Slurry is a valuable source of organic N, P and K and, if used wisely, can help reduce chemical fertiliser bills on farms.
In springtime, when suitable weather and underfoot conditions allow, aim to maximise the spreading of slurry on the fields that need it most, on a lot of farms, these will be silage fields or fields known to be low in P and K (from soil test results).
Fertiliser Value of Slurry
The typical value of 1,000 gallons of cattle slurry applied by splash plate in springtime has an available N-P-K content equivalent to a 50kg bag of 6-5-30.
The nutrient content of cattle slurry will vary with animal type and diet and especially with slurry dilution with water.
Phosphorus (P) & Potassium (K)
Cattle slurry is a rich source of P and K fertiliser and should be applied to parts of the farm that have either low soil P or K levels, or to crops with high P and K demands such as grass silage.
Targeting these areas will help reduce fertilizer bills and replenish soil P and K reserves.
Research shows that grazing fields near farmyards tend to have higher fertility levels due to more regular applications of manures. Far away silage fields tend to have low soil fertility levels plus the largest demand for both P and K.
Slurry is also a very well balanced fertiliser (P to K ratio) for grass silage crops. For grazing ground, the P and K demand will be lower and will depend on the stocking rate and the soil test results.
A soil testwill confirm the P and K status of the soil and help with targeting slurry to Index 1 and 2 soils for more efficient crop fertilization and P and K utilisation.
Nitrogen Content
Half of the Nitrogen that is in cattle slurry is in an organic form and the other half is as ammonium, the very same as the form of Nitrogen that is in purchased urea fertiliser. It is this ammonium half that can replace bought in bagged N.
Similar to urea fertiliser there are times of the year that you can expect to get the maximum value of N from slurry and this is very much weather dependent.
To maximise N uptake, apply slurry on cool, overcast or misty days. It is recommended to apply as much slurry as possible in the springtime to maximise the fertiliser N value of slurry.
Spring applied slurry is worth approximately 3 units of N per 1,000 gallons extra compared with summer application. However, irrespective of timing, applying slurry in the right weather conditions (cool, overcast, misty conditions) is advised rather than hot dry weather.
Dilution of cattle slurry will also improve the N uptake as the slurry will infiltrate faster into the soil compared to thick slurry. Diluted slurry will be washed off the grass faster resulting in reduced grass contamination.
Remember that dilution will increase the N efficiency but will reduce the P and K content of the slurry and this needs to be accounted for in balancing crops P and K requirements.
In Summary
- Slurry is a valuable source of N, P & K;
- Target slurry to fields with large P & K demands based on soil test results;
- Apply slurry on cool, overcast days in springtime to maximise N recovery;
- Switching slurry application from summer to springtime will increase N value of slurry; and,
- Using low emission technology such as band spreader or trailing shoe application methods will also increase N utilisation and value.
Anthony O’Connor, Teagasc Adviser, Galway/Clare Regional, Unit and Tim Hyde, Environment Specialist, Teagasc, Athenry.