What you will learn in Soils and Biogeochemical Cycles

1. 1. How soil is made, and how fast

   Five factors make a soil: the rock, the climate, what lives there, the slope and time. Run them forward and watch a profile grow at a rate measured in millimetres per century — and watch it fail to become soil at all if nothing lives in it.

2. 2. Reading a profile

   Dig a pit and the wall is a record: an organic layer, a dark topsoil, a bleached layer or a red one, an accumulation below, then the rock itself. Read seven profiles, from a laterite to a podzol to the black soil of the Deccan, and say what climate and what rock made each one before you look.

3. 3. Texture, pH and what plants can get

   Mix sand, silt and clay on the USDA texture triangle and read how fast water drains and how much the soil holds. Then move the pH and watch different nutrients run short at the two ends — so that no single pH is best for all of them.

4. 4. The cycles, and the balance

   Nitrogen is most of the air and still scarce, because nothing can use it until its triple bond is broken. People now break about as many as all of nature does. Phosphorus has no air to return through at all. And the soil the cycles run through forms far more slowly than a ploughed field loses it.