RAINWATER provides additional nutrients to plants through leaves. This principle is applied by spraying the foliage with solutions of desired nutrients.

Foliar fertilization is generally recommended for supplying additional nitrogen, magnesium and micronutrients but can also be used for phosphorus, potassium and sulfur.

In practical farming, it is used as a quick remedy for unexpected deficiencies, late supply of nitrogen during advanced growth stages, as a preventive measure against unsuspected deficiencies, and to overcome fixation of nutrients in soils (e.g., copper, iron and zinc). The main advantage is the immediate uptake of applied nutrients. It can be applied in limited amount. Nutrients present in inorganic salts or chelated forms can be used for foliar application.

Micronutrients application to foliage is widely practiced in fruit crop production. Boron, copper and zinc application in fruit trees has advantage of effectiveness, rapid plant responses, convenience, and elimination or reduction of toxicity. The disadvantages include low penetration rates in thick leaves, run-off from hydrophobic surfaces or getting washed off by rain, rapid drying of spray solution, limited translocation from uptake site to other plant parts, limited amounts of nutrients that can be supplied and often do not meet plant demands, and leaf damage/burn.

The effects are temporary, therefore annual sprays are necessary. In case of copper and zinc foliar nutrition, there is a reported danger of leaf and/or fruit injury. By proper timing, injury can be avoided but there is a need for better means of supplying these elements to fruit trees.

Micronutrients such as zinc, boron, and iron are required in relatively small quantities by plants (i.e., less than 100g to produce one ton yield). Thus, foliar sprays can prevent or correct a problem with relatively small amounts absorbed by the foliage but at the same time it has also been recognized that root uptake must be maximized in order to obtain the most benefit from foliar sprays.

Researchers have classified foliar-absorbed mineral nutrients into three groups: mobile, partially mobile, or immobile. The first are potassium, phosphorus, chloride and sulfur; the second zinc, copper, manganese, iron and molybdenum; and the third calcium. Very little is known about the mobility of foliar-absorbed boron (possibly partially immobile). Foliar application must consider the time of application, most commonly used effective concentration, dose per acre, dilute vs. concentrated application, mixability with pesticide sprays, and, finally, pesticides as a source of foliar nutrients.

Various surfactants like detergent surf and humactants (0.2 per cent calcium chloride) can enhance the foliar nutrition efficiency. However, calcium chloride should not be combined with boron sprays.

For orchard crops, soil application dose per plant is 175-435 gram zinc sulfate (23 per cent zinc); 45-90 gram borax (11 per cent boron) and iron sequestrene @ 3-5mg per litre of irrigation water to make up the deficiency of zinc, boron and iron, respectively.

Moreover, the dose after dissolution in 100 litres of water is, 440 gram zinc sulfate (23 per cent zinc); 450-900 gram borax (11 per cent boron) and 500-1000 gram of ferrous sulfate or one litre of iron sequestrene to make up the deficiency of zinc, boron and iron, respectively.

Cooperative agriculture extension services in the US recommend 60 gram of boron in 100 litres of water as maintenance dose while 100 gram per 100 litres of water as to correct the deficiency of this nutrient in orchard crops.

In case of zinc, the recommendations are 100-800 gram per 100 litres of water as maintenance dose, and 300-700 gram per 100 litres of water to correct the deficiency of zinc in fruit crops. For chelates or organic complexes farmers should follow the manufactures guidelines.

As some fungicides like Dithane M-45 and Zineb also contain zinc so recommended foliar doses must be accordingly adjusted. Orchards growers in the NWFP and Balochistan should be aware of the fact that zinc sulfate is not recommended for application within three days before or after applying oil to fruit trees.

There are no advantages in using chelated products of zinc in sprays as compared to their inorganic salts. Foliar-absorbed zinc is not easily translocated in plants which necessitates repeated sprays.

The addition of urea to zinc sprays might improve zinc absorption. For iron, the literature contains conflicting reports as to whether the iron chelates or inorganic salts are more effective. Too frequent spraying with boron after bloom may cause fruit drop, fruit breakdown of apples in storage, and possibly boron toxicity (dieback of shoots and veinal chlorosis).

The best timing for foliar sprays should be one or more of the followings; i) at new flush, ii) after fruit harvesting, iii) pre-anthesis/2-3 weeks prior to fruit bud differentiation, iv) at full bloom, and v) at small fruit formation stage.

Due to the restricted mobility of iron, zinc and boron in plant tissues and keeping in view plant physiology, the writers of this article are of the view that as orchard crops try to accumulate maximum amounts of essential nutrients before flower formation, so micronutrients based foliar sprays should be made preferably after fruit harvest and before flower formation, in addition to recommended deficiency doses already applied through soil.

However, after the review of literature some general recommendations regarding application timings for individual nutrients are also listed in this text for the interest of readers.

Foliar sprays of zinc, applied before anthesis may be most beneficial in terms of fruit yield in citrus and grapes. Bicarbonate-induced chlorotic leaves (in case of irrigation with high residual sodium carbonate water) should be sprayed with iron chelates twice during the growing season.

The first spray is recommended about four weeks after bloom and the second three weeks later. In literature, effectiveness of early but not late boron sprays is evidence that boron is critical for pollination or fertilization of flowers.

In apples pink flowering timing for boron sprays is often used because of the importance of adequate boron for proper pollen tube growth, flower fertilization, fruit set, and early fruit development. To improve the boron status of fruit trees for good yields, boron must be before bloom.

For foliar application to be effective, a substantial amount of the deficient nutrient must be added, but it should not cause plant damage, leaf scorching, and negative osmotic effects. The solutions must be dilute (1-2 per cent), especially if they contain nutrient salts.

Foliar fertilization is at the best a supplement to soil application and not a substitute for it. Crops are less sensitive to organic compounds because they have only a slight osmotic action. Therefore, urea is better tolerated by leaves than is nitrate or ammonia and it enables the application of concentrations up to 15 per cent with low-volume sprayers. Where urea is used for foliar nutrition, it should contain no more than 0.25 per cent biuret.

The same applies to micronutrient sprays through chelates vs. inorganic salts. With the exception of nitrogen, foliar application can supply very limited amounts of major nutrients such as phosphorus and potassium compared with their total requirements.

The situation is a little better for calcium, magnesium and sulfur, but even these can be added only in limited amounts, which are often insufficient in a single application.

The best results are obtained with micronutrients because a relatively large portion of the total requirement can be supplied in a single spraying. In cases of marked deficiencies or mobility problems within the leaf, repeated sprayings with micronutrients are essential, as in the case of iron and manganese. Foliar fertilization can be combined with crop protection spraying, but the mixed components must be compatible.

Several types of sprayers are employed for foliar application. A greater volume of solution is required per unit of area in the case of high-volume sprayers.

The commonly employed procedures involve: (i) spray about 400 litres/ha of a solution in fine 0.1–0.2-mm droplets; or (ii) high-pressure, low-volume sprays where the solution is blown at the leaves in small droplets.

Higher nutrient concentrations can be used with low-volume sprayers. In either case, there should be good adhesion of solution to the leaves. This can be improved by adding special detergents and stickers.

Spraying is most effective, and the risk of scorch is minimized, where the spray droplets do not dry rapidly. This is best achieved by spraying on cloudy days or in the early morning or late afternoon. Application of nitrogenous solutions should be avoided during the early growth stage.