Expo 2000, Science in Dialogue
Sustainable Soil Management
August 15, 2000, Hanover, Germany
Integral part of sustainable soil management
by A. Krauss, International Potash Institute, Basel, Switzerland
- Population growth and urbanization call for more, better and more varied food.
- Land and water are becoming scarce - the existing land must produce more.
- The higher yields required remove more nutrients and these must be replenished.
- Use of mineral fertilizer to support soil fertility has become common practice but fertilizer use is out of balance.
- Imbalance in crop nutrition leads to soil nutrient mining and consequently to soil nutrient depletion.
- With depletion, the soil loses its potential to cover nutrient demand of high yielding varieties and demand in stress situations.
- Balanced fertilization provides the basis for sustainable crop production; who are the beneficiaries?
Global population increases annually by about 80 millions. Although the growth rate will decline, it is expected that the global population will hit the 8 billion mark within the next 20 years. Most of the population growth will occur in developing countries, where between now and the year 2020, the population will increase by almost 40%. In contrast, over the same period in developed countries, the increase will be under 4%.
There is also a considerable shift in the ratio of people living in the rural areas and in towns. Urbanization in developed countries is currently about 74%, in developing countries 40%. However, within the next 20 years, half of the population in developing countries, in the search for jobs and food, will live in towns.
More people need more food and urbanization changes the diet towards more animal protein, wheat, rice, high quality vegetables and fruits. In China for instance, townees eat more meat (27 kg red meat) and less rice (68 kg) than their rural counterparts (17 kg meat and 103 kg cereals) (ROZELLE & JIKUN HUANG, 1999). Correspondingly, the per capita demand for meat will increase till 2020 in developing countries by 60%, in developed countries only by 5% (ROSEGRANT et al., 1995). Furthermore, in response to changing demand with urbanization the acreage under vegetables and fruits in China increased by 4 and 8 times respectively over the past 30 years whereas the area under cereals even declined slightly (Figure 1).
On the global scale, it is expected that the total demand for cereals as food and feed will increase by 2.4% annually reaching about 3.4 billion tons in 2020. However, the projected cereal production of 2.7 billion tons in 2020 would leave a gap in supply of 700 million tons unless crop yields, especially in developing countries can be increased substantially.
|Figure 1: Crop spectrum in China as affected by changes in diet|
|database FAOSTAT '98|
The global availability of arable land is decreasing and will further decline from currently 0.24 ha per capita to 0.17 ha in 2020. Most striking is the situation in Asia where, 20 years from now, only 800 m2 per caput will be available for crop production. A similar trend is expected in India. The per capita land availability will decline from currently 0.14 ha to 0.10 ha in 2025. Moreover, "... the quality of land (in India) likely to remain available for agriculture due to severe competition from urbanization, industrialization and civic needs, will be poor..." (KANWAR & SEKHON, 1998). In this context, HANSON (1992) reports that only about 12% of the soils in the tropics have no inherent constraints. Of the remaining land, 9% has limited nutrient retention capacity, 23% aluminium toxicity, 15% high P fixation and 26% low potassium reserves.
What has been said for land availability also applies for water. Withdrawal of water in developing countries will increase by 43% between now and the year 2020, in developed countries by 22%. But, in developing countries, the demand for domestic and industrial uses will double, reducing the supply for agriculture (PINSTRUP-ANDERSEN et al., 1997).
In consequence, horizontal expansion in food production is hardly possible unless further deforestation and use of marginal land is accepted. The necessary increase in production has to come through higher yields and denser cropping sequences, i.e. through higher productivity of the remaining land and water. To match the expected cereal demand of 3.4 billion tons in 2020, cereal yields must be increased from currently 2.9 to almost 4.9 t/ha, and rice yields by 60 to 70%. The current rate of increase as shown in figure 2 is far too low to meet the future demand. Had India, for example, relied on area expansion for increased grain production, it would have had to more than double the area currently under cultivation. But India like the other Asian countries does not have spare land.
