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Soil pH Zinc - 2002

In our last edition of Subtropical News I reported on a trial that David Crowley at UC Riverside and I had done trying to correct zinc deficiency in avocado.  One technique that worked well was 7.5 pounds of zinc sulfate per tree applied to the soil surface.  This was a technique used many years ago and a single application usually would last for 5 years or more. The problem with the technique is that it requires lugging around all that material on a steep hill plus the environmental issue of applying such a large amount of a heavy metal to the soil.  It certainly would not be a practice repeated on an annual basis.  In the case of citrus, foliar applications have been highly successful in correcting micronutrient deficiencies, but annual applications are required.  In fact, ideally, we would like to avoid micronutrient applications altogether, and this can be accomplished through modifying soil pH.

Much of our western soils have pH's in excess of 7, making them alkaline.  When the soil has excess or free lime, the soil is calcareous and usually has a pH in excess of 7.6 or 7.8.  A calcareous soil will effervesce or fizz when treated with a 10% solution of hydrochloric acid, or the juice of a squeezed lemon.  In some areas north of Los Angeles it is not uncommon to find excess lime in the 20% range.  This free lime can be clearly visible as old shells lying on the soil surface.  More common are soils that run 2 to 5 % free lime.  These are not as easy to detect with the acid test as with the high lime soils, but these soils can lead to similar problems of iron, zinc and manganese deficiencies.  In the presents of free lime these nutrients are precipitated by the carbonate from the lime, causing lime-induced chlorosis.

Trying to correct the pH of a calcareous soil is complicated by the shear volume of material that needs to be neutralized in order to free up micronutrients like iron and zinc.  An acre-foot of soil has an approximate weight of 4 million pounds.  With a free lime content of 1%, that would mean 20 tons of lime to neutralize.  It takes a pound of sulfuric acid to neutralize a pound of calcium carbonate.  It takes a third of a pound of elemental sulfur to neutralize a pound of lime, but this is still a stiff amount to apply, especially by hand in an avocado orchard.  In areas where large machinery can be used, bulk soil treatment with sulfuric acid and elemental sulfur are commonly performed.

Something to remember about micronutrients is that they are required in very much smaller amounts than the macronutrients like nitrogen and potassium.  It is not as important to neutralize the whole soil in order to allow a tree to take up adequate amounts to meet its needs.  A commonly used material to correct iron chlorosis is the iron chelate-FeEDDHA (sold as Sequestrene 138¨).  Only a small amount applied in concentrated spots around the tree or as a band is needed, but it is an expensive material to use.  And, unless soil pH is corrected, yearly applications of the chelate are required.

One way to get eventual pH modification is to inject sulfuric acid through the irrigation system or use acid fertilizers, such as ammonium sulfate or the new urea-sulfuric acid products.  If the system is a microsprinkler, a fairly large area is wetted, and consequently, a large amount of buffered soil is wetted.  Gradually over time the soil pH will drop, but if there is much free lime, it will take time to reduce the pH.

The nature of chlorotic trees is such that in most cases a whole orchard is not affected.  It may be a lower corner or five trees out of a whole orchard.  It may not be necessary to treat the whole orchard, in order to improve the few trees that are affected.  This makes the concept less daunting of treating individually the affected trees.  This can be done by banding 2-5 pounds of sulfur in a channel on either side of the tree or in holes at the compass points of the tree.  The elemental sulfur needs to be covered with soil so that microbes can do their work of converting it to sulfuric acid.  The microbes slowly work on the sulfur, releasing acid over a number of years.  Working with a prilled sulfur is much easier than powder, although the larger pieces of sulfur do not allow microbes to do their work as rapidly.  A prilled sulfur mixed with bentonite clay is on the market.  As soon as soil moisture soaks the sulfur lumps, the nugget dissolves, exposing the sulfur to microbes.

Sulfuric acid can also be used as a soil drench, but for safety reasons, proper attire is required.  Using acid is a much more rapid way of getting acidification.  Urea-sulfuric acid fertilizer (49% sulfuric acid) is safer than the straight acid and can be used instead, although proper gear is still important.  In tests reported by Unocal on apples, significant chlorosis correction was obtained by a 15 gallon per acre strip spray (5 inches wide) or by injecting a total of one pint of urea-sulfuric acid at four points around the tree.  With all the soil-applied acidifiers, improved correction is obtained by adding iron and zinc sulfate at the point of banding or injection (1-2 ounces per tree).

A much more gradual approach to correcting soil pH is through soil organic matter additions.  The breakdown products of decomposition result in acidification.  This is due partially to increased microbial activity that generates carbon dioxide. The carbon dioxide in soil water creates carbonic acid.  There are also a myriad of byproducts of decomposition that are acidic.  To achieve much pH correction with organic matter, fairly large additions of mulch are required, on the order of a hundred tons per acre.

A high soil pH leading to lime-induced chlorosis is often exacerbated by root rot and wet soils.  By destroying the small roots responsible for micronutrient uptake, it may not be possible to adequately correct the chlorosis until fungicides are used.  A wet soil further impairs a root's ability to take up nutrients.  Modifying the irrigation schedule to allow the soil to dry between irrigations may by the simplest way to correct a chlorosis, and this would be the best long term solution to this problem.