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Grid sampling can improve soil tests
By Bonnie Coblentz
MISSISSIPPI STATE -- Soil sampling provides a classic example of how a "one size fits all" approach to farming often is not a good idea.
Soil sampling involves taking small amounts of soil from representative areas of a garden or field and chemically processing it. The process determines the crop nutrient status of the soil, and provides a basis for recommendations on improving its production capability.
Larry Oldham, soil specialist with the Mississippi State University Extension Service, said soil samples should be taken at least every three years, but an annual test is best.
"The sticking point is to take the samples during the same time each year," Oldham said. "There are biological and chemical variations within the soil throughout the year that affect the results. Samples can be taken either in the spring or fall, as long as you maintain that schedule."
Traditionally, producers treat an entire field based on the findings from samples in one area. However, the MSU Extension Service recommends that samples represent 20 or fewer acres.
Grid sampling and the related zone sampling both go beyond traditional soil sampling to take advantage of remote sensing capabilities, software analysis and variable-rate applicators. Each method allows for site-specific treatment.
"Grid sampling is a way to study a field more intensively through soil sampling because the samples are taken from smaller units of the whole field, so you end up with more soil samples per unit area," Oldham said. "Often the smaller units within a field are plotted and identified using computerized techniques."
An increasingly popular variant of this is zone sampling, where fields are broken into smaller areas for sampling based on some feature. This feature may be color, soil texture, landscape position, or remotely sensed data on moisture content or electrical conductivity. Zone identification also may use National Resource Conservation Service soil survey information.
Either of these sampling methods can be used on areas smaller than 1 acre to as large as 20 acres.
"Grid or zone sampling provides a much more intensive array of information about a particular field," Oldham said.
Grid and zone sampling services are available commercially, but what to do with this information depends on the goals, needs and capabilities of the producer. Oldham said it has been challenging to use this wealth of information adequately.
"Variable rate liming based on intensive soil sampling has been promising in several Delta situations over the past few years," Oldham said.
Jimbo Burkhalter, Tallahatchie County Extension director, cited a recent example of the increased cost of grid sampling paying dividends for an area farmer.
"A cotton producer called me requesting assistance in determining a problem in one of his cotton fields. The yield was 444 pounds per acre, while the rest of his fields were picking from 900 to 1,200 pounds per acre," Burkhalter said.
A soil pH test showed the soil to be low in lime and recommended the producer add 1.5 to 2 tons of lime per acre on 90 of the 120 acres tested.
"The producer and I discussed the results, and we decided to try grid sampling," Burkhalter said. "The soil sample was run in 2.5-acre grids on the same 120 acres, and the results showed the pH was low on just 32 of the 120 acres."
Adding lime to 90 acres according to the first soil test would have cost $2,340. The grid soil test cost $960, but it called for just $832 of lime, bringing the soil tests and lime application cost to just under $1,800.
"The savings in the amount of lime that was needed more than paid for the $8 cost per acre to grid sample," Burkhalter said.