|Figure 2: Regional cereal
current development and projected demand
|database FAOSTAT 1998|
The need for higher soil productivity to feed future generations is evident. Higher yields remove more nutrients from the field and these must be replenished. The global mean yield of 3 t/ha cereals removes together with the straw about 81 kg nitrogen, 15 kg phosphorus and 75 kg potassium from the field, the equivalent of 3½ bags of urea, 1½ bags of DAP and 1½ bags of potash fertilizers. There are other unavoidable losses of nutrients in leaching water, by erosion, fixation, etc. as shown schematically in figure 3. These must also be replaced.
|Figure 3: Schematic nutrient flow in agroecosystems|
Native soil fertility cannot support the necessary yield increase. Nutrient inputs through depositions, sedimentation and biological nitrogen fixation are far from sufficient to satisfy the nutrient demand of high yielding varieties. Mineral fertilizer is the primary source of nutrients and usually contributes 35 to 50% to yield increases. Numerous field trials conducted by FAO and the private sector have shown that one kg of mineral fertilizer can achieve under farmer's conditions about 10 kg additional yield. Figure 4 demonstrates the rather close relationship between use of mineral fertilizers and cereal production, taking sub-Saharan Africa and developing Asia as examples. Fertilizer use in Asia grew over the past 40 years from virtually nil to more than 20 kg per capita. During the same period, per capita cereal production increased from less than 200 kg to more than 300 kg. In contrast, use of fertilizers in sub-Saharan Africa fluctuated around a rather minute level of 2-3 kg, the corresponding cereal production declined from around 150 kg per capita during the sixties to around 130 kg indicating loss in soil fertility.
|Figure 4: Per capita fertilizer use and cereal production in Subsaharan Africa and developing Asia|
|database FAOweb 2000|
Recycled plant residues, green manure or farmyard manure are important components in soil fertility management, at least minimizing the loss caused by export of food and thus nutrients into urban centers or across national boundaries. However, in principle, use of organic manure or recycling of plant residues cannot compensate for all losses unless additional nutrients enter the farm from outside like in the form of concentrate feed. In this context, Belgian dairy farms have a positive nutrient balance although fertilizer usage has declined because imports of concentrate feed for animals have increased (MICHIELS et al., 1997).
The situation in developing countries is different. In China for instance, although use of organic manure is still increasing, the share of nutrients therefrom in total nutrient input has declined steadily from 100% in the fifties to currently 30% (JIAN-CHANG XIE et al., 2000). In Pakistan, the use of farmyard manure is inversely related to farm size. Small farms up to 1 ha use on average almost 5 t/ha whereas larger farms of 10 to 20 ha apply less than 100 kg/ha (NFDC, 2000). Lack of affordable labor, misuse of crop residues as fuel and building material is reasons for this.
Use of mineral fertilizer to support soil fertility has become common practice but fertilizer use is out of balance
Global consumption of mineral fertilizers increased steadily after the war up to the late eighties, when economic constraints, especially in Eastern Europe and the FSU, ecological considerations, set-a-side programs, crop prices, etc, caused a substantial setback in fertilizer use. Subsequently, use of nitrogen fertilizer recovered fairly well whereas use of phosphate and especially of potash fertilizers is still below the level achieved during the late eighties although crop output continued to increase further. This resulted in fertilizer usage becoming unbalanced in two respects:
- Nutrient ratio, especially of N to K.
- The ratio of fertilizer nutrient to nutrient removed by crops, i.e. the input/output ratio.
The NK ratio in fertilizer use declined in the past 20 years from a roughly balanced value of 1:0.4 to currently 1:0.26. This ratio contrasts sharply with the ratio in which plants absorb N and K. Cereals for instance take up nitrogen and potassium in almost equal quantities, vegetables and root/tuber crops absorb even more potassium than nitrogen.
There are substantial regional differences in the N:K ratio in fertilizer use ranging from 1:0.43 in West Europe or North America to 1:0.1 in India or even 1:0.06 in West Asia and North Africa (the WANA Region). Developing countries in general consume roughly 65% of the global N use but only 45% of K use. Some of the reasons for the regional differences in the nutrient ratio are:
- Belief in unrestricted K supply of cultivated soils.
