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What is soil?

Everything at one point or another has been forged, extracted, nourished or enveloped by the soil. From bricks made of clay, to roof frames forged from aluminium to returning when things die, everything is able to trace itself back to the soil. For the home gardener soil is generally classified as the unconsolidated mineral (inorganic) and organic materials found on the immediate surface of the earth that provide a natural medium for the growth of plants (Soil Taxonomy, 2nd edition, 1999).  Soil is the structure that holds water, nutrients and air required to support the growth of plants. In nature there is no single process which forms it. It is a product of a vast production line which incorporates weathering, erosion, deposition, leaching and microbial activity. The variations in soil are directly related to these processes in conjunction with the climate, topography, vegetation and source of the parent material. The reason most prime agricultural lands are within valleys at the base of old volcanic hills and mountains is due to the slow process of weathering at the top of the mountain (parent material) eroding minerals vital for the growth of plants and depositing them within the valley.

Classifying the Soil through Texture

Inorganic Materials:

What is meant by inorganic materials? Basically anything that isn't part of a living thing or hasn't come from a living thing. Like the sand on a beach or the glass blown from it. They are the compounds that don't contain carbon, the precious element that has allowed life to proliferate across earth. Soil is largely comprised of inorganic materials with mineral particles being the major constituent. The inorganic material within soil is what determines physical properties, such as texture, and has a large effect on the soils density and its ability to retain water. I have taken these texture identifications from the Australian Standards for Identifying Soil Particles and then for gardening purposes simplified to the predominant 5 size fractions we will be interacting with, these are:

  • Gravel: Particles greater than 2mm in diameter
  • Coarse Sand: Particles less than 2mm but greater than 0.2mm in diameter
  • Fine Sand: Particles less than 0.2mm but greater than 0.02mm in diameter
  • Silt: Particles less than 0.02mm but greater than 0.002mm in diameter
  • Clay: Particles less 0.002mm in diameter

Sand:

  • Sand is predominantly made of quartz; it has low water retention and little chemical activity. It has a loose gritty feel and does not stick together. Individual grains can be seen or felt. You can find whether something has sandy texture by giving it a lick.

Silt:

  • Silt is generally a mixture of well-eroded quartz and feldspar, it retains the most water compared with sand and clay, has little chemical activity and is easily compacted which affects the air and moisture content. It has a smooth silky feel and will stick to other particles but can easily smear apart. Individual grains cannot be seen without a microscope.

Clay:

  • Clays are generally a mixture of clay minerals (Chlorite group, Illite group, Kaolin group, Smectite Group) with traces of oxidized metals and organic matter. Clays are chemically active, retain water and are able to hold nutrients. Their ability to retain water is what gives them plasticity. Clays are sticky to the touch, they are easily molded into shapes and do not easily smear apart. Individual grains cannot be seen without a microscope.

Understanding Textural Classes

Sand:

  • Loose gritty feel and does not stick together. Individual grains can be seen or felt.

Loamy Sand:

  • Particles barely stick together, a molded piece just holds its shape

Sandy Loam:

  • Sticks together more than a loamy sand but can be easily broken, individual sand grains can be felt.

Silty Loam:

  • Breaks into crumbs but will tend to stick together, has a smooth silky feel

Loam:

  • Breaks into crumbs but will tend to stick together, sand grains cannot be felt. A sample when squeezed will retain its shape but when rolled between the palms will break apart.

Sandy Clay Loam:

  • More easily molded into a shape than a loam, it rolls out to a thin ribbon without breaking. Sand grains can be felt. When dry it will form a tough lump.

Clay Loam:

  • More easily molded into a shape than a loam, it rolls out to a thin ribbon without breaking. When dry it will form a tough lump.

Sandy Clay:

  • Can be molded into shapes, forms long flexible ribbons with the ribbon able to be bent into a U without breaking. Sand grains can be felt

Silty Clay:

  • Can be molded into shapes, forms long flexible ribbons with the ribbon able to be bent into a U without breaking. Has a smooth silky feel.

Clay:

  • Can be molded into shapes, forms long flexible ribbons with the ribbon able to be bent into a U without breaking. When dry it will form a tough lump.

Organic materials in Soil

Organic matter within natural soils generally makes up less than 10% of the weight of the soil. It is less than 10% yet without it, the world would be predominantly deserts with what little life that can survive, struggling to grip on. A far cry from a big healthy vibrant ecosystem within a forest! It goes to show how finely calibrated these systems are. Organic matter is segregated into living and non-living sections and primarily comes from the decomposition of animal and plant products. Soil micro-organisms primarily decompose the organic matter, releasing nutrients for the growth of plants. During the initial phase the carbohydrates, starch and proteins are decomposed. Cell walls, which are made of lignin* and tannin**, are structurally more resistant to the enzymes secreted by the micro-organisms resulting in slower decomposition. They are what leave that dark color so desired in soils with high organic content. Decomposition rates are directly related to how favorable the soil environment is for microbial activity. Favorable soils for microbes are warm, moist, have good aeration, consist of necessary nutrients, have a pH around neutral (7) and are free of toxic compounds.

To learn more about increasing organic matter within soils be sure to check out; 

  1. Green Manures in the Market Garden
  2. Building Bio-intensive Market Gardens

  • *Lignin:
    • Lignin is a class of complex organic polymers that form important structural materials in the support tissues of many known plants. It is important in the formation of cell walls, especially in wood and bark. They lend rigidity and do not easily rot.
  • **Tannin:
    • Tannin is a polyphenolic organic molecule that is able to bind to and secrete proteins and other various organic compounds like amino acids and alkaloids. It is astringent and naturally occurring within grape skins.

What soil is best for plant growth?

The soil widely considered best for plant growth is a Friable Loam soil. This is because friable loam soil is a nice balance of silt, sand and clay mixed with organic matter. It wont easily form a hard crust or become brittle, it is able to retain necessary nutrients and water for plant growth while still being free draining and air moves through the soil keeping the roots well oxygenated.

Testing your soil

Identifying the predominant inorganic materials:

Collect a sample from the top 250mm of soil, trying to get an even representation of the depth. Dry it in the oven at 180C for 40mins. Fill a glass jar with water and place a couple of tablespoons of crushed soil in the water. Leave it for 15 minutes then shake the jar and wait for the layers to segregate. Your soil particles will segregate based on their density and you will be able to determine whether your soil is predominantly sand, silt or clay.

Identifying the need for gypsum:

Another test is to dry a small ball of soil and drop it in a glass of distilled water. Leave it for 5 minutes, the ball will slowly break apart, slake or disperse. This shows the clay bonds and determines whether to add gypsum/dolomite. If the sample slakes (the lump will collapse into small pieces) or disperses (the particles spread over a wide area) an application of gypsum/dolomite will be necessary. 

To learn more about Market Gardening and how to set up your own Market Garden be sure to check out;

  1. 7 Steps to Market Garden Design Success
  2. The First 5 Steps to Starting a Market Garden
  3. Building Biodiversity in the Market Garden
  4. Composting in the Market Garden
  5. DIY Bug Hotel