The diffusion-sorption drying model has been developed as a physics-based way to model the decreasing drying rate at low moisture contents. This new model is founded on the existence of different classes of water: free and bound water. The transition between these classes and the corresponding thermodynamics form distinct components of the drying model. This paper shows that the characteristics of the different classes of water and of the transition between them can be deduced from the GAB sorption isotherm. The parameters in the GAB sorption isotherm support the theory of localised sorption, establishing the existence of different classes of water. Moreover, the sorption mechanism retrieved from the GAB parameters is in accordance with the sorption mechanism, which is obtained from the moisture dependence of the net isosteric heat of sorption. This holds for experimental sorption data of corn and starch as well as for literature data on five vegetables and four fortified cassava products. An extremum in the net isosteric heat of sorption coincides with the transition between bound and free water, and the partition moisture content corresponds with the monolayer value derived from the GAB equation. This confirms that the GAB monolayer value can be chosen as model boundary between bound and free water. Moreover, it reveals that this method can be developed into a technique to estimate the bound water content in foods.

Sorption isotherms of corn and starch cylinders with immobilised catalase are experimentally determined at different temperatures for use in drying models in optimal control studies. This application of the sorption isotherm requires an accurate prediction of the sorption data at different temperatures for the low water activity range. The GAB equation is used for the prediction of the sorption isotherms. Two major problems are encountered by employing standard procedures, ie prediction of sorption at a w < 0.11 and sensitivity of the GAB parameters to the applied data range. An improved methodology is developed, consisting of extending the standard experimental procedure with additional data points in the low water activity range and changing the criterion in the regression procedure in the sum of squares, which is weighed by the variance of the experimental data. The new methodology leads to accurate, consistent and physically relevant parameters of the GAB equation, which are independent of the applied data range in the regression analysis and which result in accurate predictions of the sorption behaviour at low water activity. The sorption data at different temperatures at low water activity can be predicted in the best way with parameters obtained after direct regression based on weighed SSQ.

Measurements on a seven-wire coaxial probe carried out with a cable tester in the time domain are compared with measurements carried out with a network analyzer in the frequency domain. Results are compared in the frequency domain and in the time domain. The frequency domain results of the time domain measurements are less smooth than the direct frequency domain measurements, but similar trends can be observed. The measurements carried out with the cable tester (Tektronix 1502B) clearly have a frequency content well above 3 GHz for measurements in air but with a very low signal-to-noise ratio for the higher frequencies. The useful frequency band for measurements carried out with a seven-wire probe depends on the dielectric properties of the material being measured. The higher the complex dielectric permittivity, the lower the useful frequency band. Methods are presented for calibrating seven-wire coaxial probes and measuring the frequency-dependent dielectric properties of soil samples using a combination of frequency and time domain analyses. The approach does not depend on a choice of frequency bandwidth. A Debye relaxation curve is capable of describing the frequency domain dielectric permittivity of sandy soils containing a soil solution with an electrical conductivity of approximately 0.4 S m -•. Results indicate an effective bandwidth of at least 0-1 GHz for sandy soils. IntroductionTime domain reflectometry (TDR) has obtained a reputation as a versatile and reliable technique for the measurement of soil water content and bulk electrical conductivity. This measurement technique quantifies the modification of an electromagnetic pulse by a soil owing to water and electrical conductivity. In order to obtain an improved understanding of the measurement technique and possibly extend the application of TDR to other topics Heimovaara [1994] presented an analysis technique which allows the dielectric permittivity as a function of frequency to be obtained from TDR waveforms. The frequency-dependent dielectric permittivity of soils was also measured by Campbell [1990] with a network analyzer directly in the frequency domain.Modern cable testers currently used in soil science are rugged, portable, battery-powered instruments and are therefore practical field instruments. Network analyzers, besides being more expensive, are not particularly suited for field use. However, network analyzers have superior frequency domain capabilities. This paper presents a comparison between measurements carried out with a Tektronix 1502B cable tester (Tektronix, Beaverton, Oregon) and a Hewlett Packard 8753A

