More fluid approach needed for plant nutrition

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Consequences of zinc lock-up in high alkaline soils shows up most readily in crops like citrus which have a high zinc requirement. On the other hand, excessively acid soils approaching pH 5.0 may increase the amounts of root-available zinc to phytotoxic levels. Zinc plays an important role in plant adaptation to stress.

Targeting soluble nutrients by spraying or fertigation (nutrients dissolved in irrigation water) onto the foliage gives an altogether more efficient and effective outcome. Nutrient(s) are deposited on the foliage where the single-cell layer of leaf epidermis and any associated waxy coverings (cuticle and wax bloom) is the only barrier to rapid entry and prompt utilisation in the leaf tissues.

Nutrients are retained on the leaf surface as droplets of solution (and later as a dried deposit) before entering the foliage (leaves and green stems). These chlorophyll containing tissues are the sites of photosynthesis and where most specialist biochemicals like plant hormones and plant metabolites such as amino acids are synthesised. They are, therefore, those plant parts with most physiological activity and therefore the highest nutrient demand. Nutrients enter most readily through open stomata but also in continuous and sustained amounts directly through the leaf epidermis.

Pioneering research which first showed the huge benefits of foliar feeding over traditional soil application was carried out by Dr H B Tukey and colleagues at Michigan State University in the 1950s. By applying radio-labelled phosphorous and potassium and then using a Geiger Counter to trace absorption, movement and utilisation of the respective ions they demonstrated the 95 per cent efficiency of foliar application compared with only 10 per cent efficiency for soil application.

First requirement is to make sure that the delivery system (formulation) is physically and chemically compatible with the job in hand including a spray water pH that does not impact negatively on nutrient solubility, and a nutrient solution concentration that does not block nozzles in the application system (the sprayer).

The former may necessitate acidification of plant nutrient products unavoidably delivered in inherently alkaline water supplies. This is a big issue in Romania where powder products will readily ‘fall out’ of solution unless pH of the water used for spraying is suitably adjusted downwards to a lower pH.

Some companies claim that problems associated with soil applied nutrients are overcome by using humectants which are hygroscopic carbohydrate substances such as sorbitol.  They absorb water and are therefore claimed to assist soil penetration of nutrients dissolved in the soil water.

However, this soil supplementation approach cannot match the efficiency of foliar feeding. Leaves present the most focused and appropriate target due to their large surface area for retention, minimal barriers to nutrient entry and being the site where most of the plant’s nutrient capacity will be utilised.

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