What is Soil?

         Although a lot of people think of soil just as "dirt," soil is one of the most complex features nature produces.  It is a blend of minerals, organic matter, water, and air.  The proportions of these in a particular soil have a lot to do with how well it will support the garden plants we want to grow.  A good proportion is shown in the graph at the right.  4 Components About half should be solids and half spaces between solids, with the pore spaces about evenly divided between water and air.  A desert soil has too much mineral content with too little organic matter and too much air with too little water.  A swampy soil, on the other hand, has too much organic matter at the expense of minerals and way too 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.

How Do the Five Soil Forming Factors Work?  

Parent Material:  This yields the mineral content of the soil.  Parent material can be residual, which means it weathered from bedrock right where we now find the soil, or it can be transported parent material.  That means it weathered out of bedrock somewhere else and was carried here by an agent of erosion and deposition, such as a river, glacial ice, ocean currents, or wind.  A lot of our home gardens and farms on the Central Coast are built on transported parent material such as alluvium (deposited by rivers), beach sand (deposited by ocean waves), or dune sand (piled up from beach sand by the wind).  But some of us live on residual soils weathered from underlyling rock (usually sedimentary rock here on the coast).  Beach and dune sand tends to consist mostly of quartz crystals, with other minerals washed away, so soils developed from those parent materials tend to be coarse textured and low in soluble minerals neede5 soil forming factors d by plants. Soils whose parent material was weathered limestone are much less likely to be deficient in calcium, which many garden plants need in large supply. Soils derived from granite or sand are normally low in calcium content. Soils derived from shale or limestone are likly to be fine textured, possibly "too heavy."

Climate:  Soil develops as rainwater washes downward fine mineral and organic particles and dissolved soluble minerals, depositing them in lower layers of the soil, a process called leaching.  Heavily leached soils in hot and rainy climates tend to be deep.  They can quickly become coarse textured and low in soluble nutrients at the surface if constant recycling of nutrients between the soil and vegetation is interrupted by destruction of rainforests for farming.  Desert soils are thin because there is so little rain to move organic matter and soluble minerals downward to create soil from parent material, and little organic matter is present to be carried downward because dryness so limits the vegetation to supply it.  In fact, groundwater may move upward by capillary action rather than downward through desert soils, carrying dissolved minerals upward to be deposited as soil water evaporates at the surface.  This makes the topsoil alkaline and/or salty.  In hot climates, soil micro-organisms very actively attack organic remains on and in the soil, releasing 
gases and soluble liquids which are easily leached downward.  This can lead to deficiency in organic content if it is not constantly recycled by vegetation, such as rainforests.   But in cold climates, soil micro-organism activity is so sluggish that little organic matter is processed, and instead it accumulates, making the topsoil very acid.   The best natural soils for farming and gardening tend to be those of moderate climates, neither too hot nor too cold, and with sufficient rain but not too much.  

 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 mimicing 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 vegatation.  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 alkalin 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 vegatation 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.

What About Soil Texture?

SParticle Sizes oil texture is very important in farming and gardening.  Texture refers to the sizes of mineral particles present in the soil.  The drawing at the left shows the three size categories of soil particles all magnified by about the same amount as the head of a dressmaker's pin shown at the bottom.  Sand particles are visible with the unaided eye and range in diameter from 0.05 to 2.0 mm.  Silt particles are too small to be seen without the aid of an optical microscope, ranging in size from 0.002 to 0.05 mm in diameter.  Individual clay particles, smaller than 0.002 mm in diameter, are so tiny that they cannot be seen even with an optical microscope, requiring an electron microscope.  Soil texture is measured by the proportions of thesePie Charts Texture three sizes of materials in the soil.  Three examples are shown in the pie charts at the right.  A soil with  moderate amounts of all three particle sizes in a good proportion for most crops is called a loam.  A mixture with more sand and less silt and clay is called a sandy loam, while a mixture with more clay is a clay loam.  Other variations (not shown) have such names as silt loams, sandy silt loams, silty clay loams, etc.  The more coarse textured a soil is (such as sandy loam, loamy sand, and sand) the more easily and rapidly it can absorb water during rain or irrigation, but the less water it can retain near the surface against the pull of gravity to be available to plants between waterings.   So a sandy soil tends to dry out fast.  The more fine textured a soil is ( such as clay loam, loamy clay, and clay), the slower it is to absorb water, allowing it to puddle on top the soil or runoff without sinking in if the soil is sloping. However, once the rain finally soaks in, fine textured soils do retain large quantities of water near the surface against the pull of gravity downward.  This could provide water to garden plants in dry spells.  However, very fine textured soils (also called heavy soils) may retain so much water as to be waterlogged, with plants roots "drowning" for lack of air in the soil to provide the oxygen the roots need.  The fine soil particles also hold water to themselves so greedily that plants may wilt from inability to get it from the soil even though the soil still has quite a bit of water present.  Good gardeners learn how their soil reacts to watering and plan their irrrigation accordingly.  Sandy soils require more frequent watering with substantial amounts of water, while heavy soils need somewhat less frequent and abundant watering to avoid killing plants with waterlogged soil.  Of course, it is also a good idea to improve soil texture with soil amendments, such as adding fine-textured organic compost to sandy soils or acquiring sand or other coarse-textured materials and digging them into clayey soils.

