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Soil: the naturally occurring, unconsolidated or loose covering of broken rock particles and decaying organic matter (humus) on the surface of the Earth, capable of supporting life. The ideal soil for plant growth has the following proportions:
Air and water are pretty straightforward but what about the "mineral" portion of soil? Mineral particles in soil are divided into three categories, from the smallest sized particles to the largest: clay, silt, and sand..
The surface of soil particles and the space between particles is important because that's where much of the water and nutrient reserves are held and where many important chemical reactions take place. Too much space because all the particles are too big (sand) and nutrients get washed away. Too little space because all the particles are too small (clay) and plant roots rot because excess water is not drained away and it is too hard for roots to push through the clay soil. So, the right mix of sand, clay, and silt is important since it determines the extent to which loose minerals in the soil are available for the plant's roots to take up. The most important mineral nutrients are Nitrogen (N), Phosphorus (P), and Potassium (K) but other nutrients also play a role: Calcium, Magnesium, Iron, Copper, Zinc, Manganese, etc.
If you want to grow things successfully, you'll need to find out what your soil's nutrient levels are and its pH (acidity/alkalinity). Click here for printable information about soil pH. pH is VERY important because it determines how available each nutrient is for your plant to use to make its own food through photsynthesis. You can never "feed" a plant--your job it to ensure that the right nutrients are present and available for the plant to make its food and feed itself! The only way to know your soil's nutrient levels and how available pH is making these levels is by soil testing. Don't guess--soil test! Click on the link below to be shown exactly how to do it.
The relative percentages of clay, silt, and sand in a soil determines the "soil texture". The texture of the soil determines how good a growing medium it is for plants or the soils "tilth". How the soil particles arrange themselves into aggregates or granules is called "soil structure".
Click here to see a Shockwave presentation on Soil.