A successful end to growth and maturation of the bulb onion crop. (Image source: Omex)

When thinking about tropical crops, onions (Allium cepa) are not the first things that spring to mind but contrary to such notions, onions are grown throughout the tropics

No less than 10 African producers appear in the the top 40 world producers rankings. In ranked order of production, they are Egypt (3.1 million tonnes (mt); 3rd), Nigeria (2.0mt; 7th), Algeria (1.52mt; 12th), Sudan (1.5mt; 13th), Niger (1.0mt; 22nd), Morocco (0.69mt; 26th), South Africa (0.68mt; 27th), Senegal (0.42mt; 35th); Uganda (0.33mt; 39th) and Ethiopia (0.33mt; 40th).

Wherever onions are grown, irrespective of climate, weather and soil, the crop has exacting nutritional needs and what’s more, onions tend to be grown in the drier areas of the African continent and frequently under irrigation. On both counts this means the logical way to provide onion crops with fertiliser is by foliar feeding using spray applications of completely soluble nutrients. 

Who better to speak to on this subject than Omex Agrifluids, a leading designer and manufacturer of soluble nutrient products which are sold for use on a huge range of crops worldwide. Omex Agrifluids have just designed a programme and schedule for foliar feeding bulb onion crops throughout Africa. I travelled to the company’s headquarters at Kings Lynn in the eastern region of the United Kingdom to learn more from Ben Odunlami, technical sales manager, Africa and Oskar Winkler, export sales manager, Africa. 

"Our programme," said Odunlami and Winkler, "is based on sequential stages of development for bulb onions and provides growers with product names; Dose/hectare (ha) in 200 litres (l) water; amount of product in a 15-litre capacity, high volume, hydraulic knapsack sprayer; the method of application and the physiological function of the product and its nutrients in the onion plant."

Stage 1 - Transplanting the onion plant seedlings

Transplanting is the most precarious time for crops and bulb onion crops are no exception. It is a time when the fledgling plants stripped of the root hair, complement when ‘pulled’ from the nursery bed, require stimulation and a boost to root biomass to ensure rapid plant establishment. Odunlami and Winkler say Omex Bio 20 (a highly concentrated emulsion) does the trick and quickly too, by furnishing newly transplanted onion seedlings with all the essential plant nutrients, macronutrients and micronutrients, and biostimulation from hormonally active components derived from a specific marine alga (seaweed).  

This combination promotes root biomass and therefore maximum uptake and utilisation of water and nutrients, as well as a bulwark against physiological stress whether caused by high temperature, drought, disease or any other abiotic or biotic factor. Besides providing a ready nutritional source, it also helps the plant establish its own hormonal mechanisms controlled by auxins and other natural plant growth regulators. At this transplanting stage, Omex Bio 20 is applied as a drench in the irrigation system and at a dose rate of 200 ml/ha in 200 litres of water (equivalent to 15 ml of Omex Bio 20 in a 15L capacity hydraulic, high volume, knapsack sprayer).

Stage 2 - Establishment of the onion seedlings (2 true-leaf stage of growth)

Once the seedlings are established after attaining the 2 true-leaf growth stage, they are given a second boost from Omex Bio 20 plus a consolidation and strengthening treatment using one of Omex’s newest and most innovative products. “This novel product is called Omex Zibo”, said Ben. “It is a highly concentrated liquid suspension containing zinc (22.10% w/v) and boron (7.30% w/v), two crucially important micronutrients with many important functions. Zinc is pivotal in the formation and modulation of enzymes which are critical in the early stages of root and shoot development and is also important in the production of proteins from amino acids, and therefore clearly important during periods of rapid plant tissue growth. Zinc also helps to maintain membrane permeability, thus enhancing stress tolerance”, said Odunlami and Winkler.

Zinc deficiency results in the rapid onset of marked deficiency symptoms with a general stunting of plant growth associated with distorted auxin production. During the early stages of zinc deficiency, the younger leaves become yellow (chlorotic) with the development of pitting on the interveinal surfaces of the mature leaves. As the deficiency progresses, early symptoms develop into an intense interveinal necrosis while the main leaf veins stay green.

