# Bill’s Forage Files: Fertilizing Hay Crops: Part 4: Nutrient Movement Into the Plant:



## ZB (Apr 1, 2008)

Many farmers have tried to incorporate a foliar fertilizer into their overall fertility programs over the years with mixed results. No one can afford to rely on the mentality of "spray and pray."* There is much talk about the foliar application of nutrients, but very few guidelines for their use are available. Hotly debated at both the scientific and commercial production levels of agriculture, foliar sprays need of a set of best management practices. Plant cuticle thickness, humidity, and temperature greatly influence when a foliar application should be made. Additionally, foliar solution properties and spray application techniques greatly affect how foliar fertilizers should be applied to forage crops. In this article I will describe the situations when foliar applications have a higher probability of being absorbed by the plant. Future articles will discuss how the application of a given nutrient should be made.

Current guidelines from state extension services do not recommend relying entirely on foliar applications of nutrients to supply a crop's needs.* The current recommendations are to apply only micronutrients in foliar applications. The purpose of this article is not to endorse or discredit foliar fertilization, but to explain the situations that are more likely, though not guaranteed, to enable foliar applications to have a positive effect on forage crop growth.

The main mechanism for the uptake of foliar nutrients into a forage crop, and every other plant, is by simple diffusion across the cuticle wall. This process is the same as diffusion in the soil discussed in Part 3 of this series, except the cuticle is a barrier that slows the diffusion process. The cuticle is the outer "skin" of plants and is very complex. Figure 1 below describes the cuticle in detail. All of these layers of wax, cutin protein, and other materials have enabled plants to move inland from the oceans and waterways over the eons. The cuticle is very good at keeping water in and not letting anything else in or out.

*Figure 1 Diagram of Plant Cuticle*









Source: H. Bargel et al 2006, Structure-function relationships of the plant cuticle and cuticular waxes-a smart material? Functional Plant Biology, 2006, 33, 893-910.

This cuticle barrier's variable properties throughout the growing season and across the United States are at the heart of the problems farmers and researchers have faced when trying to improve and replicate foliar fertilization techniques. Specifically, different climatic conditions across the country greatly influence the thickness of the cuticle throughout the growing season. Microclimate conditions of individual fields also influence cuticle thickness.

Applying foliar nutrients when cuticles are thinner will result in more successful foliar nutrient applications. Young thin cuticles are more permeable than old thick cuticles because the barrier is less developed to slow or stop the flow of nutrients from an area of higher concentration outside the plant to an area of lower concentration inside the plant. This is also part of the reason why spraying weeds with herbicides when they are young is more effective than when the weeds are more mature. Additionally shaded plants often have thinner cuticles and variable success may be seen in a field that is partly sheltered by trees.

Another proposed path of nutrients moving into the plant is via the stomata (holes in the leaf that control water and gas exchange). However this has not been demonstrated repeatedly under field conditions. Diffusion across the plant cuticle is the only established foliar pathway into the plant for foliar nutrients.

Humid areas of the country and humid times of the year will experience more success with foliar fertilizations than drier, less humid regions and parts of the growing season. Waxes accumulate on plants hot environments and the cuticles thicken more quickly. Additionally under humid conditions, the cuticle of plant becomes more permeable. This is why many of the guidelines that do exist for foliar applications recommend applying the solutions in the early evening, or early morning when the dew is still present. Given that there is more time for the solution to diffuse into the plants overnight, combined with the higher humidity at the plant level, it is more likely that late afternoon and evening applications will be more successful than midday applications of foliar sprays. Morning applications have also been moderately successful when dew is present.

When humid conditions exist, warm temperatures can increase the rate of nutrient uptake. However if the temperature is too warm the plant will dry out and the solution will leave a residue of unused nutrient salts on the leaves. A more specific discussion of nutrient solutions will be included in the upcoming article describing specific nutrients.

In fruit and grain crops foliar applications occur during the transition between vegetative and reproductive growth phases of the crop. Since most forage crops are harvested in the vegetative stage the optimal growth stage to apply a foliar application has not been established, though some amount of plant material should be present. Spaying bare ground after a harvest of hay or silage is not recommended.

Below are some extension links regarding foliar fertilization. They are not specific to hay production. The ATTRA link is the most extensive and describes more of how specifically foliar applications should be made.

ATTRA General*********** *********** Arizona Turf Grasses

Wisconsin Pastures*********** *********** Iowa State Corn & Soybeans

Iowa State Mn & Soybeans*********** UC Davis Vines

U Conn General*********** *********** Minnesota Trees

Clemson K in Cotton

Next up Part 5: Nitrogen Application to Forage Crops. Post a comment below or send an email to [email protected] if you have a specific interest or question on this topic.


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