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Soil scientist outlines options for soil sampling

Aug. 25, 2014 | 0 comments


When it comes to collecting soil samples for analysis tests, farmers and crop consultants have options from where in the field the soil cores should be taken.

Those variations were described by University of Wisconsin-Extension Service soil scientist Carrie Laboski during a presentation at the summer twilight meeting of the Sheboygan County Forage Council at the Ramel families' Windy Hills Farms.

The conventional method was a whole field approach that involved taking 10 cores to represent 5-acre parcels in the field, Laboski said. The more refined methods that have evolved include taking 10 samples within a 10-foot radius in a grid format along with a computer-guided unaligned grid pattern.

Striving for accuracy

Laboski described grid point sampling for 5-acre parcels as "a bad idea" unless similar testing had been conducted within the past four years or if the field has a very high residual of phosphorus and potassium.

The top accuracy for phosphorus and potassium availability and additional need can be obtained with soil samples taken for every quarter-acre, she said. There is a 30-percent loss in accuracy when soil samples are taken to represent 1 or 2 acres. There can be gaps of 200 feet for sampled parcels covering 1 acre and of 300 feet for 2-acre parcels.

As an alternative, Laboski suggested zone or directed sampling in order to both save on testing costs and to improve accuracy. Zones with the likelihood of significant differences in soil traits and residual nutrients can be identified with the use of soil maps, yield monitoring data maps, electrical conductivity maps, remote sensory imagery from satellites and topography maps that are often correlated with soil maps, she said.

Other good sources for identifying zones of particular traits are the nutrient application history; the applications of lime and manure; and the combination of two or more former fields into one, Laboski said. She also mentioned drainage, field tiles and erosion as other factors that are worthy of recognition.

Following the zones

Once a zone is defined, one soil sample per 5 acres or 6 to 9 samples per 25 acres should suffice, Laboski said. Instead of gearing soil sampling to strict lines and parcel sizes, this revised approach should result in significant cost savings on soil sampling and testing. In many cases, she expects the costs for soil testing should pay off within two or three years.

To a question about taking soil sampling in fields where strip tillage is the practice, Laboski said the percentage of soil cores should be in line with how much of the soil surface is used for the strips and the total amount of soil surface.

The concept of using soil samples as the basis for variable rate applications of fertilizer has been around since the early 1990s, Laboski said. As the years have passed, equipment has been designed and developed to carry out that concept along with addressing the special situations that occur during field corner turns and other maneuvers by the equipment.

In turn, she said, it's appropriate to revise the approach to soil sampling by using those equipment capabilities along with other resources such as soil maps, crop yield maps and other data to recognize variability within fields, identify zones and track the results of management decisions based on that information.

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