Before tillage farmers start applying organic manures to stubble ground or crops, two questions must be addressed.
Firstly, is there sufficient allowance, specifically where phosphate (P) is concerned, to apply the organic manure?
Secondly, how to get the most from the organic manure that is to be applied?
These raw materials include: slurries; farmyard manure; poultry litter; spent mushroom compost; and sludge.
Organic manures
Head of the Crops Knowledge Transfer Department at Teagasc, Michael Hennessy, said:
“The days of applying an organic manure and largely ignoring the nutrient content have come to an end, with the introduction of the Department of Agricultural National Fertiliser Database.
“This is because the Department of Agriculture, Food and the Marine (DAFM) will have a clear view of all chemical fertilisers purchased by every farmer each year.”
According to Hennessy, every tonne or 1,000gal of organic manure applied to a farm means the allowance for chemical fertilisers must be reduced.
“P will generally be the limiting nutrient for most tillage farms,” he said.
Case example
The following example is relevant. It centres on a 40ha (100ac) block on a farm, with up to date soil samples, all showing a P index of three, and the area is planted to spring barley.
This farm will have a standard allowance of 25kg P/ha or 1,000kg P for the block.
Where the farmer chooses to use 13:6:20 compound (which contains 60kg of P in every 1,000kg of product), the total tonnes which can be bought is 16.6t (416kg/ha or 3.36 x 50kg bags/ac) based on P as the limiting element.
If the farmer imported 22m3/ha (2,000 gal/ac) of cattle slurry to half (20ha) the block (a total of 440m³ of slurry), this would apply a total of 220kg of P (1m3 of cattle slurry contains 0.5kg P).
The farmer would also incorporate the slurry within two hours of application to stubbles to ensure that nitrogen (N) losses are minimised.
Before the farmer can buy chemical fertiliser, the P in slurry (220kg P) must be deducted from the total allowance (1,000kg P), leaving 780kg P remaining to be applied to the block.
If the farmer was to stick to purchasing 13:6:20, now the farmer can only purchase 13t.
However, as 1m3 of slurry also contains 3.5kg of K (potash), therefore the slurry will apply almost all of the crop’s K requirements.
The farmer can now switch to applying straight P (4.8t of super phosphate) and balance the N with straight calcium ammonium nitrate (CAN) or urea.