Why maintain soil potash reserves? |
Potassium in soil
Potassium in soil can be thought of as existing in four pools according to the availability of the K for uptake by plant roots. It is present dissolved in the soil water, adsorbed onto particles of clay and organic matter and held within the crystal structure of clay particles.
Simplified potash cycle
Exchangeable K, which is determined by routine soil analysis, is the K that is most readily available for uptake by roots. It is the K in the soil solution and in the readily available pool. The less readily available K is K that has been retained within soil minerals from previous applications of fertilisers and manures or has been released from native soil minerals by weathering. This pool of K is difficult to measure in the laboratory and is not usually determined for advisory purposes. However, field and glasshouse experiments have shown that the K in this pool is slowly available to crops and is a vital source of potassium.
The 'mineral matrix K' is that K in native soil minerals such as feldspars and micas; some soils contain lots of these minerals some very little; the clay content of the soil is not a reliable guide as to the type of mineral. Potassium in the soil minerals is only very slowly released by weathering and the amounts available each year are not sufficient to supply the needs of crop cultivars with a large yield potential. The essential feature of this concept of the behaviour of soil K is its reversible transfer between the exchangeable and less readily available pools. Thus when K is added in fertilisers and manures it goes first into the soil solution, from where it is taken up by plant roots. Some of the K is then adsorbed onto the exchange sites, where it is readily available for release back into the soil solution. From these exchange sites K can move to be within the "silicate layers" that constitute the soil minerals. Here the K becomes part of the less-readily available pool of K. However, when K is in demand by the growing crop this K can be released back into the soil solution for uptake by plant roots. The speed and extent of the transfer of K between these pools and the factors that control the transfer are of special concern, especially if farmers stop applying potash. Little K is lost in drainage from many soils and what is lost does no harm to the environment.
How much soil potash do I need?
Research at Rothamsted through the 1960s and 70s showed that, on many soils, potash reserves which
accumulated from applications of fertilisers and organic manures increased crop yields compared to those
obtained on similar soils but without such reserves. Also applying fresh K fertiliser to the K-deficient soil,
did not increase crop yields to equal those obtained on a soil with an adequate amount of available K.
Very recently, this result has been confirmed for sugar beet in experiments at Rothamsted done on behalf
of BBRO. The inability of freshly added K to increase yield significantly is because it is almost impossible
to thoroughly mix even a large application of potash fertiliser, say 200 kg K2O per hectare, into the
cultivated soil layer that can weigh 2 million tonnes per hectare to a depth of 15 cm. On this basis,
building up sensible potash reserves and maintaining the readily available pool of soil K at Index 2- is a
'Win-Win' scenario. Money spent on potash is not wasted and correct soil potassium levels ensure
optimum yields and use of other inputs.
However, there is no need to continually build up reserves; other than on very sandy soils it is possible on
most UK soils to build up considerable reserves of plant-available K in the soil. On such soils, the yields
of crops will follow the Law of Diminishing Returns. That is, as exchangeable K in soil increases then yield
will increase rapidly at first and then more slowly until it reaches a maximum beyond which there is no
further increase in yield despite further increases in exchangeable K reserves (soil K Index).
Index 2- is the critical value for most arable crops.
The K index at which yield approaches closely to the maximum can be considered the critical value. Below the critical value the loss of yield is a financial loss to the farmer. Above the critical value, there is no justification for further increase in the available K because this is an unnecessary expense. Current recommendations for potash on arable soils are to maintain the soil at K Index 2- for most crops and at K Index 2+ for potatoes and vegetables. Thus if the crop rotation on an individual field has a significant proportion of potatoes and vegetables the soil should be maintained around K Index 2+. This is because the potash fertilisation policy should apply to the whole rotation of crops on the farm and it is most important to maintain soil fertility for the rotation. Once the soil has been brought to the appropriate K Index for the rotation it should be maintained by replacing the amount of K removed in the harvested crop. The amount of K removed from a field in the harvested crop can be calculated from the yield and the concentration of K in the crop. The latter can be found in tables published in RB209 or in PDA leaflet 'Phosphate and Potash Removal by Crops'. These published K concentrations are averages so it may be that a little too much or a little too small an amount will be applied as a maintenance (M) or replacement dressing each year. Consequently, it is sensible to check that potash applications are maintaining the correct level of plant-available K in the soil. This can be done by taking a soil sample every 4 or 5 years from each field for analysis for exchangeable K. Preferably, sampling should always be done at the same point in the rotation, at the same time of year and to the same depth.
The British Survey of Fertiliser Practice shows that livestock manures are being applied to about 20% of arable land. Thus it is important when deciding on the size of the maintenance K application to allow for this K as described in RB209.
Data from the UK Representative Soil Sampling Scheme showed that 55% of soils growing arable crops were at Index 2-. With about 20% of soils above the critical value and 25% below it, there is clearly a need for more thought to be given to potash use, especially on K-deficient soils.