My first experience of this was some years ago when a dairy farmer called me for help. His irrigated pasture, he told me, was simply not growing, despite all the nitrogen he had applied. Perhaps potassium was the problem? But when we visited his lands, I immediately saw that we were dealing with a nitrogen deficiency. It was only then that he informed me the nitrogen he had applied had been in the ammonium form. I told him that this nitrogen would not become available to the crop for some months because of the low soil temperature.
Then, last year, I was asked to visit a large cabbage grower whose crop had reached a standstill in winter. He explained that he had applied the same quantity of LAN as he had done throughout summer. The problem was that only the nitrate could be used by the crop at this time. In effect, he was applying half the required nitrogen.
A farmer should make provision for lower soil temperatures when fertilising the crop in winter. There are three main options:
Apply LAN in larger quantities in autumn, before the soil temperature drops. The amount of nitrogen in the soil in the nitrate form will be higher than required at the time, and for most crops, this is not too serious. But it can be a problem in some cases. Lettuce, for example, will be overstimulated, and become tender and more vulnerable to frost damage.
Even if the crop is not harmed, there is another hazard – nitrogen in the nitrate form is easily leached. So be careful not to irrigate too deeply and wash away the nitrate below the root zone. Only the nitrate part of LAN will be of use to the crop during winter, so double up the amount of LAN applied during winter.
The ammonium will build up in the soil until the soil gets warmer in spring. If this seems wasteful, use fertilisers such as calcium nitrate and potassium nitrate, where all the nitrogen is in the nitrate form. If the soil’s potassium level is low, potassium nitrate is definitely an option, as nitrogen makes up only one-third of potassium nitrate. However, both these fertilisers are expensive and it may still be more economical to double up on the LAN in winter, especially when the following crop can use the balance as it becomes available.
Use a urea foliar spray so that the nitrogen is taken up directly by the leaves. Apply it at a rate of 3kg/ ha to 5kg/ha, depending on the crop. Although this is not much nitrogen, the reaction of the plants is immediate. This option can be especially useful if you get caught with low nitrogen in the crop for whatever reason and a quick ‘dose’ is needed to minimise crop loss. In this case, the spray should be considered merely as a supplement. Repeat-spray whenever the nitrogen level drops in the leaves.
Nitrogen loss may also be due to anaerobic conditions in the soil, where bacteria remove the oxygen molecule from nitrogen and turn it into the gaseous form. The nitrogen then escapes into the air. This process is usually initiated by waterlogging of the soil, even for relatively short periods. I have even seen heavy clay soil lose its nitrogen in this way following persistent rain.