IPI logo

Presented at the Global Conference on Potato

6-11 December 1999, New Delhi, INDIA

Potassium and Integrated Nutrient Management in Potato

Patricia Imas1 and S.K. Bansal 2

1 International Potash Institute, Basel, Switzerland - Coordinator India,
c/o DSW, Potash House, P.O. Box 75, Beer Sheva 84100, Israel. e-mail: patricia@dsw.co.il
2 Potash Research Institute of India, Sector-19, Gurgaon-122 001, India

Contents

Abstract

Potato crop has strict requirement for a balanced fertilization management, without which growth and development of the crop are poor and both yield and quality of tubers are diminished. Among the major nutrients, potassium not only improves yields but also benefits various aspects of quality. Some of the tuber quality parameters affected by potassium nutrition are: tuber size, percentage of dry matter, starch content, internal blackening, storability and resistance to mechanical damage. Potassium also provides resistance against pest and diseases and drought and frost stresses.

At present, average yield of potato in India is about 19 t/ha, which is much below the crop potential productivity. Low use of fertilizers and serious imbalances in the N:P:K application ratio are partially responsible for this low yield. Current fertilization rates are insufficient to sustain high yields and to replenish nutrient removal by the crop.

Field experiments jointly conducted by IPI, PRII and CPRI at Shimla and Jalandhar show that a balanced N x K fertilization increased tuber yield. Increasing K doses decreased the yield of small grade tubers and increased the proportion of large marketable tubers. Potassium application also dramatically decreased the incidence of late blight. In 1998, a maximum yield of 40.8 t ha-1 was obtained at Kufri (Shimla) when applied 180 kg N ha-1, 100 kg P2O5 ha-1 and 150 kg K2O ha-1, against a tuber yield of only 14.6 t ha-1 at the control plots.

It is concluded that high yields and enhanced quality tubers can only be sustained through the application of optimal nutrient doses in balanced proportion.

back to
contents

Introduction

Maximum yield research studies, performed in small experimental plots under ideal conditions, show that fresh weight yields of potato tubers can reach at least 100 t ha-1 (Ewing, 1997). Commercial yields are certainly much lower, but can be as high as 42 t ha-1 in the Netherlands or 40 t ha-1 in Israel (FAO, 1998). This compares to 10-15 t ha-1 in many developing countries, and to the average yield of potato in India of 19 t ha-1, which is much below the crop potential productivity (FAO, 1998).

Yield increases as a result of new and improved production agrotechnologies involves fertilization. Low use of fertilizers and serious imbalances in the N:P:K application ratio are partially responsible for low yields in India. Current fertilization rates are insufficient to sustain high yields and to replenish nutrient removal by the crop. According to Grewal et al (1992), potato yield in India could be increased by almost 50% only by improved nutrient management.

Marketable yield is a function of total biomass production, the percentage of biomass that is partitioned to the tubers, the moisture content of the tubers and the proportion of tubers that are acceptable to the market, in terms of size and lack of defects (Ewing, 1997). Great opportunities exist to increase potato yield and quality by improving nutrient management.

Potato demands high level of soil nutrients due to relative poorly developed and shallow root system in relation to yield (Perrenoud, 1983). Compared with cereal crops, potato produces much more dry matter in a shorter cycle (Singh and Trehan, 1998). This high rate of dry matter production results in large amounts of nutrients removed per unit time, which generally most of the soils are not able to supply. Hence, nutrient application from external sources as fertilizers becomes essential. High yields can only be sustained through the application of optimal NPK doses in balanced proportion.

According to Perrenoud (1993), a crop yielding 37 t ha-1 removes 113 kg N, 45 kg P2O5 and 196 kg K2O per hectare. At very high yields, nutrients removal in tubers is very elevated: for example, in an irrigated field of Central Washington of commercial potato cv. Russet Burbank, yielding 79 t ha-1, N, P and K accumulation in tubers was 282 kg N ha-1, 92 kg P2O5 ha-1 and 384 kg K2O ha-1 (Fageria et al, 1997).

