March 21, 2019 7:23 am Published by KirstyH
Broccoli harvest

RICH VOLCANIC SOIL, ample rainfall and an equable climate are core advantages of growing vegetable crops in the tropical highlands. However, the naturally high soil fertility of upland soils in the tropics is not enough to satisfy the nutrient demands of fast-growing vegetable crops. Fertiliser application is a crucially important component of any cultivation schedule.

Spray application of soluble nutrients is the most efficient and targeted way of meeting the nutritional needs. Foliar feeding with soluble products is almost twice as effective in getting nutrients into a plant compared with the equivalent amount applied as solid base fertiliser to the soil.

Upland areas across Africa produce ‘high-flying’ fresh vegetables exported daily by air to lucrative markets in Europe and the Middle East. Vegetable crops are now grown extensively in upland areas of Kenya, Tanzania, Uganda, Zambia and Zimbabwe, but growers are not always sold on the benefits of foliar feeding. Some remain unconvinced, not so much about efficiency but efficacy given the high rainfall and humidity typically experienced at higher altitudes in the tropics.

Allaying farmers’ fears

The first and foremost concern for growers almost certainly relates to assumed wash off effects of frequent and intense rainfall on soluble nutrients deposited by spraying on foliar surfaces.

However, an important component of high performing foliar sprays of soluble nutrients is the speed with which individual ions (eg NO3-, Ca2+ and Zn2+) pass into the plant and integrate into the metabolism. Provided the spraying takes place during a suitably dry weather window then nutrients will have sufficient time to make their way into the plant. Once inside the plant nutrients are safe and available for use, unlike soil-applied solid fertiliser which is subject to continual solubilisation and leaching from the soil.

It is worth pointing out that the amount of solid fertiliser applied to soil, compared with soluble nutrients applied by foliar spraying, is necessarily large, precisely because of continual solubilisation and leaching with comparatively little taken up from the soil by plant roots.

The view from OMEX

To obtain a deeper insight into the advantages and benefits of using foliar feeding, even under the ultra-high rainfall conditions typically experienced in the tropical highlands, I spoke with Alan Lowes (Regional Director) and Peter Prentis (Managing Director) at OMEX Agrifluids who export their soluble nutrient products throughout the world.

“OMEX’s soluble liquid nutrient products contain an organic humectant/sticker which ensures the sustained uptake of nutrients in dry conditions and slows down wash-off during rainfall,” said Prentis.

Regional Director Alan Lowes has extensive experience with vegetable growers throughout east and southern Africa, including Kenya, Uganda, Tanzania, Zambia and Zimbabwe. “In addition to their primary plant nutrition function many of these products also impart disease resilience”, said Alan.

First on Alan’s agenda for vegetable growers, and logically so, is OMEX Bio 20. This product is particularly appropriate for seedling plants to ensure these very early stage crops are provided with a full range of nutrients and simultaneously given a biostimulant boost.

“This combination of a full complement of macronutrients and micronutrients, plus a specific seaweed derived natural biostimulant, maximises yield and quality of vegetable crops which may be at risk of physiological stress brought on by high temperature, moisture unavailability and disease,” said Alan Lowes.

He also highlights OMEX K41, a highly concentrated liquid potassium foliar feed currently used widely on vegetables in the highland regions of Latin America. OMEX K41 contains very low levels of nitrogen thereby minimising soft leafy growth to reduce disease incidence, while maximising translocation of sugars to storage organs (eg onion and garlic bulbs) and fruit such as tomatoes and capsicum peppers.

“OMEX DP98 is the classic example of a missed opportunity for vegetable growers who fail to take advantage of foliar feeding,” says Peter Prentis. DP98 is custom designed by OMEX for its high phosphorous content [N (4.0 per cent); P (37.50 per cent) and K (17.50 per cent)] but crucially with phosphorous (P) as fully water-soluble phosphite (PO3-) rather than traditional phosphate (PO4-).

“Base phosphate fertilisers applied to the soil regularly fail to furnish crops with sufficient P macronutrient” said Prentis, “due to factors surrounding the type, moisture status and pH level of soils.” OMEX DP98 can crucially ‘step in’ during critical periods of plant growth and development with boost and benefits which are far too large to be explained by plant nutrition alone. That’s because phosphite is a so called ‘elicitor’ which triggers anti-fungal and antibacterial responses in the crop plant, thereby assisting in the prevention of microbial infection and the inhibition of disease development.

OMEX DP98 also has the ability to rapidly transport cations like calcium and magnesium into plant tissue when used in a tank mix with products like OMEX CalMax (22.5 per cent weight/volume calcium and 3.0 per cent weight/volume magnesium). Rapid assimilation of these nutrients prevents their wash off in high rainfall areas.

And last but not least, is OMEX CalMax with its 22.5 per cent soluble calcium as the core component alongside magnesium and micronutrients. OMEX CalMax is a formidable tool for maintaining plant tissue resilience to disease. Calcium in calcium pectate, an ‘adhesive’ compound which cements the cellulose walls of adjoining cells together to form structurally sound plant tissues, performs key roles in cell division, tissue integrity and the permeability of the walls separating living plant cells.

Potato (Solanum tuberosum) is a crucial crop in the African highlands

As such it plays an important part in mitigation against tissue weakening effects caused by plant pathogens and disease. For instance, calcium deficiency is the primary cause of ‘blossom end rot’ in tomato, capsicum and aubergine fruit. This physiological damage is subsequently exploited by Phytophthora pathogens. Potato (Solanum tuberosum) is susceptible to shortfalls in calcium. Calcium deficiency causes internal browning of potato tuber flesh and inferior tuber skin strength and finish, thus increasing tuber susceptibility to infection by a range of potato storage diseases. These twin effects hit both tuber storage longevity and tuber marketability.

“This group of specialist nutrient and biostimulant products (OMEX Bio 20, OMEX DP98, OMEX K41 and OMEX CalMax) are crucially important for vegetable growers, including those in upland environments who must satisfy the nutrient needs of fast-growing crops while conferring maximum plant resilience to disease,” said Alan Lowes

Dr Terry Mabbett