Plants obtain their carbon from atmospheric carbon dioxide. A plant’s weight is forty-five percent carbon. Elementally, carbon is 50% of plant material. Plant residues have a carbon to nitrogen ratio (C/N) of 50:1. As the soil organic material is digested by arthropods and micro-organisms, the C/N decreases as the carbonaceous material is metabolised and carbon dioxide (CO2) is released as a byproduct and finds its way out of the soil and into the atmosphere. The nitrogen, however, is sequestered in the bodies of the live matter and so it builds up in the soil. Normal CO2concentration in the atmosphere is 0.03%, which is probably the factor limiting plant growth. In a field of maize on a still day during high light conditions in the growing season, the CO2concentration drops very low, but under such conditions the crop could use up to 20 times the normal concentration. The respiration of CO2 by soil micro-organisms decomposing soil organic matter contributes an important amount of CO2 to the photosynthesising plants. Within the soil, CO2 concentration is 10 to 100 times atmospheric but may rise to toxic levels if the soil porosity is low or if diffusion is impeded by flooding.

All the nutrients with the exception of carbon are taken up by the plant through its roots. All those brought through the roots, with the exception of hydrogen, which is derived from water, are taken up in the form of ions. Carbon, in the form of carbon dioxide, enters primarily through the leaf stomata. All the hydrogen utilised by the plant originates from soil water and participates with the carbon dioxide in the photosynthetic production of sugars and release of oxygen as a byproduct. Plants may have their nutrient needs supplemented by spraying a water solution of nutrients on their leaves, but nutrients are typically received through the roots by: Mass flow Diffusion Root interception The nutrient needs of a plant may be carried to the plant by the movement of the soil water solution in what is called mass flow. The absorption of nutrients from the soil solution with which the roots are in contact causes the concentration of nutrients in that area to be reduced. Diffusion of nutrients from areas with high concentration to those of lower concentration moves nutrients near the roots as they take up those nutrients. Plants constantly send out roots to seek new sources of nutrients in a process called root interception. Meanwhile older, less effective roots die back. Water is lifted to the leaves, where it is lost by transpiration and in the process it brings soil nutrients with it. A maize plant, for example, will use one quart of water per day at the height of its growing season. Estimated relative importance of mass flow, diffusion and root interception as mechanisms in supplying plant nutrients to corn plant roots in soils Nutrient Approximate percentage supplied by: Mass flow Root interception Diffusion Nitrogen 98.8 1.2 0 Phosphorus 6.3 2.8 90.9 Potassium 20.0 2.3 77.7 Calcium 71.4 28.6 0 Sulfur 95.0 …

Sixteen nutrients are essential for plant growth and reproduction. They are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, iron, boron, manganese, copper, zinc, molybdenum, and chlorine. Nearly all plant nutrients are taken up in ionic forms from the soil solution as cations or as anions. Plants release bicarbonate and hydroxyl (OH–) anions or hydrogen cations from their roots in an effort to cause nutrient ions to be freed from sequestration on colloids and so forced into the soil solution where they can be picked up. Nitrogen is available in soil organic material but is unusable by plants until it is made available by that material’s decomposition by micro-organisms into cation or anion forms.   Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for plant uptake Element Symbol Ion or molecule Carbon C CO2 (mostly through leaves) Hydrogen H H+, HOH (water) Oxygen O O2-, OH –, CO32-, SO42-, CO2 Phosphorus P H2PO4 –, HPO42- (phosphates) Potassium K K+ Nitrogen N NH4+, NO3 – (ammonium, nitrate) Sulfur S SO42- Calcium Ca Ca2+ Iron Fe Fe2+, Fe3+ (ferrous, ferric) Magnesium Mg Mg2+ Boron B H3BO3, H2BO3 –, B(OH)4 – Manganese Mn Mn2+ Copper Cu Cu2+ Zinc Zn Zn2+ Molybdenum Mo MoO42- (molybdate) Chlorine Cl Cl – (chloride)

The resistance of soil to changes in pH and available cations from the addition of acid or basic material is a measure of the buffering capacity of a soil and increases as the CEC increases. Hence, pure sand has almost no buffering ability, while soils high in colloids have high buffering capacity. Buffering occurs by cation exchange and neutralisation. The addition of highly basic aqueous ammonia to a soil will cause the ammonium to displace hydrogen ions from the colloids, and the end product is water and colloidally fixed ammonium, but no permanent change overall in soil pH. The addition of lime, CaCO3, will displace hydrogen ions from the soil colloids, causing the fixation of calcium and the evolution of CO2 and water, with no permanent change in soil pH. The addition of carbonic acid (the solution of CO2 in water) will displace calcium from colloids, as hydrogen ions are fixed to the colloids, evolving water and slightly alkaline (temporary increase in pH) highly soluble calcium bicarbonate, which will then precipitate as lime (CaCO3) and water at a lower level in the soil profile, with the result of no permanent change in soil pH. All of the above are examples of the buffering of soil pH. The general principal is that an increase in a particular cation in the soil water solution will cause that cation to be fixed to colloids (buffered) and a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution (buffered). The degree of buffering is limited by the CEC of the soil; the greater the CEC, the greater the buffering capacity of the soil.

Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil. More precisely, it is a measure of hydrogen ion concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms. Soil pH At 25°C an aqueous solution that has a pH of 3.5 has 10-3.5 moles hydrogen ions per litre of solution (and also 10-10.5 mole/litre OH–) . A pH of 7, defined as neutral, has 10−7 moles hydrogen ions per litre of solution and also 10−7 moles of OH– per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 is 10-9.5 moles hydrogen ions per litre of solution (and also 10-2.5 mole per litre OH–) . A pH of 3.5 has one million times more hydrogen ions per litre than a solution with pH of 9.5 (9.5 – 3.5 = 6 or 106) and is more acidic. The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of aluminium and manganese. Plants which need calcium need moderate alkalinity, but most minerals are more soluble in acid soils. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH 6.5 and organic soils of pH 5.5. In high rainfall areas, soils tend to acidity as the basic cations are forced off the soil colloids by the mass action of hydrogen ions from the rain as those attach to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil sterile. Once the colloids are saturated with …

Anion exchange capacity should be thought of as the soil’s ability to remove anions from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the solution. Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have the highest AEC, followed by the iron oxides. Levels of AEC are much lower than for CEC. Phosphates tend to be held at anion exchange sites. Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH-) for other anions. The order reflecting the strength of anion adhesion is as follows: H2PO4- replaces SO42- replaces NO3- replaces Cl- The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 g dry soil. As pH rises, there are relatively more hydroxyls, which will displace anions from the colloids and force them into solution and out of storage; hence AEC decreases with increasing pH (alkalinity).

Cation exchange capacity should be thought of as the soil’s ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution. CEC is the amount of exchangeable hydrogen cation (H+) that will combine with 100 grams dry weight of soil and whose measure is one milliequivalents per 100 grams of soil (1 meq/100 g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100 grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with a valence of two, converts to (40/2) x 1 milliequivalent = 20 milliequivalents of hydrogen ion per 100 grams of dry soil or 20 meq/100 g. The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil. Most of the soil’s CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates, due to leaching and decomposition respectively, explains the relative sterility of tropical soils.

Cation exchange, between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful. The negative charges on a colloid particle make it able to hold cations to its surface. The charges result from four sources. Isomorphous substitution occurs in clay when lower-valence cations substitute for higher-valence cations in the crystal structure. Substitutions in the outermost layers are more effective than for the innermost layers, as the charge strength drops off as the square of the distance. The net result is a negative charge. Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete at the edges of clay. Hydrogens of the clay hydroxyls may be ionised into solution, leaving an oxygen with a negative charge. Hydrogens of humus hydroxyl groups may be ionised into solution, leaving an oxygen with a negative charge. Cations held to the negatively charged colloids resist being washed downward by water and out of reach of plants’ roots, thereby preserving the fertility of soils in areas of moderate rainfall and low temperatures. There is a hierarchy in the process of cation exchange on colloids, as they differ in the strength of adsorption and their ability to replace one another. If present in equal amounts: Al3+ replaces H+ replaces Ca2+ replaces Mg2+ replaces K+ same as NH4+ replaces Na+ If one cation is added in large amounts, it may replace the others by the sheer force of its numbers (mass action). This is largely what occurs with the addition of fertiliser. As the soil solution becomes more acidic (an abundance of H+), the other cations bound to colloids are pushed into solution. This is caused by the ionisation of hydroxyl groups on the surface of soil colloids in what is described as …

In the extreme environment of heavy rain and high temperatures of tropical rain forests, the clay and organic colloids are largely destroyed. The heavy rain washes the alumino-silicate clays from the soil leaving only sesquioxide clays of low CEC. The high temperatures and humidity allow bacteria and fungi to virtually dissolve any organic matter on the rain-forest floor overnight and much of the nutrients are volatilized or leached from the soil and lost. Carbon, however, is far more stable than soil colloids and is capable of performing many of the functions of the soil colloids of sub-tropical soils. Research into terra-preta is still young but is promising. Fallow periods “on the Amazonian Dark Earths can be as short as 6 months, whereas fallow periods on Oxisols are usually 8 to 10 years long”

Humus is the penultimate state of decomposition of organic matter; while it may linger for a thousand years, on the larger scale of the age of the other soil components, it is temporary. It is composed of the very stable lignins (30%) and complex sugars (polyuronides, 30%). Its chemical assay is 60% carbon, 5% nitrogen, some oxygen and the remainder hydrogen, sulfur, and phosphorus. On a dry weight basis, the CEC of humus is many times greater than that of clay. Plant roots also have cation exchange sites.