Guidelines for managing zinc, boron, and other micronutrients in California tree crops are not satisfactory, yet growers can take steps to improve sampling practices until better tools are developed.

That was the message from Patrick Brown, pomologist and professor of plant nutrition at the University of California, Davis, speaking at the recent Fertilizer Research and Education Program (FREP) conference in Tulare.

In 2007, a FREP-funded study headed by Brown surveyed 1800 almond growers and 300 pistachio growers in the state. Of the 650 almond and 100 pistachio growers responding, nearly 80 percent of them sample for micronutrients once or more per year. Tissue sampling is the most often used guide for decisions in managing micronutrient levels.

However, Brown also learned that the University of California recommended critical values for micronutrients, or those above which no benefit is seen in the crop, were considered by about 80 percent of the growers and 90 percent of the plant nutrition consultants as having little or no usefulness.

“Apparently what we've been telling you does not jibe with what you've been doing in the field,” he said. The reason, he added, is likely because the critical values were established for sugar beets in Germany in the l890s, later modified for corn in the Midwest, and more recently adapted to tree crops in the West.

Brown went on to state that micronutrients have become more important in maximizing returns in high value crops, and the objective is not to correct any shortages, but rather to prevent them in the first place.

Many growers, Brown said, point out there is a lack of ability to translate their sampling results into action. What's more, laboratory calculations are difficult to carry out in the field. While one leaf may register an adequate amount of a micronutrient, others do not. Deficiencies in zinc and other micronutrients tend to be much more variable across a field than nitrogen or potassium.

Similar results have come from elsewhere, such as multiyear research on micronutrients in Florida citrus revealing that leaf sampling as an indicator of increases in productivity was a rough management tool at best.

So how have more successful California growers overcome the shortcomings? Over the years, Brown said, they have pushed actual values, perhaps without realizing it, above UC recommendations, “so that all individual trees across the field are above the critical values.”

The uptake of micronutrients, whether applied to the soil or to foliage, is complicated and varies by the crop and element concerned, Brown said.

Solubility of micronutrients and pH of the soil must be in balance for them to bring about improved productivity. Healthy roots are essential for movement of micronutrients from the soil, but the problem is greater than that alone.

In trees, for boron and other micronutrients, soil analysis is poor because it cannot predict where almond or pistachio roots will be. Soil tests have a role, but it is limited.

Deficiencies in other micronutrients occur when boron is low in the irrigation water, because boron is the glue for the others. Boron, critical in pollen structure and building cell walls, is very mobile in plants.

On the other hand, zinc, which functions to develop genes into protein as the first step in cell growth, moves poorly in the plants. Along with iron and manganese, it is highly impacted by soil organic matter and microbial activity.

A foliar zinc spray is useful only for the current season and the following year's flush will get no benefit from it.

Brown reminded growers and PCAs of the Law of the Minimum, propounded by German chemist Justus von Leibig in the 1860s. It states that if one element is lacking, even in the smallest amount, plant growth will be poor when other elements are abundant. But when too great an amount of that element is supplied, some other element then becomes the limiting factor.

“Don't waste your time applying the element that is not the limiting element,” Brown said.

Meanwhile, he has begun studies on relative uptake efficiencies of several foliar fertilizers and sampling protocols.

Growers, Brown added, can do their own trials to evaluate their micronutrient management. They can consider using smaller blocks for sampling, such as splitting a 100-acre block into two 50-acre blocks.

They can also keep in mind the soil conditions that contribute to the deficiency and the stage of growth when sampling is done.

The conference also heard from R. Scott Johnson, UC Cooperative Extension pomologist at Parlier, who commented on his FREP-funded research with zinc in peaches and pistachios.

He has been using the isotope 68Zn to trace the movement of the element in the trees.

In peaches, he noted, “So far, we have found the period of greatest zinc uptake to be in early fall. Young trees in the field were able to take up between 7 percent and 8 percent of the 68Zn in a labeled zinc sulfate solution painted on the leaves in mid-September.”

Johnson said the greater amount of leaves during the fall gives a better “target” for foliar treatment and fall is when growers are less concerned with phytotoxicity.

In addition to learning that zinc is taken up more efficiently from a zinc sulfate solution than from a zinc oxide suspension, he has concluded that the micronutrient moves more readily in peach than in pistachio.

Bob Beede, Kings County farm advisor, who's been collaborating with Johnson, said zinc deficiency is the most common micronutrient problem.

However, Beede stressed, “90 percent of our problems are still soil-water management.” Zinc deficiency in walnuts, for example, he added, is commonly seen near the wetter irrigation head end.

Soil and tissue analyses, he said, are no substitute for the grower's own experience and judgment, based on the cropping history and soil conditions of his farm.

But, he added, sampling is an opportunity to integrate all the factors and a talking point with other growers to address solutions.

Among zinc sampling tips, Beede said compare leaves of trees of the same age, with and without symptoms. And do not sample leaves that have been sprayed, but instead wait and sample new leaves that have emerged after the spray.

FREP, which operates under the California Department of Food and Agriculture, is funded by mill assessments on California fertilizer sales and supports research and education for improved farming practices and reduced environmental effects from fertilizers.