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Soil Sampling Season Begins for 2020 Crop Year (08/15/19)

With the beginning of winter wheat, rye and some spring wheat harvest, it is time to consider soil sampling for the 2019 crop year.

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 to soil sample. Some years past, I reviewed NDSU research into the effect of sampling time on soil nitrate values from August through April the following year and found that some values decreased, some stayed the same, and some increased, depending on site sampled. There was no relationship between trend and rainfall. Any time is as good as any other time. Some of you have noticed that NDSU N recommendations carry a ‘plus or minus 30 lb./acre rate’ to final recommendations. The ‘fuzz’ around the nitrate test with time is part of it. However, it is very important to have a soil test value on which to base an N rate. If you do not have a value or zone values for a specific field (each field has a ‘personality’ of soil fertility), then what you have is a not-very-educated guess.

Also, for P, soil pH, EC, CCE, OM, zinc and chloride, anytime is a good time to sample. I would sample for P and soil pH (and K) in an untilled field whenever possible, so the 0-6 inch sample core is consistent. Achieving consistent core depth in a tilled field is very difficult, and in some cases, impossible.

Soil test K values vary through the season. Work in Illinois, and now work at NDSU 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 the values tend to increase until freeze-up. Variation appears to be less in moist seasons and in illitic-dominated soils. Sampling anytime for K is okay, but note when K sampling was last conducted, and then sample the same time of year the next time 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-rate yield dependent (small grains, corn, sugarbeets, potato, sunflower, canola, flax, dry bean) require a soil test for nitrate-N. However, based on the high residual nitrate values before soybean in some years, I would also consider a nitrate soil test before soybean in the eastern 50 miles of the state in IDC country. High soil nitrate (greater than 50 lb N per acre) can increase the severity of IDC in 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 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 and other Corn Belt states because most of their P and K variability has the same fertilizer recommendation; it’s all in the high range. In this state, variability is much greater, and the variability is in soil test ranges requiring different rates.

4. Sampling in low pH soils

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, zone sampling should be the method, and 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 should 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. Ongoing research at the Dickinson and North Central R&E Centers so far indicate that surface lime

application is effective in overcoming aluminum (Al) toxicity. This agrees with previous studies in the US. It may also be wise to analyze for base saturation using the cation addition method for cation exchange capacity, as magnesium (Mg) values may be low in some soils. If Mg values are low, a beet lime or another calcitic liming source like city water treatment lime would only help the pH, but it might result in a Mg deficiency down the road.

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 unless substantial rates of manure have been applied. What usually has 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 sample 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 analysis of the real organic matter. One percent organic matter in 6 inches of soil weighs about 20,000 pounds/acre. The conversion from organic matter to organic carbon using NRCS figures is a factor of 0.58, so the organic C in 6 inches of soil is 11,600 pounds/acre. The C/N ratio of organic matter is about 10:1, so the N in 1% organic matter is 11,600/10, or 1,160 pounds. Therefore, it is not reasonable to think that a few years of no-till and a cover crop will result in that much C AND N becoming part of the soil; in a decade perhaps, but not in a few years. One farm that I follow has been in no-till for over 40 years, and the farmer started using cover crops about 20 years ago. The organic matter on some of his fields has increased from about 3% to well over 6% today.


Dave Franzen

Extension Soil Specialist


This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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