Introduction
Soil organic matter (SOM) contains a large part of the soil's reserve of nutrients; it enhances stability in unstable soils, improves drought tolerance on light soils and improves workability in heavy soils.

Soil health depends on the presence of a varied and active population of micro-organisms.  These release nutrients from soil organic matter and chemical compounds.  In addition, the micro-organisms break down SOM and crop residues and during this process create gums that cement soil particles together, thus stabilising soil structures.

A decline in SOM will adversely affect soil structure and stability, water retaining properties, biological activity and the retention of crop nutrients.  It may also in the medium and long term, make the soil more vulnerable to erosion, compaction, acidification, nutrient deficiency and drought.

Over the last twenty to thirty years SOM has declined by a noticeable amount on many soils. The traditional intensive cultivations used for some crops have, over time, contributed to this decline by causing oxidation of soil organic matter.

Incorporation of crop residues by shallow cultivation rather than by deep ploughing will improve structural stability and, through microbial activity in this upper layer, encourage more nutrient cycling by releasing nutrients from the incorporated matter thereby releasing some nitrogen before planting.

Adding organic manures in compliance with NVZ rules is beneficial as they encourage soil organisms to release bonding agents which stabilise the soil and thus reduce the potential for erosion, capping and compaction.

Biology and fertility as it applies to different sectors...

Potatoes
A good level of soil organic matter improves structure, workability, moisture and nutrient holding capacity.  The intensive cultivations of potato seed bed preparation can contribute to gradual loss of organic matter.  Soils contain many living organisms ranging from microscopic bacteria and fungi to burrowing animals of which earthworms are the most obvious.  All play a part in maintaining the natural processes which are vital for maintaining the fertility of the soil.  If soil pH, aeration (including drainage where necessary) and particularly organic matter are managed carefully the biological activity can function effectively and largely take care of itself.


Horticulture
By following sound management practices, including the maintenance of organic matter levels, growers seek to maintain and enhance soil microbial activity by achieving a positive balance between water and oxygen availability. Well-prepared seedbeds improve drainage and soil aeration and thus crop development, and in horticulture many crops are grown on raised beds for this reason. Some growers seek to supplement and replace organic matter by carrying out on farm-composting or by buying in municipal compost or other organic manures. By carrying out in-field packing operations, crop residues can also be left in the field rather than being removed to the packhouse, which also enhances organic matter levels. Under intensive cropping systems where high yielding crops are produced, organic matter levels can reduce over time.

Cereals and Oilseeds
Soil health depends on the presence of a varied and active population of micro-organisms.  These both release nutrients from soil organic matter and chemical compounds but also as they break down soil OM and crop residues create gums which cement soil particles together and thus stabilise soil structures. 

Physical soil quality is largely governed by soil organic matter (SOM) content, which is dynamic and responds to changes in soil management and soil texture.  A decline in SOM will affect soil structure and stability, water retention properties, buffering capacity, biological activity and the retention and exchange of nutrients.  It may also in the medium and long term, make the soil more vulnerable to erosion, compaction, acidification, nutrient deficiency and drought.

Topic Sheet 67 : P nutrition of winter oilseed rape
Topic Sheet 66 : Diagnosing & correcting S deficiency in wheat & rape
Topic Sheet 44 : Non-NPK nutrient needs of cereals
Topic Sheet 19 : Diagnosing P & K requirements of winter wheat
P&K fertiliser planning guide
"Tried & tested" leaflets on nutrient management for farms, printed in 2005 (Available from HGCA)

Sugar
Over the last twenty to thirty years soil organic matter levels have declined on many soils and thus soil structural stability and moisture holding capacity has been compromised. The traditional intensive cultivations for root crops such as for sugar beet seed bed preparation has, over time, contributed to this decline by causing oxidation of soil organic matter which releases carbon dioxide. Incorporation of crop residues in the topmost soil layers rather than by deep ploughing will improve structural stability and, through microbial interactions in this most biologically active layer, encourage more nutrient cycling by releasing nutrients from the incorporated matter, thereby, supplementing N supply pre-planting. Adding organic manures, be they farm or otherwise derived, is always beneficial as they encourage soil organisms to create gums which stabilise the soil and thus reduce the potential for erosion, capping and compaction. This is common practice on 25-30% of beet fields.  Organic manures also supply nutrients, and care should be taken that applications do not supply more organic nitrogen than is allowed under NVZ rules where these apply, and that phosphate applications on soils already high in available phosphate do not exceed phosphate offtakes during the rotation.

BBRO
Beet growers guide, sugarbeet portal, Beet review magazine

MDC
Grass+ chapter 11

MLC
Leaflets on clover
Best practice information

Web
SMI www.SMI.org.uk  - Influence of Cultivation Practices on Arable Crop Diseases (2003)
Defra http://www.defra.gov.uk/environ/pollute/rb209/  Fertiliser recommendations for agricultural and horticultural crops (RB209), 2000 available online

Publications
SAC -  Fertilizer guides

Workshops/Practical
Environment Sensitive Farming
Elm Farm - Soil biodiversity talks and presentations (UK) + others
ADAS - Nutrient & manure planning farm visits/advisory reports
SOWAP Soil biodiversity open days (UK)
Courses
Harper Adams College, ADAS, Cranfield

IGER - Grassland Development Farms