The effect of variations in calcium supply, ph value and nitrogen content of nutrient solutions on the response of lettuce and red clover to molybdenum

Lettuce and red clover were grown in nutrient solutions with varied calcium supply, pH value, and nitrogen content, and the response to molybdenum compared under each set of conditions. The calcium requirement was greater in solutions at pH 4-4 than at 6-3, but the quantity of calcium supplied did not affect the response of the plant to molybdenum. Growth was best in the more acid of a range of solutions from pH 4-2 to 8-2 in spite of a rapid levelling up to a pH between 6 and 7, but with the possible exception of the solution at pH 8-3, the need for molybdenum was unaffected by the reaction of the medium. When the calcium supply and/or the initial pH value of the solution was varied, the effect of molybdenum was most pronounced in the largest plants. When the nitrogen supply was deficient, lettuce showed a slower response to molybdenum than when it was plentiful. With both inoculated or uninoculated clover the reverse was true. This difference in behaviour is explained on the assumption that lettuce has a smaller requirement for molybdenum than clover. In both lettuce and clover the percentage nitrate-nitrogen in the dry matter of the shoot was higher when molybdenum was not supplied, but the total nitrogen content was increased in the case of lettuce only. At any level of nitrogen supplied, 5 or 10 p.p.m. molybdenum was of no more benefit than o-i p.p.m. though the liability to damage from toxicity was possibly greater when nitrogen was plentiful.