What is Soil Structure?  

Soil particles usually do not slide around independently of one another in the soil.  Instead, they are held together in small clumps by the stickiness of clay and organic matter in the soil.  These clumps can form in different sizes and shapes, giving a characteristic pattern in the soil called soil structure.  Some structures are more favorable to penetration by air, water, and plant roots while other structures may make it more likely that rain water will run off than sink in or may force roots to detour around clumps to cracks in the soil.  Platey Some soil clumps are flat and horizontal to the soil surface, a condition called platey structure, illustrated in a soil photo at the left.  As rain begins, the splash of raindrops flattens the soil clumps still further, and as they absorb water they swell, closing the pore spaces between clumps. Water then pools on top of the soil instead of sinking in, or runs off if there is a slope.  When dry, plant roots have difficulty penetrating the soil through the flat and oveColumnar rlapping plates.  Another common soil structure is called columnar or prismatic, and is illustrated in the photo to the right.  With columnar structure, the soil clumps are arranged vertically.  When the soil dries, vertical cracks develop between the columns.  Penetration of the soil by plant roots tends to occur only along the vertical cracks while horizontal spreading of the plant roots is constricted.  During rainstorms or irrigation, the soil in the columns swells sideways as water is absorbed, soon closing the cracks in the soil and pretty much sealing it Blocky shut from further penetration by more rain or irrigation water.  Blocky structure is illustrated in the photo at the left.  Here the soil clumps are extremely large and solid.  Plant roots and water must detour around the clods.  Ever seen carrots with big angles in their roots instead of nice straight roots?  They probably grew in soil with blocky structure! Plowing a field or digging in a garden too soon after a rain or a thorough watering can produce blocky structure at the surface.  However, plowing or digging soil under proper Granular conditions (not waterlogged or excessively dry) can greatly improve soil structure.  In fact, that is one of the most important purposes of plowing or digging soil.  Large blocks, columns, or plates are broken up and "corners" are knocked off, shaking the soil into the most desirable soil structure, which is crumb-like or granular. Granular structure is illustrated in the photo to the right.  Here the clumps of soil particles are fairly small and rounded, with plenty of pore space between them for penetation of water, air, and plant roots.

What Is Soil pH?

The letters pH stand for "Hydrogen Ion Potential" and constitute a scale from 0.0 to 14.0 to measure the pH Scale acidity or alkalinity of chemical solutions, such as soil water.  A pH of 7. 0 is neutral, neither acid nor alkaline, and is the pH of distilled water (rainwater is slightly acid, or very acid if industrially polluted).  A pH of less than 7.0 is acid, while over 7.0 is alkaline. The pH of some common household solutions is shown on the scale to the right.  
The mineral nutrients needed by plants each move more easily from soil particles to plant roots at a certain pH.  Most minerals are most r
eadily available to plants at a neutral pH, but some are more available under slightly acid or slightly alkaline conditions.  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 diffuculty obtaining that mineral from the soil, even though it is present.  It will be bound too firmly to the soil particles.  pH 2

Most plants need all of these minerals and therefore grow best in soils between 6.5 and 7.5 in pH because no minerals are extremely hard to obtain when the soil is nearly neutral, neither very acid nor very alkaline. 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.  Needleleaf evergreen trees such as pines and firs produce very acid leaf litter.  Soil sampling in the Sierra Nevada where lodgpole pines were invading grassy meadows showed a sharp drop in pH under the branches of lodgepoles 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 garderns 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.