1. Will the potash that was applied in 2018 for second-cut silage still be available or is it lost?

Where second-cut silage yields were low, yes the K will still be available in the soil. It may have increased the soil’s K fertility status. For example, 5t of fresh grass will remove about 25kg K (50 units). Adjust K rates based on grass yields.

.2 From a low K index soil, will the K content of silage be lower and therefore a lower K content of cattle slurry from the animals eating that silage?

Generally not as the lower soil K supply (index 1) will result in a lower grass yield with typical grass K concentrations. Therefore, the slurry K will not be lower.

3. What’s the latest time you could apply K before cutting silage to prevent animal health issues? What is the maximum units/ac? Split application?

The maximum application of K for first-cut grass silage is 90kg/ha. This can be applied in advance of closing time. On low-fertility soils where additional K is required (build-up), apply extra K after first-cut silage or in September/October to reduce risk of animal health issues.

4. Will excessive use of chemical fertiliser sources of K such as MOP compared to SOP have negative effects on soil biological activity?

There is no evidence on Irish soils due to our good soil organic matter levels and high annual rainfall. In addition, maintaining soil pH in the optimum range (6.3 to 6.5) reduces the availability of such nutrients as chloride in the soil.

5. Is there any issue with applying K in October-November if growth rates are low?

This is an ideal time to apply additional K to replenish and build soil K reserves. It will allow sufficient time for the K to move into a soil stable state/form for maximum supply at peak growth during the following growing season.

6. What K strategy should be employed for zero grazing? Six to seven cuts being taken during the year, usually 2t dry matter (DM) per cut?

Aim to fertilise this as a crop and supply sufficient levels of K based on yield during the growing season. Each tonne of DM will remove 4kg P and 25kg K. A cut of 2t DM will require about 50kg K/ha (40 units/ac). Aim to apply K build-up rates where required at the end of the grass growing season.

7. Why is it that K index levels move up relatively fast in comparison to other soil nutrients, such as phosphorus?

The build-up rates are related to the amount of additional nutrient required to improve the soil P or K indices (1 or 2) to the optimum index 3 over a five-year period. Potassium is a faster nutrient to respond to additional K applied compared to P as there are not as many soil factors such as soil type/pH/Al and Fe levels affecting the rate of K build-up.

8. What are the reasons that one-third or two-third ryegrass mixes are out yielding ryegrass only swards?

The rye grass mixture is more efficient at utilising the K in grass production. This is due to the different / deeper rooting profiles of the other grasses in the mix at exploring a large soil volume and taking up more soil K from deeper down the profile.

9. How much K spread in October is lost through leaching over the winter?

The amounts of K lost from Irish soils through leaching is relatively low unless one is on a very light soil type. Expected K losses over the winter period would range from 5kg/ha to 30kg/ha.

10. What is the ideal blend of fertiliser for silage and grazing fields?

For silage fields a fertiliser blend with a P:K ratio of 1:6 is ideal while for grazing ground a P:K ratio of 1:2 is ideal.