B. D. Kay

Conservation tillage and depth stratification of porosity and soil organic matter

Abstract Pores and organic matter take a multitude of forms in soil and their characteristics change in space and time following a change in tillage practices as a new “steady state” is approached. Information on the variation with depth (stratification) in the characteristics of pores and organic matter and the rates of change in these characteristics is vital to interpreting the short- and long-term impacts of a reduction of tillage on the productivity and hydrology of agricultural soils. This information is also of value in estimating the effect of a reduction in tillage on the sequestration of carbon in agricultural soils. Recent literature comparing conventional tillage (CT) with no-till (NT) in temperate agroecosystems with varying soil textures and climates was reviewed for the purpose of assessing rates of change in the magnitude and stratigraphy of bulk density, porosity, pore size classes, organic matter content and organic matter fractions. The influence of tillage on bulk density, macroporosity and organic matter content was found to be documented more extensively than the effects on pore size distribution, soil organic matter fractions and their interactions at different soil depths. Many of the reports documenting tillage-induced changes in soil porosity and organic matter were based on measurements at a specific time after initiating the tillage trial. Results obtained by different investigators were found to be most consistent when measurements were made ≥15 years after initiating the tillage trial. Data from different studies were used to generalize trends in pore and organic matter characteristics with depth and time. However, few studies provided measurements that permitted accurate prediction of either the rates of change or the maximum change that will occur following a change from CT to NT. Future research must enhance our ability to make these predictions if we wish to garner a better understanding of the effects of NT on the quality and productivity of agricultural soils and their ability to sequester carbon.

Applications of fractals in soil and tillage research: a review

Abstract Fractals are spatial and temporal model systems generated using iterative algorithms with simple scaling rules. This paper reviews the literature on spatial fractals as it applies to soil and tillage research. Applications of fractals in this area can be grouped into three broad categories: (i) description of soil physical properties; (ii) modeling soil physical processes; (iii) quantification of soil spatial variability. In terms of physical properties, fractals have been used to describe bulk density, pore-size distribution, pore surface area, particle-size distribution, aggregate-size distribution, ped shape and soil microtopography. In terms of physical processes, fractals have been used to model adsorption, diffusion, transport of water and solutes, brittle fracture and fragmentation. In terms of spatial variability, fractals have been applied to quantify distributions of soil properties and processes using semivariograms, power spectra and multifractal spectra. Further research is needed to investigate the specificity of different fractal models, to collect data for testing these models, and to move from the current descriptive paradigm towards a more predictive one. Fractal theory offers the possibility of quantifying and integrating information on soil biological, chemical and physical phenomena measured at different spatial scales.

Management versus inherent soil properties effects on bulk density and relative compaction

Abstract Bulk density ( D b ) is a soil physical parameter used extensively to quantify soil compactness. The D b varies with management as well as with inherent soil properties. Because of its dependence on inherent soil properties, measurements of D b are of limited value as a measure of the effect of management on soil compaction when soils with different inherent characteristics are compared. Researchers have used the concept of relative compaction or relative bulk density, ( D b-rel ), the ratio of the D b of a soil to the D b under some standard compaction treatment ( D b-ref ), in order to compare the response of different soils to stresses arising from management. This research was conducted to determine D b , D b-ref and D b-rel across a range of soils under different tillage practices and to assess the relative importance of texture, organic matter content and management on these parameters. Thirty-six paired sampling sites were located along two parallel transects in a side by side comparison of no-tillage and conventional tillage treatments. The transects crossed three soil types: Aquic Hapludalfs, Psammentic Hapludalfs, and Typic Hapludalfs. Clay content (CLAY) varied from 5.8 to 42.3%, and organic matter (OM) varied from 1.4 to 11.6%. Multiple regression analyses showed that D b was related ( R 2 = 0.83) with CLAY, OM, tillage, position, and the interactions OM∗CLAY and tillage∗position, whereas the D b-ref was related ( R 2 = 0.86) with OM, CLAY, and tillage. Normalizing D b with respect to D b-ref , effectively eliminated the influence of CLAY, OM and their interaction on D b-rel . The analyses indicated that it is possible to quantify the separate effects of inherent soil properties and management on D b , when soils derived from similar parent materials and under similar climatic conditions are considered, either by using multiple regression analyses to describe D b , or by normalizing D b with respect to D b-ref .

