What is loam?
Loam is a type of soil that naturally occurs on earth. It consists of a combination of three particles: Sand, Silt and Clay  By having the right proportion of these particles you'll get a perfect Loam-mix depending on your goal (plastering, COB, rammed earth, etc.)
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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 soil particles.  Sand, being the larger size of particles, feels gritty.  Silt, being moderate in size, has a smooth or floury texture. Clay, being the smaller size of particles, feels sticky.  
Comparative size of sands, silt and clay. If clay was the size of a dot on the page, silt and sands would be a comparative size.
Soil Texture TriangleThe Soil Texture Triangle gives names associated with various combinations of sand, silt and clay.  A coarse-textured or sandy soil is one comprised primarily of medium to coarse size sand particles.  A fine-textured or clayey soil is one dominated by tiny clay particles.  Due to the strong physical properties of clay, a soil with only 20% clay particles behaves as sticky, gummy clayey soil.  The term loam refers to a soil with a combination of sand, silt, and clay sized particles.  For example, a soil with 30% clay, 50% sand, and 20% silt is called a sandy clay loam.  [Figure 2]
 Soil Textural Triangle - Based on the triangle, a loamy soil has 40% sand, 20% clay and40% silt. A sandy loam has 60% sand, 10% clay and 30% silt. [Source: U.S.D.A.]
​​IDENTIFYING SOIL TEXTURE BY MEASUREMENT
Although this method is generally used by natural builders, it is not exact and doesn't always work. When you have a mix with high clay content it is very probable you don't see any separation between the elements.
  1. Spread soil on a newspaper to dry.  Remove all rocks, trash, roots, etc.  Crush lumps and clods.

  2. Finely pulverize the soil.

  3. Fill a tall, slender jar (like a quart canning jar) 1/4 full of soil.

  4. Add water until the just is 3/4 full

  5. Add a teaspoon of non-foaming dishwasher detergent.

  6. Put on a tight fitting lid and shake hard for 10 to 15 minutes.  This shaking breaks apart the soil aggregates and separates the soil into individual mineral particles.

  7. Set the jar where it will not be disturbed for 2-3 days.

  8. Soil particles will settle out according to size.  After 1 minute, mark on the jar the depth of the sand.

  9. After 2 hours, mark on the jar the depth of the silt.

  10. When the water clears mark on the jar the clay level.  This typically takes 1 to 3 days, but some soils may take weeks.

  11. Measure the thickness of the sand, silt, and clay layers.
  1. Thickness of sand deposit  ____
  2. Thickness of silt deposit  ____
  3. Thickness of clay deposit   ____
  4. Thickness of total deposit  ____
12. Calculate the percentage of sand, silt, and clay.
  1. [clay thickness] / total thickness]  =  ___ percent clay
  2. [silt thickness] / total thickness]  =  ___ percent clay
  3. [sand thickness] / [total thickness]  =  ___ percent sand
13.  Turn to the soil texture triangle and look up the soil texture class.
Texture by Measurement - Measure the depth of the sand, silt and clay layers.
IDENTIFYING SOIL TEXTURE BY touch
Rub some moist soil between fingers.
  • Sand feels gritty.
  • Silt feels smooth.
  • Clays feel sticky.


​IDENTIFYING SOIL TEXTURE BY BALL SQUEEZE TEST ​
Squeeze a moistened ball of soil in the hand.
  • Coarse texture soils (sand or loamy sands) break with slight pressure.
  • Medium texture soils (sandy loams and silt loams) stay together but change shape easily.
  • Fine textured soils (clayey or clayey loam) resist breaking.
​​IDENTIFYING SOIL TEXTURE BY RIBBON TEST
Squeeze a moistened ball of soil out between thumb and fingers.
  • Ribbons less than 1” (2,5cm)
  • Feels gritty = coarse texture (sandy) soil
  • Not gritty feeling = medium texture soil high in silt
  • Ribbons 1-2” (2,5-5cm)
  • Feels gritty = medium texture soil
  • Not gritty feeling = fine texture soil
  • Ribbons greater than 2” (5cm) = fine texture (clayey) soil

Note: A soil with as little as 20% clay will behave as a clayey soil.  A soil needs 45% to over 60% medium to coarse sand to behave as a sandy soil.  In a soil with 20% clay and 80% sand, the soil will behave as a clayey soil.
(Source: ©2013 Colorado State University Extension)
​DETERMINING THE IDEAL MIX FOR YOUR PURPOSE
It is difficult to deterime this ideal proportion as it also depends on other factors (grain size distribution, type of clay, additives, etc.).
Typically a 5%-12% Clay mix is used for rendering, while you can go up to 40% of clay mix for COB building.

To find out what is the best mix is just to try it out!
Usually you want to start from a clay-ish soil as it is easier to add sand afterwards. When you would have a sandy soil you would need to add clay to it, which is harder to do.

You can check the proportions by adding different volumes of sand to the soil while keeping the straw or other reïnforcement material (woodchips, hair, etc.) constant.

For instance, make 5 samples with

0% sand, 100% Soil
10% sand, 90% Soil
30% Sand, 70 % Soil
40% Sand, 60% Soil
60% Sand, 40% Soil
80% Sand, 20% soil

This method is in most cases the most reliable and it's the one I always use when working with a new batch of loam.
Additives
There are many additives that can change the properties of your loam mix:
- Lime
- Cow Dung
- Horse or cow urine
- Ashes
- Gypsum
- Linseed oil
- Casseine
- Straw, hemp, hair, ....
-....

For more info on additives, take a look at the page on loam rendering