With fertiliser prices remaining at record levels, it is important to know the value of the fertiliser that you are producing on your farm or, indeed, importing on to the farm.

Many farmers have been testing slurry this season to find out the true nutrient value of what some are now calling liquid gold. However, testing is the only way to find out if it’s six karat or 24 karat.

So far on the Footprint Farms, 13 slurry samples have been taken and the results are varying hugely within farms and from farm to farm.

Table 1 shows the results of these slurry tests. It shows the amount of nitrogen (N), phosphorus (P) and potassium (K) in the test result and then shows the value of these nutrients in units/1,000 gallons – 1,000 gallons is the equivalent of spreading a 50kg bag of fertiliser.

The average result for N, P and K across the 13 samples was 5-5-27. However, nitrogen values ranged from two units to seven units. Phosphorus values ranged from one unit to 13 units and K levels ranged from 13 to 53 units.

Two of the most attractive test results in the selection of samples had N-P-K values of 6-4-53 and 5-13-16.

The first sample with high K levels came from animals on a predominantly silage-based diet, while the second sample with a high P result was from the finishing unit of a pig farm. However, to show the variation that occurs in samples another pig slurry sample from a finishing unit had a P reading of six units/1,000 gallons.

Nutrient value

Looking at sample 10 in Table 1, it had dairy washings and slurry mixed together, which clearly diluted the nutrient value of the slurry with a result of two units/1,000 gallons of N, just one unit/1,000 gallons of P and 17 units/1,000 gallons of K.

Turning slurry test results into fertiliser savings

When you begin to multiply these figures out into what might be spread in a field, it really becomes interesting. If we take the average figures for N-P-K in the slurry results of 5-5-27 and apply this at a rate of 3,000 gallons/acre to silage ground on an index 3 soil for P and K, this will supply 94% of the P requirement, 81% of the K requirement and 12% of the N requirement. Assuming application rates of 125 units of N, 16 units of P and 100 units of K per acre.

On a 2.6t/ac spring barley crop 3,000 gallons/acre would supply 108% of the K requirement, 60% of the P requirement and 11% of the nitrogen requirement. This is assuming an application rate of 135 units of N (index 1 for N), 25 units of P (index 3 for P) and 75 units of K (index 3 for K).

How to put a monetary value on this slurry?

Table 3 shows how to calculate the value of slurry once you have the slurry test result. Using the average result again and placing CAN at a price of €700/t, superphosphate (16% P) at €710/t and Muriate of potash (50% K) at €710/t, the value of this slurry with an N-P-K value of 5-5-27 is estimated to be €36.75/1,000 gallons.