IPI International Potash Institute
IPI International Potash Institute

K effects on yield quality

Potafos IPI-PPI-PPIC Symposium on K in Brazilian Agriculture

Sao Pedro, SP, Brazil, 22-24 September 2004

K effects on yield quality

by Dr. A. Krauss


Why yield quality?
What is quality?
Where is the link between K and quality?
What can we prove?


The paper discusses the demographic factor, which demands a stronger consideration for the quality of produced crops. On the other hand, crop quality is not a singular item that can easily be measured, it is rather complex and refers to subjective and objective parameters such as nutritive value, processing properties, taste and appearance. Although components of crop quality are genetically controlled, the nutrition of plants can alter the expression to a substantial extent. Through its versatile function in plants, K in the concept of balanced fertilization plays a particular role in quality formation as shown in numerous experiments and field trials around the world. Complying with the quality standards set by consumers and environmentalists warrants farmers who apply balanced fertilization to remain competitive at the market because of an ecologically sound and economically viable production method.

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Why yield quality?

  • With urbanization, consumers demand more diverse food that is easy to prepare.
    FAO (2004) estimates that the global population will continue to grow from currently 6.07 billion to 8.9 billion in 2050. A comparable change is seen in the proportion of people living in towns. The global urbanization is expected to increase from currently 47% to more than 60% in 2030, whereby most of the growth will appear in developing countries. The latter have currently an urbanization of about 40%, which will increase to 56% within the next 30 years. Current urbanization of 81% in Brazil is as high as in EU-15 (79%), whereas in comparison with India as a representative of developing country, only some 28% of its total population live in towns.

    Figure 1: Per Capita food supply 2000 in selected countries
    Figure 1: Per Capita food supply 2000 in selected countries
    (FAO, 2004)

    Increasing population demand more food and urbanization demand more diverse food. As shown in figure 1, people in countries with a high rate of urbanization such as Brazil and EU-15 consume relatively less cereals but more fruits and vegetables as compared to India with its lower rate of urbanization.

  • With raising income, quality becomes the first choice when selecting food at the market.

    A survey in 7 European countries shows that quality, though largely a subjective property, is rated as the most important determinant of acceptability by 25% of consumers, followed by price (16%), brand name/reputation (14%) and freshness (9%) (TRAILL, 1999). Looking at the food spectrum, as already indicated in figure 1, there is a clear trend that, with raising income, the diet shifts from low value subsistence crops like cereals and root/ tuber crops towards animal protein, fruits and vegetables, and quality comes into picture (Figure 2). It is also obvious that the wealthy "high-tech" urban society wants to spend less time on food preparation and looks for packed and processed food.

    Figure 2: Relationship between income and food habit
    Figure 2: Relationship between income and food habit
    (after KERN, 2000)

    The rather close relationship between income and food spectrum is also shown in figure 3, in which the per capita GDP of major industrialized and developing countries, together with countries from South America, are plotted against the per capita supply of vegetable oil. The higher the income, the more vegetable oil is consumed.

    Figure 3: Relationship between per capita GDP and consumption of vegetable oil
    Figure 3: Relationship between per capita GDP and consumption of vegetable oil
    (FAO/CIA 2004)

  • With increasing health consciousness, consumers look for food free from pests and diseases.

    Apart from the fact that consumers would reject mouldy vegetables and rotten fruits at the market, GONZALO RIOS (1999) points out that ...in a situation of globalization, trade liberation and commitment to reduce tariffs and eliminate non-tariff barriers, sanitary and phytosanitary justification could be used as a means of introducing measures that are more protectionist than if they were only concerned with safety and health...

  • There is an increasing demand for food with functional properties.

    Our population is rapidly aging. OECD (2004) estimates that, on average of all OECD countries, the share of aged people older than 65 years will increase from currently 13% to 21% in 2030 and almost 25% in 2050. This will certainly have an impact on the food habit towards high quality processed food, functional food, but at reasonable costs because of decreasing purchasing power.

