Soil Tilth

Gardening in Colorado can be a challenge due to poor soil tilth.  Sandy soils hold little water and nutrients.  While some Colorado soils are rocky and shallow.

Along Colorado’s Front Range, many soils are clayey and compact readily.  These soils may have poor drainage, which may lead to salt problems.  Due to low soil oxygen levels root systems are typically shallow reducing the crop’s tolerance to drought and hot windy weather.

Special attention to soil management is the primary key to gardening success.  While gardeners often focus their attention on insect and disease problems, 80% of all plant problems begin with soil conditions reducing the plant’s vigor.    

The “text book” soil is composed of 45% mineral solids, 25% air, 25% water, and 5% organic matter.  Colorado soils are naturally very low in organic matter
(less than 1%) due to low precipitation that limits plant growth and decomposition of plant tissues.  [Figure 1]

Figure 1.  A textbook soil is 25% air, 25% water, 45% mineral solids and 5% organic matter.

The term soil tilth refers to the soil’s general suitability to support plant growth or more specifically to support root growth.  Tilth is technically defined as “the physical condition of soil as related to its ease of tillage, fitness of seedbed, and impedance to seedling emergence and root penetration.” 

A soil with good tilth has large pore spaces for adequate air infiltration and water movement.  (Roots only growth where the soil tilth allows for adequate soil oxygen levels.)  It also holds a reasonable supply of water and nutrients.

Soil tilth is a factor of soil texture, structure, fertility, and the interplay with organic content and the living soil organisms that help make-up the soil ecosystem.  For additional information refer to the CMG GardenNotes #212, The Living Soil.  [Figure 2]

These properties also influence how to manage the soil for successful gardening.  Many gardeners give attention to the soil’s nutrient content by applying fertilizers.  However, fertilization is only one of the keys to a productive garden

Urban Soils

Soils in urban settings differ greatly from field and natural soils in the following characteristics:

• Soil compaction is common place
• Less organic matter
• Greater variability due to construction activities moving and mixing the soil
• Frequent problems with surface crusting
• Higher pH
• Frequent drainage problems due to construction compaction and extensive hard surfaces (driveways, streets, parking lots, buildings)
• Less soil microbial activity
• Warmer soil temperatures
• Faster organic decomposition
• Longer root growth into fall
• Drying
• Waste materials (asphalt, concrete, masonry, construction debris)

Texture

Texture refers to the size of the particles that make up the soil.  The terms sand, silt, and clay refer to relative sizes of the individual soil particles.  [Table 1]

Table 1.  Size of Sand, Silt, and Clay

Name	Particle Diameter
Very coarse sand	2.0 to 1.0 millimeters
Coarse sand	1.0 to 0.5 millimeters
Medium sand	0.5 to .25 millimeters
Fine sand	0.25 to 0.10 millimeters
Very fine sand	0.10 to 0.05 millimeters
Silt	0.05 to 0.002 millimeters
Clay	below 0.002 millimeters

A fine-textured or clayey soil is one dominated by tiny clay particles.  A coarse-textured or sandy soil is one comprised primarily of medium to large size sand particles.  The term loam refers to a soil with a combination of sand, silt, and clay sized particles.  For details on how to estimate the texture of a soil refer to CMG GardenNotes #214, Estimating Soil Texture.

Clay – Clay particles are so tiny it takes 12,000 in a line to make one inch.  Clay feels sticky to the touch.  As soil with as little as 20% clay size particles behaves a sticky clayey soil, with restricted water and air movement, good water and nutrient holding capacity and being rather prone to compaction issues.

Some types of clayey soils expand and contract with changes in soil moisture.  These expansive soils create special issues around construction and landscaping.  For homes on expansive clays, limit landscaping along the foundation to non-irrigated mulch areas and xeric plants that require little supplemental irrigation.  Avoid planting trees next to the foundation and direct drainage from the roof away from the foundation.  Refer to CSU Extension fact sheet #7.236, Landscaping on Expansive Soils.

Silt – Silt has a smooth or floury texture.  Silt settles out in slow moving water and is common on the bottom of an irrigation cannel or lakeshore.  Silt adds little to the characteristics of a soil.