Potato crop is a heavy remover of soil potassium and is the nutrient taken up in the greatest quantity - the tubers remove 1.5 times as much potassium as nitrogen and 4-5 times the amount of phosphate (Perrenoud, 1993). Potato is regarded as an indicator crop for K availability because of the high K requirement (Roberts and McDole, 1985). Few soils could produce high potato yields for very many seasons without replenishing removed K.

In India, a compilation of many experiments on response of potato to optimum potassium application in the presence of optimal N and P doses, show that yield increases were as much as 52 q ha-1 in alluvial soils with 113 kg K2O ha-1, 45 q ha-1 in hill soils with 103 kg K2O ha-1, 37 q ha-1 in red soils with 125 kg K2O ha-1 and 10 q ha-1 in black cotton soils with 69 kg K2O ha-1 (Grewal et al, 1991b).

back to
contents

Potassium in physiological processes

Potassium has a crucial role in the energy status of the plant, translocation and storage of assimilates and maintenance of tissue water relations (Marschner, 1995). Potassium is not an incorporated component of plant molecules, in opposite to N and P which are constituents of proteins, nucleic acids, phospholipids, ATP, etc. Potassium predominantly exists as a free or absorptive bound cation, and can therefore be displaced very easily on the cellular level as well as in the whole plant (Lindhauer, 1985). This high mobility in the plant explains the major functional characteristics of K: as the main cation involved in the neutralization of charges and as the most important inorganic osmotic active substance (Clarkson and Hanson, 1980).

Potassium is involved in many aspects of the plant physiology (Marschner, 1995):

  • Activates more than 60 enzyme systems
  • Aids in photosynthesis
  • Favors high energy status
  • Maintains cell turgor
  • Regulates opening of leaf stomata
  • Promotes water uptake
  • Regulates nutrients translocation in plant
  • Favors carbohydrate transport and storage
  • Enhances N uptake and protein synthesis
  • Promotes starch synthesis

These multiple functions of K in many metabolic processes lead to numerous positive effects of an adequate K nutrition for potato:

  • Increases yield
  • Increases proportion of marketable tubers
  • Increases tuber size
  • Decreases internal blackening and hollow heart
  • Decreases mechanical damages to tubers
  • Decreases storage losses, enhances shipping quality and extends shelf life
  • Improves cooking and processing qualities
  • Improves chips colour
  • Improves resistance to frost and drought
  • Decreases incidence of diseases (late blight)
  • Better N use efficiency

    back to
    contents

Tuber quality

Results of experiments indicate that potassium nutrition influences tuber size, specific gravity, susceptibility to blackspot bruise, after-cooking darkening, reducing sugar content, fry color, and storage quality (Perrenoud, 1993; Martin-Prevel, 1989).

The crucial importance of potassium in quality formation stems from its role in promoting synthesis of photosynthates and their transport to the tubers, and to enhance their conversion into starch, protein and vitamins (Mengel and Kirkby, 1987). With a shortage of potassium many metabolic processes are affected, like the rate of photosynthesis, the rate of translocation and enzyme systems (Marschner, 1995; Mengel, 1997). At the same time, the rate of dark respiration is increased. The result is a reduction in plant growth and in crop quality. K influences on quality can also be indirect as a result of its positive interaction with other nutrients (especially with N) and production practices (Usherwood, 1985).

Plants require potassium for the production of high-energy molecules (ATP). Potassium maintains the balance of electric charges in chloroplasts, which is required for ATP formation. Hence, K improves the transfer of radiation energy into primary chemical energy in the form of ATP (photophosphorylation) and NADPH. This energy is required for all synthetic process in plant metabolism, resulting in production of carbohydrates, proteins and lipids, which express the quality of the crops. The high-energy status in crops well supplied with K also promotes synthesis of secondary metabolites, like vitamin C (Mengel, 1997).

