Soil structure is defined as “the spatial heterogeneity of the different components or properties of soil” Aspects of soil structure which are important for plant development, soil water balance and soil workability are reviewed briefly The different types of soil structure which occur on different size scales are placed in a hierarchical order Different mechanisms give rise to the different hierarchical orders Similarly, different physical/chemical/biological processes are involved in the stabilization of the different hierarchical orders

A number of methods for measuring soil structure are described Preference is given to methods involving direct observation of structural features by scanning electron microscopy and by optical scanning of impregnated sections and fracture surfaces These need to be supported by assessments of the stabilities of compound particles in water and of the mechanical strengths of compound particles as a function of water content

“Good” soil structure is described as one where all the hierarchical orders are well-developed and stable The greatest lack of knowledge appears to be in the 2–100 μm size range which is too large to have been studied by colloid chemists and too small to be visible to the naked eye It is suggested that more observations of soil structure should be made in this size range, as it may hold many important clues on how to manage soil structure in the field

Advances in characterization of soil structure

I Abbreviations and definitions Symbols of water status are standardized as follows: '1' ... total water potential; '1'" osmotic or solute potential; 'I' P' turgor or pressure potential. Throughout this text the term adaptation refers to heritable modifications in structures or functions that increase the probability of an organism surviving and reproducing in a particular environment (121); adaptations can be constitutive (always expressed) or stress-induced. When discussing stress metabolism, we have followed Hochachka (92) in making use of the term "design principle" in an engineering sense. In this sense, the design principles for a system are set by what that system is supposed to do under various circumstances; the design principles then determine what components must be included in the system, and how they must interact. An illustrative example: in automobile engineering. distinct design principles are involved for the two safety objectives of collision avoidance and injury mitigation (125a). Collision-avoid­ ance design principles are set by the aim of preventing accidents, and lead to the inclusion of system components such as dual-braking systems. Injury-mitigation design principles follow from the goal of protecting occupants in case of an accident; their application results in introduction of components such as energy-absorbing steering columns.

Metabolic Responses of Mesophytes to Plant Water Deficits

Publisher Summary This chapter focuses on the role of the soil structure (the soil–pore system) through which both water and roots move. The principle reason for concentrating on soil structure is that, of the soil's intrinsic physical properties, it is the one most easily, frequently, and widely altered, particularly by cultivation. The transmission porosity is responsible for rapid water movement and provides pre-existing channels for root growth. Disruption or lack of, transmission pores is common in cultivated soil. This leads to impeded water flow and root growth within the profile. Structural instability is frequently encountered at the surface of tilled soils. It gives rise to crusting and translocation of clay into the subsoil. Crusting can reduce the infiltration rate by up to two orders of magnitude. Organic matter accumulation at the soil surface, by conservation tillage practices, provides the most cost-effective remedy. Crop roots have been studied less than crop canopies, but it appears that considerable inter-and intra-specific variation exists in such properties as maximum rooting depth, lateral branch production, xylem vessel diameter, and root hair length. Mechanical impedance and transient waterlogging are structurally related causes of restriction to root development in arable soils. There is evidence that loosening of zones of high strength improves crop–water relations.

The influence of soil structure on water movement, crop root growth, and water uptake

Chapter 2 – coniferous forests and plantations

The importance of root function in water and nutrient transport is becoming increasingly clear, as constraints on agricultural resources are imposed due to water limitations and environmental concerns. Both are driven by the increasing need to expand global food production. However, the historical neglect of consideration of water and nutrient uptake processes below ground has created a knowledge gap concerning the plant responses of nutrient and water limitations to crop production. The review includes sections on (i) notation and definitions of water potential, (ii) the physical coupling of plant transpiration and plant assimilation by way of the principles of diffusion of water vapor and carbon dioxide, (iii) apoplastic and symplastic water and nutrient pathways in plants, (iv) active and passive nutrient uptake, and (v) a discussion of the current state-of-the-art in multidimensional soil water flow and chemical transport modeling. The subsequent review of water uptake, nutrient uptake, and simultaneous water and nutrient uptake addresses shortcomings of current theory and modeling concepts. The review concludes with an example illustrating a possible multidimensional approach for simultaneous water and nutrient uptake modeling. Specific recommendations identify the need for coupling water and nutrient transport and uptake, including salinity effects on root water uptake and the provision of simultaneous passive and active nutrient uptake. It considers the requirement for multidimensional dedicated root water and nutrient uptake experiments to validate and calibrate hypothesized coupled root uptake models.

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Current Capabilities and Future Needs of Root Water and Nutrient Uptake Modeling

Soil water potentials, leaf water potentials, and transpiration rates of sweet corn (Zea mays L.), growing in a greenhouse, and grain sorghum (Sorghum bicolor L.), growing in a field, were determined to evaluate the magnitude of the combined soil and plant resistances to water flow in the plant system. Using a theoretical analysis of water absorption by roots, soil resistance was estimated. Plant resistance was inferred by the difference between the measured combined resistances and the estimated soil resistance. A wide range of soil hydraulic conductivity values for the plant rooting media which included nutrient solutions, peat-vermiculite-sand mixture, and a sand and clay soil maintained at various water potentials provided variations in calculated soil resistances. Our results showed that when root density was not unusually low, plant resistance to water transport was much larger than soil resistance, until the threshold soil hydraulic conductivity reached about 10-6 to 10-7 cm/day. This conductivity usually occurred at about -1 and -8 bars for the sandy and clay soils, respectively. These findings emphasize the need to consider plant resistance in water-uptake calculations when using equations that evaluate water potential gradients along the water flow path.

D. C. Reicosky

Papers

Relative Importance of Soil Resistance and Plant Resistance in Root Water Absorption