 0.33** 0.14 0.54** 0.30** 0.03 -0.05 0.21* 0.26* -0.42* 0.23* 0.32** -0.28** -0.26* -0.22* 0.30** 0.20* 0.39** 0.32** 0.40**

TTY = total tuber yield; MTY = marketable tuber yield; UTY = unmarketable tuber yield; TTN = total tuber number; MTN = marketable tuber number; UTN =

unmarketable tuber number; SN = stem number; PH = plant height; DTF = days to flowering; DTM = days to maturity; ADM = aboveground dry matter yield;

UDM = underground dry matter yield; PDM = percent dry matter yield; HI = harvest index; SG = specific gravity; ATW = average tuber weight; TNC= tuber

nitrogen concentration; ST = yield of small sized tubers; MT= yield of medium sized tubers; LT= yield of large sized tubers.


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Appendix table 3. Simple correlation coefficients among different parameters

Plant Characters Considered

TTY MTY UTY TTN MTN UTN SN PH DTF DTM ADM UDM PDM HI SG ATW TNC ST MT LT

TTY 1.00 0.88** 0.19 0.46** 0.38** 0.21 -0.01 0.79** 0.81** 0.39** 0.88** 0.90** -0.84** -0.71** -0.55** 0.75** 0.86** 0.50** 0.79** 0.51**

MTY 1.00 0.13 0.46** 0.38** 0.21 0.07 0.79** 0.77** 0.30** 0.88** 0.86** -0.85** -0.76** -0.54** 0.71** 0.82** 0.54** 0.89** 0.69**

UTY 1.00 -0.01 0.02 0.16 0. 08 0.18 0.16 0. 10 0.17 0.10 - 0.13 -0.20 -0.12 0.14 0.20 0.23** -0.02 0.09

TTN 1.00 0.49** 0.34** 0.17 0.35** 0.49** 0.21* 0.42** 0.41** -0.39** -0.37** -0.17 0.30** 0.34** 0.17 0.46** 0.43**

MTN 1.00 0.64** 0.10 0.31** 0.37** 0.13 0.35** 0.38** -0.37** -0.32** -0.19 0.19 0.38** 0.15 0.34** 0.31**

UTN 1.00 0.21* 0.22* 0.23* 0.10 0.21** 0.21* -0.19 -0.19 -0.16 0.10 0.21* 0.11 0.19 0.24*

SN 1.00 0.08 0.81** 0.37** 0.01 0.01 0.06 -0.09 -0.04 -0.03 0.01 0.08 0.74** 0.46**

PH 1.00 0.81** 0.78** 0.80** 0.79** -0.74** -0.62** 0.47** 0.69** 0.77** 0.49** 0.04 0.04

DTF 1.00 0.31** 0.81** 0.87** -0.81** -0.70** -0.52** 0.67** 0.82** 0.51** 0.69** 0.42**

DTM 1.00 0.32** 0.46** -0.30** -0.16 -0.12 0.39** 0.28* 0.37** 0.35** 0.20*

AD

M

1.00 0.91** -0.96** -0.88** -0.68** 0.73** 0.91** 0.52** 0.79** 0.50**

UD

M

1.00 -0.92**

-0.76**

-0.50**

0.76**

0.87**

0.50**

0.85**

0.51**

PDM 1.00 0.72** 0.65** 0.76** -0.87** -0.47** 0.85** 0.51**

HI 1.00 0.65**

-0.46**

-0.75**

-0.81**

-0.63**

-0.48**

SG 1.00 -0.52** -0.62** -0.48** -0.54** -0.27

ATW 1.00 0.66** 0.36** 0.66** 0.43**

PNC 1.00 0.50 0.72** 0.44**

ST 1.00 0.44** 0.32**

MT 1.00 0.62**

LT 1.00

TTY=total tuber yield; MTY=marketable tuber yield; UTY=unmarketable tuber yield; TTN=total tuber number; MTN=marketable tuber number;

UTN=unmarketable tuber number; SN=stem number; PH=plant height; DTF=days to flowering; DTM=days to maturity; ADM=aboveground dry matter yield;

UDM=underground dry matter yield; PDM=percent dry matter yield; HI=harvest index; SG=specific gravity; ATW=average tuber weight; TNC= tuber nitrogen

concentration; ST=yield of small sized tubers; MT= yield of medium sized tubers; LT= yield of large sized tubers.

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