Only 32% of the soils on Tullamore Farm are at optimum levels for phosphorus (P), potassium (K) and soil pH. That is a soil pH of 6.5 and P and K index levels of 3 or 4. This is above the estimated 20% of soils at optimum levels nationally, but it is still low.

Soil pH levels are good overall, largely due to the soil types on the farm. The average soil pH is 7.34 and the range in soil pH is 6.4 to 8.0.

Sixty-three per cent of the soil test results were at index 3 and 4 for phosphorus index levels, 45% are at index 3 and 4 for K, and 14% are at index 1 for both P and K.

Twenty-three per cent of soils are at index 2 for phosphorus and 41% of soils are at index 2 for potassium.

Building soil P and K levels

Soil K levels are generally lower where silage has been produced and these levels need to be built. However, if the K is low in the soil it is often low in the silage and therefore the slurry, so it is hard to build soil K levels through that slurry.

Using farmyard manure which has straw might be an option to build soil K levels, but the process will be slow.

Soils low in P can benefit from the slurry from finishing cattle as these animals are receiving ration and the P should be higher in the slurry as a result.

Why increase soil indices?

If soils do not receive optimum P and K to grow crops each year then they will become depleted as the P and K is being harvested from the soil and is not being replaced at the same rate. When soils are at optimum levels for P and K then that P and K needs to be applied at a maintenance rate. If soil P and K levels are low then a building rate needs to be applied and if soils are at index 4 then P and K levels can be reduced.

Ensuring the correct levels of K are applied to produce silage is important, for example. Many of the fertiliser options out there which are commonly used for silage do not contain enough K for maintenance. So if you are not applying any slurry with this fertiliser, the K level should really be topped up.

Slurry test results

To get a better idea of the slurry which should be targeted at different fields, we tested the slurry on Tullamore Farm. This also allows the amount of artificial fertiliser needed to meet requirements to be calculated.

In spring 2022 we tested slurry from three different slurry tanks and all three varied hugely in their N, P and K values.