Physical Properties of Soil

Soil Properties Overview

Soil is overlooked often even though the composition of soil heavily effects plant growth and growing success. Soil compaction has become a problem when dealing with lawns, gardens, agriculture, and many other scenarios. The physical mechanics of how water moves and is retained in soils is misunderstood as well. Minerals in soil contribute heavily to plant growth as well.

Due to the misunderstandings of physical soil properties, soils have become overused and damaged. Understanding the physical properties of soil allows quick diagnoses and cures for common and often neglected soil problems.

Feeling dirt helps determine soil type.
Feeling dirt helps determine soil type. | Source

Soil Texture

Soil texture refers to the way the soil "feels." Soils can be very coarse to very fine and anywhere in between. Sand, silt, and clay determine the texture of soils. A soil containing equal amounts of sand, silt, and clay is called clay loam. Regular loam contains a 40-40-20 percentage of the three soils.

Soil Texture Importance
Soil texture affects physical and chemical properties that affect plant growth. Soil texture directly or indirectly impacts soil structure, aeration, water movement and retention, mineral weathering, bulk density, tilth, and nutrient supplies. Soil texture also has a significant impact on crop yields.

Sandy soils retain little water due to the spacing between each particle. Nutrients are easily leeched away from root zones due to the spacing of particles. Wind can cause severe erosion to sandy soils if wind occurs regularly or is sustained for extended periods of time. Clay soils are quite the opposite. Clayey soils are hard to till into an adequate seed bed, and water drains very slowly from a clayey composition. Medium textured soils are preferred for planting due to the balance of sand, clay, and silt.

Particle Size and Test
Particles of sand, silt, and clay vary greatly in size. A grain of coarse sand is much larger compared to a particle of coarse clay. A soil particle test can easily be performed at home using a jar, soil, and water. Sand particles settle first, while clay particles can remain suspended in water for over an hour or more.

Particle Surface Area
Clay particles are extremely small, but have a vast surface area due to the tiny, flat layers that make up clay particles. Water is easily retained between each tiny layer. To put it into perspective, an acre of clay that is 7 inches deep has a surface area equivalent to 25 million acres. This is why a little bit of clay can ultimately impact plant growth.

Soil Feel Test
Soil type can be estimated accurately enough for home gardens, flower beds, and other plantings by simply rubbing and squeezing the soil together. Organic matter can change the way soils feel. Soils rich in organic matter may not provide adequate results.

Feeling for Soil Texture

Soil Type
Dry
Wet
Sand
Loose and gritty. Falls apart after being squeezed.
Holds form when squeezed, but very easily falls apart when touched.
Sandy Loam
Clumps are easily crushed. Slightly smooth, but mainly gritty.
Holds form when squeezed, but can be broken easily.
Loam
Clumps can be firm. Slightly smooth when rubbed, but relatively gritty.
Holds form well when squeezed and is not fragile.
Silt Loam
Clumps are firm to hard. Flour-like texture when crushed.
Holds form well when squeezed. Slight ability to hold ripples via squeezing.
Clay Loam
Clumps are difficult to crush by hand. Somewhat gritty due to clumpiness.
Holds form very well when squeezed. Ribbons form when squeezed between thumb and finger.
Clay
Clumps are extremely hard to crush. Gritty texture when crushed.
Holds form extremely well when squeezed. Satin feel when rubbed. Sticky when wet. Retains plasticity when moist.
Soil horizons
Soil horizons | Source

Soil Structure

What is Soil Structure?
Soil structure is the arrangement of primary soil particles into aggregates of definite shape. Water movement, root penetration, aeration, and bulk density are all determined by soil structure. Soil particles tend to stick together, unless the soil is sandy. Soils are held together by clay, organic matter, bacterial gums, iron and aluminum oxides, silica, lime, root hairs, and fungal mycelia. Very sandy soils are composed of single grains and do not bind together.

Soil Granules Determine Structure
Granular soils can easily be worked into a manageable seedbed. The finer the granules are, the better and more manageable the soil is. Intensive row cropping, inadequate soil management, and erosion have destroyed much of the granular soil structure across the Earth. Heavy equipment compacts soil and roots do not like encountering a compacted layer, which results in a poor root system with horizontal growth instead of vertical growth.

Restoring Soil Structure
Large dirt clods are a symptom of poor soil management. Restoring soil structure involves growing native grasses and legumes, amending with barnyard manure and crop residues, controlling erosion, and minimizing tillage and traffic to reduce compaction. Tilling deep into the soil is not a cure-all for compaction and drainage. It may provide temporary improvement, but will not provide a long term fix.

Mulch around plants.
Mulch around plants. | Source

Organic Matter in Soil

Organic matter plays a factor in soil structure. It also plays an important role in storing plant nutrients and improving a soil's capacity to retain water. Mulch resides onto top of soil while organic amendments are worked into the soil.

Mulch
Mulch is organic matter that is applied to the surface of soils. Mulching has many benefits such as moisture retention and pathogen suppression. Mulch should be applied 2 to 3 inches thick above the topsoil. Some benefits of mulch are:

  • Suppression of weeds & diseases
  • Evaporation reduction
  • Visually pleasing
  • Reduction of winter damage to roots
  • Added nutrients to the soil as mulch decays

A few common fine-textured mulches are twice-shredded bark, compost, cocoa hulls, and grass clippings. Grass clippings should not be applied in thick layers because the clippings can smother the soil and seedlings. Do not use clippings from grass that was exposed to weed killer. Herbicide residue on lawn clippings can kill plants when applied as mulch. Coarse-textured mulches commonly available are straw, bark, and wood chips.

Organic Matter in Soil
Organic matter provides a food source for plants, beneficial bacteria, earthworms, and many other organisms that reside in soil. Plants and organisms break down dead matter into nutrients while promoting drainage and good soil structure. Introducing organic matter into the soil has a downside - limited usefulness. Only about 800 pounds out of 10,000 will remain as stable organic matter. The rest will be turned into carbon dioxide via microbes or broken down into water and simple salts. Quickly changing a soil's composition and nutrient content using organic matter is not very feasible on a large agricultural scale, but it can benefit small gardens and flower beds.

Mineral Matter in Soil

Soil contains many kinds of minerals. A mineral is a naturally occurring inorganic substance whose chemical composition varies only within prescribed limits and which has definite physical properties. Some minerals are unused by plants, while others provide a good source of nutrients.

Common Minerals
Feldspar is a common mineral that contains potassium, calcium, magnesium, copper, iron, manganese, and zinc. Ferromagnesium silicates are high in magnesium and iron. Oxides of iron, manganese, aluminum, and magnesium are commonly found in clayey soils. Oxides are important sources of magnesium, boron, copper, iron, manganese, molybdenum, and zinc. Dolomitic limestone is a source of calcium and magnesium. Apatite contains phosphorus and gypsum has calcium sulfate.

Soil Types and Minerals
Soil types play a huge factor in mineral content. Sandy soils are mainly composed of quartz and contains no usable minerals. Silty soils contain good amounts of feldspar and micas. Clay soils hold potassium and magnesium that are available on the soil surface when introduced to an electrical charge.

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