The structure of the soil depends on how well the textural components (or particles) are joined together. The presence of organic matter and clay causes the particles to clump together into “crumbs,” and connecting them is a network of pores, through which water, dissolved nutrients, and air can circulate.
soil structure influences the supply of resources to plant roots through water retention, nutrient supply, aeration, rain infiltration, and drainage. overall, it determines the soil’s productivity. approximately 60% of well-structured soil is made up of pore spaces; in poorly structured soils this can be as low as 20%. The action of roots, worms, and microorganisms plays an important role in soil structure, as does the shrinking and expanding of the soil in hot and cold weather. cultivation helps to improve soil structure.
The inclusion of well-rotted organic matter is one way to improve soil structure, particularly if one particle type (sand, silt, or clay) is in abundance. It helps clay soils to “flocculate” (form crumbs), as does the addition of lime and gypsum. The presence of calcium ions makes for good flocculation, as they are attracted to the negatively charged clay particles. soil structure is at its most vulnerable when wet, because the flocculants (the “glue” that holds the crumbs together) become soluble. Just walking on wet soil can damage its structure, resulting in compaction
blue plantain lily (Hosta ventricosa). Hostas need a fertile,
matter and recycle nutrients. While some plants are not too fussy about soil pH, some are very specific. Plants that only grow on acid soil are known as “ericaceous,” and they include rhododendrons and blueberries
(Vaccinium). some plants have a preference but not a requirement; for example, many fruit trees produce higher yields on a slightly acid soil (approximately pH 6.5).
The pH of a soil is literally set in stone, by its bedrock. It rarely fluctuates or changes. It is possible to influence the pH of a soil by incorporating mineral additives, such as sulfur to acidify a soil, or lime to increase the pH of a soil, but it can be expensive and over a relatively short time the pH will revert. The addition of organic materials with a high or low pH—such as spent mushroom compost (alkaline) or pine needles (acid)—often has little impact on soil pH. The best thing for gardeners to do if the plants they want to grow are not suited to the soil pH is to grow them in containers, using a potting compost that suits the requirements of the plant.
you can easily test the pH of your soil by buying a testing kit from your local garden center. Take a soil sample a few inches deep from several points in the garden so that you get an average reading. If it is a big garden, there is a chance that soil pH may vary from one place to another.
T
he term pH will be heard a great deal by gardeners. It is a measure of acidity and alkalinity and is expressed on a scale of 0 to 14, with a value of 0 being very acidic, 7 neutral, and 14 very alkaline. Most soils tend to range from 3.5 to 9. Theoptimum pH range for most plants is between 5.5 and 7.5. “chalky” and “lime-loving” are terms associated with alkaline soils, and “ericaceous” and “lime-hating” are used in reference to acidic soils.
soil pH is largely controlled by the parent rock and the mineral ions that leach from it. Magne- sium and calcium ions are the most significant, and on soils rich in calcium, such as chalk or limestone, the pH tends to remain
high (alkaline). Many soils are unable to fall below pH 4, and alkaline soils rarely exceed pH 8.
The acidity and alkalinity of a soil profoundly affects its behavior, mainly because it controls the solubility of different minerals. This means that at different pH levels, some minerals are available to plant roots, and others are not. This is why some plants will grow on acidic soils (these plants are called calcifuges) and others will not (calcicoles).
soil pH also has an effect on soil structure (through the availability of calcium ions, see p.143), and on the activity of soil organisms that break down organic
S o i l p H
Vaccinium uliginosum, bog bilberry
Moderately
alkaline Slightly alkaline Neutral Slightly acidic Acidic acidicVery acidicVery
8.5 8.0 7.0 6.5 6.0 5.0 4.0
E x T E r n a l F a c T o r s
humus becomes less sticky and better aerated. soils contaminated with heavy metals are sometimes treated with organic materials, as they can form strong chemical bonds with the heavy mineral ions, reducing their solubility.
Humus can hold up to 90% of its weight in moisture, and so its presence will help to increase a soil’s water capacity and nutrient retention. soils rich in humus will take on a much darker color, which has a beneficial effect in spring as a dark soil will absorb more of the sun’s energy and hence warm up more quickly.
The rate of humus breakdown is influenced by many factors. conditions such as extreme acidity, waterlogging, or nutrient shortages can inhibit the action of microorganisms, which leads to a build-up of surface litter and, in extreme and site-specific cases, peat. such soils can be improved by the addition of lime, fertilizer, and better drainage.
as a general rule, forest soils have the highest levels of organic matter, followed by grasslands and then
farmland. sandy soils have less organic matter than clay soils. an easy and
effective way for gardeners to increase the organic content of their soil
is by spreading any bulky material such as compost, leafmold, and well-rotted manure. This can either be dug in or spread as a mulch, leaving the worms to
take it into the soil.