SENSITIVITY OF SOIL STRUCTURE TO CHANGES IN ORGANIC CARBON CONTENT : PREDICTIONS USING PEDOTRANSFER FUNCTIONS

Pedotransfer functions (PTFs) were used to assess the sensitivity of the structural characteristics of coarse- and medium-textured calcareous illitic soils at different levels of relative compaction (RC) to changes in the organic carbon (OC) content. The analyses predicted that an increase in the OC content of 0.01 kg kg−1 would:• increase the available water content from 0.02 to 0.04 m3 m−3 with the largest increases occurring in coarser-textured soils and not being strongly influenced by RC;• decrease the air-filled porosity at field capacity from 0.01 to 0.04 m3 m−3 with the largest decreases occurring in the finer-textured soils and not being strongly influenced by RC;• decrease the soil resistance to penetration with the decreases most pronounced at lower water potentials and higher RC; at the permanent wilting point and a RC of 0.95 the decrease would range from 1.2 to 3.8 MPa;• increase the least limiting water range from 0.01 to 0.05 m3 m−3 with the increase varying with clay content.A comparison ...

Plant response to mechanical resistance and air-filled porosity of soils under conventional and no-tillage system

Roots may respond to restrictive soil physical conditions and send signals to shoots to control plant growth. Soil mechanical resistance and aeration can be managed to improve the soil physical conditions for plant growth by using different tillage systems. The objective of this study was to quantify the influence of no-tillage and conventional-tillage systems on plant response to soil mechanical resistance and aeration. The study was carried out on a farm, cultivated with corn, with a side-by-side comparison of no-tillage and conventional-tillage systems. Thirty-two paired sampling sites were located along two transects, located one in each treatment. Soil water content, bulk density, and plant growth were measured in each treatment. Based on the soil water and bulk density measurements, the air-filled porosity values were computed for each treatment. Soil water contents and bulk density values were converted to soil mechanical resistance by using the soil resistance curve. Plant growth varied positively with soil air-filled porosity, and negatively with soil mechanical resistance in both tillage systems. However, the decrease rates/increase rates were dependent on the tillage system. The no-tillage system somehow improved the soil physical conditions for the plants, especially when they were more restrictive, allowing them to attain greater values of growth.

Comparison of functions for characterizing the dry aggregate size distribution of tilled soil

Functions to characterize the dry aggregate size distribution (DASD) are needed for evaluating tillage implement performance. We compared the log-normal, fractal and Rosin-Rammler functions as descriptors of the DASD after tillage. These functions were fitted to data from flat and rotary sieve analyses. Energy input was kept constant. Comparisons were made in terms of physical implications for aggregate fragmentation, goodness of fit and parameter sensitivity to soil properties. All three functions resulted in a more accurate description of seed bed conditions than the use of individual size classes. However, the fractal and Rosin-Rammler functions were theoretically superior to the log-normal function, which was unable to accomodate scale dependent probabilities of failure during aggregate fragmentation. In terms of goodness of fit, the fractal function consistently gave the highest R2 values (mean > 0.999 across a wide range of DASDs). The fractal parameter D, showed the greatest sensitivity to inherent soil properties at a site with different soil types and a common cropping history. Clay content was the most important inherent property influencing D; cloddiness (denoted by decreasing D) increased with increasing clay content. Despite lower model R2 values, the Rosin-Rammler parameter, α, showed a greater sensitivity to transient soil properties than D at a site with similar soil types and different crop rotations. Wet-aggregate stability at time of plowing was the most important transient property influencing α; cloddiness (denoted by increasing α) increased with decreasing wet-aggregate stability. We suggest the fractal parameter, D, be used for studies comparing seed bed conditions across a range of soils, and the Rosin-Rammler α parameter be used for studies comparing management practices on a single soil.

Spatial variability of soil penetrometer measurements at the mesoscopic scale

Abstract Laboratory penetrometer measurements are reported for 10 undisturbed cores from two soil types: a Typic Eutrocrept and an Aeric Ochraqualf. Resistance to penetration was measured at numerous points within each core, which permitted an analysis of spatial variation at the mesoscopic scale (10 −3 –10 0 m). The objectives were to examine the extent of autocorrelation in the data, determine the optimum number of penetrations per sample, and identify physical properties influencing variability. Two different penetrometers were used: a constant-load needle penetrometer and a constant-rate micro-penetrometer. Data were expressed as a percentage of the maximum depth attainable with the needle penetrometer (PMP), and as the slope ( m ) and intercept ( n ) of linearized percent penetrability vs. tip pressure relationships for the micro-penetrometer. No correlation was found between the m and n parameters, suggesting they represent independent physical properties contributing to soil strength. The PMP and m measurements were positively skewed, whereas the n measurements were normally distributed. None of the parameters exhibited any spatial autocorrelation. The average fractal dimension for all three parameters was 2.89 ± 0.09, indicating Gaussian random variation. For a given bulk density, the mean PMP decreased exponentially with decreasing moisture content. Conversely, at constant water potential, mean values of m and n decreased linearly with increasing bulk density. Coefficients of variation (CV) were relatively uniform for the n parameter (11.5–20.1%), but depended upon soil moisture and bulk density in the case of PMP and m , respectively. The CV for PMP increased exponentially with decreasing soil moisture, while a curvilinear relationship was found between the variation in m and bulk density. This was modelled with a second order polynomial, which predicted a maximum CV of 36.1% at 1.14 Mg m −3 bulk density. The amount of variability encountered was greater than previously reported for laboratory penetrometer measurements. The fact that this variation was not independent for two of the three parameters investigated, has important implications for sampling at the mesoscopic scale. Before a study is undertaken, preliminary measurements should be made to determine the optimum number of samples for the parameter under consideration. Sampling requirements for soil penetrability appear to increase with decreasing moisture content and bulk density.

