The demand for food and especially vegetables is increasing in the RSA, as a result of e.g., rising living standards. Higher prices are being fetched for better quality agricultural products. The producer must utilize this opportunity to apply additional and more intensive cultivation methods, to cope with the demand. This will also ensure a higher income for the farmer.
Intensive cultivation methods usually result in higher production costs in the form of larger capital outlay. Running costs are also higher and have to be recovered from higher product prices. If this is not done, it means a financial loss for the farmer making use of these more specialized methods.
In view of the high cost of construction and operational costs associated with the conventional tunnel or greenhouse for the cultivation of products such as flowers and vegetables, a need has arisen for the design of similar structures at lower cost. A study has therefore been made on three different designs, whereafter a comparison was made between them.
Plants are able to fulfil their biological function best when the temperature of the plant is between 15°C to 30°C. To achieve this and still ensure that enough sunlight reaches the plant, materials such as plastic and glass are used as cover materials for greenhouses. These materials, however, do not allow circulation of air, therefore cooling and heating systems have to be used to keep the temperature inside the structure within limits. These systems increase the capital as well as the operational cost of the structures considerably.
In the research that was undertaken, investigations into the use of shade-net covers for structures were done. Shade-net allows air circulation through the structure and shields against direct radiation, which ensures that the plants are within their optimum temperatures. The temperature within the structure may sometimes be outside the optimum temperature levels of the plant, but it has been found that these deviations do not justify the high cost of cooling and heating in regions with a moderate climate.
For research purposes, a wooden shade-net structure was erected by researchers on the ARC NRE-Agricultural Engineering Campus. The consideration was that the small-scale farmer will probably not possess the necessary equipment, expertise, or capital for the erection of the steel structure. A welder and knowledge of welding are also required for erecting the steel structure, but not for the wood structure.
ERECTION OF THE WOODEN SHADE NET STRUCTURE
Planting of poles
Before holes are dug for the planting of the poles, pegs have to be driven into the ground where the poles have to be planted. This will ensure that the poles are planted in line.
To ensure that the poles are planted in an accurate rectangle, a peg is driven in where the one corner post should be. A peg is then driven to where the next corner post should be, by measuring the distance. To determine the position of the third corner post, two measuring tapes are used. Mark the correct distances on the measuring tapes and bring the two marks together from the first two poles. The fourth corner post is planted in the same way as the third.
To ensure that the structure is erected correctly, the ground surface where it is to be erected must be as level as possible. If not, the poles will each have to be planted individually at the correct depth, but the structure will appear distorted.
After the poles have been planted, all the cables are connected and then secured with the anchors. All these cables (anchor and other cables) are braced evenly to prevent contortion of the structure. The cables can be anchored to the ground very cheaply, by planting a 1,2 m pole 1 m into the ground. In order to obtain more resistance against the pulling force of the cable, the pole must be at a slight angle away from the pull direction of the cable. The anchor poles can be seen clearly at the top end of the structure.
The structure is now covered with shade netting. First, measure a length of approximately 12 m of shade-net and thread steel wire (2,5 mm thick) through the eyes at the edges of the shade-net. The shade-net is then pulled over one of the roof ridges. The wire as well as the shade-net is secured to the one side of the structure. The wires are now braced in the valleys, then the shade-net over the length of the structure. Leave approximately 1 m of the shade net at the ends of the structure to cover the openings beneath the ridges. To cover the “walls” of the structure with shade net, the correct length of net is measured to cover the whole structure. A steel wire is threaded only on one side of the eyes. The wire and shade-net are now secured to the corner post where the entrance should be, where the wall and ridge join. The shade-net is now braced at the correct height right around the structure. The shade-net is hammered to the pole where bracing began, and a wire is threaded through the shade-net at ground level. This wire is for pulling the shade net down.The full article is for subscribed members only. To view the full article please subscribe. It’s FREE!Log In Register
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