    Concerning functional food, there is an increased public interest in this type of food that may have a potential to lower body fat, cure gut maladies, provide gender and age-related medical needs, improve skeletal strength, lower cholesterol or improve the optical vision, etc. (KERN, 2000). Ingredients such as lycopene in tomato, allicin in garlic or isoflavones in soybean are associated with prevention or treatment of cancer, diabetes, hypertension and heart disease (BRUULSEMA, 2000). The diet can also play a critical role in a wide spectrum of diseases. KEEN & ZIDENBERG-CHERR (2000) refer to the antioxidant effects of vitamins C and E in human plasma, which is prolonged in the presence of the phytochemical catechin, a phenolic compound found in fruits, vegetables, green tea, red wine and chocolate.

  • The food and processing industry demands crops with high contents of ingredients like protein or oil to save transport and processing costs.

    As an example, OEHLUND (1999) calculated that, in order to produce one ton of sugar from low quality beet (e.g. 80% extractability and 13% sugar content typical of unbalanced fertilization), about 10 t of roots would be needed. But, with high quality beet having 95% extractability and 17% sugar content, less than 7 tons would suffice. The savings in energy required to transport and extract 3 t of a lower quality beet are obvious. The same principle applies to cane.

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What is quality?

  • Quality is ...an intrinsic property of food by which it meets predefined standard requirements. Determinants of food quality can be grouped into several properties. Food quality therefore refers to the value, which is subjectively or objectively attached to food with respect to quality properties... (ALABAKA, 1999).
    It considers:

  • Nutritional properties
  • Functional properties
  • Organo-leptic properties
  • Hygienic properties
  • Environmental compatibility
  • Food safety

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Where is the link between K and quality?

K is the most versatile nutrient in plants, highly mobile and involved in numerous regulation mechanisms, e.g.

  • N metabolism

    As a cation, K accompanies the nitrate anion as it is transported from the roots to the shoot (Figure 4) where nitrate is reduced to NH3 to be incorporated into amino acids, the precursors of protein. K deficient plants have a repressed activity of the enzyme nitrate reductase.

    Accompanied by malate, K is then retranslocated from the shoot to the roots, where K-malate is oxidized, yielding KHCO3, which exchanges for KNO3, and the cycle continues.

    Figure 4: Model for the circulation of potassium in relation to nitrate and malate transport
    Figure 4: Model for the circulation of potassium in relation to nitrate and malate transport (after MARSCHNER, 1995)

    Plants inadequately supplied with K fail to transport nitrate efficiently into the shoot. This leads to nitrate reduction and accumulation of amino acids in the roots, which signals, via a feedback effect, to the roots to close down further nitrate uptake although nitrate might be present in the rooting zone (MARSCHNER et al., 1996). Any surplus nitrate in the rooting zone of K deficient plants is likely to be leached into the groundwater or lost to the atmosphere as NOx gas. And finally, accumulation of nitrate in K deficient plants leads to reduced protein contents.

  • Sugar transport

    Phloem loading of sucrose is enhanced by K. Accordingly, HARTT (1969) showed that, in cane adequately supplied with K, more than 40% of 14C applied to a leaf blade was translocated within 90 minutes out of the fed leave, half of it was already translocated into the stalk below the fed blade. In contrast, in K deficient plants more than 90% of the applied 14C was retained in the fed blade and almost nothing was found in the stalk. 2.5 hrs, later 30% of the applied 14C was contained in the stalk of cane with adequate K, whereas K deficient plants had only 5% of the applied 14C in the stalk. Similarly, also HAEDER (1975) showed that in potato plants receiving adequate amounts of K, 80% of foliar applied 14C was translocated within 2 hrs into the tubers, whereas K deficient plants retained more than 50% of the absorbed 14C in the shoot.

