Determination of Soluble Phosphorus Content in Black Walnut Husk Compost via Formation of Molybdenum Blue

Abstract

Walnut fruit husk compost is a by-product of the commercial processing of walnuts. These outer husks collect into generous size piles and are normally left to decompose. Black walnut (Juglans nigra) husk compost was analyzed for phosphorus content to determine its potential as a phosphorus fertilizer or soil additive. Twenty-two samples of walnut husk compost of varying decomposition times were collected and dried. Soluble phosphate was extracted with distilled water and reacted with ammonium molybdate and stannous chloride to form the brightly blue-colored heteropoly-molybdenum blue which was analyzed spectrophotometrically at 650 nm. Soluble phosphorus (as mass percent of dried samples) decreased by nearly 50 percent in the first year from 0.1708 ± 0.0199 percent in three-week-old compost (n = 7) to 0.0967 ± 0.0099 percent in 52-week-old compost (n = 8). In conclusion, the water soluble phosphorus found in walnut husk compost is within a suitable range for plant usage, but as a fertilizer it would be very mild. Due to the known allelopathic effects of this species, this level of phosphorus might make such compost an acceptable candidate for herbicidal applications with concurrent fertilizing benefits.