Coverage and absorption

By Dr Brian de Villiers

April 2026

There are two vital processes that ensure the efficacy of systemic crop protection products (CPPs). The first is coverage, facilitated by retention and droplet spreading, and the second is absorption. Coverage is useless without any absorption, and absorption is useless with no coverage. Coverage is achieved by effective application techniques, deposition agents, and surfactants or oil adjuvants. Surfactants and oils play a further role on the leaf by helping the absorption process. The role of adjuvants in both these processes is essential; therefore, product selection is vital.

Coverage

Spray droplets with no adjuvants normally have a high surface tension. Therefore, when they impact the leaf, a significant percentage will bounce or roll off. The droplets that are retained don’t spread much and have a limited contact area with the leaf. It is important to decrease the spray solution surface tension, so that droplets don’t bounce or roll off easily. The droplets that are retained then need to spread to increase the contact area with the leaf. Surfactants and oils play a vital role to decrease surface tension, thereby ensuring coverage. Surface tension is the key factor, therefore adjuvants that reduce it are used extensively to increase coverage.

Absorption

Droplet spreading is not only important for coverage, but also plays a vital role in the absorption process. After the spray droplet has dried, it leaves the spray droplet residual that must be absorbed into the leaf. Effective absorption is necessary to ensure that the CPP isn’t degraded on the leaf by poor weather conditions. Research has shown that absorption is sometimes affected by how much the droplet residual is spread out. Not all
CPPs have the same optimal degree of spreading; therefore, it is crucial to use the correct surfactant or oil.

The next function of surfactants and oils is to hydrate the waxy layer on the leaf. This creates a path for systemic CPPs to reach the site of action. Different adjuvants have different wax hydration ability, therefore only use the adjuvant that has been tested and registered with a particular CPP or tank mixture. The use of a product with less wax hydration, will lead to poor efficacy.

Villa’s stance

Lower surface tension facilitates more coverage by reducing droplet bounce and by increasing droplet spreading. Droplet spreading is not only important for coverage, but it also play a vital role in the absorption process, together with surfactant chemistry. These vitally important efficacy processes highlight the need to use the correct adjuvant. Surfactants and oils are not all the same, therefore is critical to use the registered product. Contact your Villa agronomist for more information about adjuvants and water quality.