Guys, here is an interesting piece on foliar feeding by a guy called Eyal Ronen who is the Regional Manager South America for Haifa Chemicals.
FOLIAR FEEDING
Another Successful Way of Feeding Plants.
Foliar feeding is a reliable method of feeding plants when soil feeding is inefficient. In this article, the author highlights when foliar feeding should be considered, how nutrients actually penetrate plant tissue, and some technical limitations to this method of plant feeding.
Plant nutritionist have traditionally considered the obvious way to feed plants is through the soil, where plant roots are meant to uptake water and nutrients, but in recent years foliar feeding has been developed to supply plants with their nutritional needs.
The development of pressurised irrigation equipment such as drip irrigation has promoted the need for water-soluble fertilisers, as clean and purified as possible in order to diminish the possibility of emitters clogging. It is not really clear when foliar feeding started, but after the development of water-soluble and liquid fertilisers farmers have begun to use these fertilisers with sprayers, the same as it is used with applications of pesticides. At the beginning, this technique of spraying nutrients was used for correcting deficiencies of minor elements. However, fast curing has shown that plants can absorb some elements through their tissue. As a result, foliar feeding has gone through further development.
These days foliar feeding is considered among the major techniques used for plant nutrition, supplementing the ground application. In this article I will review the whole concept of foliar application - when it should be considered, how nutrients actually penetrate plant tissue, and some technical limitations of this method of plant feeding.
The case for foliar feeding
Foliar feeding is a ‘by-pass’ approach, overtaking conventional ground fertiliser applications whenever it does not perform well enough. Foliar application overcomes soil fertilisation limitations like leaching, insoluble fertiliser precipitation, antagonism between certain nutrients, heterogenic soils unsuitable for low dosages, and fixation/absorption reactions like in the case of phosphorus and potassium. Foliar feeding can also be used to overcome root problems when they are suffering from limited activity due to low/high temperatures (<100, >400C), lack of oxygen in flooded fields, nematode attack damaging the vascular system, and a decrease in root activity during the reproductive stages where more of the photosynthetic creation is transferred for reproduction with less for root respiration (Trobisch and Schilling, 1970. Foliar feeding has proved to be the fastest way of curing nutrient deficiencies and boosting plant performances at specific physiological stages. With plants competing with weeds, foliar spraying focuses the nutrient application on the target plants. Fertilisers have also been found to be chemically compatible with pesticides, thus saving labour costs. Certain types of fertilisers can even slow down
the hydrolysis rate of pesticides/growth hormones (GA3) owing to lowered pH of the solution and this may improve performance or cut costs.
Fertilisers applied through the plant leaf canopy have to face several structural barriers, unlike pesticides which are mainly oil-based and don’t face difficulties to penetrate the leaf tissue. Nutrients, which are salt based (cations/anions) may face some problems penetrating the inner plant tissue cells. General leaf structure is based on several cellular and non-cellular layers. The different layers support protection against desiccation, UV radiation and various kinds of physical, chemical and (micro) biological agents.
The different layers are characterised by electrical negative charge, which influences the way and rate of penetration of different ions. Some layers are hydrophobic and therefore repulse water-based spray.
The first layer from outside is a wax layer, which is extremely hydrophobic. The epidermal cells synthesize the wax and it crystallises in an intricate pattern of rods, tubes or plates. The wax layer can change during the plant growth cycle.
Cuticle proper
Pectin & Cuticle layer
Primary wall
Secondary wall
Plasma membrane
Cytoplasm
The second layer, referred to as the ‘cuticle proper’, is a non-cellular protective layer surrounded by wax to the upper side and the bottom one as well and made mainly from ‘cutin’ (macromolecule polymer consisting of long-chain fatty acids creating a semi-hydrophilic character).
The following layer is ‘pectin’, negatively charged and made of Polysaccharides that form sugar-acid based gel-like tissue (cellulose and pectic materials).
Next is the outer side of the cells starting with the primary wall. The cuticle has negative charge density as well due to the pectin and cutin (Franke, 1967; Marschner, 1986).
How do nutrients penetrate plant tissue?
When we refer to penetration of nutrients we can define two movements – into the tissue from outside, which is referred to as absorption, and movement from the point of penetration to other parts of the plant that is referred to as translocation.
Penetration/absorption can be done through several organ elements that exist in the tissue. Main penetration is done directly through the cuticle. The penetration is done passively. First to penetrate are the cations as they are attracted to the negative charge of the tissue, and they move passively in accordance to the gradient – high concentration outside and low one inside. After a certain period the cations that have moved inside change the electrical balance in the tissue causing it to be less negative and more positive. From this point on the anions start to penetrate the tissue in the same manner as described for the cations
Since the penetration is a passive one, the rate of diffusion across the membrane is proportional to the concentration gradient, therefore achieving a high concentration without scorching the tissue - may dramatically improve the penetration.
Figure 3
Penetration also occurs through the stomata, which are aperture controlled for gas exchange and transpiration. It is known that these apertures differ between different plant species, their distribution, occurrence, size and shape. In broadleaf crops and trees, most of the stomata are on the lower leaf surface, while grass species have the same number on both surfaces.
Size may differ, for example, sorghum stomata are four times larger than bean stomata. High penetration is estimated to be due to high cuticle pore density in cell walls between guard cells and subsidiary cells (Maier-Maercker). In addition, the pores near the stomata guard cells seem to have different permeability characteristics (Schonherr and Bukovac, 1973 An opposite opinion exists, claiming that penetration through open stomata does not play a major role since a cuticle layer also covers the surface of the guard cells in stomata cavities and because ion uptake rates are usually higher at night when the stomata are relatively closed.
Another path that nutrients can penetrate is through hair-like organs known as ‘trichomes’, which are epidermal outgrowths of various kinds. The importance of this pathway depends on the trichomes rate and position, dependent on leaf age and its origin (Hull et al., 1975; Haynes and Goh, 1977).
Translocation
After the ions have penetrated, transportation to different parts of the plant starts and this is referred to as translocation. Translocation is done through two mechanisms: cell-to-cell transport is referred to as ‘Apoplast movement’, and transport through the vascular channels is referred to as ‘Symplast movement’.
The Apoplast movement describes the ion movement from one cell to another. This is done by three mechanisms
• Passive transport involves diffusion according to the gradient and mass flow through the water/fluid movement between cells.
• Absorption by cytoplasm membrane surface via plasmodesmata, which are microscopic channels connecting one cell wall to another, enabling transport and communication between them.
• Active transport (ATP) against the gradient, enabled due to energy investment of ATP molecules.
The Symplast movement describes the ion discharge into the vascular system. This is done through two systems.
• Phloem – translocation is energy dependent and more suitable to the divalent cations (C2+); anions are very limited since the cell wall is negatively charged (Van Steveninck and Chenoweth, 1972). Phloem transport is important for distribution from mature leaves to growing regions in the roots and shoots. Phloem movement regularly follows the ‘sink-source’ relationship, from locations where carbohydrates are created (source) to places where they are consumed (sink).
Active transport (ATP)
Plasmodesmata
Diffusion/Mass flow
• Xylem – translocation is flux regulated and driven by water potential differences between soil, leaf and atmosphere.
Translocation differs between different ions, thus, nutrients are divided into three groups (Bukovac and Wittwer, 1957) – mobile, partially mobile and not mobile.