Some years are unusual 1984 will be remembered for high yields, others like 1975, 1976 and this year in England, for drought. 1976 was dry but this year was worse because the drought started earlier, more like 1975. What to do about nitrogen was a big question would nitrogen already applied be lost, should rates be increased, was it worth applying late nitrogen for protein? If ammonium nitrate was used, there should be very little loss but crops could still look nitrogen deficient. This was not because there was too little nitrogen in the soil but because it was too dry to be taken up. If urea had been used, the dry conditions might have led to some loss of nitrogen to the air but in most cases the problem would still have been uptake, not supply. Applying more nitrogen would have been a waste of time and money. Deciding on late nitrogen for protein was like betting no certainty and any decision would probably turn out wrong. Crops that had been badly affected by drought probably would not respond and those that were little affected would respond normally so these were easy. The difficulty was with crops that had been set back by early May but that continued to grow. Useful rainfall in the following week or two might allow a response to late foliar nitrogen but who could predict this? It really was luck of the draw.
We might be seeing seasons like this more frequently so it s a good thing there are several large research projects underway to adapt cropping for drought. It looks like there is enough genetic variation in drought tolerance and water use efficiency to give breeders scope for improved varieties and that s likely to be good news for nitrogen utilisation too.
Effects of drought on use of other nutrients shouldn t be forgotten. If yield has been reduced, there will be less offtake of phosphate and potash so perhaps some saving on the next crop. I wouldn t bank on it though national usage of phosphate and potash already is too little to maintain soil fertility. In 2009, average GB fertiliser applications to wheat were equivalent to around 2 kg P2O5 and 3 kg K2O per tonne of grain. As typical offtakes are 7.8 kg P2O5 and 5.6 kg K2O per tonne of grain, indices will be falling, perhaps to critical levels, in many fields.
The key is regular soil analysis - typically it costs less than 15p/ha/year - and maintaining target Indices of 2 for P and 2- for K. Latest data from the Professional Agricultural Analysis Group (PAAG) show just under 30% of samples analysed were at target P or K Index. 28% of samples were below P Index 2 and 37% below K Index 2- where yield is at risk. So, unless recent soil indices are known to be at or above target, where savings can be made and the risk of nutrient losses reduced by not making applications, phosphate and potash are best applied to replace offtake.
Ian has over 30 years experience in fertilizer use and crop nutrition. He now runs his own consultancy, Ecopt, involved in all aspects of nutrient management, including provision of the FACTS technical information service and he is also a member of the steering groups for revision of RB209 in 1999 and the Fertiliser Manual from 2007.
No comments have been made.