Growing Tomatoes Under Glass

The tomato, originally a small prostrate spreading plant from South America, belongs to the genus Lycopersicon and two distinct species, Lycopersicon esculentum and Lycopersicon pimpinellifolium, and several botanical varieties exist. The large-fruited tomato, as we know it today, still yearns for the high light conditions and warmth inherent in its makeup.

Most crops of tomatoes are likely to be earlier and more successful in areas of good light in well-sited, modern, light-admitting greenhouses than they are in dull areas in greenhouses of poor design. For early crops the level of heating should also be sufficiently high and effectively distributed.

Nutritional standards for tomatoes

Tomatoes are greedy plants, especially when grown under glass and coerced into producing their maximum weight of fruit. It is generally accepted that for 125-150 tonne/ha (50-60 tons per acre) which is 4-4.5kg (9-10lb) of fruit per plant, the following nutrients are necessary. Commercial yields are now much in excess of this — as much as double or treble.

Allowing for loss through drainage and ‘fixing’ of phosphorus, especially in soil-based growing media, it is general to provide for about one-third of the plants’ needs in the base dressing, this being made up to almost half by the nutrient reserves present in most soils or growing media, the remainder of the nutrients being supplied by liquid feeding or top dressing applications during the growing season. The calcium is supplied by adjustment of the pH figure. When border soils are intended for tomato culture it is advisable to have the soil analysed, the acceptable standards being as follows:

pH 6-6.5; 5.5-6 for soilless media

Nitrogen

Owing to the extreme variability of the nitrogen figure and its presence in different forms and at different temperatures, ie as ammonia, nitrate, etc. this figure is not always available from soil laboratories, but when it is it should be stated as:

• 100-150 mg/litres (ppm) N Index 2-3
• Phosphorous 41-70 mg/litres (ppm) P Index 5
• Potash 500-700 mg/litre (ppm) K Index 5
• Conductivity Index 3-4.

A point of some importance which will be repeatedly stressed is the need to avoid high salt contents, especially at the propagating stage. It should also be noted that figures for nutrients vary according to analysis procedures, but index factors are reasonably standard.

Deciding on Cultural Methods

The many and varied cultural methods for tomatoes which have been developed over the last decade tend to make a decision on any particular cultural method difficult. The main reason why so many methods have developed is to avoid soil sickness, persistent diseases such as deep-seated virus infection and brown root rot, and pests like the cyst-forming potato root eelworm. In addition, variation of cultural methods avoids most of the physiological disorders arising from monoculture.

Comparison of Cultural Methods

Hydroponics

With hydroponics the tank method as distinct from NFT is most likely to appeal to gardeners.

The advantages and disadvantages are as for the above systems.

Planning for tomato growing

The two basic decisions which must be made concern the growing method and the timing of the crop, all other cultural aspects revolving around them. Given a new site on which a greenhouse is erected, where the soil by physical examination is of good structure and drainage, by laboratory analysis of acceptable nutritional status, and by eelworm determination found to be free of potato eelworm cysts containing live larvae, there is much to be said for using the existing soil.

It should be borne in mind, however, that as the border soil becomes progressively more sick, sterilization by either heat or chemicals will invariably be necessary if it is intended to continue border culture without grafting. Here again cost cannot be ignored, nor indeed can the practicability or otherwise of sterilization. Where sterilization or soil renewal is not feasible then the way is open for alternative cultural methods.