When Potash may be damaging your soil – Should you use it? download full version in pdf
Potash is measured in your soil in terms of K2O (potassium oxide) but is taken up by your crop in the elemental form of K+. The single ‘+’ added to the ‘K’ symbol is because it is a cation with a single positive charge and as such is weakly held in the soil on the clay and humus particles.
Now imagine this: calcium Ca++ has two positive charges and will easily ‘knock off’ K+ from the clay particles, in which case the poor old K (potassium) has nowhere to go except a) into your crop or b) down the river.
So when you apply lime (especially if you use magnesium lime) you will increase the amount of potassium (K) in the soil solution, your plants cannot help but take it up and you can then have excessive potassium and low magnesium (Mg), because the Mg++ will be strongly held in the soil.
It is this excessive potassium that will cause imbalances and may result in Staggers; it will also cause the grass to become bitter and unpalatable.
Excessive potassium will also flocculate the soil causing it to become hard when it dries out and traps too much water when it does rain – grass does not require high levels of potash.
The recommendations are usually made to replace what you have taken off and if you are taking 2 or 3 cuts of silage and not using any FYM or slurry, then you may indeed need to consider applying potash – but that is rarely the case and often more than enough is applied to maintain levels.