Seven crop residue treatments were initiated in 1931 to measure long‐term residue management effects on soil organic matter in a wheat‐fallow cropping system on Pacific Northwest semiarid soils. There was evidence at this time of substantial organic matter (OM) loss during the first 50 years of wheat cultivation in the Great Plains. Organic carbon (C) and total (N) were measured at approximately 11‐year intervals over a 45‐year period to determine residue effects on the rate of change in soil OM content.Only the addition of 22.4 metric tons of manure/ha to straw residue before incorporation prevented a decline in soil N and C. The addition of 45 or 90 kg fertilizer N or of 2.2 metric tons of pea vines/ha to straw residue before incorporation reduced N and C loss when compared to straw only incorporation. Burning of straw in the fall following wheat harvest accelerated the loss of N but not C. Burning of straw in the spring just prior to tillage had no effect on N or C loss.Changes in N and C were primarily confined to the top 20 cm of soil. Soil C/N ratios in 1976 differed between treatments proportional to the rate of N loss; they were highest in burn or straw only treatments and lowest in the manure treatment.In all treatments, changes in soil N were best described by a linear function of time; slope within the linear function depended upon residue treatment. This linear function of time over a 45‐year period following approximately 50 years of previous cultivation suggests that 100 or more years may be required before N levels become stationary. Residual effects confirm that the new stationary level will depend on past crop residue management practices.Changes in soil C correlated highly with the amount of organic C supplied by each treatment, regardless of the different kinds of residue applied. Thus, changes in soil organic matter levels were controlled primarily by the amount of organic C supplied in crop residue. Regression equations indicate that approximately 5 metric tons of mature crop residue ha−1 year−1 are needed to maintain soil organic matter at its present level when cropped in wheat‐fallow rotation in this climatic zone.

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Maintaining or improving soil organic matter has high priority in agriculture because of its beneficial effect on soil physical, chemical, and biological properties. Soil organic N and C were measured 44 yr after establishment of a long‐term experiment to evaluate tillage and fertilizer effects in a winter wheat (Triticum aestivum L.)‐fallow rotation on a coarse‐silty mixed mesic Typic Haploxeroll. Main treatments consisted of three primary tillage systems, one conventional (moldboard plow) and two stubble mulch (offset disc, subsurface sweeps). Subplots consisted of six N treatments, 493, 728, 986, 1221, 1714, and 2207 kg N ha−1 applied over 44 yr. Organic N and C in the top 75 mm of soil were 26 and 32% higher, respectively, in the two stubble mulch systems than in conventional tillage, and equal below 75 mm. Stubble mulch plots contained 245 kg more N ha−1 than conventionally tilled plots, representing the conservation of 5.7 kg N ha−1 yr−1. Nitrogen fertilization increased soil N linearly in all tillage treatments, with 18% of the applied N incorporated into the soil organic fraction. Applied N also increased soil C linearly on plots with previous S application. Soil C was higher on plots with no previous S than on comparable plots with previous S, however, which suggests an S deficiency that altered S, but not N, transformations in soil. Identical N fertilization effects on soil organic N and C in both stubble mulch and conventional tillage suggests that N transformations were the same in both systems.

SYNOPSISComparison of agronomic characters between semidwarf selections and commercial short and very short strawed varieties suggests that seini-dwarf growth habit represents an important development in breeding winter wheats better suited to Pacific Northwest conditions. Bushel-per-acre yields of selections were higher than Br'evor and Elmar; selections were approximately two-thirds as tall as Brevor; they did not equal the sheaf weight of Brevor but averaged higher in grain yield and lower in straw tonnage. Average protein content was only slightly lower than Elmar and Brevor.

