Potash for Grassland

Losses

Potash is not lost directly to the atmosphere and losses by leaching are generally small. Moreover potash leaching has no known adverse environmental effects.

It has been shown that most soils lose only about 1 kg/ha of potash for every 40 mm of excess rainfall. Therefore losses by leaching for lowland grass production are normally less than 15 kg/ha K₂O annually. Losses are particularly low when there is a good balance between the supply of K and N and grass yields are high.

However it is important to recognise that significant potash losses may occur via drainage or by run-off on shallow sandy soils and when fertiliser and particularly manures are applied under inappropriate conditions. Losses are greatest when:

• large quantities are applied to very dry, deeply fissured soils or to waterlogged land or frozen land which rapidly thaws;
• heavy rain occurs within about four days of application, particularly in autumn and winter. Potash losses may amount to half the amount applied under such conditions. This is a serious financial loss and the associated phosphate loss may provide an environmental hazard.

Losses also occur from manure and in silage effluent, with up to 50% of available potash at risk in uncovered stores. Potash is normally freely soluble and loss of the liquid fraction of animal wastes, crop effluent, etc. will lose most potash.

Principles of nutrient management

"Let the soil feed the crop, use fertiliser and manures to feed the soil"

Potash reserves in the soil are more effective at supplying the plant than fresh fertiliser applications. Impoverished soils will often not produce the same yields as fertile soils even if higher fertiliser rates are applied. An adequate reserve of phosphate and potash to feed the crop should be maintained in the soil by using fertilisers and manures to replace what is removed by cutting or grazing. For soils with low reserves, extra fertiliser should be used to restore fertility to target levels. For soils with high reserves, fertiliser usage should be reduced or omitted.

	Target Soil Fertility
		Soil P Index	Soil K Index
	Sand & peat soils	2	1+
	All other soils	2	2-

Soil analysis

Knowledge of the fertility level in the soil is essential to determine fertiliser policy.

It is impossible and irresponsible to guess correct fertiliser use without soil analysis which should be undertaken about every 4 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, Soil Analysis, Key to Nutrient Management Planning).

Because soil fertility varies within fields and according to soil conditions and climate, soil analysis is not a precise measurement and the index system has therefore been developed to indicate 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.

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	Unnecessarily high reserves	4	401-600

For soils under cut grassland sampled in 2002/3, 9%, 41% and 26% had K indices of 0, 1 and 2- respectively, with only 24% having index of 2+ and above. K status was marginally better in grazed grassland, with 3%, 33% and 32% at K indices of 0, 1 and 2- respectively, and 32% having index of 2+ and above.

Herbage analysis

Herbage analysis can be used as an adjunct to soil analysis or as an extra diagnostic tool in specific circumstances.

The maintenance of an adequate level of soil K can be achieved at low cost and will normally ensure that crop needs are being met without the need for tissue sampling and analysis. Where tissue testing is used, tissue water (cell sap) measurement is recommended. This indicates whether K concentrations are at or below optimum levels and is more reliable and meaningful than measurement of K content in the dry matter. %K in dry matter varies widely with season, stage of growth, part of plant, fertiliser (N or K) application and weather or other factors which affect nutrient uptake and rate of growth. Where dry matter analysis is used, 1.8-3.0% K normally represents a satisfactory level, but a better yardstick is the N:K ratio in herbage which should be between 0.9:1 and 1.1:1