IPI International Potash Institute
IPI International Potash Institute

Optimizing Crop Nutrition, Potassium in Soil, Plant and Agro ecosystem

Presented at IPI-PRII-KKV Workshop on:

Recent Trends in Nutrition Management in Horticultural Crops

11-12 February 1999, Dapoli, Maharashtra, INDIA

Quality Aspects of K Nutrition in Horticultural Crops

Patricia Imas - International Potash Institute, Coordination India. c/o DSW, Potash House, P.O.Box 75, Beer Sheva, 84100, Israel. E-mail: patricia@dsw.co.il

Contents

Introduction
Involvement of potassium in physiological processes relevant to crop quality
Energy
Photosynthesis
Enzymes
Translocation
Tomato
Citrus
Banana
References

Introduction

All vegetable and fruit crops have strict requirement for a balanced fertilization management, without which growth and development of the crop is poor and the harvested part (roots, leaves, tubers, flowers or fruits) lacks visual qualities such as color and shape. Many horticultural crops are heavy removers of nutrients and high yields can only be sustained through the application of optimal doses in balanced proportion. Among the major nutrients, potassium not only improves yields but also benefits various aspects of quality. Hence, potassium fertilization results in a higher value product and therefore in a greater return to the farmer.

The term quality refers to several aspects which affect the marketability of the product, and includes:

  • Attractiveness:
    • Uniform size
    • Big size
    • Good color
  • Organoleptic:
    • Enhanced flavor
    • Enhanced aroma
  • Nutritional value:
    • % protein, % oil, vitamin C, etc.
  • Intact state:
    • Free of blemishes or unusual markings (mechanical injuries)
    • Free of any sign of disease
  • Long shelf life
  • Adequate processing quality for industry.

Potassium is often referred as the quality element for crop production (Usherwood, 1985). Potassium has been widely proven to have a crucial role in many crop quality parameters. For example, in citrus, K increases fruit size, reduces fruit creasing and cracking, improves fruit color, increases vitamin C and soluble solids content and peel thickness. Potassium also enhances storage and shipping quality of bananas, tomatoes, potatoes, onions and many other crops, and also extends their shelf life (Usherwood, 1985; Geraldson, 1985; Koo, 1985; Von Uexkll, 1985; Bhargava et al., 1993; Mengel, 1997).

The crucial importance of potassium in quality formation stems from its role in promoting synthesis of photosynthates and their transport to fruits, grains, tubers, and storage organs and to enhance their conversion into starch, protein, vitamins, oil etc. (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 nitrogen) and production practices (Usherwood, 1985).

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Involvement of potassium in physiological processes relevant to crop quality

Potassium has a crucial role in the energy status of the plant, translocation and storage of assimilates and maintenance of tissue water relations. 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. K+ 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 in leaves

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Energy

Plants require potassium for the production of high-energy molecules (ATP) which are produced both in photosynthesis and transpiration processes (Willingford, 1980). 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 (FeIII cyanide reduction in the chloroplasts (Figure 1). This energy transfer is a fundamental process in the plant and an adequate K supply guarantees high levels of energy in the form of ATP and NADPH (Pfluger and Mengel, 1972). 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).

Figure 1: Photoreduction (reduction of FeIII cyanide) and photophosphorylation in chloroplasts as a function of K concentration (K1 suboptimum K supply, K2 optimum K supply) (Pfluger and Mengel, 1972).

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Photosynthesis

Potassium affects the photosynthesis process in many levels, affecting several processes (Marschner, 1995):

  • Synthesis of ATP needed for the photosynthesis reaction
  • Activities and efficiencies of the enzymes involved in photosynthesis (like RuBP carboxylase)
  • CO2 uptake into the leaves(stomata opening)
  • Balance of electric charges needed for photophosphorylation in chloroplasts
  • Counterion to the light-induced H+ flux across the thylakoid membranes

The rate of photosynthesis is measured as the rate of CO2 assimilation. Photosynthesis requires adequate K levels in leaf tissue: in corn, maximum CO2 fixation happens when leaf K concentration is 1.7-2.0%, lower K levels decreases photosynthesis very sharply (Smid and Peaslee, 1976). Table 1 shows the role of potassium in CO2 assimilation in alfalfa leaves, the increase in CO2 assimilation is accompanied by an increase in photorespiration and a decrease in dark respiration. The effect of K in stomatal regulation and in the activation of ribulose biphosphate carboxylase is also shown (Marschner, 1995).

Table 1: Relationship between potassium content in leaves, CO2 exchange and RuBP carboxylase activity in alfalfa (Marschner, 1995).

