Soil health (also called 'soil quality') is "the capacity of a soil to function, within ecosystem and  land use boundaries to sustain biological productivity, maintain environmental quality and promote plant and animal health" (Doran and Parkin, 1994). Soil health is an aggregate concept affected by chemical, physical and biological factors.

Chemical indicators.

Total organic carbon, total organic nitrogen, pH, mineral N, available P, K.

Physical indicators.

Soil texture, depth of soil and rooting, soil bulk density and infiltration, water holding capacity, water retention, water content, soil temperature.

 Biological indicators.

Microbial biomass C and N, potentially mineralizable N, soil respiration, biomass C/biomass ratio.

Fertile topsoil (Clarion)

Photo courtesy of NRCS

http://photogallery.nrcs.usda.gov

1. Crusts
2. Plow Layer
3. Subsoil

Crusts

Crusts are caused by the impact of raindrops on the soil surface. If the soil is unprotected and has poor structural stability, soil fines will disperse and fill large pores. Infiltration capacity is reduced quickly. When wet, this thin layer is called a seal, and when it dries up it becomes a crust. Crusts can significantly harm germination of young seedlings which have trouble breaking through the crust. Increased runoff due to crusts and seals is a threat to surface water quality.

Plow layer

A plow layer is created by repeated use of moldboard or disk plows at the same depth. The soil just below the tillage tool is compacted and becomes denser than the rest of the soil below or above it. Penetration resistance peaks at the depth of the plow pan, then decreases below it. Soils with high clay contents at tillage depth are especially sensitive to this type of compaction. One of the consequences is inhibition of root penetration through the plow pan. Roots can grow horizontal if they are unable to grow through the plow pan. Water will stagnate on top of the plow pan. This causes problems with trafficability, aeration, and compromises the effectiveness of artificial drainage systems. Increased runoff can ensue.

Subsoil

Subsoil compaction is caused primarily by heavy machinery traffic at times when the soil is moist (in the plastic state). Subsoil compaction is primarily related to axle load. It affects root penetration and water percolation.

1. Equipment traffic and load distribution
2. Timing of tillage and traffic as it relates to soil water conditions
3. Tillage methods

Equipment traffic and load distribution

Surface compaction is primarily caused by surface pressure, which can be the result of animal hoofs, tires inflated to high pressures, steel wheels, etc. The threat of this type of compaction is primarily within the top 12 inches of the soil. Subsoil compaction is primarily due to axle load. When axle load exceeds 10 tons the threat of subsoil compaction is great.  If the load can be distributed over a larger footprint area, surface compaction is reduced. If the load can be distributed over multiple axles, subsoil compaction is reduced.

Timing of tillage and traffic as it relates to soil water conditions

When soils are tilled or trafficked in their plastic state they are highly sensitive to compaction, while the soils are sensitive to rutting in their liquid state. In the friable state soils are less sensitive to compaction.

Tillage methods

The moldboard and disk plows are probably the two tillage tools that pose the greatest threat to soil compaction because they are the most likely to cause formation of a plow pan. In-furrow moldboard plowing is the worst because, in addition to the tillage tool causing compaction, the tires or animal hoofs cause direct subsoil compaction.

Repeated and frequent tillage at the same depth with a tool such as a moldboard plow or disk will cause compaction below the tillage tool. When this is repeated year after year, a tillage pan may form. High intensity traffic at times when the soil is above the plastic limit will cause compaction. Heavy equipment such as grain carts and manure spreaders can cause subsoil compaction.

1. Aeration
2. Aggregation/Structure
3. Soil Strength
4. Runoff and Erosion
5. Drainage

Aeration

Soil compaction reduces pore volume, especially of the larger pores. This reduces air-filled porosity and aeration. This affects root function, because most crop roots start malfunctioning below 10% air-filled porosity. A poorly aerated soil leads to greater denitrification losses. Iron oxides will be reduced to ferric iron (Fe²⁺), which is colorless and mobile. The soil will take on the color of the soil matrix (usually gray). Many soil organisms will suffocate and die in poorly aerated soils. The microbial community may go predominantly anaerobic.

Aggregation/structure

Soil structure will degrade upon compaction. Most damage is done when soils are trafficked or tilled in their liquid state, therefore when these soils dry out they tend to become hard. Roots will have problems penetrating this hard soil. Water will tend to stagnate, causing aeration problems.

Soil Strength

Soil strength increases in a compacted soil. Roots will have difficulty penetrating compacted soil.

Runoff and Erosion

 Runoff and erosion will increase as an effect of compaction because of the reduced pore volume and especially macro-pores.

Drainage

Drainage will be compromised because of the reduced pore size and subsequent water percolation rate in compacted soils.

