Potash for Potatoes

Form of potash and quality

Principles of potash manuring

The normal principles of sound potash manuring are :-

• to replace nutrient removed where soil reserves are adequate
• to apply more than removed on soils which are below an adequate level, and
• to reduce or omit applications on soils which are above an adequate level.

Applying these principles for potatoes gives the following recommendations. The 'replacement' value is also referred to as the 'maintenance' (M) application.

The 'replacement' or 'maintenance' quantity is calculated by multiplying the yield in t/ha by the potash content per tonne of potato tubers harvested (5.8 kg/t of fresh weight) i.e the quantity of potash removed from the field in the harvested crop.

Thus for a yield of 50 t/ha of maincrop potatoes, the replacement value is 290 kg K₂O/ha. For a 30 t/ha early and seed potato yield, a potash replacement of 175 kg K₂O/ha is required.

The additional applications of +50 and +25 kg K₂O/ha at Indices 0 and 1 respectively will go some way towards increasing the soil K Index, and may be as much as can reasonably be charged to the crop being grown. However, significantly more potash will be required in order to raise the Index to a satisfactory level in the short term, and such additions should be costed as 'capital improvement' items, not as a variable cost against the crop in the field.

These are general guidelines which will in most situations ensure an adequate supply of K to achieve optimum yield. Soil analysis should be used as a starting guide rather than a final definition of requirement. Site specific consideration should also be given to soil structure, soil depth/volume, stone content, organic matter level, possibility of drought, possibility of K leaching on very light soils and other relevant factors.

Removal and replacement

There are large variations in uptake and removal in practice. However, the usual method for calculating the replacement K requirement is to multiply the appropriate K removal factor by the yield as shown below. Thus the requirements of crops with very different growing periods and yields (e.g. earlies, canners, main crops, seed, etc.) are covered by the same approach.

Potato responses to potash; estimated potash removal

In a replicated trial on a loamy sand soil at K Index 1 (K 79 mg/L) at ADAS Gleadthorpe in 2002 (funded by BPC), 0 and 325 kg K₂O/ha (RB209 recommendation for the site) were applied to 8 potato varieties and their tuber yields measured and K offtakes estimated.

Although not a potash response trial and whilst the study did not identify optimum potash application rates, the data clearly demonstrate large yield responses to potash fertiliser for all the potato varieties tested on this K Index 1 site. Mean response of all varieties was 12 t/ha giving an excellent benefit:cost ratio of 12:1.

The amount of potash removed was estimated using actual yields and a standard potash content (see above). The average offtake of 459 kg K₂O/ha for all varieties was 134 kg/ha larger than the 325 kg K₂O/ha applied and the range of K₂O soil deficits was from 41 to 203 kg/ha for different varieties. Thus, there was a depletion of soil reserves for all varieties. The current principles of potash use recommend that this shortfall should be avoided by matching potash use to offtake plus an addition of 25 kg/ha for this loamy sand at the low end of Index 1 (459 + 25 = 484 kg K₂O/ha).

High yielding potato crops remove very large quantities of potash and a manuring policy based on replacement, with supplements where K reserves are low, will generally provide adequate potash to ensure optimum yield not only of potatoes but of other crops grown in rotation.

Timing

Apply up to 190 kg K₂O/ha in the seedbed. For higher rates some should be applied earlier in the spring or in the previous autumn/winter. Where amounts over 300 kg/ha are required, half the total should be applied in the autumn/winter and half in the spring; this is to avoid any risk of excessive 'salt' concentrations adversely affecting root growth. On lighter soils the winter application should be made after the year-end.

Placement

Placement of nutrients in the vicinity of the seed can improve fertiliser use efficiency and this can be important on soils of low nutrient status. However, placement of large amounts of soluble nutrient close to the seed can cause damage to roots especially under dry conditions. It is not recommended to place more than a total of 250 kg/ha of nitrogen and potash near to the seed.

Manures

Around a third of the national potato crop receives some manure but there appears to be only a small allowance for the potash contribution from this source. Estimates of nutrient contribution from manures can be made easily using typical values (see PDA leaflet 'Nutrient content of manures') and fertiliser rates should be adjusted accordingly. An excess of nutrient not only causes unnecessary expense but may give additional problems.

Foliar application and top dressing of potash

The interest in foliar phosphate has lead to consideration of foliar potash. However the large quantities of potash required and the lower responsiveness of the crop to potash compared to phosphate does not suggest a likely response to foliar potash and there is inadequate evidence to support any general recommendation here. Also unless great care is taken there is a risk of scorching the leaves.

Where large quantities of potash are needed because of high potential yields on light soils with low potash reserves, a case can be made for top dressing potash in conjunction with nitrogen at tuber initiation, especially where irrigation is available. Again there is insufficient evidence at present to support this with firm recommendations.

Potassium : Magnesium imbalance

There is some evidence that potash availability and uptake can be reduced on soils where magnesium levels are excessive, even though the level of soil K may not be deficient. This situation is most frequently associated with long term use of magnesian limestone to correct pH. It is suggested that where the soil concentration of Mg is more than double that of soil K, potash applications may need to be increased above the normal recommended rate to achieve adequate potash nutrition of the crop. Further studies are being undertaken.