Measures clay content: Mapping cart shows soil textures

Its generic name is a mouthful — soil electrical conductivity mapping cart — so it's no wonder that farmers and researchers have taken to calling it simply the Veris machine, taken from the manufacturer, Veris Technologies, Salina, Kan.

The carts have created quite a buzz in variable-rate circles for their ability to measure clay content in soil. Combined with GPS, the carts can provide very detailed field maps showing the distribution of soil texture in a field — from sandy to heavy clay. These maps have a number of variable-rate application uses, according to Maurice Wolcott, a researcher at the LSU AgCenter, Baton Rouge, La.

Wolcott, who has worked with soil electrical conductivity for seven years, is participating in a LSU AgCenter project to help bring precision agriculture techniques to Mid-South agriculture. In these studies, soil electrical conductivity mapping carts have been used to map soil texture for potential variable-rate applications — from nematicides to plant growth regulators.

Soil electrical conductivity carts consist of six rolling coulters. Two are sending-coulters which transmit a measured small electrical charge. When the coulters come in contact with the soil, this current is conducted by the soil to receiving-coulters. The difference between the charge leaving the sending-coulter and measured by the receiving-coulter represents the soil's capacity to conduct electrical current at that location.

Soil electrical conductivity technology is not new. Original soil electrical conductivity sensing devices were pulled to a spot by a tractor, then dropped into the soil for a reading. Lately, sensors have become mobile with adaptations to coulters and GPS technology, which provides precise location updates as the device rolls through the field. Soil electrical conductivity carts can collect between 60 and 100 data points per acre — depending on speed and swath width.

The devices are useful in California to detect areas of fields that have salt buildup due to irrigation, according to Wolcott. “In that situation, salt becomes the dominant conductivity factor. It will dominate the other soil properties.”

In the alluvial soils of the Mid-South, clay content is the most dominant factor driving changes in soil electrical conductivity, according to Wolcott. Soil electrical conductivity mapping carts can create a very detailed map for a number of variable-rate field activities in the Mid-South.

“I know of several individuals using these maps for variable-rate nitrogen applications,” Wolcott said. “They want to apply more nitrogen in the heavier soils in the field and less nitrogen in the lighter soils in the field.”

The soil maps can be used for variable-rate planting in cotton, “in areas of fields with ideal textures for seed germination and planting, where the plant populations can be reduced a little bit. When you get into heavier zones where you have more of a problem maintaining good seed-to-soil contact, you want to get the seeding rate up a little bit. The maps can also be used for directed soil sampling, or zone sampling, for fertility management.

“We could be using this technology even more extensively if we were applying more soil-applied herbicides with residual effects. For example, with the application of Sencor- and Lexone-type materials to soybeans or Cotoran to cotton in highly variable fields, one rate just did not fit all. But now that we've moved to the Roundup technology, we're not using as many soil-applied residual herbicides.”

Nematode management is another area with possibilities. “There are some good relationships between nematode damage, yield suppression and soil textures.”

Information gathered by the mapping carts should be ground-truthed, or verified, according to Wolcott, “even though some farmers who have been farming fields for a long period of time can quite often ground-truth from their own historical knowledge of the fields.”

An soil electrical conductivity image has some similarity to a true bare soil aerial image, “except an aerial soil image does not tell you what goes on deeper in the profile. The clay area that's showing up dark on an image may have a sandy subsoil underneath.”

Soil electrical conductivity carts see deeper than an aerial image, according to Wolcott. “Coulters that are placed closer to the sending-coulter will generally look at the top foot of soil. Coulters that are placed farther away from the sending-coulter give you an idea of what's going on down to 36 inches.”

The ability of the machine to reliably conduct electricity is based somewhat on soil temperature and soil moisture, noted Wolcott. “The field should be dry enough that you can get in the field and do tillage operations, but with enough moisture to germinate seed.”

Adequate moisture is especially critical in heavy clay soils that can crack when dry and give lower readings. “When we have adequate moisture, we have a straight line relationship with clay content.”

As for most variable-rate implements, the economics of soil electrical conductivity mapping depends on the degree of variability in the field, according to Wolcott. In addition, “if the producer is soil electrical conductivity mapping as a base layer for management decisions and he's able to use the same data set for 10 years or more across several applications, the data is very cheap. The more you use it, the more you spread the costs over all the different uses.”

There are indirect cost benefits as well, according to Wolcott. “A producer may have highly variable soils, but he's fertilizing at the maximum nitrogen rate to get the rate he needs in the heavier soil. At the same time, he's over-fertilizing in the lighter soils, which he has to fight with numerous applications of plant growth regulators all season. With variable-rate applications, we can treat the cause, not the symptoms.”

Wolcott notes that most soil electrical conductivity mapping is provided as a service to farmers, “even though there are farmers who have bought and used the devices. I know of several consultants and service providers that are mapping electrical conductivity as a service.”

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