Crop production is based on chemical - biological processes, which utilize natural resources (CO₂, water and O₂) as well as available and supplied mineral nutrients. These are essential processes in plants and they need to function efficiently in order to obtain crops of high yield and quality.

Some 179 years ago, Sprengel, and later von Liebig, established the theory of mineral nutrition of plants. As scientists continued to explore and improve mineral nutrition, some clear interactions between nitrogen and potassium were revealed. Researchers realized that potassium is actively involved in various processes such as nitrogen uptake and conversion to protein, and nitrogen fixation in legumes - all which lead to higher efficiency of nitrogen utilization either applied as fertilizer or fixed from the atmosphere. While N is the building block of protein and hence considered to be the 'fuel of food production', potassium "is the most important cation not only in regard to its content in plant tissues but also with respect to its physiological and biochemical functions." (Mengel and Kirkby, 1987).

Nitrogen Use Efficiency (NUE) and balanced fertilization with potassium.
The need to use N fertilizer efficiently (a higher NUE) is greater than ever before because of the increasing demand for food and the consequent greater N fertilizer usage set against the negative role that reactive nitrogen has on the environment. The immediate gain from balanced fertilization with potassium is described in Fig. 1: Higher nitrogen uptake is clearly the result of higher potassium supplied. Hence, balanced fertilization, also as part of what we call 'Best Management Practices' (BMP), can reduce 'N leaky practices' which leads to N cascading into and within the environment.

The International Potash Institute (IPI) is involved in many research projects around the world to study and demonstrate the benefits from potash application. Through the years, IPI has conducted thousands of on-farm experiments and demonstrations in many regions showing the role of potassium in effective and sustainable nutrient management. We have chosen in this case-study paper to present a few examples which demonstrate how to achieve higher NUE through improved potash fertilization practices in Asia and Europe (see Table 1). In the examples shown, we compare a constant level of N application with increasing levels of applied potassium. In addition to the increased yield obtained (yield increase, kg/ha), we have calculated the increase in NUE (%) by attributing the higher yield to the same N application (see Table 1). As shown in Table 1, a typical gain of 10 to 30 per cent increase in NUE is achieved by applying a moderate dose of potassium to maize, rice, wheat, rye and sunflower. When combining potassium application and advanced water management (e.g. fertigation), gains result in a much higher (70 per cent increase in NUE in fertigated sugarcane in India; see Table 1).

NUE in legumes?
Measuring NUE in legumes in the terms described above is not relevant, because the Leguminosae fix nitrogen from the atmosphere and hence N fertilization rates are much lower in comparison with other crops. Nevertheless, as these crops provide an important protein source, it is of significant interest to cultivate them to achieve higher protein yield by
enhancing both the rate at which N is fixed and its conversion to protein. Experiments in Pigeon Pea (Cajanuscajan L. Millsp.) showed that the application of P and K markedly increased both grain yield and protein yield (Fig. 2, adapted from "Fertilizer Use and Protein Production", 1975). This clearly demonstrates the benefits of potassium and phosphorus supplements in addition to a small rate of N to pulses in the production of much higher grain and protein yields.

In summary, the profits from balanced fertilization are as follows:

• Direct income: Typically we obtain a ‘Value Cost Ratio’ (VCR) with potassium application which is 3-7 times greater; hence the additional potassium fetches the farmer increased net income for the same N level applied. This extra income can be measured directly.
• Additional gains: Increased NUE means that approx. additional 30 kg of N/ha (e.g. 25 per cent increase in NUE at 120 kg N/ha) are utilized by the crop and are prevented from entering the environment. Unfortunately our understanding in computing the costs of these leakages to the environment (e.g. nitrates in groundwater, NOX to the atmosphere and extra CO₂ released for production and logistics of nitrogen fertilizers) is still insufficient. What is clear though is that this N loss is a real cost to the environment and society. Praise is therefore due to those farmers who increase NUE by the deployment of advanced nutrient management techniques as part of a whole BMP approach.
• Additional protein: In legumes, the use of potassium (and other deficient nutrients) increases protein yield and thus contributes to the yield and quality of these crops. This is of significant value to crops which are used for direct consumption by humans (e.g. pigeon pea) as well as those used for animal feed (e.g. soybean).

Sources and further reading:

• Crews, T.E. and M.B. Peoples. 2004. Legume versus fertilizer sources of nitrogen: ecological tradeoffs and human needs. Agriculture Ecosystems & Environment 102:279-297.
• Fertilizer Use and Protein Production. Proceedings of the 11th Colloquium of the International Potash Institute held in Rønne-Bornholm, Denmark, 1975. International Potash Institute, Horgen, Switzerland.
• Gething, P.A. 1993. Improving Returns from Nitrogen Fertilizer; The Potassium-Nitrogen Partnership. IPI Research Topic No. 13 (2nd revised edition). International Potash Institute, Horgen, Switzerland. 51p.
• IPI website at http://www.ipipotash.org/publications/detail.php?i=72
• Mengel, K. and E. A. Kirkby. Principles of plant nutrition. 1987. 4th edition. ISBN: 3-906 535 03 7. International Potash Institute, Horgen, Switzerland.
• The Third International Nitrogen Conference (N2004); Impact of Population Growth and Economic Development on the Nitrogen Cycle: Consequences and Mitigation at Local, Regional and Global Scales. Nanjing, China, October 12-16, 2004. V 48 special issue. ISSN 1006-9305.