|Abstract or Summary
- The semi-arid dryland wheat-producing areas of the Pacific Northwest are characterized by cool, moist winters and dry, hot summers. The amount of annual precipitation is generally quite variable and inadequate for annual cropping. Where soil depth is adequate, the predominant loessal soils generally supply sufficient moisture for small grain production through the practice of summer fallowing. Though inefficient in moisture storage, fallowing reduces the risk of crop failure by storing a portion of the precipitation received during the 14-month fallow period for later use during a 10-month winter-grain growing season. Nitrogen fertilizer rates must be balanced with available moisture supply for maximum yields and desired protein contents. In order to develop predictive moisture availability equations, five fallow-crop precipitation patterns characteristic of the 250-350 mm precipitation zone of eastern Oregon were simulated in the field on a commercial dryland farm near Moro, Oregon, beginning in 1977; two 24-month fallow-crop cycles were completed during a three-year period. Moisture measurements made to a minimum depth of 270 cm at different times during both the fallow and crop periods have provided useful agronomic information. The soil moisture content continued to decline through upward movement from the beginning of the fallow period to the period of winter precipitation; vertical movement depended on the tension gradient. The greatest efficiency in moisture storage occurred during the first winter storage period when the fallow soil was generally dry; cumulative storage efficiency was highest on the plots which had received the least precipitation. The greatest rate of measured moisture loss occurred immediately after the fallow winter storage period. Soil moisture continued to decline during the remainder of the fallow period, especially in the 0-90 cm soil profile, but at a much lower rate due primarily to the development of a soil mulch through spring tillage. On the average, there would have been no moisture storage advantage to fallowing when the level of net storage in the spring of the fallow period was 194 mm or greater in the 0-270 cm soil profile (r² = 0.94). After the crop winter storage period, comparison of precipitation treatments that eventually would have equal amounts of cumulative precipitation indicated that a greater amount of stored moisture was stored in the plots which had the wetter fallow period, and in these plots more moisture was stored in the 90-180 cm soil profile than in plots with a drier fallow period. The plots with the wetter fallow also showed more soil moisture removal by the crop than plots with a drier fallow. A significant increase in grain yield, water-use-efficiency, and soil moisture extraction occurred as nitrogen fertilizer rates increased from zero; increases in soil moisture extraction due to fertilizer were most pronounced in the 90-180 cm soil profile. Linear regression analyses showed the relationship between the maximum grain yield (kg/ha) in each main plot, and the sum (mm) of the precipitation (P) and moisture depletion from the 0-180 cm soil profile (SM) between early spring of the crop period and harvest to be defined by the following equation: Y = 21.1 (SM + P - 62); r ² = 0.80 Soil moisture held between 1/3 and 15 bars tension was not uniformly depleted to the 15-bar level throughout the root zone, but rather the amount extracted tended to decrease with increasing soil depth. Extractable moisture, defined as the difference between the highest measured volumetric soil moisture content in early spring of the crop period and the soil moisture content at harvest, more accurately reflects the amount of soil moisture utilized through evapotranspiration than does the amount of moisture held between 1/3 and 15 bars tension. Linear regression equations were developed to estimate the amount of extractable soil moisture in the 0-90, 90- 180, and 180-270 cm soil depths from soil moisture measurements in early spring of the crop year. Estimates of extractable moisture, together with an estimate of anticipated crop season precipitation, can be used to predict the potential grain yield, which in turn is necessary to calculate the optimum amount of nitrogen fertilizer.