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Title: A new soil test for measurement of plant available and adsorbed boron

item Goldberg, Sabine
item Suarez, Donald

Submitted to: Proceedings of the International Salinity Forum
Publication Type: Abstract Only
Publication Acceptance Date: 2/26/2014
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Agricultural drainage waters and treated municipal wastewaters are often elevated in B, posing a potential hazard to crop production if these waters are reused for irrigation. In arid areas, such as the Western United States, elevated B concentrations, of potential toxicity to crops, are also encountered in many brackish groundwaters of potential use for irrigation. Boron is a micronutrient trace element required by plants in trace amounts. However, the concentration range between deficiency and toxicity symptoms is very narrow such that it is possible for crops to experience both deficiency and toxicity symptoms throughout a single growing season. A variety of soil tests exist to measure available B for plant uptake, to diagnose B deficiency, and to make B fertilizer application recommendations. Historically, B extracted by various methods has been correlated with crop growth and response, with a primary focus on diagnosing B deficiency. The B concentration of a saturation extract is the standard method used to diagnose B toxicity conditions in arid zone soils. Sugar alcohols have been used as extractants of soil B because they contain cis-diol groups that complex strongly with B. An optimal soil test would measure B capacity which includes soluble, organic, and adsorbed pools of plant available B. Determination of the total available pool is required to evaluate the extent of leaching necessary for reducing B concentration of arid and semi-arid land soils to below toxic levels and to provide management recommendations for waters and soils elevated in B. We conducted a study with the following objectives: 1) to determine plant available B using a set of currently recommended soil tests; 2) to develop an improved B soil test by determining the optimal combination of soil:solution ratio, reaction time, and sugar alcohol type to measure all pools of plant available B; 3) to evaluate the new B soil test for its ability to recover adsorbed B pools from a diverse set of soils. We selected a diverse group of soils from California, Iowa, Oklahoma, Belgium, United Kingdom, the Netherlands, France, and Spain. Available B concentrations were determined using hot-water-soluble, calcium chloride-mannitol, potassium chloride, and DTPA-sorbitol extractable soil testing solutions. We compared the sugar alcohols: xylitol, mannitol, and sorbitol and found the greatest amount of B extraction to occur using sorbitol. Calcium chloride and DTPA were not required to aid in the B extraction of these solution. Optimum reaction time and soil:solution ratio that maximized B release were evaluated using 0.2 M sorbitol for three California soils ranging in native B content from very low to high to very high. Boron release increased considerably with reaction time and decreased with increasing soil:solution ratio. Traditional soil tests are commonly done at high soil:solution ratio and therefore extract only a portion of plant available B. We evaluated B recoveries of previously adsorbed boron following three extractions at a soil: solution ratio of 100 g soil/L. For two California soils, recoveries were 104% and 96% indicating quantitative extraction by the new B soil test. To further simplify the soil test, we determined percent recoveries on these two soils using a single extraction and found them to be 98% and 97%. These values for one extraction were not statistically significantly different from those obtained for three extractions. The single extraction 0.2 M sorbitol test was applied to five additional soils from California, Iowa, and Oklahoma. The resulting B recoveries ranged from a low of 89% to a high of 102%. Our study shows that low solid:solution ratios (less than or equal to 100 g/L) and long reaction times (greater than or equal to 24 hours) significantly improve extraction of plant available B. Therefore, we recommend