hydroponic greenhouse plant populations for tomato production in the eastern cape

Plant Population Ten years ago, the benchmark plant population for tomatoes in a closed/ ventilated environment was 3 plants/m² (30 000 plants/ha). Effectively set at a row spacing of 0.85m apart and in-row spacing of 0.33m.

This spacing can still be effective in areas experiencing low humidities during the warmer months. In the coastal regions, it started becoming a problem especially when higher yields were being achieved with improved varieties and vegetative growing conditions. Due to the increased vegetative growth, there was reduced air movement around the plants and in humid coastal conditions, the incidence of fungal diseases such as Botrytis became a large problem wiping out a huge percentage of the increased yield.

As a result, farmers adjusted the plant population down to 2.7 plants/m² (27 000 plants/ha) effectively set at a row spacing of 0.875m apart and in-row spacing of 0.37m.

In a standard 30m x 10m tunnel growers would usually have 10 rows running the length of the tunnel which are fed by 5 dripper lines. So, the dripper lines were set at 1.70m apart feeding 2 lines of plants on either side. It is now common practice to maintain 5 dripper lines but reducing the 10 lines of plants into only 5 lines of plants. So effectively the in-row spacing is reduced to 0.185m, the dripper row spacing remains the same but increases normal row spacing considerably. This practice allows for a larger pathway between the plants, increases the air movement in the structure and reduces the risk of mechanical damage to the plants especially from staff working in the house or from moving spray machinery. It also improves the picking operation from a standpoint of ripe fruit visibility. It furthermore improves the amount of sunlight the plants receive, as close double rows always shadow the inside of the double row above the dripper line particularly after 8 – 10 weeks onwards.

Fertiliser Application Tomatoes are grown in hydroponic environments ideally require an EC of 1.9mS/cm. Anything above this is wasteful and can create toxicity. It is important to have the water source tested before use in a laboratory to establish the mineral content and more importantly the EC of the raw water. Mineral content cannot always be balanced but the overall EC is paramount. For example, if the EC of the raw water is 0.6mS/cm then this is added to the plant EC requirement of 1.9mS/cm giving a total EC requirement of 2.5mS/cm. The grower has to ensure that the inflow of

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water/fertilizer mix is 2.5mS/cm. The outflow EC should not exceed more than 0.5mS/cm above the inflow (e.g. 3.0mS/cm would be the maximum). The outflow can be manipulated by the percentage of water run-off daily. For example, 10 – 15% run-off of the total inflow will maintain the outflow EC within the parameters of the inflow EC. If there is a deficit of run-off then the salt residue in the bag will begin to rise and the run-off EC will climb to damaging levels. This can be corrected easily by flushing the bag at night to bring the run-off EC down.

Recommended pH is between 5.8 – 6.0. This window allows maximum uptake of all the nutrients being applied in the fertilizer mix. Natural water is normally around 6.5 – 7.0. The introduction of acids in the mix will bring the alkalinity down to 5.9. Be sure to adjust the fertilizer programme to accommodate the acid application, as acids can come in the concentrated form of Sulphuric, Phosphoric or Nitric acid at 80%.

Summary 2.7 plants/m² - 27 000 plants/ha (effectively set at a row spacing of 0.875m apart and in-row spacing of – 0.37m.

EC – 1.9mS/cm + Raw water EC = Required EC at inflow.