Potassium plays an important role in the transport of assimilates and nutrients. The photosynthesis products (photosynthates) must be transported from the leaves (sources) to the site of their use or storage (sinks). Potassium promotes phloem transport of photosynthates (mainly sucrose and aminoacids) to the physiological sinks the tubers (Mengel, 1997). Potassium plays a positive role in phloem loading with sucrose, in increasing the transport rate of phloem-sap solutes and in phloem unloading (Herlihy, 1989). This role of K is related to its contribution to the osmotic potential in the sieve tubes and to its function in ATP synthesis which provides the energy for the loading of photosynthates (Marschner, 1995).

back to
contents

Potassium and market use

The intended market use of the harvested tubers influences the application of this nutrient more than N or P. Size of the tubers is always reported as enhanced by K, increasing the proportion of large tubers relative to small ones (Martin-Prevel, 1989). Potassium promotes larger size of potato tubers by increasing water accumulation in tubers resulting in a lowering of dry matter content and specific gravity (Perrenoud, 1993).

Potato chips: this market prefers high dry matter content, which confers robustness and crispness to the slices, and low reducing sugar content, to avoid dark browned chips. The lowering in the percent of dry matter caused by heavy K applications can eliminate tubers from the potato chips market.

French fries: percent dry matter is less important for most frying varieties, for which tuber length, blockiness and weight are ideal.

Tablestock: dry matter content is unimportant and big tubers are preferred. For these two markets, tablestock and frystock, K is applied to increase tuber size grades.

Seedstock: on the contrary, for seed production, the target is to keep tubers smaller than the other markets, therefore less K application is desired.

back to
contents

Tuber composition

Dry matter content: Potassium influences the water content of the plasma volume, thus affecting the water content of fleshy storage tissues, like the tubers. Elevated K concentrations in the tubers of above 2% DW can therefore lead to above-normal water contents and lower contents of dry matter (Bergmann, 1992).

Starch content: Potassium is involved in the activation of the enzyme starch synthase which is responsible of the synthesis of starch. Potassium is the most efficient cation stimulating the activity of this enzyme that catalyzes the incorporation of glucose into long-chain starch molecules (Mengel and Kirkby, 1987). Although potassium activates enzymes involved in starch formation, K can reduce starch content through an increased water content in the tubers (Perrenoud, 1993). It is generally agreed that starch content is enhanced by K application, so long as this is to correct K under-nutrition, whilst heavy doses of K may decrease starch content (Martin-Prevel, 1989).

Reducing sugar content: in order to receive chips with a desirable light color, glucose+fructose content of potato should not exceed 0.25% (Perrenoud, 1993). Potassium deficiency changes carbohydrate metabolism, with negative consequences such as accumulation of soluble carbohydrates and in decrease in starch content (Mengel and Kirkby, 1987). Accumulation of reducing sugars and decrease of starch in potato tubers are the cause of undesired dark-colored potato chips which occur under low K nutrition levels (Martin-Prevel, 1989; Perrenoud, 1983; Usherwood, 1985).

Vitamin content: High concentrations of K usually lead to an increase in organic acid concentration, also having a beneficial effect on ascorbic acid levels (Bergmann, 1992). Some experiments show an increase of vitamin C and in the tubers (Perrenoud, 1993).

back to
contents

Physiological Disorders

Internal blackening (also enzymatic blackening, or browning, or black spot): Responsible for significant postharvest losses, the blackening is caused by the oxidation of phenolic substances, mainly tyrosine to melanin (Martin-Prevel, 1989). Usually it is associated with to over-fertilization with nitrogen and low soil potassium availability. There is much evidence that potassium reduces susceptibility to internal blackening (Roberts and McDole, 1985). Potassium deficient potatoes have a higher content of soluble, non protein N and tyrosine (Usherwood, 1985).

Hollow heart: Consists of cavities in the tuber which are lined with necrotic, brown tissue (Bergmann, 1992). Nelson (1970) and Jackson and Mc Bride (1986) found that potassium application reduced the incidence of hollow heart. The application of KCl was more effective than K2SO4 in reducing the percentage of of tubers affected by hollow hearts and browning (Jackson and Mc Bride, 1986).

