SOILS, Watersheds, and Plant Communities

JEAN D. WHEELER, PROFESSOR EMERITA
GEOGRAPHY DEPARTMENT
CALIFORNIA STATE UNIVERSITY, LONG BEACH

What is Soil?

much water at the expense of air.  We think plants take in carbon dioxide and give off oxygen, which green plants do overall.  But the roots of plants consume rather than produce oxygen and the roots of most garden plants need to get it from air in the soil (not from oxygen dissolved in soil water).  Soil is produced by the interaction of five soil forming factors:  parent material (ultimately weathered out of bedrock), climate, topography, vegetation and animal life, and time.  These factors not only cause variations in the proportions of minerals, organic matter, water, and air but also cause soils to vary in such factors as color, texture, structure, and pH (acidity versus alkalinity).  Most of these have very important implications for gardeners.

Topography:  On steep slopes, soil is very thin and coarse textured, because fine textured minerals and organic particles are quickly washed away down to the valleys below.  On mountainsides, the soil may be very little more than somewhat weathered parent material.  On gentler slopes, the finer materials are deposited, and resulting soils are deeper and finer textured. Flat bottomlands often have nothing but the finest sized particles and are excessively heavy.  On flat upland semiarid areas, where rainwater tends to sink in a few feet rather than run off, fine particles and dissolved calcium tend to be washed out of the surface soil to be deposited a few feet down where the rainwater evaporates into the soil air.  This can form a layer of fine-textured particles and lime mimicking cement and known as hardpan.  The best soils tend to occur on gently sloping land.

Vegetation and Animal Life:  This is, of course, the source of the organic component of the soil. Dead leaves, tree branches, fallen tree trunks, and animal feces and remains become mixed with the surface soils. These are acted upon by insects, earthworms, and soil micro-organisms to produce fine organic particles and, ultimately, organic minerals and gases in the soil which can be recycled by the roots of the vegetation into living plants and animal life.  The nature of the biotic complex living in and above the soil can have quite an impact on the nature of the soil.  Particular kinds of plants differ greatly in which soil nutrients they require in the greatest amounts and tend to keep those they need most cycling between the soil and the living vegetation.  Meanwhile soluble minerals less needed by the local vegetation are more likely to be lost to leaching.  Needleleaf evergreen forests use the more acid types of minerals and less calcium and other alkaline minerals, so they tend to produce acid soils deficient in calcium. Grasses, on the other hand, require great amounts of calcium, so grassland soils tend to keep calcium cycling in the topsoil.  Most of our staple crops are themselves members of the grass family (cereals like wheat, rye, barley, oats, corn, and grain sorghums) so the fact that prairie and steppe grassland soils are considered fertile and soils under pine forests infertile should come as no surprise.

Time:  All this movement of particles and dissolved minerals through the soil (downward leaching by rainwater or upward capillary action from groundwater) to mix organic matter and soluble minerals through parent material in various layers in the soil requires time.  The amount of time a particular parent material has been exposed to the altering processes of climate, topography, and biotic activities is therefore also regarded as a factor in determining what a soil will be like.  "Immature" soils usually reflect the overwhelming influence of parent material.  As time goes on, the effects of climate, topography, and vegetation and animal life become more and more dominant and the influence of parent material much less so. The great soil regions or "zonal soils" of the world are based on fully mature soils in which the influence of climate and natural vegetation are overwhelming.

How Important is Soil Color?

Frankly, not very.  It can indicate something about the soil-forming processes that have been active in an area.  However, relying too much on color can lead to serious misconceptions about fertility.  Dark brown or black colored soils are often assumed to be very fertile because those colors often come from organic matter in the soil, usually good for gardens plants and crops.  However, some soils with very little organic matter may still look very black because they are are immature soils derived from parent material dominated by very black minerals, and they may even have some minerals present in toxic amounts.  Then again, if the black color is from organic matter, the soil might have way too much raw humus which is too acid to be good for most crop or garden plants.  Red and yellow colors usually come from oxidation of iron compounds in the soil.  Such colors are common among excessively leached soils of southeastern United States or the tropics.   However, in desert areas these colors may be inherited from bedrock parent material in which the rock formed in more hot and humid past geologic time while the soil on them today is anything but leached.

The chart below shows a number of minerals our crop and garden plants need. Each mineral can most easily move from soil particles into solution in soil water and be taken into plant roots where the black band for that mineral in the chart is widest.   Where the band is narrowest, plants will have great difficulty obtaining that mineral from the soil, even though it is present.  It will be bound too firmly to the soil particles.

However, some species have evolved in areas of somewhat more alkaline soils or rather acid soils.  These plants may require much higher amounts of minerals best available in that pH zone and can tolerate very low levels of nutrients limited in availability there.  For example, azaleas and rhododendrons evolved in cool, wet climate areas which tend to have acid soils. They need high amounts of minerals best available under acid conditions, notably iron.  Their leaves may turn pale or even yellow from deficiency of iron when grown in soils that are slightly alkaline, such as those of semiarid or desert climates in California.  Gardeners trying to grow them in most of our state add much organic compost to raise these plants because compost yields  organic acids as it decomposes.   Gardeners coping with soils excessively acid for the plants they want to grow add lime to the soil, as it is alkaline in pH and can neutralize an acid soil.  Heavy watering will also help to leach out excessive alkaline minerals.

Natural vegetation can have a strong impact on soil pH.  Needle leaf evergreen trees such as pines and firs produce very acid leaf litter.  Soil sampling in the Sierra Nevada where lodge pole pines were invading grassy meadows showed a sharp drop in pH under the branches of lodge poles only 6-7 years old as compared with the general pH of the grassy meadow, and grasses were dying under the pines.  The Central Valley of California and the Great Plains have neutral to slightly alkaline soils because they have developed for a long time under grasses.  Grasses need very high amounts of calcium, which is alkaline in its chemical reaction.  Desert soils can be so high in alkaline minerals that they may need much heavy watering, with drainage pipes below the topsoil to carry away the water.  This leaches out the alkaline minerals and also the chloride salts such desert soils often have at the surface.  Growing and plowing under crops such as alfalfa for several years can also help neutralize an alkaline desert soil by providing organic matter and the acids such organic matter yields.

Although the soils in our gardens may have developed their peculiarities over a very long time, there is much we gardeners can do to improve our soil and customize it for the particular plants we want to grow.  The main thing is to recognize the nature of the soil we start with and the requirements of the plants we want to grow.  By digging deeply and shaking loose the soil when it is neither too wet nor too dry, by working in compost and other appropriate soil amendments, and by adjusting our watering regimen to our soil texture and structure as well as to our plant needs, we can produce a soil more moderate in texture, granular in structure, and adjusted in pH for the ornamentals or crops we desire.

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