nutrico.org

Recently, I received an old question that I have encountered numerous times, this time from a marketer whose clients often ask about foliar feeding as a method of fertilizing plants.  As he says, “All the water soluble fertilizer companies advertise the practice all the time, while the water insoluble fertilizer companies do not advertise the practice at all.” What, he wondered, was, what was my actual opinion of the practice? First of all: Foliar feeding involves spraying the foliage of target plants with water-based fertilizers or liquid micronutrients. The logic for the practice is based on scientific research from the 1950’s, which demonstrated that leaves can take up minerals through their stomata, and in some cases through their cuticles. This research is consistently cited in the argument that foliar feeding is 8, 10, or even 20 times more effective than traditional soil application. In assessing the advertised claims for foliar feeding of trees, I had particular questions that are answered in bulleted lists below (all bullets are directly from marketing media from various companies): 

(1) What are the advantages of foliar feeding over soil application?• Immediate results• Prolong bloom• Increase crop yields• Increase storage life of food crops• Boost growth during dry spells• Increase cold and heat tolerance• Increase pest and disease resistance• Maximize plant health and quality• Help the internal circulation of the plant   

(2) When should one use foliar nutrients sprays?• When the soil is too cold for conversion of nutrient elements into usable forms• When it is at least 40°C• Any time except when it is too hot or too cold• Transplant time• Bloom time• When a quick growth response is desired• After fruit set• Every 2-3 weeks• Any time of stress• As long as the plant has leaves that aren’t dormant• When the soil is deficient in nutrients (3) What time of the day, and in what quantity, should you apply foliar fertilizers?• Early morning• Until it drips from the leaves• There is no improper way 

(4) How long will material last on the leaves?• 24 hours• 1-2 days• Four weeks

(5) What nutrients are critical components of foliar feed fertilizers?• Nitrogen• Phosphorus• Micronutrients As one company states, “In our opinion, foliar feeding is by far the best approach to use to insure maximum growth, yields, and quality by overcoming limitations of the soil and its ability to transfer nutrients into the plant.” 

The Reality If these laundry lists look more like a multiple choice test rather than solid information, it’s not surprising. Foliar feeding is yet another agricultural practice best suited to intensive crop production under specific soil limitations rather than management tools. Thus, advertisers take great liberties with the facts, often resulting in contradictory messages. Rather than individually refute the numerous errors in the claims, I’ll explain when foliar feeding might actually be beneficial.The original 1950’s research came from Michigan State University and was particularly useful in understanding how nutrients move within plant tissues. As explained by Dr. Tukey in his testimony to the Joint Committee on Atomic Energy, use of radio labelled nutrients allowed his team to discover “…that a leaf is a very efficient organ of absorption. The amounts may at first seem relatively small, but to offset this handicap, the efficiency is high.” From this advertisers claim that foliar feeding is 8, 10 or 20 times more effective than soil application. This is not accurate for several reasons.Obviously, materials applied directly to a leaf are more likely to enter the leaf in large quantity than the same materials applied to the soil. Leaching, chemical reactions, microbial activity, etc. can decrease what actually reaches the roots and is taken up into the plant. But materials applied to the leaf do not necessarily travel throughout the entire plant as effectively as they do through root uptake. They often remain in the same or adjoining tissues but travel no further. This is especially true of those elements recognized as “immobile” within plant tissues (apart from root uptake and xylem transport). Research over many decades has explored the mineral uptake and transport of many species of fruit trees. Results have been mixed in many cases, with some species responding well to treatment and others remaining unaffected. Generally, the results suggest that foliar application of particular nutrients can be useful in crop production situations where soil conditions limit nutrient availability. For instance, alkaline soils do not readily release many metallic nutrients, especially Iron and Manganese. Zinc, Copper, Magnesium, Molybdenum, Boron, and Calcium are other micronutrients required in small quantities that have been applied to foliage in an effort to relieve deficiencies and combat fruit disorders.Fruit, as adjacent tissue, can benefit from foliar spray. But this is a localized application that does not affect the trunk or roots – and therefore is not a solution to soil imbalances. In fact, researchers consistently state that foliar treatments are a specialized, temporary solution to leaf and fruit deficiencies in tree fruit production but will not solve larger soil management issues.On the other hand, macronutrients, such as nitrogen, phosphorus and potassium, are needed in larger quantities. While many of these are mobile in the plant, it is pointless to apply them to foliage as leaves cannot take up enough material to supply the entire plant’s demands. Furthermore, foliar application of high concentrations of such nutrients often results in leaf burn as water evaporates and the fertilizer salts remain behind. Substituting numerous, lower concentration applications would not be cost effective. Species differ widely in their ability to take up nutrients through their leaves. Differences in cuticle thickness, stomatal resistance, and other genetic factors will influence uptake, as will environmental conditions. Plants in a protected situation (like a greenhouse) have thinner and more porous cuticles than plants in the field and take up foliar sprays much more readily. Likewise, plants adapted to arid environments naturally have thicker, less penetrable cuticles than those from more moderate locations. A better management solution to the problem of nutrient availability is to choose plants that can adapt to the existing soil conditions. It would obviously have benefit for those landowners with fruit trees that perpetually have flower or fruit disorders associated with micronutrient deficiencies. Applying fertilizers to leaves (or the soil) without regard to actual mineral needs wastes time and money, can injure plant roots and soil organisms, and contributes to the increasing problem of environmental pollution.  The Bottom Line

