Dan Orchard on N soil tests

Dan Orchard
Dan Orchard

CCC agronomy specialist Dan Orchard writes:

When looking at soil sample results, it’s important to understand the information contained in the results, and how to use this information. Applying fertilizer in the fall has its benefits, and without a soil sample growers are doing some guessing — particularly with nitrogen (N).

Nitrogen is quite mobile in the soil, and amounts can fluctuate year to year and depth to depth. Although some growers have success by using removal rates and target yields when making N decisions, a soil sample can be a great indicator and a good way to ensure intended rates aren’t too high or too low.  

When determining how much nitrogen to apply, there are a couple of important factors growers should look at:

1. Determine the total amount of N the crop will require to achieve the yield target. Past research showed a requirement of close to three pounds of N for every bushel produced, while more recent research with high yielding hybrids suggests a revision. The new range is as low as 1.7 lb./bu. of total N required when yields approached 75 bu./ac. and as much as 2.7 lb./bu. for yields around 25 bu./ac. If we aim for the mid point of that range — 2.2 pounds of N per bushel of canola — then a 50 bu./ac. crop requires 110 lb./ac. of available N during the growing season. Although with the suggested “range” it could require more or less, also hinging on rainfall and environmental conditions.  Required N will come from organic matter (OM) mineralization, soil residual N, and added fertilizer or manure.
2. The amount of N from mineralization will vary from year to year depending on environmental conditions and soil type, but around 8-10 pounds of N per percentage point of OM is a reasonable estimate. Topsoil with 3% OM will contribute 24-30 lb./ac. of N through mineralization.

A scenario: Canola with a target yield of 50 bu./ac. needs 110 lb./ac. of available N. If 25 lb./ac. of N comes from mineralization and 10 lb./ac. remains in the soil, as determined by a soil test, that leaves 75 lb./ac. to come from fertilizer. Given that some nitrogen is always lost through immobilization, volatilization, denitrification and leaching, the applied rate should probably be closer to 100 lb./ac. This recommendation will help allow for losses, some nutrient tie-up, and compensates for the range of nitrogen use.