Evaluation of Commercial Potting Mixes for Optimizing Growth of Citrus in Containers

Optimal plant growth is crucial in research experi ments as well as in nursery operations. Two major factors that influence plant growth are soil mix and fertilizer. Use of com mercial soil mixes would be desirable for smaller operations which often lack facilities for mixing and sterilization. Several commercial potting mixes were compared to the UC (Universi ty of California) mix for growing citrus plants in our green houses at the Citrus Research and Education Center in Lake Alfred, FL. The UC mix is part of the UC system that uses a peat moss based soil mix and fertigation for nutrient supply. Com mercial mixes were compared against the original UC mix us ing the UC system. A fertilizer formulation was used that was previously adjusted for Ca content in the irrigation water. Plant Florida Agricultural Experiment Station Journal Series No. N-01928. The authors wish to thank Bill Sidwell, Jason Zellers, Deborah van Clief and Stephen Futch for their help. growth and soil pH were measured, and plants were monitored for nutrient deficiencies. Two mixes were found to be compa rable to the UC mix in terms of plant growth and lack of micronutrient deficiency. No commercial mixes were found to give better plant growth or fewer deficiency symptoms than the original UC mix. The choice of mix appeared to affect mostly the soil pH and development of deficiency symptoms. Good plant growth and freedom from micronutrient de ficiencies are crucial in greenhouse experiments in which dif ferences among treatments are shown in plant growth and disease symptoms many times show as patterns on the leaves (Nauer et al, 1967b). Micronutrient deficiencies can mask or otherwise interfere with the development and observation of these symptoms. It is therefore very important to develop a system for optimal plant growth free of micronutrient defi ciencies. Also, in nursery operations, optimal plant growth is essen tial for maximization of productivity. Healthy plants without deficiencies will ensure optimal graft take, will sell at better prices, and will grow into more healthy and vigorous trees once they are planted in the field. The University of California (UC) system for growing plants in containers was developed in the 1940s by the Univer sity of California to meet the need for more efficient produc tion of ornamental plants in nurseries (Baker, 1957). The resulting system addressed the major issues found to limit nursery production: diseases, salinity, and toxicity. Answers were found in a system using a peat moss-based soil mixture that was easily replicated, fertigation and sanitary measures. 130 Proc. Fla. State Hort. Soc. 113: 2000. Plants were trained to a single shoot. Plant height was measured five months after planting. The pH of the drainage water was measured as described by Roistacher (1991) to ob tain soil pH after five months. Initial soil pH was measured in the same fashion from one sample of each mix. Also, the pH of the irrigation water was measured. Micronutrient deficien cy symptoms were evaluated at five months after planting us ing the following rating system: 0 for dark green healthy leaves; 1 for slightly chlorotic leaves without visible patterns; 2 for leaves with slight deficiency patterns, but with all parts green; 3 for leaves with clear deficiency patterns including chlorotic areas; 4 for leaves with clear patterns and visible growth reduction. The experiment was set up and analyzed as fully random ized (Lauckner and Fielding, 1991). Duncan's multiple range test with a confidence level of 0.95 was used to differentiate treatments using soil pH, plant length and deficiency ratings.