Replacing used P and K

A recent Teagasc Beef Newsletter article addresses the decline in soil phosphorous and potassium levels due to reduced fertiliser application, its impact on nitrogen efficiency and farm emissions, and provides recommendations to mitigate these effects.

Fertiliser sales of phosphorous (P) and potassium (K) have fallen considerably over the last two years. This is not surprising with the large increases we saw in the price per tonne. Many farmers also decided to take a ‘holiday’ from applying P and K to lessen the impact further.

We are now seeing the result of this reduction in the soil sample results that are coming back. Soil P and K levels are trending downwards and this will considerably reduce the efficiency of the nitrogen (N) that is spread on these soils.

Long term, this means more losses, i.e., reduced returns (€) for each kg of N spread (through less grass grown) but also more emissions of greenhouse gases from reduced efficiency and more N needed to get the same level of growth.

Lower prices

Prices quoted this year for compound fertilisers are back considerably compared to the last two springs. Grass farmers who have a P allowance for their farm should aim to apply enough P and K to at least replace what they are going to remove over the next six months.

The table shows what beef and sheep farmers need to spread if they are using 18-6-12 as their product of choice on grazing ground.

If your P and K indices are low and you can afford to build some of the lost nutrients back up, the extra bags of 18- 6-12 per acre needed are also shown. This will ultimately save N, as its efficiency increases at a soil P and K index of 3.

Silage harvests take huge amounts of P and K out of fields and, if they are not replenished, soil fertility levels can deplete very quickly. For example, a first-cut crop of silage will remove 20kg of P per ha (16 units per acre) and 125kg of K per ha (100 units per acre). Cattle slurry at a rate of 2,500-3,000 gallons per acre will replace the majority of the offtakes.

Table 1: Grazing ground P and K advice

Read the full Teagasc Beef Newsletter