Nutrient requirements of forage crops | print version (pdf 128kb) |
Nutrient removal
The study provided useful figures for phosphate and potash removal from the field by high yielding forage crops. The basis of phosphate and potash manuring depends upon estimation of nutrient removed - see the section on basic P and K policy below. The nitrogen offtake is also shown in order to indicate the relative quantities of nutrient present in the crop. (Nitrogen removal is not a guide to the quantity of N required). It is notable that in all cases considerably more potash was removed than nitrogen. Potash removal was between 3-6 times greater than phosphate removal.
Average annual yields and nutrient removal
| Fresh yield t/ha |
DM yield t/ha |
Nutrient
removed kg/ha |
|||
|---|---|---|---|---|---|
| N | P2O5 | K2O | |||
| Whole crop wheat | 39 | 14.4 | 181 | 69 | 212 |
| IRG | |||||
| Cut 1 | 34 | 5.2 | 126 | 48 | 229 |
| Cut 2 | 31 | 5.5 | 115 | 41 | 197 |
| Cut 3 | 19 | 3.8 | 79 | 28 | 109 |
| Cut 4 | 6 | 1.0 | 52 | 5 | 34 |
| Total per year | 90 | 15.5 | 341 | 122 | 570 |
| PRG/white clover | |||||
| Cut 1 | 31 | 5.0 | 121 | 41 | 208 |
| Cut 2 | 27 | 4.5 | 105 | 35 | 190 |
| Cut 3 | 18 | 3.2 | 81 | 24 | 116 |
| Total per year | 76 | 12.7 | 307 | 100 | 514 |
| Fodder beet | |||||
| Tops (returned) | 44 | 4.4 | 114 | 28 | 241 |
| Roots | 78 | 12.6 | 113 | 55 | 314 |
| Kale | 58 | 6.7 | 164 | 53 | 287 |
| Maize (1 year only) | 33 | 8.9 | 130 | 55 | 216 |
Estimating P and K removal
It is interesting to compare the P and K offtake of the forage crops in this trial with standard published figures. The general trend in this study was for phosphate removals to be slightly lower than standard and potash removals to be higher. The standard figures for fodder beet derive from the sugar beet crop and in view of the large difference for potash, it is clear that the standard figure for fodder beet should be revised.
Phosphate and potash removed
| kg per fresh t of yield | |||||
|---|---|---|---|---|---|
| Kingshay study | Standard figures | ||||
| P2O5 | K2O | P2O5 | K2O | ||
| Whole crop wheat | 1.8 | 5.4 | na | na | |
| IRG | 1.4 | 6.3 | 1.7 | 6.0 | |
| PRG/white clover | 1.3 | 6.8 | 1.7 | 6.0 | |
| Fodder beet | 0.7 | 4.0 | 0.8 | 1.7 | |
| Kale | 0.9 | 5.0 | 1.2 | 5.0 | |
| Maize | 1.7 | 6.5 | 1.4 | 4.4 | |
Phosphate fertiliser requirements
The manure dressing provided a total of 124 kg/ha P2O5. This was similar to the offtake by the IRG but for other crops provided between 24-70 kg/ha more phosphate than was removed. Over a period of years, this rate of manuring would increase soil P levels. Phosphate inputs to the rest of the rotation should be reduced or omitted to avoid levels being built up beyond index 3.
Potash fertiliser requirements
The manure application provided a total of 234 kg/ha K2O which was sufficient potash to replace that removed in the wholecrop wheat and maize but left a deficit of 80 kg and 53 kg after the fodder beet and kale. The very large offtake of potash in the intensively cut grass resulted in a serious annual shortfall of 280-314 kg/ha of potash which would need to be made up by application of potash fertiliser each year to prevent soil reserves from being depleted rapidly.
Basic P and K policy
Total nutrient requirements should be calculated on the following basis :-
| Index 0 | Crop offtake + 50 kg/ha |
| Index 1 | Crop offtake + 25 kg/ha |
| Index 2 | Crop offtake |
| Index 3 | 50% of crop offtake |
| Index 4 | Nil |
The nutrient contribution of manure should then be estimated using standard values of total content of phosphate and potash if the soil index is 2 or more and using available content if soil index is 0 or 1.
60% of total phosphate and 90% of total potash in FYM or slurry can be regarded as available.
Fertilisers should be used to make-up the difference between total requirements and nutrients provided by manures.
Soil analysis should be used every 4-5 years to check on any changes in soil reserves.
With acknowledgements to Kingshay Farming Trust, Bridge Farm, West Bradley, Glastonbury, Somerset BA6 8LU. Tel: 01458 851555, www.kingshay.com