Extraction of Available Boron

Most procedures for extracting available boron from acid and alkaline soils are similar. The colorimetric and other methods of determining boron in the soil extract remain the same for testing on acid and alkaline soils. Methods have been extensively reviewed by Bingham (151). There are a number of methods for extracting available boron from soils (151). The most common extractant is hot water because soil solution boron is most important with regard to plant uptake. Hot water and other common extractants will be discussed in this section.

Hot-Water-Extractable Boron

The measurement of hot-water-soluble boron is a very popular method for determining available boron. Berger and Truog (152) established a hot-water method for determining available boron in soil that served as a reliable indicator of plant-available boron; however, the method was time-consuming. Additional modifications were made by Dible et al. (153), Baker (154),Wear (155), Jeffery and McCallum (156), and methods were summarized by Bingham (151).

Gupta (157) further modified the hot-water procedure by extracting soils with boiling water directly on a hot plate. Boron is then determined in the filtrates by a carmine colorimetric method (157) or by an azomethine-H procedure (158).

However, Gupta found that a cooling period of more than 10 min before filtering the hot-water extracts resulted in slightly less recovery of boron. Yellow coloration that appears in some soil extracts interferes with the Azomethine-H procedure. The positive error due to yellow coloration can be reduced by refluxing soils in 10 mM CaCl₂. If the yellow color persists, the addition of not more than 0.16 g of charcoal per sample should be used. Too much charcoal tends to adsorb boron and reduce measured boron values (159,160). Gupta (158) reported that quantities of more than 0.8 g charcoal were necessary on soils containing more than 4.1% organic matter.

Extraction of hot-water-soluble boron is the most effective way to evaluate available boron to plants in most agricultural soils. Generally in the soil solution, less than 0.2 mg B L^-1 is considered deficient for crops, whereas greater than 1 mg L^-1 is considered toxic (161). On a soil mass basis, less than 1 mg B kg^-1 is considered marginal for boron-sensitive crops whereas greater than 5 mg B kg^-1 is considered toxic (119).

Boron from Saturated Soil Extracts

Saturation extracts of soils generally contain 0.1 to 10 mg B L^-1. The main advantage of a saturation extract is that it is easier to obtain than hot-water-soluble boron. Since the amount extracted by this method is less than that by hot-water extraction, this procedure has an advantage in determining the boron availability in toxic boron soils but would be less useful in soils containing low quantities of boron.

Other Soil Chemical Extractants

Li and Gupta (162) compared hot water, 0.05 M HCl, 1.5M CH₃COOH, and hot 0.01 M CaCl₂ solution as boron extractants in relation to boron accumulation by soybean, red clover, alfalfa, and rutabaga. They concluded that 0.05 M HCl solution was the best extractant (r = 0.82) followed by 1.5M CH₃COOH (r = 0.78), hot water (r = 0.66), and hot 0.01 M CaCl₂ solution (r = 0.61) for predicting the available boron status of acid soils. Aitken et al. (163) stated that hot water as well as hot 0.01 M CaCl₂ solution were far superior to mannitol and glycerol methods as a predictive test for plant boron requirement. They added that the levels of boron extracted with mannitol and glycerol were low compared to those displaced from the soil by the refluxing procedures. They suggested that mannitol would not be an effective extractant for boron in acid soils. Tsadilas et al. (164), working on high-boron soils, found that hot-water-soluble, 0.05 M mannitol in 0.1 M CaCl_2- extractable, 0.05 M HCl-soluble, and resin-extractable boron strongly correlated with each other. The coefficients of boron determination improved when the soil pH and clay content were included in the regression equation.

Mineral acid extraction of boron, especially with sulfuric acid, creates a number of problems for detection by complexing agents before the introduction of azomethine-H. Baker (165) found that phosphoric acid was a less suitable extractant than hot water for assessing the amount of soil boron available to sunflower during a short growing period. Gupta (166) found that sulfuric acid extraction of soils leads to high boron values due to interference with absorbance of the boron carmine complex. The HCl extracts were filtered easily, and no interference was encountered. Furthermore, the percentage recovery of added boron to soils was good and reproducible when extracted with 6 M HCl. No boron was lost when 6 M acid solutions were heated for 2 h at 100°C in a hot-water bath.

Another extractant, ammonium bicarbonate-diethylenetriaminepentaacetic acid (AB-DTPA), was suggested for determining boron in alkaline soils. The resultant filtrate is analyzed by inductively- coupled plasma spectroscopy (167). The AB-DTPA extractant has proven effective for determining boron and other nutrients on alkaline soils. It has been shown that this soil test alone was not as effective as the hot-water extractant in assessing boron availability to alfalfa (167). This soil test required the inclusion of percentage clay, organic matter, and soil pH to be effective. Gestring and Solanpour (168) further improved the AB-DTPA extractant on alkaline soils (pH 7.3 to 8.4) by the inclusion of ammonium acetate-extractable calcium into the regression equation of soil boron versus crop yield. This addition resulted in significantly increased correlation from r2 = 0.50 to 0.77, suggesting a possible effect of calcium in boron toxicity. Studies conducted by Matsi et al. (169) showed that the AB-DTPA-extractable boron was significantly greater than the saturated extract and similar to the hot-water extract, and was correlated significantly with hot-water or with saturation extracts. They included cation-exchange capacity in the regression equation for boron determination.

Correlating an extractant for boron with plant growth is a key for determining the effectiveness of that extractant. The hot-water extraction method appears to be the most effective procedure for assessing B availability to plants on alkaline soils.