Potash for Potatoes

Potash yield and deciding the need for potash

There are three reasons why potash should be applied for potatoes :-

• to ensure no loss of yield due to shortage of K and reduce unit costs of production, "Response requirement"
• to maintain soil K fertility and yields of future crops, "Replacement requirement"
• to achieve specific "Quality requirements"

It is important to identify the relevance and interaction of these different reasons.

"Response requirement"

A very large number of experiments from many countries have confirmed that potash supply has a major influence on yield. Therefore, in the short term a grower needs to know how much potash to apply for the best economic return. However, the increase in yield to an application of K fertiliser will depend on the supply of K from the soil, i.e. the K Index of the soil on which the crop will be grown. For the UK, current recommendations in RB 209 are based on the average amount of K required to achieve optimum yield for crops grown on soils at different soil K Indices. As in all experimental data, there was considerable variation for different sites and years. In general, however, the response to added K is usually large at low levels of soil K and the requirement for added K tends to decrease as soil K level increases. There will always be variations from this general interpretation and it is important to attempt to identify site specific factors that may cause such variation. These include the volume of soil explored by roots, the availability of water and nitrogen and phosphorus. For example, increases in yield can occur at high soil K levels because too little of the volume of soil was explored by the roots. There are also cases where little or no increase in yield to applied K was obtained at low soil K levels, perhaps because the roots were very efficient in exploring the volume of soil available to them, and there was adequate moisture throughout the growing season. In the latter case, such results have been used to argue that the amount of potash applied can be reduced or omitted. This would, however, represent a negative potash supply/removal balance and lead to reducing soil K reserves. Whilst there is no precise critical value for the soil K Index, maintaining soils at the upper end of K Index 2- and the lower end of K Index 2+ is still seen as an appropriate target range to aim for on most soils. These levels are unlikely to be achieved on true sands and loamy sands because they have so little ability to retain exchangeable K and for these soils it is essential to apply adequate K as fertiliser and/or manure each year.

"Replacement requirement"

If the response requirement is less than the amount of potash removed by the crop it would run soil reserves down and would not be sustainable in the longer term. As soil K reserves are reduced, the response to added potash fertiliser would rise and the optimum "response" dressing would become larger. This may be a theoretical ideal economic optimum approach but assumes far more precision in adding K fertiliser than is possible in practice and ignores the fact that potash held in historically accumulated reserves is more efficiently taken up and used by the plant than fresh fertiliser K.

It is also argued that a "response" dressing approach to K manuring is applicable for the increasing area of the crop that is grown on land rented for a single year. Again if the "response" dressing is less than the amount of K removed, soil fertility would be depleted by this approach. This is not equitable for the landlord and cannot be generally recommended as a sound soil management practice.

"Quality requirement"

Potash supply also contributes to quality characteristics that may affect marketability of potatoes such as dry matter, specific gravity, sample size and tuber number, starch content, fry colour, fat absorption, internal blackening, susceptibility to mechanical bruising, cooking quality and flavour. Many other factors also affect these characteristics (often to a greater degree than potash) and additional potash supply will not improve the characteristic where it is being controlled by an excess or limitation of another factor.

If adequate potash is available to the crop for full yield, additional nutrient is unlikely to provide cost effective additional quality benefits.

Photo courtesy of Grimme (UK) ltd