Why maintain soil potash reserves?

Other effects of declining levels

Loss of yield is not the only result of potash deficiency in soil. Lack of potash results in:

• inefficient use of other nutrients, especially nitrogen, a financial cost to the farmer with the risk of environmental pollution through nitrate leaching and emissions of nitrous oxide;
• enhanced susceptibility to crop diseases and the likely need to increase pesticide use - more cost and more risk of pollution;
• less natural vigour and resistance to stress from pests, diseases and adverse weather;
• weaker straw with greater risk of lodging;
• reduced grain quality.

The decrease in yields with declining exchangeable K in these long-term experiments also emphasise the importance of balanced nutrition - that is maintaining the supply of adequate amounts of all nutrients. Balanced nutrition involving N x P x K interactions needs to be more fully appreciated and responded to by those wishing to achieve optimum economic yields of good quality produce. The yields of sugar shown in the table illustrate such interactions. With too little nitrogen (N), the sugar beet did not fully exploit the soil to find sufficient K to achieve optimum yield; simply adding extra K fertiliser did not solve the problem. The appropriate amount of readily plant-available phosphorus (P) in soil was essential and in this case more important than K. The maximum yield of just over 10 t sugar/ha was achieved with 180 kg N/ha and P or PK.

Interactions between N and K are especially important. Many crops contain as much K as N during rapid vegetative growth. Too little K causes ineffective use of N and the potential for N losses to the environment. Experiments with winter wheat at Saxmundham show that, on soils with a low K index (Index 1), first wheats did not respond to more than 120 kg N/ha. Maximum yield only required 160 kg N/ha when the soil was at K Index 2-.

Cost of rebuilding potash reserves

There is not much research upon which to base a calculation of the costs of rebuilding potash reserves if they fall below the critical level. Again, the long-term experiments at Rothamsted provide the best basis. Taking into account the movement of potash from 'Readily Available' into 'Less Readily Available' forms during build up, but ignoring crop removals, researchers found that it took 10 kg K₂O/ha in excess of crop requirements to increase the exchangeable soil K by 1 mg/kg. It would therefore require an application of potash in excess of normal crop requirements of approximately 600 kg K₂O/ha to move from the middle of Index 1 to Index 2- and 1200 kg K₂O/ha to move from the middle of Index 0 to 2-.

The cost of buying and applying this amount of potash fertiliser is appreciable. For example, at £140/t for muriate of potash (23p/kg K₂O) this comes to £140/ha to move up from Index 1 and £280/ha to move up from Index 0.

As can be seen from the Saxmundham data above, the penalty for being in Index 1 compared to Index 2- (when using N rates which are optimised for correct reserves) is 11.15 - 9.12 = 2 t/ha. This currently represents an annual value which is greater than the total cost of returning soil K to the Index 2- target.

The practical conclusion to be drawn from these restorative costs is not to allow soil K to fall below target levels and this is further endorsed by consideration of the crop value penalties involved.

However, it is not recommended to restore low soil K with such large quantities in a single year. Besides the cost of buying and applying such large amounts of potash fertiliser there is a risk that unless the fertiliser is applied at the right time and is very well mixed into the cultivated soil there could be a serious risk of 'salt' damage to germinating seedlings.

If the exchangeable K is below the desired value then it is preferable to apply each year a little more K than the maintenance or replacement dressing. The current guidelines (Defra RB209, 7th edition) recommend 25 kg/ha potash in excess of replacement per year at K Index 1 and 50 kg/ha at K Index 0 and thus aim to restore reserves which have been run down over a longer period. The cost of 25 kg/ha of potash is less than £6 and clearly is an input that should not be sacrificed under any circumstances.

	Conclusions
	Maintain soil K at Index 2- for most arable crops, but at K Index 2+ for potatoes and vegetables. Allowing soil K levels to fall below this is a false economy. Insufficient potash costs money in lost yield and probably wastes money in lost N and extra pesticide use.