Principles of P, K and Mg manuring

The principle of manuring is to maintain plant-available soil nutrient levels within a target range depending upon crop rotation and soil type, by replacing nutrients removed at harvest. Soil analysis shows the status of soil nutrients relative to the target values and allows changes as a result of husbandry to be monitored. Where soils are below the target level, nutrient applications should provide more than is removed by the crop to help ensure full yield response and to improve nutrient levels. Nutrient applications for soils above the target range may be reduced or omitted until the soil reserve approaches the target value. Additional nutrients may be applied before very deficiency-sensitive crops such as potatoes with the surplus balance being allowed for in subsequent less sensitive crops. The overall nutrient balance in the rotation should be estimated and then checked by regular soil analysis.

Target levels

	Soil P	Soil K
Arable and forage crops	Index 2 (Olsen P) 16-25 mg/l	Lower index 2 120-180 mg/l
Vegetables and potatoes	Index 3 (Olsen P) 26-45 mg/l	Upper index 2 181-240 mg/l

Sand soils

On true sand-textured soils (not loamy sands or sandy loams) it is not practical to attempt to maintain a soil K Index of 2- and the target K level should be adjusted to 100-120 mg/l (Index 1+).

Chalk and limestone soils

At very high pH, potash additions are more likely to be converted to slowly available reserves in the soil and it will be more difficult to raise the soil K Index. However soil K targets and replacement principles for application remain the same as for other soils.

Other factors affecting interpretation

Soil structure is also very important because any restriction to root growth may restrict the plants ability to obtain an adequate nutrient supply despite a satisfactory value indicated by analysis. The remedy is not to apply more nutrient but to improve the soil structure.

Organic matter - The level of organic matter (humus) will also affect the availability of nutrients in a soil and regular addition of manures so that the physical conditions and biological activity is improved will increase the plant-available nutrients.

Stone content can also have a large effect upon nutrient supply. Very stony soils have little fine earth yet it is the nutrients in this fine earth fraction that are measured. In consequence it is advisable to maintain very stony soils at slightly higher levels of available P and K than are required on deep stone-free soils.

Soil depth - Crops frequently access some nutrients from below sampling depth and from subsoil (especially potash). Soils with greater rooting depth potential offer a greater soil volume, and provide larger total quantities of nutrient than shallow soils. It may be possible to manage deep, well textured soils at slightly lower concentrations than the stated targets given above. However shallow rooting crops which do not explore the full soil volume will still require normal nutrient concentrations.

Changes in soil nutrient status

Changes in nutrient status relate to the balance between nutrients applied and removed or lost (little P, K or Mg is leached or lost from most properly managed UK soils). Nutrient balances per hectare are normally small especially in comparison with the total quantity of soil nutrients per hectare. Where the balance is positive not all the residual phosphate or potash applied remains as immediately available soil P and K and therefore only small changes in soil status should be expected. Large changes, where not related to very large nutrient balances, need to be investigated.