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Proper soil sampling ensures correct fertilizer management

Proper soil sampling is the first step in cropland fertility management, a vital ingredient for maximum crop production with minimum expense, especially with soaring fertilizer prices.

“Texas High Plains producers generally do not soil sample often enough, and their usual sampling techniques may not be adequate,” says Kevin Bronson, professor of soil fertility and nutrient management at the Texas AgriLife Research and Extension Center at Lubbock.

According to Bronson, irrigated fields should be sampled for nitrate nitrogen every year, and dryland fields should be sampled every two to three years. Both irrigated and dryland fields should be sampled every two to three years for phosphorus, potassium, and micronutrients.

“Improper sampling can result in incorrect fertilizer applications, loss of production and profits, and can cause nutrient contamination issues,” Bronson said.

Soil sampling locations should be based on soil type, topography, and/or zones defined in yield maps.

“A distinct zone should be delineated and sampled anywhere there is an obvious change in the land characteristics, such as topography or soil type. But within a given zone, the land characteristics should be relatively uniform,” Bronson said.

Site-specific sampling should be used when a potential exists for fertilizer levels to be much higher or lower than that of adjacent areas. For example, if manure has been applied to various portions of the field, that area should be in a separate zone.

“Using site-specific sampling can help mitigate excess nitrogen buildup in the subsoil and minimize losses below the root zone,” Bronson said. “Avoiding excessive phosphorus fertilizer applications by site-specific sampling and fertilization can minimize movement of phosphorus in surface runoff.

“To get a reliable estimate of nitrate-nitrogen, samples should be taken from both the zero- to 6-inch layer and the 6-inch to 24-inch layer because clayey subsoils can contain up to 70 pounds per acre of nitrate nitrogen, about four times as much as we usually find in the zero- to 6-inch top layer. If the producer sampled only the top 6 inches, he would likely apply far more nitrogen than needed. Not only is the application of unneeded nitrogen an unnecessary expense, the nitrogen may be lost below the effective root zone,” Bronson said.

Sampling the top six inches of soil is sufficient for phosphorus, potassium, micronutrients, and pH analyses. If phosphorus has been banded in previous applications, soil samples should be taken from the bottom of the furrow, top of the bed, and sides of the bed.

Soil samples can be taken with a shovel, auger, or tube.

“If a shovel is used to take a zero- to 6-inch sample, a v-shaped hole is dug and a 1-inch-thick slice is taken from the smooth side of the hole. A trowel can be used to take from the center of the slice a sample approximately one-inch square and six inches long,” Bronson said.

“If a 6-inch to 24-inch sample is also taken, a tube or auger should be used. Take the zero- to 6-inch sample first, then, in the same hole, take the 6-inch to 24-inch sample. The samples from the two different depths should be kept separate,” Bronson said.

Four to six samples should be taken from each zone, deposited in a clean plastic bucket or paper container, and mixed thoroughly. A single sample of about one pint should be taken, stored in a moisture-proof bag, and appropriately labeled including the depth: “zero to 6 inches,” or “6 to 24 inches.”

“A map showing where the samples were taken should be made and coded so analyses results can be matched to the proper zone. These maps should be filed and reviewed each time an additional soil sampling is performed. This will enable the producer to see changes in fertility levels over time, and will facilitate improved management practices,” Bronson said.

Additional information about soil testing, fertilizer rates, etc., for High Plains producers may be obtained from the Soil Fertility site at

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