Salinity, Root-Knot Nematodes and Continuous Tomato in a Plastic House
In a 0.35 ha plastic greenhouse near Larnaca, tomatoes have been grown continuously for 11 seasons. The soil is a sandy loam with organic matter 0.8%, pH 7.9, EC 3.6 dS/m in the 0-20 cm layer and nitrate-N 165 mg/kg. Yields fell from 9.2 to 6.4 kg/m2. Roots show galls and cortical necrosis; a soil test finds Meloidogyne javanica juveniles and high Fusarium counts. The grower uses 60 t/ha of cattle manure each year, drip irrigation with borehole water (EC 2.5 dS/m, chloride 280 mg/L) and foliar copper every 10 days. He wants to stay organic, keep tomatoes as the main cash crop, and asks for a three-year plan that reduces salinity and nematodes without losing income.
1. Diagnose the interaction between salinity, nutrition and the root disease, and explain why more manure or more water would not solve the problem.
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Four problems interact. Salinity: an EC of 3.6 dS/m with irrigation water at 2.5 dS/m and chloride at 280 mg/L means salt is accumulating faster than the crop can tolerate; tomato yield begins to fall above roughly 2.5 dS/m in the saturation extract. Nitrate at 165 mg/kg plus 60 t/ha of manure is a large nitrogen surplus, which raises EC and pushes leaf vigour at the expense of fruit. Organic matter at 0.8% is low despite the manure because it mineralises rapidly in a warm, moist greenhouse and much is lost as ammonia. The galls and juveniles confirm root-knot nematode, and Fusarium is favoured by continuous tomato and stressed roots.
More manure would add salt and nitrogen, raise EC further and increase ammonia loss. More water would leach some salt but with 2.5 dS/m water the leaching fraction needed is large, and over-irrigation waterlogs the root zone and spreads nematodes and Fusarium. The priority is to stop adding salt and nitrogen, reduce nematode pressure, and rebuild stable organic matter with mature compost rather than raw manure.
2. Set out a three-year rotation and nematode-management plan that keeps tomato as the main crop.
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Year 1 in the current house: remove crop residues and roots, do not replant tomato immediately, and apply 20-25 t/ha of mature compost rather than raw manure. In summer, incorporate a sorghum-sudangrass or brassica cover, irrigate, and cover with clear polyethylene for four to six weeks of solarisation, aiming for soil above 40 C at 15-20 cm; this reduces nematodes and weed seed. Then plant tomato on a nematode-resistant rootstock and keep the root zone healthy.
Year 2: rotate the house to a non-host cash crop such as sweet corn or a brassica, or a leafy crop only if the variety is not a Meloidogyne host, and sow Tagetes or a cereal cover in the off-season. Do not follow tomato with pepper, eggplant or potato.
Year 3: return to tomato with a different resistant rootstock and a cover crop in the rotation. Map nematode hotspots and sample soil each autumn. Solarisation, biofumigation and resistant rootstocks are the core tools because nematicides are not available in organic production. Expect two seasons before populations and yield recover, so plan cash flow around the rotation crops rather than promising an immediate recovery.
3. Explain how the grower should manage irrigation and salinity over the next two seasons, including what to measure and when to leach.
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Measure root-zone EC weekly, using a 1:2 soil:water extract or suction cups, and keep a simple chart. Maintain a leaching fraction of 15-25% and leach mainly in autumn when the house is empty, using the lowest-EC water available. The borehole at 2.5 dS/m is marginal for tomato; blend it with rainwater harvested from the greenhouse roof, or reserve it for the leaching phase. Chloride at 280 mg/L will accumulate, so avoid overhead wetting and keep drip lines properly wetted.
Reduce manure, switch to compost and fertigation with permitted potassium and nitrogen sources, and split nitrogen so nitrate does not spike. Use tensiometers or a similar sensor to irrigate to crop need rather than by calendar, and mulch to reduce evaporation and salt rise. Test soil at the start and end of each season. If EC stays above 4 dS/m despite leaching, the house needs a break crop and a fallow leaching year before the next tomato cycle; continuing to plant into a saline, nematode-infested soil will only deepen the yield decline.