Not available – first paragraph follows: Since the Citrus Experiment Station began in 1907, a major concern has been development of more efficient fertilizer practices. Early experiments by CES scientists, primarily in the orchards of cooperating growers, showed that on most, but not all, California soils, citrus trees do not respond to potassium and phosphorus application. Substantial amounts of nitrogen, however, have been required annually to maintain good vigor and yield. Later (after World War I), it was found that animal and green manures could be replaced as nitrogen sources by chemical sources, which, among other advantages, were more efficiently used by the tree and became progressively cheaper per unit of nitrogen.