Don't forget about potash and sulphur
Alan Galbraith, Countryside Management Branch, Department of Agriculture and Rural Development (DARD)
Silage making is in full swing and decisions must be made now on fertiliser type for second cut. Two issues are coming to the fore regarding silage swards, these are potash and sulphur deficiencies.
What level of potash?
Silage swards are commonly low in potash leading to reduced yields. Silage swards can quickly become deficient in potash, particularly if low rates of potash fertiliser or slurry are applied. The recommended amount of potash to be applied varies significantly depending on fertility status.
| Soil K | |||||
|---|---|---|---|---|---|
| Index 0 | Index 1 | Index 2 | Index 3 | Over 3 | |
| 2nd cut silage (potash kg/ha) | 120 | 100 | 90 - 60 | 40 | 0 |
If you have not had your soil tested recently and your silage remains uncut, herbage analysis can be carried out to assess the potash level in the sward. A potassium (K) concentration below 2 percent indicates a deficiency.
What level of sulphur?
Sulphur levels are falling and increasingly this nutrient is becoming deficient. Deficiencies were traditionally found in second cut silages on sandy soils only, but now they can also exist in first cuts on all soils. At this time of the season, herbage analysis is recommended to give an indication of the sulphur status. A concentration below 0.25 percent for total sulphur would indicate a deficiency. Apply 25-40 kg/ha SO3 (Sulphate) where a deficiency exists or as an insurance dressing.
Bringing back a balance
The majority of silage fields, in terms of nutrients, are currently out of balance. There is a tendency to apply excessive amounts of nitrogen and phosphorus whilst at the same time under supply potash and sulphur. Find out now through herbage analysis or later in the year by soil sampling whether your swards are balanced. Applying all the nutrients required is the key to good sward growth.