Potash - crop rotations, yield and quality

Do some farmers consider the annual cropping and fertiliser inputs for each field on the farm in isolation and without taking into consideration their cropping rotation and existing level of soil fertility? Unfortunately the answer might be 'yes', because fertiliser recommendations are usually given on the basis of individual crops and also sectoral interests sometimes seek to decrease the apparent cost of growing 'their' crop by suggesting lower than optimum inputs of phosphate and potash. If soil reserves are not below the critical P and K Index level, (P Index 2, and K Index 2- for cereals and 2+ for potatoes and vegetables), then at least the off-take of phosphate and potash from the field should be replaced, if not for each crop then certainly for a rotation.

Using recent data for potash published in the British Survey of Fertiliser Practice, it is possible to see how the K balance has changed, on average, for a number of crops between 1994 and 2003. For sugar beet with tops ploughed in, the K balance has changed from being positive (+51 kg K₂O/ha) in 1994 to becoming negative (-6 kg K₂O) in 2003. For winter wheat with straw removed the situation is more serious, a negative value of -35 kg K₂O/ ha in 1994 has worsened to -45 kg K₂O/ha in 2003. While in some fields when K has been applied the amount has been less than the offtake, the big driver for this national negative balance is the fact that about 40% of the wheat crops in Britain did not receive any dressing of fertiliser potash in 2003. Some of these fields will have received organic manure, but most did not.

Continued negative K balances for all crops in a rotation should be a cause for concern unless the soil is at K Index 3 or above. Not only is overall soil fertility at risk but yield and quality are also jeopardised and nitrogen inputs may not be used efficiently. This is especially so for winter wheat where large N inputs to high yielding cultivars grown to achieve bread making quality are not balanced by the availability of sufficient K.

The first-year results from a recent study at Rothamsted have shown that, in a trial where the amount and timing of N to winter wheat were not fully optimised, there was increased lodging of the crop that affected yield on soils at K Index 0 and 1 (Table 1). On a farm scale, increased lodging not only increases the time required to harvest a field but it can also adversely affect yield and grain quality.

Table 1: The effect of soil K status on wheat yield, as affected by lodging at low K levels.

There was an appreciable loss of yield on the K Index 0 soil caused particularly by lodging when the N was applied in three split applications because there was not sufficient soil K to meet the crop's demand relative to the availability of N.

Sufficient K helps minimise lodging because N increases tissue hydration, i.e. the amount of water in the cells, but this additional water requires extra K to maintain cell turgor (rigidity) and cell size. The larger the cell size and the more a leaf is 'held upright' by ridge cells the greater the opportunity for photosynthesis and thus larger yields. The rigidity of the stem will also help minimise lodging.

This study builds on earlier Rothamsted work on the response of winter wheat to N on soils with different levels of K (Table 2).

Table 2: Effect of soil K supply on the response of winter wheat to nitrogen in 1984-86.

Yields were reasonable on the low K soil but the application of more than 120 kg N/ha was not justified. With more available K, yield was increased by more than 1 t/ha when 160 kg N/ha was given. Applying more N than a crop can use when too little K is available is an unnecessary cost and the loss of unused N can have adverse effects on the environment.

There is another danger in allowing soils to become deficient in K: deficiency is not easily corrected. Studies even on heavier textured soils have shown not only smaller yields on low K soils but that applying K to such soils did not immediately increase yields to the level of those on soils with an appropriate level of K. So both yield and quality are at risk on low K soils. On lighter soils it is essential to apply K for each crop because such soils do not retain sufficient K.

Additionally much recent work outside the UK has looked at the role of K in helping plants overcome stresses, like those due to pests and diseases and water shortage, and has shown that sufficient K does indeed help plants overcome such stresses.

All the evidence points to the fact that an adequate supply of K in the soil is clearly as important for cereals as for other crops and that it may not be prudent to 'take a holiday' from applying K. It is essential that the soil contains sufficient K to ensure yield and quality, and thus the financial viability of a farming enterprise.

Key messages:

1. Crops grown on soils with K reserves which are less than the critical levels (K Index 2- for cereals and 2+ for potatoes and vegetables) are unlikely to be able to achieve their full yield and quality potential, so adequate soil reserves must be maintained.
2. If the amount of K removed from a field over a rotation is more than that which is applied, then the soil K reserves will fall. This applies equally to P, and leads to a reduction in the fertility of the soil.
3. The K required by the growing crop is likely to be considerably more than the quantity removed at harvest. Sufficient must be available to satisfy crop demand, especially in relation to nitrogen supply.
4. Crops grown on soils with inadequate K reserves are more likely to suffer from stresses caused by drought, pests or disease.
5. It is often difficult to raise the K reserves in light soils to levels which are sure to provide an adequate supply and annual applications of K are essential.

Johnny Johnston.
Rothamsted Research, February 2005

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

update: February 2005