Soil fertility is one of the basics of modern farming, yet during the busy off-season many growers fail to get the knowledge they need to make optimum decisions for the upcoming cropping seasons.

North Carolina Crop Consultant Danny Pierce has been monitoring soil quality for state farmers for more than 25 years. He says,” The practice of a fertilizer program that applies the same rate and kind of fertilizer on each field is no longer economically feasible. This practice just a few years back cost $20-$30 per acre, but now is $50-$75 per acre. Each field must be sampled each year.

“I work in the sandy Coastal Plain approximately 50 miles east of Raleigh, N.C. For row crops, many growers here broadcast 200-300 pounds per acre of a fertilizer containing a lot of potash, since a lot of our soils leach potash badly.

“Our soils vary greatly. Some are very sandy with clay more than 30 inches deep. In general, these soils need potash annually and actually the application should be split up. It should be put out at two different times.

“I have already sampled soils of this nature this year where 150 pounds of K20 was applied split and the potash level is very low — in some cases it is gone.

“On the opposite side of this are heavy soils (finer textured) with clay much closer to the top of the ground. They do not leach potash badly. If soil samples have been taken properly and the potash is high, no potash is needed on these soils.

“Throughout the Southeast there are an infinite number of soils between the two mentioned here. Some of these may also not need potash, while others may need 90 pounds per acre,” Pierce says.

“Just like with fertilizers, the cost of lime has made guesswork very costly. “Lime is approximately $50 per ton spread. We can’t afford to apply lime when we think there might be a need. Apply exactly what you need,” he adds.

With proper soil sampling growers can accurately determine how much fertilizer is needed. With accurate application equipment, especially variable rate applicators, soil test information can be a make or break input for many growers.

Two soil testing systems, zone sampling and grid sampling came on the scene in the mid-1990s and both have slowly added more soil fertility information for farmers over the past few years.

Grid sampling uses a systematic approach that divides the field into squares or rectangles of equal size (usually referred to as grid cells). Soil samples are collected from within each of these "cells." The location of each "grid cell" is usually geo-referenced using global positioning system technology.

Some of the soil and production criteria that favor the use of grid sampling include:

• A measure of non-mobile nutrients is the primary concern; with no movement, distribution will be affected less by topography and other fixed properties.

• The soil test levels in the field range from very high to very low with substantial acres in both the very high and very low categories; management practices used in the past will override natural variability.

• There is a history of manure use.

• Small fields have been merged into large fields; differences in past management may have larger influence on soil test levels than natural variability.

• The field history is not known.

Zone sampling uses a more subjective and intuitive approach to divide any field into smaller units. Soil samples collected at random from within each zone are bulked together and analyzed to provide an average sample value for each unit.

Criteria that favor the use of zone sampling include:

• Cost of sampling and analysis is a major concern; zones may be larger than grid cells thereby lowering sampling costs.

• A measure of mobile nutrients is the primary concern.

• Relatively low rates of fertilizer have been applied in recent years.

• There is no history of manure application.

• The history of the field is known and can be used to divide the field into smaller units; a more accurate judgment can be made when all available information is used.

Modern technology has provided some new tools that growers can take advantage of to maximize the investments they make in fertilizer and other valuable crop inputs.

Clemson University Researchers Ahmed Khalilian and Will Henderson have worked with a Veris machine that measures electric conductivity of the soil for the past few years, including some work on grower fields.

Though it sounds high tech, and the science behind it has been tech, the use of electrical conductivity, or EC, is really an extension of the good old common sense farmers have been using for thousands of years.

The Clemson scientists use a Veris 3100 machine that measures electricity movement in the soil. The higher the clay content, the better the soil conducts electricity, and thus the higher the EC number. Sandier soils have less conductivity. “Unless soil is in a noticeably wet area or erosion area, the higher the EC number, the more productive the soil will be,” Khalilian says.

Simple in design, the Veris system uses electrically-charged disks or coulters that send electric impulses to a box mounted on the machine's frame. One set of sensors is set at a 12-inch depth and a second set at 36 inches. The composite data provides an accurate understanding of soil textures up and down the root zone of a plant. By using global positioning satellite imagery maps and overlaying the Veris data, the researchers can build a field map that is laid out in zones based on the electrical conductivity recorded in the soil by the Veris machine.

High tech fertilizer application rigs with on-the-go monitoring of crop needs are now feasible and coming down in price, but this equipment is only as good as the information fed into it. Knowing what is going on with soil below the surface is critical to the success of high tech plant monitoring and variable rate application.