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

The Holocene sedimentary record in southeastern Australia is present in a range of landscape settings, such as upland swamps (dells), internally drained lake basins, alluvial fans, and mid-catchmen...

Mind the gap: an absence of valley-fill deposits identifying the Holocene hypsithermal period of enhanced flow regime in southeastern Australia:

Fine-grained micas are present in nearly all soils. Frequently, they are the most abundant component of the clay fraction. In many soils, micas are the principal source of native potassium for plants. On weathering, micas are altered to vermiculite and sometimes to smectitelike minerals and, as such, significantly increase the cation exchange capacity and affect the relative selectivity by soils for various exchangeable cations.

Fine-grained Micas in Soils

The Bellinger River catchment in the New England Fold Belt on the mid-north coast of New South Wales is characterized by an assemblage of stepped late Quaternary alluvial units. Late Pleistocene terraces were formed by large, more competent rivers that eroded almost entire valley floors; however, a decline in discharge prior to the Holocene has resulted in the abandonment of these deposits as elevated terraces or residual alluvium, onlapped by contemporary floodplains. Intrinsic controls on floodplain formation appear to be superimposed over an early–mid-Holocene climatic signature. A fluvially active period, known as the Nambucca Phase, from 10 to 4·5 ka, eroded Late Pleistocene terraces. Two floodplain surfaces, one higher than the other, both started to accrete vertically from 4 ka but with some valley locations remaining vulnerable to episodes of erosion, resulting in substantial units of even younger basal alluvium. The high floodplain is dominated by horizontally laminated, vertically accreted sequences, while the low floodplain, which overlaps in age, is characterized by pronounced cut-and-fill stratigraphy. Terraces and floodplains in partly confined settings can have similar elevations but be polycyclic, with very different basal ages. In such landscapes the classical assumption that individual terrace or floodplain profiles along a valley represent periods of coeval formation is shown to be frequently invalid. Copyright © 2007 John Wiley & Sons, Ltd.

Topographically associated but chronologically disjunct late Quaternary floodplains and terraces in a partly confined valley, south-eastern Australia

Summary Soil stratigraphy concerns the chronological ordering of pedological episodes. It therefore has important implications in the reconstruction of paleoenvironments and the interpretation of stratigraphic sequences where soils and weathering zones occur. Because of the lateral variation in soil mantles, the principles of soil stratigraphy involve some different considerations from those of rock stratigraphy. Researchers in North America, Europe, and Australia, conditioned by different landscapes and soils, have studied soil mantles using different approaches. Soil-stratigraphic units such as geosols, pedoderms, soil facies, and paleocatenas emphasize the geographic extent of phases of soil formation taking into account local variations within soil mantles. Understanding of episodic development in the pedosphere is tied to concepts of separate identity, lateral continuity, ascendancy and descendancy, and pedogenic persistence. Providing a basis for soil-stratigraphic studies, these concepts are used in the stratigraphic ordering of soils after proofs of existence have been established for independent (buried) soil layers. Correlation of soil-stratigraphic units with 8OO stages in the oceanic record refines incomplete sequences recognized on land.

Stratigraphic principles and practices as related to soil mantles

Five soil profiles on alluvial terraces in the Macleay River Valley, N.S.W., have been studied to determine changes in soil development with increasing age. The criteria used were mainly macromorphology, micromorphology (basic fabric, evidence of weathering of lithorelicts and biotite, and depth functions of illuviated clay), depth functions of grain-size distributions, and clay mineralogy. The evidence suggests that the four youngest soil profiles represent successive stages of profile development by weathering for the most part and, to a lesser extent, formation of an horizon of illuviated clay; data for the oldest soil indicate a different history of profile development.

R Brewer

Papers

Weathering and soil development on a sequence of river terraces