Improving soils with low nutrient status

Soil analysis measures the average nutrient concentration in the depth of soil sampled; theoretically a concentration of 1 mg/l to a depth of 10 cm in soil represents 1 kg/ha of (elemental) nutrient. In practice however, plants also obtain nutrients from below sampling depth and from the slowly available pool that is not measured by analysis (see earlier diagram). This often gives rise to some confusion especially as these sources are very variable depending on soil type and past fertilisation and manuring. It is therefore possible to have a soil at 125 mg/l of K (Index 2) which supplies a cereal crop with a peak uptake of 250 kg/ha K (300 kg/ha K₂O).

Conversely the addition of 100 kg/ha of phosphorus, potassium or magnesium (in fertiliser terms this represents 230 kg P₂O₅ 120 kg K₂O and 170 kg MgO) will not result in an increase of 100 mg/l of readily plant-available soil P, K or Mg. This is because some of the nutrient applied will not remain in the readily available pool measured by analysis. Unfortunately the proportion remaining available and thus affecting soil analysis values will differ widely with different soils and conditions and is not reliably predictable.

With present knowledge it is not possible to provide more than a very rough guide as to how soil values will change with additions (or removals) of nutrient. The following figures have been suggested but it is likely that the range in practice is even wider than indicated:

To increase soil Olsen P by 10 mg/l requires approximately 400-600 kg/ha P₂O₅

To increase plant-available soil K by 50 mg/l requires approximately 300-500 kg/ha K₂O

More potash will be required on heavier soils where the clay type can make a difference. Where either the P or K status needs to be increased, triple superphosphate and muriate of potash (KCl) respectively are cost-effective nutrient sources.

Treatment of high fertility soils

No additions of nutrient (fertilisers or manures) need be made to any soils above Index 3 (arable) and 4 (vegetables).

Soils at Index 3 should receive maintenance dressings of phosphate but there is no need for potash for most arable crops (except potatoes) or grassland.

Some clay soils contain very large reserves of potash which are not reflected in the level of available K shown by normal analysis. In these cases the amount of slowly available K released each season may be sufficient to replace some or all of the potash removed in combinable crop rotations, especially where the straw in not removed (the higher demand of root crops and cut grass normally requires some addition). The soil K value may not change over many years cropping even though little or no potash is used to replace that removed. Not all heavy soils have this ability and where less nutrient is applied than is removed, soil levels must be regularly monitored for any reduction in plant-available K.

Conclusion

Whilst soil analysis is not a perfect tool, it is the most effective and practical means of assessing soil fertility in respect of pH and plant-available P, K and Mg. Analytical data should be used in conjunction with other knowledge such as soil type, structure and crop offtake as the basis for deciding on fertiliser and manure use.