• Plant species differ dramatically in their ability to absorb foliar fertilizers.

• Proper plant selection relative to soil type is crucial to appropriate mineral nutrition.• Foliar spraying is best accomplished on overcast, cool days to reduce leaf burn.• Micronutrients are the only minerals that are effectively applied through foliar application.• Foliar application will not alleviate mineral deficiencies in roots or subsequent crown growth.• Foliar spraying is only a temporary solution to the larger problem of soil nutrient availability.

• Minerals (especially micronutrients) applied in amounts that exceed a plant’s needs can injure or kill the plant and contribute to environmental pollution. 

Benefits of Foliar Fertilization In soilless systems such as hydroponics, many nutrient interactions can occur within the root zone which makes it difficult for the plant to absorb certain minerals. Even well run hydroponic systems can become deficient in nutrients - either because of nutrient depletion, antagonism between certain elements, or due to elements becoming ‘bound’ and therefore unavailable for plant uptake. Imbalances in the combination of nutrients, pH levels which are too high or low for maximum plant uptake and poor physical properties of the media surrounding the root zone, including oxygen starvation are more common in soilless systems than many growers realize. Furthermore, elements such as iron, an essential trace element, can not only be prone to becoming unavailable for plant use at moderate to high pH levels, but uptake by the plant can also be severely limited under certain environmental conditions such as cool temperatures. Iron chlorosis in many crops which are overly stressed by low temperatures can be a common problem where root uptake is the only source of iron for the plant. Any situation which damages the root system or restricts its growth, development or physical process such as respiration affects the uptake of minerals. Plant pathogens such as fusarium pythium and phytophthora can not only rapidly destroy a crop, but low, less damaging levels can restrict function of the root zone to the point where mineral uptake is affected. While the crop may not show signs of severe infection, mineral and water uptake can be restricted and therefore crop yields and quality are affected. Other plant stress conditions such as anaerobic conditions in the root zone where oxygen is deficient, can limit nutrient uptake, with trace elements such as iron often affected to the greatest degree. Any other condition which stresses the plant - temperature stress, high or excessively low humidity levels, lack of light, high radiation levels, high plant densities, presence of pests or disease, will affect the efficiency of the root system in taking up mineral elements. These conditions are common and occur in many growing systems from time to time without the grower even realizing that plant growth and mineral uptake is being limited in some way. It is under these types of situations that foliar feeding has its greatest advantage. Since plant stress is dependent on a number of factors — many are environmental, which growers have limited control over, foliar fertilization provides an ‘insurance policy’ against yield and quality loss from limitations in root mineral absorption and transportation.   The Process of Foliar Fertilization Foliar feeds are conventionally applied as solutions of dissolved salts to the plant leaves. Recent research has demonstrated that, initially, the feed moves mainly through the stomata, while later on, penetration across the cuticle is the primary pathway. The foliar feed solution contains both positively charged cations and negatively charged anions, which enter the plant through stomata, found mostly on the undersides of the leaves. The stomatal pores are the main route for the entry of carbon dioxide (CO₂) into the plant and the exit of water vapour from the plant. Once the plant surface has been hydrated, the main entry of dissolved salts into the leaf is via the cuticle and its polar pores; the smaller molecules entering faster than the larger ones.A problem with the conventional application of foliar feeds is that the initial uptake of the dissolved salts into the plant is dependent on the regulation of the opening of the stomatal pores. At least two mechanisms trigger the opening or closing of stomatal pores. Firstly, in most plants the stomata are closed at night and the CO₂ is depleted during the dark phase of photosynthesis. At dawn, light triggers the stomata to open and resume CO₂ uptake. Secondly, when the amount of water entering the leaf via the roots is less than the amount of water exiting the leaf in the form of water vapour, such as would occur in hot weather, the stomatal pores close in order to protect the leaf from further dehydration. Thus it is essential to apply the foliar feed to the plant’s leaves at specific times in the day when the stomatal pores are open for efficient uptake of the nutrients by the plant. However, water deficiency and temperature can cause the stomata to close during the day, and thus it is not guaranteed that the stomatal pores will be fully open when the foliar feed is applied to the leaves. In addition, the time of application for optimal uptake of the foliar feed by the leaves is not always predictable. This is problematic as it can result in wastage of the foliar feed together with increased costs. Feed solution settling on astomatous parts of the plant may enter the plant by penetrating the waxy covering (cuticle), either directly through the matrix or via the cuticular micropores. These openings in the cuticle are several orders of magnitude smaller and vastly more numerous than stomatal openings cf 20,000 stomata to 10 billion micropores. The majority of dissolved salts in conventional feed solutions have large molecules and therefore, typically, slow penetration of the dissolved salt molecules occurs through the cuticle. Once the plant surface has dried, penetration of dissolved salts through the cuticle ceases. This results in a greater loss of feed solution containing large molecules, than that with small molecules.   

