Effect of age upon the absorption of mineral nutrients by sugar cane under field conditions

A study has been made of changes which occur with age in the mineral composition of the sugar cane plant. It was found that the percentage compositions (mineral) of the leaves and of the stalk of the cane plant are markedly influenced by the age of the plant, particularly during the early months of growth. Hence, danger underlies the indiscriminate utilization of data pertaining to the percentage composition of the cane plant as an indication of the nutrient status of the soil. The dry matter of dead cane leaves was found to contain much lower concentrations of potassium and nitrogen than that of green leaves. This is accounted for on the basis that these nutrients migrate from the leaves back to the stalk before the leaves become physiologically inactive. The cane plant was found to absorb the necessary mineral nutrients in widely differing amounts. Potassium and silicon were taken up to the greatest extent, while nitrogen and phosphorus were absorbed in relatively moderate quantities. Of the nutrients studied calcium and magnesium were absorbed in least amount. The rates at which the several mineral nutrients were absorbed were found to vary with the age of the plant, but not always in the same degree. The rates of absorption of all the elements studied, excepting silicon, reached maximum values by the early age of 3 months. During this period approximately 10% of the first year's uptake of phosphorus and potassium occurred. The corresponding quantity of nitrogen was much greater, amounting to nearly 25%. After the age of 6 months in the case of nitrogen and after about 10 months in the case of calcium, magnesium, and potassium, the rates of absorption diminished. Uptake of silicon and phosphorus, on the other hand, continued at essentially constant rates until the conclusion of the experiment at 14 months. The rates of absorption of potassium and nitrogen were found to decrease immediately following the acquirement of maximum quantities of these nutrients by the leaves. From the results of this study it appears that the absorption of nitrogen and potassium, and probably other nutrients, by sugar cane is not primarily a function of the rate of growth, but of the age, or of the stage of development, of the plant. Pronounced differences were found in the distribution of the elements between the components of the crop. These differences were most marked in the instances of phosphorus and silicon. The quantities of these nutrients in the stalk (at the final harvest) amounted to 60 and 15%, respectively, of the totals taken up by the plant. Evidence is adduced which indicates gross luxury consumption of silicon by Hawaiian-grown canes. Examination of the distribution of potassium in the stalk suggests that as the meristematic tissues mature they lose potassium by upward migration of the nutrient.