The release of nutrient reserves in soil. Taken from The Potash Development Association
10. Soils invariably contain total quantities of plant nutrients that greatly exceed the amounts that are immediately available to plants. Two methods of increasing plant availability of this store of nutrients are being suggested, neither of which seems to have any reasonable chance of success.
One suggestion is to apply ammonium sulphate to acidify the soil locally and increase nutrient availability. Acidifying H+ ions are released when soil microbes convert ammonium ions, NH4+, to nitrate ions, NO3–, but the resulting acidification of the soil occurs only in the immediate vicinity of the ammonium sulphate crystal, which may be remote from where the roots are. In consequence, even if nutrients are released there will be no benefit to the plant unless the root moves into the acidified soil zone. This method of acidification is likely to be much less important than that resulting from roots themselves excreting H+ ions that acidify the soil immediately around them and helping mobilise nutrients in soil in close contact with the root. In addition, when ammonium sulphate is applied to calcareous soils there is a risk of gaseous ammonia being produced and lost to the atmosphere.
The other suggestion is that increased microbial activity will unlock significant reserves of soil nutrients. This does not occur. The idea was suggested in the early 1800s based on the fact that soil microbes produce carbon dioxide (CO2) by respiration. The CO2 dissolves in the soil solution to produce a weak acid, which, it was suggested, could attack soil minerals releasing K, Ca and Mg. Experiments at Rothamsted in the mid 1800s tested this idea by feeding the soil microbial population a carbon rich diet to get them to produce more CO2 and hence more acid. But yields on the treated plots were not increased above those on the untreated soil and these yields were very small and today would not be economic.
What is needed is the addition of soil organic matter in the right condition rather than simply a carbon rich material – if this is done according to THIS web site crops can increase yields 100-300%. Richard Higgins, CEO Sustainable Agriculture London
This is probably the most effective, reliable and fastest method of releasing soil nutrients.