  • Water utilization

    K controls water uptake, transport and utilization. Plants adequately supplied with K wilt less under water stress because K has the major responsibility for turgor changes in the guard cells of stomata during stomatal movements. As shown by MARSCHNER (1995), the K concentration in the guard cells of open stomata is about 20 times higher than of closed stomata, and the osmotic pressure in guard cells as caused by K and its accompanying anion (Cl-) almost doubles from 1.9 MPa in closed to 3.5 MPa in open stomata of bean plants. The better the K supply of plants the more rapid is the stomata movement. The osmotic effect of K also helps to extend the shelf life of leafy vegetables in particular.

  • Stress resistance

    CAKMAK (2003) summarized the beneficial effect that K has in plants subjected to environmental stress. K deficient plants are highly light-sensitive, and become very rapidly chlorotic and necrotic when exposed to increased light intensity. These plants suffer from oxidative damage catalized by reactive oxygen species, ROS, because photosynthetic CO2 fixation is severely reduced at K deficiency, which ultimately results in excess of photosynthetically produced electrons and thus stimulating ROS production. However, there is increasing evidence that improving the K status of plants can greatly lower ROS production.

    Concerning the relationship of K and plant health, PERRENOUD (1990) reviewed some 2450 references and showed that in 40 to 70% of all quoted cases K induced a significant reduction in pest and disease incidences and substantial yield increases at the same time. The ratio of N:K in plants plays an important role in the host/pathogen relationship. Plants supplied with excessive N/deficient K have usually a high content of low molecular assimilates such as sucrose and amino acids because of impaired phloem transport and N metabolism. The soft and often injured tissue gives easy access to invading pathogens and exhibit less chewing resistance. The content of repelling secondary plant substances such as phenolic compounds is rather low. The relationship between plant nutrition and secondary plant substances as shown in figure 5 is well described by GRAHAM (1983).

    Figure 5: Synthesis of pathogen repellents in plants as affected by the level of nitrogen supply
    Figure 5: Synthesis of pathogen repellents in plants as affected by the level of nitrogen supply
    (schematic after GRAHAM, 1983)

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What can we prove?

  • K improves the nutritive value

    Field trials conducted for instance by IPI and their cooperators in India (and other countries) showed that, by applying K, the grain yield as well as the protein content in the grains could be increased (Figure 6). The beneficial effect of K refers to the impact that K has on phloem loading and N metabolism.

    Figure 6: Yield and quality of wheat grains in India as affected by K
    Figure 6: Yield and quality of wheat grains in India as affected by K

    A 6% increased seed yield of oil seed rape in China together with a 3% higher oil content at balanced fertilization resulted in a 9% higher oil output.

    Applying adequate K (100 kg/ha K2O) in split application to soybean in Madhya Pradesh, India, increased the seed yield by 26% and oil yield by 34%. This represents an added net value of about US$ 120/ha in seeds or about $ 150/ha in oil and soybean cake. The higher oil yield also provides additional 22 people with the annual allowance of 6.57 kg edible oil. As a further effect, soybean fed with balanced fertilizer showed considerably less incidences of girdle beetle, semilooper and aphids.

    The influence of K on oil contents and yields in oilseeds appears to be more indirect in nature and is controlled by the rates of photosynthetic CO2 fixation and the utilization of fertilizer N. Lipids in leaves are mostly associated with chloroplasts. Enhancement of protein synthesis by balanced fertilization with adequate K leads to an increased chloroplast synthesis and hence to higher contents of lipids. WEBER (1985) also refers to the fact that K ions are required by two enzyme systems in the pathway of fatty acid biosynthesis, namely the acetyl-CoA synthetase and acetyl-CoA-carboxylase.

