May 22, 2022

Managing fall P and K conundrum

BLOOMINGTON, Ill. — High fertilizer prices, supply shortages and the weather converged this fall as farmers turn their focus toward nutrient management plans for the 2022 crop.

Emerson Nafziger, University of Illinois professor emeritus, examined fall fertilizer application strategies in a recent webinar hosted by the Illinois Fertilizer and Chemical Association.

Here are Nafziger’s comments on fall phosphorous and potassium management.

On P And K

There a legitimate ways to do phosphorous and potassium that we can postpone it, but my message is going to be at some point we have put P and K back in the soil that we took it out of with the recent crops.

Most of our Illinois soils in have the ability to go a year, in some soils even more than that, without fear of yield loss. But postponing the decision or the expenditure to put P and K back into the soil is the thing that we are all going to have to deal with. What are the risks of doing that?

Corn takes out about 0.37 of a pound P2O5 per bushel and about 0.24 of K2O per bushel. Soybeans is 0.75 of a pound of P2O5 per bushels and 1.17 pounds per bushels of K2O. These are all lower than the values that we used for the last decades.

Using those figures, 200 bushels of corn removes 74 pounds of P and 48 pounds of K, and 65 bushels of soybeans remove 49 pounds of P and 76 pounds of K.

I just pulled those numbers out as kind of typical numbers and if you put those numbers together we get almost identical P and K removed over the previous two crops.

So, if you took 200-bushel corn off in 2020 and 65 bushels of soybeans off this year you would have removed 123 pounds of P and 124 pounds of K over two years based on these numbers.

On Delay Or Not Delay

If the soil tests levels in medium to heavy-textured soils currently exceed 25 to 30 parts per million of P and 150 to 175 ppm of K, applying none for the 2022 crop carries little risk of yield loss.

One caution, based on what we have seen, root-restricted conditions — no-till, dry soils — next season may result in K deficiency symptoms even if soil test levels are adequate.

P and K removed by crops will need to be replaced eventually, maybe not fully if soil tests levels are high, and future supplies and prices are uncertain. That additional P and K that’s there, if soil test levels are high, is almost certainly not contributing to higher yields, but it’s a bigger cushion.

On Placement

Subsurface, or deep-banding, application of P is being promoted by some. Some equipment companies are making applicators that do that. The main reason is we can get some movement on sloping soils off the soil surface.

Most research has now shown a yield advantage to doing subsurface application. It does help decrease runoff of P and K on sloping soils. It also disturbs the soil and may increase soil erosion. It’s more costly and time-consuming than broadcast.

All considered, broadcasting P and K in the fall or spring is fast, inexpensive, and effective in most fields.

Planter-applied P and K help provide nutrients early, but this seldom boosts yield if soil test levels are adequate. Planter-applied, or liquid, forms are more expensive per pound of nutrient.

On Timing

Fall broadcast application is usually OK, but the loss of P from slopes is possible if it rains a lot after application, nitrification of nitrogen from MAP/DAP is more likely with earlier application in the fall on warm soils and a loss is possible.

We do know from our research that if you put DAP on when the soils are cooled around the first of November we were able to find we got pretty much full value of that nitrogen comparing it to spring UAN.

Spring broadcast applications works well. More of the nitrogen from MAP/DAP is likely to remain for the crop. Runoff may be less likely, and unlikely if tilled after application. Logistics may be an issue, especially if the spring is wet.

Applying P and K after planting is complicated, particularly the phosphorous. If we have a pretty low soil test and we really need P and K to prevent yield loss for the crop that we just planted, it’s probably not particularly effective to put on the surface.

We can’t really guarantee with any certain that P and K are going to get down where they’re needed if soil test levels are low. If soil tests are pretty good it’ll probably work in enough.

Planter-applied P and K provide nutrients to plants early, but seldom boost yields if soil test levels are adequate, and planter-applied, or liquid, forms are more expensive per pound of nutrient.

Tom Doran

Tom Doran

Field Editor