By Misha Manuchehri and Brian Arnall, Oklahoma State University Extension

There are many factors that influence the persistence and uptake of a herbicide that has soil activity. One of those factors is soil pH or the amount of hydrogen (H) ions present in the soil solution. Some herbicides will persist for an extended amount of time or rapidly degrade when outside the pH window of 6.0-7.0.

The triazines (atrazine, simazine, etc.) and sulfonylureas (chlorsulfuron, metsulfuron, etc.) are two herbicide chemical families that are especially affected by soil pH (Table 1). The dinitroanilines, and the active ingredient clomazone also can be affected by low and high soil pH; however, degradation by light and/or volatility are more important when it comes to the activity of these herbicides.

Generally, the triazines and sulfonylureas persist longer and are more available for plant uptake in higher pH soils (>7.0) while the opposite is true for imidazolinone herbicides (imazamox, imazapic, imazethapyr, etc.). Imidazolinones persist and are more available for plant uptake in lower pH soils (<6.0). 

The persistence of the triazines and sulfonylureas in high pH soils is a result of a decrease in chemical and microbial breakdown, a trend that is often observed in high pH soils where neutral herbicide molecules are loosely adsorbed to the soil and are more available for plant uptake. Conversely, in low pH soils, triazine and sulfonylurea herbicides become charged and are more tightly adsorbed to the soil where they are more susceptible to breakdown.

A key management factor that must be considered when evaluating a field’s soil pH is whether or not the field is no-till and for how long it has been in no-till. Tillage will impact how deep you should take soil samples to determine soil pH.

pH and herbicidesIn no-till and minimum tillage fields, the traditional method of 0-6 inch or 0-8 inch soil cores may not be adequate. Instead, a 0-2 inch core depth and a 2-6 inch core depth may be needed, since application of limestone to the surface may increase surface pH more than expected or application of nitrogen fertilizer to the surface may cause a drop in pH at the surface.

In many long term no-till fields with historic surface applications of N and no lime applications, soil pHs in the low 4s have been observed while the 3-6” depth will be at a 6.0. Since herbicides with a soil residual are affecting plants just below the soil surface, this is the soil zone we are the most interested in.

Oklahoma and Kansas production fields can have a wide range of soil pH from field to field and within field. In a dataset of over 300 grid sampled fields from Oklahoma (259 fields) and Kansas (47 fields), the average field pH was a nice 6.0. However, the average range in the lowest and highest soil pH within the fields was 1.9.

This means the average field had a pH range from 5.0 to 7.0. It should be noted that more than 25% of the fields had a pH range of 3.0 units. This range of highs and lows has helped explain the presence of spotty herbicide issues on several fields in the past and should be taken into account when planning crop rotations.

It is extremely important to know and understand the pH of your soils and the herbicides you plan to use and how they will react. Soil testing is the only way to know your soil pH and reading your herbicide label is a great way to learn if soil pH affects the herbicide you are applying.