Post Harvest Assessment Tools of Soil Fertility Practices

Prepared by John Heard, MB Ag Soil Fertility Specialist

As harvest nears completion, it is time to take stock of the past year’s soil fertility program. The grower’s financial yardstick of soil fertility success is the amount and quality of harvested yield. A couple of other post harvest plant analysis, plant and soil measures can be used to gauge the appropriateness of your grower’s current fertility program. These tools include grain protein, stalk nitrate levels for corn and forage feed analyses. Some special considerations may be needed in interpreting these results due to this year’s continued wet weather.

Grain protein

Wheat protein can provide a general measure of whether sufficient nitrogen was supplied for the crop to meet full yield potential. If hard red spring wheat is less than 13.5% protein¹, or hard red winter wheat is less than 11.0-11.5% protein², then insufficient nitrogen was supplied to meet that crop’s yield potential. One shouldn’t consider these values as actual final targets for protein since it is often profitable to apply more N than this in order to capitalize on protein premiums for spring wheat.

For planning purposes it is important to identify the possible causes of low protein in what was otherwise an appropriately fertilized crop. Possible reasons for wheat protein falling below these benchmark values may be:

• Growing conditions that permitted yield potentials exceeding the supplied fertilizer level (these were observed on fields with good natural surface and internal drainage).
• Losses of nitrogen through higher risk application methods such as fall applications, broadcast rather than banded or surface applications. Most losses this year will be associated with excessive wetness (i.e., leaching below the root zone or gaseous loss through denitrification).
• Insufficient nitrogen supply, especially for newer, very high yield potential varities.

Stalk nitrate test in corn(³)

This is a test to gauge the success of nitrogen fertilization in corn. Nitrogen taken up by the corn plant tends to accumulate as the nitrate form in the lower stalk. During grain-fill, plants running low in N, mobilize more of this N out of the stalk. Conversely if the plant has ample N, it draws sparingly on this N, and if nitrogen is available in surplus amounts, then nitrate levels can be quite high in the lower stalk. This amount of nitrate-N remaining in the stalk at maturity can be compared to benchmark values to indicate whether the crop was inadequately, excessively or appropriately fertilized with nitrogen.

Many of the soil test labs currently offer this test. An 8” section of the corn stalk is cut from the 6-14” height above the soil. Our Manitoba testing of this technique has found it to be useful as long as one samples within a couple weeks after reaching maturity (kernel black layer). If sampling is delayed too long after harvest and rainfall continues, it is possible that nitrate may leach from the stalk, producing low stalk levels.

Forage analysis

Few forage producers conduct in-season tissue testing of the crop, yet many take a feed analysis before exporting high-value dairy hay or feeding their own livestock. The feed analysis can provide a general measure of crop nutrient sufficiency. These feed tests do not provide a fertilizer recommendation amount but will underline the need for a soil test that can do so.

Table 1. Critical nutrient levels of nutrient sufficiency.

From Manitoba Soil Fertility Guide⁴. Table 18.

Forages testing lower than these levels in Table 1 might be suspected to be inadequately fertilized for yield potential. Legumes do take up and require greater levels of nutrient cations such as potassium, calcium and magnesium, which may make interpretation of mixed grass/legume stands more difficult.

But factors other than inadequate fertilization can lead to lower forage nutrient levels. For example rained-on forage will tend to have potassium leached from the leaves. And rained on hay that is excessively raked will have greater leaf loss resulting in lower crude protein and N levels. Similarly very mature crops will tend to have lower nutrient levels than early cut and harvested crops.

Summary

A number of post harvest plant tissue assessment techniques may aid advisors and growers in evaluating their fertilization practices. Recognize that our very wet conditions may challenge the reliability of some of these techniques.

Routine fall soil testing should still remain the standard in planning the next year's fertility program.

References

1. Flaten and Racz. 1997. Nitrogen fertility and protein in red spring wheat

2. Heard and Gares. 2000. Nitrogen sufficiency in winter wheat for yield and protein based on soil, tissue and chlorophyll measurements.

3. Heard. The end-of-season stalk nitrate test for corn: A novel approach to evaluating your fertility program

4. Manitoba Soil Fertility Guide.