IPI International Potash Institute
IPI International Potash Institute

Importance of balanced fertilization to meet the nutrient demand of food crops

IPI-NFS International Workshop

on Importance of potash fertilizers for sustainable production of plantation and food crops in Sri Lanka

1-2 December 2003, Colombo, Sri Lanka

Importance of balanced fertilization to meet the nutrient demand of food crops

by Dr. A. Krauss

Contents

Abstract
The steadily increasing population of South Asia demands more and better food
Balanced fertilization is one of the essentials to increase food production
The K dynamics in soils, a decisive factor in balanced nutrition of annual crops
Consumer not only want more but also high quality and healthy food
Conclusion
References

Abstract

The demographic development and the shrinking natural resources in South Asia make it mandatory to increase the productivity of the remaining land. Balanced use of mineral fertilizers is one of the options to increase yields and to improve quality and stress tolerance of annual crops. However, use of mineral fertilizers in S-Asia is highly unbalanced to the detriment of potassium. Although plants absorb K in equal quantity as N, the use of N in S-Asia exceeds that of K by about 10 times. Soil K mining due to negative K balances is prevalent in the region. One of the problems is seen in the dynamic exchange processes, to which soil K is subjected and which makes precise estimations of soil K reserves and corresponding K recommendations rather difficult. On the other hand, numerous on-farm trials of IPI in S-Asia prove repeatedly the beneficial effect of balanced fertilization with adequate K on yield, quality and tolerance to pests and diseases. With higher yields and better quality, the farmer can generate more income that, on the other hand, contributes to the rural development. However, further research on soil K and more education on the benefits of balanced fertilization are still necessary.

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The steadily increasing population of South Asia demands more and better food

As shown in figure 1, the population of South Asia is still rapidly growing. From less than 500 million inhabitants 50 years ago, the population increased to currently 1.3 billion and will reach the 2 billion mark in some twenty five years from now. At the same time, the acreage of arable land has hardly changed, it stagnated in the last 25 years at around 192 million ha, which in turn reduces the per capita land availability from 0.33 ha in 1960 to currently 0.15 ha.

Figure 1: Evolution of the population of S-Asia (data source FAO, 2003)
Figure 1: Evolution of the population of S-Asia
(data source FAO, 2003)

Figure 1 also reveals that the share of urban on the total population is steadily increasing. This means that increasingly more food and thus nutrients will be transferred to the urban centres, from where the nutrients will hardly be returned to the arable land where they came from. An increasing urban population also changes the food habits from subsistence food to packed and processed food. And finally, with higher incomes, the urban inhabitants also look for better quality as well as safe food, and whether the food has been produced in context with the environment.

As far as the supply of more food to a growing population is concerned, the FAO statistics show that cereal production in S-Asia almost trebled within the last 40 years. This was mainly caused by an increase in yield from about 1 t/ha in the early 60ies to currently 2.4 t/ha. With the higher cereal output, the per capita production remained fairly stable at about 210-230 kg.

Figure 2: Per Capita food supply in S-Asia
Figure 2: Per Capita food supply in S-Asia

The impact that the urbanization has on the food habits can be seen in figure 2. The supply with cereals has only slightly increased in the last four decades. However there is a remarkable increase in the per capita supply with vegetables, fruits and vegetable oils, which reflects the changing demand according to the income development.

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Balanced fertilization is one of the essentials to increase food production

The use of fertilizers is, next to the genetic potential of the crops, the crop management and irrigation, one of the major inputs to increase food production. As shown in figure 3, fertilizer use, nitrogen in particular, has increased substantially during the last decades. Potash use lags seriously behind N and P, the decreasing NK ratio indicates that the imbalance in fertilizer use worsens in S-Asia.

Figure 3: Fertilizer use in S-Asia
Figure 3: Fertilizer use in S-Asia

Concerning the nutrient ratio in fertilizer use, the FAO statistics also show that Sri Lanka still has a better NK ratio in fertilizer use as compared to the other countries of S-Asia. However, the NK ratio in Sri Lanka deteriorated rapidly from a balanced ratio of N:K = 1:0.65 during the 70ies to currently N:K = 1:0.28. This compares with a current NK ratio in Southern India of N:K = 1:0.33. The NK ratio in All-India and Bangladesh varies between N:K = 1:0.10 to 1:0.15, whereas Pakistan has a depressing imbalance in fertilizer use of N:K = 1:0.01, meaning that, for each kg N, a Pakistani farmer uses only 10 g K2O.

