SOIL medium is an essential and basic component of any agriculture system. Maintenance of soil quality by proper management is important for successful crop growth. While soil provides essential requirements like water and nutrients, change in basic soil reactions through excessive cultivation and degradation takes place constantly. Loss of soil quality and property reduces production. The challenge lies in the development of technological innovations to combat food inadequacy.

The science of food production demands supply of agricultural inputs, while protecting the environment through proper management. Foliar application of nutrients is not a substitute for soil fertilisation but can be an alternative where crop needs are critical and soil application expensive, time-consuming or ineffective.

The leaves of a plant absorb nutrients supplied in a liquid or aqueous medium. This capacity is exploited in many agronomic practices like application of inorganic nutrients, growth regulators and herbicides. Foliar supply of nutrients has many advantages over root-feeding. There has been considerable interest in the practical use of this technique and several accomplishments have taken place not only in understanding the mechanism involved in foliar uptake, but also in the development of chemical and surfactants.

Several inorganic nutrient elements are generally required by plants for their growth and development among which 25 per cent have been identified. Plant roots have two important roles to provide anchorage and abstract water and nutrients from the soil. The root and shoot are mutually dependent. The shoot receives inorganic nutrients from root, and metabolites from shoot are transported to root. The medium for plants contains all factors necessary for growth - water, carbon dioxide, inorganic salts and diffuse sunlight. All plant parts undergo two vital processes – absorption of solutes and the process of photosynthesis.

The evidence that the leaves could be fed with inorganic salts effectively came from a number of scientists in the last century. Reports on beneficial effects of foliar feeding with compounds containing nitrogen (N), manganese (Mn), calcium (Ca) and boron (B) on several crops appeared during the next few decades.

Urea sprays for N were practiced with many crops in several countries. The introduction of radioisotopes in early 1950s marked the beginning of our understanding of foliar uptake of inorganic nutrients and organic substances.

The need to exploit the capacity of plant leaves to absorb inorganic nutrients has increased greatly for a number of reasons (i) adverse soil conditions which favour fixation of nutrients and thus render many essential ones unavailable for root absorption; (ii) root absorption is slow for some elements and also results in poor translocation; (iii) relatively large amounts of fertilisers are required for root supply and heavy application loads to soil-water pollution. Foliar supply of nutrients can result in increasing the photosynthetic efficiency and it is possible to modify the physiology of leaf.

The knowledge on the mechanism of foliar uptake cuticular penetration, absorption by leaf cells, cell-to-cell transport within the leaf and the transport out of the leaf to other regions has widened in the last few years. Furthermore, there has been a revival of interest in this field, both for basic research and practical application.

Mechanism of foliar uptake: The inorganic nutrient elements given in the form of aqueous sprays have to be first absorbed by the leaf before translocation to other parts. The mechanism of absorption by leaf cells has been studied using leaf disks and its slices. Light is found to enhance the absorption.

The relative mobility of various elements was studied as early as 1957 with the aid of radioisotopes and grouped into three categories based on their rate of transport out of the leaf. Since then, the techniques of application and measurement of transport have been refined by several workers and based on these, the relative mobility of elements can be grouped as follows: i) Freely mobile: N, phosphorus (P), potassium (K), rubidium (Rb), sodium (Na), molybdenum (Mo); ii) Partially mobile: iron (Fe), Mn, zinc (Zn), Mo, copper (Cu), B; iii) Relatively mobile: Ca, magnesium (Mg). There are environmental factors like light, temperature and relative humidity which directly and indirectly affect foliar absorption. Young leaves have been found to absorb nutrients more effectively than the one matured ones.

Of major nutrients, the N has been extensively used as foliar sprays on many crops while urea is the source. Urea sprays for fruit trees, and crops like sugarcane and pineapple have been in commercial practice in many countries for a long time. Workers revealed that post-harvest urea sprays resulted in greater N uptake and distributed to new growth in apples. The interest in foliar application of N has been renewed in the last three decades, especially for timely supply of N to soybean during the ‘grain-filling’ period. There are two contrasting features in soybean with regard to N nutrition. Being a leguminous crop, N application depresses N fixation. On the other hand, the N requirement for soybean is one of the highest among field crops. Foliar nutrition studies have been extensively carried out and application of N sprays to cereals is a regular practice particularly for increasing the longevity of leaves. Study indicates that foliar sprays with N, P, K and S at late reproductive stage are superior to early application on the ear length of corn.

Foliar sprays of urea during winter are effective in increasing N levels and not the yield of passion-fruit. Foliar application of P and K has been reported to benefit the groundnut crop grown under salt stress. A new role for foliar nutrition is in the regulation of water use efficiency of fruit trees. Sprays with complete nutrients and with KCl help apple trees withstand water-stress. Studies have shown that foliar application of N, P, K and S increase grain yield of corn.

Plants with large leaves provide more area for the retention and absorption of spray liquids and they respond better to foliar nutrition. Addition of the CaCl2 augments the effect of N, P, K spray.

In sugar beet, which has a large leaf area, foliar sprays with three per- cent N, P, K increased the fresh weight of roots and sugar yield by 31 and 40 per cent, respectively. One area where foliar application is most effective is in the control of micronutrient deficiencies. Foliar application for the control of B, Cu, Mg, Mn and Zn has been in vogue for a long time and in many crops because of prompt increase the yield of vegetables grown in the acid soils of Florida everglades. The effectiveness of micronutrient spray application in different crops is listed as: Fe (sorghum, sugarbeet, fruit, trees, vegetables, ornamentals, cacao); Mn (fruit, trees, vegetables, soybean); Zn (apple, pear, citrus, grapevine); B (apple, tomato, cantaloupe turnip); Cu (apple, pear, citrus, onion); Mo (citrus, field crops); Mg (apple, fruit trees, tomato); Ca (apple, tomato, strawberry). It is clear from the above discussion that foliar applications have far-reaching effects on the growth and developments of plants.