Last week we covered the five ped types, of which the granular type is the one best for plant growth. This type is synonymous with soils of good fertility, drainage, aeration, friability, and tilth.

Not all soils are this good unfortunately, and you may have no choice but to dig a hole into substandard soil to plant your tree. You can improve on that soil however, with time, effort and patience. This was mentioned briefly here, in the ‘Soil Properties’ section, and we’ll go into greater detail over the following weeks. Here we will explore the impact of organic matter and microbial activity on crumb formation.

Soils left undisturbed and under pasture for years have much better structure than continually cultivated soil. Grasses have a fibrous root system, which enables them to push through soils in all directions more easily than plants with the thicker roots of a tap-root system. The soil immediately around roots of any kind is known as the rhizosphere, and is always more abundant in microbial life than the rest of the soil. Thus the finer and more numerous roots of a grass makes a larger volume of soil available to microbes.

Roots grow and die, and the dead roots become a rich food source in the rhizosphere. Leaf litter on the surface eventually fragments into smaller and smaller pieces from animals breaking them underfoot, ultraviolet (UV) radiation, and the pulverising actions of rain. The very small fragments will in turn slip between the grass blades to settle directly on the soil surface. This contact exposes these fragments to soil microbes, which decompose them further.

Soil microbes include bacteria and fungi. While you may think of fungi as toadstools and mushrooms, those are merely the visible, fruiting bodies on the surface that release reproductive spores. The fungi themselves are underground, microscopic, and spread through the soil via long sticky fliaments called hyphae (singular is hypha). Hyphae are the means by which fungi feed, and one gram of pasture soil could contain 2-5 km of hyphae! A clump of hyphae is called a mycelium, and can be large enough to be visible.

The bacteria in soil, while not sticky themselves, excrete a sticky slime as they feed on organic matter (you may have seen similar material on rotting meat). 

Both the sticky hyphae and microbial slime bind organic and mineral particles in the soil together, thus initiating crumb formation. These early crumbs are loosely held together and somewhat unstable — disturbing soil every few months whilst crumbs are in this stage of development will break them up and undo this work.

But over time, as soil is left undisturbed for longer and longer, these crumbs become more and more tightly formed due to several forces. Roots as they grow and push through the soil will become thicker and thicker, and this compresses the crumbs they travel through. The organic matter and minerals are pushed closer together under this pressure, and the crumb becomes more stable. Crumbs expand and shrink repeatedly through regular wetting and drying cycles within the soil, and this also forces their components more tightly together with time. Still another factor is the presence of humus, the highly decomposed, black, jelly-like remains of organic matter breakdown. Humus is an extremely stable component of soil that binds to clay inside a crumb, further holding it together.

Organic matter will bind to sand and silt minerals, but not as tightly as to clay minerals and are more likely to fall apart. Thus the presence of clay ensures better forming and more stable crumbs. While too much clay in a soil can make for a cloddy structure, the presence of organic matter will cause some clay minerals to bind to that rather than to other clay minerals. Over time clods will become smaller and smaller as more and more organic matter takes up the surface area that would otherwise have bound to other clay minerals. Crumb formation will develop given time and enough organic matter.

Unfortunately you’ll have no choice when planting a tree in untouched ground but to disturb that soil to dig a hole. But do take the opportunity to dig in a good amount of well-composted organic matter as well. (This material must be well-composted and aged, and never fresh material like food scraps or lawn clippings, for reasons we’ll go over when discussing organic matter more deeply later.)

A tree tends not to be moved once planted, and thus the soil it is in tends also to be left undisturbed. Leave them both be, and allow grass to regrow in the bare soil. Allow leaf litter to remain on the surface. These actions together with the newly-created rhizosphere you introduced with the tree’s root system will boost the soil microbe population significantly, and allow the process of crumb formation to begin anew.

Next week we will zoom in a little closer on our crumbs, and look more deeply at their two very important components: clay and humus.