Maintaining high soil nutrient levels for greenhouse tomatoes without excess salt accumulation.

Experimental and commercial plantings of greenhouse tomatoes received supplemental fertilizers in the form of KH₂PO₄, (NH₄)₂HPO₄, KNO₃ and NH₄NO₃ in quantities greater than normally applied. Monthly changes in soil nutrient levels and soluble salt contents were determined and crop behavior observed. A total of 2,922 pounds of nitrogen, 3,054 pounds of P₂O₅, and 4,662 pounds of K₂O per acre were applied during two growing seasons in experimental plantings at the Michigan State University. Soil tests near the end of the growing season for the second year showed 10 pounds of nitrate-N, 188 pounds of P, and 402 pounds of K per acre by the Spurway “active” method. The soluble salt reading for a 1:2 soil/water mixture was 28 × 10⁻⁵ mho. Yields up to 100 tons of fruit per acre were harvested resulting in total crop removal of 350, 105, and 580 pounds per acre of nitrogen, phosphorus, and potassium, respectively. Cooperative studies were conducted with seven commercial growers. The fertilizer program followed closely that used at the University. Results of soil tests and soluble salt readings were similar to those obtained at the University greenhouse.