NDSU Extension - Griggs County


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August 24

The Extension Connection

By Megan Vig

It’s harvest time, and it is also time to think about soil sampling for next year’s crops.  This week I share information from Dave Franzen, NDSU Extension Soil Specialist on important considerations for soil sampling.

Time of sampling is an important consideration.  For soil nitrate there is no perfect nor stable time. Some years past, the effect of sampling time on soil nitrate values from August through April was investigated and found that at some sites, values decreased, some stayed the same, and some increased. There was no relationship between trend and rainfall. Any time is as good as any other time. You may have noticed that NDSU Nitrogen (N) recommendations carry a ‘plus or minus 30 lb./acre rate’ to final recommendations. This is part of it. However, it is very important to have a soil test number on which to base a N rate. If you do not have a number from a specific field (each field has a personality of soil fertility), then what you have is a not-so-educated guess.

For Phosphorus (P), soil pH, Electrical Conductivity (EC - salt index), Calcium Carbonate Equivalent (CCE), Organic Matter (OM), zinc and chloride, anytime is a good time. Sample for P and soil pH (and K) in an unworked field whenever possible, so the 0-6-inch sample core is consistent. Achieving consistent core depth with a worked field is very difficult, and in some cases, impossible.

Soil test Potassium (K) values vary through the season. Work conducted in Illinois, and now work conducted in North Dakota shows that highest K values are in early spring. As the season progresses, K values decrease, achieving their lowest values in August through mid-September, then increase until freeze-up. Sampling anytime is OK, but note when K sampling was last conducted, and then sample the same time of year for K in future. Sampling time for K is important. The soil test K values vary slowly with K fertilization, so probably every two years is good enough, although I would not argue with anyone wanting to sample every year.

Soil sampling for crops that are N yield dependent (small grains, corn, beets, potato, sunflower, canola, flax) require a soil test for nitrate-N. However, when high residual nitrate values occur before planting beans, it is recommended to test as well.   High soil nitrate (greater than 50 lb. N per acre) can increase the severity of IDC to soybeans and a grower needs to know if it is present.

                The days of a composite soil test should be over. Knowing how a field varies in fertility from boundary to boundary is manageable and economically advantageous. Zone sampling should be the rule in this state. The only part of a field where a grid of more than 1 sample per acre should be used is the area where high rates of manure have been applied within the past 20 years. A 2.5-acre grid is not a substitute for a good zone sampling and a 2.5-acre grid

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