Calcium-boron ratio as an important factor in controlling the boron starvation of plants

In order to determine if boron is fixed by the soil colloid in a manner similar to the fixation of the phosphate ion, portions of an electrodialized colloid from a Miami soil and an electrodialized humus extract from a Brookston loam were adjusted to 16 pH levels, ranging from pH 4.1 to 11.5. Three and one-half millimoles of H(3)BO(3) per 100 grams of colloid were added, the suspensions shaken 24 hours, aliquots centrifuged, and boron determined by the method of Truog and Berger (11). All of the boron added was recovered in all cases. The same quantities of boron were shaken with water containing amounts of Ca(OH)2 equal to those added to the inorganic soil colloid-boric add systems. All the boron was recovered from each of the solutions regardless of its pH or calcium concentration. From this it was concluded that boron is not absorbed by the clay or humus complexes, or made insoluble with calcium. Four series of samples of a Crosby silt loam soil were limed at 0, 1/8, 1/4, 1/2, 1, and 2 times the lime requirement. One series did not receive boron or nutrients (nitrogen, phosphorus, and mannitol), a second received 2 p.p.m. of boron, a third received nutrients, and the fourth received 2 p.p.m. of boron and nutrients. At the end of 10 weeks, analysis for available boron showed that neither the calcium supply nor the added nitrogen, phosphorus, and mannitol, affected the amount of boron recovered, although there was an indication that a small part of the added boron becomes less available since all of the boron added was not recovered. Corn and tobacco plants were grown in pots containing silica sand to which was added a nutrient solution. The solution did not contain boron, except as an impurity in the chemicals. The solution was modified to make one series 0.0025 molar and the other 0.02 5 molar in calcium. The pH of these solutions was adjusted to range from 4.4 to 7.6. After 3 weeks, four of the plants were removed from each pot and the remaining four were treated 4 days with nutrient solution containing 1 p.p.m. boron. Neither the active calcium nor the pH affected the uptake of boron by these plants. The growth of Turkish tobacco grown on a Norfolk sand in greenhouse pots appeared normal when the calcium-boron ratio in the plants did not exceed 1,340:1. A calcium-boron ratio of 1,500:1 in the plants was correlated with severe boron starvation symptoms. The information reported here, together with results of other investigators, indicate that boron starvation results when the calcium-boron in the plant becomes unfavorable. Undoubtedly the most favorable calcium-boron ratio for different kinds of plants will vary. Further investigation should determine this as well as the ratios at which boron is toxic and where it is deficient. It appears that there is a possibility of using these ratios of calcium to boron in the plant as a guide in determining the need of boron fertilization.