California is a leader in the organic production of processing tomato (Solanum lycopersicum L.), with >3,400 ha produced annually (Klonsky, 2010). Nitrogen is often the most limiting nutrient in organic vegetable production systems, especially in high N demand crops such as tomato (Gaskell and Smith, 2007). Not only must organic soil management provide sufficient plant-available N to maximize crop productivity, but also do so in a timely manner to ensure that crops are not N limited at any growth stage. Achieving synchrony between soil N availability and crop N demand has been recognized as a central challenge of organic N management (Berry et al., 2002; Bhogal et al., 2001; Gaskell and Smith, 2007; Gill et al., 1995; Hatch et al., 1991).
Processing tomato has a high N requirement for peak production. Hartz and Bottoms (2009) reported a mean aboveground biomass N accumulation of 296 kg·ha−1 in high yield conventionally farmed fields. Crop N uptake was slow in the initial month after transplanting, with the rate increasing to >4 kg·ha−1·d−1 during fruit set and early ripening. This pattern of crop N uptake highlights the problem of synchronizing N availability in organic production. The rate of Nmin from cover crops or organic amendments incorporated before planting has been shown to peak before the period of highest crop N demand (Gaskell and Smith, 2007). This suggests that measuring SMN (NH4-N and NO3-N) concentration after transplanting, but before rapid crop N uptake, might be useful in predicting the N sufficiency of organic fields (Muramoto et al., 2011). The value of considering residual SMN in determining sidedress N requirement has been documented in conventional production of many crops (Breschini and Hartz, 2002; Fox et al., 1989; Hartz et al., 2000; Heckman et al., 1995, 2002; Krusekopf et al., 2002), but the utility of this practice in organic vegetable production has not been widely investigated.
The ability to predict seasonal soil Nmin potential would also be useful to inform in-season N management. In recent years, several laboratory methods for soil Nmin prediction have been proposed that could be practical for routine agronomic use. Short-term Cmin following rewetting of dry soil has been reported to correlate well with soil Nmin, as measured in longer term, aerobic soil incubation (Haney et al., 2001, 2008a, 2008b). The measurement of WEOC and WEON has been reported to be highly correlated with Cmin, and this additional information regarding the labile C:N ratio may improve soil Nmin prediction (Haney et al., 2012).
Plant tissue analysis has been widely used to document crop N sufficiency. For tomato, leaf N and petiole NO3-N sufficiency standards are available (Hartz et al., 1998; Jones et al., 1991; Lorenz and Tyler, 1983). Historically, tissue N analysis has been used as a measure of current sufficiency; it is unclear from existing literature the degree to which tissue analysis can give inference on future N fertilization requirement.
There is little data available on efficient N management in organic processing tomato production, or the adequacy of current practices to maximize productivity. Similarly, data are lacking on the effectiveness of soil and tissue N monitoring in guiding in-season N management. This study was undertaken with two objectives. First, to document current N management practices in organic tomato production in the Sacramento Valley of California. Second, to investigate the utility of soil and plant N monitoring in predicting seasonal crop N sufficiency, and the need for in-season N fertilization.
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