https://www.youtube.com/watch?v=OiLITHMVcRw
What it said about how much carbon the world's soils hold?
The number is large. What it does not tell you is why. Two fields can sit side by side under the same rain, take the same fertiliser, and end up with different yields and very different amounts of carbon locked away underneath them. Nothing about the weather explains that. The explanation is in the handful of soil itself, and it comes down to two things: what the soil is made of, and how fast things rot in it.
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5.1.12 Soil texture defines the physical make-up of the mineral soil. It depends on the relative proportions of sand, silt, clay and humus.
Soil texture can be determined using a key, a feel test or by mixing with water and separating the layers in the laboratory.
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Pick up a handful of soil from Site A, the field on moraine. Most of what you are holding is mineral: fragments of rock broken down by weathering, which you met last lesson as a transformation. Those fragments come in three size classes, and the proportions of those three are what soil texture means.
Sand is the coarsest, from 0.05 to 2 mm across. You can feel the individual grains. Silt runs from 0.002 to 0.05 mm and feels smooth, almost like flour, when it is dry. Clay is anything below 0.002 mm, far too small to feel as separate particles, and sticky when wet. Those figures are the USDA size classes, which is the set most widely used.
Size is not a trivia question. It decides two things that matter enormously. Large particles leave large gaps between them; small particles leave tiny ones. And for the same mass of soil, small particles offer vastly more total surface. Grind a fixed mass of soil from 1 mm sand down to 0.001 mm clay and its total surface area goes up by roughly a thousand times. Everything in the next phase follows from those two facts.
The diagram below is worth a careful look before you go on, because the span involved is hard to picture. A sand grain can be a thousand times the diameter of a clay particle, and every silt and clay particle is below the size your eye can separate unaided.
Then there is the fourth component, and the statement is deliberate in naming it alongside the three mineral fractions. Humus is fully decomposed organic matter, dark and crumbly, lying beneath the leaf litter and formed by the partial decay of dead plant material. It does not sit in the soil as a separate layer so much as coat the mineral particles and stick them together into crumbs.
A soil with a reasonable balance of all three mineral fractions and a decent amount of organic matter is called a loam. Hold that word. You will find out in the next phase why it turns up in every list of good farmland.
Task
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Coming later: you will work out the texture of a real soil yourself, by feel and by settling it out in water, when you do the soil investigation.
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5.1.13 Soil texture affects primary productivity through the differing influences of sand, silt, clay and dead organic matter, including humus.
Humus contributes significantly to the texture of soils in which it is abundant. It is a dark brown or black substance lying beneath the leaf litter. It has a loose, crumbly texture formed by the partial decay of dead plant material. It influences mineral nutrient retention versus leaching, water retention versus drainage, and aeration versus compaction or waterlogging; these influences affect primary productivity.
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A plant growing in a soil needs four things from it at once: nutrients it can absorb, water it can reach, drainage so it is not sitting in a bath, and air in the gaps so its roots can respire. The awkward part is that texture cannot give you all four at maximum. Every fraction wins one side of a trade-off and loses the other.
Nutrient retention against leaching. All that clay surface holds dissolved mineral nutrients where roots can still get at them. Sand offers almost no surface to hold anything, so nutrients dissolve and leach downwards, out of reach, exactly as the nitrate did on its way to the aquifer last lesson. So clay holds nutrients and sand loses them.
Water retention against drainage. The tiny gaps between clay particles hold onto water. The wide gaps between sand grains let it straight through, so a sandy soil is dry again within hours of rain. But there is a sting here that catches people out: a clay soil holds the most water in total, and grips some of it so tightly that a root cannot pull it out. Silt turns out to hold the most water that is actually available to a plant.
Aeration against compaction or waterlogging. Roots and soil organisms respire, so they need oxygen in the pore spaces, which is what aeration delivers. There are two ways to lose it. A soil is waterlogged when water fills the pore spaces where air should be, which is what happens to clay that cannot drain. A soil is suffering soil compaction when pressure, usually from heavy machinery or trampling, has crushed the pore spaces out of it altogether. Different causes, same result: roots short of air.
So there is no best fraction. There is only a balance, which is what a loam is.
Humus is the exception that makes it all easier. It holds water, it holds nutrients, and it glues mineral particles into crumbs, which creates larger gaps between the crumbs than existed between the particles. That last one is why organic matter improves water retention and drainage in the same soil at the same time, which sounds impossible until you see how it works. Adding organic matter pushes all three trade-offs in the right direction at once, which is why the statement lists it alongside the three mineral fractions rather than as an afterthought.
Task
https://revise.alltheway.ing/simulations/embed/soil-texture-explorer