Leaf Potassium

Stomatal resistance

Photo Synthesis

RuBP carboxylase activity

Photo respiration

Dark respiration

mg/g dry wt s/cm mg CO2/dm2/h µmol CO2/mg protein/h dpm/dm2 mg CO2/dm2/h
12.8 9.3 11.9 2.8 4.0 7.6
19.8 6.8 21.7 4.5 5.9 5.3
38.4 5.9 34.0 6.1 9.0 3.1

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Enzymes

Potassium is involved in the activation of more than 60 enzymes, including synthetases, oxidoreductases, dehydrogenases, transferases and kinases. These enzymes are necessary for essential plant processes such as energy utilization, starch synthesis, N metabolism and respiration (Wallingford, 1980).

Let us take as an example the enzyme starch synthase which is responsible of the synthesis of starch. Figure 2 shows the effect of different cations on the activity of starch-synthase that catalyzes the incorporation of glucose into long-chain starch molecules (Mengel and Kirkby, 1987). It can be seen that potassium is the most efficient cation stimulating this enzyme. Optimum K nutrition results in higher concentration of starch in the plant, and therefore on quality of crops grown for this material. Vegetables that storage starch can have a total carbohydrate content of more than 30% (Mengel, 1997).

The high-energy status provided by starch accumulation is also of importance for water stress and winter hardiness. On the contrary, 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).

mµ Moles
ADP/hr/µg
Protein
  Cation concentration, nM
Figure 2: Effect of different monovalent cations on the activity of starch synthetase isolated from sweet corn (Mengel and Kirkby, 1987).

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Translocation

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 (fruits, roots, tubers, seeds and grains) (Mengel, 1997). K 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. In plants well supplied with K, the concentration of potassium, the osmotic potential of the phloem sap and the volume flow rate, are all higher than in plants supplied with a lower K level. As a result, sucrose concentration in the phloem sap is increased (Marschner, 1995).

Potassium not only promotes the translocation of newly synthesized photosynthates but also has a beneficial effect on the mobilization of stored material (Mengel and Kirkby, 1987).

Potassium plays also an important role as counterion for nitrate transport in the xylem. After nitrate reduction in shoot, charge balance has to be maintained by corresponding net increase in organic acid anions. Part of these organic anions (mainly malate) can be retranslocated with K as the accompanying cation through the phloem to the roots (Marschner, 1995 Figure 3).

Figure 3: Circulation of K between shoot and root in relation to nitrate and malate transport. PEP= phosphoenol pyruvate (Marschner, 1995).

 

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

  • Increases root growth
  • Improves drought resistance
  • Reduces water loss and wilting
  • Enhances winter hardiness
  • Improves resistance to pests and diseases
  • Builds cellulose and reduces stalk lodging
  • Increases nodulation of legumes

The specific effects of K on quality improvement are:

  • Increases protein content of plants
  • Increases starch content in grains and tubers
  • Increases vitamin C and solid solubles content
  • Improves fruit color and flavor
  • Improves size of fruits and tubers
  • Increases peel thickness
  • Reduces physiological disorders (creasing and cracking in citrus, blotchy ripening complex in tomato, etc)
  • Reduces incidence of pests and diseases
  • Enhances storage and shipping quality
  • Extends shelf life

Three important horticultural crops tomato, citrus and banana - were chosen here to illustrate the positive influence of potassium on yield and quality parameters. Similar results were obtained for a wide range of other horticultural crops.

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Tomato

Potassium is absorbed in amounts largest of any the other nutrients: total K uptake is 150-300 kg K2O/ha for outdoor crop yielding 40-50t/ha, and 600-1000 kg K2O/ha for greenhouse crop yielding over 100 t/ha (IFA, 1992). Potassium deficiency results in slow and stunted growth, and reduction in yield and percent of fruits suitable for marketing (Usherwood, 1995).

High rate of K is needed to achieve not only highest total fruit production but also the greatest percentage of fruit production suitable for marketing. With proper K nutrition, tomato fruit is generally higher in total solids, sugars, acids, carotene and lycopene, as well as longer shelf life (Usherwood, 1985). With increased K doses, uneven ripening, hollow fruits and irregularly shaped fruits were decreased (Figure 4). In the same experiment, the titrable acidity increased at higher applied K (Windsor, 1973).

Red color development in fruits is due to carotenoid pigments, particularly lycopene, which is synthesized more at adequate K levels (Usherwood, 1985).

A high-unbalanced N:K ratio is associated with poor set fruit and poor carrying quality. According to Geraldson (1985), at the beginning of the season the N:K ratio must be 1:3 and then the ratio should be increased progressively to a 1:1 ratio.

%
  K applied (kg/ha)
Figure 4: Effect of potassium on some parameters of tomato fruit quality (Windsor, 1973).

