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Organic matter

The soil’s bank 6 min read

The short version

Organic matter is everything in the soil that was once alive, or still is. It feeds the soil’s life, holds minerals, banks most of the soil’s nitrogen and sulfur, and glues the crumb. Its biggest gift for water is letting the rain soak in; the water it holds itself is modest. It builds slowly, and roots build it best, so keep something growing.

Everything that was once alive

A soil’s organic matter is everything in it that was once alive, or still is: fresh roots and stubble, the soil’s life itself, the remains of what has died, and humus, the dark, settled part that’s left once the life has eaten the rest. In most cropping soils it’s only a few parts in every hundred by weight. It does far more work than that share suggests. See kelp, fish, humic and fulvic for humus and the acids in it.

The soil’s life: bacteria. A fungus’s fine threads. Humus: the dark, settled part.

  • One crumb of soil, magnified: small grains of sand, silt and red-brown clay stuck together with dark coatings and open pores between them, a fine pale root running through it, and a piece of old straw against it.

    One crumb of soil

    magnified; the bacteria drawn bigger than true

    organic matter is a fresh root, stubble, the soil’s life and humus. The life glues the grains into a crumb.

A soil’s organic matter is everything in it that was once alive, or still is: fresh roots and stubble, the soil’s life itself, the remains of what has died, and humus, the dark, settled part that’s left once the life has eaten the rest. The life that feeds on it, and the sugar roots pay out, glue the soil into crumbs, with spaces between them for air and water. An illustration, simplified.

What it does

  • It feeds the soil’s life. Bacteria, fungi and the creatures that eat them live on it, and they’re what turn it, bit by bit, back into minerals the roots can take up. See soil health.
  • It holds minerals. Like clay, it carries a charge that holds calcium, magnesium and potassium where roots can trade for them, so it adds to what a soil can hold. In a sand, organic matter may do most of that holding. See reading a soil test.
  • It’s a bank of nitrogen, sulfur and phosphorus. Most of a soil’s nitrogen and sulfur sits in its organic matter, and so does a good share of its phosphorus. The life pays them out as it breaks the organic matter down, faster when the soil is warm and moist. See sulfur.
  • It builds the crumb. The life that feeds on it, and the sugar roots pay out, glue the soil into crumbs, with spaces between them for air and water. See dig a plant.
  • It steadies the pH, so a soil rich in it swings less when something acid or alkaline goes on.

How much water it holds

Organic matter helps a soil drink. Its crumbs and the channels its life leaves let the rain soak in, rather than run off or sit on top, and that’s where most of its gain in water comes from. You’ll often hear big figures for the water each extra bit of organic matter holds itself. When soil scientists brought the measurements together, the gain was real but modest, most of it in sands and least in clays. So build organic matter for everything it does, and value the water it lets in more than the water it holds.

Water. Wet soil.

  • A cut-away of pale, dense topsoil in rain, its sealed surface holding a puddle and water running off.

    Low in organic matter

    the topsoil, in rain

    the surface seals, and the rain runs off before it can soak in

  • The same ground, dark and crumbly under stubble, the rain soaking in through root channels and worm burrows.

    Rich in it

    the same ground, the same rain

    the crumbs and the channels its life leaves let the rain soak in

Organic matter’s biggest gift for water is letting the rain in, through the crumbs and the channels its life leaves behind. On a soil low in it, the surface seals and the rain runs off. The water it holds itself is real, but modest. An illustration, simplified.

Building it is slow

In a cropping soil, organic matter builds slowly, over years, and falls faster than it builds. Working the soil hard wakes the life, and it eats through the organic matter. A bare fallow gives the life nothing new to eat. Burning stubble sends the carbon straight up as smoke.

Roots build it best. Carbon from roots, and from the sugar they pay out to the life around them, stays in the soil longer than carbon from what’s left on top. It’s one more reason Steve says that to change a soil, you’ve got to put a plant in there. Keep something growing as many days of the year as you can. See soil health.

Sugar, paid out by the roots. The soil’s life. A fungus’s threads. The roots’ carbon, building up round them.

  • A cut-away of a paddock between crops: pale stubble lying on the surface and bare brown soil below, with no living roots.

    Stubble on bare ground

    a paddock between crops

    the stubble on top breaks down, and its carbon is gone sooner

  • The same ground with young oats and vetch growing through the stubble, fine white roots all through the soil, white fungal threads and small pink nodules on the vetch roots.

    A living cover

    the same ground, with oats and vetch growing

    living roots, the sugar they pay out and the life they feed build organic matter that stays

Roots build organic matter best. Carbon from living roots, and from the sugar they pay out to the life around them, stays in the soil longer than carbon from stubble lying on top. Keep something growing as many days of the year as you can. An illustration, simplified.
  • Keep the stubble, and work the soil no more than you have to.
  • Grow a cover crop between cash crops, so living roots keep feeding the life.
  • Put compost or manure on where you have it, and feed the life with the carbon in each spray. See kelp, fish, humic and fulvic.

Carbon on a soil test

A soil test reports organic carbon, not organic matter. The organic matter is roughly twice the carbon figure. Labs measure it in different ways, and the ways don’t agree, so you can’t compare one lab’s figure with another’s. Test the same depth with the same lab every few years, and watch the trend. It moves slowly: a change you can trust takes years to show.

Where it fits in a program

  • Keep living roots in the ground for as much of the year as you can.
  • Keep the stubble on, and work the soil as little as the crop allows.
  • Feed the life as well as the crop, with carbon in each spray.
  • Test organic carbon every few years, at the same depth and with the same lab.