Soil and Why Plants Need It
THE ROOTS of many plants are not limited to the so-called ‘top-’. To learn the conditions their roots will have to meet it is necessary to look beneath the surface by digging down to about 3 ft. or even more to disclose the soil ‘profile’.
Most soils dug in this way show a series of definite layers, one above the other, with different colours and different properties. These layers have been produced through the centuries by the action of climate and vegetation on mineral matter, and are known collectively as the soil profile. This is the basis of soil classification. The main things to look for when examining a profile are depth, texture, structure, colour and drainage.
A soil profile usually consists of two or more layers lying parallel to the land surface. For instance, often a foot of sandy soil overlies several feet of clay soil. The upper layer, which is generally called the top-soil, is, as a rule, darker in colour than the lower layers owing to its higher humus content. In this upper layer life in such forms as bacteria, fungi, insects and other small animals is most abundant. Being near the surface it receives the rainfall first, so that its nutrients and lime are washed down and lost more easily than in deeper layers. In addition, the humus slightly acidifies the soil water, so that its solvent power is in-creased, and a number of salts are washed out. It is in this way that lime is lost, so even in shallow, chalky soils it is not unusual to find the upper layer acid. On steep slopes the upper layer is commonly thin, and on building sites it is often completely destroyed by the builders, or covered over with raw earthy material from excavations.
This is the name usually given to the layer below the top-soil. It is generally harder to dig when dry, and is much stickier when wet, owing to its higher clay content, much of which has been washed out of the top-soil. The subsoil often has a brighter colour, due to the washing down of red oxides of iron and other elements, which in some cases collect and cement the soil particles together into hard continuous layers known as pans. These, of course, very effectively prevent roots from penetrating any farther.
Below the top-soil and the subsoil — the layers which constitute the true soil — lies the mineral matter of geologic origin which may or may not be the parent material of the soil above. This mineral matter may be loose and porous to great depths, and may have accumulated in place by the breakdown of hard rocks, or it may have been moved to its present position by water, wind or ice. Both this layer and the one above are poorer in humus and in organic life than the top-soil, and when dug should only be brought to the surface bit by bit for fear of spoiling the top-soil. Deep soils hold more moisture and nutrients than shallow soils. Plants will grow in shallow soils but, just as when grown in pots, do need more frequent feeding and watering than when grown in deeper soils.