Extractability, Plant Yield and Toxicity Thresholds for Boron in Compost

The increasing use of a variety of postindustrial wastes believed to be benign in the compost process raises the concern of presence and toxicity of xenobiotics, defined as compounds that have been introduced into the environment by artificial means at any point in their useful cycle, and which may or may not later exert harmful influences if present in composts. Boron (B) is a trace element that is infrequently found at very high levels in composts( Purves and Mackenzie 1974) . Boron may be introduced into composts as a type of point-source and in others as nonpoint origin. Examples of the former are in the use of treated gypsum board and coal fly ash, both of which contain high levels of boron, the first by deliberate addition of borated fire retardant chemicals and the latter as an unavoidable residue of environmental contamination (UREC) from coal mining. Apparently, as much as 62% of coal fly ash from power utilities goes unutilized (Stevens & Dunn 2004; USEPA 1988) and has been useful in agriculture under controlled conditions. Examples of the analogous nonpoint source pollution in the case of boron would be MSW composting, where numerous household products and industrial chemicals and textiles are comingled in the final compost, each having boron from prior use in products such as PVC (for softening) and textiles and synthetic mattresses (for fire-resistance treatment). Boron is an interesting element since it is essential to crop growth in small soil concentrations (0.2-1.5 ppm) yet may produce plant toxicity in plants as the soil concentration increases over 2.5 ppm (Reisenauer et al. 1973). The margin between boron sufficiency and toxicity is therefore very narrow for a very wide variety of crops. Concentrations of boron in plant tissue only slightly above the required level have been shown to cause serious injury to the plant (Smith et al. 1997). Therefore, composts containing the listed ingredients may potentially exceed agronomic B levels, especially when used at heavy rates, and therefore be of great concern to end-users. A complicating factor in addressing boron from composts is that the bioavailability of boron is depenCompost Science & Utilization, (2008), Vol. 16, No. 2, 114-118