Leaf mould. Mould from a tree. Peat. About 1 lb. soil from a wood, a well-manured garden and a field; also some subsoil. Six crucibles or tin lids. Six tripods, pipe-clay triangles, and bunsen burners or spirit lamps. Six beakers and egg-cups [1].
In the autumn leaves fall off the trees and form a thick layer in the woods. They do not last very long; if they did they would in a few years almost bury the wood. You can, in the springtime or early summer find out what has happened to them if you go into a wood or carefully search under a big hedge in a lane where the leaves were not swept away. Here and there you come across skeleton leaves where only the veins are left, all the rest having disappeared. But generally where the leaves have kept moist they have changed to a dark brown mass which still shows some of the structure of a leaf. This is called leaf mould. The top layer of soil in the wood is soft, dark in colour, and is evidently leaf mould mixed with sand or soil.
Leaf mould is highly prized by gardeners, indeed gardeners will often make a big heap of leaves in autumn and let them "rot down" and change into mould. If you can in autumn collect enough leaves to make a heap you should do so and leave it somewhere where the rain can fall on it, but cover it with a few small branches of trees to prevent the wind blowing the leaves away. The heap shrinks a great deal during the first few months, and in the end it gives a supply of mould that will be very useful if you want to grow any plants in pots.
Some of the little hollows in the bank under a hedge, especially on chalky soils, are filled with leaf mould which has sometimes changed to a black powder not looking at all like leaves.
You can also find mould in holes in decayed trees; here it has formed from the wood of the tree.
It appears, then, that dead leaves, etc., slowly change into a black or brown substance, shrinking very much as they do so. For this reason they do not go on piling up year after year till finally they fill the wood; instead they decay or "rot down" to form leaf mould: the big pile of the autumn has changed by the next summer to a thin layer which mixes with the soil.
We want now to see what happens on a common or a piece of waste ground that is not cultivated. Grass and wild plants grow up in summer and die during winter; their stems and roots are not taken away, but clearly they do not remain where they are, because next year new plants grow up. We may suppose that the dead roots and stems decay like the leaves did, and change to a brown or black mould. It looks as if we are right, because on digging a hole or examining the side of a freshly cut ditch we shall find that the top layer of soil, just so far as the living roots go, is darker in colour than the layer below.
We must, however, try and get some more proof, and to do this we must study some of our specimens a little more closely. We will take some leaf mould, some black mould from a hollow in the bank, some from a tree, soils from a wood, a well-manured garden, a field and some subsoil. All except the subsoil have a dark colour, but the wood and garden soils are probably darker than the field soil. Now weigh out 2 grains of each of these and heat in a dish as you did the soil on p. 4; notice that all except the subsoil go black and then begin to smoulder, but the moulds smoulder more than the soils. Then weigh again and calculate how much has burnt away in each case. Here are some results that have been obtained at Harpenden:—
Amount Percentage
Colour before of loss on Colour of
burning smouldering burning residue
Leaf mould dark brown much 78.3 light grey
Mould from
dead tree black much 60.6 light grey
Soil from
wood dark brown less 43.4[1] white
Soil from
garden almost black less 10.1 red
Soil from
field brownish still less 5.4 red
Subsoil red none 2.0 red
The mould nearly all burns away and its dark colour entirely goes, so also does the dark colour of the soil.