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Humic acid defined at Actagro seminar

Products sold as extracts of manures and composts that profess to contain humates or humic acid to promote crop growth and yields may not always be genuine, according to Patrick MacCarthy, professor of chemistry at the Colorado School of Mines, Golden, Colo.

MacCarthy, an expert on the often confusing substances, defined them in a talk during a day-long seminar sponsored by Actagro Plant Nutrients at Fresno, Calif.

Humates, MacCarthy said, are not single, pure substances, but rather “an extraordinary complex mixture of organic molecules that originate from the decomposition and humification of plant and animal matter, including microorganisms.”

Humification, he added, “refers to the process whereby the plant and animal residues undergo major structural modification and essentially lose the primary character of the biological materials from which they originated.”

Humate sources include peat, sediments, soils, and a low-grade coal known as leonardite, which has a high concentration of humic acid recoverable in processing.

“Humates, which exist around the globe, are critical to the development and sustenance of botanical life everywhere,” he said.

A special chemical property of humates, which have the ability to hold clay particles, retain moisture, and transport nutrients, is their resistance to rapid microbiological breakdown in the environment.

Although various processes to chemically isolate humates from the various sources dates back to the late 18th century, MacCarthy warned of a tendency in recent years to promote any number of so-called humates from spurious sources, including roasted coffee beans, molasses, and even beer.

More intense interest in products that claim to enhance crop productivity has come as a result of the shrinking supply of highly arable land.

To prove a product is indeed from humates, MacCarthy has a three-step procedure. First is to establish the original source as a true geologic deposit, such as leonardite or other material that has undergone extensive humification.

Second is an extraction procedure using a strong base followed by treatment with a strong acid to separate humic acid, fulvic acid, and humins. Third, the three fractions, known as the “organic acids,” are dried and weighed.

Noting that the verification of the original source is key, MacCarthy stated that no single chemical analysis can measure and identify humate content of a substance.

Further describing leonardite, he said it is an oxidized form of lignite, or soft coal, found in strata above lignite. Since it is more oxidized than coal, it is less suitable as a fuel. Leonardite used in Actagro products is mined in North Dakota.

While other companies market raw, crushed ores or extracts from composts or manures, he said “these materials may be good, but that does not justify calling them humates, and it's good for anyone involved in the purchase of these products to question where they originally came from.”

The humification process, according to Actagro sources, requires at least 20 to 30 years, and only then is the small amount remaining from manure or compost sufficiently decomposed to be considered humate or stable humus.

Under regular tillage, complete loss of compost- and manure-based carbon will occur long before the organic matter goes through humification.

Addressing the seminar on management of micronutrients in tree and vine crops, Patrick Brown, professor of plant nutrition at the University of California, Davis, said some of those principles used in California were inherited from agronomic crops.

“With high-value and high-investment horticultural crops, you never want to be in a reactionary circumstance with micronutrients. It's a matter of prevention, not cure,” he said.

The University of California and other institutions have issued critical value guidelines setting limits for micronutrients, but these were mostly based on sugar beets and corn where fertilizers were not applied unless an immediate response was expected.

Soil and leaf sampling technologies and data interpretation are fundamentally flawed because they do not account for variability across an orchard or vineyard. In most cases, several samples are taken and combined for a single but irrelevant value, and tremendous variations also occur on leaves from the same tree.

What's more, deficiencies do not occur uniformly throughout the calendar year. For example, a recommendation for zinc in almonds is 15 ppm in July, although “the real role for zinc in almonds occurs in the spring,” he said.

Rather than over-applying to ensure that all parts of the field receive adequate amounts of a fertilizer, management with precision amounts is likely to make greater gains in efficiency of nutrients than any other practice. Fertigation is one way to move toward greater precision.

Consequently, Brown recommended, “As you increase field size and management unit, don't decrease management intensity. That is a false economy.”

More often than not, he continued, nutrient deficiencies are caused by environmental factors rather than an absolute nutrient shortage.

For example, water is necessary for nutrient uptake, and there must also be warm soil, adequate roots, and photosynthesis.

Manganese, nickel, iron and other micronutrients are all restricted in how far they move, about 1/64 of an inch, in the soil, so roots must be able to find the elements. Boron, however, is quite mobile in water and will move to roots.

Species differ in uptake of manganese, zinc, and iron, but rootstocks can help if uptake is a primary limiting factor. Nemaguard, Brown noted, the most widely used rootstock is about the worst in getting manganese, copper, and zinc out of the soil.

“The most important factor determining micronutrient availability, with the exception of boron, is the soil pH.

For iron, manganese, and zinc, as the soil pH increases, their availability decreases dramatically,” he said.

Plant roots, however, can change the soil pH around them by encouraging microorganisms and excreting organic acids or other compounds. Up to 40 percent of all carbon that is fixed in the plant by photosynthesis is released into the soil, not the crop.

“It's important to recognize that roots are living, breathing, and manipulating the environment in which they are growing,” he said.

The goal of a soil test, Brown added, is to imitate what the plant might be able to find in that same soil. “But bear in mind when you take a soil test, you obviously have to take it from where the roots are. The sample has to represent the active root layer.”

TAGS: Management
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