No answers in the short term| Cash
crops usually are well fed with potash because crop quality pays like in Haryana/India |
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| after Tandon & Sekhon, 1988 |
Fertilizer use in India is highly unbalanced with respect to potassium. Nevertheless, yields and production of food grains are still increasing. This apparent paradox relates to the special properties of K, the nutrient that does not immediately reveal its presence -nor its absence. Lessons learnt in India are no less relevant elsewhere.
As a general observation, total fertilizer use in India has developed in parallel with production of food grains. And, as is common elsewhere, the use of K lags behind that of N and P. In subsistence and staple crops, this imbalance is even more marked but, where farmers are growing cash crops, and need to produce high quality to attract premium prices, use of K is more common. Potatoes are a good example. Farmers in Haryana achieve a positive balance of potassium by applying farmyard manure, a more profitable use of this resource than selling it for fuel. However, this is an exception and soil mining of K is widespread.
The availability of soil K to plants is subject to dynamic processes controlled by soil texture and water. Because K movement depends on diffusion, the distance it can travel is restricted to a few millimetres. Whether or not plants can exploit soil K reserves depends not only on soil structure but also on the plant's root system, particularly its root length and density. This is a property that varies with genotype. For example, cereals have a dense root system which can efficiently exploit the immobile potassium in the soil. In contrast, sugar beet, potatoes and pulses, have a rather coarse and less dense root system. They are less able to exploit soil reserves and therefore respond more immediately when K is applied. It is for this reason that seasonal trials often fail to reveal crop responses to K, especially in cereals.
Long-term field trials over many years reveal the effects of continuous K mining from the soil. Such mining alters the configuration of soil minerals and K fixation increases. This explains why, when K is subsequently applied to an impoverished soil, crops often fail to respond immediately. The only solution is to rehabilitate the soil by applying sufficient potash to restore soil reserves. But, in general, rehabilitating impoverished soil costs three to five times more than maintaining soil fertility in the first place.