CROP RESPONSES TO WINTER AND SUMMER CO₂ ENRICHMENT

The technique is especially useful in winter when poor light limits the growth and development of crops. Raising the CO₂ concentration inside a greenhouse from the normal ambient level of 335 to 1000 vpm counteracts the adverse biochemical effects of atmospheric oxygen on photosynthesis and growth. During the wintertime this can boost net photosynthesis by up to 50% and increase crop yields by 20–40%.

Threefold CO₂ enrichment is normally practised throughout the winter and early spring. By the end of April it becomes increasingly uneconomic to maintain a constant level of 1000 vpm CO₂ in the daytime as ventilation is frequently required to limit the rise in temperature from solar heat gain. During the late spring and summer the rate of ventilation is often insufficient to prevent crops from depleting the atmosphere of CO₂, i.e. the concentration inside the greenhouse falls below 335 vpm.

CO₂ depletion of the greenhouse atmosphere is undesirable because it results in reduced rates of photosynthesis and growth. Preventing CO₂ depletion in greenhouses during the late spring and summer can increase the yield and value of long-season cucumber and tomato crops by 5–15%. Crop productivity can be further improved by enriching with CO₂ to maintain levels above ambient, e.g. a 10% rise in the mean daytime CO₂ concentration can increase the output of marketable cucumbers by 5%.

Commercially, carbon dioxide for enrichment purposes can be either supplied in liquid form or produced directly by burning hydrocarbon fuels with a low-sulphur content in the atmosphere. Realistic cost comparisons between the different CO₂ sources are difficult to make as the charges for recurrent and capital expenditure depend on many variables.

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