- Spectacular crop response to nitrogen when mineral fertilizers are introduced, so relating yield mainly to N fertilizer usage.
- Rather discreet crop response to potash fertilizers in terms of quality, disease and stress resistance.
- Misinterpretation of soil test results.
- Simply lack of knowledge.
- Unawareness, unavailability of potash.
- Inadequate promotion of balanced fertilization in developing countries.
Concerning the nutrient output/input ratio or the balance between nutrient removal by crops and fertilizer use, fertilizer imbalance is especially evident in developing countries.
|Figure 5: Fertilizer use in relation to nutrient removal by crops in developing countries|
|data source: FAO Yearbooks|
As shown in figure 5, use of nitrogenous fertilizers in developing countries has reached the level of N removed by crops. Increasing use of phosphate fertilizers also started to close the gap. However, potassium removal by crops exceeds by far use of potash fertilizers, the negative balance in potassium is steadily growing. India for instance has currently a deficit of about 7 million tons K2O whereas N use more than covers N removal by crop. China's use of potash lags more than 11 million tons K2O behind potash removal; but N use exceeds N removal in crops by nearly 7 million tons. The situation in sub-Saharan Africa is even worse because all 3 major nutrients are in deficit. IFDC estimates the actual crop nutrient requirement to be 3.9 million t N, about 2 million t P2O5 and 2.9 million t K2O. The current fertilizer use is about 0.7 million t of N, 0.4 million t P2O5 and 0.23 million t K2O, i.e. use of mineral fertilizers in Sub-Saharan Africa covers around 20% of the need for N and P and less than 10% for potassium (HENAO & BAANANTE, 1999).
The nutrient balance in developed countries evolved rather differently from developing countries (figure 6). In the former, fertilizer use up to the late eighties exceeded nutrient removal by crops; thus there was a substantial build-up of soil fertility. This applies to all 3 major nutrients. During the early nineties, fertilizer use in developed countries decreased sharply, mostly in response to uncertainties in land titles, lack of credits, etc. during the economic reform in East Europe and the FSU. As indicated earlier, economic constraints, ecological considerations and set-aside programs in Western countries also caused some reduction in fertilizer use. N and P fertilizer usage are close to the nutrient removal by crops. K deficit is also evident in some developed countries.
|Figure 6: Fertilizer
use in relation to nutrient removal by crops in developing countries
mean of 3 years
|data source: FAO Yearbooks|
Imbalance in crop nutrition leads to soil nutrient mining and consequently to soil nutrient depletion
Extended soil nutrient mining as observed in Africa not only depletes the nutrient reserves, it also affects chemical soil characteristics such as pH, base saturation and the content of free aluminium (table 1). The latter is toxic to roots of most cultivated plants. Root length is drastically reduced, which restricts exploration for soil nutrients and water. Not only is yield much reduced, the plants are also very susceptible to drought.
Detailed information is available on the impact of soil K mining on soil fertility. As readily available K is depleted, K supply to the soil solution depends more on K release from the non-exchangeable or slowly available fraction, the K reserve. As shown by CHENG MINGFANG et al. (1999) on soils in North China, the rate of release of K from the exchangeable fraction is much higher (5-9 mg K/kg/min) than that from the non-exchangeable fraction (0.1-0.5 mg K/kg/min). Consequently, with depletion of soil K and thus with increase of the contribution from non-exchangeable K, the yield declines because the release intensity cannot cope with the demand of high yielding crops (figure 7).