Sand – Sand, being the larger size of particles, feels gritty.  When it comes to sands there is a major difference in soil characteristics between fine sands and medium to coarse sands.  Fine sands add little to the soil characteristic and do not significantly increase large pore space.  An example of fine sand is the bagged sand sold for children’s sandboxes.

For a soil to take on the characteristics of a sandy soil it needs greater than 50-60 percent medium to coarse size sand particles.  Sandy soils have good drainage but low water and nutrient holding capacity.

Texture directly affects plant growth and soil management as described in the table below.  A soil with as little as 20% clay may behave as a fine-textured, clayey soil.  A soil needs at least 50% to 60% medium to coarse size sand to behave as a sandy soil.  [Table 2]

Table 2.  Comparison of fine-textured (clayey) soil and coarse-textured (sandy) soil

	Fine-textured Clayey	Coarse-textured Sandy
Water holding capacity	high	low
Nutrient holding capacity	high	low
Drainage	slow	fast
Warming in spring	slow	fast
Crusts and packs, restricting root growth	yes	no

Structure

Structure refers to how the various particles of sand, silt and clay fit together, creating pore spaces of various sizes.  Sand, silt, and clay particles are “glued” together by chemical and biological processes creating aggregates (clusters of particles).  Mycorrhizae, earthworms, soil microorganisms and plant roots are responsible for driving this aggregation.  [Figures 3 & 4]

Figure 3.  The size of pore spaces between soil particles plays a key role in plant growth.  Pore spaces are a factor of soil texture and structure. 

 

The term peds describes the soil’s individual aggregates or dirt clods.  Soils that create strong peds tolerate working and still maintain good structure.  In some soils, the peds are extremely strong making cultivation difficult expect when the soil moisture is precisely right.  Soils with soft peds may be easy to cultivate, but may readily pulverize destroying the soil’s natural structure. 

To maintain good structure avoid over working the soil.  Acceptable ped size depends on the gardening activity.  For planting vegetable or flower seeds, large peds interfere with seeding.  In contrast, when planting trees peds up to the size of a fist are acceptable and pulverizing the soil would be undesirable. 

Primary factors influencing structure include the following:

• Texture
• Activity of soil mycorrhizae, earthworms and other soil organisms – For additional details refer to the CMG GardenNotes #211, The Living Soil, and #212, Earthworms.
• Organic matter content – For additional information refer the to the CMG GardenNotes #241, Soil Amendments and #244, Cover Crops and Green Manure Crops.
• Soil moisture (year round)
• The freezing/thawing cycle
• Cultivation – Obviously tilling a soil has a direct impact on structure breaking apart aggregates and collapsing pore spaces.  Avoid tilling except to mix in organic matter, control weeds (limited use), or to prepare a seedbed.
• Soil compaction – For additional information refer to CMG GardenNotes #215, Soil Compaction.

Pore Space

Pore space is a factor of soil texture, structure and the activity of beneficial soil organisms.  Water coats the solid particles and fills the smaller pore spaces.  Air fills the larger pore spaces.

To help understand pore space, visualize a bottle of golf balls and a bottle of table salt.  The pore space between golf balls is large compared to the pore space between the salt grains.

The relative percent of clay size particles versus the percent of medium to coarse sand size particles influence the pore space of a soil.  Silt and fine sand size particles contribute little pore space attributes.  Note in Figure 5 how large pore space is non-existent to minimal until the sand strongly dominates the soil profile.  Organic matter also plays a key role in creating large pore space.

The quantities of large and small pore spaces directly affect plant growth. On fine-texture, clayey and/or compacted soils, a lack of large pore spaces restricts water and air infiltration and movement thus limiting root growth and the activity of beneficial soil organisms.  On sandy soils, the lack of small pore space limits the soil’s ability to hold water and nutrients.

Soil Profile

The soil profile describes how the soil changes in texture, structure (including pore space), and organic content as it moves down through the soil layers.  Changes in soil structure and texture are readily observed in road cuts or construction excavations.  Sometimes the change may be dramatic; other times minimal.  [Figure 6]

 

 

 

 

Figure 6.  Down through the soil profile, dramatic changes in soil texture and structure impact water movement and root growth.

