Potash for Potatoes

Potash supply

All soils contain large quantities of potassium but only a small proportion of this is available for crop uptake in a season. Potassium is held by the clay minerals and organic matter in soil and is not potentially leached or lost in the same way as nitrate and sulphate. On the lightest textured and shallow soils some potash may move down the profile from the topsoil to a greater depth than can be reached by the roots of shallow rooting crops. However, in the majority of soils if more K is applied than is removed in the harvested crop, the excess will simply increase the amount of soil K reserve for following crops. If offtake is greater than the quantity applied, soil reserves will be depleted. Hence normal manuring policy is to identify the appropriate level of soil K for the individual soil type and calculate potash use to maintain it, primarily by replacing the quantity of K removed in the harvested crop.

Potato plots at Rothamsted illustrate the value of soil K reserves. Four potash fertiliser treatments on soil at K Index 2 (left), compared with the same treatments on soil at K index 1 (right).

Most crops take up nutrients mainly from the ploughed or cultivated depth of soil. Where crops root more deeply some nutrient may be taken up from supplies at greater depth. Potato root systems are generally far less extensive than those of crops such as winter cereals or sugar beet. Consequently nutrient uptake from the deeper soil horizons is much less important for potatoes. However, improved soil cultivation and de-stoned bed systems increase the potential soil volume available to the crop for nutrient and water uptake. Medium and heavy soils may also release some potash from non-exchangeable reserves which are not measured by a normal soil analysis. However the potato crop is commonly grown on lighter soils which do not retain large amounts of non-exchangeable K.

Effect of soil K residues
Relative yields from the trial shown above. Application of fresh potash fertiliser could not overcome the effect of the low K soil.

Potash may be taken in via the leaves from foliar applications of soluble K fertilisers but only a very small proportion of the large total K requirement can be supplied by this route and foliar application tends to need a more expensive source of K than for soil application. This is because it is necessary to use a very soluble K source and to avoid the risk of the salt scorching the leaves.

Potatoes need an amount of potash in the soil that can satisfy both the peak rate of demand during early vegetative growth and the total uptake in late summer. Where soil reserves are insufficient to meet either of these two requirements, the soil supply needs to be supplemented by adding K in fertiliser or manure. However, these additions may not be as efficient in providing K as are the reserves in the soil. Long term experiments at Rothamsted and Woburn have shown that yields from impoverished soils may not match yields on fertile soils even if large amounts of K fertiliser are applied (see illustrations and chart above). Also, it can take several years to improve a soil with a low K status once soil K reserves have been run down.

Soil analysis

Reliable methods of soil analysis have been developed over many years in the UK to indicate the ability of a soil to supply a specific nutrient to a crop and to show the need for any additional input. Samples are normally taken to 15 cm and thus the analysis does not indicate possible supply of a nutrient from deeper in the soil profile. Also for K current methods of analysis do not predict the release of non-exchangeable K which can supplement available K on some soils. As explained earlier neither of these sources is as important for potatoes as for some other crops.

Soil analysis is not a precise indicator of a soil's ability to supply K to a plant for a number of reasons. Invariably there are large natural variations in the level of plant available K within a field. Also the uptake of K is affected by how easily plant roots grow within a soil to take up the nutrients that are there. However, the Index system that has been developed provides a practical and worthwhile guide relating to crop response as shown below. Compared to earlier editions of MAFF Fertiliser Recommendations Book RB209, in the 7th edition published in 2000 the system was modified with K Index 2 being divided into an upper half (2+) and a lower half (2-):

Varietal differences

The increasing recognition of varietal differences in nitrogen requirement and response has lead to interest in whether there should be similar discrimination for potassium. It has been suggested that indeterminate varieties (e.g. Cara) develop a better root system that may improve uptake and utilisation of K from the soil and so affect potash requirements. However, there is insufficient evidence to differentiate recommendations for K depending on variety except where certain varieties produce large yields and require larger potash dressings to replace the greater offtake.