Zinc Nutrition of Soils and Crops in the Salinas Valley

Zinc (Zn) is an essential micronutrient for plant growth; it plays a critical role in the function of enzymes and in nitrogen metabolism of plants. Although it is essential for their growth, crops take up relatively small amounts of zinc (generally < 0.5 lb/acre/crop). Zinc is a divalent cation (Zn²⁺) and its radius is about the same size as iron and magnesium, which allows it to substitute for these ions in soil minerals such as hornblende and biotite. Most soil Zn occurs in mineral structures, but Zn also occurs as salts of varying solubility (e.g. ZnS, ZnCO₃ and ZnO) and on the exchange sites of clay minerals and organic matter. Historically, Zn deficiency was widespread in California, particularly on weathered soils.  However, due to the widespread application of Zn fertilizers over years, Zn deficiency is now much less common.

The bioavailability of Zn is determined by several factors, the most important of which is pH. The solubility of Zn decreases with increasing pH. For instance, in the range of 5.5 to 7.0 the concentration of Zn in the soil solution may decrease 30 to 45 times for each unit increase in soil pH. Other factors that decrease the availability of Zn in the soil solution include: high clay content, high phosphorus and low soil temperatures. However, the bioavailability of Zn at a given pH may also depend on the quantity of natural chelates from organic matter, and other factors.

Zinc deficiency causes distinct symptoms such as interveinal yellowing, stunting and leaf distortion. On lettuce zinc deficiency appears as severe stunting and yellowing along the edges of the leaves that turn brown and “papery”. Zn sufficiency levels for annual crops range from approximately 15 to 30 ppm in leaf tissue, with values above 200 rarely observed.  Leaf levels of 300 to 400 ppm can be toxic to crop plants.

The most common soil Zn test is DTPA extraction, a non-aggressive extractant that gives a reasonable measure of plant-available Zn. The following chart shows the DTPA soil test levels at which Zn fertilization would be likely to improve crop growth.

In 2013 we conducted a survey of over 50 fields throughout the Salinas Valley. Levels of DTPA extractable soil Zn varied from 1.3 to 4.3 ppm, with an overall average of 2.4 ppm. These data indicate that most soils in the valley have adequate quantities of Zn for optimal crop growth.

In that survey the average levels of Zn in head lettuce, romaine and spinach were 27, 30 and 73 ppm, respectively; levels of Zn in spinach varied greatly and ranged from 41 ppm to 112 ppm. The highest Zn levels were on heavy soils in the Blanco, while the lowest were along the river on the sandy soils. All of these fields appeared healthy and the observed levels of leaf Zn appeared to be adequate, based on published tissue sufficiency levels for these crops.

When choosing a Zn fertilizer the primary considerations are solubility and cost (see the table below). Highly soluble Zn forms are likely to be more plant-available for the first crop after application than less soluble forms. However, over time Zn in soil solution will combine with other minerals to form less soluble compounds, the exact compounds formed depending on soil pH. Less soluble Zn fertilizers can be effective over time, and are generally less expensive per unit of Zn than highly soluble forms.

Source: International Zinc Association, www.zinc.org