IPI International Potash Institute
IPI International Potash Institute

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

Presented at the

AFA 10th International Annual Conference

Cairo, Egypt 20-22 January 2004

Balanced fertilization, the key to improve fertilizer use efficiency

by Dr. A. Krauss

Contents

Balanced fertilization, the key to improve fertilizer use efficiency
Excessive fertilization - one of the causes for low fertilizer use efficiency
Unbalanced fertilization - another cause of low fertilizer use efficiency
The link between the balance in K use and efficiency in fertilizer use
Balanced fertilization helps to improve fertilizer use efficiency
Conclusion
References

Balanced fertilization, the key to improve fertilizer use efficiency

With the invention of the Haber-Bosch process in 1913, which converts inert atmospheric N2 into ammonia, and the ability to increase nitrogen (N) fertilizer production, the rapidly growing food demand of an exploding global population in the second half of the 20th century could be met. But this has meant that increasing amounts of reactive N, in form of fertilizers, are being used in agriculture. Simultaneously, with developing industrialization and transport, progressively more fossil fuel is burnt and this also releases reactive N in form of NOx. Today, the release of reactive N into the environment in the form of ammonia, nitrates and their derivatives as well as in the form of organic N is in the order of 165 Mt (OTTER and SCHOLES, 2003), which is about 15 times greater than the human contribution in 1860 and twice the current amount of reactive N from biological fixation.

The increasing release and accumulation of reactive N in the environment have an impact on the global climate (greenhouse gas, ozone destruction), and on acidification of terrestrial and aquatic ecosystems to mention only the major components. Figure 1 symbolizes in the nitrogen cascade the relationship between the source of reactive N and its sinks.

Figure 1: The Nitrogen Cascade (after GALLOWAY et al., 2003).

TOWNSEND et al. (2003) consider that the changing global N cycle has effects on human health well beyond the associated benefits of increased food production and that excessive air- and waterborne N is linked to respiratory ailments, cardiac diseases and several cancers. These authors also see a link between increasing release of reactive N and the increased allergenic pollen production as well as with the changed dynamics of several vectorborne diseases like malaria and cholera. In other words, they suggest that the benefit to the public that comes from better crop yields and nutrition and improved transport and heating, will be more and more concealed by the negative impact of the increasing release of reactive N to the environment and its effects on air and water pollution and ecological feedbacks.

The use and manufacture of N fertilizers contribute about 60% of the total release of reactive N. Thus to control the release of reactive N, especially from agriculture, some countries like Denmark within the European Union have issued legislative measures (Nitrate directives) within which farmers have to obey certain upper limits in the use of N from organic and inorganic sources, otherwise there will be a financial penalty. However, being squeezed within a legislative frame that reduces N use by 10%, the Danish farmer not only looses yield but also crop quality, especially the protein in cereals. Taking the value of protein into account, the farmer loses up to 75 €/ha when growing winter wheat (KNUDSEN, 2003).

Without doubt, the producers and users of fertilizers are under progressively greater public pressure with respect to the protection of the environment. However, before more stringent legislative measures are put in place, especially on farmers, the stakeholders should look at ways to control and reduce the loss of nutrients from mineral fertilizers into the environment, and in particular, N and phosphorus (P).

One of the alternatives is to improve the efficiency of fertilizer use, which could be much better. WITT (2003), for example, reported that the recovery of fertilizer N in irrigated rice in Asia, ranges from only 24% in China to 50% in the Mekong Delta of Vietnam, i.e. 50 to 76% of the applied N is lost to the environment. Extrapolated to about 130 M ha of rice in Asia and an average use of 100 kg/ha N, the loss of reactive N from fertilizers would amount to 6.5 to 10 Mt N valued at some $1.2 to $2 billion as fertilizers. On the global scale, assuming an overall efficiency of one third, the quantity of reactive N lost to the environment would be about 55 Mt or the equivalent of $11 billion, indeed a heavy burden.

What has been said about N applies also to the other major nutrients, P and K, although their impact on the environment is quite different. WITT (2003) reported recovery efficiencies in irrigated rice of 0 to 40% for P and 20 to 60% for K, depending on the application rate. Both nutrients are not volatile and, due to intensive interactions with soil particles (exchange and retention processes), leaching losses of both nutrients are restricted and mostly confined to light textured soils. Nutrient losses do occur with eroded soil. If lost to the aquatic environment, P contributes to eutrophication, whereas practically no environmental or health hazards are known for K.

