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Soil Test Information
How Often To Test
Soil nitrate-N levels can change drastically from year to year and from field to field.
As a result all fields should be tested for nitrate-N every year.
Time
Sample anytime after small grain harvest and before planting for all tests involving
nitrate-N. For testing conducted before September 15, add 1/2 lb/acre of nitrate-N for
each day the test was taken before that date. For example, a soil test taken on August 16
would subtract 30 X 0.5 = 15 lb/acre nitrate-N from the soil test N recommendation because
of the sampling date adjustment.
Tests involving P and K or salinity sampling may be done any time. Take note of previous crop for future soil test history.
Where to sample
To represent the majority of a field through a composite test-
The soil samples submitted from each area must be representative of that area. To obtain a representative set of two samples, from the 0-6 and 6-24 inch depth, randomly select at least 20 places in the area to be sampled. Dry the soil, crush as fine as possible, mix thoroughly and submit about 1/2 pound to the soil lab. Do not submit pieces of soil cores from a random handful out of a bucket.
To represent management zones-
Choose management zones from knowledge of topography, gross yield monitor histories,
aerial photographs or satellite images of crop growth habits, especially in very dry or
very wet years and personal knowledge of past crop and farming history. In each zone, take
a minimum of 8 soil cores to mix for lab analysis. All soil must be ground and mixed for
analysis, not a handful of core pieces.
To sample by grid-
Grid sampling should be as dense as practical. For 0-6 inch core and with a hand probe
and analysis of non-mobile nutrients (P, K, pH, OM), an initial sampling of at least 1
sample per acre is recommended. For 0-24 inch sampling or deeper that is necessary each
year, less dense grids are often used, but should not be considered as stand-alone
fertility patterns when the maps come back. A 4-5 acre grid should be considered a survey
of variability, and combined with future topography, aerial images, yield monitor
histories, and other pattern revealing methods to determine future sampling zones.
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