Early fall soil tests and fertilizer applications

September 27, 2017 - Issue 27

Some growers are already doing soil tests and making fall fertilizer applications. While this may not be the ideal timing for soil tests or fertilizer applications (closer to freeze up is better), this timing may satisfy a number of other issues, including custom soil-sampler availability, wanting test results before fall application, and checking to see if current soil nutrient levels could explain lower- or higher-than-expected yields this year.

Soil testing now. A key with soil testing is to be as consistent as possible for year over year comparisons. If you always soil sample in late September, then keep going with that practice. By early October, soil will be cooling and the microbial activity that can influence soil nutrient availability will be slowing down.

Sampling moist soils. Any time you sample wet soil, available nitrogen in the sample will change rapidly. Try to air dry it (lay samples on a piece of wax paper) or refrigerate the samples and send them to the lab as soon as possible. Sampling to greater depths (24”) can be useful following wet years if there is a possibility that mobile nutrients have leached during the growing season.

Sampling dry soils. If you can get the probe into dry soils, these tests can actually be more valuable than a soil test after more “normal” moisture and yield results. That’s because in dry conditions with low yields, soil nutrient reserves may be higher than expected. This can save money because fertilizer application rates for next season may not need to be as high as usual.

Fertilizing now. Fertilizing early in the fall can be more economically risky because N losses can be higher. If banding ammonia into warm, moist fall soils with potential for many more warm days to come, losses from gassing off can be high. Investing in a stabilizer product can preserve more of the fertilizer for the spring. Banding is preferred to broadcast applications, and deeper bands will also help reduce N losses.

Further reading:

Timing and depth for fall fertilizer
How to reduce fall fertilizer N loss
Canola Encyclopedia section on fall fertilizer

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