Ornamental growers must provide fertilizers to the substrate when growing crops in containers because the organic substrate contains few nutrients. The cost of fertilizer use, around 8% of total production cost, ranks third among all production costs for growing container crops in Miami-Dade County.
Fertilizers used for most ornamental crops in our area, if not all, are controlled-release fertilizers (CRF) that contain both macro- and micronutrients. It is important to be aware that many factors can affect the performance of fertilizer in the container media, according to Dr. Altland, a scientist of USDA-ARS on container substrate, who gave a presentation on nutrient management in a recent workshop.
CRFs can be applied by incorporating into the substrate before potting, or by top dressing or dibbling after transplanting the crops. It is best to pot the substrate soon after it is delivered when incorporating CRFs, or keep the substrate dry to avoid nutrient loss and potential crop burn after planting.
More importantly, the pH of the substrate will affect the availability of nutrients to the plants (Fig. 1), particularly micronutrients such as iron. Because micronutrients bind to organic matter in the substrate and move downward more slowly than macronutrients, the effect of pH on their movement may be more significant when CRFs are applied by top dressing. If macronutrients are applied with irrigation water, Dr. Altland recommends incorporating a granular premixed micronutrient package into the substrate for optimal distribution and maximum plant availability (Fig. 2).
Fig. 1. pH affects nutrient availability.
Fig. 2. Micronutrient movement and distribution when applied with different approaches.
Because most substrate components have a pH of 4.5 or lower, Limestone is commonly used to adjust and stabilize the pH to the ideal range of 5.5 to 6.5. However, the solubility of limestone is inversely affected by the pH of the substrate and reaches a stationary state at around pH 6.5 (Fig. 3), corresponding to a limestone rate of about 5 kg/m3 (8.5 lb/yard3). Because of the high pH of the groundwater used for irrigation in our area, the limestone rate can be further reduced to 4.5 kg/m3 (7.5 lb/yard3), as used in previous studies. Also, the groundwater has a very high calcium concentration, so additional calcium, such as gypsum (CaSO4) or Calcium Nitrate (Fig. 4), may not be needed in the substrate.
Fig. 3. Dolomitic lime rates and substrate pH
Fig. 4. Record of Calcium application from a local grower.
Because growers can adjust the components of the substrate for container plants, your knowledge of what to add and how to provide optimal conditions is critical to grow uniform crops. Learning from experts on the substrate and nutrient management is essential for expanding your knowledge and getting the most out of your investments.
Credits: The featured picture and those in Figures 1-4 are from Dr. Altland’s presentation.
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