Many gardeners (and that used to include me) are confused by phrases like loam soil, garden loam or loam based and are not sure exactly what is meant by “loam”.
An on line search for a definition of loam will probably result in more questions than answers as well as different meanings of the word depending on the dictionary used.
Some writers say that if you stack lawn turfs upside down and allow them to rot, loam is the result where as some dictionaries say it is a mix of sand, silt and clay – Hardly the same thing!
So, what is loam?
Technically, loam is a mix of sand, silt and clay and this does constitute a large part of what soil is but it is not soil. It is a s
ort of framework that holds the water and air, the nutrients, the humus, the organic matter, the microbes, fungi, arthropods – in fact everything that makes up and is in the soil – but it is not actually soil.
For most gardeners though, loam soil, means “good” soil, the right blend of clay, sand and silt together with sufficient humus and nutrients.
The scientific definition of loam is: “A blend of sand, silt and clay in a 2:2:1 ratio.”
But what defines sand, silt and clay?
It is simply the size of the granules, grains or particles! – nothing to do with what the material is or where it came from!. If there is a higher percentage of sand it might be described as “sandy loam” or for a higher percentage of silt “a silty loam.”
Sand, as anybody who has walked on a beach will know consists of tiny granules of stone, rocks or minerals.
The material doesn’t matter. If the grains are smaller than 2mm then they are sand grains and if they are bigger, then it is gravel. Sand of course can be much finer than 2mm but if the grains are less than 0.05mm it is technically silt. Again the material doesn’t matter it is the particle size. It is a fact that even if the silt is from the same location as the sand it is likely to be composed of softer stone or minerals than the sand simply because natural weathering and erosion will have reduced the particle size more easily.
Clay has smaller particles still. About 1/1000th the size of sand grains or 0.002mm in size – too small to see individualy with the naked eye, you would need an electron microscope to look at them. The particles in clays are more likely to have been formed as a result of chemical actions or the results of thermal activity long ago. These particles absorb moisture. They can also contain aluminium, iron, magnesium and other trace minerals. The moisture in the clay particles sticks them together so that they feel slippery rather than gritty like sand
Why is this mix of sand, silt and clay important?
To understand this we need to think about not just the size of the grains or granules but their surface area. It is the surface area of the particles that determines their capacity to hold water. Water drains from sand fairly easily but the sand will remain wet because water will cling to the surfaces of each grain. If you compare the surface area of say a teaspoon of sand grains with the surface area of a teaspoon of clay particles you will find that the clay particles although smaller, have approximately one million times the surface area and so it is able to hold very much more water than sand.
So, loam, with a sand, silt clay ratio of about 2:2:1 is an ideal mix for holding on to about the right amount of water (by the clay and the silt) and at the same time allowing excess water to drain away (from the sand) which in turn allows air (and oxygen) to be drawn back into the mix.
For this “loam” to become loam soil, it needs organic matter. The organic matter in soil ranges from recently dead plant (or animal) material through to humus. Humus is decayed organic material which has completely decayed to the point where it has become a stable material. “Well rotted” compost or manure , worm castings etc are in the process of becoming humus and may contain humus material but may take as long as 10 years to actually become humus. The amount of organic matter in garden loam may be 5-10% by volume, (3 – 5% by weight).
The final ingredient (or ingredients) in loam soil, are all of the living things that make up the Soil Food Web. Tiny animals, creatures and microbes, beneficial bacteria, fungi, nematodes, arthropods, protozoa and many more. Consuming dead matter, supplying food chains which are linked and cross linked and importantly for organic and natural gardeners, producing plant available nutrients. As organic gardeners and growers know, it is the biology in the soil that constitutes the soil food web and this is what allows our natural flora, our wild flowers, trees and forests to survive without the need for highly processed and man made chemical fertilisers, pesticides and herbicides.
By Bryan Davenport