Farm Ireland

Friday 19 January 2018

Three the magic number for soil

Step 3 – Aim to have soil test P and K in the target index 3 in all fields

Soil index 3 is the optimum soil index to maximise grass quantity and quality on an annual basis.

Phosphorus and potassium levels change very slowly over time.

The aim is to maintain soils at index 3 (see Table 2) by replacing P and K removed in grazed grass or grass silage.

For example, 1,000 litres of milk will remove approximately 1kg/ha of P and 1.5kg/ha of K. A cut of silage yielding 5t DM/ha will remove 20kg/ha of P and 120kg/ha of K.

Soils at index 1 and 2 have a lower nutrient supply and will require additional annual nutrient applications to reach soil index 3.

Soils at index 4 have a high nutrient supply and offer the opportunity to save on fertilisers.

It is recommended to omit P applications for a number of years and re-test soils to monitor soil P changes.

Also Read

For K it is recommended to omit K for one year and revert to index 3 advice (especially on silage ground) until soils are re-tested.

Standard off-take values of P and K in grassland and tillage crops can be calculated using the online fertiliser calculator available at www. calculator

Irish Independent