Lately, it has been suggested that the suppresion effect of KCl is due to the chloride application rather than potassium effect (Fixen, 1993).

back to
contents

Storage and shelf life

Potassium enhances storage and shipping quality of potatoes, and also extends their shelf life (Martin-Prevel, 1989; Perrenoud, 1993). The effects of potassium on shelf life are dominantly favorable, both through slowing of senescence and through a decrease of numerous physiological diseases (Martin-Prevel, 1989). Potassium application reduces storage losses of tubers, and this was related to reduction in the activity of catalase and peroxidase enzymes (Perrenoud, 1993).

In India, application of K tended to decrease gradually the weigth loss of tubers from 20% to 16% (Grewal et al, 1991b). In other experiment, dry matter loss with application of 100 kg K2O ha-1 was only 5.6% as compared to 20.3% without K application (Perrenoud, 1993).

back to
contents

Frost injury

Plants receiving an inadequate K supply are often more susceptible to frost damage (Marschner, 1995). Improved frost hardiness is attributed to a number of physiological and morphological factors like: healthy, deep roots, large xylem vessels, high content of sugars and reserve carbohydrates, reduced transpiration and water loss (Kemmler and Krauss, 1989). Potassium acts positively on most of these factors thus decreasing winter injury.

Adequate supply of K to plant can increase the osmotic potential in cell vacuoles, and thus increasing the plant chilling tolerance. It is recommended to keep high K concentrations in the soil and in the plant in order to increase the soluble carbohydrate content that may reduce the damage to plant tissues in case of a cold stress (Kafkafi, 1990).

Grewal and Singh (1980) found for potato an inverse relationship between potassium content in leaves and the percentage of foliage damage by frost in 14 field experiments conducted in India. Increasing K application, increased tuber yield and the potassium content in leaves, which in turn reduced frost damage. Another experiments in India show that the application of 200 kg K2O ha-1 reduced frost damage from 38% to 7% (Perrenoud, 1993).

In India, KCl has established its superiority over K2SO4 in developing frost resistance in potato (Grewal et al, 1992). Therefore, in the north-western plains of India where frost is a problem, the application of KCl is recommended.

back to
contents

Diseases

The role of potassium in increasing crop resistance to diseases caused by bacteria and fungi was widely reviewed by Perrenoud (1990). In general, potassium application improves plant health and vigour, making infection less likely or enabling a quick recover (Perrenoud, 1993).

Potassium probably exerts its greatest effects on disease through specific metabolic functions that alter compatibility relationships of the host-parasite environment. Potassium in plant increases the production of disease inhibitory compounds, such as phenols, phytoalexins and auxins around infection sites of resistant plants. Under low plant K conditions, inorganic N accumulates and phenols, that have fungicidal properties, are rapidly broken down (Kiraly, 1976). In addition, K deficiency leads to thinner walls and slower growth of meristematic tissue, making easier for the parasites to penetrate the epidermis (Bergmann, 1992).

In potato, potassium fertilization was found to decrease the incidence on several diseases, such as late blight (Phytophtora infestants), dry rot (Fusarium ssp.), powdery scab (Spongospora subterranea) and early blight (Alternaria solanii) (Perrenoud, 1993; Marschner, 1995). In India, experiments conducted at Jalandhar (Punjab) and at Shimla (H.P.) showed that the application of potassium reduced significantly the incidence of late blight (R.C. Sharma and J.P. Singh, personal communication).

back to
contents

Potassium source

Field experiments in Wisconsin, where K was applied as KCl or K2SO4, did not show statistically significant differences in total tuber yield between the two sources of K fertilizer studied (Panique et al, 1997). A survey of different experiments in India shows that both sources are almost equally effective in increasing tuber yield (Grewal et al, 1991b; Grewal et al, 1992). In alluvial soils of Punjab, both KCl and K2SO4 were equally efficient with regard to yield, processing and keeping quality of tubers (Singh et al, 1996). Westermann et al (1994) suggested that K fertilizers can be applied to potato according to their soil test values and the crop requirements, without considering the K fertilizer source.

Under good irrigation management and moderate rates, very little difference in tuber yield and quality has been observed in the field. Because of the higher cost of potassium sulfate and potassium nitrate and the lack of consistent improvement in tuber yield, these sources of K are not recommended over potassium chloride. Application of K2SO4 may be preferred in S-deficient soils; or under conditions of soil salinity and/or saline irrigation water.

