Three sources of boron including high‐grade Fertilizer Borate, Colemanite and Howlite were compared as to their water‐soluble boron contents, effect on plant growth and movement within the soil profile. Based on mill‐run samples, the water‐soluble boron in Fertilizer Borate was 5 times greater than in Colemanite and 25 times greater than in Howlite. Particle size had only a slight effect on the water‐soluble boron recovered from Fertilizer Borate, but for Colemanite and Howlite the amount of water‐soluble boron recovered was 6 to 8 times greater for finer than 80‐mesh material as compared to coarser than 10‐mesh material. This indicates the importance of particle size in controlling the rate of solubility of the more slowly soluble sources of boron. Although the water‐soluble boron in Fertilizer Borate, Colemanite and Howlite was in the ratio of approximately 1:5:25, the boron contents and toxicity symptoms of turnips and soybeans grown in the greenhouse in Norfolk loamy sand indicated a requirement of about twice as much Colemanite as Fertilizer Borate and 2 to 3 times as much Howlite as Colemanite to produce the same degree of toxicity. Results of the field leaching study on Norfolk loamy sand indicate that high‐grade Fertilizer Borate leached out of the topsoil very rapidly and collected in the lower zones of 8 to 16 and 16 to 24 inches. Twelve months later most of the water‐soluble boron had been leached past the 2‐foot depth. Howlite leached out of the topsoil slowly; the concentrations of water‐soluble boron remained fairly constant for the 6 and the 12‐month periods. Colemanite was found to be intermediate between the highly soluble sodium borate and the less soluble borosilicate from the standpoint of loss by leaching.

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