Investigation into the fractal scaling of the structure and strength of soil aggregates

The hierarchical nature of soil structure is examined by measuring the physical properties of a range of aggregate sizes obtained using repetitive fracture. Fractals are used to assess the change with aggregate size of the specific volume, the proportion of pre-existing cracks which link to form the aggregate failure surface, and the aggregate failure stress. The pore size distribution, evaluated using mercury porosimetry and the application of the box counting algorithm to thin sections and thick sections, is also used to obtain a fractal dimension, D. Our results show that D depends upon the measurement approach for mass fractal scaling. This finding may limit the application of fractals to predict the scaling behaviour of soil physical properties.     

Diffusion from instantaneous sources of differing geometries

The effects of geometry upon diffusion from instantaneous sources is considered. Although surface area is important initially, it is the square of the volume of the source which determines flux from the source at large times. This asymptotic shape independence is demonstrated for spherical and cuboidal sources.     

The apparent antagonism between vitamin A and carotenoids in the fowl

These experiments show that the pigmentation-suppressing factor in fish-liver oils is vitamin A. Carotene fed at comparable levels does not exert a suppressing effect on pigmentation. Inhibition of pigmentation takes place when there is a sufficiently large bodily store of vitamin A; it is not exclusively an intestinal phenomenon.     

Strontium-90 content of deciduous human incisors

The concentrations of strontium-90 in deciduous incisor teeth of children born in St. Louis between 1949 to 1957 are in accord with estimated bone levels, suggesting that human deciduous teeth are useful as an index of strontium-90 accumulation during the time the teeth are formed.     

A Computer-based Extension Delivery System: Progress and Outlook1

The development of pest management programs in extension has placed severe strains upon traditional information delivery systems. The implementation of new monitoring procedures and management strategies has emphasized the need for systems which allow rapid data summarization, interpretation and dissemination to the agricultural community. Thus, new extension information delivery systems have been developed which incorporate modern, high-speed communications technology. Such a system is operational in Michigan. The computer-based aspects of Michigan's system are referred to as PMEX, or the Pest Management Executive System. The objectives, development and structural components of PMEX have been described in the literature. This paper shall only briefly review these aspects to provide a basis for understanding the significance of recent changes. Any useful information delivery system must be designed with flexibility so as to incorporate changes demanded of it. As Michigan's pest management programs evolve from their crude beginnings, they demand more from and put more emphasis on such systems. Three progressive changes in the system are discussed: 1) a new interactive-feedback program providing interpreted reports from specialists to field personnel based on monitoring input; 2) expansion of the environmental monitoring capabilities and 3) a generalized phenology modeling program or PETE and its implementation in an action pest management program in tree fruit production. Finally, the future uses of the PMEX and related systems shall be explored, especially with reference to a holistic view of farm management. Le developpement des programmes de protection integree demande une amelioration des systemes de diffusion d'informations. La mise en ceuvre des nouveaux systemes de recensement et des strategies de management requiert le recours aux procedes permettant une analyse et une interpretation rapides des donnees qui doivent ensuite atteindre I'exploitant dans les meilleurs delais. Profitant du progres de la technologie moderne des communications ultra-rapides, I'Etat du Michigan a passé a un systeme d'information tres perfectionne. Ce systeme, appele PMEX, donc le but et les structures ont ete decrits (Pest Management Executive System) repose sur I'emploi d'ordinateurs. Le present article a pour objet de revoir I'essentiel du procede afin de faciliter la comprehension de la signification des changements qui sont intervenus. Tout systeme de diffusion d'informations doit etre conyavec suffisamment de souplesse afin qu'il soit possible d'y incorporer les modifications necessaires. Avec le developpement des programmes de management au Michigan, le recours a la telematique s'est intensifie et il est question dans le present article de trois     

Water retention models for fractal soil structures

A review of water retention functions based upon fractal soil structures is presented. We consider the modelling approach for a fractal fabric, a fractal pore boundary and a fractal pore space, identifying the latter case as one of particular complexity. In each case, the water retention function is derived from the pore volume distribution arising from the structural model in question. We examine published models and highlight problems, namely lack of generality and inconsistency with the assumed fractal structure. The models considered in this paper do not account for the effects of pore connectivity, and as such their validity as a necessary condition for the existence of fractal structure is questionable.