Features Agronomy Fertility and Nutrients
Foliar fertilization fallacies

Foliar fertilization in various crops has its place as a rescue treatment for some macro and micronutrients. Just where though, has been much debated and maligned. Rigas Karamanos, manager of agronomy with Westco Fertilizers in Calgary, Alberta, sets the issue straight, with reviews of research trials and peer-reviewed papers.

“Foliar fertilization can be very efficient under certain conditions. However, the quantity of nutrients absorbed by plant leaves is very small in comparison to overall plant requirements,” explains Karamanos.
In general, foliar fertilization decision-making should consider four criteria: the amount of nutrient required by the crop; the effectiveness of the application at the growth stage that best matches the crop response; the suitability of a product formulation for uptake; and the impact of method, time and concentration of applied foliar product on crop foliage.

Most macronutrients not suitable
Karamanos explains that the difficulty with macronutrients is that the application of adequate amounts is
difficult without causing foliar burning. Very low rates are necessary, in the range of one to two percent
concentration (less than 0.4 percent P) to avoid foliar damage. At these concentrations, the amount of nutrient applied is usually not high enough to influence crop development. “Foliar is a very, very inefficient way to apply macronutrients,” says Karamanos.

Research through the years has found that post-emergence nitrogen (N) fertilization can help improve grain protein content in wheat. However, foliar application was not necessarily any better than dribble-banded N. Growth chamber experiments using 15N-labelled urea found that plants were better able to access soil placed N than foliar applied N.

“Whether top dressing with foliar or soil application, the majority of N found its way into the plant through the roots and not through the leaves. The majority of foliar applied N is taken up by the roots after it washes off the plant leaves and stems and reaches the soil,” explains Karamanos. “That is generally where we see the response coming from.”

Looking at phosphorous (P) and potassium (K), the challenge is to supply the nutrients in adequate amounts without severely burning the leaves or using excessively high water volumes or multiple applications to compensate for the burning. “I wouldn’t recommend P and K because the efficiency of application is extremely low, around one to two percent,” says Karamanos.

Sulphur (S), on the other hand, is a good candidate for foliar fertilization. Research at AAFC in Melfort by Dr. Malhi showed that foliar application of 15lb/ac S as ammonium sulphate at bolting and flowering resulted in correction of S deficiency in canola. However, Malhi also found that application of sulphate-S fertilizer at seeding was more effective than a foliar application at bolting and early flowering stages.

“Yield increases with foliar S is similar to S application at seeding. Growers can use foliar S as an option for rescue treatments, but they may be able to save on costs if they applied S in a fertilizer blend while seeding,” Karamanos says.

Micronutrient applications can be effective
Generally, foliar application of micronutrients can be an efficient and effective way to apply to crops, when needed. In western Canada, foliar application of micros is considered an emergency treatment, but at the same time, one of the most effective methods of micronutrient application, explains Karamanos. “Foliar application is usually more effective than soil applications, especially if the micronutrient interacts with other nutrients in the soil.”

Foliar application of copper (Cu) is usually considered an emergency treatment, as it is not always as effective as a broadcast and incorporated application of 4.0lb/ac Cu in the form of copper sulphate.

Karamanos says this is especially the case on crops grown on severely Cu deficient soils, when a foliar application comes too late to help plants overcome a Cu deficiency.Research has shown that foliar Cu application is best conducted at Feekes Growth Stage 6 (first node visible), but a second application at Feekes 10 (early booting) may also be required to maximize grain yield. Conversely, a single application at Feekes 10 was not as satisfactory. Additionally, Karamanos says that application at the early growth stage Feekes 2 (four to six leaf stage) has proven to be ineffective and should be avoided.

Manganese (Mn) response on mineral prairie soils is extremely rare. However, Karamanos’ research found foliar Mn at Feekes 6 and Feekes 10 were effective in alleviating deficiencies on organic (peat) soils.

In western Canada, boron (B) deficiencies are rare. In these circumstances, it has been found that B uptake by barley through the root system is more effective than through the leaves.

Significant yield increases to foliar zinc (Zn) at 0.3lb/ac Zn have been obtained in corn, lentils and field
beans in western Canada. Multiple applications, though, may be required since plant leaves form a cuticular barrier that prevents absorption. Additionally, foliar application may not be as effective as soil applied Zn. In one experiment, foliar applied Zn only provided a response 50 percent of the time compared to when Zn was soil applied at 5.0lb/ac.

Approach multi-nutrient products with caution
Multi-nutrient foliar fertilizer products containing macro and micronutrients are used in some parts of the world as providing an ‘optimum’ balance of all nutrients. Their usefulness in western Canada, though, remains in question. Thirteen experiments on wheat and 21 experiments on barley from 1989 through 1994 investigated whether such a product could provide increased yield and economic returns to a farmer. The product, 15-20-20, had an analysis of 15 percent N, 15P, 20K, 2S, 0.15Cu, 0.1Fe, 0.1Mn, 0.08B, 0.0005Mo, 1Zn. A product with similar analysis, 18-20-20 plus identical micro content, is currently available in western Canada.

The foliar blend provided statistically significant yield increases in two of 13 experiments in wheat and five of 21 experiments in barley. Yield increases were small and unpredictable, even when exceptionally high yields were obtained. “Non-targeted application of micronutrient mixes at all sites proved to be agronomically inefficient and economically unviable under western Canadian conditions,” cautions Karamanos.

While foliar application of a single nutrient may be warranted under certain circumstances, Karamanos says farmers may be better off with soil applications on deficient soils. The key is to understand the characteristics of the nutrient and to look at the research to understand when foliar is the best choice. n