Summary.1Under controlled conditions in water cultures soluble silicate was found to have little effect upon the growth of barley if phosphorus were also present, but if the latter were absent a significant increase in dry weight was induced by the silicate.2The addition of silicate caused an appreciable increase in the height of the main shoot, which was most marked in phosphate‐free solutions, becoming less evident as the quantity of phosphate present was increased.3Leaf development was retarded by phosphate deficiency and hastened by the addition of silicate.4A close association exists between the amount of phosphate present, and the effect of silicate upon the rate of tillering and the number of tillers developed.5Soil cultures with barley and mustard in pots with various forms of silicate showed that soluble silicates are more active than glass silicates, tending to cause increase in dry weight with deficient mineral manuring, and in some cases also with complete manuring.6Further soil experiments revealed variations in the response of barley and mustard to silicate on different types of soil. A general improvement occurred with increasing doses of silicate together with various combinations of manures, notably when phosphorus or potash was deficient.7The significance of the results obtained has been examined statistically, and an attempt made to formulate the effect of added silicate in terms of an increase in the efficiency of the superphosphate presen