Potash is needed for full response to nitrogen

Cereals require a balance of nitrogen and potash to obtain full yield response to applied nitrogen. Careful optimisation of nitrogen is a waste of time if potash supplies are not adequate. As shown in the diagram, both the level of yield and the shape of the nitrogen response curve are totally altered by potash limitations.

If potash supply is limiting, the uptake and utilisation of nitrogen will be restricted. If soluble forms of nitrogen remain in the soil and are not taken up there is increased risk of leaching when through-drainage occurs. Ready availability of both nutrients at peak crop demand helps the uptake of the large requirements of nitrogen and potash. During rapid vegetative growth, the rapid uptake of nitrogen as negatively charged nitrate ions (NO₃^-) is normally balanced by a similar uptake of positively charged potash ions (K⁺) which maintains the electrical neutrality of the plant. Adequate potash is clearly important in the production of quality wheat as it assists the conversion of nitrate to protein.

Identifying potash deficiency

Visual symptoms
Potassium deficiency causes yellowing and chlorosis to the edge and tip of older leaves, with progressive senescence. Plants may be stunted and exhibit excessive basal tillering. Symptoms are not easily distinguished from those resulting from many other causes especially from physiological stress - drought, water-logging, wind etc.

By the time such symptoms are seen it is almost certainly too late to remedy the situation and avoid crop loss. Potassium deficiency is sometimes referred to as 'hidden hunger' because losses occur despite any recognisable visual symptoms. Two crops that differ in yield by 10% at harvest cannot be distinguished visually during growth. Visual diagnosis is thus a totally unreliable means of detecting potassium shortage. Soil analysis is the most cost-effective approach providing an indication of adequacy or deficiency for future crops.

Soil analysis

Knowledge of the fertility level in the soil is clearly essential to deciding fertiliser policy. It is impossible and irresponsible to guess correct fertiliser use without soil analysis, which should be undertaken every 4-5 years. Soil analysis provides a measure of the nutrient level available to the crop - the total level of potash (and phosphate) in any soil is very much larger but is not relevant to plant use. The physical condition of a soil is also of vital importance, e.g. structure, depth of soil, level of organic matter and stone content. These must be considered first before interpreting soil analysis (see PDA Leaflet 24).

A very large number of cereal trials on different soil types in different regions has been undertaken to establish how cereal performance is related to the available nutrient supply as measured by standard methods of soil analysis. Yields increase with increasing soil K up to what is described as a critical level, beyond which further improvement in soil potash fertility has no effect. Similar data confirm the same principle for soil P. Because soil fertility varies within fields and according to soil conditions and climate, soil analysis is not a precise measurement as these data indicate. The Index system has therefore been developed to provide the general pattern of crop response to added nutrient. Soil K Index 2 is divided into a lower and an upper half denoted by minus and plus signs, with 2- as the safe Index for arable crops.

Soil analysis and cereal crop response
Soil P Index	mg/l	Yield response to added nutrient	Soil K Index	mg/l
0	0-9	Large response likely	0	0-60
1	10-15	Response likely	1	61-120
2	16-25	No response	2-	121-180
		No response	2+	181-240
3	26-45	No response. Good reserves	3	241-400
4	46-70	Excess nutrient present	4	401-600