Unbiased estimation of the fractal dimension of soil aggregate size distributions

The fractal dimension has been used to characterize the size distribution of soil aggregates produced by tillage. The fractal dimension is normally estimated from the slope of the cumulative number-size distribution plotted on a log-log scale, D1. It can also be estimated non-linearly from the cumulative number-size distribution, Dn1. In both methods, the fitting is done over the entire range of scales for which data are available. However, fractal models indicate deviations from ideal behavior during the initial stages of fragmentation. Inclusion of data from the early stages of fragmentation may result in biased estimates of D1 and Dn1. We used the derivative of the cumulative number-size distribution plotted on a log-log scale after the fourth stage of fragmentation as an unbiased estimator of the fractal dimension, Dd. Values of D1 Dn1 and Dd were compared for soil aggregate size distributions resulting from the fragmentation of dry clods collected from seven cropping/ tillage treatments and crushed at five specific energy levels. Estimates of the fractal dimension ranged from 1.179 to 2.803. The range in D1 was much wider than the ranges in Dn1 or Dd. No significant relationship existed between D1 and Dd. The relationship between Dn1 and D1 had an R2 of only 0.183 (P<0.05), and was significantly different from 1:1. In contrast, a highly significant 1:1 relationship existed between Dn1 and Dd (R2 = 0.965, P<0.01). These relations indicate estimates of D1 are biased towards data from the early stages of fragmentation. Since both Dn1 and Dd provided unbiased estimates of the fractal dimension, either could be used in future studies. Values of Dn1 and Dd were used to predict scale-invariant probabilities of failure for the different cropping/tillage treatments. The calculations indicated minimally tilled clods are more likely to fail than conventionally tilled clods when crushed to the same energy level. This result may be explained by differences in clod shape observed between tillage treatments.

Soil tests for predicting the N requirement of corn

Several soil N tests were compared with the one currently used for predicting the N requirement for corn in Ontario. The current test involves a measurement of nitrate (NO3−, 0–30 cm) before N fertilizer sidedressing. The study was done to determine the efficacy of other tests for N fertilizer prediction. The tests chosen varied in the quantity of N “extracted” and included hot KCl-extractable NH4+, anaerobically released NH4+, extractable NH4+ following autoclaving in CaCl2 solution and total N of soil sampled to a depth of 30 cm. The 3-yr study was conducted on a sloped (simple) field site, which provided a wide range in soil organic matter (SOM) contents. A corn crop was grown each year following a barley crop with or without red clover cover crop residues incorporated in the spring and with or without N fertilization. Corn grain yields were obtained at the end of the growing season. Grain yields were lowest at the shoulder and backslope locations and highest at the footslope and toeslope locations. Co...

Soil dissolved organic carbon: Influences of water-filled pore space and red clover addition and relationships with microbial biomass carbon

Soil dissolved organic carbon (SDOC) plays an important role in organic C cycling and translocation of nutrients and pollutants in the soil profile. Soil microbial biomass C (MBC) has been used as an indicator of soil quality. Both SDOC and MBC may be affected by management practices and indigenous soil properties, which however are not fully understood. Using a laboratory incubation technique, we determined the effects of red clover (Trifolium pratense L.) addition and soil water saturation as expressed in water-filled pore space (WFPS, 20-95%) on soil SDOC and MBC in three soils from Ontario. The levels of SDOC were the greatest at 20% WFPS, and decreased with increase s in WFPS up to 95%. In comparison with the control, addition of red clover increased SDOC by up to 72% at 20% WFPS, but the effect was minimal or insignificant at WFPS above 50%. Reduction of SDOC with increases of WFPS both with and without red clover was attributed to the increased mineralization of labile organic C, as indicated by CO...