As second-cut silage is now cut or near cutting stage, slurry spreading will be the next job to follow. This year has been particularly awkward for livestock farmers on many fronts, and that includes emptying slurry tanks. The main spreading time on farms is when the deadline opens in the spring, after first-cut silage and then again in autumn or after second cut, before the spreading window closes again.

After some particularly difficult spring and summer conditions this year, slurry tanks in yards are probably less empty than usual. As a result, one of the key objectives over the coming weeks will be to bring down these levels as much as possible before the winter. The deadline for spreading organic manures has been extended by just over two weeks, and while this isn’t long, it should help the situation.

As the tankers and agitators roll out once again, it is important to understand what we are spreading and how we use this organic fertiliser as an asset, rather than looking for the quickest way to get rid of it.

Nitrogen (N)

The N content in organic fertilisers is normally present in two forms; ammonium, which is readily available for plant uptake and N in organic compounds, which are less available to plants. Therefore the amount of ammonium compared to the total N content will determine the N availability in the organic fertiliser being applied.

In general, slurries will contain around 40% to 60% more ammonia relative to the total N content, compared with solid farmyard manures. The availability of ammonium N also depends largely on the weather conditions at the time of application. Spreading in cool, moist weather will increase the amount of ammonium that is captured by the crop and therefore increase the overall N uptake and organic fertiliser value.

It is estimated that 1,000 gallons of typical cattle slurry is the equivalent to 6.5 units of N.

Phosphorus (P) and potassium (K)

The rate of release of P and K from organic manures depends on the soils P and K status. When soil P is at index 1 or 2, then P in organic manures is deemed to be 50% available. Therefore, it is recommended to only supply 50% of P crop requirement with organic manures and the remaining 50% with chemical fertiliser P. When the soil P status is medium to high at index 3 and 4, organic fertilisers can be used to supply 100% of the crop P requirement.

Similarly for K, it is advised that only 75% of the crop requirement should be applied as organic fertiliser on soils with low K status of index 1 and 2, whereas organic fertilisers can supply 100% of crop requirement on soils with K index 3 and 4.

It is estimated that 1,000 gallons of typical cattle slurry contains 5 units of P and 30 units of K.

Maximising potential

There are two factors in maximising your return from the application of slurry.

Firstly, planning the applications of all fertilisers in such a way that nutrients are not being applied in excess of crop demands. There is no need to be applying too much chemical fertiliser where cattle slurry or other organic manures have already supplied some level of nutrients. Nutrients supplied that are not required by the crop or the soil is an additional cost on the farmer. As well as that, balancing organic and chemical fertiliser applications so that the N, P and K are supplied on the correct proportions is crucial in order to ensure maximum value for money from fertiliser.

The second factor is to apply the organic fertiliser at a time and in a method that maximises nutrient availability. This is of particular significance for N in organic fertilisers – P and K levels tend not to be affected by time or method. Ammonia losses to the air are highest when soil and air conditions are dry and warm.