  • K affects the functional properties

    Sugar from cane and beets, starch from potatoes and oil from oilseeds will play an increasing role as basic source of alternative fuel, Brazil is an excellent example. Consequently, processing plants look for products with a high content of the particular ingredients in order to improve the conversion economy. Correspondingly, sugarcane in Egypt responded to increasing soil K content with higher yield and higher sucrose content in stalks (Figure 7). The same IPI on-farm trials also showed that increasing soil salinity had a deleterious effect on both the fresh weights of stalks and their sugar content. The link of decreasing sucrose content in cane stalks with increasing salinity could be that Cl, together with K, accumulates in the storage tissue where it raises the osmotic potential. This impedes sucrose transport into storage cells and competes with Cl for storage space. Furthermore, Cl in storage tissue decreases the pool of plant potassium available for other metabolic functions in the plant (KRAUSS, 1991).

    Figure 7: Relationship between K absorption ratio (K in % of CEC) and stalk size and sucrose content of cane in Egypt
    Figure 7: Relationship between K absorption ratio (K in % of CEC) and stalk size and sucrose content of cane in Egypt

    IPI trials in India with cane on farmer's land showed an increase of sugar yield from 15.1 t/ha at NP control to 18.5 t/ha at NP+169 kg/ha K2O, i.e. an increase by 22%. Each Rupee invested in potash returned almost 10 Rupees with higher yield.

    Sugar beet in Hungary receiving adequate potash gave a higher root yield with increased sugar content and consequently a much higher sugar yield (KULCSAR & DEBRECZENI, 1997). The latter counts when quality-based procurement systems are established. Farmers in Germany earned an additional $140/ha from sugar beet at 360 kg/ha K2O in spite of higher fertilizer costs because, at adequate soil K levels, quality of beet improved and commanded a higher procurement price (ORLOVIUS, 1996).

    Potatoes in Poland increased starch yield by 18% when K was added to NP, and increased further to 45% on top of the NP control when Mg supplemented NPK.

  • K is involved with organo-leptic properties

    In field experiments, the effects of K and Mg nutrition on the quality components of black tea, oolong tea and green tea were studied in China. The results show that K and Mg increased the contents of free amino acids and caffeine of the various tea types. The content of polyphenols, theaflavins and thearubigins in black tea were also increased by K application. Flavour compounds in brewed oolong tea improved with K and Mg (JIANYUN RUAN et al., 1999).

    Potato trials in Bulgaria revealed that the content of reducing sugar dropped from 0.56% at NP to 0.04% at balanced fertilization with K and Mg together with NP (NIKOLOVA, 1999). A low content of reducing sugar in potato tubers is mandatory for tasty and bright coloured potato chips and crisps.

    Figure 8: Impact of balanced fertilization with K and Mg on quality of tea (China)
    Figure 8: Impact of balanced fertilization with K and Mg on quality of tea (China)

  • K controls hygienic properties

    Field trials with potatoes in Germany showed that the incidence of black spots decreased with increasing K content of tubers. Improving the K supply led also to increased starch content of potato tubers, the beneficial effect was more pronounced with sulphate of potash than with potassium chloride. The higher starch content returned a higher sales price with a quality bonus (ORLOVIUS, 1996b).

  • K enhances the content of functional compounds

    Applying adequate potash to cabbage increased yield by 20 t/ha to 90.8 t/ha and improved vitamin C content by 12% (PROKOSHEV, 1998). BRUULSEMA (2002) reported on 13% higher concentration of isoflavones in soybean seeds in response to applied K fertilizers (Figure 9).

  • K keeps plants healthy and hence the food

    Less aphids on mustard plants, less pests damages on leaves of soybean and less late blight in potatoes at adequate K supply in India are few of numerous results from IPI field trials worldwide. A summarizing report on K and biotic stress was also given earlier by KRAUSS (2001).

