The Potash Development Association (PDA) has issued an advisory update on the release of nutrient reserves in soils.
Maintaining the fertility of agricultural soils is of paramount importance not only for the present but also for the future.
In addition, soil analysis and its interpretation are important management tools in assessing the need to apply nutrients in fertilisers and/or manures to maintain soil fertility.
According to the PDA, soils invariably contain total quantities of plant nutrients that greatly exceed the amounts that are immediately available to plants.
With the dramatic increase in fertiliser prices over the last few months, there will be increased focus on the requirement for these inputs and, no doubt, increased attention on alternative means of accessing these nutrients.
The PDA is pointing out that two methods of increasing plant availability of the store of nutrients in soils have previously been suggested.
However, neither have any reasonable chance of success.
Ammonium sulphate
One suggestion is to apply ammonium sulphate to acidify the soil locally and increase nutrient availability.
Acidifying hydrogen ions are released when soil microbes convert ammonium ions to nitrate ions.
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 hydrogen ions. Such ions acidify the soil immediately around them and help 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.
Nutrient reserves and microbial activity
The other suggestion is that increased microbial activity will unlock significant reserves of soil nutrients.
But, according to the PDA, this does not occur.
The idea was suggested in the early 1800s based on the fact that soil microbes produce carbon dioxide by respiration.
The gas subsequently dissolves in the soil solution to produce a weak acid, which, it was suggested, could attack soil minerals releasing potassium, calcium and magnesium.
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. Such yields were very small and, today, would not be economic.
The PDA points out that the Index system of differentiating soils on the basis of soil analysis and likely response of crops to fertiliser additions currently provides the best practical approach to plant nutrient management.
Basing fertiliser recommendations on soil analysis and the index system offers considerable benefits as it ensures that nutrients are applied if needed and provides a guide to the amount required.
Current methods of interpretation simply aim to ensure that there is sufficient amounts of each nutrient in readily plant-available forms to ensure optimum yields and the financial viability of the farm enterprise.