Note: This article is intended as a general guide to herbicides and soil pH. It is not a substitute for herbicide labels, nor promotes or discourages the use of any herbicide(s). All herbicides are names are trademarks of their respective manufactures.
Soil pH can make a big impact on soil-applied herbicides
Low soil pH (<6.2) will cause the triazine herbicides (Atrazine, Sencor) to be bound to the soil. When herbicides are adsorbed they are not effective at controlling weeds since they are not available in the soil solution. This is why pH sensitive herbicides like Atrazine, and Sencor can be used with less risk of crop injury in low pH soils. At low soil pH higher rates are need to control weeds. Crop injury increases when soil pH is higher.
When higher rates of herbicides are used in an attempt to get better weed control in low pH soils, herbicide residues in the soil increase. These bound herbicides are released if the soil is over-limed. If ag lime is postponed until just before planting, this release of bound herbicide can have serious detrimental effects on sensitive crops.
Sometimes there are problems when soils are limed with large amounts of ag lime. Spreading high rates of lime than required or quickly raising a very acidic soil can cause crop injury. If there is a long history of triazine herbicides used, liming can release these chemicals and kill sensitive crops. Decreased crop growth because of "over-liming" injury is usually associated with lowered availability of phosphorus, potassium, or boron. Over-liming acidic sandy soils can produce zinc and copper deficiencies.
Poor crop performance due to nutrient deficiency is often blamed on Atrazine, and Sencor since problems do not develop until 2 to 3 weeks after emergence. Moldboard plowing can reduces phytotoxicity of Atrazine, and Sencor by diluting the herbicide residue in a large volume of soil. The best way to avoid these problems is to consistently maintain the soil pH above 6.2. Applying SuperCal 98G minimizes the adsorption of triazine herbicides to the soil and results in improved crop safety and performance. Properly limed fields will reduce the residual herbicide in the soil and avoid large release of bound herbicide causing crop injury.
Poor Performance and Carry-over
The half-life of many herbicides varies with soil characteristics and environment. For example, the half-life of atrazine in Georgia on a soil with a pH of 6.8 was reported to be 39 days, whereas in Minnesota the half-life was 261 days on a soil with a 7.9 pH. Whether a herbicide has basic, acidic or neutral properties can determine its ability to exist in the soil solution or adsorbed by soil solids. In general, herbicides whose pH is close to the pH of the soil are strongly adsorbed and are not subject t