In potatoes for processing, high dry matter content is desirable. Using K2SO4 usually gives a higher dry matter content than KCl. In addition, K2SO4 normally gives higher starch content than KCl (Perrenoud, 1993).

back to
contents

Application of potassium fertilizers

The potato plant needs potassium from early stage of plant growth because of its positive effect on root growth, therefore K application at planting is needed. It is a common practice to surface broadcast K fertilizer before planting with incorporation into the soil during seedbed preparation (Roberts and McDole, 1985).

Deficiency symptoms of K usually manifest at tuber initiation, when K uptake rate is maximum (Roberts and McDole, 1985). Thus in sandy soils, where K may be lost by leaching, it is recommended to apply K in two splits (half at planting and half at earthing up). This practice may give better results than the entire dose applied at planting (Grewal et al, 1991b).

Banding potassium fertilizers near the potato seed is the most efficient method of application. However, if a soil test recommends the addition of a large quantity of potassium then it should be split between a pre-plant broadcast and an at-plant banding to avoid potential problems with salt toxicity and/or K leaching.

back to
contents

Potassium deficiency symptoms in potato

Deficit of K is most likely in leaching soil types especially sandy soils, and/or in acid soils with a low cation exchange capacity. Due to the mobility of K ions, the first symptoms of K deficiency appear in the older leaves. Early symptoms of K deficiency are a dark greening or bluish greening of foliage. Leaves appear glossy, and tiny, light green spots develop between the veins of larger leaves. In the upper canopy, leaf margins curl down and leaflets are small, cupped and crowded (Bergmann, 1992; Perrenoud, 1993; Singh and Trehan, 1998).

Leaf scorch is also a typical potassium deficiency symptom. Older leaves turn bronze then brown (necrotic) and die early. The key symptom is the overall bronzing of the canopy. A severe deficiency results in short plants with shortened. internodes, poor root growth and shortened stolons. Tubers are predisposed to black spot bruising and hollow heart and are disease-susceptible (Bergmann, 1992; Perrenoud, 1993; Singh and Trehan, 1998).

back to
contents

Recent experiments on potassium fertilization conducted in India

In the north-western states of Punjab and Himachal Pradesh, potatoes are grown on nearly 50,000 ha; with a mean tuber yield of 150 q ha-1 (Grewal et al, 1992). Unbalanced fertilization in favour of nitrogen and lack of potassium application is one of the major reasons responsible for these low yields (Singh, 1999). Therefore, field experiments were conducted during 1996 to 1998 to study the effect of K and N fertilization on tuber yield and quality in two potato growing zones of India: Jalandhar, Punjab (plains) and Shimla, H.P. (hills).

The treatments consisted of 3 levels of K (as KCl), in split application (half at planting and half at earthing up): 0, 75 and 150 kg K2O ha-1. Potassium doses were combined with 3 levels of nitrogen (as urea), in split half application: 60, 120 and 180 kg N ha-1 (in Shimla) and 80, 160 and 240 kg N ha-1 (in Jalandhar). Phosphorus was applied as SSP basal dose of 100 kg P2O5 ha-1. The variety was Kufri Jyoti.

The experiments at Jalandhar were conducted at six farmers fields. The soils were Typic Ustrochrepts, with available K content ranging from 100 to 206 kg K2O ha-1.

The results of the 1998-99 season (Table 1) show that tuber yield increased with increasing N and K application: potato yield was 147, 180 and 193 q ha-1 due to main effect of 80, 160 and 240 kg N ha-1 respectively; and 134, 184 and 202 q ha-1 due to main effect of 0, 75 and 150 kg K2O ha-1 respectively. There was a significant positive interaction between N and K: at each level of N, increasing levels of K application increased the tuber yield. Application of 150 kg K2O ha-1 increased the tuber yield over no K treatment by 35%, 54% and 61% at 80, 160 and 240 kg N ha-1 respectively.

