Graduate Thesis Or Dissertation
 

Soil moisture and temperature relationships under fallow in eastern Oregon

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  • Time of stand establishment is a critical factor affecting yields of winter wheat and barley in the fallow-crop rotation areas of the Pacific Northwest. Farmers in this winter-rainfall region are dependent on residual moisture in the seed zone for germination, because significant precipitation does not usually occur until after the optimum planting dates. Moisture is maintained near the soil surface through the summer of the fallow period by the use of a soil or stubble mulch. The rate of moisture loss is relatively low during the summer, but seems to accelerate in late August and September. This loss dries the seed zone and forces either deeper planting to reach adequate moisture or delayed seeding until precipitation re-wets the seed zone. Both practices can result in late, less vigorous stands which have lower yield potential and provide less protection from erosion. The objectives of this study were to: 1) quantify changes in seed zone water content prior to seeding; 2) determine the cause of the accelerated loss; 3) substantiate the effect of planting date on the yield of one variety of winter wheat and one variety of winter barley; 4) investigate the effect of soil temperature on the rate of first and 70% emergence of these species in the field; and 5) develop a means of predicting the average last date of planting after which stand establishment is excessively delayed at one location in eastern Oregon. Although both years were abnormally wet in late summer and fall, significant losses of seed zone water content occurred in 1976. At 6 cm, the loss period occurred in early September; at 9, 12, 15, and 18 cm, the losses occurred in late September. The measured losses were not as great as expected. No significant losses were observed in 1977 because of frequent precipitation. My hypothesis was that increasing nighttime vapor pressure gradients from the moist seed zone to the soil surface develop because of the combination of warm days and cool, clear nights characteristic of late August and September in this area. Larger vapor pressure gradients would cause increased water losses from the profile. However, no correlation was found with calculated vapor pressure gradients, the occurrence of low surface temperatures at night, or average temperature gradients in the upper soil profile. Computer simulation of isothermal liquid flow was used to discern the relative contributions of evaporative losses and of long-term redistribution of water In response to gravitational and potential gradients in the profile. Redistribution accounted for 60% of the water loss in the soil beneath the seed zone from mid-July to early August, and accounted for none of the loss from early August to early September. Planting date had a significant effect on yield of both wheat and barley; the optimum planting dates were late September to early October. On each planting date, the seeds were placed in moist soil and covered with approximately 5 cm of soil with a deep furrow drill, so temperature was the primary factor affecting rate of first and 70% emergence. Regression equations of rate of emergence on average 10-cm soil temperature from planting to emergence were highly significant. The degree days needed for first and 70% emergence for wheat were 149 and 210 using a base temperature of 0.7 and 0.4 C, respectively, and for barley were 92 and 159 using base temperatures of 6.1 and 3.5 C, respectively. Soil temperatures from 1963 to 1977 were used to develop a means of predicting average daily 10-cm soil temperature. Using this long-term average and the regression equations of rate of emergence and stand establishment versus temperature, the average last date to plant and still obtain 70% stand in 14 days was 25 September. If seeding is delayed until 15 October, lower soil temperatures will cause the average days to 70% stand of wheat to approach 22-24 days, while barley will require 24 - 2 9 days to reach 70% stand at these temperatures.
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