The effects of nitrogen rates ranging from 0 to 2,000 pounds of N per acre yearly and of 40-, 60-, and 90-day harvest intervals on the yield and composition of Napier grass and on soil acidity, were determined for three consecutive years. Yields increased with nitrogen fertilization to at least the 800-pound level during all seasons. Crude-protein contents and protein yields increased with nitrogen fertilization up to the 2,000-pound level. More than 60 percent of the fertilizer nitrogen was recovered in the forage at all rates up to 1,200 pounds per acre yearly, but efficiency of utilization in terms of dry matter produced per pound of nitrogen decreased beyond the 400- pound level. The phosphorus and potassium contents of the forage decreased, but the lignin content increased with increasing nitrogen rates. The calcium and magnesium contents were not markedly affected by nitrogen fertilization. Dry-matter and protein yields and lignin content of the forage increased, while the protein, phosphorus, calcium, magnesium, and potassium contents decreased with length of harvest interval. With a 60-day harvest interval and 800 pounds of nitrogen per acre yearly, which seemed to be the optimum combination, Napier grass yielded 44,561 pounds of dry matter, or about 130 tons of green forage, per acre yearly, containing 9.7 percent of protein. With this treatment, Napier grass removed 674 pounds of nitrogen, 554 of potassium, and 120 each of calcium, magnesium, and phosphorus per acre yearly from the soil. Over-all yields did not decrease during the 3 years of experimentation but seasonal yields varied by as much as 70 percent of the average. The treatments affected residual yields obtained more than 6 months after the experiment was terminated. The application of 800 pounds of N as ammonium sulfate per acre annually over a 3-year period caused a drop of 3 pH units and a loss of 10.4 m.e. of exchangeable bases per 100 gm. of soil in the upper 6 inches of soil.
Effects of Nitrogen Fertilization and Frequency of Cutting on the Yield and Composition of Napier Grass in Puerto Rico
