Life in the Soil

The University of Chicago Press

Contents

Chapter One

A. INTRODUCTION

Every rock has what it takes to be part of a soil someday. Even the hardest of rocks will eventually succumb to the unremitting action of weather and plants. In the early days of the earth there was neither soil nor living creatures-only rocks, water, the wind, and the sun. But these were ingredients enough for the making of the earth's first mineral soils. Four major factors worked together to form the first mineral soils: the weathering of rocks from which soil originated (parent materials) was influenced by the slope of the land (topography) where rocks were exposed to wind, rain, and sun (climate) as well as by the length of time that rocks were exposed to weathering. Soil began to form slowly, imperceptibly as large rocks changed into small rocks, and small rocks changed into even smaller rocks (plate 1).

The ultimate result of the weathering of most rocks is the formation of sand, silt, and clay-the three main mineral particles that make up all soils and that give each soil its distinctive texture. New soils and their mineral particles of sand, silt, and clay are born as rocks weather and disappear. With the appearance of these three types of mineral particles, soils begin to take on particular characteristics.

The particular combination of mineral particles found in a soil determines that soil's texture. Sand particles that range in diameter from 0.05 mm to 2 mm impart a coarse texture to sandy soil. Minuscule particles of clay, on the other hand, which are smaller than 0.002 mm in diameter impart a sticky texture to clay soil. Silt particles, whose diameters are smaller than those for sand particles but larger than those for clay particles, give silt a silky or powdery texture (fig. 3).

A soil in which the stickiness of clay particles, the silkiness of silt particles, and the coarseness of sand particles contribute almost equally to the soil's texture is known as loam soil. All other soil textures with names like clay loam, sandy loam, silty clay, or sandy clay represent combinations of soil particles with textures lying somewhere between any two of the three major types of soil texture: sand, silt, and clay (fig. 4).

The soil-forming forces are as effective now as they were when the earth was much younger. Soil is being born today just as it has been since the early years of the earth. But the birth of soil is a slow and labored process. By one liberal estimate, just an inch of soil takes on the order of 500 years to form; and by a more conservative estimate, one inch of soil forms every 1,000 years.

A complete soil represents the marriage of the mineral or inorganic world with the organic world. It is a good marriage; and as in all good marriages, the two partners work together in harmony. Each partner's attributes are often enhanced in the other's company. Minerals come from the breakdown of rocks; organic matter arises from the decay of animals and plants. Minerals provide many essential elements for plant life, but so does organic matter. The marriage of the mineral world and the organic world is a marriage that improves the longer the two partners work together. In addition to the four major factors (climate, topography, parent materials, time) that contribute to the formation of mineral soils, living organisms represent a fifth and final factor that not only controls but also completes the formation of soil. All these factors work together to create the great diversity of the earth's soils, which differ so much from place to place.

Mineral soils form from rocks, air, and water. These soils, however, are missing one final element that is essential for the survival of all creatures. This element is nitrogen. Ironically, soil is surrounded by this element. Even though roughly three-fourths of the air we breathe is nitrogen in the form of dinitrogen gas ([N.sub.2]) and 34,500 tons of this gas lie over every acre of land, very few of our fellow creatures can use nitrogen as it exists in the air-nor can we. Animals depend on plants and other animals as sources for their nitrogen; and the only forms of nitrogen that green plants can use are ammonia (formed by combining nitrogen with hydrogen) and nitrates (formed by combining nitrogen with oxygen). In the early years of the earth, lightning was the only agent that could transform nitrogen of the air into nitrogen of nitrates. As life began to appear on earth, a few specialized bacteria, blue- green algae or cyanobacteria (cyano = dark blue; bacteria = rod), and certain actinomycetes that live in the soil began to take on the task of converting dinitrogen gas to ammonia, a process known as nitrogen fixation.

Rain, wind, sun, and ice all help convert rock to soil, but it is the living creatures that clearly make the soil hospitable for other living forms. Only hardy and intrepid pioneers can move in and eke out a meager existence on the barren and nutrient-poor stretches of uninhabited soil and rock. New pioneers appear and new nutrients are added as old pioneers pass away. As early generations of microbes, plants, and animals die and decay, later generations of creatures take up residence among their remains. Once the first pioneers are established on a rocky and barren soil, they begin to create soil particles from rock and from the remains of the creatures that preceded them. The slow and often imperceptible breakdown of rocks and the buildup of soil by the weather now proceed a little faster with help from plants, their roots, and their various partners.

