Broadly, garden plants consist of two different but integrated structural systems—the above-ground system of stems, branches, green leaves, flowers and seeds; and the below-ground system of roots and root hairs, arranged about a common axis.
Though complementary, the root system is fundamentally the more important to plant performance. Apart from anchoring a plant in its environment, it has to supply it with water to sustain its functions and growth.
And with a wide range of nutrient mineral elements, largely as soluble salts, to help form cell and organic structures and enable its leaves to manufacture foodstuffs photosynthetically for further healthy function and growth.
This flow of watery nutrient solution or sap from root hair to leaf cell influences growth profoundly, and in quality and quantity is decided by the ease with which the roots can penetrate the soil and tap its resources.
In turn this depends upon the nature and condition of the soil; the thin surface skin of earth on which plant, animal and human life depends.
Soils and Their Nature
Individual soils will vary considerably in composition, texture, structure, color and kind, but have five properties in common:
1) A mineral content of rock particles of very variable sizes.
2) An organic content of humus forming from dead and decomposing remains of plants and animals.
3) Water from rains.
4) Air filtering in and through from the atmosphere.
5) A flora and fauna of tiny plants, algae and fungi, and of bacteria, microorganisms, earthworms and insects. From the ferment of their biological and chemical interactivity comes the capacity of a soil to nurture plant growth, or its fertility.
The dominant and permanent part of a soil is its mineral content. It usually consists of a mixture of particles varying in sizes from stones and gravel, through coarse and fine gritty sands, smoother, silky silt to the—very fine mud smearing particles of clay.
The mineral particles, by their kind and proportions, determine the texture of a soil, and soils are classified accordingly.
Soil particles vary in shape as well as size. They do not lie solidly close together. There are interstices or spaces, known as the pore space, through which roots grow, and air and water circulate.
The larger the soil particles, the larger the spaces between; the more quickly water percolates and drains through, and the more easily roots can grow, and the more readily they absorb warmth from the sun and lose it again.
For convenience mineral particles are classed as:
1) Stones and gravels when larger than 2mm in diameter.
2) Sands when less than 2mm but greater than 0.02mm.
3) Silt when less than 0.02mm but greater than 0.002mm.
4) Clay when less than 0 002mm down to ultra-microscopic sizes.
They are an important source of mineral elements needed by plants. But whereas gravels, sands and silt are rather inert and very slowly soluble, clay is different.
Its minute particles are more chemically active, and when wetted become colloidal or glue-like and hold soluble plant nutrients well. Any soil containing clay is inherently more fertile than one without.
Soils are designated according to the class of particles most predominant in their mineral make-up. Soils containing 80 percent or more by weight of the coarser particles are either stony, gravelly or sandy.
They are light and easy to work, open-pored, drain readily of moisture, and are well-aerated. Soils containing 20 percent or more of the finest particles are clayey, heavy to work, fine-pored, dense, sticky, retentive of moisture, ill-aerated and slow to warm under the sun.
Soils containing little sand or clay but a preponderance of fine particles are silts, with fine pore spaces, lacking the openness of sand and the stickiness of clay, tending to consolidate inert, infertile and unresponsive unless well managed.
The better balanced the mineral matrix, however, the less extreme the soil characteristics, and it is termed a loam, though it may be a sandy loam, a clay loam or a silt loam, according to which group of particles remains most influential in its texture.
The Organic Content
The purely mineral part of a soil is infertile and cannot nurture garden plants. It needs an organic content, derived from the dead remains of plants, animals and myriads of microorganisms accumulating and actively decomposing in the soil.
This material is food for numerous soil organisms and bacteria which break it down into small fibrous particles and brown and black amorphous substances known as humus.
The organisms—bacteria, minute fungi, earthworms—themselves excrete compounds of value to plant life, release nutrient elements from the organic matter, and in dying decompose and perpetuate the buildup of a soil’s organic content.
The fine fibers and organic particles help to fill in the large pore spaces between the larger mineral particles, and being absorbent, slow the movement and loss of water and soluble nutrients from the soil.
But the key to soil fertility is humus. This is a highly complex substance or entity; largely composed of colloidal particles, extremely minute, and forming a gel that can enter all pore space, and transform inert soil into a biochemically reactive living earth.
Physically, it acts as a weak cement, grouping mineral particles together; particularly those of clay, into crumbs or granular structure.
When wetted it swells and holds moisture to the benefit of open sandy soils, while in clay its formation of crumb structures or aggregates of soil particles improves aeration and drainage.
Humus also darkens soils, so that they absorb the sun’s heat more readily and retain it longer.
Biologically, humus is a center of micro organic activity; chemically it excites the release of soluble elements from all soil particles, and is itself a reservoir of nutrient for plants which roots reach into and exploit.
Eventually, humus itself is broken down to simpler residues, such as nitrogen and carbonic acid, and disappears. Other things being equal, the more humus a soil contains, the more fertile it is.
Unlike the mineral matrix, the organic content of a soil is subject to fluctuation, and under cultivation, usually needs building up.
The Role of Lime
Another critical factor in soil fertility and plant growth is the soil’s content of lime. Lime controls the acid-alkaline balance in the soil and of the nutrient soil solution on which plant roots feed.
Rains percolating through the soil slowly but steadily leach or wash lime out, and it becomes more acid. Increasing acidity is discouraging to the activity of many soil organisms breaking down organic matter.
It is also detrimental to the growth of most garden plants and crops, with the exception of certain plants, chiefly rhododendrons and related members of the Ericaceae or Heath family, native to acid, lime-free soils.
Most garden plants grow best where the soil contains some lime and is neutral or only slightly acid overall.
Too little lime not only denies plants the essential calcium as a nutrient but allows—other elements of a toxic nature to become more freely available.
Too much lime can also adversely affect many plants, and some soils, derived largely from chalk or limestone rocks, need cultivations that will increase acid reactions. It is, however, easier to counter soil acidity than alkalinity.
With this background knowledge, it is now possible to assess your type of soil, identify its good and bad characteristics, and plan its improvement, for few garden soils are anywhere near perfect.