The obvious preference for N fertilizers and the sparse use of potash contradict the ratio at which crop plants absorb and remove the nutrients from the field. As shown in figure 4, K is removed by the crop at about the same quantity as nitrogen. Therefore, if fertilizer is applied with a NK ratio of 1:0.10 as currently in S-Asia but crops remove both nutrients at almost equal quantities, substantial soil K mining has to be assumed.

Figure 4: Nutrient removal by crops (indian conditions; FAI 2002)
Figure 4: Nutrient removal by crops
(indian conditions; FAI 2002)

Comparing the nutrients removed by the crops with the use of mineral fertilizers in S-Asia, it shows that the resulting balance for nitrogen is becoming increasingly more positive (Figure 5). Also the P balance improves with time. The K balance in contrast shows the reverse order, namely an increasingly negative balance. This means that the farmers in S-Asia remove annually some 50 kg/ha K2O more from the field than they return with potash fertilizers. It is also obvious that with the widespread misuse of farmyard manure as fuel, the increasing deficit in the K balance cannot be closed with more use of organic manure, because of its restricted availability as soil amendment.

Figure 5: Apparent nutrient balance of S-Asia
Figure 5: Apparent nutrient balance of S-Asia

More detailed calculations of the nutrient balance made for several States in India confirm the simplified calculation of IPI. For instance, VINOD KUMAR et al. (2001) estimated for Haryana a K deficit that nose-dived from -35 kg/ha during the 60ies to currently -90 kg/ha K2O. The N balance of -4 kg/ha N is almost in equilibrium. YADAV et al. (2001) calculated for U.P. a current deficit of annually 1 million t K2O that contrasts sharply a surplus of 1.2 million t of N and 0.5 million t of P2O5 in U.P. YADAV and co-workers also concluded by saying "…over-mining of K… as evident through apparent K balance sheets, is not reflected in available K status in most cases, and the soil-testing laboratories go on reporting medium to high K fertility rating. … In view of the established contributions on non-exchangeable K in meeting plant K demands, this form of K should also be used as an index of K availability in the soil. Threshold levels of non-exchangeable K for different soils and crops have to be worked out, and used for the purpose of fertilizer K recommendations…".

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The K dynamics in soils, a decisive factor in balanced nutrition of annual crops

The timely pattern of nutrient uptake by annual crops like maize differs substantially between the nutrients as shown in figure 6; about 70 days after planting (day 197) when little less than one third of the total dry matter has been accumulated, the crop had taken up about half of its P requirement, three quarters of its N requirement and virtually all of its total K needs. The uptake of the other nutrients proceeded at a steadier rate (CORAZZINA et al., 1991).

Figure 6: Nutrient and dry matter accumulation of maize (redrawn after Corazzina et al., 1991)
Figure 6: Nutrient and dry matter accumulation of maize
(redrawn after Corazzina et al., 1991)

The rapid increase in K accumulation implies that the K uptake rate at the early stage of plant development is also considerable, reaching levels of 5 to 20 kg/ha/day, or, as in the reported case with maize, more than 19 kg/ha K2O per day. This in turn requires that the soil contains adequate amounts of K to be in the position to release within a short period of time enough K into the soil solution to meet the needs of the plant.

Correspondingly, JOHNSTON et al. (2001) found that barley, cultivated on soils with 300 ppm exchangeable K, had a maximum daily K uptake of almost 6 kg K/ha, whereas the same crop but grown on a soil poor in K, namely 50 ppm exchangeable K, took up only 0.5 kg K/ha a day. The total K uptake and ultimately the final yield differed correspondingly. Barley on the soil poor in K took up around 30 kg K/ha and produced 4.2 t/ha biomass, the crop on the soil with adequate K took up 160 kg K/ha and yielded 7.5 t/ha biomass.

