Organic Nutritional Needs
March 1, 2007 Due to the broad range of soil types in Florida, the best way to plan for future crop nutritional needs is through periodic soil tests to monitor soil nutrient concentrations. In general, the rotation schedule will influence the soil-sampling schedule. Samples should be collected prior to each rotation when the rotation is short (less than three years), and at least every two to three years for longer rotations.
To avoid contamination of surface and groundwater associated with over-application of fertility inputs, producers are advised to submit samples of soil amendments to a reputable laboratory for analysis if a guaranteed analysis is not available from the supplier. During the growing season, plant tissue analysis is the most accurate method to assess fertility levels. Plant-tissue analysis can also provide information on nutrients that are not easily measured in soil analysis such as nitrogen, boron, and sulfur.
The University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) Extension Soil Testing Laboratory (ESTL) offers six types of analysis, including soil and tissue analyses, for a modest fee. Interested producers should contact their county Extension office or visit http://soilslab.ifas.ufl.edu/ for more information. In addition to the ESTL, a number of private laboratories offer similar services.
Making The Measurements
Crop-fertility recommendations are the same in organic as they are in conventional systems, and materials must be applied at rates compliant with existing nutrient-management guidelines and in a manner that minimizes environmental degradation. Most plant nutrients are supplied by natural sources of fertility, including compost, animal manures, and leguminous cover crops. These natural sources may be supplemented by the application of mined sources of mineral nutrients and various formulations of minor nutrients.
Because organic fertility sources listed above are slowly available in the soil, nutrient amendments are typically preplant incorporated, with occasional side dressing or fertigation, depending on crop production recommendations. Most fertigation products are fish meals and emulsions that can be purchased as wettable powders.
These products typically have a low nitrogen analysis and can be expensive, so providing a substantial fraction of the necessary nitrogen through fertigation may not be economically feasible. To prevent nutrient losses associated with leaching and volatilization, a number of strategies should be employed, including but not limited to:
• Splitting fertilizer applications;
• Incorporating nitrogen into moist but not saturated soil;
• Using precision irrigation for site-specific irrigation;
• Covering the surfaces of beds with plant or plastic mulches;
• Submitting organic materials to a laboratory for analysis prior to use to avoid over application;
• Avoiding the application of materials during periods of heavy precipitation; and
• Planning for effective tillage.
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