Alliances between organisms have been very important, if not essential, in colonizing soils for the first time. By sharing their talents and working together, the partners survive where either one alone would have perished. Four very successful partnerships are (1) the algae and fungi that form lichens, (2) the plants and bacteria that form nitrogen-fixing root nodules, (3) the fungi and plants that form mycorrhizae, and (4) the diggers and decomposers that circulate and recycle the soil nutrients that plants need for survival.

Lichens and algae are usually the first obvious signs of life on barren, uninhabited surfaces of the earth. The rugged, tenacious lichen is half alga and half fungus and combines the best attributes of each partner. The algal partner not only captures the energy of sunlight to produce sugars and oxygen but can also fix nitrogen from the air while the fungal partner provides water and essential elements for survival and growth. Lichens, of all creatures, are especially gifted when it comes to breaking down rocks. First of all, lichens are extremely abundant and occupy 8 percent of the earth's land surface. Second, a lichen can live an awfully long time in one place on a rock-a few hundred, even a few thousand years (plate 2). A lichen can afford to be unhurried and persistent in its affairs. Third, lichens can penetrate and, after many generations, ultimately eat their way into their rocky homes. What lichens have that other organisms do not is the ability to produce a variety of acids, many of which are found nowhere else in the kingdoms of life. The names of some of the acids are taken from the scientific names given to the lichens that produce them: usnic acid of Usnea lichens, lobaric acid of Lobaria lichens, gyrophoric acid of Gyrophora lichens, and evernic acid from Evernia lichens.

Mosses too can gain footholds on bare rock. They grip the surfaces of rocks with tiny rootlets or rhizoids. These green pioneers manage to obtain enough water and elements from surfaces of rocks to sustain them as generation after generation of mosses claim the bare rock for their home. Mosses, unlike lichens, are not known to produce special acids that can digest rocks, but no one has really looked into this matter very carefully. Even if mosses only occupy space on rocks and do not send their rootlets into the rock, they still shelter bacteria, fungi, tardigrades, and other tiny organisms among their rootlets that may help transform rock to soil. Certainly some fungi can chew away at rocks. The acids secreted by the thin, delicate tips of fungal strands or hyphae allow them to perforate even the most substantial rocks.

Atop barren rocks and the sands of deserts one of the first signs of life is the formation of a thin, fragile crust, known among ecologists as a cryptobiotic (crypto = hidden; bios = life) soil, that harbors bacteria, lichens, algae, protozoa, some fungi, and some mosses (plate 3). These pioneers settle down in these sterile environments, adding nitrogen, storing water, stopping erosion, and just generally enriching the soil by intertwining their living filaments with the lifeless grains of sand and the solid crystals of rocks. But none of these pioneers has roots that can delve deep beneath the surface of the new soil. Hardy plants are drawn to these fragile sites, extending their roots and soon transforming the sites to green oases where other creatures can gain a foothold in the young soil. These plants with roots get a great deal of help along the way from bacteria, from fungi, and from animals of the soil that form some remarkable partnerships with their green allies.

B. HOW SOIL FORMS FROM ROCKS AND WEATHER

Sun, ice, water, and wind are all constantly changing the face of the earth. Where once a field of boulders covered the landscape, grasses and forbs may spread their roots into a rich, dark soil. Where once the granite roots of a mountain anchored it to the earth, massive trees now spread their own roots and branches.

As the sun beats down on a rock, its surface heats up, while just beneath the rock's surface, where the heat does not penetrate, the temperature is many degrees cooler. The rock's surface expands during the heat of the day and contracts during the cool of the night, while the well-insulated core of the rock remains unperturbed by the rising and falling temperatures. After many cycles of expansion and contraction, the outer layers of the rock begin to flake off , and a first step in the conversion of rock to soil has begun. The deserts of the world are the best places to observe this gradual wearing down of rocks by the intense heat of the midday sun and the cold of the desert night.

As water freezes in cracks of rocks, it expands to make more and wider cracks. Every time water freezes, it expands almost 10 percent in size. Freezing water can expand enough to completely split a rock. By acting as a wedge in crevices and cracks, ice contributes to rock breaking and the early stages of soil formation.

Wind works best as a former of soil when it works together with dust and sand, blowing, blasting, and scraping whatever rocks lie along its path. The rocks are eventually worn smoother and smaller as a few more mineral particles are sloughed off by each gust of wind.

Over centuries and over millennia, rain, snow, and flowing water wear away at rocks by the force and friction of their movements. Look at any rock canyon or rocky shoreline to see how water has sculpted and shrunk its surface, carrying off tiny particles of rock that may some day form part of a rich soil, sometimes far from the rocks where they first arose.

