Organic production has become a significant sector of the vegetable industry in California. Providing sufficient soil nitrogen availability to reach maximum yield potential can be a challenge in organic culture. Cover cropping is generally the most economical way to provide plant-available N in organic systems (Gaskell and Smith, 2007). In California, cover crop production is typically done over the winter. There are circumstances in which the production of a cover crop is impractical. Water availability or cost to irrigate a cover crop can be an issue. Foregoing cash crop production for the 3 months or more required for cover cropping may be difficult to economically justify on high-value land, particularly when there is a significant seasonal market premium for produce. Unpredictable spring weather can complicate cover crop residue management and delay spring planting. Even where a cover crop is produced, its N contribution may be inadequate to supply high-N demand crops, and other organic N sources may be required to maximize crop productivity.
Application of manure-based compost is a common organic production practice, but the low N mineralization rate typical of compost limits its N contribution. Hartz et al. (2000) reported that most composts mineralized <10% of initial N content in the 4 to 6 months following soil incorporation. A high compost application rate can make a significant contribution to soil N availability, but in practice, application rate is constrained by cost and by the potential for water quality degradation from high phosphorus (P) loading (Sharpley et al., 1994).
Dry organic fertilizers such as fishery waste, feather meal, and seabird guano are widely used; these products have high N content (>10% of dry weight) and relatively rapid N mineralization. When applied to agricultural soils, such products mineralize 60% to 80% of N within 4 to 8 weeks (Hadas and Kautsky, 1994; Hadas and Rosenberg, 1992; Hartz and Johnstone, 2006). A limitation of these dry fertilizers is that they are not easily applied through irrigation systems. Drip irrigation is becoming widely used by organic growers in California, and interest is high in organic fertilizers that can be applied through drip systems. In recent years, a number of liquid organic fertilizers has come to market, but there is little data available on their N mineralization characteristics or the challenges they pose for application through a drip system. This study was undertaken to document the N mineralization behavior of a representative group of liquid organic fertilizers and to evaluate their suitability for fertigation in drip systems.
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