Generally boron is extracted from the soil with boiling water, but care must be exercised during the extraction so that the boron is not volatilized into the atmosphere. The extract containing the boron must then be concentrated. The actual analytical procedures in use today usually require that a very strong, concentrated sulfuric acid solution be used in measuring the boron. Such solutions are unpleasant and dangerous to use.

The chemical analysis of plant tissue, as a routine procedure for the evaluation of soil fertility, has not achieved as common acceptance as the chemical analysis of the soil. Nevertheless, many investigators have shown that deficiency symptoms for the various plant nutrients appear when the nutrient in question drops to certain concentration levels within the plant. The complex relationships among the various nutrients required by plants make the interpretation of plant analyses fully as difficult as the interpretation of the soil analyses.

Further complicating factors in plant analyses are the stage of growth of the plant when it was sampled and the part of the plant (stems, leaves, whole plant, roots) used in the analyses. These factors are subject to wide variations. Probably the best method of evaluating the fertility status of a soil is one that is based on a combination of soil and plant analyses. Whenever such an approach develops on a more widespread basis, the use of the spectrograph would be feasible. Nearly all elements important in plant growth, except nitrogen, can be analyzed concurrently with the spectrograph, and only one sample need be used.

BORON FOR FERTILIZERS Comes from mineral deposits in Death Valley and the Mojave Desert and from the brine of Searles Lake, all in California.

Borax (sodium tetraborate) and other sodium borates have been the most commonly used materials for agriculture. These materials contain 10.5 to 13.6 percent of boron, depending on whether or not it is a regular or high-grade fertilizer borate.

New boron minerals for fertilizers have been introduced. Colemanite, a calcium borate, is less soluble than sodium borate. It is recommended for use on sandy soils in areas of high rainfall because it leaches out of the soil less rapidly than borax. It contains 10.1 percent of boron.

Some sodium borates have been developed especially for use as sprays or dusts. Such materials are highly soluble and are highly concentrated in boron. They are applied directly to the foliage of fruit trees, vegetables, and other crops in areas of high soil alkalinity.

All boron materials may be mixed with regular fertilizers, which are then known as borated fertilizers, before being applied to the soil. In several States it is recommended that all fertilizers for legumes contain a certain amount of boron.

Boron often is applied to the soil as a contaminant in other soil amendments or soil fertilizers. Barnyard manure may contain approximately 20 p.p.m. of boron. Superphosphate contains 5 to 20 P.P.M. of boron. Even lime contains boron, 1 ton containing the equivalent of 1 ounce of borax.

Since the boron content of the usual fertilizer materials is low and the rate of application of these materials is also low, these secondary sources of boron cannot be expected to provide an adequate amount of boron for crops.

The amount of boron that should be applied to a soil deficient in available boron depends on a number of factors the crop, the soil, the season, the method of application, and the source of boron. For a given crop, boron fertilizer can be applied in larger amounts to soils that have a high organic-matter content, a high exchange capacity, or a high pH, than could be applied to light, acid soils of low organic-matter content. The rate of application of borax fertilizer for alfalfa varies from 15 to 60 pounds an acre. The lower rates of application are generally used on the light, acid soils of the Coastal Plains. The heavier rates are recommended in the Midwest and the West.

A few words of caution are necessary. Borax is one of the oldest weedkillers known. The active ingredient is boron. In regions of low rainfall, the boron content of the soil is high. In these regions, the boron content of water used for irrigation is sometimes great enough to cause injury to crops.

Carl S. Scofield and L. V. Wilcox, of the Department of Agriculture, found that 0.5 p.p.m. of boron in irrigation water injured some crops. A boron content of 1.0 p.p.m. caused injury to most crops, even those with high boron requirements.

The boron that occurs in injurious concentrations in irrigation water may be derived from the solution of exposed outcrops of soluble boron minerals, from underground waters, or directly from volcanic gases dissolved in percolating waters. The areas in which boron toxicity may occur are not large; nevertheless, the injury to crops in some of these areas is serious. These areas are located primarily in southern California, western Nevada, and parts of Arizona.

Certain control measures are available for eliminating or preventing the accumulation of toxic concentrations of boron in soils. Thorough leaching of contaminated soils is usually recommended when possible. Mixing irrigation water high in boron content with water low in boron content is recommended as a method of utilizing all the water available for irrigation without building up toxic concentrations.

Borax, or other boron fertilizers, therefore must not be used indiscriminately. The effect may be disastrous. Fortunately, the effect in humid regions is temporary; the boron soon leaches from the soil. In the absence of official recommendations, farmers should use borax at the rate of only a few pounds to the acre and only on small areas until experience shows the need for boron and the proper amount to use.

Boron fertilization is becoming a necessary and accepted practice in many areas of the United States, just as plastic is becoming more widely used in the kitchen. But just as the housewife knows that plastic has its limitations, the farmer knows that the use of boron has its limitations.