The effect of waste products from the guayule rubber mill on the growth of various crops

Plants of some crops commonly raised in the Salinas Valley of California were grown in soil to which various waste products of the guayule mill were added. These waste products consisted of the dried parboiled leaves shaken from the plants prior to milling, the liquid effluent in which the roots and stems were ground, and the bagasse consisting of the ground stems and roots from which the rubber had been extracted. The addition of the parboiled leaves and filtered effluent to the soil always increased the dry weight of the plants of such crops as lettuce, cotton, corn, and pinto beans. Immediately after the dry leaves were added to the soil a temporary inhibition of growth occurred, but this effect disappeared after a few weeks and then the dry leaves added to the soil resulted in an increase in dry weight of the plants. If the effluent was not filtered so as to remove practically all of the bagasse, some retardation of growth occurred in the seedling stages only. When bagasse was added to the soil the growth of the plants was always retarded and in some cases of heavy applications the seedlings died soon after emergence. It was determined that the effect of the various mill wastes on the growth of the plants was primarily related to their effect on the available nitrogen in the soil and not the presence of any growth-inhibiting substances in the guayule material. The leaves and effluent increased the available nitrogen, while the bagasse tended to make the nitrogen already present in the soil unavailable because of its high carbon, low nitrogen character. This was pointed out by the fact that the nitrate content of the soil and the plant was greatly reduced when bagasse was added to the soil and that the inhibiting effects of the bagasse could be eliminated by the addition of ammonium sulfate with the bagasse. The results indicate that the leaves from the guayule mill can be safely applied to soils and that the effluent can be used for irrigation without retarding the growth of crops; but that bagasse can be safely applied to soil only where a great abundance of nitrogen is available or along with a supply of nitrogen.