SOIL MICRONUTRIENTS

(TRACE ELEMENTS)

There is a growing awareness of the need for knowing the trace element content of soils and plants. Although only required in small amounts by plants, their deficiency or toxicity can have just as much effect on crop production as any of the major elements.

The trace elements which are at present receiving most attention for which we are analyzing are boron (B), zinc (Zn), manganese (Mn), copper (Cu), iron (Fe), molybdenum (Mo), and in some instances chloride (Cl) and arsenic (As).

Other elements that have received some attention as affecting plant growth such as aluminum (Al), barium (Ba), chromium (Cr), cobalt (Co), fluorine (F), lead (Pb), nickel (Ni), selenium (Se), strontium (Sr), titantium (Ti), tungsten (W), and vanadium (V) may be included in the future for soil trace analysis.

The rate of availability of soil trace elements is a vitally important factor. The availability of trace elements is more susceptible than major elements to the influence of various soil factors such as pH, water availability, soil structure, and most importantly, the ratio of the other plant nutrients present in the soil. Therefore, it is necessary to determine which portions of the trace elements in the soil are available to plants and how much each plant species requires.

This measurement of availability and plant requirement is made more difficult by the small quantities of element being dealt with, usually in the parts per million range, but sometimes in parts per billion. Because of this, sampling and analysis should be done with the utmost care and precision. (See section on Soil Sampling Techniques for Trace Elements, and see section on Toxicity.)

Using Trace Element Analyses

Several considerations should be kept in mind when trace element analyses are being used.

1. Very small amounts are required and the margin between deficiency and toxicity is quite narrow for some trace elements such as boron and molybdenum.

2. Analysis for, or application of, minor elements should not be given serious consideration until the major elements are in balance.

3. Toxicities or deficiencies can be induced by the following conditions.

a - When pH is rapidly changing up or down. (Large limestone applications, soil acidification).

b - Soil sterilization, both steam and chemical.

c - Irrigation water high in an element (i.e., boron) or material that accumulates by deposition or concentrates during soil moisture evaporation losses.

d - Application of fertilizer compounds which form soluble toxic substances.

e - Leached or accumulated spray material.

f - Guessing as to what is needed and applying a "shotgun" mixture.

REMEMBER: IT IS FAR EASIER TO CORRECT DEFICIENCIES THAN TO COUNTERACT TOXICITIES: SO TEST BEFORE CORRECTIVE MEASURES ARE SUGGESTED.

Since there are many factors that affect the availability of micronutrients and as the levels needed by plants vary a great deal, only very general ranges of deficiencies or toxicities are given. As mentioned previously, the balance of the major elements and the pH can have a great effect on minor element utilization. To better interpret the test results the major element test should accompany the minor element analysis.

As more information becomes available from other sources, as well as the correlated data from our own soil trace element analyses, plant tissue analyses, and field response data, an attempt will be made to publish more exact ranges of soil micronutrients for various crops.

Your interest and assistance in this area will always be welcomed.

(C) 2003 Northwest Argicultural Consultants. Back to Main Page