Water hardness and pH-value have a strong impact on the stability of the spray mix. Farmers observe that the spray mix can change in its effectiveness when different water sources are used. To avoid the negative effects of an adverse pH-value and high water hardness, specialized tank mix adjuvants containing buffer systems are used to adjust the pH-value and reduce water hardness.
Often different pesticides, formulation types (e.g. EC, SC or OD) or adjuvant oils are added in one and the same spray mix. Several of these combinations show a poor compatibility with their mix partners. This leads to a rapid phase separation, or a sedimentation, etc. When the spray mix is not homogeneous it causes an uneven spray distribution on the crops, where some of the crop area receives too high or too low concentration levels of the active ingredient. The result is a poor disease control, a phytotoxic effect or even a build-up of resistance in the target organism. Corresponding tank mix adjuvants contain specialized surfactants, which prevent or slow down the effects of phase separation. Thus, a spray mix which usually separates within minutes forms a long-term stable uniform mixture when the right adjuvant is used.
The dilution of pesticide concentrates in water sometimes leads to the formation of foam which may become an issue. While foam does not impact the quality of the spray mixture, it does cause a complication by foaming out of the spray tank and possibly endangering the operator. In this case, defoamers can be added to the tank mix to reduce or prevent any foam formation.
When the operator is spraying the spray mix onto the crop during windy weather conditions a significant amount of the spray is blown away and misses its targeted leaves. This can reduce the success of the disease control and lead to a contamination of the environment. The spray drift is strongly dependent on the droplet size. Even when there is a low wind speed small and light droplets are able to easily be carried away, compared to heavier and larger droplets. When the spray nozzle is set to an increased droplet size, it may reduce the drift but it also generates too large droplets which easily run off the leaf. A specialized tank mix adjuvant is able to reduce the drift significantly by changing the physical properties of the spray liquid such that the droplet size distribution becomes narrow. In combination with a well-adjusted spray nozzle, most droplets will be just the right size and will neither run off the leaf nor be carried away by the wind.
Once the sprayed droplets reached the leaf surface it is important that they also remain there. Pesticides running off the leaf onto the ground cause an inefficient disease control as well as environmental issues. Therefore, special adjuvants which are called retardants are used to modify the droplets to be inelastic and sticky which makes them spread on the leaf but not bounce or flow off of the leaf.
In humid and rainy areas, the challenging issue is the loss of the active ingredients by rain which washes off the spray mix residue on the leaf. To reduce this loss, tank mixes with hydrophobic adjuvants are used. These adjuvants spread on the leaf and are able to repel the rain. They bind to the leaf surface, either physically, chemically or in the form of a film such that the active ingredient is protected from the rain. Thereby, such adjuvants extend the time for the active ingredients to penetrate into the leaf.
Optimizing the bioavailability of active ingredients in order for them to unfold their full biological potential, is one of the strongest pursuits within agrochemistry. On the one hand, it saves the crop by securing the effectiveness of the plant protection agents. On the other hand, it reduces the amount of plant protection agents and helps to minimize the exposure of xenobiotics to the environment. Tank mix adjuvants that support the bioavailability in any way (e.g. wetting agents, penetration enhancers, etc.) are called activators.
As soon as a spray droplet hits a leaf it should adhere to its surface and spread immediately to achieve the optimum coverage of the active ingredient on the leaf. Measuring the surface tension of a liquid is a general way to determine the ability of the droplet spreading onto the leaf´s surface. For pure water, the surface tension is about 72 mN/m and is reduced down to 20 mN/m with a potent wetting agent. Another important measure is the timescale in which the process takes place. The time a droplet covers the distance from the spray nozzle onto the leaf is usually about 20 milliseconds. Thus, a good wetting adjuvant not only reduces the surface tension and contact angle on a leaf but it also acts fast enough so that this process will take place while the droplet is still on the leaf.
In order to allow the active ingredient to penetrate into the tissue of the leaf, it is important to keep it in a mobile form. When the spray coverage dries on the leaf the active ingredients becomes immobile due to agglomeration or precipitation as crystals. Thus, it is important to delay the drying-up of the spray coverage. Adjuvants that maintain a liquid spray coverage are called humectants. They can either be hygroscopic salts or polymers, polyols, etc.
The penetration of active ingredients through the outer layers of the target plant is another important and the rate-limiting step in the application of plant protection agents. Even if all other physicochemical factors are optimized, active ingredients that are absorbed too slow lead to an ineffective disease control. Furthermore, an insufficient uptake of plant protection agents increases the risk of building up resistances or contaminating the environment. Penetration enhancers enable or support the active ingredients to pass through the leaf's outer boundary layer into the plant.