Potassium is also known to reduce the incidence of physiological disorders in the tomato fruits which affect their marketing quality, such as puffiness, blotchy ripening complex, greywall, gold flecks, and greenback (Table 2, Kinet and Peet, 1997).

Table 2: Physiological disorders in tomato fruits and potassium nutrition (Kinet and Peet, 1997).

Disorder Symptoms Remarks
Puffiness
Hollowness
boxiness
Puffy fruits lacks some or all of the gel normally surrounding the seed, leaving a gap between the placental tissue and the outer wall of the locule. Externally, fruits are angular rather than round. High potassium decreases puffiness.
Blotchy ripening
complex
Green to greenish-yellow to waxy-white areas near the calyx of the otherwise normal red tomato fruit. In some cases, fruit symptoms are accompanied by foliar symptoms of deficiency. The incidence is highest on soil with low potassium, adding K reduced BR incidence in 5 different cultivars.
Greywall The outer locular wall turns brown or grayish brown and the area may become slightly depressed and roughened. Internally severe browning appears in the outer pericarp, especially in regions associated with the vascular bundles. Can be promoted by low potassium levels. High levels of added K reduced incidence of greywall.
Gold Fleck,
Fruit Pox

Gold specks or flecks are observed around the calyx and shoulders of mature fruit. These specks decrease the attractiveness of the fruit and reduce shelf life. Produced by excess calcium in the fruits. C an be reduced by increasing the K:Ca ratio which prevents excess Ca uptake.
Greenback,
Yellow shoulder
Lack of uniform ripening. Is most severe in potassium-deficient plants.

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Citrus

Potassium is essential for producing quality citrus. The quantities of potassium contained in one metric ton of fresh fruit are: orange 3,194; mandarin 2,465; lemon and lime 2,086; and grapefruit 2,422 g K2O/t fruit (IFA, 1992). Potassium is removed by citrus fruit more than any other nutrient element (Koo, 1985), and the large amounts of K are reflected in the high K content of citrus juice (IFA, 1992).

Potassium nutrition influences size of fruit, thickness of the rind, and fruit color. The improved yield is due, in part, to reduced fruit fall from the tree and larger fruit size. Potassium also improves and citric and ascorbic acid (vitamin C) content in juice, while influences other juice characteristics, like the acid/sugar ratio and soluble solids content (Koo, 1985). The effect of K on increasing vitamin C is related with the improved sugar metabolism in the plant under proper K nutrition (Mengel, 1997). Quality of citrus fruits during storage is also influenced by K nutrition in the tree: the incidence of stem-end rot and green mold is reduced as K fertilization rate is increased, therefore fruit loss during transport is reduced and shelf life in the supermarket is increased (Koo, 1985).

Application of potassium decreases fruit granulation, which is an undesirable characteristic as it leads to harder and dry juice sacs. Bhargava et al. (1993) quotes that foliar spraying with K reduced granulation in mandarins from 74% to 50%. Some fruit disorders are likely to occur under at low potassium conditions and at high N:K ratios; these disorders result in reduced fresh fruit pack-out (Tucker et al., 1994):

  • Creasing: Narrow sunken furrows on the rind surface; the fruit is not desirable for processing as the peel disintegrates easily.
  • Plugging: Removal of the peel in the stem end area of the fruit; increased incidence of postharvest decay
  • Splitting: Vertical split at the styler or blossom end and opening longitudinally towards the stem end.

Table 3 presents the results of a K fertilization experiment in sweet oranges, showing the positive effect of K on yield, fruit size and quality (Bhargava et al., 1993), and Table 4 summarizes citrus response to K fertilization (Koo, 1985).

Table 3: Effect of potassium on yield and quality of sweet oranges (Bhargava et al., 1993).

K2O Fruit weight Yield Juice TSS Acidity Vitamin C
g/tree g Kg/tree % % % mg/100 ml
0 165.2 31.9 46.3 9.77 0.549 52.8
200 173.1 36.2 47.2 9.89 0.542 54.1
400 178.0 37.5 47.2 10.06 0.533 55.9

Table 4: Response of citrus yield and quality parameters to K fertilization as indicated by leaf K * (Koo, 1985).

Parameter Leaf K
(g/kg dry wt)
Deficient Low Optimum High
< 7 7-11 12-17 18-23
External fruit quality
Size
Weight
Rind color
Rind thickness
Rind disorders
Creasing
Plugging
Splitting
Storage decay
Stem end rot
Green mold
Sour rot
Internal juice quality
Juice content
Soluble solids
Acid
Vitamin C

* The thicker the arrows, the stronger the effect

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Banana

Bananas dwarf most other crops in terms of potassium uptake. Potassium uptake is 18-30 kg K2O/t whole bunch (Cavendish type varieties) and up to 60 for other varieties (IFA, 1992). A banana plantation yielding 50 t/ha requires approximately 1625 kg K/ha, being K absorption the largest during bunch growth (Von Uexkll, 1985).

