The Potash Development Association (PDA) has highlighted the key role of sulphur in crop production systems in its latest crop update.
Sulphur in the soil acts in a similar way to nitrogen. It arises naturally from the breakdown of organic matter, and to some extent from soil minerals.
According to the PDA, soils that are organic, or heavy textured, are better able to supply adequate sulphur than light and inorganic soils.
The organic sulphur compounds are broken down to inorganic forms, which are then useful to the plant.
The sulphur cycle demonstrates these processes, which are driven by microbial action, so moisture, warmth and time are required.
Sulphur in crop production
Organic sulphur in the soil comes from organic matter, whether from applied organic materials or from grazing/crop residues. The organic sulphur has to be “mineralised” into the sulphate ion (SO42-) in order to be taken up by plants.
These sulphate ions reside in the soil, which means they are liable to be leached, depending on the soil texture and rainfall, just like nitrates.
This risk must be taken into account when nutrient planning.
According to the PDA, immediately available sulphur in the soil can be measured by lab analysis, but its variability both down the soil profile and over time means that the normal four-yearly soil analysis (pH, P, K, Mg) regime is not appropriate as both the organic processes and the leaching potential cause levels to vary by month and by year.
Some research work has been done by measuring sulphur in deep core samples taken for soil mineral nitrogen testing in oilseed rape crops.
However, clear conclusions on guide levels are still being determined.
The major role of sulphur in all plants is in support of nitrogen in protein production, which is hugely important for high crop yields.
For bread-making wheat, protein production and its quality are even more important for loaf volume. In brassicas, (vegetables, oilseed rape, kale, etc.) sulphur is also in glucosinolate compounds, which give them their ‘hot’ taste.
Sulphur also reduces the likelihood of immature rapeseed, which would face penalty pricing in the market.
This explains the greater requirement and importance of sulphur applications to brassicas.
In grass, sulphur is probably more important for improving the quality of grazing and silage, in terms of protein, than the yield increase achieved.
Sulphur deficiency symptoms
Deficiency symptoms in cereals, grass and brassicas show up in the younger leaves first.
Symptoms are a pale yellow appearance (chlorosis) and, later on, stunting. Much later, oilseed rape flowers will have pale yellow, or almost white, petals.
Symptoms in some crops are easily missed, or confused with nitrogen deficiency, and may not be noticed at all, especially in cereals and grass.
Because routine soil analysis cannot be used to predict sulphur deficiency, plant tissue testing is often used. For oilseed rape and cereals, there is the Malate Sulphate test, which can be done early in the crop’s life.
But it may need to be repeated later at stem extension.
Alternatively plant tissue analysis, for nitrogen and sulphur concentrations, can be carried out for oilseed rape and other crops.
But the results may not become available until too late to correct for the current crop. However, it should give good indications for future policy.
