To the practical gardener, the origin of his soil is of interest, but not vitally important. What is important to the gardener is the nature of his soil, wherever it came from: whether it is light, meaning composed of large particles like sand; heavy, meaning composed of very small particles like clay; or something in between. It is important to know: whether it is organic soil, which means it is composed of decaying vegetable matter; whether it is acidic or alkaline — sand is inclined to be acidic, clay alkaline; whether it is naturally well drained or not; what lies underneath it—soil above limestone is very likely to be alkaline.
Fortunately, whatever your soil is like, you can improve it. There is scarcely a soil in the world that will not grow good crops of some sort if it is properly treated. Excess acidity is easily remedied by adding lime; excess alkalinity by adding compost or manure. Waterlogging can always be cured by drainage. Trace-element deﬁciencies can be cured simply by adding the missing trace elements.
Above all, everything about your soil can be improved by the addition of one thing: humus.
Humus is vegetable or animal matter that has died and been changed by the action of soil organisms into a complex organic substance that becomes part of the soil. Any animal or vegetable material, when it dies, can become humus. Humus has many beneﬁcial effects on the soil. All of the following have been established experimentally by soil scientists—they are not just the optimistic conjectures of a humus enthusiast: humus protects soil from erosion by rain and allows water to percolate gently and deeply; it reduces erosion by wind; its slimes and gums stick soil particles together and thus turn a very ﬁne soil, or clay, into a coarser one; it feeds earthworms and other useful soil organisms; it lowers soil temperature in summer and increases it in winter; it supplies nutrients to plants, because it contains all the elements that plants need and releases them slowly at a pace that the plant can cope with; it enables the soil to hold water like a sponge, and minimizes the loss of water by evaporation; it ensures that chemical changes are not too rapid when lime and inorganic fertilizers are added to the soil; it releases organic acids that help to neutralize alkaline soils, and help to release minerals from the soil, making them available to plants; it holds the ammonia and other forms of nitrogen in the soil in an exchangeable and available form—without it, nitrogen is lost quickly because of the action of denitrifying bacteria; it keeps down many fungal diseases and the notorious eelworm.
Clearly one of your main aims as a self-sufﬁcient gardener should be to increase the humus content of your soil as much as possible. Soils ranging from the heaviest clay to the purest sand can be improved and rendered fertile by the introduction of sufﬁcient humus. There is no soil that does not beneﬁt from it, and there is no crop, that I know of, that is not improved by it.
Now, any organic material that you put into the soil will produce humus. Compost, green manure, farmyard manure, human excreta, peat, leafmold, seaweed, crop residues: anything that has lived before can live again. Bury it in the soil and it will rot and make humus. Leave it on top of the soil; it will rot, the worms will drag it down, and it will still make humus.
Humus is the ﬁrm basis of good gardening. It is possible to grow inferior crops on humus-deﬁcient soil by supplying all your plants’ chemical require- ments, mainly in the form of nitrates, out of a fertilizer bag, but if you do this, your soil will progressively deteriorate and, ultimately, blow or wash away, as the topsoils of so much of the world’s surface, abused by humankind, already have.