“Role of Boron is no less important and together with zinc is closely involved with the synthesis of cell wall material and the transport of sugars,” said Odunlami. "And therefore has crucially important tissue strengthening and growth consolidation roles," added Winkler.  

Boron deficiency is common in crops growing in soils with a pH of less than 6.5. Boron deficiency in onions results in blue/green coloured leaves with the younger leaves becoming mottled with distorted, shrunken areas. Cracks may appear on the upper surface of the lower leaves, which become stiff and brittle. Root development is poor. Omex Bio 20 and Omex Zibo are applied as foliar sprays at respectively, 1.0 l and 250 ml per ha in 200 l of water, which is equivalent to 75 ml and 18 ml, respectively, in a 15 l capacity knapsack sprayer.

Stage 3 - Vegetative growth of onion plants (4 to 7 true leaf stage of growth)

Starting at the 4 to 7 true leaf stage of growth, the onion crop is ready for another twin burst and boost from Omex Bio 20 and Omex Zibo, applied in the same way and at the same rates as previously applied. The twin targets are fast and sustain root growth power and support for a spurt in sustainable above ground vegetative growth. With this achieved, the plant can maintain a fast rate of photosynthesis for the next and perhaps most crucial stage in the bulb onion crop, which is bulb initiation.

Stage 4 - Onion bulb initiation (stage at which the diameter of the bulb is twice that of the neck of the onion plant and when the plant is carrying 8 to 12 true leaves)

Bulb onion growers are recommended to continue using Omex Bio 20 albeit at half the rate used previously in the plant establishment and vegetative growth stages. “And now is the time to introduce a new product”, says Ben and Oskar, “and in the form of Omex CalMaxB which is also applied by foliar spray application”.  

Omex CalMaxB contains calcium and boron at concentrations of, respectively, 22.50% w/v and 1.53% w/v, plus micronutrients, nitrogen and magnesium. “This combination continues to provide the growth boost to onions and essential to ensure bulb filling continues unabated,” said Winkler, with Odunlami adding how the calcium/boron combination in CalMaxB complements growth with a tissue strengthening dimension. “This ensures the onion bulb gets off to robust start in its journey through bulb development and into maturity,” they said.

Calcium deficiency in onions is characterised by die back of leaf tips and short lengths of leaf and without any previous yellowing (chlorosis) of the leaf. This causes the top of the leaf to fall over and die. Calcium is a crucially important nutrient for strong tissue growth and good storage quality of onions. Both Omex Bio 20 and Omex CalMaxB are applied as foliar sprays at doses of, respectively, 0.5 l and 1.0 l per ha in 200 l of water. This is equivalent of 37 ml and 75 ml, respectively, in a 15L capacity knapsack sprayer.

Stage 5 – Bulb development (this point is reached about 100 days after planting depending on the onion variety. The leaves continue to grow and the number of leaves remains the same)

The fifth and final stage of bulb development, and ensuring the grower’s previous investments and efforts are not wasted, is reached in approximately 100 days post planting. The strengthening benefits of Omex CalMaxB are continued alongside the application of Omex K41, a highly concentrated, water soluble emulsion containing potassium (41.0% w/v), magnesium (2.50% w/v), sulphur (4.80% w/v) and nitrogen (3.00% w/v). To ensure optimal water relations during the final phase in the onion crop, which is thanks to potassium at the bumper concentration of 41.0% w/v.

“Potassium is called the ‘gatekeeper nutrient’ because it has such deep-seated and far-reaching influence and effects on the onion plant’s water relations, including stomatal movements, and therefore photosynthesis, but also, and crucially, the transport of soluble sugars from the leaves and down into the now fast growing bulb,” said Odunlami and Winkler.  

Potassium is very mobile within the plant which means that deficiency symptoms will only develop on young leaves and in the case of extreme deficiency. Tip burn symptoms, leaves turning dark green and erect and promotion of bolting are all symptoms of potassium deficiency in onions.

Omex Bio 20 and Omex K41 are applied as foliar sprays at dose rates of one litre each and equivalent to 75 ml of product in a 15 l capacity knapsack sprayer.
“Overall, this combination of products applied in the fifth and final developmental stage promotes and ensures bulb growth, consistency within the crop and high quality onion bulbs, while maximising the shelf-life of onion bulbs in storage”, they said.