How I recommend fertilizer nutrient ratios

This is how I think everyone should do it. Make nutrient applications that have a site-specific ratio of nitrogen (N), phosphorus (P), and potassium (K). I write this because I’ve had some questions about ratios and about fertilizer nutrient analysis, and I want to explain this in a general way.

Grass uses the quantity of nutrients that it takes up as it grows. That quantity is the maximum amount of nutrients that the grass has used. For the purposes of this post, I’ll call that quantity nutrient use and will refer to it again. If the grass could get all its nutrient use from the soil, then fertilizer would not be required.

That actually is the case for some elements like calcium, magnesium, and micronutrients. The nutrient use by the grass of those elements is almost always supplied in its entirety by the soil. Consequently, those elements are almost never required as fertilizer.

With the macronutrients N, P, and K, it’s a different story. One generally wants to have grass growing more rapidly than it would if no N were applied, so it is typical to have a site-specific N rate. If one applied no nutrients at all, I’d call that a 0:0:0 ratio.

This six month old turf grew with no fertilizer after planting as sod: that's a 0:0:0 ratio of N, P, and K.
This six month old turf grew with no fertilizer after planting as sod: that’s a 0:0:0 ratio of N, P, and K.

The grass uses relatively large amounts of P and K, also, but if the soil has enough P and K to supply all the nutrient use, and the soil often does, then there is no need to add those elements as fertilizer.

In that scenario, in which one applies N and in which the soil can supply all the P and all the K, the N:P:K ratio is 1:0:0. I don’t mean by that ratio that the fertilizer nutrient analysis should be 1:0:0. No, I mean that for every unit of N applied, there should be 0 P and 0 K applied. The types of fertilizer to use to supply all the nutrient use could be anything with that ratio: a 10-0-0, 21-0-0, 46-0-0, etc.

One can do a soil test to find out if the soil is able to supply enough P and K to the grass, or if P or K may need to be added as fertilizer. The MLSN method works particularly well for that, because it is based on modern turfgrass management and the soil conditions in which high performance turfgrass is grown today.

Remember that MLSN has nothing to do with nitrogen in terms of making a recommendation for that element. The turfgrass manager decides how much N to apply. Then MLSN works by considering how much of the other elements will be required as fertilizer, given that N rate, and given the site-specific soil conditions, to ensure the nutrient use quantities of those other elements are supplied to the grass.

I find it easy to make recommendations in terms of nutrient ratios when P and K are required as fertilizer. Within the reasonable ranges of N fertilizer that might be applied for a particular grass on a particular site, one can test the soil and find what amount of P and K might be applied to the grass in terms of a ratio with N.

That may be a 1:0:0, as I mentioned above. But it is more common, especially in sand-based rootzones, to have something like an 8:0:3 or 8:1:4 or 8:2:5 type of ratio. This specific ratio will be dependent on the soil test result and on the turfgrass species. If one applies N, P, and K to the grass in approximately the ratio that is determined from the soil test, it is certain that the grass will be supplied with 100% of its nutrient use.

These are ratios. Not an 8-2-5 fertilizer, for example, but a P rate that is 25% of the N rate and K rate that is 62.5% of the N rate. One can mix and match fertilizer applications that over time converge on something like the recommended ratio.

This is how I like to manage turf and make fertilizer recommendations. This ratio approach makes practical sense. Adding a bulk amount of product to try to change K in the soil, for example, may not work the way one expects. If the soil happens to be low in K, then obviously the soil doesn’t hold much K. Adding a lot of K in a futile attempt to increase the soil K content will certainly fail to increase the soil K as much as one expects. In fact, one may fail to supply the grass with all its nutrient use in that case. A more effective approach is to consider the soil conditions as being, essentially, what they are, and to use the information from the soil test to determine how one might best adjust the N:P:K ratio supplied as fertilizer.

Anyone who’s done soil testing with ATC over the past few years will have seen that method employed in the way fertilizer recommendations are made. This approach is based on ensuring that the grass gets 100% of nutrient use, and this approach doesn’t try to add nutrients to reach illusory target levels in the soil.

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