Soil Types – Important for Plant Growth
Mineral soils are derived from the long-term chemical and mechanical breakdown of the earth’s crust, a process which has been going on for millions of years. The size of particle to which the rock has been reduced varies greatly, from coarse gravel, through sand, down to silt and clay. Few mineral soils are, however, ‘pure’, having become interlaced with the organic remains of plants and animals, in addition to a vast population of micro and macro-organisms such as bacteria, fungi, worms, and other forms of life.
All these living organisms have differing functions and in the course of satisfying their own nutrient needs they decompose or chemically change a great many of the contents of the, rendering complex chemicals into a soluble form suitable for absorption by the living plant.
Mineral soils are classified largely according to their mineral particle size and are broadly divided into sandy soils or loams, which have a predominance of large particles; heavy clay loams, which consist of a large percentage of fine particles; and medium soils or loams, which have a reasonable proportion of all particle sizes. Size of particles in different soils is as follows:
Particle size in mm
|Coarse sand||2.000 — 0. 200|
|Clay||less than 0.002|
Particle size has a profound effect on the ability of air and moisture to penetrate freely through a soil, the sandy soil allowing free movement of air and moisture through the large pore spaces, the clay soil tending to lock up moisture and air because the particles adhere very closely together. Without movement of water, plants are unable to extract it for their needs and when there is a lack of air the chemical cycle is restricted, with profound effects on the availability of plant nutrients. This is why the leaves of overwatered plants in pots frequently go yellow.
The organic matter content of the soil, coupled with chemicals, micro- and macro-organisms and worm activity can, however, greatly influence the behaviour of a soil, especially if of a clay type, by causing the particles to adhere together in a crumb formation instead of existing separately. This is due both to the adhesive effect of humic gums and to the electolytic effect of positively and negatively charged chemical elements. When this can be encouraged to happen to a high degree, a soil is said to have a good structure. Conversely, when soil is badly lacking in organic matter, or where micro-organism activity is reduced to a low level due to waterlogging or other factors, a soil may lose structure and become inhospitable to desirable plants. Organic as opposed to mineral soils are derived from plant and animal remains, fen or peat soils being classic examples, although few, if any, soils are 100% organic.
It would be wrong to make too large an issue of the ideal soil or loam consisting of a good distribution of mineral particle size, organic matter in reasonable quantity, an excellent crumb structure, and a supply of both immediately available and long-term nutrients. While such soils do exist and are capable of growing excellent plants, research work and practical growing experience in recent years have shown that plants are grown highly successfully in a very wide range of soil types, from sandy soil with negligible organic matter content, to pure peat with barely any mineral content at all, provided there is a constant supply of readily available plant nutrients.
It would be more correct to say that the nearer ‘ideal’ a soil is, the better the results from plants without so much attention and without recourse to additional nutrients. For greenhouse culture of border-grown plants, or culture in pots using soil-containing media, the same principles will apply.