With the beginning of winter wheat, rye and some spring wheat harvest, it is time to consider soil sampling for the 2019 crop year. The following are important to consider:
1. Sampling time
For soil nitrate, there is no perfect, stable time. Some years past, I investigated the effect of sampling time on soil nitrate values from August through April 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. Some of you 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 an 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.
Also, 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. I would 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 here 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 next time. 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.
2. What crops should have nitrate sampling?
Traditionally, all crops that are N yield dependent (small grains, corn, beets, potato, sunflower, canola, flax) require a soil test for nitrate-N. However, based on the high residual nitrate values before soybean last fall, I would also consider a nitrate soil test before soybean in the eastern 50 miles of the state in Iron Deficiency Chlorosis (IDC) country. High soil nitrate (greater than 50 lb N per acre) can increase the severity of IDC to soybean and a grower needs to know if it is present.
3. How to sample
The days of a composite soil test should be over. There is enough equipment around owned by farmers or their suppliers that 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 will not identify the fertility patterns within most fields. It works in Iowa because all of their variability has the same fertilizer recommendation; it’s all in the high range. In this state, variability is much larger and the variability is in soil test ranges requiring different rates.
4. Sampling in low pH soils
As addressed in an earlier 2018 Crop and Pest Report, the number of acres in North Dakota with soil pH values near or below 5 continues to increase, particularly in no-till fields that have had N applied to or near the surface for many years. When sampling these fields, it is important to sample the 0-2 inch depth, and the 2-6 inch depth for pH. Application of liming materials, such as beet lime will go onto the soil surface, and knowledge of pH with depth will not only provide information on whether a surface application will be effective, but the farmer can track pH progress after lime application.
5. Trends of Organic Matter Following Movement to No-Till
I smile when I read that a farmer increased organic matter a full point one or two years after transition to no-till. This is not possible. What happened is that whoever is taking the soil sample pushes the soil probe through the no-till residue, which is not really organic matter, and the residue becomes part of the soil sample result. Growers transitioning to no-till must insist that their soil sampling people get out of the truck, kick the residue aside and then sample the cleared area with a 0-6 inch core. That will provide a real trend in real organic matter.