Potassium and nitrogen application also improved tuber size by increasing the medium and large grades and decreasing the medium and small sized tubers (Table 1). In average, K application increased percentage of large tubers from 29% (0 kg K2O ha-1) upto 40% (75 kg K2O ha-1 ) and further upto 44% (150 kg K2O ha-1).

Treatments had no effect on tuber dry matter, specific gravity, and content of vitamin C, reducing sugars and starch (not presented).

Table 1: Effect of N and K application on yield and tuber size of potato at Jalandhar (mean of six farmers fields, 1998-99 season).
Application rate Tuber yield
N K Total Large Medium Small
kg N ha-1 kg K2O ha-1 > 75 g 25-75 g < 25 g
q ha-1 q ha-1 % q ha-1 % q ha-1 %
80 0 123 33 27 75 61 14 11
80 75 154 58 38 82 53 13 8
80 150 166 62 37 88 53 15 9
160 0 138 41 30 82 59 15 11
160 75 188 69 37 106 56 13 7
160 150 213 97 46 101 47 16 7
240 0 141 44 31 81 57 17 12
240 75 211 94 45 101 48 15 7
240 150 227 104 46 108 47 15 7
CD (5%)
N 8 8 - 7 - NS -
K 8 8 - 7 - NS -
N x K 14 15 - NS - NS -

The experiments at Shimla were conducted on acid brown hill soils. The results show a highly significant response to N and K application (Table 2). The tuber yield increased in average by 10% (75 kg K2O ha-1) and by 33% (150 kg K2O ha-1) as compared with no K application. The effect was more evident on the yield of larged sized tubers (not presented). Similarly, N application also increased tuber yield upto an application rate of 180 kg N ha-1. In 1998, a maximum yield of 408 q ha-1 was obtained at when applied 180 kg N ha-1, 100 kg P2O5 ha-1 and 150 kg K2O ha-1, against a tuber yield of only 146 q ha-1 at the control plots without any NPK application.

Application of K increased the N concentration in tubers and N uptake (Table 2), which suggests that potassium improved N utilization by the plant. Treatments had no effect on tuber dry matter and content of vitamin C, reducing sugars and starch (not presented). However, new high-yielding processing varietes recently released by CPRI, like Chipsona and Anand, might be more responsive to nutrient management regarding quality parameters.

 

Table 2: Effect of N and K application on tuber yield (1996-98) and on NPK uptake (1998) at Kufri, Shimla.
Application rate Total tuber yield Nutrient uptake (1998)
N K 1996 1997 1998 N P K
kg N ha-1 kg K2O ha-1 q ha-1 kg ha-1
60 0 238 204 208 55 9 75
60 75 262 208 254 57 10 89
60 150 285 229 296 72 12 111
120 0 254 221 292 78 13 125
120 75 272 258 334 90 14 134
120 150 297 279 358 98 16 146
180 0 276 264 303 82 12 108
180 75 258 297 325 100 14 127
180 150 260 300 408 118 16 159
CD (5%) for N/K 18 23 27 7.1 1.6 13.3

At both experiments, potassium application helped in imparting resistance against late blight. This effect was specially notable in 1997-98, year of severe late blight incidence.

Increase in total yield and the yield of large tubers due to K fertilization may stem from the stimulating effect of potassium on photosynthesis, phloem loading and translocation, as well as synthesis of large molecular weight substances within storage organs, contributing to the rapid bulking of the tubers (Singh, 1999).

The optimum doses of N and K for Kufri Jyoti in both regions have been presented by Grewal et al (1992): 220 kg kg N ha-1 and 150 kg K2O ha-1 in the alluvial soils of Jalandhar; while in acid hills of Shimla the recommended application rates are 130 kg kg N ha-1 and 91 kg K2O ha-1. According to the results of the experiments presented here, K optimal dose at Shimla can be further increased, as further positive effects of K were achieved at an application rate of 150 kg K2O ha-1.

Fertilizer recommendations can be refined if soil test values are taken into consideration. For alluvial soils of Jalandhar, the critical limit for soil available K (ammonium acetate) is 105 ppm; while for acidic soils of Shimla the limit is 120 ppm (Grewal et al, 1991a). Fertilizer adjustment equations based on targeted yields and soil tests have been calibrated for different varieties and soil types (Grewal et al, 1992).

