Knowing a soil’s texture gives us insights into its other properties. This was touched upon briefly here, but now that you’ve perhaps determined your own soil texture with a glass jar and some water, let’s go further into what the contributions of sand, silt and clay components are.
Soils high in sand, but low in silt and clay, are easy to cultivate, but are usually deficient in organic matter as they lack the negatively-charged clay particles that bind it. Water infiltration is generally greatest in sandy soils, though some sandy soils are water-repellent. Most do uptake water readily, though they drain easily as they are unable to hold this water. Sandy soils are less likely to compact under traffic (whether from people, animals or machinery) than other soil types. Higher silt and clay concentrations will improve water and nutrient retention.
Soils with around 40-50% clay (so-called ‘heavy’ soils) are difficult to cultivate, but do hold organic matter, minerals and water well. Water infiltration is poorest in clay soils. Water is slow to enter and percolate even dry clay soils as a rule, owing to the nature of the clay particles. However, some clays crack so deeply when dry that heavy rainfall causes rapid flows and deep penetration of water until the clay minerals swell and close the cracks. Too much water in a clay soil can lead to a sticky consistency and even waterlogging. Much of the water in a waterlogged soil isn’t actually plant-available — plants just can’t ’soak it up’. Rather, too much water in soil displaces the oxygen plant roots need, leading to poor overall plant growth at best and root rot and plant death at worst. Clays are also susceptible to compaction to the point that plants may struggle to grow in them. Higher sand and silt concentrations in otherwise clayey soils makes them more ‘open’ and better draining.
Loams have mixtures of sand, silt and clay that give them more intermediate properties to those of sands and clays. The best ones are easy to cuiltivate and hold good amounts of water and nutrients whilst still draining well. Loams in general hold the greatest amount of plant-available water, and silty loams hold the most of all. As a group these are regarded as the best soils for plants, with overall properties that encourage good root growth and nutrient retention. But as a group they also vary a lot in their proportions of sand, silt and clay, and some have less than desirable properties. Some loams set rock-hard, some develop surface crusts impervious to water, and some have very poor nutrient retention.
Knowing a soil’s texture can reveal a lot about its properties, as you’ve seen. But so far all we’ve done is extrapolate these properties from what we know about the properties of sand, silt and clay as distinct particles.
Soil is one entity to which the phrase “the whole is greater than the sum of its parts” could apply. The individual sand, silt and clay particles in soil group together to form aggregates or peds, and it’s the arrangements of these peds that give soil a characteristic more important than texture: its structure. Structure plays a major role in water infiltration and retention, nutrient retention, and how readily (or not) plant roots grow. A high clay soil with good structure could theoretically outperform a loam with terrible structure.
Next week we’ll begin a study of structure with first a look at soil horizons (the vertical layers that become apparent when you go deep enough). After that we’ll zoom in a little closer and describe the soil peds that make those horizons. And after that we’ll zoom in closer still and go over ‘exchangeable cations’ and ‘anions’. These are all well worth knowing, even if just for their own sake, as they will enhance your knowledge — and perhaps appreciation — of soil all the more!
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