Soil Sampling Tips

Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

Fall is the time of year that most soil samples are taken and soil test results are only as good as the sample that was taken. The following are some reminders for taking good soil samples.

● Use a soil coring tool (soil sampling probe). Always use a soil probe designed for soil sampling. Insert the probe into the ground straight (upright and perpendicular to the soil) and not at an angle. Start with clean plastic buckets and clean soil probes (avoid contamination). Thoroughly discard the portion of soil that is not used for the composite sample between each sampling.

● Take soil samples from a uniform depth. Standard soil tests have been calibrated at the depth of the plow layer (6-8 inches), no shallower and no deeper. A 7 inch depth should be targeted. Cores taken from varying depths will give more variable results. Cores that are too shallow will skew results toward higher soil nutrient levels. Too shallow of sampling is often an issue in dry, hard soils. Cores that are too deep will give lower nutrient levels. Avoid excess surface organic materials in the core. Discard any cores that you cannot take from the proper depth. Discard any cores that appear unusual in any way. If the core cannot be extracted intact, discard it.

● In no-till you should take a standard 7″ depth sample. You may also want to take a separate 2″ depth sample to check for pH depression on the surface.

● Ideally, a core should be taken no less than one every acre. Ideally, a minimum of 20 cores should be taken and composited for a sample. This will reduce the effects of an errant core (however, more than 30 is generally overkill). When you mix the cores to form the samples, make sure that you have mixed them well before you take the subsample to send to the lab. All clods or core pieces need to be broken up. The most common mistake in soil sampling is not mixing cores adequately.

● Ideally, large fields should be divided up so that a sample represents no more than 20 acre units. So for a 100 acre field, you should have 5 samples, each composited from 20 or more cores. This is especially critical in fields that have high variability. Very uniform fields can be sampled in larger acreage increments (but no more than 40 acres).

● The sampling pattern in a field area being sampled should be representative of the site. You cannot just walk diagonal across a field. Each soil sample should represent only one soil type or soil condition. Different soil types within a field should be sampled separately and areas treated differently should be sampled separately (areas where different crops were planted, areas where different varieties were grown with widely different yields, different planting dates, different fertilization, etc.).

● If fertilizer has been banded in a field, samples should stay off of the band if at all possible. If the band is not known, than plan to take extra cores. This also applies where plastic mulched beds were used for vegetable production.

● Avoid any features in a field that might skew the test results. Examples would be areas where spoil was spread, old roads or fence lines, areas where buildings stood, wet pockets, or poultry manure stockpile areas. Do not take cores from these areas for the field composite. If you are interested in the fertility of these areas, take separate samples.

● Intensive grid sampling is used for variable rate fertilizer and lime applications. For grid sampling large fields, sample 2.5 acre units with 5 or more cores from each unit forming the sample. For smaller fields that are grid sampled, use 1 acre units.

● Take samples at least once every crop rotation cycle with a minimum of once every 3 years. In a corn-wheat-soybean rotation or corn-soybean rotation, take samples once every 2 years. Consider yearly sampling in rotations that vary considerably, where vegetables are included, or where other high value crops (fruits, nursery, specialty crops, etc) are being grown. Yearly samples are also appropriate for intensively managed cropping systems and high yielding irrigated production. Perennial hay and pasture fields should be sampled at least once every 3 years.

● Target soil sampling at the same time of year each time you take the sample and at the same point of time in the crop rotation (for example, every 2 years after corn and before wheat in a corn-wheat-soybean rotation). Time of year is not as important as being consistent in when you take the sample. Late summer or fall, after crop harvest, is an ideal time to take samples for practical reasons.

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