Mulch

Water and heat: mulch in a Mediterranean climate

In a Mediterranean climate the summer is the bottleneck. For months hardly any rain falls, while sun and wind pull water out of the ground every day. On bare soil the surface heats up strongly, dries out first and then draws moisture from deeper layers. The upper centimetres, the very zone where most soil life and the fine roots live, become a hot, dry crust.

Mulch breaks this mechanism. The figures vary with material, amount and soil, but the direction of the evidence is clear. In field trials on the North China Plain, where little rain falls during the wheat season, straw reduced evaporation from the soil by about a fifth at a light rate and by about two fifths at a heavy rate; at the same time the daily maximum of soil temperature fell and the minimum rose. Trials under rice straw in Spanish citrus orchards, in a semi-arid climate with hot summers, measured mean topsoil temperatures 0.5 to 1.2 degrees below bare soil, and under the straw the soil let water through two to fourteen times faster, a sign of more coarse pores. Trials in Mediterranean vineyards showed that barley straw lowers runoff and soil erodibility from the moment it is laid down.

These are no small effects in a landscape where every week of moisture counts. A soil that stays cooler and moister for longer in June and July keeps its soil life active, and young plants survive the first summer more often.

Why there is no stable humus without mulch

Humus does not come from a bag; it is the stable remainder of organic matter that soil life has processed. It forms where two things come together: a constant supply of organic material and a soil environment in which life can work. Mulch delivers both.

In hot, dry climates this is all the more decisive. Heat speeds up the breakdown of organic matter, so a large share of what enters the soil is respired away rather than stabilised. A bare surface also exposes the soil to the sun, kills off fungal networks and leaves the first heavy rain an open target. Under these conditions humus is lost faster than it forms. Only when the soil stays covered over years does the balance tip. In the trial in the Spanish citrus orchards, organic carbon in the topsoil rose about ten percent more under straw than under bare soil over nearly three years. That is the slow, real pace at which humus grows.

This is why Terra Humana treats permanent soil cover, whether dead mulch, living plants or the shade of a canopy, as the precondition for any lasting soil building in the Mediterranean, not as one measure among many. Amendments such as biochar or compost only take effect in a soil that is protected from above.

Organic, living, mineral: the kinds of mulch

Organic mulch is the classic form: straw, leaves, grass cuttings, wood chips, pruned branches, the cut biomass of support plants or green manure. It feeds the soil and gradually disappears into it, so it has to be renewed. Coarse material lasts longer and lets rain through better; fine material breaks down quickly and can mat. A special form is ramial chipped wood, known in French as bois raméal fragmenté. It goes back to work by Gilles Lemieux at Université Laval in Quebec and uses chipped young branch wood rather than trunk wood. Thin branches carry a large share of a tree's nutrients, and their lignin feeds above all the fungi that build crumb structure. Sheet mulching is the layered version of organic mulch, used to build up beds and planting positions.

Living mulch is a low plant cover that grows alongside the crop and protects the soil while alive. In the Mediterranean it typically consists of annual clovers and medics that grow in winter and dry to a straw layer in summer. Related to it are cover crops between trees and green manures grown specifically for their biomass.

Mineral mulch means gravel, stones, sand or volcanic ash. It hardly decays, lasts for decades and does not burn, but it adds no organic matter. Its classic example is the volcanic ash on Lanzarote.

Mistakes, and how mulch gets through the summer

Most mulch failures come from a few avoidable errors.

Too thick. A heavy layer intercepts light rain before it reaches the ground, keeps air out and provides cover for voles and mice. A moderate layer renewed regularly does more than a deep pile laid once.

Against the trunk. Mulch heaped against the bark keeps it permanently damp, invites rot and gives rodents a hidden place to gnaw. A hand's width around the trunk stays free.

Wrong material. Hay full of seeds sows the next weed problem. Treated or painted wood, plastic and contaminated material do not belong on soil. Fresh, fine sawdust or wood chips worked into the soil tie up nitrogen while they decompose; laid on the surface the effect is much smaller.

Wrong timing. Mulch on soil that is already dry keeps the dryness in. The right moment is spring, while the soil is still moist from the winter rains, or autumn after the first good rain.

For the summer itself, a few rules follow from the climate. Mulch is laid before the heat, not during it. The cover is renewed in winter and early spring, when cut material is available and rainfall still exceeds evaporation. Shade comes first: wherever plants cast shade in summer, they stay standing, because shade protects the soil better than any layer of straw. And in fire-prone landscapes dry mulch is kept away from buildings, paths and anywhere a spark could land; the protection it gives the soil must not become a risk for the site.

How Terra Humana understands mulch

For Terra Humana mulch is less a technique than a principle: bare soil is a wound. On land with five dry months a year, the question is never whether to cover the soil, but with what, and where the material comes from. The answer we follow is to produce it on site, from support plants, prunings and ground cover, instead of bringing it in from outside. That keeps the cycle closed and makes the land itself the source of its own protection.