PH and Weed Control

With planting wrapping up in most areas, the focus shifts to protecting yield. While scouting to evaluate weed control, it is common to find areas where the weeds persist or see areas with crop injury.

Consider pH when evaluating herbicide effectiveness. PH changes occur in cropping systems for a number of reasons, including the conversion of nitrogen fertilizers?which creates H+ ions, lowering pH. In some cases, this issue can be more pronounced in reduced tillage systems because soil is not mixed on a regular basis.

Soil pH impacts herbicide activity and persistence both positively and negatively depending on the product. According to a study from the Washington State University Extension, herbicides carry a net positive or negative charge. Net positive herbicides will be attracted to negatively charged soils. Negatively charged herbicides will be repelled by negatively charged soils. In some weak acid herbicides, lower pH can alter the charge and affect the persistence of the herbicide. Imidazolinones and triazolopyrimidines are weak acids and will have increased persistence at lower soil pH. Sulfonylureas are more likely to have greater persistence in higher pH soils.

In some cases the increase in persistence is due to the effect of high pH on the process of hydrolysis, which is a breakdown method for some herbicide families. If herbicides are not broken down in the soil due to an imbalance in pH they can persist in the soil leading to crop injury the following year.

As you scout your fields, if you see areas or patches that seem particularly weed-ridden for no reason or discover crop injury, it would be a good idea to take a soil analysis and measure the pH. A pH of below 5.5 or above 7.5 could be a potential issue that requires investigation to rectify.

photo courtesy of University of Wisconsin Extension