Raindrops help convert rock to soil by two different routes. The force of raindrops falling from the sky physically wears away at rock surfaces to form particles of sand and silt. Raindrops also mix with the carbon dioxide in the air to form a weak acid called carbonic acid-the acid that is responsible for the fizz of carbonated beverages like champagne and sodas. Carbonic acid is very effective at corroding surfaces of rocks like limestone.

Chemical corrosion of rocks by rain is a process of interaction and exchange of chemical elements that can be expressed in the simple, straightforward shorthand of a chemical equation. The elements in the equation are chemicals that cannot be broken down to other chemicals with different properties, but these elements can join to form compounds that are made up of more than one element. For example, hydrogen and oxygen, which are elements, combine to form water, a compound. Carbon, in turn, can combine with hydrogen and oxygen to form simple sugar compounds as well as long chains of sugars like cellulose. Each element in a chemical reaction is represented by a capital letter or a capital letter and a small letter (for example, C = carbon; Ca = calcium). Some elements and some compounds have either a negative charge or a positive charge; such charged particles are called ions. When nitrogen (N) and oxygen (O) join they often form negatively charged nitrates (N[O.sub.3.sup.-]), but when nitrogen and hydrogen join they form positively charged ammonium (N[H.sub.4.sup.+]). In every chemical equation, the number and the types of elements as well as the number and types of charges on the left side of the equation (the reacting chemicals) must equal the number and types of elements and charges on the right side of the equation (the chemical products). Therefore another and a quicker way to say that rain reacts with carbon dioxide to form carbonic acid is to write:

[H.sub.2]O + C[O.sub.2] -> [H.sub.2]C[O.sub.3] rain carbon dioxide carbonic acid

The carbonic acid of raindrops is corrosive and chemically transforms rocks by removing certain elements from them. What actually happens is that the positively charged hydrogen in carbonic acid replaces other positively charged elements in the rocks and over time dissolves rocks like limestone and granite. Each rock is made up of one or more minerals, and each mineral consists of several elements. Caves are formed and calcium ions are liberated by the corrosive action of carbonic acid on limestone, otherwise known as the compound calcium carbonate (CaC[O.sub.3]).

[H.sub.2]C[O.sub.3] + CaC[O.sub.3] -> [Ca.sup.+2] + 2O[H.sup.-] + 2C[O.sub.2]

A variety of clays are also formed as rocks like granite and schist encounter carbonic acid. Like all acids, carbonic acid releases positively charged hydrogen ions that continually exchange places with other positively charged elements like aluminum, magnesium, iron, sodium, and potassium that make up many of the compounds found in rocks and soils. The more rain that falls, the more carbonic acid forms, and the more hydrogen ions are added to the soil. These hydrogen ions by their sheer numbers continually displace other positive ions from rocks and contribute to the slow and steady conversion of rocks to particles of clay, sand, and silt.

The most abundant minerals of many rocks such as granite are known as feldspars. Feldspars come in a variety of forms. What they all have in common are the elements aluminum (Al), oxygen (O), and silicon (Si). All feldspars contain either one or two additional elements. These can be potassium (K), calcium (Ca), sodium (Na), or barium (Ba), with potassium being the most common element of the four. When feldspars encounter carbonic acid, the two of them react and leave behind several compounds.

2K(Al[Si.sub.3][O.sub.8]) + [H.sub.2]O + [H.sub.2]C[O.sub.3] -> [K.sub.2]C[O.sub.3] + Feldspar water carbonic potassium Acid carbonate

[Al.sub.2][Si.sub.2][O.sub.5][(OH).sub.4] + 4Si[O.sub.2] Clay sand/silt

One of the products of the reaction is formed when carbonate anions (negative ions) join with potassium cations (positive ions). Large particles of silicon dioxide, better known as sand, as well as smaller particles of silicon dioxide, better known as silt, result as different elements change partners during the reaction. The particles of clay are a mixture of aluminum, silicon, oxygen, and water.

C. PLANT ROOTS AND THEIR BACTERIAL PARTNERS

As soil begins to appear on the surface of sand or rock, every now and then a seed lands, germinates, and extends its newly formed roots into the young soil, testing the new environment in which it has happened to alight. Even though these soils have most of the mineral nutrients that a plant needs to get a start, they are still deficient in that one essential nutrient-nitrogen.

(Continues...)

Excerpted from Life in the Soilby JAMES B. NARDI Copyright © 2007 by The University of Chicago. Excerpted by permission.
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