Maintain sufficient K availability from the soil.

	Nutrient uptake by oilseed rape over the vegetative period
	Nutrient uptake by sugar beet over the vegetative period
	source: IFA

When margins are tight and costs rising it is easy to fall into the trap of assuming that the 'background' needs of the crop will be OK, at least for the current year, and that it will be sufficient to focus attention on the nitrogen and agrochemical requirements. Such an assumption is an extreme form of risk-taking and is neither good agronomic practice nor economically wise. By 'background' needs we are talking about soil fertility, and there is no reason just to assume it is satisfactory when such major aspects as nutrient levels and pH can be measured so easily and cheaply by soil sampling and analysis.

The two charts on the right, reproduced from the IFA World Fertiliser Use Manual, clearly illustrate the very large requirement by crops for potassium, which must be available during the vegetative growth stages if they are to yield to their full potential. If the supply of any essential nutrient is insufficient, crop performance will suffer, and it is clear that potash is the nutrient required in the greatest quantity by most crops during vegetative growth. These quantities frequently exceed 300 kg or even 400 kg K₂O/ha. Smaller amounts of potash are likely to be removed at harvest, for example if only the grain of cereals is taken off.

It is useful to estimate the quantity of potash that might be available from a soil at the bottom of K Index 2, i.e. with 120 mg K/L (~120 mg/kg). The top 25 cm of soil weighs about 2,500 tonnes per hectare and if the soil contains 120 mg/kg exchangeable K this is equivalent to only 300 kg K/ha, or 360 kg K₂O/ha. But crop roots do not explore the whole volume of soil and cannot access all this K. Thus while the supply of potash from agricultural soils is quite well buffered, at an Index level often accepted as providing a satisfactory reserve of K in the soil, the balance is potentially very tight. At the levels of yield for which we are striving today, even a low K Index 2 soil may not provide a sufficient margin of safety as far as K supply is concerned, especially when considering the probable spatial variability of K reserves within a field.

Updated PDA Leaflet #26 Nutrient Requirements of Forage Crops

A copy of the latest revised PDA leaflet is enclosed with this Newsletter. This Leaflet 26 on forage crops has proved very useful since it was first published, and continues to provide valuable information. The detailed trial work carried out by the Kingshay Farming Trust, supported by the PDA, confirms the point raised on the previous page, that the uptake of potash by actively growing crops can be very large indeed, with about 700 kg K₂O/ha being measured in both fodder beet and kale. Wholecrop wheat contained approaching 300 kg/ha, and grass and grass/clover crops contained 300 kg or more, just at the first cut! This leaflet also gives the quantities of phosphate and potash removed at harvest in the trial, and compares them with those published in the literature. There is agreement in general, although the offtake of potash by fodder beet was more than double the standard figure, which was originally derived from work on sugar beet.

Crops which are harvested green invariably contain large quantities of potash, compared with the amounts removed in crops harvested for grain. However, even if the quantities in grain are not large, the growing crop will have had a high demand for potash during vegetative growth. This leaflet helps to illustrate the levels of potash which can be taken up during the growing season by a wide range of plant species, with whole-crop wheat and kale representing cereals and brassicae. With potash (and phosphate) application rates frequently not matching offtakes in recent years, particularly for combinable crops, there is concern that many fields are now not able to supply the potash needs for the crops they are capable of growing, resulting in lower-than-expected yields.

For further information please contact:
PDA info@pda.org.uk
Potash Development Association

update: Nov 2007