Cation Exchange Capacity (CEC)
Cation-exchange capacity (CEC) is the maximum quantity of total cations that a soil is capable of holding, at a given pH value, available for exchange with the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect groundwater from cation contamination. It is expressed as centi-mol of Hydrogen per kg (cmolc/kg or 100 meqc/100g). Most of the soil's CEC occurs on clay and humus.
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 soils from 3.5 (very acid) to 9.5 (very alkaline). The effect of pH is to remove from the soil or to make available certain ions. Soils with high acidity (<5.5) tend to have toxic amounts of aluminium and manganese. Soils with high alkalinity (>8.5) tend to disperse. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH 6.5.
Base saturation percentage
There are acid-forming cations (hydrogen and aluminium) and there are base-forming cations (calcium, magnesium, potassium and sodium). The fraction of the base-forming cations that occupy positions on the soil colloids is called the base saturation percentage. When the soil pH is 7 (neutral), base saturation is 100 percent and there are no hydrogen ions stored on the colloids. Base saturation is almost in direct proportion to pH and except for its use in calculating the amount of lime needed to neutralize an acid soil, it is of little use.
Sixteen nutrients are essential for plant growth and living organisms in the soil. These fall in two different categories namely macro- and micronutrients. The macronutrients include Carbon (C), Oxygen (O), Hydrogen (H), Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulhpur (S) and are the most essential nutrients to plant development whereby a high quantity of these is needed. The micronutrients on the other hand are needed in smaller amounts, however they are still crucial for plant development and growth, these include Iron (Fe), Zinc (Zn), Manganese (Mn), Boron (B), Copper (Cu), Molybdenum (Mo) and Chlorine (Cl). Nearly all plant nutrients are taken up in ionic forms from the soil solution as cations or as anions.
Organic soil carbon
The carbon that is fixed by plants is transferred to the soil via dead plant matter including dead roots and leaves. This dead organic matter creates a substrate which soil micro-organisms respire back to the atmosphere as carbon dioxide or methane depending on the availability of oxygen in the soil. Soil organic carbon can also be oxidized by combustion and returned to the atmosphere as carbon dioxide. Some of the carbon compounds are easily digested and respired by the microbes resulting in a relatively short residence time. Others, like lignin, humic acid or substrate encapsulated in soil aggregates, are very difficult for the biomass to digest and have very long residence times. Soil organic carbon improves the physical properties of the soil. It increases the cation exchange capacity (CEC) and the water-holding capacity and it contributes to the structural stability of clay soils by helping to bind particles into aggregates. Soil organic matter, of which carbon is a major part, holds a great proportion of nutrients, cations and trace elements that are of importance to plant growth. It prevents nutrient leaching and is integral to the organic acids that make minerals available to plants. It also buffers soil from strong changes in pH. It is widely accepted that the organic carbon content of the soil is a major factor in its overall health, is a major part of the Carbon Cycle and an important factor in the mitigation of climate change effects.
Nitrogen is the most critical element obtained by plants from the soil and when deficient is a bottleneck in plant growth. Plants can use the nitrogen as either the 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. Nitrogen is also available in gas forms in the soil, however these quantities are very small and difficult to detect such as :nitrous oxide (N2O), nitric oxide (NO), nitrogen dioxide (NO2) , ammonia (NH3) and molecular nitrogen (N2) present in the air space of the soil.
Salts can be transported to the soil surface by capillary transport from a salt laden water table and then accumulate due to evaporation. Salinization occurs when irrigation practices are carried out without due attention to drainage and leaching of the salts out of the soil. Salts can also accumulate due to seawater intrusion, or may occur naturally. As soil salinity increases, salt effects can result in degradation of soils and vegetation. The most common salts are combinations of the cations sodium, calcium, magnesium and potassium with the anions chlorine, sulfate and carbonates.
Sodicity refers to an excess of exchangeable sodium in the soil. Sodic soils tend to occur general within arid to semiarid regions and are often unstable, exhibiting poor physical and chemical properties, which impede water infiltration, water availability, and ultimately plant growth.
Calcium carbonate content
Calcium carbonate CaCO3,is a salt that is not very soluble and occurs in various forms and concentration in soils. Calcium carbonate in moderate amounts is favorable for soil structure and is often used to correct the pH of acidic soils, but when the level of calcium in the soil exceeds the capacity of the soil to absorb it, it binds with other elements and forms insoluble compounds that are difficult for plants to absorb. Excess amounts of calcium may restrict the availability of phosphorous, boron and iron to plants.
Calcium Sulfate (Gypsum) content
In soils substantial secondary accumulation of gypsum (CaSO4.2H2O) may occur, particularly in the driest climates. The soils affected by gypsum are developed in mostly unconsolidated alluvial, colluvial and aeolian deposits of base-rich weathering material. The natural vegetation on soils with high gypsum content is sparse and dominated by xerophytic shrubs and trees and/or ephemeral grasses.