Introduction
Crops need soil, firstly, for anchorage, then to supply nutrients, water and air.  Whatever the crop, its nutrients are mainly supplied from the top 20 to 30 cm of soil.  However, roots need to go much deeper to find sufficient water, typically around 1 metre deep.  At the same time, a good supply of air must be maintained around the roots. 

To allow roots to easily take up nutrients and achieve high yields the topsoil needs to have an open soil structure like a sponge.  The aggregates, which are the building blocks of soil, need to have many pores and be soft enough for roots to penetrate or easily pass round in order to collect nutrients.  Having said that, the soil also needs to be firm to give good contact with roots - but not compacted. 

For roots to travel to depth there must be an open network of cracks travelling vertically down from the soil surface and well into the subsoil.  Again, the subsoil needs to contain many pores and fine fissures.  It is important that these cracks are vertical so that roots can go as deep as possible and thoroughly explore the subsoil in search of water.  Air also penetrates through these fissures so that the roots are not damaged by waterlogging.  If horizontal cracks are present in soil or the soil is densely packed together, these are signs of poor structure which has been caused by compaction.    

The only sure way to check the soil's structure is to use a spade.  Digging to around 40 cm depth is sufficient for a good indication.  Check for desirable vertical fissures and "sponginess".  Also check that roots are travelling vertically and not horizontally.  In spring it is helpful to check that there are no wet layers, particularly at the junction of topsoil with subsoil.

Maintaining an open soil structure to depth is also important for minimising soil loss from fields into water because it allows the water to penetrate the soil surface and move downwards rather than running across the surface.

BBRO
Beet growers guide, sugarbeet portal, Beet review magazine

HGCA
Soil and crop management issues insert in Crops sent to 25000 in Sept 2004. 
Soil2crop initiative 2003/2004 1500 growers attended.  Funded by HGCA and BPC, led by Vic Jordan of SMI.  Included consideration of effects of compaction, how to avoid and sub soiling as the solution
Topic sheet 65:  Reducing costs of establishing and managing wheat.  Winter 2002/2003.  2 page document reporting findings of research by arable research centre on direct drilling, min tillage and ploughing.
Summary of soil2crop.

MDC
Grass+ chapter 10

SMI - SMI.org.uk - publications
Environment Sensitive Farming - Environmentsensitivefarming.co.uk
FWAG - www.fwag.org.uk
SAC - www.sac.ac.uk
Environment Agency - www.publications.environment-agency.gov.uk/epage - Best Farming Practice (2003)
NSRI www.silsoe.cranfield.ac.uk/nsri/pdfs/structure_brochure.pdf - A guide to better soil structure 2001
Macaulay Land Institute www.macaulay.ac.uk/aweg/soilphysicaldamage.pdf - Fact sheet on soil physical damage

Publications not available on-line
SMI - Guide to crop establishment
Improved soil management for agronomic and environmental gain by SMI 2002
Visual soil assessment booklet (free from Vaaderstadt)
Contributions and interactions of cultivations and rotations to soil quality, protection and profitable production (2004)
NSRI - Free booklet on managing soil structure, reports soil structural assessments, soil types, their cropping and management
SMI/ADAS Video 'Changing Cultivation Practices'

Workshops
SMI
FWAG
TAG
NRSR
SAC
Elm Farm
Courses - Harper Adams College
Cranfield
TAG

IGER - Grassland Development Farms