Temporal fluctuations in soil structural stability within cropping treatments are often as large as differences between crops during the growing season. The relative importance of soil moisture, roots, and microbial biomass as factors contributing to this variation were investigated. Research was conducted on a fine‐silty, mixed, mesic Typic Eutrochrept intergrading to a fine‐silty, mixed, mesic Typic Haplaquept soil near Elora, ON. Six perennial forage treatments, established 2 yr previously, were compared with spring‐seeded conventional and zero‐till corn (Zea mays L.). The plow layer of each cropping treatment was sampled at monthly intervals. A combination of wet sieving and turbidimetry was used to provide a rapid assessment of wet aggregate stability (WAS) and dispersible clay (DC), respectively. On average, the forages had significantly less DC and greater WAS than the two corn treatments. Structural stability was found to decrease with increasing soil water content (r = 0.80 for DC and r = 0.74 for Was, both significant at P = 0.01). The corn soil experienced less extreme drying in the spring. Stepwise multiple‐regression analyses selected soil moisture and microbial biomass (in that order) as significant predictors of structural stability within the growing season. Using both parameters, it was possible to explain up to 85% of the temporal variation in DC and WAS on a by‐treatment basis.

The thermal conductivity of the thin seasonally freezing and thawing soil layer in permafrost landscapes exerts considerable control over the sensitivity of the permafrost to energy and mass exchanges at the surface. At the same time, the thermal conductivity is sensitive to the state of the soil, varying, for example, by up to two orders of magnitude with varying water contents. In situ measurement techniques perturb the soil thermally and are affected by changes in soil composition, for example through variations in thermal contact resistance between sensor and soil. The design of a sensor for measuring the temperature of the soil rather than the axial heating wire temperature has consequences for the modeling of heat flow. We introduce an approximation of heat flow from a heated cylinder with thermal contact resistance between the cylinder and the surrounding medium. This approximation is compared to the standard line source approximation, and both are applied to data measured over a one-year period in northern Alaska. Comparisons of thermal conductivity values determined numerically using the line source solution, line source approximation and the analytical form of the heated cylinder model fall within 10% of accepted values, except for measurements made in pure ice, for which all methods of calculation under-predicted the thermal conductivity.Field data collected from a complete freeze-thaw cycle in silty clay show a seasonally bimodal apparent thermal conductivity, with a sharp transition between frozen and thawed values during thaw, but a three-month transition period during freezing. The use of soil composition data to account for changes in heat flow due to the effect of latent 2 heat during phase change results in a relationship between soil thermal conductivity and temperature.

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Data on rates of change in soil structural stability under different cropping systems are scarce. Field measurements of changes in dispersible clay (DC) and wet aggregate stability (WAS) were made during 3 yr in six forage treatments and conventional and zero‐till corn (Zea mays L.). The study was conducted on a silt loam soil (fine‐silty, mixed, mesic Typic Haplaquept intergrading to fine‐silty, mixed, mesic Typic Eutrochrept) near Elora, ON, Canada. The DC and WAS changed exponentially with soil water content at time of sampling and were adjusted for this parameter before calculating changes with time. No increases in structural stability occurred during the 3‐yr time period within the corn treatments. In contrast, all of the forage treatments resulted in highly significant rates of structural improvement. A half‐life parameter was used to quantify these changes. Calculated half lives for the different forages were similar in magnitude, being approximately 2.7 and 5.4 yr for DC and WAS, respectively. Across the treatments, highly significant correlations were found between rates of structural stabilization and mean root length (r = −0.57 for WAS, significant at P = 0.01), mean root weight (r = −0.56 for DC and −0.65 for WAS, significant at P = 0.01) and the coefficient of variation for soil water content (r = −0.64 for DC and −0.56 for WAS, significant at P = 0.01). Forages may promote structural improvement through their influence on the amplitude of wetting and drying cycles. In addition, large increases in stability were observed over the intervening winter months, suggesting a freeze‐thaw effect.

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