Figure 7: The potassium cycle in the soil-plant-animal system (from SYERS, 1998)
Figure 7: The potassium cycle in the soil-plant-animal system
(from SYERS, 1998)

Soil K is subject to dynamic exchange processes between different fractions. A widely accepted concept divides soil K into four pools or compartments (Figure 7):

  1. the soil solution K (Ksl)
  2. the exchangeable K (Kex)
  3. the fixed or non-exchangeable K (Kf)
  4. and the K in the lattice of certain primary minerals (Kl).

The amount of K in each fraction varies and depends on the past cropping history, past fertilizer and manure use, i.e. the K balance, soil pH and soil water content. GOLAKIYA et al. (2001) for instance reported that the content of soluble K in the calcareous soils of Gujarat, India ranges from 0.003 to 0.21 cmolc/kg soil, the exchangeable K varies from 0.03 to 2.00, non-exchangeable or fixed K from 0.32 to 21.7 and total K from 1.10 to 20.30 cmolc/kg soil.

As indicated in Figure 7, the fractions Ksl, Kex and Kf are related to each other through reversible exchange processes. K removed by uptake of plants and/or leached into the subsoil is replenished by K released from both the Kex and Kf pools. There can be simultaneous release of K from both pools to the soil solution or a linear exchange process, from the non-exchangeable to the exchangeable pool, and from the exchangeable pool to the soil solution. Whether one or other of these two possible mechanisms predominates is of little practical importance provided that the K in the soil solution is replenished quickly enough to meet the maximum demands of a rapidly growing crop. When there is surplus K in the soil solution, after the addition of fertilizer or manures, K is transferred to both fractions through exchange and fixation processes.

The exchange processes and the possible involvement of two fractions in the replenishment of solution K, namely the fraction of Kex and the fraction of Kf can also be a source of misinterpretation. JOHNSTON and co-workers (2001) reported that in the Garden Clover Experiment in Rothamsted, a total balance of 1667 kg K over a period of 10 years increased the content of exchangeable K by 690 kg/ha, which is only 41% of the K balance (Figure 8). Almost 60% of the K balance had gone into a pool of K that did not belong to the Kex fraction always assuming that no large amount of K was lost through leaching. On the other hand, in the same experiment the subsequent long-term omission of K, and thus, a negative K balance of 1494 kg K/ha due to crop removal, resulted in a reduction in the content of Kex by only 563 kg K/ha, i.e. a reduction by 38%. The other 62% of the K removed had to be supplied by K in other soil K pools.

Figure 8: K balance and changes in Kex (after JOHNSTON et al., 2001)
Figure 8: K balance and changes in Kex
(after JOHNSTON et al., 2001)

Therefore, finding a smaller increase in Kex than the K balance, and removing more K from the soil than was indicated by the decline in Kex at cropping without application of K, questions the validity of relying only on one parameter when assessing the soil K status as it is also stated above by YADAV and co-workers.

Figure 9: Release of fixed (non-exchangeable) K related to initial exchangeable K (adapted from JOHNSTON & MITCHELL, 1974)
Figure 9: Release of fixed (non-exchangeable) K related to initial exchangeable K
(adapted from JOHNSTON & MITCHELL, 1974)

The concept of a 'tripartite' relationship is supported by the findings of JOHNSTON and MITCHELL (1974). They showed that the release of K from the non-exchangeable pool was linearly related to the content of initial exchangeable K (Figure 9). There was also a close linear relationship between the decrease in exchangeable K and the release of K from the non-exchangeable pool. The higher the initial content of exchangeable K, the more K was released from the non-exchangeable pool, and with the decline in the uptake of K from the Kex pool, the release of K from the Kf pool also declined. In the soil used by JOHNSTON and MITCHELL, the uptake of K from the non-exchangeable pool was twice the amount taken from the exchangeable fraction, suggesting there was an equilibrium between the K in the Kex and Kf pools. It can be assumed, however, that the ratio of K released from these two fractions will differ with the type of clay minerals, degree of weathering, etc. Nevertheless, it is probable that the K in the Kf pool is of importance especially for crops with a restricted root system such as potato. As shown by JOHNSTON and co-workers (1998), the poorer the root density, the higher should be the concentration of K in the soil solution to sustain a particular K uptake rate. The same authors also showed that with increasing clay content and/or decreasing soil moisture the solute K concentration has to be increased proportionally to support the required K uptake rate.