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Excessive fertilization - one of the causes for low fertilizer use efficiency

The global use of mineral fertilizers increased steadily up to the 80s but then lost its momentum during the last two decades (Figure 2) in the developed countries. Here total fertilizer use decreased sharply after the collapse of the Former Soviet Union at the end of the 80s. Other reasons for the decline of fertilizer use in the developed countries can be seen in higher use efficiency, better utilization of organic sources of nutrients, set-a-side programs for land use and economic and ecological considerations.

Figure 2: Global and regional consumption of mineral fertilizers (FAO, 2003)

In contrast, the fertilizer use in developing countries has shown a steady increase. China and Vietnam, for example, increased N use from 5 and 2 kg/ha respectively 40 years ago to currently 170 to 180 kg/ha. In China, the use of more than 1000 kg/ha N for vegetables is not uncommon. However, CASSMAN et al. (2002) showed that the recovery efficiency of N is greatly affected by the amount of N used and by the synchrony between N demand and supply (Figure 3).

Figure 3: Recovery efficiency in relation to the degree of synchrony between crop N demand and the N supply from indigenous resources (CASSMAN et al., 2002).

In general, N recovery efficiency is high if fertilizer use is relatively low and if crop demand is high. Over-fertilizing the crop on sites that have a relatively high indigenous nutrient supply, and not limiting supply and ensuring synchrony between demand and supply, reduces substantially the recovery efficiency. In Figure 3, the degree of synchrony is given by FN = the amount of applied N, INS = N supplied by indigenous resources, UN = N uptake from fertilized plots. Smaller values on the abscissa in Figure 3 indicate greater synchrony between N supply and demand. This relationship applies also to P and K.

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Unbalanced fertilization - another cause of low fertilizer use efficiency

Apart from the changes in the quantity of fertilizer used, there has also been a change in the nutrient ratio, especially the NK ratio. As shown in Figure 4, the NK ratio of fertilizer use in developed countries decreased continuously from a fairly balanced ratio of 1:0.8 in the 60s and 70s to a current N:K ratio of 1:0.36. The NK ratio for fertilizer use in developing countries has changed little and remains very wide at 1:0.23, ranging from 1:0.10 in the WANA region, 1:0.13 in South Asia to 1:0.22 in East Asia.

Figure 4: Development of the regional NK ratio in fertilizer use (data source FAO, 2003)

An exception is South America with an N:K ratio of 1:0.96, because of the large amount of soybean that is grown. And soybean is a crop that is very responsive to potash.

The very wide NK ratio for fertilizer use for instance in Asia contrasts sharply with the NK ratio in plants. Cereals take up N and K in almost equal amounts, root and tuber crops, leguminous crops and vegetables take up even more K than N. In consequence of the wide NK ratio in fertilizer use, the ratio of K input to K output has become highly unbalanced. Figure 5 shows that the N balance in developing countries - represented by the WANA region - improved over time to be in equilibrium, i.e. the harvested crops removed as much N as was applied in mineral fertilizers. The K balance of the WANA region shows the opposite trend, the annual deficit in K use has reached some 40 kg/ha K2O or the equivalent of 7 Mt of potash fertilizers. The highly negative K balance indicates considerable mining of soil K reserves.

Figure 5: Partial N and K balance of the WANA region and in Western Europe (WE)

In developed countries, represented in Figure 5 by Western Europe, there was up to the late 80s an increasing N balance, followed by a rather sharp decrease to currently around +40 kg/ha N but the N balance is still positive. On the other hand, the K balance for Western Europe declined steadily from a rather comfortable surplus of almost 40 kg/ha K2O to currently less than 10 kg/ha. Livestock farms for instance in Germany still have a positive K balance due to the import of K in feed concentrates, whereas arable farms already have a negative K balance. The input/output balance of P is intermediate between that of N and K.

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The link between the balance in K use and efficiency in fertilizer use

The uptake of K during the ontogenesis of plants is much more advanced than that of N (Figure 6). During shooting of cereals (EC around 40) when the plants have reached about 40% of the final biomass weight, the plants have already taken up 65% of the total N and 100% of the total K. During this stage, the soil has to have the capacity to release enough K into the soil solution to meet the daily K uptake rate of up to 15 kg/ha K2O. Soils depleted in K due to negative K balances, do not have the release capacity to meet the demand of the plant. With an inadequate K supply the yields remain low, which decreases the efficiency of the inputs, especially of N fertilizers.

Figure 6: Nutrient uptake of winter cereals during ontogenese in relation to biomass production after FRÜCHTENICHT et al., 1993

Because of the relationship between N uptake and biomass production, care is needed in N management. When a basal dressing is above the absorption capacity of the plant, only part of the applied N is taken up from the rooting zone and any N leached below this zone is lost to the water or to the air. On the other hand, a shortfall in supply during the phase of greatest N uptake (EC stage 30-50) prevents the realization of the genetic yield potential.

