Chapter 16. Micronutrients


Micronutrients have various functions, some not yet understood. Most of those that are known involve metabolic activities.

Micronutrients are required in small amounts and only in small amounts.

Soil pH and organic content are two important factors affecting their availability in soil.

Several possibilities for inorganic micronutrient fertilizers exist, but organic materials offer a double advantage: a diversity of micronutrients and a natural chelate capability.

Table 25. Sensitivity Of Plants To A Micronutrient Deficiency lists sensitivity of crops to a micronutrient deficiency. Table 26. Micronutrient Content Of Various Materials lists the micronutrient content of organic residues. Table 27. Typical Application Rates Of Micronutrient Fertilizers contains typical fertilizer application rates.

Micronutrients In Plants

Most of the discussion in this chapter is from [50].

Some minerals are important to plants only in small amounts; large quantities can be toxic. Most likely not all such minerals been recognized yet, and the functions of those which are known are not well understood. These minerals constitute the trace elements or micronutrients.

This chapter describes those six which have received the most attention. However, there are others. Two whose function is at the borderline of our knowledge are chlorine, which is essential in one phase of photosynthesis; and silicon, which gives plants a mechanical strength, minimizes water loss, and resists disease.

In addtion, others are important in various unnrelated or indirect ways. Cobalt is necessary to bacteria that fix nitrogen; vanadium is necessary to soil organisms in an as yet unknown way. Trace amounts of cobalt, iodine, selenium and chromium are essential to human and animal health.

The six which are discussed here are boron, copper, iron, manganese, molybdenum and zinc. All except boron control metabolic reactions, together with specific plant enzymes. Copper, manganese and zinc are involved in a variety of activities. Molybdenum and iron are important in nitrogen fixation and in reducing nitrate to ammonia for the synthesis of proteins. The pink color associated with healthy legume root nodules is due to the existence of an essential iron-molybdenum-enzyme combination. In addition iron is necessary for the production of chlorophyll.

The importance of molybdenum in nitrate metabolism has been confirmed by research with potatoes showing that additions of sodium molybdate can reduce nitrates and toxic glycoalkaloids [8]. This presents a conflict. Potatoes are traditionally grown in acid soils in order to reduce the onset of potato scab; but the availability of molybdenum is low in these soils. Although there may be an alternate way to avoid scab1, and even though scab is only a cosmetic defect, it is a dilemma for market gardeners.

Boron exists in cell membranes, and, in some as yet unknown way, it is influential in tissue production. It is also important in nitrogen fixation. Boron seems to have a common function with calcium, and a deficiency of either can inhibit the growing points of the plant and disrupt cells. Boron is thought to contribute more than any other micronutrient to the quality of produce.

Our lack of knowledge about the effects of trace elements in plants is due partly to two problems. One is the minute quantity needed in plants and the effect of increasing amounts on plant functions. The usual behavior with major nutrients is that, within a reasonably wide range of fertilizer application rates, plant growth increases with increasing application rates.

The response of plants to micronutrients, however, is an all-or-nothing affair. Within a narrow range of application rates, a plant grows well, indifferent to the precise rate; but outside this range it manages poorly or dies. Boron is deficient for some crops at less than 1 part per million (ppm) in the soil and toxic for others at more than 3 ppm.

The second problem is the complexity of the interactions with each other and with the major nutrients, in both plants and soil. Iron, copper, manganese and zinc are mutually antagonistic. Excess potassium or magnesium may lead to a manganese deficiency. Sulfur or copper may cause a molybdenum deficiency, but phosphorus has a beneficial effect with molybdenum. Phosphorus and iron together may affect zinc. Nitrogen fertilization may lead to deficiencies in iron, copper, or boron, and prolonged phosphorus fertilization may cause a copper deficiency. Zinc availability requires a well-aerated soil, but iron needs some period without oxygen. It is difficult to keep track of these conflicts.

Table 25. Sensitivity Of Plants To A Micronutrient Deficiency [50], [57], [51], [22] lists the degree of sensitivity of a variety of crops to a micronutrient deficiency.

Micronutrients In The Soil

Some soils are extreme in their micronutrient content. Quartz and sandstone soils are low in zinc, while igneous soils can be high. Boron is higher in sedimentary than in igneous rocks. Boron is often high in arid or semi-arid soils, but boron, copper, manganese, molybdenum and zinc may be low in leached sandy soils.

In most cases, however, the total content of micronutrients is adequate, and the two most important factors controlling their availability are the pH and the organic content.

Iron, manganese and zinc are often unavailable in soils of high pH, but molybdenum tends to be unavailable in acid soils. Boron is not as sensitive to pH as it is to the presence of lime, to which it becomes strongly attached, and recently limed soils may develop a temporary boron deficiency.

For most soils, the best compromise between the availability of molybdenum and the other micronutrients occurs when the pH lies somewhere between 6 and 7. Along the Atlantic coast and in the southeast, soils of low organic content may be deficient in total manganese; their pH should not be much above 6.

Organic matter is often essential to maintaining the availability of micronutrients; without it, micronutrients tend to be bound to inorganic minerals so strongly that their release during the growing season is low.

The importance of fresh organic residues is due to their ability to chelate the cation trace elements (copper, zinc, iron, manganese)2.