Yield and quality are strongly influenced by K nutrition. Potassium improves fruit weight and number of fruits per bunch, and increases the content of total soluble solids, sugars and starch (Table 5, Bhargava et al., 1993). Low potassium nutrition results in thin and fragile bunches with shorter shelf life (Von Uexkll, 1985).

In addition, K stimulates earlier fruit shooting and shortens the number of days to fruit maturity. Potassium has also a significant effect in improving resistance to diseases such as leaf spot and banana wilt (Von Uexkll, 1985).

Low K also induces poor buoyancy or ability to float, creating difficulties while packing, when the detached fruits must float in tanks for their cleaning and sorting (Martin-Prevel, 1989).

Table 5: Effect of K levels on yield and quality of bananas (Bhargava et al., 1993).

K2O
(g/pl)
Bunch weight
(kg)
Yield
(t/ha)
Total sugar
(%)
TSS
(%)
Acidity
(%)
Plant Ratoon Plant Ratoon Plant Ratoon Plant Ratoon Plant Ratoon
0 12.0 12.1 30.0 30.2 11.0 11.9 15.9 16.0 0.59 0.59
240 13.4 14.2 33.5 35.5 12.6 12.6 16.5 16.4 0.55 0.55
480 15.2 15.3 38.0 38.2 13.1 13.1 17.0 17.0 0.53 0.52

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References

Bhargava, B.S., H.P. Singh and K.L. Chadha. 1993. Role of potassium in development of fruit quality. In: Advances in Horticulture, Vol.2 Fruit Crops: Part 2. (Eds. K.L. Chadha and O.P. Pareek). Malhotra Publishing House, New Delhi. p. 947-960.

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

Geraldson, C.M. 1985. Potassium nutrition of vegetable crops. In: Potassium in Agriculture (Ed: R.S. Munson). ASA-CSSA-SSSA, Madison, WI. pp. 915- 927.

International Fertilizer Industry Association (IFA). 1992. IFA World Fertilizer Use Manual. IFA, Paris. 632 pp.

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

Kinet, J.M. and M.M. Peet. 1997. Tomato. In: The Physiology of Vegetable Crops. (Ed. H.C. Wien). CAB International, New York, NY. pp. 207-258.

Koo, R.C.J. 1985. Potassium nutrition of citrus. In: Potassium in Agriculture (Ed: R.S. Munson). ASA-CSSA-SSSA, Madison, WI. pp. 1077-1086.

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. Department of Agriculture and Water Supply, International Potash Institute and Fertilizer Society of South Africa. Pretoria, October 1985. p. 95-113.

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

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

Mengel, K. 1997. Impact of potassium on crop yield and quality with regard to economical and ecological aspects. In: Food Security in the WANA region, the essential need for balanced fertilization (Ed: A.E. Johnston). Proceedings of the Regional Workshop of the International Potash Institute held at Bornova, Izmir, Turkey, 26-30 May 1997. IPI, Bern, Switzerland. pp. 157-174.

Mengel, K. and E.A. Kirkby. 1987. Principles of Plant Nutrition. 4 th Edition. International Potash Institute, IPI, Bern, Switzerland. 685 p.

Pfluger, R. and K. Mengel. 1972. The photochemical activity of chloroplasts from various plants with different potassium nutrition. Plant and Soil 36: 417-425.

Smid, A.E. and D.E. Peaslee. 1976. Growth and CO2 assimilation by corn as related to K nutrition and simulated canopy shading. Agron. J. 68: 904-908.

Tucker, D.P.H., L.G. Abrigo, T.A. Wheaton and L.R. Parsons. 1994. Tree and fruit disorders. Fact Sheet HS-140, Horticultural Sciences Department, Institute of Food and Agricultural Sciences, University of Florida. 15 p.

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.

Von Uexkll, H.R. 1985. Potassium nutrition of some tropical plantation crops. In: Potassium in Agriculture (Ed: R.S. Munson). ASA-CSSA-SSSA, Madison, WI. pp. 929-954.

Wallingford, W. 1980. Function of potassium in plants. In: Potassium for Agriculture. Potash and Phosphate Institute, Atlanta, GA. pp. 10-27.

Windsor, C.W. 1973. Nutrition. In: The UK Tomato Manual (Ed. H.G. Kingman). Grower Books, London. pp. 35-42.

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