The test of
time
The test of time Current status of crop responses to fertilizers in different agro-climatic zones (of India) is the title of an interesting report by A.Swarup and Ch.Srinivasa Rao, discussing "experiences of All-India co-ordinated research projects on long-term fertilizer experiments" (Fertiliser News 44, No. 4, pp. 27-43, 1999).
| Figure 14: Crop response to fertilizer nutrients in India over time | |
|---|---|
| mean of 16 crops and 8 locations | |
| kg grain per kg fertilizer nutrient | |
 |
|
 1973-77 |
 1992-96 |
| from SWARUP & SRINIVASA RAO, 1999 | |
Although permanent plot trials were first set up in India in 1905, 'long term' in this context means nearly thirty years. Results show that the addition of K to N and P increases crop yield irrespective of the crop type and the agroclimatic zone. Crop yields improve when farmyard manure is supplied in addition to NPK, indicating the value of the micro-nutrient content of the manure and that the NPK formula used in the long term experiments does not, on its own, maintain correct nutrient balance. Results also show that the fertility of soils receiving only N, or NP, declined substantially over time whereas yield level could be maintained or even increased with NPK, especially when complemented with farmyard manure. At some of the trial sites, the soil K status decreased even with NPK application, indicating that K rates were too low to compensate K loss through harvest and leaching. Major changes in soil K occurred in the non-exchangeable fraction (extracted by H2SO4 and HNO3) which indicates substantial depletion of K reserves. This fraction is not monitored by routine standard soil tests (K extraction by NH4OAc). Crop response to individual nutrients (kg grain per kg nutrient) changed considerably over time. Response to N declined by about 40% whereas response to P increased by 20% and to K by more than 160% (Figure 14). The use of mineral fertilizer led, in most cases, to an increase of the organic C content in soils.
The authors point out that the long-term fertilizer experiments clearly show that unbalanced and non-integrated fertilizer use can be disastrous. They add that intensive cropping systems generate a marketable surplus of commodities, including for export, and therefore the quality of produce should also be measured in relation to nutrient supply.