Foliar fertilization is the process of applying fertilizer nutrients directly to plant leaves. In addition to roots, leaves are also capable of absorbing plant nutrients. The nutrients are absorbed through the stomata, which are openings in the leaves that allow gases to move in and out of the plant. Petiole nutrient concentrations typically respond very quickly to foliar fertilization, since you don't have to wait for roots to pick up nutrients moving around in the complex soil matrix. In other words, through foliar fertilization, you can cut out the middle man (soil/root interactions) and get the nutrients directly to the plants.
It should be mentioned that foliar fertilization differs from fertigation. In fertigation, fertilizers are field-applied through an irrigation system and then followed by a regular irrigation cycle. The goal of fertigation is to get the fertilizer to the soil, and not to the leaves. Fertigation is more commonly used than foliar fertilization to correct or prevent nutrient deficiencies; however, foliar fertilization does offer a few benefits over fertigation.
1) When to consider foliar fertilizers
Foliar fertilizers can be very effective toward correcting micronutrient deficiencies. Managing micronutrients in the soil is challenging under alkaline soil conditions common to Idaho and eastern Oregon and Washington. Micronutrients like zinc, iron, manganese and boron in the soil become less and less available to plants as the soil pH increases above 7.5. By bypassing the soil and applying these micronutrients directly to the plant, these micronutrients don't get a chance to be tied up in the soil.
Fertigation applications of phosphorus fertilizers can be very inefficient if there are appreciable levels of calcium and/or magnesium in the irrigation water. Calcium and magnesium will readily bind with phosphates to form precipitates, or solids. These precipitates can clog up sprinkler systems. Foliar applications of phosphorus fertilizers can be an effective method for avoiding sprinkler clogging issues related to calcium- and magnesium-rich irrigation waters.
Finally, late-season foliar applications have been found to be beneficial toward tuber bulking. Bryan Hopkins (BYU) has reported that late-season foliar applications of a product containing molybdenum and boron appears to help with tuber bulking by triggering senescence to start a little earlier than normal.
2) When foliar fertilizers don't work
While effective in some cases, foliar fertilizers are not very effective toward correcting macronutrient deficiencies (nitrogen, phosphorus, potassium, magnesium, sulfur, calcium). By definition, plants need a much larger quantity of macronutrients than micronutrients to support plant growth. The leaf stomata can absorb nutrients, but it is impossible to transfer enough nitrogen, phosphorus, potassium and other macronutrients through the leaf to meet the needs of the potato plant.
Most people can readily agree that all of the plant needs cannot be met through foliar applications; however, many disagree on the effectiveness of macronutrient foliar applications to boost falling petiole nutrient concentrations. Fertigation is the most common application method for getting nutrients like nitrogen, phosphorus and potassium to plants, but some growers have concerns that the nutrients will get to the plant too late to serve any benefit. Researchers have found mixed results in supplementing basic fertility programs with foliar applications. Due to the high cost of foliar applications, in many cases it may not be worth the financial risk to foliar-apply
Tips and Tricks
If you decide that the benefits of foliar fertilization are worth the cost, here
are a few suggestions:
Be mindful of application rate. Concentrations less than 1-2 percent will minimize the potential for leaf burn.
Early morning or late evening applica- tions will help to reduce the potential for scorching.
Droplet size should be as fine as pos- sible to achieve uniform wetting on plant surfaces.
Tank-mixing surfactants can improve nutrient absorption through the leaf surfaces.
Do not foliar-apply to moisture-
stressed plants.
Follow foliar fertilizer sources and University of Idaho recommendations for fourth petiole nutrient concentra- tions of Russet Burbank potatoes
during tuber bulking.
macronutrients, with the possibility of little to no improvement in yield or quality.
There are also limitations to micronutrient foliar applications that should be considered. According to Hopkins, foliar-applied micronutrients are not mobile in the plant. This lack of mobility forces the need for repeated sprays to supply new tissue with the micronutrient.
3) How to avoid the need for foliar sprays
Ultimately, the best method for using foliar fertilizer applications is to avoid it altogether. Foliar applications should be considered as a last resort for any fertility program, primarily because it is a cost-prohibitive practice. Application of foliar sprays is far more expensive than pre-plant, topdress and fertigation applications. The most economical, practical and generally most effective method for getting nutrients to plants is through pre-plant, topdressing and fertigation. Foliar applications of micronutrients can also be avoided through field applications of chelated micronutrients, as well as soil acidification methods such as applications of elemental sulfur.