Electromagnetic Mapping

All soils consist of sand, silt and clay. The difference between soils is their percentage of these properties and the types of clay that constitute them. Sand, silt and clay have different conductivity or resistivity which is measured by the EM instrument. MISS uses a non-contact DualEM sensor that uses electromagnetic induction (EMI) to map apparent electrical conductivity. The sensor is pulled in a plastic sled and takes a simultaneous topsoil and subsoil reading every second. This information makes a precise map to be used in variable rate nitrogen, variable rate irrigation (VRI), determining ideal moisture probe placement and creating zones for sampling as well as fertilizer, lime and seed applications. With thousands of data points collected per field the maps are much more accurate than the NRCS maps.
The point cannot be emphasized enough that an EM map is a must for any variable rate application and positively essential for some agronomy practices such as variable rate irrigation.
Two layers at different soil depths are collected at the same time.

Top Soil EM Mapping

Topsoil EM measures roughly 1/3-meter deep which help define the soil which then can be used for yield goals, nitrogen usages, variable rate planting, along with other agronomic factors in the field. (The majority of the plant’s roots are located in the top soil layer).

The following maps are from Northwest Iowa.

Top Soil EM

NRCS soil type map

NRCS Soil Type Map

Soybean yield

Soybean yield 2003

Corn Yield

Corn Yield 1999

 

 

 

 

The maps shown above display the correlation that an accurate soils map has with yield. Some of the reasons for this could be characteristics, such as water holding capacity of the soil, basic productivity of the soil and emergence problems due to soil characteristics. As a general rule, EM soil maps in the MISS trade area correlate closely with yield maps as the second set of figures also show. Therefore, using EM soil maps are a very useful tool in agronomic management for change of seeding rates, using different varieties of seed, applying different amounts of chemical or fertilizer or any situation that calls for soil information. The USDA/NRCS maps are still very valuable for providing information on productivity, permeability and other soil type characteristics.

yield map

Yield Map

EC soil map

EM Soil Map

nrcs soil map

NRCS Soil Map

Generally, the lower the EM reading the more sand content and, thus, poorer water holding capacity. The higher the reading, the more clay content and higher water holding capacity; therefore, the soil is usually more productive. However, especially in glacial till soil, the highest readings are not necessarily the most productive. These soils can have very fine clay particles that have very poor internal drainage, causing lower productivity from denitrification in wet soils and poor root penetration.

MISS has mapped a number of fields under very wet and very dry conditions. The resulting maps are slightly different, as expected, but are close enough in results that either map could be used for creating management zones. MISS uses EM technology in the spring and summer when the soil has completely thawed and are at preferred planting conditions.

Subsoil EM Mapping

Subsoil EM measures soil types at a deeper depth. The composition of the subsoil is critical in the rate of water infiltration, root penetration, microbial activity, etc… To make variable rate irrigation possible, knowing the subsoil characteristics is crucial. Also by seeing the soil type at 1-meter depth, a tile drainage contractor can utilize this method of mapping for defining the areas of sand, silt and clay that may alter a drainage design.
Subsoil EM Soil Mapping allows tile designers to look into the soil profile without setting a foot in the field. Soil borings may be taken in advance in these “pockets” to determine the percentage of sand, silt and clay in the soil. With this information, the tile designer can change the tile spacing or the type of tile to be installed depending on the soil profile. Better tile design produces better drainage. Below are some examples of the soil profile at a 3-meter depth and soil borings with their respective sand, silt and clay.
top soil subsoil surface maps
Although the above shallow and deep surface maps look similar, they are very different. A drainage system based and designed on the shallow surface information would not be a good match for the deep soil profile.
partcial separation value map
This example shows the deep surface break lines and the corresponding soil boring locations and values. The same information can be used for drain tile Installation or variable rate irrigation to exactly define the soil properties as identified with the EM system.

Sampler pulling EM Sled

New EM and elevation mapping system using Trimble GPS and display equipment and DualEM sensor

New EM and elevation mapping system using Trimble GPS and display equipment and DualEM sensor

 

Additional Resources
Site-Specific Management Guidelines – Soil Electrical Conductivity Mapping
Precision Farming Tools: Soil Electrical Conductivity