Potash for heavy soils

Fertiliser Strategy for medium & light soils

For these soils, fertiliser policy should be based on soil analysis every 4 years or so (preferably at the same time of year and at the same point in the rotation - especially if this includes crops with a high potash demand e.g. roots, cut grass etc.) Soil index gives a general guide to soil fertility but it is more useful to monitor the actual nutrient values given in mg/l (ppm). This will permit changes in fertility to be identified more readily. Soil K differences of less than 10-20mg/l are unlikely to be significant.

The principle of manuring is to keep these soils above the critical minimum value relevant to the crops grown. In practice because soil nutrient levels cannot be raised and lowered over short periods, the appropriate status will be that for the most demanding crop grown in the rotation. Fertiliser policy should then maintain this level by replacing the nutrients removed by the rotation. For lighter soils applications must be annual or even split within the season. On medium or heavier soils rotational manuring may be followed, applying 2 or 3 years requirement before the most responsive crop e.g.. roots, legumes etc. Annual applications are normally advisable where grass is cut to avoid possible luxury uptake.

Strategy for Heavy Soils

Some clay soils contain very large reserves of "non-exchangeable" K which are not revealed by normal soil analysis results. Unlike other soils however these soils can sustain combinable crops for many years without any reduction in exchangeable soil K because this is being topped up from non-exchangeable reserves.

Regular soil analysis may be undertaken every 4 years but an alternative strategy is provided by the Reserve K Analysis. This can be used to test typical blocks of land and indicates the number of years cropping possible without further soil analysis.

Levington Reserve K analysis*

Trials involving 40 soils showed that the longer term uptake of K by crops receiving no fertiliser could be related to the combined results of three separate soil tests. Different extractants are used to measure both available and reserve (non-exchangeable) K. Results of the tests are combined to give an index - R1 to R4 - indicating the level of reserve K in a soil. Lighter soils maybe in R1 whilst boulder clays are in R4.

* Reserve K analysis can be arranged through:-

Agronomy Enterprise
3 Orchard Place
Wickham Market
Woodbridge
IP13 0RU
England
Tel. and fax: 01728 746579

Individual crop recommendations

For individual crop recommendations use the PDA PK Arable Kalculator and refer to individual crop leaflets.