Using Nitrogen Smartly

I often come across a farmer who thinks his land might have a nitrogen deficiency, but then rules this out as he has fertilised with nitrogen at the usual rate and no leaching rain has fallen.

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Invariably, though, a look at the soil reveals undecomposed crop residue. This is a ready food source for soil organisms, which then grow in number, in turn absorbing large quantities of nitrogen.

A hectare of soil can easily contain 2t of bacteria. The carbon:nitrogen (C:N) ratio of bacteria is 5:1. This means that 200kg of nitrogen is tied up in each ton of bacteria, the equivalent of 15 bags of LAN.

Simultaneously, fungi, actinomycetes, protozoa and other organisms that feed on both bacteria and plant residue increase, tying up even more nitrogen. In total, a hectare of soil can contain between 5t and 6t of living organisms.