A tentative estimation method of soil water soluble boron; using "Kenaf" plants as an indicator plant is proposed. 1) Prepare the three kinds of culture solutions that is (A) solution (basic nutrient culture solution free from boron), (B) solution (0.01 p.p.m. boron added to A solution) and (C) solution (some quantity of soil water extract added to A solution). 2) Plant "Kenaf" in above each solution and afterwards make? a comparative study of the lowest position of petioles curved in each plants. 3) The position of plant grown in (C) is higher than one grown in (A) but lower than (B), then boron content in soil extract used as 0-0.01 p. p. II!. 4) If the position of petioles curved in (B) and (C) are equal, consider the boron content in added soil extract is 0.01 p. p. m. 5) By changing the quantity of soil extract, make the position of petioles curved in (C) equal to the plant grown in (B). Boron content in soil extract used, can estimate from this experiment. By converting this value into soil weight, then soil water soluble boron also can estimate.