Editing Soil structure (section)

== Overview ==
Soil structure describes the arrangement of the solid parts of the soil and of the pore spaces located between them (Marshall & Holmes, 1979).<ref name="Dexter1988">{{cite journal |last1=Dexter|first1=A.R.|title=Advances in characterization of soil structure |journal=Soil and Tillage Research|date=June 1988 |volume=11|issue=3–4|pages=199–238|doi=10.1016/0167-1987(88)90002-5|bibcode=1988STilR..11..199D }}</ref> Aggregation is the result of the interaction of soil particles through rearrangement, flocculation and cementation. It is enhanced by:<ref name="Dexter1988"/><ref>{{cite journal|last1=Masoom |first1=Hussain|last2=Courtier-Murias|first2=Denis |last3=Farooq |first3=Hashim|last4=Soong|first4=Ronald|last5=Kelleher|first5=Brian P.|last6=Zhang |first6=Chao|last7=Maas |first7=Werner E. |last8=Fey|first8=Michael|last9=Kumar|first9=Rajeev |last10=Monette|first10=Martine|last11=Stronks|first11=Henry J. |last12=Simpson |first12=Myrna J.|author-link12=Myrna Simpson|last13=Simpson|first13=André J.|title=Soil Organic Matter in Its Native State: Unravelling the Most Complex Biomaterial on Earth|journal=Environmental Science & Technology|date=16 February 2016 |volume=50 |issue=4 |pages=1670–1680 |doi=10.1021/acs.est.5b03410|pmid=26783947|bibcode=2016EnST...50.1670M}}</ref> the precipitation of oxides, hydroxides, carbonates and silicates; the products of biological activity (such as [[biofilms]], [[Hypha|fungal hyphae]] and [[glycoproteins]]); ionic bridging between negatively charged particles (both clay minerals and organic compounds) by multivalent cations; and interactions between organic compounds ([[hydrogen bonding]] and [[hydrophobic]] bonding).

The quality of soil structure will decline under most forms of [[Tillage|cultivation]]—the associated mechanical mixing of the soil compacts and shears aggregates and fills pore spaces; it also exposes organic matter to a greater rate of decay and [[oxidation]].<ref>Young, A &amp; Young R 2001, ''Soils in the Australian landscape'', Oxford University Press, Melbourne. {{page needed|date=January 2018}}</ref> A further consequence of continued cultivation and traffic is the development of [[soil compaction|compacted]], impermeable layers or 'pans' within the profile.

The decline of soil structure under [[irrigation]] is usually related to the breakdown of aggregates and dispersion of [[clay]] material as a result of rapid wetting. This is particularly so if soils are [[sodic soils|sodic]]; that is, having a high exchangeable sodium percentage (ESP) of the [[cation]]s attached to the clays. High sodium levels (compared to high [[calcium]] levels) cause particles to repel one another when wet, and the associated aggregates to disaggregate and disperse. The ESP will increase if irrigation causes salty water (even of low concentration) to gain access to the soil.

A wide range of practices are undertaken to preserve and improve soil structure. For example, the NSW Department of Land and Water Conservation advocates: increasing organic content by incorporating pasture phases into [[crop rotation|cropping rotations]]; reducing or eliminating [[tillage]] and cultivation in cropping and pasture activities; avoiding soil disturbance during periods of excessive dry or wet when soils may accordingly tend to shatter or smear; and ensuring sufficient ground cover to protect the soil from raindrop impact. In irrigated agriculture, it may be recommended to: apply gypsum ([[calcium sulfate]]) to displace sodium cations with calcium and so reduce ESP or sodicity, avoid rapid wetting, and avoid disturbing soils when too wet or dry.<ref name=NSW1991/>