Figure 10: Effect of K on sucrose content and sugar yield of cane in India (IPI on-farm trials 2001)
Figure 10: Effect of K on sucrose content and sugar yield of cane in India (IPI on-farm trials, 2001)

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Consumer not only want more but also high quality and healthy food

K, after being absorbed by the plant roots from the soil solution affects yield and quality of the crop in a multiple way:

  • K supports the plant's metabolism by stimulating some 50 different enzyme systems, some of which are directly or indirectly involved in quality development.

  • One major aspect of the functions of K is to stimulate assimilation, which is the conversion of CO2 from air with the help of sun energy into sugar that can be translocated through the plant to storage organs and converted to starch, etc. The latter is an important parameter in food processing like chips production from potato tubers and in fibre quality.

    Figure 11: Effect of K on yield and quality of wheat in India (IPI on-farm trials 2001)
    Figure 11: Effect of K on yield and quality of wheat in India (IPI on-farm trials, 2001)

  • K also assists to transfer the assimilates from the leaves to the storage organs like tubers, roots, fruits or the stalk, a process called phloem loading. The higher sugar content of fruits and sugarbeets or cane at adequate K supply refers to the nutritive value and the processing properties in the quality management. An example is given in figure 10 that shows the impact of K application on the sucrose content and the corresponding sugar yield in cane.

  • Another important aspect is the role of K in the nitrogen metabolism. With adequate K, the absorbed nitrate-N is rapidly incorporated into amino acids, the basic constituents of protein. At inadequate K, less nitrate-N is metabolized, nitrate accumulates in the plants and the protein contents remain low (Figure 11). This has also an impact on the food safety (nitrate) and on the nutritive value (protein content).

    Figure 12: Effect of K on yield and quality of oilseed rape in China (IPI on-farm trials, 2001)
    Figure 12: Effect of K on yield and quality of oilseed rape in China (IPI on-farm trials, 2001)

  • Plenty of carbohydrates in plants adequately supplied with K allow ample conversion to fatty acids and oils apart from the demand for the N-metabolism (Figure 12). This improves the nutritive value as well as the processing quality of plant products.

  • Also the translocation of nitrate from the roots to the shoot is stimulated by K acting as the counter-ion. At inadequate K, less nitrate is transferred from the roots to the shoot, the building-up of higher nitrate concentrations in the root signals to reduce N uptake which consequently lowers the N fertilizer use efficiency (Figure 13). More residual N in soils at low uptake increases the risk of environmental pollution of the groundwater and the atmosphere, and the farmer cannot claim to produce his crops in an environmentally friendly manner.

  • K is the main component in regulating the water household of plants. K deficient plants desiccate rapidly. Early wilting is indeed a good indicator of inadequate K supply to plants. Desiccating green vegetables will not sell at the market, the poor appearance and attractiveness of the produce from K deficient plants repel potential purchasers.

  • Wilting plants at inadequate K supply also cease assimilation rapidly. This lowers the yield and reduces the content of quality components like sugar, starch or protein.

    Figure 13: Impact of site-specific nutrient management, SSNM, as compared to farmer's fertilizing practice, FFP, on rice yield and recovery efficiency of N, REN (DOBERMANN, 1999)
    Figure 13: Impact of site-specific nutrient management, SSNM, as compared to farmer's fertilizing practice, FFP, on rice yield and recovery efficiency of N, REN
    (DOBERMANN, 1999)

  • On the other hand, efficient assimilation at adequate K supply leaves plenty of carbohydrates available in the plant to produce the so-called secondary plant constituents such as vitamins, lycopene, isoflavones, phenolic compounds. The first group is associated with health care of the consumer, the phenolic compounds are also important repellents in disease and pest control.

