Farm Progress

Spring into fall soil sampling now

Eye on Crops: Fall soil samples will reveal any phosphorus or potassium deficiencies.

August 29, 2017

3 Min Read
SAMPLES AND MAPS: Use soil samples and yield maps together to determine crop nutrient amounts for next season based on actual numbers rather than estimates. If nutrients are needed, then you can either apply them any time after harvest, or wait until spring prior to planting.

By Mark Glady

Continued low commodity prices may be prompting you to look for ways to reduce nutrient costs in 2018.

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MARK GLADY

Identifying fields that might be able to handle a lower fertilizer rate — as well as those that cannot — will help you more accurately apply nutrients where they’re needed, and pull back where you can, without jeopardizing return on investment potential.

Especially in no-till or reduced-till systems, or if you do variable-rate fertilization, comprehensive soil samples are imperative to achieve more accurate fertilizer and soil amendment application. And once this year’s crop is removed, it’s easier to gather soil samples and evaluate whether available nutrients will support next year’s crop.

Levels to look for
Fall soil samples will reveal any phosphorus or potassium deficiencies, while yield maps will show the amount of nutrients removed by this season’s crop. Use soil samples and yield maps together to help plan crop nutrient amounts for next season based on actual numbers rather than estimates. If samples identify phosphorus or potassium deficiencies, you can either apply them any time after harvest, or wait until spring prior to planting.

It is also particularly important to check soil pH levels in the fall, since pH has a direct effect on soil nutrient availability.

Consider a partial nitrogen application
Some farmers are turning to partial nitrogen applications in the fall, then testing soil nitrate levels in the spring to determine the additional amount needed to meet next season’s crop performance goals. Fall nitrogen applications should not be made until soil temperature cools to 50 degrees F or lower, when soil bacteria begin to go dormant. In the fall, nitrogen must be applied with a stabilizer to prevent it from converting into a mobile nitrate form that could move off target.

Tips for achieving accurate results
As you work with your agronomist to collect soil samples, here are some rules of the road.

1. In conventional tillage fields, take soil samples at a consistent depth of 6 to 8 inches across the field. Making a permanent mark on the probe at the selected depth will help ensure a consistent sampling. Being off by just an inch across a field can make a significant difference in accuracy.

2. In no-till fields, take two samples in each location, with the first sample at 2 inches and the second sample at 6 inches. This helps determine which nutrients have worked their way deeper into the soil.

3. Pull 12 to 15 individual cores for each composite sample. Submit one or two composite samples for testing per 80-acre field.

4. For variable-rate nutrient applications, divide the field into 2.5-acre grids and take four to six core samples per grid. Grid sampling data will help find the precise amount of plant nutrients needed in each area of the field for greater application efficiency and optimal ROI potential. 

5. Use a plastic bucket when collecting samples to ensure accurate results. The zinc in galvanized pails can raise zinc level readings and skew results.

Be sure to allow adequate time for soil test results to be returned from the lab, so that your agronomist can promptly interpret them and make timely recommendations. Sampling time is relatively flexible. However, don’t wait too long — especially in central and northern portions of Minnesota, where a sudden drop in temperature could make for impenetrable soil that’s too cold to accurately test. Also, sample at the same time each year if you intend to compare results from one year to the next.

Glady is a regional agronomist with WinField United in southwestern Minnesota. Contact him at [email protected].

 

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