Uptake, removal and crop requirement for K

Recommendations for potash application are based on removal in crop products, with adjustment up or down depending on soil Index. Much more potash is actually taken up by the crop than is removed at harvest, not least because of all the potash in non-harvested parts such as straw and roots. The supply of available potash must be enough to meet the total crop requirement, not just removal. This is especially critical at low soil Indices where any shortfall in application will reduce the uptake of potash and therefore also the crop yield. A 10 t/ha wheat crop for example will remove around 56 kg K₂O in the grain but total uptake by the crop can be well over 200 kg K₂O/ha. It is this total uptake which the potash supply from soil, fertiliser and any manures must provide. As well as meeting total crop requirement, potash supply must also support the rate of uptake at peak growth periods. Spring barley for example can take up around 7 kg K₂O/ha/day.

Inadequate potassium supply to crops will result in serious penalties. Unfortunately the effects of low soil K cannot usually be 'seen' and any crop deficiency symptoms may not be identifiable as shortage of K, or may not be evident until too late to remedy.

Penalties of low K

• Serious yield loss especially in adverse growing conditions, e.g. in dry summers.
• Weaker growth resulting in higher lodging risk.
• Lower crop vigour giving greater susceptibility to disease, pests and weed competition.
• Poor grain sample - low grain size, poor specific and 1000 grain weights, more tailings.
• Inefficient N uptake and utilisation leading to poor N response and less protein synthesis.
• Adverse effect on marketability.
• Lower tolerance to physiological stress - wet, drought, frost, wind.

Individually these effects may not be great but overall the result is lower crop value and higher costs per tonne.

If low soil K reserves are suspected, or have been found by soil analysis, or if the soil is not expected to be able to satisfy the high daily crop requirement for K in the spring (for example a light sandy soil), then the topdressing of K, probably with a nitrogen application, can help prevent the adverse effects of a shortage of potash.

Grass plots demonstrate need for K

The results from the monitoring of the PDA plots at the NAC clearly demonstrate how important it is to replace the potash removed in silage (see updated PDA leaflet 5b):

• Large quantities of potash are removed where silage is taken. Over 320 kg K₂O/ha per year was needed in these plot comparisons to maintain soil K reserves.
• At soil K Index 1 responses to fertiliser potash are very worthwhile. If soil fertility is allowed to fall the response to K increases, for example in the fourth year of this work yields of grass were doubled by the application of potash.
• Where the potash supply is limiting, nitrogen response will be impaired, with increased risk of N loss to the environment through inefficient use by the crop of applied nitrogen.
• A ratio of 1:1 for N:K in herbage was associated with the highest yields. The quantities of nitrogen and potash removed in silage are therefore similar and need to be replaced in similar proportions, depending upon soil K contribution.

Clover needs extra K

Potash is important for keeping a good proportion of clover in a sward. A good mixed grass/clover sward needs more potash than does grass alone. Field experiments over three years where herbage was either cut and removed or grazed showed the effect of applied potash on clover yields. Where 70 kg K₂O/ha was applied each year, clover dry-matter yield was increased by 53% on the grazed sward and by 72% where the herbage was cut.

For further information please contact:
PDA info@pda.org.uk
Potash Development Association

update: February 2007