  • Last but not least, plants supplied with adequate K within the concept of balanced fertilization proved to be more tolerant to soil borne and climatic stress such as drought, frost, salinity, and resist better to pests and diseases (Figure 14). Less variable yields of stress tolerant plants contribute to food security, and freedom from pests and diseases is essential for instance in food export and marketing. Less need of agrochemicals in plant protection at balanced fertilization with adequate K reduces the production costs and complies with the demand for safe food.

    Figure 14: Effect of K application on pest incidences in soybean (IPI on-farm trials 2001 in india)
    Figure 14: Effect of K application on pest incidences in soybean (IPI on-farm trials 2001 in india)

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Conclusion

The benefits of balanced fertilization of annual crops are obvious: with higher yields the farmer increases his profits, with better crop quality he remains competitive at the market, and the better resistance to pests and diseases reduces the production costs. His higher income will also attract other business, it creates jobs and thus, contributes to the rural development. At the same time, the nation benefits from higher food security, less dependency from food imports and even from export opportunities.

However, the short duration of annual crops and consequently, the quick turnover of nutrients require a judicious nutrient management to optimize the efficiency of the costly inputs and to minimize the environmental impact of fertilizers. Potassium plays a particular role in these aspects.

Unfortunately, the use of potash fertilizers lags seriously behind the requirement of the plant. One of the factors responsible for this is certainly lack of knowledge, especially as far as the assessment of soil K is concerned.

There is also need to convince farmers to re-invest in soil fertility not only with N and P but also with K. With increasing K deficits due to unbalanced fertilizer use, which is common in South Asia, soil fertility degrades and the farmer looses the basis for a sustained production and income. It is indeed often fairly difficult to visualize the effect of potash use but this should not prevent us to demonstrate the effect of balanced fertilization on yield, quality and stress tolerance in the farmer's field, to educate the extension service and the fertilizer dealers on the effectiveness of potash in concert with other fertilizers, and to inform the government on the consequences if soil K mining continues.

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References

Corazzina, E., Gething, P.A. and Mazzali, E. (1991): Fertilizing for high yield: Maize. IPI-Bulletin No. 5, 2nd ed. International Potash Institute, Basel, Switzerland, 87 p.

Dobermann, A. (1999): Reversing diminishing growth of rice yields in Asia. 6th IFA Annual Conference, 17-20 May 1999, Manila, The Philippines.

FAI (2002): Fertiliser Statistics 2001-2002. Fertiliser Association of India, New Delhi, India.

FAO (2003): Food and Agriculture Organization of the United Nations, Rome, Italy, website www.fao.org

Golakiya, B.A., Gundalia, J.D. and Polara, K.B. (2001): Potassium dynamics in the soils of Saurashtra. Poster at the IPI-PRII International Symposium on the "Importance of potassium in nutrient management for sustainable crop production in India", 3-5 December, 2001, New Delhi, India.

Johnston, A.E. and Mitchell, J.D.D. (1974): The behaviour of K remaining in soils from the Agdell experiment at Rothamsted, the results of intensive cropping in pot experiments and their relation to soil analysis and the results of field experiments. Rothamsted Experimental Station Report for 1973, Part 2, 74-97.

Johnston, A.E., Barraclough, P.B., Poulton, P.R. and Dawson, C.J. (1998): Assessment of some spatially variable factors limiting crop yield. Proceedings No. 419, The International Fertilizer Society, York, UK, 48 p.

Johnston, A.E., Poulton, P.R. and Syers, J.K. (2001): Phosphorus, potassium and sulphur cycles in agricultural soils. Proceedings No. 465, The International Fertiliser Society York, UK, 44 p.

Syers, J.K. (1998): Soil and plant potassium in agriculture. Proceedings No. 411, The International Fertiliser Society York, UK, 32 p.

Vinod Kumar, Antil, R.S. and Kuhad, M.S. (2001): Nutrients mining in agro-climatic zones of Haryana. Fertiliser News 46 (4), 81-92.

Yadav, R.L., Dwivedi, B.S., Singh, V.K. and Shukla, A.K. (2001): Nutrient mining and apparent balanced in different agro-climatic zones of Uttar Pradesh. Fertiliser News 46 (4), 13-31.

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