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Fluid Journal : Fluid Journal 2008-2009
constituents to longer chain more hydrophobic (water repelling?) lipids, thereby further impeding the uptake of foliar-applied nutrients. Mechanisms Cuticle vs. stomata. There are two possible channels for penetration of foliar-applied compounds into the leaf before they can produce a response. One is through the external cuticle and the other is through the stomata. However, it is generally accepted that most nutrient uptake occurs through the cuticle. Based on recent evidence, showing that the uptake of large anions can occur through the stomata, there is general agreement that the stomata might indeed represent a possible pathway through which a limited amount of nutrient might gain entry into the leaf. Organosilicon surfactants can also help, reducing aqueous surface tensions to about 20 Mn m-1 and allow entry via the stomates. Further stomatal penetration can occur only in the brief period after application while spray deposits remain liquid. In cotton, it is unlikely that direct penetration of solutes from the leaf surface through open stomata, into the leaf tissue, plays an important role. The most important consideration for efficient and profitable foliar fertilization is that this practice will only be effective if the applied nutrient ultimately reaches the target site for its use--i.e., the growing points in a vegetative cotton plant and the developing fruit in a more mature reproductive plant. Rate and timing The timing of foliar sprays, particularly in regard to growth stages, can be critical in relation to the optimum efficacy of the foliar treatment. More attention should be given to it because the seasonal pattern of nutrient uptake varies with growth rate and stage but generally follows a sigmoidal pattern with sharp increases occurring as the boll load develops. The developing fruit load (the sink) has a high requirement for NPK, in particular, and this demand is not always completely met by the soil, especially when adverse conditions occur and as root growth declines. Absorption Nitrogen. Results of field research clearly demonstrate the uptake of foliar-applied nitrogen (15N-labeled) urea by cotton leaves and translocation to the developing bolls. Foliar-applied 15N was rapidly absorbed by the leaf to which it was applied (30% within one hour) and translocated into the closet boll within 6 to 48 hours after application. The 15N moved progressively into adjoining bolls for the next few days with no translocation to other leaves. The cotton uptake of foliar applied 15N was highest in the early morning and late afternoon, and lowest at midday. Water deficit significantly reduced the absorption of the foliar-applied nutrient. Total leaf wax of field-grown cotton leaves increased with increase in leaf age and this was associated with a significant decrease in 15N from foliar application. This may account for Dr. Oosterhuis is Distinguished Professor of Crop Physiology, Department of Crop, Soil, and Environmental Sciences, University of Arkansas. the decrease in yield response to foliar-applied urea three weeks after flowering as has been reported and may warrant the use of increased rates or frequency of application of N and the use of adjuvants. Potassium. Potassium (K) fertilizers have a high pH in solution, and adjusting the solution to a pH of 4 to 6 significantly increased uptake and yield. Furthermore, KNO3 and K2SO4 were superior to the other K fertilizers tested, whereas K2CO3 and KOH gave the poorest results. Advantages/disadvantages Advantages of foliar feeding include: Low cost Quick plant response Lack of soil fixation Independent of root uptake Small quantities needed Combines easily with other agrochemicals Higher yields. Disadvantages include: Possible foliar burn Solubility problems Ideal weather required when applying Inefficient absorption when pH is too high Possible incompatibility with certain chemicals.
Fluid Journal 2005-2007