Foliar Nutrient Application One very important criterion of the effectiveness of nutrient sprays is the rate at which the foliar applied nutrients are absorbed by the leaves and translocated within the plant. The uptake of nutrients is further affected by a number of interacting factors of which only part are known at the current time - these are shown in Table 1. It would be difficult to ensure that all of these factors shown in Table 1, are optimal for foliar feeding at any one time, but some are more important than others. The use of a good quality, non ionic wetting or sticking agent is vital for foliar feeding. Wetting agents are necessary to ensure the adherence of droplets on difficult to wet leaves as well as assisting with the absorption of the fertilizer solution into the plant tissue. The foliar fertilizer solution should then be applied as a fine mist until ‘run off’ so that the entire leaf surface is wetted. The time of day when the solution is applied is also important. Spraying early in the morning, while it is light, but temperatures are still cool, or in the evening is best and conditions that allow the leaf to dry rather than stay wet for an extended length of time is also important to consider. Foliar solutions should not be applied during hot, bright conditions, if the plants are wilting or under water/osmotic stress as the plants stomata are likely to be closed making application ineffective. Foliar feeding can by carried out on a regular, weekly basis, or can be limited to the times when the crop comes under high nutrient demand such as early fruit set and heavy fruit loading. Often the greatest response to foliar feeding will occur during the active growth phases of plants (period of exponential growth). During these active growth stages, leaves show a particularly high efficacy for absorbing nutrients. If a period during which the plants have difficulty in absorbing nutrients via the root system should coincide with a period when there is a particularly vigorous demand for nutrients, the result will be a significant loss in yield potential, without the grower seeing any visible signs of deficiency. Under such conditions foliar fertilization can give particularly impressive results.   

Table 1 Influences determining the efficacy of foliar nutrient sprays

The Role of Foliar Fertilization in Hydroponic Crop

While hydroponic crops may appear to be supplied with optimal nutrition via a well balanced and formulated nutrient, they still benefit from the application of foliar fertilizers. Studies have shown that hydroponic crops such as capsicum, treated with a micro nutrient foliar applied solution, had an increase in fruit yield over control plants and also an increase in the compound capsaicin in the fruit tissue. Hydroponically cultivated potato plants also showed similar results. Potato plants given foliar fertilization treatments with a micro nutrient solution not only had a greater tuber harvest, but also higher dry matter of the whole plants. Rockwool grown tomato crops have shown both yield and fruit quality improvements when given a weekly foliar feed of a micro nutrient complex. These results would also be expected on a number of hydroponic crops with similar nutrient requirements and thus the process of foliar fertilization is a cultivation technique that should be considered as more than just a ‘quick fix’ for mineral deficiency symptoms.