ARCHIVE: Fertiliser Manual (RB209)

Phosphate, Potash and magnesium for field crops

(Section 8 contains additional information for grassland)

Phosphate, potash and magnesium applied in fertilisers and manures move only slowly through the soil and many soils can hold large quantities of these nutrients in forms that are readily available for crop uptake over several years. Consequently managing the supply of these three nutrients for optimum yield is based more on maintaining appropriate amounts in the soil for the needs of the rotation than on those of an individual crop. In practice, this means maintaining target soil Indices that ensure optimal phosphate, potash and magnesium nutrition.

As the amount of crop-available phosphate or potash in the soil increases from a very low level, crop yield increases, rapidly at first then more slowly until it reaches a maximum. Typically, maximum yield of arable crops or of grass is reached at Index 2 for phosphorus and Index 2- for potassium (see Target Soil Indices below). The principle for phosphate and potash management is to maintain the soil at the appropriate target Index. If the Index is lower than the target, yield may be reduced and additional phosphate and potash should be applied. If the Index is higher than the target, applications can be reduced or omitted until the soil level falls to the target Index. Effective use of target Indices depends on representative soil sampling. If it is felt that significant areas of the field could differ in P or K Index, these areas should be sampled and treated separately.

To maintain soils at the correct Index it is usually sufficient to replace the amount of each nutrient removed from the field in the harvested crop. This amount can be calculated from the yield and an average concentration of the nutrient in the harvested product as shown in Appendix 5 (see worked example on page 38). To check that this approach is maintaining the required P or K Index (i.e. the phosphate and potash status of the soil), soil sampling should be carried out every 3-5 years, at a suitable time in the crop rotation. Maintaining the appropriate level of phosphate and potash in the soil is especially important as these nutrients move slowly in the soil to plant roots. Once deficiency has occurred, a fresh application of phosphate and potash is most
unlikely to be available for uptake by roots in time to benefit the crop being grown.

Yield response of some crops to soil phosphorus and potash concentrations

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