FOLIAR feeding of crops is the process of spraying nutrients on pant parts including leaves and stem, where they get readily absorbed and reach the site of activity. Foliar feeding has resulted in cost effectiveness of fertilisers, improved crop tolerance against stresses like heat, drought, insect pests, diseases etc., superior food quality due to enhanced protein contents and ultimately increase in yields.

Nutrient absorption efficiency may be improved by 12-20 times in case of foliar feeding as compared with conventional application of fertilisers resulting in high fertiliser use efficiency to minimise expenditures incurred on fertiliser purchases. However, this efficiency will only be realised if the principles and precautions of foliar feeding are followed strictly.

Vegetative development, fresh and dry weight of the plant tissues, plant height, flowering commencement, flower numbers, fruit initiation, fruit size and fruit weight are positively influenced by foliar feeding eventually resulting in better quality and quantity of produce.

An important benefit of foliar feeding is the increase in uptake of nutrients by plants from the soil as with provision of nutrition through foliage causes the plants to exude more sugars into root zone which promote microbes’ activity to enhance biological nitrogen fixation as well as availability of essential plant elements. Moreover, the technology has great significance for crops sown on problem soils where the normal uptake of the nutrients by crop plant is restricted due unfavourable growing conditions.

Foliar feeding, although being used for a wide variety of crops including cereals, flowers, fruits, pulses, oilseeds and vegetables, is most suited to horticultural crops which economically are more important. For instance, the technology is being used on large scale to cure zinc deficiency in grapes.

Foliar feeding is equally effective for application of both minor and major plant elements. Deciding the time for foliar feeding is very important. Therefore, agricultural scientists have suggested some approaches to decide for the appropriate timing of foliar feeding. A very simple technique is ‘deficiency history or deficiency symptoms, which make use of the previous knowledge about a certain crop or field. For example, if rice fields are found deficient in zinc, they can be sprayed with zinc on the basis of past experience or field history.

The other, most authentic and reliable method to decide for foliar feeding involves the tissue analysis of crops for deficiency of a certain nutrient element. A great deal of care is required in collection and transfer of sample tissues to laboratory for analysis so that they must be true representative of the whole field and be shifted in packed form. Utilisation of refractometer is another method to determine nutrient deficiency and subsequent need for foliar feeding, which make use of dissolved salts concentration in the plant tissues as indicator of nutrient status of the plant tissues.

Foliar application of nutrients is as simple as the spray of herbicides or insecticides. Conventional hand knap sack sprayer is more suitable for small scale farming while growers with large acreage can use tractor mounted boom sprayers. Calibration of the sprayer to calculate the quantity of water to be used on a specific area is the first step. More or less 100 liters of water is used for one acre. Electrostatic sprayers and sonic blooms are the modern technologies being used in developed countries which impart charge to spray particles and make use of sound respectively, to enhance absorption of the nutrients on plant parts.

The concentration of nutrients in the solution to be used for foliar spray is very crucial. Highly concentrated nutrient solutions if used will result in crop foliage burn especially in case when the salts contain chloride as constituent e.g. potassium chloride. An optimum range for different fertiliser salts to prepare solutions for foliar feeding has been suggested by the researchers which can further be modified depending on deficiency level, plant species, stage of plant growth etc.

Recommended concentrations of different nutrients for foliar feeding:

Cleanliness and quality of water to be used for fertiliser solution preparation is of utmost importance. Water should not only be clean from soil or straw to avoid clogging of the nozzles but must also be free from disease pathogens and possessing a neutral pH range (6.0-8.0). pH of water, if not neutral can be adjusted by adding vinegar or baking soda to it in order to decrease or increase pH respectively. Hydrogen peroxide and chlorine can be mixed in trace amounts in water to kill disease pathogens and bacteria. Water if chlorinated, must be stayed for overnight to remove harmful effects of chlorine from water.

Weather conditions like temperature, wind, relative humidity and the day time have a strong influence on the success or failure of foliar application of nutrients. For instance, a temperature of below 25æC, minimum wind speed, high relative humidity and early dawn or evening times are recommended for foliar application. High temperatures reduce enthrallment of nutrients by foliage while humid condition especially the presence of dew on leaves facilitates absorption.

Pressure and the angle of the sprayer influence the nutrient absorption by crop plants. Increase in pressure of sprayer, adjusting boom sprayer nozzles at an angle of 45æ and use of mist blower result in very fine mist which gets readily absorbed into plant system. Absorption is further enhanced when the spray particle reach on the underside of the leaf where the stomata are located and also by the addition of surfactants in the solution which increase retention of nutrients on plants by decreasing surface tension.

For economical application of nutrients using foliar feeding, fertiliser solutions can be mixed with insecticides or herbicides if the time of spray for both coincides. However, both the solutions must have no antagonism to avoid damage to plants as well as clogging of the sprayer nozzles. This can be checked by mixing a small quantity of both and taking observation either some precipitation is formed or not. In case, no precipitation results, both the solutions are compatible and can be safely sprayed in a single application.

Though research is being conducted in the country on the foliar feeding technology in various institutes as Narc (National Agriculture Research Centre), UAF (University of Agriculture, Faisalabad) etc., however, still too much efforts are required to strengthen the knowledge of the subject and proliferate technology quickly to farmers for gaining enormous benefits.