Summary
The production of vegetable crops in the low desert region of the southwestern United States is over a 2 billion dollar industry and product is shipped nationally during the fall-winter-spring period. Inorganic contaminants, such as heavy metals, are of concern in edible fresh fruits and vegetables. Many of the alluvial soils used for crop production in the low desert contain low levels of several metals, including cadmium (Cd). We are particularly concerned about spinach which has a propensity to accumulate heavy metals. The large number of spinach samples we collected to date indicate 50% of the spinach produced in the desert would exceed the EU Maximum Levels (ML) of 200 ug/kg fresh weight (FW). Studies have shown soil tests to be potentially useful for predicting Cd accumulation by plants, however, proper sampling protocols and a predictive soil test need to be calibrated for desert soils. The objectives of these studies was to collect high resolution soil samples from typical production blocks in the desert to assess in-field variability and develop sampling protocols. We also collected paired soil and spinach tissue samples to evaluate the important soil properties affecting Cd concentrations of spinach. These data were needed as a prerequisite to developing soil testing as a management tool. The observed generally low in-field variation and normal distribution of observed values for Cd suggest that a good composite representative soil sample can be collected from the 3 to 10 ha spinach production blocks typically used in the region. While the DPTA soil test was among the best evaluated, it was not sufficiently predictive of spinach Cd concentrations to be utilized as a management tool alone. Data show that in addition to soil test Cd, soil test Zn, soil test P, and soil salinity were correlated to spinach Cd concentrations and need to be incorporated into any soil test algorithm used to predict spinach Cd concentrations. While spinach Cd concentration were negatively correlated to soil test Zn levels, the application of high rates of Zn fertilizer to mitigate Cd uptake was only marginally effective. However, modest rates of Zn fertilization to correct Zn nutrient deficiency appear justified.
Research Objectives
The objectives of the project were to expand current sampling efforts during the current Yuma growing season to develop tools and management strategies to reduce heavy metals in vegetable crops. Studies included the development of a soil test that growers can use to predict the potential for a given field to produce crops with problematic levels of Cd. This evaluation included soil test calibrations and high resolution soil sampling to assess the in-field variability and guide sampling protocols. Studies for evaluation of Zn fertilization as a potential strategy to reduce Cd accumulations in leafy vegetable crops were also performed. While some of this work was in progress with Specialty Crop Block Grant funding, CPS Rapid Response funding allowed the following: • The collection and analysis of high resolution soil samples to assess in-field variability; • Doubling of the soil test calibration database, creating more reliability and confidence in the results; • Evaluation of Zn fertilization on additional sites from the original plan (5 versus 2). We focused on sites where the soil has higher background Cd, and tried various rates
Findings & Recommendations
It seems that except for the EU ML, nearly all the soil used for spinach production, and the spinach itself produced in the in the desert, tests below most emerging public and industry selfimposed standards for Cd. The industry in the desert should not be overburdened with excessive sampling and monitoring costs related to Cd compliance at this time. Continued research to fully develop a predictive soil test is needed now that we have identified important soil properties affecting Cd uptake. We will continue this effort with funding from a new Arizona Specialty Crop block grant that begins October 1, 2016. The development of this test will provide a short term management solution. A more effective long term solution should be aimed at developing spinach cultivars with a lower propensity for Cd accumulation as our research show genetic variation in spinach germplasm. These data show that the application of large rates of Zn to mitigate Cd uptake in desert production is inconsistent, only marginally effective, and cannot be economically justified. However, more modest rates of Zn fertilizer to correct Zn nutrient deficiencies appears justified for many desert soils.