More fluid approach needed for plant nutrition

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Water soluble plant nutrients are marketed as either powder (crystalline) or fluid formulations which can, in principle, be applied through any system that delivers water to the crop whether by spraying or over-head irrigation. The only restrictions rest with the application system itself.
For instance, centre pivot irrigation systems used for fertigation invariably create self-wash off problems where nutrients landing on the foliage are perpetually washed off into the soil thus defeating the whole object of the fertigation process.
Omex Agrifluids is actively encouraging farmers to move away from fertigation with centre-pivot systems to the application of plant nutrients by power sprayer. “It may be ‘easier’ to apply nutrients via over-head irrigation but is considerably less cost effective due to the problem of ‘self-wash-off’,” said Peter Prentis.
Where crop spraying is concerned the requirement to apply high concentrations of nutrients may create blockage problems when used through ultra-low volume (ULV) and controlled droplet application (CDA) machines with correspondingly ultra-fine nozzles and spray liquid flow rates. However, hands-on experience has shown this is not a problem on cotton in Africa where Manganese (Mn), Boron (B) and Zinc (Zn) micronutrients, as well as NPK (Nitrogen:Phosphorous:Potassium) macronutrients have been successfully applied through hand-held spinning disc CDA sprayers (the Micron Ulva+) in Zambia.
Soluble nutrients may be applied by a whole range of spray application machinery whether self-propelled, tractor-mounted or trailed and portable sprayers including lever operated knapsack sprayers, pneumatic (pump-up) sprayers, shoulder-mounted low volume mistblowers and hand-held spinning disc ULV/CDA applicators.
Soluble plant nutrients are also being applied by aerial spraying using fixed-wing aircraft and helicopters over broad-acre crops like cereals in East Africa. Plant nutrients are traditionally applied to greenhouse crops through cold fogging machines and highly specialised aeroponic/hydroponic fogging systems which have been custom-designed for hydroponically grown crops.
The beauty of foliar feeding by spraying is that plant nutrients can be tank-mixed with insecticides and fungicides provided any issues with pH and compatibility can be avoided or overcome. “In terms of application soluble nutrients give farmers and growers a much greater freedom and flexibility and of the same order offered by fungicides and insecticides,” said Lowes. “The close relationship between plant health, including tolerance and resistance to insect pests and diseases, and a balanced plant nutrition is the obvious connection when using pesticides and nutrients within an integrated application package,” said Prentis.
Foliar feeding should not take place during periods of intense sunshine and ideally not in temperatures above 27°C especially when the atmosphere is dry and water vapour is leaving the cells. Low temperature conditions should also be avoided wherever possible.
Selective application timing avoids those periods when crops plants are under maximum stress with stomata (pores), usually concentrated on the abaxial (lower) leaf surface, being closed and therefore presenting a barrier to the rapid entry of plant nutrients into leaves.
As a general and practical rule farmers and growers are advised to select the coolest time of a hot day and the warmest time of a cool day. And spray when humidity is high which invariably means early morning or late afternoon and evening. More specifically, spraying is ideally carried out in the early morning with temperatures at 22°C or less, when leaf cells are turgid (full of water) and dew has collected on the leaf surface.
The pH of plant nutrient sprays should be between 6.2 and 7.0 which is weakly acid to neutral. This is critically important for keeping nutrients in solution during the entire spray operation, ensuring rapid entry of nutrients from retained spray droplets into the leaves and subsequent efficient utilisation by the leaf tissues as seen by fast growth responses.
Research shows that the more alkaline (over pH 7.0) is the nutrient spray solution then the lower the resulting plant growth rate achieved. Some recommendations suggest that a pH closer to 7.0 is better for plants with lots of young growth while one closer to pH 6.2 is better for those with a preponderance of established growth.
Uniform coverage over the surface of the crop is the aim but there are specific parts of the canopy where spray droplets will secure the best retention, dried nutrient deposits are best protected from rainfall or irrigation and nutrients will secure the most rapid and complete entry into the leaves.
These are the abaxial (lower) leaf surfaces and especially those of leaves inside the canopy. The lower leaf surface lacks a thick cuticle and wax bloom and is where most if not all of the stomata are situated. Tree crops like coffee and citrus which display a thick waxy cuticle on the upper (adaxial) leaf surfaces have stomata completely confined to the lower (abaxial) leaf surface. The lower leaf surface is essentially protected from the wash-off effects of rainfall and overhead irrigation and is always the last part to dry out, especially for leaves in the middle of the canopy.
Air assisted sprays are especially useful for opening up the canopy to incoming spray and maximising deposition of nutrient laden droplets on the abaxial surfaces of the leaves. The leading edge of the spray stream removes the layer of still air surrounding the outer canopy before flipping the leaves upwards to open up the canopy and expose the under-surface (abaxial surface) to incoming droplets following on in the main body of moving air.

By Dr Terry Mabbett

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