Studies on fertilization for lettuce. I

The absorption of nutrients at different stages of growth of head lettuce (Great Lakes) was studied using plants grown in sand supplied with the solution containing 200ppm of N, 50ppm of P, and 100ppm of K.The peak of absorption rate occurred coinciding with the stage of the most rapid growth: this fell within the two week period around the starting of head formation. The total amounts of nutrients absorbed were N 1.19g, P 0.32g, and K 0.77g per plant. To elucidate the effects of nitrogen and potassium levels and their balance on the growth of lettuce, plants were grown in 12 plots given 100, 200 or 400ppm of nitrogen with 0, 50, 100 or 200ppm of potassium. With regard to the nitrogen level, the growth was best in the plots given 400ppm of nitrogen regardless of the potassium level. As to the potassium level, however, the optimum concentration for the growth varied according to the concentration of nitrogen. Nitrogen content of the head leaves in the plots. in which high yields were obtained ranged from 3.0 to 3.5%. The optimum potassium content differed according to the nitrogen level. Nitrogen and potassium contents in the N-400, K-100ppm plot in which the highest yield was obtained were 3.03 and 1.7%, respectively. Analysis of head leaves of the lettuce produced by some growers showed that the nitrogen contents were within the range mentioned above, but the potassium contents were much higher than the, value which was sufficient for obtaining high yield in the sand culture experiment. Therefore, it is assumed that the growers may be applying excessive amounts of potassium to the lettuce in their field.