Water Movement

Soil water coats the mineral and organic particles and is held by the property of cohesion (the chemical process by which water molecules stick together) in the small pore spaces.  Air fills the large pore spaces.

Water movement is directly related to pore space.  In small pore space of clayey soils, water slowly moves in all directions by capillary action.  The lack of large pore space leads to drainage problems and low soil oxygen levels.  On sandy soils, with the large pore space, water readily drains downwards by gravitational pull.  Excessive irrigation and/or precipitation can leach water-soluble nutrients, like nitrogen, out of the root zone and into ground water.  [Figure 7]

Figure 7.  Comparative movement of water in sandy and clayey soils

Texture Interface

Down through the soil profile, wherever there is an abrupt change in texture (actually pore space), known as a soil texture interface, it creates a line that affects the movement of water, air infiltration, and root growth.  Water and air is very slow to cross a texture interface.

When water moving down through a sandy soil layer (primarily large pore space) hits a clayey and/or compacted soil layer (primarily small pore space) water accumulates in the soil just above the interface.  This back up is due to the slow rate that water can move into the small pore space of the clay.  It is like a four-lane freeway suddenly changing into a country lane; traffic backs up on the freeway.

Likewise, when a clayey and/or compacted soil layer (primarily small pore space) is on top of a sandy soil layer (primarily large pore space) water accumulates just above the change.  Water is slow to leave the small pore space of the clayey soil due to the water properties of cohesion (water molecules binding to water molecules). 

This change in water movement creates a perched water table (overly wet layer of soil) generally 6 inches thick or greater just above the change line.

Managing Soil Tilth

Gardening on Coarse-Textured, Sandy Soils

The major limitation of sandy soil is its low capacity to hold water and nutrients.  Plants growing on sandy soils do not use more water; they just need irrigated more frequently but with smaller quantities.  Heavy irrigation wastes water as it leached below the root zone.  Water-soluble nutrients, like nitrogen, also leach below the rooting zone with excessive irrigation or rain. 

The best management practice for sandy soils is routine applications of organic matter.  Organic matter holds 10+ times more water and nutrients than sand.  Sandy soils with high organic matter content (up to 5%) make an ideal gardening soil.

Gardening on Fine-Textured, Clayey Soils

The limitations of clayey soils arise from a lack of large pore space, restricting both water and air movement.  Soils easily waterlog when water cannot move down through the soil profile.  During irrigation or rain events, the limited large pore space in fine-textured soils quickly fills with water, reducing the roots’ oxygen supply. 

The best management practice for clayey soils is routine applications of organic matter and attention to fostering the activity of soil microorganisms and earthworms.  As soil microorganisms decompose the organic matter, the tiny soil particles bind together into larger clumps or aggregates, increasing large pore space.  This improvement takes place over a period of years.  A single large application of organic matter does not do the trick.  A gardener may start seeing improvement in soil conditions in a couple of years. 

As the organic content increases, earthworms and soil microorganisms become more active, this over time improves soil tilth.

On clayey soils, also take extra care to minimize soil compaction.  Soil compaction reduces the large pore space, restricting air and water movement through the soil, thus limiting root growth.  Soil compaction is the primary factor limiting plant growth in urban soils.  Soils generally become compacted during home construction.  Methods of minimizing or reducing soil compaction are discussed in CMG GardenNotes #215, Soil Compaction.

Additional Information – CMG GardenNotes on Soils, Fertilizers and Soil Amendments:

#211 The Living Soil
#212 Earthworms
#213 Managing Soil Tilth
#214 Estimating Soil Texture
#215 Soil Compaction
#216 Mulching with Wood/Bark Chips, Grass Clippings and Rock
#217 Soil Drainage
#221 Soil Tests
#222 Soil pH
#223 Iron Chlorosis
#224 Saline Soils
#231 Plant Nutrition
#232 Understanding Fertilizers
#233 Calculating Fertilizer Rates
#234 Organic Fertilizers
#241 Soil Amendments
#242 Using Manure
#243 Using Compost
#244 Cover Crops and Green Manure Crops

[ top ]