Wheat (Triticum aestivum L.) response to N in semiarid climates is highly influenced by precipitation and tillage, leading to inefficient use of fertilizer and greater potential for groundwater contamination. To improve fertilizer recommendations, we measured white winter wheat response to 45, 90, 135, and 180 kg N ha−1 for 10 crops grown in a wheat‐fallow rotation with three tillage treatments, one conventional (moldboard plow) and two stubble‐mulch (offset disk, subsurface sweep). The soil was a Walla Walla silt loam (coarse‐silty, mixed, mesic Typic Haploxeroll) with three depths to bedrock (208, 132, and 111 cm). Grain yield averaged 5.51, 4.07, and 3.55 Mg ha−1 with above‐ (147), near‐ (88), and below‐normal (61 mm) growing‐season precipitation (GSP). Nitrogen increased yield 3.0, 1.9, and 0.4 Mg ha−1 in these precipitation regimes. Grain yield was not affected by soil depth when GSP was above normal, but 10 to 20% less on the shallower soils when GSP was below normal. There was a trend towards higher yield with conventional tillage when GSP was above normal. The amount of applied N required for optimum yield was >135 kg N ha−1 with above‐normal GSP and <45 kg N ha−1 with below‐normal GSP. Excess N decreased grain yield when GSP was near normal, but not when above or below. Applied N increased straw yield curvilinearly, with little influence of tillage or soil depth. Straw/grain ratio averaged 1.53 and 1.70 on the 208‐ and 111‐cm soils, respectively. Nitrogen uptake tended to be greater in conventional than in stubble‐mulch tillage, and was not affected by soil depth. Grain protein exceeded the desirable level for white wheat only when applied‐N was above that needed for optimum yield. Precise selection of the amount of N to apply each year was difficult because of the strong influence of GSP on yield and the need to apply N before GSP is known.

Continued use of ammonium‐based (NH4‐N) fertilizer can lower soil pH to levels deleterious to crop growth. The depth and intensity of acidification is affected by rate and type of N fertilizer, and differs with tillage systems. Change in soil pH in relation to applied N was determined for one conventional (moldboard plow) and two stubble‐mulch (offset disk, subsurface sweep) tillage treatments in a winter wheat (Triticum aestivum L.)‐fallow rotation receiving cumulative N application of 493, 728, 986, 1221, 1714, and 2207 kg N ha−1 over 44 yr. Nitrogen was applied as (NH4)2SO4 from 1940 to 1961 and as NH4NO3 from 1962 to 1983. The soil was a Walla Walla silt loam (Typic Haploxeroll) containing 200 g clay and 11 g organic C kg−1 in the upper 30 cm. Soil pH (1:2 soil/0.01 M CaCl2) decreased linearly with increasing N application in both conventional and stubble‐mulch tillage. Acidifying effects were concentrated in the top 7 cm of stubble‐mulched soil but distributed to 22 cm or below with moldboard plowing. The rate of pH decline in the top 22 cm of soil was greater for moldboard plow than for disk or sweep tillage, 0.38 vs. 0.21 and 0.28 units, respectively, per Mg of applied N ha−1. Linear regression analysis predicted that 2.7, 4.8, and 3.5 Mg N ha−1 would acidify the 0‐ to 22‐cm soil depth to a pH deleterious to wheat growth with moldboard plow, offset disk, and sweep tillage, respectively. However, the more rapid decrease in pH in the 0‐ to 7‐cm zone with disk or sweep tillage may sooner predispose stubble‐mulch systems to negative effects of acid conditions within the seed zone. Nitrogen applied as NH4NO3 produced more acidity per unit of N than did (NH4)2SO4, but NH4NO3 was applied at much higher rates at a later date so unbiased comparison is difficult.

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The wheat line C.I. 7090 (Triticum aestivum L.) was shown by previous work to carry an unidentified gene(s) for resistance to many races of common [Tilletia caries (DC.) Tul. and T. foetida (Wallr) Liro], and dwarf bunt (T. controversa Kuhn). C.I. 7090 was crossed with completely susceptible cultivars (‘Triumph’ and ‘Elgin’) and with bunt‐host differentials. The F3 and F4 progenies of these crosses were analyzed to determine the identity and number of genes involved in resistance. The results showed that a new gene, designated Bt9 was involved and the Bt9 plus Bt7 conferred resistance in C.I. 7090 to 36 of the 41 reported races of common bunt. Inheritance for Bt9 is independent of that for Bt1, 2, 4, 6, and 7.