In this context, it is understandable that there was little or no response to K on alluvial soils of the Nile, Indus and Ganges systems so long as the yield level was rather low. On these heavy textured soils, the rate of release of K from the reserves was high enough to meet the K demand of a small crop. But with transition from subsistence to market oriented agriculture, the introduction of high yield varieties, double cropping and intensive use of nitrogen, the demand for soil K increased to the extent that the native K supply of these soils could not cope with it. With declining native K reserves, crop response to potash increases as observed in India. Evaluation of long-term experiments revealed that during the last 25 years crop response to N has decreased, whereas the response to P increased by 20% and to K even by 160%.
|Figure 7: K uptake and yield of Sudangrass at sequential cropping|
|from SRINIVASA RAO & KHERA, 1995|
With progressive exhaustion of soil K reserves, applied potassium is more strongly fixed. Because applied K is trapped in the interlayer spaces of clay minerals crops may fail to respond to normal K dressings. Unfortunately, the occurrence of K fixation and resultant absence of crop response often misleads the fertilizer advisor and this worsens the situation. At the same time, K depleted soils tend also to fix NH4, thus lowering the availability of fertilizer N.
With depletion the soil loses its potential to cover nutrient demand of high yielding varieties and demand in stress situations
Results from 650 field trials in Germany showed that, at the lowest level of exchangeable K, cereals lost 18% and leafy crops like potato or sugar beet almost 40% of yield opportunity (figure 8). In long-term field trials at Rothamsted UK, barley receiving 144 kg/ha N yielded less than 2 t/ha on exhausted soil with 3ppm P and 68 ppm K but 5 t/ha on fertile soil with 140 ppm P and 329 ppm K (JOHNSTON, 1994). Lack of sufficient P and K impaired utilization of natural resources and reduced the efficiency of inputs like N fertilizer, causing a burden to the environment because the content of residual N in soil is much higher following a poor crop than after a high yielding crop.
|Figure 8: Loss in opportunity yield at different levels of soil K status|
|soil K status|
|after KERSCHBERGER & RICHTER, 1987|
The value of lost yield opportunity due to unbalanced fertilization can be considerable. For instance, Bulgaria lost around 135'000 t wheat, 28'000 t maize and 53'000 t sunflower or the equivalent of almost $30 million due to unbalanced fertilization (NIKOLOVA & SAMALIEVA, 1998).
Apart from lower yield, unbalanced fertilization impairs the quality of the crop. This refers in the same way to nutritional, hygienic, organoleptic and functional properties as well as to the environmental compatibility in production. Numerous field trials conducted by IPI in India, Egypt, Hungary or China showed higher contents of protein in wheat, oil in groundnut, rapeseed or soybean, sugar in cane or beet, and more aromatic components in tea at balanced fertilization.
Tolerance to climatic and biotic stress is also much affected by soil fertility. Early wilting in drought is a typical indication of K deficiency. Water stressed plants have significantly reduced photosynthesis with ultimately lower yield. Yield of triticale was more than halved by drought on soils inadequately supplied with potassium (WYRWA et al., 1998). Use of K alleviated the effect of drought stress and increased yield to about 83% of unstressed plants because potassium, as an osmotically active ion, improves the water economy of the plant and regulates stomatal movement.
Frost hardiness is also improved by balanced fertilization. Finally, adequate potash use in relation to N decreased incidence of fungal diseases in 70% of investigated cases. There was a similar improvement in resistance to bacterial (69%) and virus (41%) diseases and to insects and mites (63%) (PERRENOUD, 1990).
At the same time, the farmer lowers input costs by needing less agrochemicals for more tolerant plants, a contribution to a more environmentally compatible crop production.
Balanced fertilization provides the basis for sustainable crop production; who are the beneficiaries?
Numerous long-term field trials prove that the initial level of soil fertility and thus crop yield can be maintained provided the nutrient balance is in equilibrium. As indicated earlier, the introduction of modern high yielding varieties affects the nutrient balance by removing more nutrients with higher biomass. Of course, there are genotypical differences in nutrient use efficiency. At the same level of soil fertility, yield of modern wheat varieties is 50% higher than that of less advanced land races (KARPENSTEIN et al., 1986). Better spatial exploitation of the root system, release of acidifying and/or chelating substances can explain genotypical differences in uptake efficiency.
