Rod origin of prolonged afterimages

Afterimages fade against any unchanging background but generally reappear if the background changes suddenly. Under some conditions, however, a change of background color fails to revive a faded afterimage. This happens only if the interchanged backgrounds equally stimulate the rod receptors. It follows that afterimages seen under these conditions are generated by rods.     

Diffusion-based soil water simulation for native grassland

A previously developed diffusion-based soil water simulation model (Hayhoe, 1981; Hayhoe and De Jong, 1982) was tested under native grassland conditions in the semiarid region of Saskatchewan, Canada. The model, which made extensive use of measured soil physical parameters and seasonal crop growth characteristics, was ‘driven’ by daily precipitation and potential evapotranspiration data. Model predictions of crop water use and soil water content in the root zone agreed well with measured values and a realistic separation of the components of actual evapotranspiration was obtained. By decreasing the lower limit of water availability to plant roots during dry periods a better correspondence between measured and simulated water use was obtained, thereby supporting previous work that the soil water content at 15 000 cm H₂O suction does not necessarily represent the permanent wilting point.     

Divisive field-scale associations between corn yields, management, and soil information

For corn crops on soils with fine texture in eastern Canada, research has been ongoing on the relationships between corn grain yields and management practice, soil strength/compaction, and soil nutrient status. However, these variables can operate in a complex manner that may not be detectable via uni-structural models. This study used regression-tree analysis, in an exploratory context, to model corn grain yields over a relatively flat clay to silty loam field on the basis of several soil physical, soil nitrate, and management variables. It was found that corn grain yields were first subdivided by length of time under corn production (1st and 3rd year corn groups). These yield groups were further subdivided by cone penetration resistance, elevation, soil nitrate, and soil texture information. Generally, higher corn grain yields were associated with 1st year corn, lower relative soil strength/compaction, and higher post-growing season soil nitrate levels. The modeled results helped to identify equifinality, context dependent relationships, and spatial continuity in inferred formative mechanisms; issues important in many field and/or landscape studies of spatial processes.     

Measurements of Mouldboard Plow Draft: I. Spectrum Analysis and Filtering

Measurements of mouldboard plow draft can potentially provide a useful tool for continuously mapping, as a surrogate variable, crop limiting soil properties such as soil strength/compaction. Numerous researchers have shown that loading on tillage tools can be periodic. One source of higher frequency periodic loading has been related to failure modes in the soil. Lower frequency periodic loading on tillage tools, assuming tillage depth is relatively constant, more often reflects local spatial variability in soil physical properties such as soil moisture, soil texture, organic matter, and/or soil strength/compaction. This study presents a spectrum and cross-spectrum analysis of mouldboard draft recorded during normal fall plowing operations along adjacent transects. One objective of this study was to determine the dominant periodic components in order to elucidate meaningful frequencies which might be better related to variation in soil physical properties. Spectral densities were generally dominated by signals with a few cycles per 300-m transect. The cross-amplitude and squared coherency between draft recorded on adjacent transects provided further indication of the frequencies which characterized the signals because they provide an indication of correlation as a function of frequency. Spatial dependence in the data was quantified using spatial autocorrelation and cross-correlation. An additional objective was to use the spectral information to help design more physically meaningful filters for the raw draft data. Taking the Fast Fourier Transform of the raw draft data and filtering in the frequency domain using an optimal filter informed via the spectrum and cross-spectrum analysis, provided a convenient method for isolating a more physically meaningful signal.     

Measurements of Mouldboard Plow Draft: II. Draft-Soil-Crop and Yield-Draft Associations

The primary objective of this investigation was to evaluate some potential causal associations between draft derived from a 3-bottom mouldboard plow during normal fall field operations and soil-crop properties at a field site near Winchester, Ontario, Canada. A secondary objective was to investigate, in a preliminary manner, associations between draft and crop yield (corn, soybean, and wheat). Regression tree analysis indicated that draft variability was best explained by field location, crop type, soil cone penetration resistance in the plow layer, and soil texture in the plow layer. Draft was found to generally increase with cone penetration resistance in the plow layer and soil clay content. Corn yields were negatively associated with draft. The reverse was true for the soybean yields, and a combination of negative and positive draft vs. yield relationships existed for the wheat plots. The overall results indicated that draft data collected during normal field operations can be useful for producers interested in identifying areas in the field where soil strength/compaction might be problematic with regard to crop yields.