Potash for heavy soils | printer friendly version |
Clay Minerals
Clay minerals consist of lattices and layers and cations are held in various positions in and around the layers. Different cations are held with different strengths according to their electrical charge:-
| Strongly held | Weakly held |
Al+++, Ca++, Mg++, K+, NH4+, Na+ |
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Measurement of Soil K
When a soil is analysed a chemical extractant (usually an ammonium salt) is used to remove a proportion of the K+ ions from the clay lattice. The amount "exchanged" by the extractant is similar to the amount of potash that a plant would be able to obtain from that soil. This quantity together with the soil solution K is then related to yield response data from many field experiments to give a measure of soil K fertility of the soil. The nutrient level is measured in mg/l and frequently shown on an index basis to give broad indications as to whether the level is deficient (0), low (1), satisfactory (2), good (3) or high (4 or above).
Some of the K+ held more strongly by the clay lattice (non-exchangeable) may be released slowly during the season and this will add to the exchangeable supply. On soils with large non-exchangeable reserves the amount released may well be sufficient to maintain exchangeable soil levels so as to fully sustain combinable crops for a number of years. The quantity of non-exchangeable K+ released will depend upon the amount and type of clay and on other aspects such as moisture, pH and especially manuring history.
Distribution of heavy soils
The distribution of clay soils in England and Wales is shown on the map. Considerable local variation of soil type and texture will of course occur within these areas and even within fields. Precision technology may permit such localised variation to be identified and managed separately within fields. Where this is not possible, fertiliser policy should be adjusted to the average fertility of the field omitting the extremes from soil samples.