The need for higher fertilizer doses than the presently recommended, may stem from the intensive potato-based cropping systems which may led to a decline in available K due to removal in excess of applied K. Sharma et al (1999) state that fertilizer doses for potatoes grown in 300% cropping intensity should be raised by 50% of the recommended one in order to get more profit. Potato and other heavy feeder crops (rice, sunflower) in the rotation may cause severe net balance of K in soil in short time spans of 4 years (Singh and Trehan, 1998). It is concluded that high yields can only be sustained through the application of optimal nutrient doses in balanced proportion.

back to
contents

Acknowledgements

We would like to express our gratitude to CPRI (Central Potato Research Institute of India) - our partners in the potato experiments on nutrient management; at Shimla: Dr G.S. Shekhawat (Director CPRI), Dr R.C. Sharma and Dr K.C. Sood; at Jalandhar: Dr Kang (Director CPRS), Dr J.P. Singh and Dr S.P. Trehan.

back to
contents

References

1. Bergmann, W. 1992. Nutritional Disorders of Plants. Gustav Fischer Verlag, New York.

2. Clarkson, D.T. and J.B. Hanson. 1980. The mineral nutrition of higher plants. Ann. Rev. Plant Physiol. 31: 239-298.

3. Ewing, E.E. 1997. Potato. In: The Physiology of Vegetable Crops (Ed.: H.C. Wien). CAB International, UK. pp. 295-344.

4. Fageria, N.K., V.C. Baligar and C.A. Jones. 1997. Growth and Mineral Nutrition of Field Crops. 2 nd Edition. Marcel Dekker Inc., New York.

5. FAO. 1998. Production Yearbook. Vol. 52. FAO Statistics Series No. 125. Food and Agriculture Organization of the United Nations, Rome.

6. Fixen, P. E. 1993. Crop responses to chloride. Adv. Agron. 50: 107-150.

7. Grewal, J.S. and S.N. Singh. 1980. Effect of potassium nutrition on the frost damage to potato plants and yield in alluvial soils of Punjab. Plant and Soil 57: 105-110.

8. Grewal, J.S., R.C. Sharma and S.S. Saini. 1992. Agrotechniques for Intensive Potato Cultivation in India. Indian Council of Agricultural Research, New Delhi.

9. Grewal, J.S., K.C. Sud and R.C. Sharma. 1991a. Soil and plant tests for potato. Technical Bulletin No. 29. Central Potato Research Institute, Shimla, India.

10. Grewal, J.S., S.P. Trehan and R.C. Sharma. 1991b. Phosphorus and potassium nutrition of potato. Technical Bulletin No. 31. Central Potato Research Institute, Shimla, India.

11. Herlihy, M. 1989. Effect of potassium on sugar accumulation in storage tissue. In: Proceedings of the IPI 21 st Colloquium on: Methods of K Research in Plants, held at Louvain-la-Neuve, Belgium, 19-21 June 1989. International Potash Institute, Bern, Switzerland. pp. 259-270.

12. Jackson, T. L. and R.E. McBride. 1986. Yield and quality of potatoes improved with potassium and chloride fertilization. In: Special Bulletin on Chloride and Crop Production No. 2. (Ed.: T. L. Jackson). Potash & Phosphate Institute, Atlanta, Georgia. pp. 73-83.

13. Kafkafi, U. 1990. The functions of plant K in overcoming environmental stress situations. In: Proceedings of the IPI 22 nd Colloquium on: Development of K Fertilizer Recommendations, held at Soligorsk, USSR, 18-23 June 1990. International Potash Institute, Bern, Switzerland.

14. Kemmler, G. and A. Krauss. 1989. Potassium and stress tolerance. In: Proceedings of the Workshop on: The Role of Potassium in Improving. Fertilizer Use Efficiency, held at Islamabad, Pakistan, 21-22 March, 1987. National Fertilizer Development Center, Islamabad, Pakistan. pp. 187-202.

