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Fluid Journal : Fluid Journal 2008-2009
between years, and has been developed as a means of removing uncertainty. This rate of application, however, represents 2.5 times the six-year average tree N export (0.35 kg/tree), and an overall NUE of less that 33 percent across 6 years. This apparent gross inefficiency can be traced to 1) the marginal cost of additional N, 2) the inability to predict or measure field variability, and 3) lack of adequate tools to measure and monitor nutrient status. In the absence of any alternative approach, the logic behind grower decisions to fertilize in this manner is both clear and reasonable. However, if a grower were provided with the tools to predict and fertilize to meet actual demand (solid line), NUE could immediately be increased from 33 to 45 percent. However, even this simple tool does not currently exist. What to do In high value crops, it is concluded that tissue sampling strategies that only provide knowledge of 'mean' field nutrient status are of limited value unless they also provide an estimate of field variability. The every tree is satisfied, this approach is economically viable only because fertilizer costs are a small part of operating expenses. This approach to fertilization is also a consequence of the lack of technology available for variable rate fertilization in orchards that are managed as a single uniform fertigated unit. The impact of this approach to fertilization can be further exacerbated in crops that vary unpredictably in their yield. Pistachio undergoes strong yield fluctuations and growers currently have neither the means to predict the current year's yield, monitor in-season nutrient status, nor apply variable-rate fertilization within a single management unit (typically an orchard 40 to 100 acres in size). As a consequence there is a tendency over time to select a fertilization regime that ensures that every tree receives adequate fertilization every year. The outcome of this approach is highlighted in Figure 4. In this example, the grower established a fertilization rate of 0.9 kg per tree, ensuring that >95 percent of all trees received adequate N in all years. This level of fertilization takes into account the variability within and Dr. Brown is Professor of Plant Nutrition, Department of Plant Sciences, University of California, Davis. Figure 3. Leaf samples were collected using standard practice (10 pairs of leaves pooled from each tree) from 100 individual pistachio trees across a 10-acre orchard containing 1,250 individuals. Leaf tissue N was analyzed. Established critical values for pistachio are marked with a blue arrow and the corresponding blue box outlines the percentages of trees that are below this value. constraints of tissue sampling are further exacerbated by the perennial nature of tree crops and the inability to effectively predict yield or to conduct early-season tissue sampling and fertilizer adjustment for which standards of practice have not been established. The limitations of current sampling strategy are further exacerbated by the constraints to nutrient management (which is now largely applied through fertigation) which limit the ability of growers to manage within field variability. This, coupled with the relatively low cost of fertilizer as a component of overall production costs, has resulted in the adoption of fertilizer regimes that are inefficient. To address these issues, several new initiatives are: 1) Tissue sampling strategies that must provide information of in-field variability. This will require: a. Development of new sampling strategies b. Development of low-cost handheld, remote, or in situ probes to monitor plant and/or soil nutrient status c. Research into modeling approaches to nutrient demand and nutrient status determination 2) Required yield prediction models. For most high-value crops, extractive yield represents the primary determinant of nutrient application. Yield prediction models that allow for early-season adjustment of fertilization strategies will be required. This will require: a) Development of yield monitors and predictive technologies b) Research into yield determinants and model development 3) Variable rate application technologies will be required for high value species. It is counterintuitive that precision technologies have not been applied to high-value crops and that the adoption of fertigation as the primary source of nutrition has reduced the ability to conduct variable-rate fertilization. We recommend: a) Development of engineering approaches to provide differential with field fertilizer delivery b) Research into the effects of timing and product form on crop response. Figure 4. Nutrient demand was calculated as the product of yield x nutrient content of the exported crop. Yield was measured in every individual tree over the six-year experimental period. Box and whisker plots show mean (25th, 75th and 95th percentiles) of yield and N removal in each year. Dashed lines represent current N fertilization rate and calculated N removal in each year. The solid line represents a theoretical annual fertilization regime that would maintain fertilization of 95% of individuals based upon real yield in that year.
Fluid Journal 2005-2007