The Texas Cooperative Extension Soil, Water and Forage Testing Laboratory does thousands of plant analyses a year at $15 each, and many are unnecessary, said the Extension soil chemist and director of the lab.
Soil tests, when the sample is properly collected, is nearly always a more accurate tool and sufficient in diagnosing most commercial production and home landscape problems. Soil testing should always precede tissue analysis, said Dr. Tony Provin, and one of the featured speakers at the Annual East Texas Fruit & Vegetable Conference, held here Feb. 17.
“Having tissue analysis done without a soil test is like installing new wipers on your car without replacing a broken-out windshield first,” said Provin, whose presentation was titled “Tissue Analysis and Foliar Sprays.”
Provin cited tests from a large pecan orchard in the San Antonio area. Tissue analysis showed toxic levels of the trace element boron. The owner was considering shutting down irrigation wells and buying irrigation water from the city of San Antonio, a very expensive proposition.
“Eventually, it was not the tissue analysis that helped us find the problem, but a follow-up soil test,” Provin said.
Boron is present in most soils, but a low soil pH made the small amounts of boron that were present highly available to the trees. Liming the soil caused the excess boron to chemically bind with aluminum compounds in the soil, which made it unavailable to the trees, and corrected the problem.
Tissue analysis can be a valuable tool when the standard diagnostic tools rule out possible problems such as insufficient soil nutrient availability, poor water quality, endemic disease or pest infestations. Tissue analysis is most appropriate for deep-rooted trees and shrubs where surface applications of fertilizer are less relevant.
To be of any use at all, tissue samples must be taken according to strict guidelines. For instance, samples must be collected from appropriate parts of the plant, and this varies according to the species. For example, with most trees, leaves are collected from the middle third of the canopy; for cabbage, the first mature leaves; and for asparagus, the middle of the stem. For any plant, call the tissue analysis lab to learn where to collect the sample, Provin said.
Samples should also be large, not just a single leaf or stem section. Wash the samples in a weak acid solution. This is because tissue analysis is most useful for diagnosing trace element problems where the nutrient is measured in parts per million. Windblown dust or improper handling can introduce extraneous compounds that can compromise the accuracy of the analysis.
An acid wash is performed using a diluted solution of muriatic acid, available at most hardware stores. Typically, muriatic acid comes in 20 percent concentrate. This should be further diluted as one part acid to 19 parts distilled water.
At this dilution, the acid is fairly benign, but rubber gloves should still be worn while cleaning the sample.
The lab will perform the acid wash if desired, he said. If the sample is sent unwashed, it should be sealed in a press-and-seal plastic bag and labeled accordingly.
“There are so many factors that influence accuracy of tissue sampling. More often than not, there's more information found by a properly taken soil sample,” Provin said.
More information can also be found at the soil testing laboratory at: http://soiltesting.tamu.edu.