The physiological background between K supply and the N fertilizer use efficiency can be explained as follows (Figure 7):

Figure 7: Conceptual of the KNO3 cycle from
BEN-ASHER and PACARDO, 1997
  • K is a highly versatile and mobile nutrient in plants.
  • K is involved in all major physiological processes, from the assimilation and the transport of assimilate to its conversion into storage products such as sugar, starch, protein and oil/fats.
  • K also plays a prominent role in the N metabolism.
  • As a cation, K accompanies the nitrate anion as it is transported from the roots to the shoot where the 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.
  • K accompanied by malate is then retranslocated from the shoot to the root, where the K-malate is oxidized, yielding KHCO3 which exchanges for KNO3, and the cycle continues (Figure 7).
  • 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 plants inadequately supplied with K is likely to be leached into the groundwater or lost to the atmosphere as NOx gas.
  • Accumulation of nitrate in K deficient leads to a reduced protein content.
  • Furthermore, plants supplied with excessive N and/or inadequate K are more susceptible to pests and diseases and less resistant to soil-borne and climatic stress than plants with balanced nutrition, which also lowers the yield and thus, affects the fertilizer use efficiency.

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Balanced fertilization helps to improve fertilizer use efficiency

There are numerous results from field trials that demonstrate the beneficial effect of balanced fertilization on the use efficiency of mineral fertilizers as shown by the following examples.

  • Effect of the K status of the soil on the efficiency of N fertilization: In a longterm field trial with spring barley, JOHNSTON et al. (2001) demonstrated that the grain yield increased by more than 50% with the same amount of N fertilizer only when the plants were grown on a soil well supplied with K (Figure 8). The plants grown on the soil with inadequate Kex (60 ppm) were not able to accumulate enough K to meet the physiological demand of the plant. Similarly, barley cultivated on a soil poor in P (2 mg/kg Olsen-P) yielded only half of the crop, which was grown on a soil with 6 ppm P although receiving the same amount of fertilizer N.
Figure 8: Effect of soil K status on the efficiency of N use by spring barley (JOHNSTON et al., 2001)


  • Nutrient recovery in maize on a degraded soil in SE Asia: HAERDTER and FAIRHURST (2003) showed that the recovery of N from fertilizers increased from 16% at traditional NP fertilization to 76% at balanced NPK supply (Figure 9). The better N recovery in presence of K (treatment NK) was due to the involvement of K in N metabolism (Figure 7). Also the recovery of P from fertilizers improved with balanced fertilization, namely from 1% at NP to 13% at NPK, and the recovery of K increased from 22% at NK to 61% at NPK fertilization. Addition of FYM had only a minor effect on the recovery of the nutrients.

    Figure 9: Nutrient recovery in maize on a degraded soil in SE Asia (after Haerdter & Fairhurst, 2003)


    Of course, the relatively low recovery of P has to be considered differently from that of N because fertilizer P interacts with the different P pools in the soil. Excessive P in soil solution will be absorbed and/or precipitated but is eventually again available - at least in part (residual effect) - when the P concentration is reduced due to P absorption of the plants. Similar reversible exchange processes are known for K, where solution K exchanges with the fraction of exchangeable and non-exchangeable K. In other words, P and K, in contrast to N, are not really lost to the environment except where there is runoff and erosion.

 

  • Yield increase and better fertilizer N recovery in rice with balanced fertilization: In the multinational field trial program RTOP (Reaching Toward Optimal Productivity) for intensive rice systems organized by IRRI, there are 179 farm sites in 7 countries. Rice grain yield was increased by 7% by balanced fertilizer use although less N was applied (Figure 10). At the same time, the agronomic efficiency of N increased from 11.5 kg grain per kg N to 14.8 kg, and the recovery efficiency of rice from 31% to 40% (Dobermann et al., 2002).

    Figure 10: Performance of site-specific nutrient management (SSNM) in comparison to farmers fertilizing practice (FFP) in irrigated rice in Asia (Dobermann et al., 2002)


    Of course, applying the nutrients in a balanced manner is only one of the options. As important is the realtime management of N. To support the farmers in their decision as to the timing and amount of N to apply, the RTOP project promoted the use of leaf colour charts, LCC (Figure 11). The farmers can easily determine in the field whether the standing crop is short of N by comparing the leaf colour with the corresponding colour on the LCC. This simple tool is replaced in precision agriculture by sensors in front of a tractor, which control the amount of fertilizer N applied by the spreader.

