Cattle slurry has an integral role in improving soil fertility, especially when it comes to rising fertiliser bills and meeting the new chemical nitrogen (N) allowance.

It is the largest national source of on-farm organic fertiliser and is a valuable source of N, phosphorous (P) and potassium (K).

Nitrogen in slurry comes in two forms - 50% ammonium N, a plant-available form, and 50% the organic N form that is not plant available at the time of application.

The amount of ammonium N retained will determine the fertiliser N replacement value of the slurry in the year of application. The ammonium N is easily lost to the air as ammonia if the weather is warm, sunny and dry. Cool, damp and overcast conditions will result in a reduction in ammonia losses at the time of application.

Technologies

Using low emissions application technologies such as the band spreader, trailing shoe and direct injection method will reduce N losses to the atmosphere, therefore increasing the fertiliser value of the slurry. The timing of application is also important in terms of maximising the fertiliser value of the slurry.

For example, farmers should aim to spread slurry in the spring instead of the summer.

Applying slurry in the springtime with a trailing shoe will recover 40% of the available N in slurry. In comparison, a summer application will see this fall to 25% N recovery.

P and K

In terms of P and K fertiliser replacement values, this will depend on the soil P and K fertility levels. For index one and two soils, P is approximately 50% available. On index three soils, slurry P is 100% available.

For K, index one and two soils saw slurry K 90% available, while on index 3 soils, slurry K is 100% available. It is important to keep this in mind, especially for P on low index soils, to make sure that P requirements are met for grazed or cut grass swards.

Farmers should aim to supply 50% of P in slurry and the remaining 50% as bag fertiliser.

According to Teagasc, 81% of the nutrients in cattle slurry are P and K, while only 19% is N.

Therefore, cattle slurry should be targeted to fields that have significant P and K requirements which are based on soil test results or fields which will be cut for silage.