Fertilizer requirement identified with 3 numbers

At a Vancouver Island Golf Superintendent Association meeting in spring 2014, I spoke about turfgrass nutrient requirements. The question a turfgrass manager must answer is this one:

Can the soil supply enough of an element to meet the grass requirements, and if it cannot, then how much of the element must be applied as fertilizer?

The presentation handout and slides contain the detailed answer to that question. The short answer is that we can identify the amount necessary as fertilizer from just 3 numbers:

1. the amount we have (soil test amount)
2. the amount we need to have in the soil (the MLSN guideline)
3. the amount the grass will use

To find how much of each element is required as fertilizer, we look at the total amount that must be present, which is the amount the grass will use (3) added to the amount we need (2) in the soil.

Then we compare that amount required (2 + 3) to the actual amount we have (1). If the amount we have is more than the amount required, that element is not needed as fertilizer. If the amount we have is less than the amount required, that difference is the amount needed as fertilizer. The handout has the full explanation of how to get these numbers.