30 Mar Building Soil Fertility

Fertilisers should supply nutrients in the correct balance for the crop and the soil fertility level.

• Soil P and K levels should be at index 3 to maximise grass production – building soil P and K levels can take a number of years.
• The approach to maintaining soil fertility is to replace the nutrients removed in product.
• For example, one kg of P is removed from the farm in approximately 1,000 litres of milk or in 100 kg of animal live weight.

Phosphorus: Apply 50-75% of P requirements in the spring with the remainder in the summer

If taking 1 cut silage add 16 units P/acre, if taking 2 cuts add 28 units of P/acre to the grazing requirements. The extra P can be balanced by slurry – 1000 gals slurry = 5 units of P.

Potassium: Apply maintenance rates during the growing season and build up rates in August/September.

If taking 1 cut silage add 100 units K/acre. If taking 2 cuts add 180 units of K/acre. Again this extra P can be balanced by slurry – 1000 gals slurry = 30 units of K

• Low P fields – get extra P out early on low P fields, e.g. 10-10-20
• Low K fields – apply extra K in the autumn, e.g. Muriate of potash.

Compound Fertiliser – a mixture of nutrients, typically N, P & K. The name of the compound reflects the % of N, P & K in the product for example 18-6-12 contains 18% N, 6% P and 12% K. A 50kg bag of 18-6-12 will supply 18 units of N, 6 units of P and 12 units of K. (convert to kg/Ha multiply by 1.25, e.g. 12 units K/acre x 1.25 = 15kg/Ha)