Apply base, or precrop, fertilizers after soil steaming and leaching, and incoiporate them into the greenhouse soil by rototilling. Add these fertilizers to the limestone that might be required for adjusting the pH level of the soil (see Table 7). In general, add as much of the required calcium and phosphorus as possible as a base dressing because these nutrients can be stored effectively in the soil and their absence from liquid feeds prevents most clogging problems of the irrigation system; the calcium should be in the form of limestone and the phosphorus in the form of superphosphate, both finely ground. Furthermore, these nutrients, by nature of their source and their ability to bind to soil particles, are released slowly into the soil solution and therefore do not raise the total amount of salts dramatically, nor do they upset the nutrient balance of the soils to which they are added as a base dressing.
Because tomatoes require a large amount of potassium, supply a good portion of it, along with magnesium, as base fertilizer; the ratio of potassium to magnesium in the soil should be 2:1. Avoid applying nitrogen. Make the final decision on base fertilization after receiving the soil test results and consulting with your horticultural crop adviser. Treat the recommended rates of base fertilizers (Table 8) as a general guide only.
Table 8 Base fertilizer recommendations
Superphosphate (o-20-0, fine grade) 250 kg/ha
Potassium sulfate 500 kg/ha
Magnesium sulfate 250 kg/ha
The following can be added in combination, if needed:
Peat 800 bales/ha
Calcitic limestone 800 kg/ha
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