Given that the soils in the Lower Colorado River Region are calcareous, one might anticipate limited availability of many metal micronutrients, such as iron (Fe), zinc (Zn), manganese (Mn) and Copper (Cu). Remarkably, vegetable crops in the region have historically not shown economic responses to micronutrient fertilization, although occasional crop response to Zn fertilization is observed in more recent studies (14). 

Micronutrient metal availability is typically tested using diethylenetriaminepentaacetic acid (DTPA) extraction (54). One should not anticipate a response to Zn fertilizer application when DTPA Zn test results exceed 1.5 ppm. A recent survey of soils across the Yuma vegetable production area shows that about 51% of the fields tested above 1.5 ppm and 49% tested in the range of 0.5 -1.4 ppm DTPA Zn (12). None of the sites tested <0.5 ppm. The occasional crop response to Zn fertilization mentioned above was observed for soil Zn tests ranging between 0.5 and 1.5. The author proposed Zn fertilizer recommendations that vary between 0 and 15 lbs/acre, depending on DTPA soil test Zn levels. We make the same recommendation for all crops based on a DTPA Zn test (Table 2-5-1). While both soil and foliar applications are often used to correct Zn deficiencies, soil application is recommended here because foliar application failed to produce consistent crop response (14). 

Applying the small amounts of required Zn to soil uniformly presents a challenge, and there are alternative strategies to help improve uniformity of Zn fertilizer application. One possibility is to have the fertilizer dealer blend the Zn with the pre-plant P fertilizer, but the differing granule size and density may reduce homogeneity of the blend and hence reduce uniformity of application. Another option is to apply P fertilizer products where Zn is incorporated into P fertilizer granules. In this product line, two commercial formulations are available: MicroEssentials SZ (12-40-10S-1Zn) and 40-Rock (12-40-6.5S-1Zn). Results of studies summarized in Table 2-5.2 show that 40-Rock is a reliable source of phosphorus and Zn, under soil conditions where Zn is the limiting nutrient. This fertilizer is a form of monoammonium phosphate (MAP), but with Zn and carrier sulfur added to the granules. There are also technologies where micronutrients are added to granular nitrogen fertilizers. The author, however, does not have experience with these products. 

Zinc fertilizer can be applied in a number of ways; adding with starter fertilizers and insecticides, fertigation with drip irrigation systems, and direct treatment of transplanted seedlings with Zn fertilizer are options for certain production scenarios. The author has had some success with these methods of application. However, further studies are needed before these options can be recommended as alternatives to traditional soil application methods (14).

The author has not observed vegetable crop responses to any of the other micronutrients (i.e., iron, manganese, and copper) in Arizona. Potential crop deficiencies in any of these metal micronutrients can be assessed through the use of the DTPA soil test. There are no specific guidelines for Arizona relating to the minimal requirements of vegetable crops for these nutrients. However, there are generalized guidelines for the Western United States which specify minimum thresholds of 2.5, 0.5, and 0.2 ppm for Fe, Mn, and Cu respectively (46, 47). The soil concentration of another important micronutrient, boron, is often evaluated using hot water extraction method (55), and a value under 0.25 ppm is considered low for vegetable crops. Alternatively, one might monitor the micronutrient nutritional status of crops using the plant tissue tests based on known sufficiency ranges (see section 4.3 of this report).