15. Kiraly, Z. 1976. Plant disease resistance as influenced by biochemical effects of nutrients in fertilizers. In: Proceedings of the IPI 12 th Colloquium on: Fertilizer Use and Plant Health, held at Izmir, Turkey, 1976. International Potash Institute, Bern, Switzerland. pp. 33-46.

16. Lindhauer, M.G. 1985. The role of potassium in the plant with emphasis on stress conditions (water, temperature, salinity). In: Proceedings of the Potassium Symposium. Pretoria, October 1985. Department of Agriculture and Water Supply, International Potash Institute and Fertilizer Society of South Africa. pp. 95-113.

17. Marschner, H. 1995. Mineral Nutrition of Higher Plants. 2 nd Ed. Academic Press, London.

18. Martin-Prevel, P.J. 1989. Physiological processes related to handling and storage quality of crops. In: Proceedings of the 21 st IPI Colloquium on: Methods of K Research in Plants, held at Louvain-la-Neuve, Belgium, 19-21 June 1989. International Potash Institute, Bern, Switzerland. pp. 219-248.

19. Mengel, K. 1997. Impact of potassium on crop yield and quality with regard to economical and ecological aspects. In: Proceedings of the IPI Regional Workshop on: Food Security in the WANA Region, the Essential Need for Balanced Fertilization, held at Bornova, Izmir, Turkey, 26-30 May 1997. International Potash Institute, Bern, Switzerland. pp. 157-174.

20. Mengel, K. and E.A. Kirkby. 1987. Principles of Plant Nutrition. 4 th Edition. International Potash Institute, Bern, Switzerland.

21. Nelson, D.C. 1970. Effect of planting date, spacing and potassium on hollow heart in Norgold Russet potatoes. Am. Potato J. 47: 130-135.

22. Panique, E., K.A. Kelling, E.E. Schulte, D.E. Hero, W.R. Stevenson and R.V. James. 1997. Potassium rate and source effects on potato yield, quality and disease interaction. Am. Potato J. 74: 379-398.

23. Perrenoud, S. 1990. Potassium and Plant Health. IPI Research Topics No. 3. 2 nd Edition. International Potash Institute, Bern, Switzerland.

24. Perrenoud, S. 1993. Fertilizing for High Yield Potato. IPI Bulletin 8. 2 nd Edition. International Potash Institute, Basel, Switzerland.

25. Roberts, S. and R.E. Mc Dole. 1985. Potassium Nutrition of Potatoes. In: Potassium in Agriculture (Ed: R.S. Munson). ASA-CSSA-SSSA, Madison, WI. pp. 800-818.

26. Sharma, R.C., S.P. Trehan, S.K. Roy and D. Kumar. 1999. Nutrient management in potato. Indian Farming 49: 52-54.

27. Singh, J.P. 1999. Potassium fertilization of potatoes in north India. In: Proceedings of IPI Workshop on: Essential Role of Potassium in Diverse Cropping Systems, held at the 16 th World Congress of Soil Science, Montpellier, France, 20-26 August 1998. International Potash Institute, Basel, Switzerland. pp.123-127.

28. Singh, J.P., J.S. Marwaha and J.S. Grewal. 1996. Effect of sources and levels of potassium on potato yield, quality and storage behaviour. J. Indian Potato Assoc. 23: 153-156.

29. Singh, J.P. and S.P. Trehan. 1998. Balanced fertilization to increase the yield of potato. In: Proceedings of the IPI-PRII-PAU Workshop on: Balanced Fertilization in Punjab Agriculture, held at Punjab Agricultural University, Ludhiana, India, 15-16 December 1997. pp. 129-139.

30. Usherwood, N.R. 1985. The role of potassium in crop quality. In: Potassium in Agriculture (Ed: R.S. Munson). ASA-CSSA-SSSA, Madison, WI. pp. 489-513.

31. Westermann, D.T., T.A. Tindall, D.W. James and R.L. Hurst. 1994. Nitrogen and potassium fertilization of potatoes: yield and specific gravity. Am. Potato J. 71: 417-431.

back to
contents