    Figure 9: Concentration of isoflavones in soybean seeds as affected by K fertilization
    Figure 9: Concentration of isoflavones in soybean seeds as affected by K fertilization

    With respect to healthy food, the main public concern is high dietary nitrate intake that may increase the risk of gastro-intestinal cancer, or infantile methaemoglobinaemia, the blue baby syndrome. However, studies undertaken in an attempt to establish whether dietary nitrate is directly linked to gastrointestinal cancer, produced conflicting and often contradictory results. More recent experiments, as shown by LEIFERT et al. (1999), revealed that nitrite from nitrate entering the normal acidic stomach appears to be rapidly converted to reduced N-forms that cannot nitrosate secondary amines to form nitrosamines. On the other hand, as said by the authors, there is "...growing evidence to suggest a beneficial physiological role of dietary nitrate in the gastrointestinal protection against food-borne pathogens which is increasingly recognized to be associated with gastric malignancy...". Irrespectively, the authors feel "...that a sensible limit for levels of nitrate in drinking water should be enforced. A rise in nitrate levels is a symptom of improper use of nitrogen fertilizers and poor agricultural management practice...".

    In this context, IPI on-farm trials in China showed that celery receiving adequate K had a nitrate content of 427 ppm, which was 27% lower than with unbalanced nutrition (HAERDTER & KRAUSS, 1999).

  • K improves the appearance and shelf life of many crops

    BANSAL & SHAHID UMAR (1998) reported from field trials in India that supplying potato with adequate potash increased tuber yield and dry matter content. At the same time, for potatoes grown for processing, chip colour was improved and storage loss of fresh tubers was reduced. The latter is of particular economic importance because the market price of potatoes increases considerably for some weeks after harvest. As an example, during the season 1997-98 the price increased from Rs/kg 4.5 at harvest to Rs/kg 6 two weeks later and Rs/kg 8 four weeks later. This increased the VCR of potash from 16.6 at harvest to 21.4 and 28.5 two and four weeks later, respectively.

    In Russia, as reported by PROKOSHEV (1998), cabbage fertilized in a balanced manner and stored for 4 months improved shelf life (weight loss 27%) and reduced considerably the incidence of spot necroses. In contrast, substantial loss in weight (35%) and serious incidence of spot necroses occurred when there was unbalanced fertilization with NP only. After 4 months, the heads at balanced fertilization had also a rather favourable K:N ratio of 1.18, whereas the cabbage heads at NP control contained substantially more nitrate (K:N 0.49). Comparable results were also found in carrots and red beet in Russia.

  • K use meets environmental requirements

    Consumers will ask more than before whether the crop has been produced in context with environmentally friendly practice. One indicator is the amount of N left in soils after harvest. Trials like those conducted by IPI in China showed a substantial decrease of residual N at balanced fertilization. Under farmer's practice, some 140 kg/ha nitrate-N was measured in subsoils, and farmer's practice in China means N first of all. The NO3-N content decreased to about 100 kg/ha after adding K to high N. It declined further to less than 40 kg/ha NO3-N when N was well balanced with potassium.

    The increase of the recovery efficiency of N fertilizers in rice from 31% with farmer's practice (117-18-30 kg/ha N-P-K) to 40% with site-specific nutrient management (112-19-58 kg/ha N-P-K) is another indirect indicator to meet the environmental requirements with balanced fertilization (DOBERMANN et al., 2002).

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Globalization, liberalized trade, competition, selective and critical consumers are the challenges to farmers whilst producing crops for food and processing. Concerns about safeguarding natural resources and protecting the environment are future control mechanisms for agricultural entrepreneurs. N directives and N taxes, as already imposed in some EU countries, will dictate crop management unless the farmer can proof that, with good agricultural practice and balanced fertilization, he can produce his crops ecologically sound on a sustainable basis. Quality auditing, to show the customers how he grows the crops will credit his produce and he will become competitive at the market, of course assuming a profitable production method.

To produce high quality, the genetic potential is made available by the plant breeders. It is up to the farmer to utilize the potential by supplying the nutrients to plants in a balanced manner, irrespective to the source, in order to capitalize the genetic potential. The knowledge is also available albeit not always accessible. It could be task of the fertilizer industry to assist in transferring the information on how best to produce quality. And last but not least, quality based procurement systems will give further incentives to head for quality produce.

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