Soil, Le Sol
When I first started gardening one of the most confusing statements I came across was that a plant required 'a moisture retentive, free draining soil'. I thought this sounded like a contradiction in terms until I decided to try and further my gardening knowledge by studying for the RHS General Examination in Horticulture.
What follows is taken from my notes and although only touches on the subject, (I am definitely not a soil scientist), I hope will make the above statement a bit clearer.
To try and put it fairly simply, soil is made up of a mixture of sand, silt and clay, with organic matter and soil life, (worms, bacteria, funghi and the like).
The triangle above provides a diagrammatical representation of the various combinations of particle types. Much more detail may be found at: soil texture at Wikipedia
Sandy Soil
If your soil feels gritty and you can hear a rasping noise when you rub it together you know you have a sandy soil. Sometimes you can see the individual grains of sand (angular chunks of quartz) if the soil is very sandy. Sandy soil won't hold together easily even when wet. As soon as you try and roll it into a sausage shape it tends to fall apart. Sand also allows water to drain freely, very useful for working on it after rain, but it soon dries out. Because it is inert ie, does not carry an electrical charge, it doesn't hold on to nutrients, hence the need to feed little and often. Plenty of organic matter helps with both the water retention and the nutrient holding ability of sandy soils.
Clay Soil
Clay feels smooth and sticky with particles shaped like plates. The particles literally stack like plates with very little space between. This is why clay soils are often slow to drain after rain, and feel smooth and sticky as the tiny plates slide over each other. Clay has its advantages though, as unlike sand, the particles carry a negative electrical charge which enables them to hold onto nutrients, therefore preventing the nutrientents being washed out of the soil. These are then released slowly over time, making many clay soils fertile. When a clay soil dries out it becomes very hard and very difficult to break up. When wetted however, clay will stick together as anyone who has worked it will know, and in its purest form of course it is used to make pots. A clay soil can be shaped into a sausage shape easily and can also be made into a ring. Clay will 'polish' when rubbed between fingers and thumb and feels sticky and plastic.
For over 20 years I have gardened on sand overlying very sticky clay. The front garden has the most sand and the back the most clay. 20 years of organic matter has made a huge difference to the soil, helping the sand to hold onto more water and the clay to form a better structure by helping the clay particles stick together as tiny crumbs rather than plates and therefore helping it drain better. The soil I have now have is rich and dark and where more sandy still drains freely after rain which means I can work it almost immediately. Where there is more clay it still cracks as it dries out but is much easier to work than it used to be.
Silt is somewhere between the two and is composed of a mixture of very small sand particles and larger clay particles. I haven't gardened on silt so therefore my knowledge is academic as opposed to being based on experience.
Silty Soil
Silt has a silky or soapy feel to it and produces a fine 'floury' dust when crushed while dry . Most silt particles are inert like sand but some are able to hold onto nutrients. Silty soils have good water holding ability, but while the particles will pack closely together closely, they are not good at forming stable crumbs (clumps of particles). This means that the soil structure is easily damaged and silty soils can be difficult to work.
Organic matter
I have mentioned the importance of organic matter in soils. Below is a brief explanation of why. Organic matter such as compost, manures etc release gels and gums as they break down, which glue particles in the soil together. It also encourages soil life such as worms, bacteria and funghi. On sandy soils it improves the water holding capacity of the soil, while on clay soils it helps crumb formation, turning the platey structure into clumps which allows excess water to drain from the soil more quickly. As more organic matter is added over time it makes the soil darker allowing it to absorb heat faster and therefore warm more quickly, which stimulates plant growth and soil life giving better crops. Like clay, organic matter is good at holding onto nutrients, in fact it is better. In more acidic soils pH 6 or less, organic matter is not broken down so effectively and so builds up in the soil. The more air and bacteria in the soil the quicker the decomposition, which is why there is less organic matter in tilled soils and the need to keep adding organic matter to sandy soils.
Water
Water in soil with a good structure, ie particles glued together into crumbs with spaces or 'pores' between the crumbs is held within the crumbs. Water can drain freely from the pores between the crumbs which hold air, (also necessary for most plants to grow well). This then is a 'moisture retentive (within the crumbs), free draining (between the crumbs) soil'. Simple really.
I am now looking forward to finding out about my soil in France.
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