Microbial soil amendments do little to improve citrus tree performance in Florida soils

Arnold W. Schumann, James P. Syvertsen, James H. Graham


The profitability of Florida citrus groves can be limited by poor soil conditions, including low nutrient and low water-holding capacity. There are many advertised soil amendments that claim to “condition” unproductive soils by improving the balance of beneficial microbes even though there is no known research demonstrating the effectiveness and profitability of these products on citrus. We tested four commercially available microbial-based liquid soil amendment products that have been recommended by their manufacturers to be beneficial for citrus. In three repeated greenhouse studies, products were applied at recommended and higher rates to seedlings of Carrizo citrange grown in pots of native Candler sandy soil. In Experiment I, total plant growth tended to increase in response to amendments of two of the products (B, C), but growth responses to increased rates of B and C were not conclusive. Nitrogen leaching and leaf N responses were not remarkable. The other two products (A, D) had no effect on seedling growth or N budgets. Greenhouse experiments were repeated using B and C at high or low nutrition, but seedling growth and mineral nutrient status were little affected. Nonetheless, products B and C were tested in three field-scale experiments at three locations (Southern flatwoods, Indian River, and central Ridge). The two products were applied at recommended rates with or without biosolids (sludge) to provide additional soil organic matter (SOM) microbial substrate. During 3 years of repeated applications and monitoring, the microbial products did not consistently affect any measured parameter in the soil or citrus crop. Soil measurements included SOM, pH, CEC, P, K, Ca, Mg, Fe, Cu, microbial respiration, PWP, and soil Phytophthora populations. Leaf analyses included color (SPAD), N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, B. Tree canopy heights and volumes were measured, and fruit quality was expressed as fruit size, percentage juice, brix, acid, and ratio. Fruit yield could not be rigorously assessed because in 2 years the harvest was damaged by hurricanes, and one experiment was destroyed due to the canker eradication rule in Florida. Based on greenhouse and field studies, these microbial-based liquid soil amendment products were of little or no benefit. The additional cost and labor of incorporating these products into Florida citrus production programs could not be justified.

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Proc. Fla. State Hort. Soc.     ISSN 0886-7283