Stewart B. Wuest

Crop residue position and interference with wheat seedling development

Unweathered crop residues can produce growth-inhibiting substances, stimulate pathogen growth, and immobilize nutrients. The location of seed relative to residue may be an important factor in the early health of a crop. This greenhouse study simulated sowing conditions possible under annual dryland winter wheat (Triticum aestivum L.) production to evaluate the likelihood of inhibitory effects. We placed newly harvested, unweathered winter wheat residue on the soil surface, mixed with the seed, immediately above the seed, or 3 cm below the seed. Treatments using a plastic residue substitute and treatments using pasteurized soil and residue provided comparisons to the natural soil and wheat residue. Residue mixed with or placed above the seed caused a temporary delay in emergence. Since this occurred with both wheat and plastic residue, the delay is explained by the physical impedance of coleoptile growth. Wheat residue 3 cm below the seed reduced the height and rate of wheat plant development, indicating a biological inhibitory effect of the wheat residue. This reduction in height and development rate at 20 days after planting did not occur when the soil and residue were pasteurized. We conclude that winter wheat seedling growth can be inhibited if roots encounter unweathered residues.

Vapour is the principal source of water imbibed by seeds in unsaturated soils

The assumption that seeds imbibe most of the water required for germination as liquid through seed-to-soil contact has been a dominant theme in germination research and seeding technology. Under most conditions, seeds are also exposed to water vapour during imbibition, but the relative contributions of liquid and vapour are difficult to assess. In water uptake models that include vapour, procedures used to estimate potential vapour imbibition have underappreciated the effect of distance on diffusion rate. At the same time, the amount of seed-to-soil contact and the liquid bridge from soil water to the seed tend to be greatly overestimated, considering the soil water contents often found in the field. Most researchers have recorded an approximately equal time to germination at soil water contents ranging from field capacity to nearly permanent wilting point, and little response to bulk density, soil type or seed‐soil contact. While hydraulic conductivity decreases by several orders of magnitude as soil water content, bulk density and seed-contact decrease, relative humidity remains near 100%. There are several experiments demonstrating timely germination in water vapour alone. The combined evidence contradicts the assumption that seed‐soil contact is important for imbibition of water by seeds. Water vapour should be considered the primary source of water for seeds in unsaturated soils.

Earthworm, infiltration, and tillage relationships in a dryland pea–wheat rotation

Dryland farming in the Mediterranean climate of the Pacific Northwest, USA supports extremely low earthworm populations under conventional tillage. Increases in earthworm populations are being observed in fields under no-till cropping systems. A3 0+ year experiment with four tillage levels in a pea (Pisum sativum L.)-winter wheat (Triticum aestivum L.) rotation was evaluated for earthworm populations and ponded infiltration rates. Where tillage has been limited to 2.5 cm depth, Apporectodea trapezoides (Duges) mean population was 25 m −2 . Plots subject to tillage by plow (25 cm depth) or chisel (35 cm depth) averaged less than 4 earthworms m −2 . The shallow tillage treatment also had the highest average infiltration rate of 70 mm h −1 compared to 36 for chisel, 27 for spring plow, and 19 mm h −1 for fall plow treatments. The highly variable nature of earthworm counts and infiltration measurements prevented conclusive correlation between the two, but increases in both can be attributed to minimum tillage. Published by Elsevier Science B.V.

Rapid Quantification of Soilborne Pathogen Communities in Wheat-Based Long-Term Field Experiments

Rainfed experiments operated continuously for up to 84 years in semiarid eastern Oregon are among the oldest agronomic trials in North America. Disease incidence and severity had been quantified visually but quantification of inoculum density had not been attempted. Natural inoculum of 17 fungal and nematode pathogens were quantified for each of 2 years on eight trials using DNA extracts from soil. Crop type, tillage, rotation, soil fertility, year, and their interactions had large effects on the pathogens. Fusarium culmorum and Pratylenchus thornei were more dominant than F. pseudograminearum and P. neglectus where spring crops were grown, and the opposite species dominances occurred where winter wheat was the only crop. Bipolaris sorokiniana and Phoma pinodella were restricted to the presence of spring cereals and pulse crops, respectively. Helgardia spp. occurred in winter wheat-fallow rotations but not in annual winter wheat. Gaeumannomyces graminis var. tritici was more prevalent in cultivated than noncultivated soils and the opposite generally occurred for Rhizoctonia solani AG-8. Densities of Pythium spp. clade F were high but were also influenced by treatments. Significant treatment effects and interactions were more prevalent in two long-standing (>50-year) annually cropped experiments (29%) than two long-standing 2-year wheat-fallow rotations (14%). Associations among pathogens occurred mostly in an 84-year-old annual cereals experiment. This survey provided guidance for research on dynamics of root-infecting pathogens of rainfed field crops and identified two pathogens (Drechslera tritici-repentis and P. pinodella) not previously identified at the location.

One-pass Tillage for Summer Fallow Under Arid Summer Conditions

Millions of acres of cropland with as low as 6 inches annual precipitation are used for production of winter wheat (Triticum aestivum L.) in the Pacific Northwest of the USA. Despite soil conservation advances, soil erosion continues to be a problem. This on-farm study analyzed seed-zone soil water under farmerimplemented fallow tillage practices to find out if very low-disturbance systems are possible. A low-disturbance, wide-blade undercutter sweep treatment was similar or superior to the farmer’s more intensive conventional tillage system. A subsequent test at four paired no-till-conventionally-tilled summer-fallow sites demonstrated that a single pass of an undercutter sweep in the no-till field could preserve seed-zone moisture comparable to the more intensive multiple-pass conventional tillage. Despite conventional wisdom, summer-fallow soil mulches do not need to be finely pulverized or repeatedly tilled to be effective.