The present trend in plant breeding, especially in developing transgenic lines, focuses more on specific traits like resistance to a particular insect or virus, on increase in toleration of a particular herbicide. Development of transgenic lines for high-yielding crops has obviously a lower priority because a large number of genes are thought to be responsible for yield. Of course, as limiting factors like disease susceptibility are eliminated, thus increasing yield, an indirect effect may be to increase nutrient requirement. As far as potassium is concerned, a future possibility could be to improve the efficiency of K uptake systems in order to utilize low external solute concentration or to improve the competitiveness of K uptake in presence of excessive Na under saline conditions.
Who benefits from balanced fertilization?
The farmer: reaps higher yields of better quality resulting in lower production costs and higher profits but also better competitiveness on the market. Increasing the stress tolerance of crops also improves yield probability under adverse conditions, which gives the farmer confidence to reinvest in agriculture. Table 2 summarizes the economics of potash use achieved in on-farm trials in India. By investing one rupee in potash the farmer gets returns between 5 and 20 rupees.
The nation: increased food production coupled with better quality improves food security; it lowers import requirement and increases export opportunities. Higher yields increase purchasing power in the rural area, attracting other business; creating jobs and reducing migration. In this way, balanced fertilization contributes to social security in the rural area.
The natural resources: by raising yield, balanced fertilization improves the efficiency of land, water and energy. In so doing, it reduces pressure on land and water resources, prevents deforestation and protects marginal land. Concerning the energy gain achievable with balanced fertilization, IPI on-farm trials with wheat in India showed for K nutrition an output/input ratio of 41 to 136. Although the yield increment and thus the energy gain with potash is lower than usually obtained with nitrogen, the energy output/input ratio is considerably higher than those shown for N use on wheat in Germany (BIERMANN et al., 1999). Potash has a lower energy equivalent (3 MJ/kg K2O) than nitrogen (35 MJ/kg N).
The environment: improved fertilizer use efficiency leaves smaller nutrient residues in the soil. This protects the groundwater and reduces losses of N by volatilization. In China, after harvesting cabbage, under farmers' practice, i.e. primarily N and P, some 140 kg/ha NO3-N was left in the soil. It declined to less than 40 kg/ha when N was well balanced with K (figure 9). Vigorous plant growth as resulting from balanced fertilization covers the ground more quickly and stabilizes the soil surface. This helps to reduce soil erosion and runoff, the major sources of nutrient losses for instance in Sub-Saharan Africa.
|Figure 9: Residual nitrate
in subsoil after harvest of cabbage as affected by fertilization practice
results from IPI trial in China
|after HAERDTER, 1999|
'Fertilizers feed the world' is the slogan of the international fertilizer industry. Global transfers of food and thus nutrients, and especially the transfer from the arable land into towns, means that farmers have to rely increasingly on external sources of nutrients to replace those lost from their fields by selling the produce. Whether the external source is a bag of mineral fertilizer or the nutrient in concentrate feed via FYM is irrelevant as long as the deficit in the nutrient budget is closed. Full respect is paid to those who advocate organic farming but systems based only on recycling, green manure, etc, will fail in the long run because of inadequate compensation for exported or otherwise lost nutrients.
Balanced fertilization involves considering the whole spectrum of nutrients. The focus on nitrogen and phosphorus during the initial phase of fertilizer use is understandable because of the spectacular response of the crop to N. The demand for K emerges as a result of increased yields produced by N or N P in the farmer's first experience with fertilizer. A demand for S will follow this and, sooner or later, Mg and the trace nutrients. There is growing need for site specific fertilizer recommendations according to the crop type, yield level and soil conditions. Balanced fertilization should include the use of organic manure as an integral part of programs designed to secure sustainable soil fertility.
The soil nutrient capital is not an inexhaustible resource and must be replenished according to the nutrient withdrawal. With the obligatory need for intensification of crop production, the demand of crops for readily available soil nutrient increases. Soil-borne nutrients may suffice for small crops in subsistence farming but are insufficient for a crop destined for the market.
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