Water is more than just a carrier

By Dr. Brian de Villiers

Water is the carrier in which we mix crop protection products (CPPs) to apply them evenly over an area or volume. However, the perception that the sole function of water is just to dilute and apply CPPs, is misleading because it has properties that play a direct role in CPP efficacy. These properties include both chemical and physical factors which can all contribute to reduced efficacy. It is important to understand these properties and to address them with the correct adjuvants to ensure effective control.

Properties of water

Spray water doesn’t only consist of H₂O molecules but also contains dissolved cations. These cations have an influence on the absorption and efficacy of certain post-emergence herbicides. Water also has a pH that indicates how acidic or alkaline it is, and a buffering capacity that determines how readily the pH can be manipulated. pH has an influence on the alkaline hydrolysis (chemical breakdown) of certain CPPs and it also plays a major role in the solubility and compatibility of products and tank mixtures. Water physical properties then affect the droplet spectrum, which in turn influences the coverage of crops and weeds, as well as drift. The high surface tension of water results in droplet bounce, which in turn reduces retention and coverage of the spray mixture on the leaf. It also influences how much of each individual spray droplet makes contact with the leaf. Water is a relatively volatile liquid. This volatility affects the evaporation of spray droplets on the leaf. All these properties and variables make water quality one of the most important hurdles to overcome to achieve effective control.

Adjuvants that address water

Fortunately, there are adjuvants that address all these factors. Ammonium sulphate adjuvants eliminate the effect of dissolved cations on postemergence herbicides. Good quality buffers reduce the pH to levels where alkaline hydrolysis plays an insignificant role, regardless of the buffering capacity of water. pH should be adjusted judiciously so it does not contribute to insolubility and incompatibility. Deposition agents then manipulate the droplet spectrum to enhance coverage and to reduce drift. Surfactants and oils reduce droplet bounce, while increasing spreading and the contact area between the spray droplet and the leaf. Furthermore, with the inclusion of humectants, the evaporation rate of spray droplets is reduced, and the absorption period is increased. Therefore, the correct adjuvant choice can reduce the negative properties of water to ensure effective control.

Villa’s stance

Water remains one of the biggest challenges affecting CPP efficacy. All the negative properties of water must be addressed with adjuvants to ensure effective application and control. Contact your Villa agronomist for more information about adjuvants and water quality.