Soil parts and functions

Soil is the basis for essentially all agriculture (hydroponic farming would be one exception), and a good, fertile, “healthy” soil is vital for growing healthy, nutritious crops.  As covered in the Agricultural Ecosystem weblink, crop plants are eaten by humans and some animals, so most of the life on earth depends on soil, directly or indirectly.  Yet we seldom appreciate the real value of this precious natural resource.

Through misuse and abuse, about 7 to 10 tons of topsoil per acre (an acre is about as large as a football field) are being lost to erosion each year in the American Midwest (the amount can be much higher in the worst areas).  It may take several hundred years for one inch of soil to form under natural conditions; thus, the high rates of soil loss (and toxic pollution of soil) threaten our children’s future as much or more than nuclear war or climate change.

 What is soil?

Soil is basically weathered, broken-up rocks from the earth’s crust, ranging from just sand to rich loam to peat (mostly partly-decayed plant matter).  But if we concentrate on typical, more-or-less good quality agricultural soils, we find that typical soils are composed of four parts:

  1. Minerals (about 45%), which include the rock particles ranging in size from gravel and sand to finer silt and clay, as well as the potentially soluble mineral nutrients that plants may use, such as calcium, magnesium, potassium, phosphorus and the trace minerals (zinc, copper, iron, manganese and others).
  2. 2.      Water (about 25%), which is found on and between the soil particles and which is used by plants and soil organisms.
  3. 3.      Air (about 25%), which provides oxygen to roots and soil organisms, as well as nitrogen to certain microbes which can convert it to forms useable by plants.
  4. 4.      Organic matter (about 1 to 5%), which includes the living soil organisms and the dead organic matter.  When organic matter has been decomposed (broken down) to very tiny particles, it is called humus.

 Soil organisms

Good soil is really “alive” with mostly tiny organisms.  According to one estimate, one teaspoon of rich grassland soil could contain 5 billion bacteria, 20 million fungi and 1 million protozoa and algae.  That’s nearly as many microbes as there are humans on earth!  Most soils also have some roundworms, earthworms, mites and insects.  Some of these critters eat each other, but most, especially earthworms, bacteria and fungi, help break down organic matter (such as manure and plant remains) to form humus.  Not only does the humus improve the soil’s structure [see next section], but it also is a storehouse of certain plant nutrients, especially nitrogen, phosphorus and sulfur.

Certain bacteria in the soil are able to change nitrogen from the air into forms that plants can use (nitrate and ammonium).  Some of these “nitrogen-fixing” bacteria live inside root nodules of legumes.

Some fungi, called mycorrhizae, live in or on the roots of most plants, where they aid the roots in absorbing water and certain nutrients (nitrogen, phosphorus, zinc, copper and others).

The roots of healthy plants in good soil are covered by certain beneficial bacteria and fungi which protect the roots from many disease-causing microbes.  The roots repay the beneficial microbes by producing carbohydrates that the good microbes use for food – a type of symbiotic relationship.

Soil structure

In a good soil, the soil particles are not found in a compact mass, but instead are clumped together into small clusters called crumbs or aggregates, with spaces for air and water between them.  You could compare a good soil’s structure to the texture of cake.  The very tiny particles (clay, silt) are held together in aggregates by sticky glue-like substances produced by plant roots and soil organisms.  Humus is one of the best sources of materials that improve soil structure.  The burrows of earthworms and ants also contribute to a good, loose soil texture.

Good soil structure not only allows air and water to enter, it also allows plant roots to grow more easily, and it soaks up precipitation readily.  In contrast, a soil with poor structure tends to be compact and hard, with little air.  Roots have trouble growing, and rain may run off rather than soak in (high runoff often causes serious soil erosion and damaging floods).  Crop root pests and diseases often flourish in hard soils.

In most soils, the uppermost several inches (the topsoil) have fairly good structure, while below that, the subsoil is usually denser, with poor structure.  In agricultural soils, there is often a very hard layer below the topsoil called a hardpan (or plow pan or claypan).  A hardpan usually forms when a farmer plows or tills the soil down to a certain depth, with soil particles just below being compacted by the plow or other tillage tool.

 Soil acidity

Most farmers and gardeners worry about how acid or alkaline their soil is.  They have been told that acid soil is bad, and that they should put lime on it to “sweeten” the soil, or correct the acidity.  Actually, it’s a bit more complicated than that.

Acid and alkaline conditions of soil (or anything else) are measured on a scale of numbers ranging from 0 (most acid) to 14 (most alkaline), with 7 being neutral (neither acid nor alkaline).

soil structure acid versus alkaline


This is called the pH scale, named for the amount of hydrogen ions (H+), which cause acidity.  Examples of the pH numbers of some common materials include: lemon juice = 2.0, black coffee = 5.0, pure water = 7.0, sodium bicarbonate solution = 8.2, ammonia water = 11.0.  Typically, soil pH varies from 4 to 10, but most crops do best under slightly acid conditions (pH 6.0 to 6.8).  Soil pH affects the availability of nutrients as well as what types of soil organisms can grow (most bacteria do not live in very acid conditions, while many fungi can).  You can measure soil pH with special paper test strips or an electronic pH meter.


Most plants can grow fairly well under a range of pH conditions, but again, from 6.0 to 6.8 works best for most plants.  Blueberry, cranberry and rhododendron are some plants that prefer very acid soils, while asparagus, beet and onion do well in alkaline soils.  Soils in the western half of theUnited Statestend to be alkaline, and those in the eastern half are usually acid.


For most crops, the thing to avoid is extremely acid or extremely alkaline conditions, since these can reduce the availability of several nutrients (such as phosphorus, nitrogen, potassium, sulfur, calcium and magnesium) to the plants, thus creating nutrient deficiencies, poor growth and unhealthful food for animals and humans.


Too acid soil is usually corrected by adding some kind of “liming material.”  The most often used is ground limestone, or “lime” (but generally not the harsh products, quicklime and slaked lime).  Other forms of lime are ground oyster shells, basic slag and marl.  The more finely the material is ground, the faster it will neutralize acidity (a mixture of fine and somewhat coarser material gives both fast and long-range effect).  Since most liming materials contain calcium, it happens that they also supply this plant nutrient, usually very beneficial for plant growth and nutritious food.  Wood ashes can be used in a garden, but they do not contain calcium (they will supply potassium).

Too alkaline soil (above pH 8) can be made more acid by adding gypsum, iron sulfate or sulfur.

Another good way to improve soil with extreme pH is to add organic matter, usually compost, since compost tends to buffer acid soils and increase acidity in alkaline soils [see the link on Composting].

 Soil aeration

One aspect of agricultural or garden soils that is often overlooked is having an adequate supply of air (or oxygen), since both plant roots and most of the beneficial soil organisms need oxygen.  Well-aerated soil is sometimes called aerobic, while soil with little or no oxygen is called anaerobic.  Without much oxygen, roots can die, and certain types of microbes which can cause disease (mainly certain bacteria) tend to flourish.  Also, toxic substances can be formed due to several processes.

Soils that are dense and compacted, or which have a crust on top, are most likely to be anaerobic.  Another situation leading to anaerobic soil is too much water, which fills up soil pores and excludes air.  Such waterlogged soil can also result after a rain when there is a hardpan that reduces water infiltration into the topsoil.  An adequate level of humus should improve the soil’s “crumb structure” and reduce anaerobic conditions.

Thus we see that ordinary soil, which many dismiss as “just dirt,” is actually a very complex and dynamic substance, responsible for the well-being of the earth’s inhabitants.