A routine method for determining extractable soil B in the 0-10 .mu.g B/g range was required. The modified curcumin method showed promise. The method, which utilizes rosocyanin as the colored complex instead of rubrocurcumin, is very stable, sensitive and was shown to be relatively free from interference. Recovery of B added to acetate buffer (pH 4.8) extracts of 4 soils was acceptable. The modified curcumin method has potential as a method for the routine analysis of soil extracts.