soil characteristics?

Answer 1

Soils are made up of four basic components: minerals, air, water, and organic matter. The minerals, rated by particle size, make up the texture but it requires all four components to create good structure.
Optimum soil mineral percentages are: Sand 30-50%, Silt 30-50%, Clay 20-30%, Organic material 7-10%. So good soil generally consists of 90-93% total mineral and 7-10% bio-organic substances.
The bio-organic parts are ~85% humus, ~10% roots, and ~5% edaphon.
The edaphon consists of microbes, fungi, bacteria, earthworms, micro/meso fauna, and macro fauna. Insects, earthworms, and other arthropods churn the soil and promote formation of soil structure, aeration, and nutrient cycling through degrading plant & animal detritus.
The last two components, air and water, must be able to move freely though the soil. Once soil is compacted air and water are excluded destroying the healthy structure. Minerals represent only around 45% of the total soil volume, water and air each occupy nearly 25%, and organic matter from 2% to 5%.
Some of the nutrients Nitrogen/Phosphorus/Potassium get lost through leaching in wet weather especially in sandy soils. Highly weathered soil tends to become very acidic. High pH limits certain nutrient availability. Acid soils can lead to deficiencies of phosphorus, calcium, magnesium and molybdenum, as well as toxic levels of manganese and aluminum. Soils high in sand or silt do not retain water well so plants that live in these soils must tolerate periodic drought conditions if rainfall is low.

The mineral content of the underlying parent rock from which the soil was formed determines acidity or alkalinity of the soil. Soils developed from basic rocks generally have higher pH values than those formed from acid rocks.
Soils that develop from weathered granite are likely to be more acidic than those developed from shale or limestone. Granite, it decomposes into a sandy soil made up of quartz grains. Basalt decomposes more rapidly to clay minerals than iron compounds and forms soil that is generally darker and less permeable to water than granitic soil.

Clay in the soil comes from the weathering of feldspars. Such soil is usually permeable and can easily nourish vegetation. Clay can act to retain the nutrients sand looses but also excessive amounts of water once the clay exceeds 30%. This tends to drown or allow disease in plants intolerant of constantly wet roots. Clay swells with water in winter but once it dries it shrinks and can form hard cracked plates. High clay content may also have high pH. Alkaline soils may lead to deficiencies in iron, manganese, boron, copper and zinc. These will limit the kinds of plants able to grow in these conditions.

Another soil type is high in organic matter rather than mineral. These occur in peat rich soils. The soil may be very acid, very wet in winter but dry in summer, and it may be fairly infertile due to the high pH. Unlike most other soil types peat rich soil is more than 20% organics so holds water like a sponge but can dry out totally in summer then it can be difficult to re-wet once it is dried out.
Most plants grow in the pH 6.0 - 7.0 range. A pH of 6.5 is neutral in soil as most plant nutrients are available for uptake by the roots at that pH level. Soil neutrality is measured using pH that is generally considered neutral at pH7 but soil has a scale based on all nutrients not just hydrogen mobility.
High acid soil is pH5.0
medium is pH5.5
slight is pH6.0
neutral ranges from pH6.5 - 7.0
mildly alkaline is pH7.5 while
pH8 is moderately alkaline

Answer 2

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