    Figure 11: Leaf colour charts for real-time N management in rice (from WITT, 2003).


    The RTOP project also introduced the "omission plots2 in order to monitor the indigenous nutrient supply. As mentioned earlier, the indigenous nutrient supply determines to a great extent the recovery efficiency, because, with decreasing supply of indigenous nutrients, the synchrony between demand and supply improves and thus, the recovery efficiency of the nutrient applied as fertilizer. Figure 12 gives an example of the response of maize on a degraded soil in North Vietnam. It shows that K is next to N as the most limiting factor in crop production. Use of FYM had only a small response, which shows the rather restricted nutrient supply capacity of organic manure or the limited amount available.

    Figure 12: Effect of nutrient omission on maize yield in North Vietnam (from HAERDTER & FAIRHURST, 2003)

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Conclusion

To improve fertilizer use efficiency is essential and should be a common practice in farming. It not only helps to increase yield and income, it is also a major requirement to improve the image of farming. One of the quality criteria with which the consumer selects food at the market is whether the food is produced in environmentally acceptable ways. Another aspect is to be pre-active in improving fertilizer use efficiency before harsh legislative measures are introduced, which can be to the detriment of the farmer's welfare.

All stakeholders should join efforts to search for better fertilizer use efficiency. There are many ways to achieve this goal. They span from better nutrient management, time-wise and quantity-wise, the balanced use of nutrients, precision agriculture and the development and use of smart fertilizers, which release the nutrients according to the demand of the plants. However, irrespective which strategy is chosen, it is essential that the fertilizer industry transfers the knowledge from the research site to the farmer's field.

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References

Ben-Asher, J. and Pacardo, E. (1997): K uptake by root system grown in saline soil: a conceptual model and experimental results. In: Proc. Regional Workshop of the International Potash Institute, Bornova, Izmir, Turkey, 26-30 May 1997, pp. 360- 369.

Cassman, K.G., Dobermann, A. and Walters, D.T. (2002): Agroecosystems, nitrogenuse efficiency, and nitrogen management. Ambio 31:132-140.

Dobermann, A., Witt, C., Dawe, D., Abdulrachman, S., Gines, H.C., Nagarajan, R., Satawathananont. S., Son, T.T., Tan, P.S., Wang, G.H., Chien, N.V., Thoa, V.T.K., Phung, C.V., Stalin, P., Muthukrishnan, P., Ravi, V., Babu, M., Chatuporn, S., Sookthongsa, J., Sun, Q., Fu, R., Simbahan, G.C. and Adviento, M.A.A. (2002): Site-specific nutrient management for intensive rice cropping systems in Asia. Field Crops Research 74: 37-66.

Früchtenicht, K., Heyn, J., Kuhlmann, H., Laurenz, L. and Müller, S. (1993): Plant nutrition and fertilization (in German). In: Faustzahlen für Landwirtschaft und Gartenbau, Hydro Agri Dülmen, Germany, pp. 254-295.

Galloway, J.N., Aber, J.D., Erisman, J.W., Seitzinger, S.P., Howarth, R.W., Cowling, E.B. and Cosby, B.J. (2003): The nitrogen cascade. BioScience 53: 1-16.

Haerdter, R. and Fairhurst, T, 2003: Nutrient use efficiency in upland cropping systems of Asia. IFA Regional Conference, Cheju Island, Korea, 6-8 October 2003.

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.

Knudsen, L. (2003): Nitrogen input controls on Danish farms: agronomic, economic and environmental effects. Proceedings No. 520, The International Fertiliser Society, York, UK.

Marschner, H., Kirkby, E.A. and Cakmak, I. (1996): Effect of mineral nutritional status on shoot-root partitioning of photo-assimilates and cycling of mineral nutrients. J. Exp. Botany 47: 1255-1263.

Otter, L.B. and Scholes, M.C. (2003): Nitrogen mobilisation and redistribution: a global perspective. FSSA Journal 2003: 13-18.

Townsend, A.R., Howarth, R.W., Bazzaz, F.A., Booth, M.S., Cleveland, C.C., Collinge, S.K., Dobson, A.P., Epstein, P.R., Holland, E.A., Keeney, D.R., Mallin, M.A., Rogers, C.A., Wayne, P. and Wolfe, A.H. (2003): Human health effects of a changing global nitrogen cycle. Front. Ecol. Environ. 1(15): 240-246.

Witt, Ch. (2003): Fertilizer use efficiencies in irrigated rice in Asia. IFA Regional Conference, Cheju Island, Korea, 6-8 October 2003.

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