Soil

When bacteria feed on soluble forms of nitrogen (ammonium and nitrite), they temporarily sequester that nitrogen in their bodies in a process called immobilisation. At a later time when those bacteria die, their nitrogen may be released as ammonium by the processes of mineralisation. Protein material is easily broken down, but the rate of its decomposition is slowed by its attachment to the crystalline structure of clay and trapped between the clay layers. The layers are small enough that bacteria cannot enter. Some organisms can exude extracellular enzymes that can act on the sequestered proteins. However, those enzymes too may be trapped on the clay crystals. Ammonium fixation occurs when ammonium replaces the potassium ions that normally exist between the layers of clay such as illite or montmorillonite. Only a small fraction of nitrogen is held this way.

In a process called mineralisation, certain bacteria feed on organic matter, releasing ammonia (NH3) (which may be reduced to ammonium NH4+) and other nutrients. As long as the carbon to nitrogen ratio (C/N) in the soil is above 30:1, nitrogen will be in short supply and other bacteria will feed on the ammonium and incorporate its nitrogen into their cells. In that form the nitrogen is said to be immobilised. Later, when such bacteria die, they too are mineralised and some of the nitrogen is released as ammonium and nitrate. If the C/N is less than 15, ammonia is freed to the soil, where it may be used by bacteria which oxidise it to nitrate in a process called nitrification. Bacteria may on average add 25 pounds nitrogen per acre, and in an unfertilised field, this is the most important source of usable nitrogen. In a soil with 5 percent organic matter perhaps 2 to 5 percent of that is released to the soil by such decomposition. It occurs fastest in warm, moist, well aerated soil. The mineralisation of 3 percent of a soil that is 4 percent organic matter would release 120 pounds of nitrogen as ammonium per acre. In symbiotic fixation, Rhizobium bacteria convert N2 to nitrate by way of nitrogen fixation. They have a symbiotic relationship with host plants, wherein they supply the host with nitrogen and the host provides the bacteria with nutrients and a safe environment. It is estimated that such symbiotic bacteria in the root nodules of legumes add 45 to 250 pounds of nitrogen per acre per year, which may be sufficient for the crop. Other, free-living nitrogen-fixing bacteria and blue-green algae live independently in the soil and release nitrate when their dead bodies are converted by way of mineralisation. Some amount of …

Nitrogen is the most critical element obtained by plants from the soil and is a bottleneck in plant growth. Plants can use the nitrogen as either the ammonium cation ammonium (NH4+) or the anion nitrate (NO3–). Nitrogen is seldom missing in the soil, but is often in the form of raw organic material which cannot be used directly. Carbon/Nitrogen Ratio of Various Organic Materials                              Organic Material C:N Ratio Alfalfa 13 Bacteria 4 Clover, green sweet 16 Clover, mature sweet 23 Fungi 9 Forest litter 30 Humus in warm cultivated soils 11 Legume-grass hay 25 Legumes (alfalfa or clover), mature 20 Oat straw 80 Straw, cornstalks 90 Sawdust 250 Some micro-organisms are able to metabolise the organic matter and release ammonium in a process called mineralisation. Others take free ammonium and oxidise it to nitrate. Particular bacteria are capable of metabolising N2 into the form of nitrate in a process called nitrogen fixation. Both ammonium and nitrate can be lost from the soil by incorporation into the microbes’ living cells, where it is temporarily immobilised or sequestered. Nitrate may also be lost from the soil when bacteria metabolise it to the gases N2 and N2O. In that gaseous form, nitrogen escapes to the atmosphere in a process called denitrification. Nitrogen may also be leached from the soil if it is in the form of nitrate or lost to the atmosphere as ammonia due to a chemical reaction of ammonium with alkaline soil by way of a process called volatilisation. Ammonium may also be sequestered in clay by fixation. Nitrogen is added to soil by rainfall.

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.