1. Drainage
2. Texture

Drainage

A poorly drained soil will be in the liquid or plastic state for longer periods of time of the year and therefore will be more susceptible to compaction.

Texture

Soils with high clay content are highly sensitive to soil compaction. This is partly due to the  fact that they dry out slowly and this causes them to be in the liquid or plastic state for long periods of time, and partly because the clay platelets easily slide over each other causing them to pack more easily than sand or silt particles which are not plate-like.

Root growth is restricted, and shoot growth may be compromised. Seedlings will have difficulty emerging in compacted soils, while shoot growth will be reduced after emergence due to the effects of a reduced root system, nitrogen deficit due to denitrification,  and inhibited root function due to lack of aeration. Crop yields can be reduced 50% in very severely compacted soils, but usually compaction losses will be hard to perceive because they are in the 2-5% range. Effects of surface compaction on crop yields can last for many years, but can usually be remediated in 1 (sandy soils) to 5 years (clay soils). Subsoil compaction, on the other hand, can affect crop yields for more than 10 years.

Soil strength is highly correlated with water content. When the soil is dry, soil strength is high, and when it is wet, soil strength is low. The effects of soil compaction on soil strength are especially severe on root penetration in dry periods of the year (summer).

 The penetrometer can be a useful tool in soils that don't have many rocks. It needs to be used when the soil is at field capacity (thoroughly moistened to depth of penetration resistance measurement). The penetrometer needs to be pushed into the soil at a rate of about 1 inch/second.

Most penetrometers have indentations on the rod every 3 inches. Use this to determine at what depth the penetration resistance increases above 300 psi. Push the penetrometer in further, and record at what depth the penetration resistance decreases below 300 psi. If there is a mild plow pan, penetration resistance might not quite increase to 300 psi, but even above 200 psi root penetration is affected.

Use of a penetrometer

image source (left):

http://www.benmeadows.com/images/xl

/EIJKELKAMP-Hand-Penetrometer-BEN_i_bmw221001.jpg

image source (right): http://images.meredith.com/ag/images/buyersguide

/sidebyside/0509soilprobes/l_cover.jpg

1. Energy requirements
2. Pesticide use
3. Runoff and water quality

Energy requirements

Energy requirements of soil engaging tools such as tillage implements increase significantly due to compaction.

Pesticide use

Because crops are less vigorous, they are more likely to suffer from weed infestations, disease pressure, and insect attacks.

Runoff and water quality

Compaction increases runoff and compromises surface water quality.

Photo courtesy of NRCS

http://photogallery.nrcs.usda.gov

• Prevent compaction by avoiding traffic in the field or tillage when the soil is wetter than the plastic limit.
• Reduce tire pressure to increase tire footprint and to avoid surface compaction.
• Use tracks to increase footprint and avoid surface compaction.
• Use doubles on the tractor and reduce tire pressure in them to avoid surface compaction.
• Reduce axle load to avoid subsoil compaction (don't exceed 10 tons/axle)
• Reduce use of moldboard and disk plows.
• Don't run tractor wheel in furrow.
• Increase organic matter content to make soil resist compaction more.
• Increase root occupation in times when soil is moist (winter period)
• Reduce the field portion impacted by tires by increasing swath width.
• Avoid trafficking the whole field by respecting field roads and traffic lanes.
• Try precision traffic, driving in same traffic lanes always.

Planting using precision farming techniques to reduce field traffic

Photo courtesy of NRCS

http://photogallery.nrcs.usda.gov

1. Deep tillage
2. Organic matter additions and cover crops
3. Reduced tillage

Deep tillage (subsoil compaction)

Deep tillage can help to remediate subsoil compaction and can break through plow pans. The effects of deep tillage are usually short lived unless management adopts strategies to avoid further compaction, and to make soil resist compaction better.

Organic matter additions and cover crops (plow layer compaction; subsoil compaction when using deep-rooted cover crops)

Organic matter can make the soil resist compaction better, whereas growing cover crops in the fall and winter when the soil is moist can help create pathways for the roots of summer crops to follow. The roots also make pores that improve infiltration and aeration. 

Reduced tillage (plow layer compaction)

Shallow tillage in the top 0-12 inches can help to remediate rutted soil and to address surface compaction. The effects of compaction are not typically completely eliminated and follow-up needs to make sure compaction is not caused again.

No-till planting

Photo courtesy of NRCS

http://photogallery.nrcs.usda.gov

• Soil health is measured chemically (nutrient content), physically (texture, depth, water content, etc), and biologically (microbial population and biomass).
• Soil compaction is a common problem in high-traffic fields, and can be somewhat remedied by tillage.  Tillage has its own drawbacks, however.  Many factors must be considered when planning to implement a tillage system.