Determining soil type important for successful preemergent weed control

Successful preemergent weed control with herbicides depends on properly identifying soil type and correctly applying the proper rate for that soil. How comfortable are you at making that call?

Timely herbicide application has become an important economic tool for most producers. When using soil-applied herbicides, achieving adequate weed control without crop damage is dependent on correctly applying proper rates at the proper time. Certain amounts of most of these herbicides are inactivated since they become bound by soil particles. That is why herbicide labels provide a range of application rates largely based on soil type with rate generally proportional to the soils cation exchange capacity (CEC). The lower the CEC, the lower the rate; the higher the CEC, the higher the rate. In a previous Michigan State University Extension article, “Anions and cations in plants, oh my! But why do we care?,” I described CEC and why it varies with soil type.

Most preemergent herbicide labels generally give four soil categories: coarse, medium, fine and organic (peat or muck). The challenge for growers is to determine which soil texture they have in their field. Again, knowing the CEC value of the site can help in making this decision. Table 1 provides soil textures, soil types and general CEC ranges for those soils listed. Note that there is overlap between CEC values for some soil types and there are also gaps. Percent organic matter (OM) also comes into play since it binds certain herbicides and is located on the soil surface where preemergent herbicides are active. The information in Table 1 is meant to be only one guideline in making your final decision on herbicide rates.

Table 1. General soil texture class and soil types in that class with the general cation exchange capacity range.

Texture

Soil type

General CEC range

Coarse

Sands

Loamy sands

Sandy loams

1-5 light colored

5-10 dark colored

Medium

Sandy clay loams

Sandy clays

Loams

Silt loams

Silts

11-15 light colored

15-20 dark colored

Fine

Silty clay loams

Silty clays

Clay loams

Clays

20-25+

Organic

Peat

Muck

50-100

CEC is a soil characteristic provided by all soil tests. In a glaciated state like Michigan, it is possible that a field of any size can have a variety of soil types and therefore a variety of CEC values. Generally, there is only a small CEC difference when transitioning between adjacent soil types, but it is possible different herbicide rates may need to be applied to different areas of the field to get the desired weed control. This is especially true if the field in question changes elevation significantly.

MSU Extension annually publishes herbicide recommendations for fruit, vegetable and field crops. These bulletins can be found with a search at the MSU Extension Bookstore website and obtained through your local county MSU Extension office.

For more information on commercial vegetable production, contact Ron Goldy at 269-944-1477 ext. 207 or .(JavaScript must be enabled to view this email address).’

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