Foliar Feeding

Fertilization exists to supply essential nutrients to plants to achieve desired color, growth and pest resistance. As the demands of the turf are increased from lower mowing heights and increased traffic levels, additional fertility is needed to promote growth and recovery, especially in sand-based soils. Nitrogen (N) is the key element in turfgrass fertility programs. Nitrogen strongly influences the color, growth and density in turfgrass; therefore, turf managers place an emphasis on this particular nutrient. However, as the trend towards low-input turfgrass maintenance programs increase, practices such as foliar fertilization, or foliar feeding, have become more popular. But how effective is this application technique in regard to nutrient uptake by turf foliage?

FOLIAR FEEDING

Foliar feeding is the entry of small amounts of liquid fertilizer through the surface of plant shoots. This allows for rapid nutrient utilization by the plant, and also provides the applicator the ability to blend the fertilizer with other products, such as pesticides and micronutrients. Current formulations of liquid fertilizers are believed to penetrate mostly the transcuticular pores on foliage, which are open virtually all the time compared to stomata. Nutrients also enter stomata, but these often are closed due to environmental stresses and darkness. Also, the majority of the stomata are located underneath leaves, away from fertilizer spray patterns. Drawbacks to foliar feeding include the inability to apply large amounts of N, phosphorous (P) and potassium (K) without potentially burning the foliage. Therefore, frequent applications at a low volume are required to maintain consistent color and plant growth.

FOLIAR ABSORPTION

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Many products claim to be true foliar fertilizers that can solve most fertility problems. However, the majority of these products lack published research to substantiate these statements. A primary concern is whether you are applying a true foliar-absorbed fertilizer. Research on banana plants determined whether foliar applications of urea were absorbed through foliage or washed off and absorbed through roots. Up to 65 percent of the foliar-applied urea was absorbed within 25 minutes, with the majority being absorbed by the lower (or younger) surface of the foliage — where the greatest number of stomata exist. Similar findings were reported in coffee, cacao and McIntosh apple. The lower leaf surfaces and younger leaves rapidly absorbed urea from foliar applications as compared to older leaves and upper leaf surfaces. Complete absorption of the urea occurred in coffee and cacao in less than 24 hours and in banana by 30 hours. The absorption of urea by the lower surface of younger McIntosh apple leaves was as high as 85 percent in a two-hour absorption period, compared to the lower leaf surface of older leaves.

Many foliar applied products also contain “hidden” ingredients, such as iron, not specified on the label. So while you may be satisfied with the results, you may not realize which ingredient, specifically, is providing this response. One way to identify it is to know exactly which elements can be absorbed and moved through the plant. Mobile elements such as N, magnesium and sulfur are transported through phloem tissue in leaves. Meanwhile, immobile elements such as calcium and boron will not move through the phloem, thus, would not be as effective if applied as a foliar fertilizer.

Previous research has reported approximately 55 percent of 15N-urea applied was absorbed by tall fescue and Penncross creeping bentgrass. Similar results occurred among eight cool-season turfgrasses with absorption of N ranging by cultivar from 31 to 61 percent over 72 hours. Lower values have been observed in Kentucky bluegrass and perennial ryegrass as the absorption of applied 15N-urea ranged from 43 and 35 percent, respectively, over 48 hours. In 2002, N adsorption rates were compared among six warm-season turfgrasses. Long-term N recovery analysis showed that St. Augustinegrass exhibited the most efficient recovery of 84 percent of applied N allocated to new leaf tissue, while centipedegrass was the least efficient at 63 percent allocated. Also, 15N uptake analysis proved Tifway bermudagrass exhibited more efficient 15N adsorption than Meyer zoysiagrass, with the majority of the N being located in the shoots (38 to 63 percent).

Previous research in McIntosh apple focused on parameters hindering foliar urea uptake. These included existing N levels in the foliage, pH of the spray, temperature and the influence of wetting agents. High existing nitrogen levels in the foliage and low temperatures (~21°C compared to 32°C) promoted best absorption. Also, incorporating a wetting agent into the spray (Tween 80 and Tween 20) approximately doubled the percent of urea absorbed, compared to a pure water solution.

Most research indicates that with urea, for instance, liquid and dry (granular) formulations produce little differences in turf growth and quality. However, previous research with urea noted foliar feeding accounted for 95 percent of plant use compared to approximately 10 percent use from soil applications. In an attempt to address efficacy questions, studies compared fluid and foliar nutrition programs to conventional programs (conventional programs designed by select golf course superintendents in the state of Nebraska) on Providence creeping bentgrass. Fluid and foliar programs were comparable to the conventional programs in terms of color and density while incorporating 25 to 80 percent less N. Furthermore, it was suggested foliar fertilizers should not replace conventional fertilizer programs (liquids and water-soluble controlled-release granular fertilizers). However, “true” foliar fertilizers can increase the growth and vigor in turf under high maintenance, especially under stresses such as increased heat. Also, with the increased attention placed on N and phosphorous leaching, liquid fertilization could be very beneficial. The low input required by foliar applications could pose a smaller risk to the environment in terms of leaching.

CURRENT RESEARCH

If you couple a foliar-absorbed product with a sound, granular soil-based fertility program, the potential to produce high-quality turfgrass with minimal rates is feasible. To build upon previous research and to determine advantages and disadvantages between dry and liquid fertilizers applied to turf, research is being conducted at Clemson University to address: (1) what is a true foliar fertilizer; (2) what is their effectiveness; and (3) long-term effects on plant vigor and recuperative potential of foliar vs. granular fertilization programs. Researchers are evaluating various annual N rates with various ratios of liquid vs. granular products on creeping bentgrass, with emphasis on the long-term response to this. In addition, laboratory studies will identify how much liquid fertilizer is actually being foliar absorbed. Such research could supply turf managers with knowledge needed to determine the effectiveness of these foliar products and their cost effectiveness.

Wesley Totten is a graduate research assistant and Bert McCarty, Ph.D., is professor of turfgrass science, both in the Department of Horticulture at Clemson University (Clemson, S.C.).

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