Potash for Cereals |
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Recommendations
The recommendations below are calculated to achieve full yield and maintain, improve or reduce soil P and K according to the soil index and soil type. The approach firstly identifies the amount of nutrient removed by the cereal crop grown (the maintenance amount or M) and then adjust this according to the current soil index level.
Phosphate
| Maintenance requirement M (kg/ha) | ||
| Grain only - All cereals | yield t/ha x 7.8 = M | |
| Grain + Straw | ||
| Winter wheat/barley | yield t/ha x 8.6 = M | |
| Spring wheat/barley | yield t/ha x 8.8 = M | |
| Oats | yield t/ha x 8.8 = M | |
| Soil fertility adjustment (kg/ha) | |||||
| Index | 0 | 1 | 2 | 3 | 4 |
| mg/l | (0-9) | (10-15) | (16-25) | (26-45) | (46-70) |
| Phosphate requirement | M+50 | M+25 | M | M-50 | nil |
Potash
| Maintenance requirement M (kg/ha) | ||
| Grain only - All cereals | yield t/ha x 5.6 = M | |
| Grain + Straw | ||
| Winter wheat/barley | yield t/ha x 11.3 = M | |
| Spring wheat/barley | yield t/ha x 13.7 = M | |
| Oats | yield t/ha x 17.3 = M | |
| Soil fertility adjustment (kg/ha) | ||||||
| Index | 0 | 1 | 2- | 2+ | 3 | 4 |
| mg/l | (0-60) | (61-120) | (121-180) | (181-240) | (241-400) | (401-600) |
| Potash requirement | M+50 | M+25 | M | M-25 | M-70 | nil |
Special cases
Sand soils
It is not economic to increase soil K beyond 100 mg/l on these soils because of their very low capacity to hold nutrients. Improvement can be achieved by many years dressing with FYM. Nutrients should be applied 'little and often' on these soils (see PDA Leaflet 22).
Loamy sands
It is not economic to increase soil K beyond 150 mg/l on these soils because of their low capacity to hold nutrients. Improvement can be achieved by many years dressing with FYM (see PDA Leaflet 22).
K releasing clay soils
Some clay soils can release potash over many years. Normal potash rates can be reduced by 50 kg/ha and omitted completely at index 3 (see PDA Leaflet 19).
High pH soils
It is sometimes suggested that calcareous soils may have a different requirement for phosphate and potash. The principles of response and the recommendations given above are applicable to these soils. However for the shallower chalk/limestone soils there can be greater risk of loss of potash, even though they have a higher clay content than sandy soils, because of their depth and a high proportion of stone in the topsoil (both these factors restricting the volume of soil for nutrient provision) Target fertility in these cases must be adjusted as with other low retentive soils. Calcareous soils with pH typically over 7.5 and high organic matter ('puffy chalk soils') may 'lock-up' a higher proportion of applied potash than other soils, so that low index levels may take longer to improve. But this does not alter the amount of nutrient required to achieve full yield.
Manures
Only 15% of winter cereals and 30% of spring barley nationally receive any manure but it is important that nutrient contribution from this source is estimated and fertiliser use adjusted accordingly. Whilst there are often higher priorities for manure use, e.g. for roots, vegetables and conserved forage crops, regular measured applications of manure will benefit cereal crops, especially on the extremes of soil type - very light or very heavy - by improvement of physical characteristics as well as nutrient supply.