Foliar Feeding

Fertilization without the fixation.

What is Foliar Feeding? Why Use It?

The need for fertilizers in agricultural systems is well known. Many regions have local nutrient deficiencies that must be amended in order to grow the healthiest crops and achieve the best yields. For centuries, growers have applied fertilizers and organic amendments (mulches, manures, etc.) to the soil to balance the nutrient levels. However, poor nutrient management and low-quality fertilizers can end up creating more problems than they solve. When fertilizers are applied to the soil, nutrient antagonisms and fixation can occur. This is especially true in alkaline calcareous soils, where the high pH renders many nutrients immobile. This results in a high total nutrient content with poor bioavailability, preventing crops from utilizing the fertilizers efficiently. Overuse of inorganic fertilizers can also cause a buildup of salts and decrease soil quality. In arid regions, low water availability can hinder nutrient uptake, and regions with heavy rainfall can experience nutrient leaching, which leads to environmental degradation.

Fertilizers can also be applied to the above-ground parts of the plant through what is known as foliar feeding, or a foliar spray. Foliar feeding avoids many of the issues common with soil fertilization. Many micronutrients are more bioavailable when applied to the foliage compared to when applied to the soil. Compared to soil fertilization, foliar sprays result in higher nutrient utilization, regardless of soil pH, and prevent negative effects on the health of the soil and environment. Nutrient absorption through foliar feeding is typically 5-10x higher than nutrient absorption from soil applications of the same fertilizer. Foliar feeding has shown to be effective in vegetables, flowers, tree crops, and cereals. However, foliar feeding is not a replacement for soil fertilization, just a supplement to fine-tune plant nutrition. If you have lab-tested your crops and identified particular nutrient deficiencies, foliar feeding is an excellent way to address those issues.

All nutrients are absorbed through the leaf easily, and are able to pass through the waxy leaf cuticle as well as the stomata. Transport through the leaf cuticle is a slow process, whereas stomatal intake is relatively quick. Therefore, it is best to apply a foliar spray at times of day when the stomata are most open, such as early afternoon when the sun is high but temperatures are still somewhat low. Rainy and windy days should also be avoided when applying foliar sprays.    Foliar feeding is most effective when combined with surfactants and chelating agents, although the best mixture will depend on the nutrient you are delivering to the crop. Chelating agents, like organic acids and sugar alcohols, can increase the nutrient uptake of foliar fertilizers and promote plant growth. Surfactants are mixed into the solution to improve its ability to stick to plants, allowing more of the nutrients to be absorbed.

Benefits of Foliar Feeding

Foliar feeding cannot completely replace soil fertilization, but is an indispensable tool that can supplement soil fertilization to more precisely deliver nutrients at the right times. Some nutrients are mobile and can be transported throughout the plant as needed, whereas other nutrients are immobile. For the immobile nutrients, a foliar spray is often needed multiple times a year, or is applied every time there are new green leaves. Foliar feeding typically has a quicker plant response than soil applications, with results often visible in three to four days. In addition to improving plant nutrient status, foliar feeding has also been used to protect plants against certain abiotic stresses. Foliar feeding with potassium (K) increases abiotic stress tolerance by improving osmotic potential, regulating stomatal closure, and increasing water uptake, therefore allowing plants to cope with drought more easily. But given that K is a macronutrient, foliar applications are unable to meet the entire needs of a plant. Foliar applications of silicon (Si) promote stress tolerance under salinity and drought stress, and may help protect against pathogens and insects.

Some crops require different amounts of nutrients at different stages of growth, which is difficult to manage with soil applications. For example, grapes require more potassium during the berry expansion phase than at the blooming phase. Foliar feeding can be fine-tuned to deliver the exact amount of nutrients needed to grow the tastiest grapes.

For some micronutrients, there is a fine line between a beneficial and toxic dose, and it varies from crop to crop. Some sources say that 10-200 ppm of boron is a healthy level, while other sources state that above 50 ppm is toxic to crops. When an extra 50 or 100 ppm could mean the difference between improving or inhibiting your crop, it’s best not to risk over-applying a soil fertilizer, which involves uncertainty around how much will be available to the plant. Foliar sprays allow the grower to control the exact dosage of nutrients applied, and you can rest assured that a very high percentage of the applied nutrients will be absorbed by the plants. Foliar feeding can also be used to address nutrient deficiencies when root uptake is limited by flooding or drought.

Macronutrients can be applied with a foliar spray as well, although these foliar sprays do not replace well-balanced nutrients in a healthy soil. There is evidence some plants reduce their root uptake of nutrients during the reproductive stage. To improve nutrient status when root uptake is poor, foliar nitrogen sprays can improve growth, increase yield, and even increase protein content in grains. Similarly, phosphorus, potassium, and sulfur can all be applied as foliar applications, although phosphorus is sprayed less often due to its poor solubility and high plant demand. But again, care should be taken to use these fertilizers in a precise manner and only use them as needed. Given that nutrients used in foliar sprays must be soluble in water, many of them are inorganic salts. These salts can cause nutrient burn and leaf scorching. With regards to nitrogen fertilizers, urea seems to be the best suited for foliar feeding compared to ammonium nitrate and ammonium sulfate, as it has a low salt content. However, nutrient burn is easily caused by foliar applications of N, especially in cereal crops. There have been recent advances in nanoparticle technology which have drastically improved the bioavailability of certain nutrients, including in foliar applications. Foliar applications of calcium, nitrogen, and zinc nanoparticles have all been shown to be more effective at improving plant health than their respective salts, at lower doses.

Further Reading

Fageria, N. K., Filho, M. P. B., Moreira, A., and Guimaraes, C. M. (2009). Foliar fertilization of crop plants. Journal of Plant Nutrition.

Haytova, D. (2013). A review of foliar fertilization of some vegetable crops. Annual Review & Research in Biology.

Niu, J., Liu, C., Huang, M., Liu, K., and Yan, D. (2020). Effects of foliar fertilization: A review of current status and future perspectives. Journal of Soil Science and Plant Nutrition.

Noreen, S., Fatima, Z.., Ahmad, S., Athar, H. -u. -R., and Ashraf, M. (2018). Foliar application of micronutrients in mitigating abiotic stress in crop plants. In Hasanuzzaman, M., et al. (Eds.), Plant Nutrients and Abiotic Stress Tolerance. Springer Nature.

Singh, J., Singh, M., Jain, A., et al. (2014). An introduction of plant nutrients and foliar fertilization: A review. In Ram, T., Lohan, S. K., Singh, R., and Singh, P. (Eds.), Precision Farming: A New Approach. Daya Publishing House.

Yaseen, M., Ahmed, W., and Shahbaz, M. (2013). Role of foliar feeding of micronutrients in yield maximization of cotton in Punjab. Turkish Journal of Agriculture and Forestry.