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Soil Sampling- When, How, How Deep (08/18/16)

Soil sampling is an essential part of good crop nutrient management.

Soil Sampling- When, How, How Deep

                Soil sampling is an essential part of good crop nutrient management. The primary reason for soil testing in our region is for nitrate-N for crops that require N in our nutrient recommendation system. In my research over the past 20+ years, yields within an experiment due to N rate did not make sense unless the soil nitrate to 2 feet in depth was known. Nitrogen rate without a soil test is a poor guess. If a fence-row to fence-row excellent soybean field was harvested, and a guess of about 30 pounds residual N was made, that would be a pretty good guess. However, look at the number of fields with drown-outs, IDC distressed areas and other issues around the region, and then consider that zoning these fields for residual N would be an excellent idea.

                Sugar beet growers used to have their fields sampled to a 4 foot depth. Currently soil sampling tools do not have the capability to sample this deep. For sugar beet fields that have been managed for N well through long rotations, considering beet-top N credits, avoiding excessive N application and soil testing for corn and small grains and other crops in the rotation, the need for a 4 foot soil sample is low. We assume that about 30 pounds N per acre is in the 2-4 foot depth in these fields, and that assumption is almost always correct.                                     

                However, if a grower intends to plant sugar beet in a field where sugar beet has not been grown before, expect the 2-4 foot nitrate-N to be much higher and a 4 foot core taken somehow would be a critical management tool.

                There is no ‘sampling date adjustment’ for soil sampling for nitrate-N in our recommendations. A close examination of the change in soil nitrate from all points in North Dakota over a series of months to spring after small grain harvests showed that sometimes the nitrate-N value increased, sometimes it decreased and sometimes it stayed the same. So being unable to predict what it might do, whether taken in August, or November, taking the sample directly behind the combine whether August on, is better than not taking it all (much better). Yes, there is ‘fuzz’ around the data, but the ‘fuzz’ is built into the recommendation. Sampling before the chisel plow arrives also results in a much better 0-6 inch soil core and therefore a much better phosphorus and soil pH recommendation that can be traced through time.

                Soil potassium (K) levels change through the season. They are greatest in early spring after snow melt, decline rapidly after June1, are at the lowest point of the year between July and mid-September, then the K soil test gradually increases until permanent freeze-up. It is important for the best predictability of K need and predictability to choose a time of year to sample for K and stick with it over the rotation.

                Prediction of crop sulfur (S) availability through use of the soil S test is blind luck. It is the poorest soil test of any offered by any laboratory anywhere in the world and I would not use it anytime. Our region is greatly susceptible to S deficiency, and high rainfall in the previous fall, high snowfall or high spring rain would prompt my recommendation to apply some S to all but saline areas. This spring, despite low fall through early spring precipitation, S deficiencies were common on hilltops and slopes within fields not fertilized with sulfate/thiosulfate S.

                The preferred sampling method in this region is using a zone approach. There are several tools to use in order to prepare a field for zone sampling. A 2.5 acre grid might be acceptable in the ‘I’ states, because their soil P, K levels are all in the ‘high’ recommendation range, but ours are not. Our soil P and K levels are patterned due to topography and erosion artifacts and natural fertility influences of our largely under-fertilized crop history. Gridding in our region is a mistake unless the field has been heavily fertilized (100 pounds per acre of MAP is not heavy. 400 pounds per acre of MAP applied for a crop for several years is heavy) or has a consistent history of high manure application. To see how well a 2.5 acre grid might do, grid a field at the recommended 1 sample per acre grid (yes, 1 sample per acre grid), then do the back mapping to see that the 2.5 acre grid depiction might as well be a field in Saudi Arabia.

Dave Franzen

NDSU 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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