Not only does chelation provide a source of micronutrients, but it also buffers the soil against an excess, because only a fraction of the chelated material is released. Tests have shown organic matter in orchard soils to hold as much as 1,000 ppm of the copper that had accumulated from the use of copper-based fungicides. Organic matter is also capable of chelating lead, chromium and other toxic metals. As a buffer, moreover, organic matter decreases the effect of the soil pH. It furnishes micronutrients if the pH is too high, and it inhibits toxicity if the pH is too low.

The alternative of supplying trace elements with inorganic fertilizers while trying to keep track of the antagonisms that exist among them is a nuisance. Also, trace elements from inorganic fertilizers often remain available for too short a time to be useful.

In the U.S. and Canada, the micronutrient most likely to be deficient is boron: its availability is less influenced by organic matter than other trace elements, it leaches easily, and it may be locked up by fresh lime. Boron is often particularly low in subsoils. Consequently, in dry weather it may be deficient as plant roots grow downward in their search for water.

Sources Of Micronutrients

Organic Matter

It is difficult to find information on the micronutrient content of organic fertilizers, because more attention has been given to determining the NPK content. This is unfortunate, because in our times, with the availability of purified commercial fertilizers, the micronutrients in organic residues may be more important than the major nutrients.

Table 26. Micronutrient Content Of Various Materials lists the micronutrient content of those residues which have been tested. Owing to the small amounts of trace elements present, large fluctuations can exist. The values illustrated are useful only in showing the possible magnitude of the micronutrient content. The high value for the boron content of legume hay was taken from a single test of alfalfa and should be suspect.

Table 26. Micronutrient Content Of Various Materials also shows the micronutrient content of an average crop. For example, approximately 7 tons of fresh cow manure should, without help from the soil, supply enough boron and molybdenum for 1 ton of crops (dry weight), more than enough copper, iron and zinc, but not enough manganese. Two tons of fresh cage layer manure should furnish enough of all of the trace elements for 1 ton of crops. Two tons of dried seaweed should provide nearly enough, although enough fresh seaweed to supply this much dried seaweed would be difficult to accumulate and haul to the field.

Straw should have approximately the same micronutrient content as hay; most of the trace elements remain in the mature plant tissue, owing to their low mobility. Similar castoffs, such as leaves, should also be high in trace elements. Wood ashes, one of the few materials on which information does exist, contains many trace elements.

As the data for poultry manure infers, seeds have a high content of micronutrients. Commercial products such as cottonseed meal and soybean meal should, therefore, be a good source.

Inorganic Fertilizers

Inorganic micronutrients can be applied in accurately measured amounts in three ways:

Inorganic salts are the easiest to locate and usually the cheapest. Sulfates and oxides are common. Borax supplies boron, and ammonium molybdate and sodium molybdate furnish molybdenum. These can either be added to the soil or sprayed on the foliage. Borax dissolves slowly in water, so a more soluble, proprietary chemical may be preferable.

Synthetic chelating agents mimic the chelating properties of organic matter. Like the inorganic salts, they can be applied to the soil or the foliage.

Glass frits are microscopic glass particles containing micronutrients. Borosilicate glass was one of the first used. Frits have a very large surface area and are slowly dissolved in the soil, after which their nutrients become available. They are not sold directly to the public, however, but only through fertilizer manufacturers, who mix them in predetermined proportions for specific crops. They are applied only to the soil. Compared to the inorganic fertilizers and synthetic chelates, they appear to have promise, but not much information is available from independent sources3.

For acid, highly leached sandy soils naturally low in micronutrients, inorganic salts are satisfactory when applied to the soil and are sometimes suitable in alkaline soils. The major difficulties are with copper, iron and manganese. Copper sulfate dissolves very fast and is soon locked up, but before this happens, it may be present in toxic concentrations. the toxicity problem is partly overcome by using copper oxide or copper dust, which dissolve more slowly. Iron and manganese salts are usually useless in alkaline soils.

Chelants are finding increasing use, especially in supplying iron, but they are expensive. Manganese chelants are often unsatisfactory, because the manganese can be replaced by other metals before it reaches the plants.

Foliar sprays - either inorganic salts or chelants - are more efficient than fertilizers applied to the soil, and less is necessary. Exceptions are some salts: ferrous sulfate produces mixed results, and copper sulfate may scorch the leaves.

Inorganic salts are satisfactory for supplying boron and molybdenum either to the soil or foliage, although seed treatment is the preferred approach for molybdenum. No chelants exist for either.

Typical application rates for the more common fertilizers are given in table 27. Typical Application Rates Of Micronutrient Fertilizers [50], [57], [51], [22]. No micronutrient fertilizers, however, should be used without good reason; inorganic fertilizers are not buffered, and they are concentrated. The necessary quantities to apply are so small that uniform spreading is critical.

If these conditions are difficult to keep in mind, reconsider organic residues.

1 for example a cover crop of soybeans [16], [54]    [return to text]

2 Molybdenum and boron are anions. They are not chelated, but soil organisms require molybdenum and boron, so they are present in residues. Molybdenum is required by plants in such small amounts that the quantity released by decomposition should be sufficient for plant needs if the pH is satisfactory    [return to text]

3 For a list of manufacturers of glass frits as well as other trace elements, see [2].    [return to text]

© 2013 Robert Parnes

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