Role of foliar application in plant nutrition

The predominant route for nutrients to enter plant tissue is via uptake through the roots. However, there are many situations where foliar fertilizer application to leaves, fruit or shoots can be a very effective complementary delivery system.

Leaves are not ideal for water and nutrient uptake, because their architecture is to protect crops against water loss as well as fungi and pest attack. The cuticle, (the outer cell layer on leaves sealed with wax) works as a barrier to the entrance of nutrients into the plant.  However, nutrients in the ionic form can penetrate either the cuticle via small cracks or through the stomata, the pores in leaves for gas exchange (Fig. 1).

Fig. 1: Leaf surface illustrating possible ways of foliar nutrient uptake (schematic)

Foliar applications are either aqueous solutions or suspension of nutrients. Application rates are low, 0.5 to 10 kilograms of foliar fertilizer per hectare, dissolved in 50 – 2500 litres of water. The water rate depends on crop size and density. Foliar fertilizers are often used to supplement micronutrients (Mn, Fe, Zn, Cu, B, Mo) and occasionally the secondary nutrients (Ca, Mg).   

Foliar application to treat absolute deficiency

All nutrients play an essential role for growth and development of crops. The shortage of a single nutrient becomes a bottleneck of production since vital metabolic pathways are impaired. As a consequence shape and colour of crops deviates from those fully nourished as shown in figure 2.

Fig. 2: Zinc deficiency of maize. Symptoms are stunted growth, shortened internodes and leaf chlorosis along the midrib. (source: www.intra.yara.com ->Our knowledge -> AIM -> CropCheck)

Visual symptoms can tell an experienced grower which nutrient is lacking. Foliar application of the nutrient concerned will provide an immediate relief of the problem and a normal crop yield can be achieved.

Sometimes there are no visible symptoms, but foliar sprays are applied to treat latent deficiencies (hidden hunger). The need for such treatments is determined by a chemical analysis of the crop. For almost all crops critical nutrient values are available which indicate the adequate nutrient content in the tissue. If the analysis for a particular nutrient is below its critical value, foliar applications can be made to correct the problem. Yield responses to foliar applications can be substantial (fig. 3).

Fig: 3: Effect of foliar application on yield of winter barley. Average of 5 field trials, conducted in Germany on sites with latent manganese (Mn) and copper (Cu) deficiency.

Foliar application to prevent nutrient deficiencies in fruits

Nutrient availability is not the only bottleneck for adequate nutrient supply to crops. Limited nutrient transport to different plant parts can also cause localized deficiencies. In particular the transport of boron and calcium in the plant relies on the transpiration flow, which means that tissue with low transpiration rates, eg. fruit, can easily become deficient in calcium and boron.

For instance in apples, the low transpiration rate of developing fruit is the main reason for bitter pit, a physiological disorder caused by calcium deficiency (fig. 5). To enhance apple quality up to 8 calcium sprays to the fruits during bulking are recommended.

Fig. 4: Field with winter barley suffering from soil induced manganese deficiency in the pale green patches next to dark green tracks with adequate manganese supply.

Foliar application to prevent nutrient deficiencies in fruits

Nutrient availability is not the only bottleneck for adequate nutrient supply to crops. Limited nutrient transport to different plant parts can also cause localized deficiencies. In particular the transport of boron and calcium in the plant relies on the transpiration flow, which means that tissue with low transpiration rates, eg. fruit, can easily become deficient in calcium and boron.

For instance in apples, the low transpiration rate of developing fruit is the main reason for bitter pit, a physiological disorder caused by calcium deficiency (fig. 5). To enhance apple quality up to 8 calcium sprays to the fruits during bulking are recommended.

Fig. 5: Incidence of bitter pit

Efficient foliar fertilizers

Efficiency of foliar applications depends on crop type, leaf coverage, weather conditions and product quality. It is estimated that foliar treated crops will absorb about 15 – 40% of nutrients applied. Efficiency of the foliar fertilizer is improved when the formulation contains additives like wetting agents to enhance the coverage of leaves or stickers that prevent nutrients being washed off by rainfall.

Together with our alliance partners Phosyn, Akzo Nobel and SQM, Yara offers growers an extensive portfolio of foliar fertilizers combined with the knowledge about optimum application rate and timing. On-going joint research with our partners will develop new products and applications to further contribute to improved human and animal health through improved plant nutrition.