The Potash Development Association (PDA) has updated its potash, phosphate and sulphur recommendations for cereal crops.
It has also published a new advisory leaflet which explores the value of magnesium as a nutrient source for wheat, barley, oats and other crops.
Trial work, carried out over the past 30 years across the UK and Ireland, has confirmed that insufficient potash has been applied to cereal crops relative to the quantities of nutrients removed in cereal grain and straw.
This has resulted resulted in an inevitable reduction of soil potash reserves.
A similar imbalance has developed for some other main arable crops and this also applies to phosphate.
These trends have continued and will lead to serious consequences for yield and quality.
The length of time before financial penalties occur will depend on soil type, manure use and the phosphate and potash reserves of individual fields.
Crops
The area of cereals receiving no annual phosphate or potash at all has also increased significantly in recent years with over 60% of the winter wheat area sown receiving no fertiliser phosphate or potash.
As only around 20% of tillage soils are at an index where phosphate and potash can safely be omitted for cereals, this is further evidence of imprudent cost-cutting.
It must not be assumed that because cereals are known to be efficient scavengers of phosphate and potash and because no visual symptoms are seen, that such cost-cutting is without penalty.
Serious economic consequences can occur without any visible symptoms of deficiency. Soil phosphorus (P) and potassium (K) indices may decline only slowly but as they fall below accepted threshold levels, the risks of yield and quality losses increase.
According to the PDA, such risks are not worth taking because of the essential requirement of these nutrients for the maintenance of yield and quality.
Potash affects both yield and quality of grain as well as the general health and vigour of the plant.
Cereal crops need at least as much, if not more, potash than any other nutrient – including nitrogen.
Potash is needed in such large amounts because it is the major regulator of solution concentrations throughout the plant.
It controls cell sap content to maintain the turgor of the plant and supports the movement of all materials within the plant.
Potash supply is thus essential for all nutrient uptake by the roots and movement to the leaves for photosynthesis and for the distribution of sugars and proteins made by the green tissue for plant growth and grain fill.
Sulphur
Immediately available sulphur in the soil can be measured by laboratory analysis.
But its susceptibility to leaching as sulphate and its variability both down the soil profile, and over time, means that the normal FOUR yearly soil analysis regime is not appropriate.
Both the organic processes and the leaching potential cause soil sulphur 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, the predictability of potential reserves is still being determined.
Because routine soil analysis cannot be used to predict sulphur deficiency, plant tissue testing is sometimes suggested.
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.
Plant tissue analysis looking at nitrogen and sulphur concentrations may also help to give a guide.
But the results may not become available until too late to correct for the current crop.
Alternatively, grain analysis for nitrogen and sulphur concentrations can be carried out. Although the results will certainly be too late to influence that crop, they should give good guidance for future policy.
