Applying Phosphorus if required
Alan Galbraith, Countryside Management Branch, DARD
Why buy something you don’t need. Why use it if it is damaging the environment? Last week’s article on phosphorus explained that chemical fertilisers containing phosphorus (P) can only be applied if a need exists and that the requirement can only be established through a soil test. This article looks at how the soil test results determine if a requirement exists.
How does a soil test indicate a requirement for P?
A soil test indicates the phosphorus (P) index of the soil. Using this index the maximum crop requirement per hectare for P applications can be found in RB209, the technical standard for fertilisation. The table outlines the maximum requirement for some of the common crops in Northern Ireland. Farmers must not exceed the amount of P required according to the soil index. The P within organic manures that will be applied must also be taken into consideration when calculating how much chemical P fertiliser is required. (Approximately 5.4 KG of phosphate is contained within 4.5m3(1,000) gallons of typical cattle slurry).
Maximum crop requirement for grass and some crops for P2O5 in KG per hectare
| Soil Index | |||||
|---|---|---|---|---|---|
| 0 | 1 | 2 | 3 | 4 | |
| Grazed grass ( total for whole season) | 60 | 40 | 20 | 0 | 0 |
| First cut silage | 90 | 65 | 40 | 20 | 0 |
| Second cut silage | 25 | 25 | 25 | 0 | 0 |
| Third cut silage | 15 | 15 | 15 | 0 | 0 |
| Grass establishment | 120 | 80 | 50 | 30 | 0 |
| Winter wheat , winter barley | 120 | 95 | 70 | 20 | 0 |
| Spring wheat, spring barley | 105 | 80 | 55 | 0 | 0 |
| Winter and spring oats | 105 | 80 | 55 | 0 | 0 |
| Winter Oilseed rape | 100 | 75 | 50 | 0 | 0 |
| Spring Oilseed rape | 80 | 55 | 30 | 0 | 0 |
| Maize | 110 | 85 | 60 | 20 | 0 |
| Maincrop potatoes | 270 | 230 | 180 | 130 | 50 |
| Early seed potatoes | 270 | 230 | 180 | 130 | 50 |
Is P required?
Where your soil test shows that P is required it can be applied up to the maximum application rate. For example if a grazing field tests as having an index of 2, up to 20 KG of P can be applied. Where optimum yield is not required, as in an extensive farming system, application rates should be reduced. On extensive grassland farms maintaining an index of 1 on grazing land is satisfactory.
If your soil test shows that P is NOT required, then you must not apply chemical P.
If you require further information on soil testing to determine whether or not you need to apply chemical P, or if you would like more information on soil testing, contact Countryside Management staff at your local DARD office.
