Copper studies with Oregon soils

Experiments are described which were made on four typical Oregon soil types to study the causes of erratic response to copper sulfate fertilization of cane fruits. These experiments included a solubility study using extractants of increasing strength; a fixation study in which amounts of copper as copper sulfate up to 160 p.p.m. were added to the soils; greenhouse work using low-copper and high-copper applications; and an availability study of fixed copper using beans The following conclusions appear to be justified: 1. An acid extraction of soils using boiling 1 and 5 normal nitric acid established arbitrary boundaries with copper solubility that were reproducible. 2. Acid extraction of the four soils studied indicated that, although the soils differed in total native copper contents and in the amounts extractable by neutral normal ammonium acetate and by different strength boiling nitric acid, the quantity of copper available was sufficient for plant growth. 3. Soil copper exists in an equilibrium between available, slowly available, and extremely slowly available forms. These forms contain sufficient copper for plant needs, hence yield increases may not be obtained from soluble copper additions to the soils studied. 4. Copper fixation absorbed most of the addition whether the soil was subjected to an alternate wetting and drying or to a moist storage. 5. Oat plants gave little or no response to copper additions. 6. Fixed copper is not available to growing bean plants. 7. None of the four soils studied responded definitely to copper additions by increased growth and it is doubtful that use of copper supplements in these soils would prove economical. 8. Erratic response obtained in the field appears to be due to factors other than a deficiency of copper in available forms in the soils studied.