Potash in the Spring
Lodging risk is higher if potash is low
With the unusually dry winter, many cereal crops are forward, well supplied with nitrogen and with a bulky canopy - ideal conditions for lodging problems later in the season. The roles of nitrogen, fungicides and growth regulators in managing lodging are well known but the importance of potash is less well appreciated. Potassium significantly affects the strength of straw through its influence on lignification and cell wall thickness. Any deficiency of potash will reduce straw strength and increase the risk of lodging. This is illustrated by measurements in wheat crops at different levels of potash application over two years:
The cost of the 95 kg K2O/ha needed for maintenance of soil fertility where straw is removed with an 8 t/ha wheat crop would be covered by less than a third of a tonne of feed wheat. It doesn’t make sense to risk yield and quality by neglecting potash.
A revised 'NK Arable Top Dressing' leaflet is available, and provides an appropriate reminder that in situations where there is a shortfall in the K supply for winter cereals and rape, this can be overcome by a spring NK top dressing. This is particularly relevant on light soils which are often unavoidably low in K. (see http://www.pda.org.uk/leaflets/pdf/PDA-lf22.pdf)
'Grain Legumes need Potash'
A revised version of PDA Leaflet 18 is published: 'Grain Legumes need Potash'. As is shown in the leaflet, legumes have a large K demand during the growing season and beans will remove 12 kg K2O/t grain at harvest, with another 16 kg K2O in each tonne of straw. It is also important to remember that, although beans can root to a reasonable depth, the root system of legumes is considerably less extensive than that of cereals. This explains why these crops are more susceptible to poor performance in low K situations. The leaflet illustrates that the critical soil K status for beans is Index 2+, whereas winter cereals, with their more extensive root system, can usually tolerate a K Index of 2- without depression of yield or quality.
'Potash and the Environment' - new PDA Leaflet 29
Organic manures and fertilisers are much in the news these days in the context of losses of nutrient to the environment. The nutrients which can be at issue are nitrogen and phosphate, but loss of potash is not an environmental concern.
There are however two significant points to note. The combination of economic pressure on growers and the concerns to minimise P losses to watercourses can have an effect on potash usage, due largely to the fact that P and K tend to be associated when planning fertiliser applications. It is very important that the large potash requirements of growing crops continue to be met, and that if there are reasons to reduce phosphate application rates this must not have the knock-on effect of reducing potash applications.
The second major point is that potash can have an environmental impact if soil K reserves are insufficient for crop needs. A deficiency of potash will have an effect on the ability of the crop to take up and make efficient use of nitrogen, which could lead to increased N losses.
For further information please contact:
PDA info@pda.org.uk
Potash Development Association
update: March 2006
