LECTURE 10 Introduction to some chemical properties of soils : Factors affecting plant growth (2)

What chemical properties affect nutrient availability? Amount of clay and humus Cation Exchange Capacity (CEC) Soil pH Amount of moisture Temperature

Why is the amount of clay and humus important? The exchange of nutrient ions between soil particles and roots is one of the most important processes in nature (others are photosynthesis and respiration). Ion: “An atom or group of atoms that are positively charged (cations) because of the loss of electrons, or negatively charged (anions) because of a gain of electrons.” – Soil Science Society of SA Ion exchange occurs mostly on the surfaces of the finer or colloidal fractions of both inorganic matter (clay) as well as organic matter (humus).

Properties of soil colloids… Extremely small (too small to be seen with an ordinary light microscope). Large surface area (surface area of 1g of colloidal clay is at least 1000x that of 1g of coarse sand). Surface charges Most soil colloids have negative charges, but some mineral colloids in very acid soils have a net positive charge. Influences both physical and chemical properties. Adsorption of cations and water.

Why are most colloids negatively charged? Ionization of hydrogen from hydroxyl ions on clay surfaces. Higher pH = more ionization of hydrogen. Isomorphous substitution. Definition: “The replacement of one atom by another of similar size but not necessarily the same valence in a crystal structure without disrupting or seriously changing the structure.” – Soil Science Society of SA

Colloids and cation exchange… Cation exchange occurs at the surfaces of colloidal particles. Cation exchange = “The interchange between a cation in solution and another cation adsorbed on the surface of any surface-active material such as a clay colloid or organic colloid.” – Soil Science Society of SA

Why is cation exchange important? Exchangeable K, Mg and Ca are important sources of plant K. The amount of lime required to neutralize a soil inc. as CEC inc. Cation exchange sites slow losses by leaching. Cation exchange sites hold fertilizers and so reduce their mobility. Cation exchange sites adsorb many metals from wastewater, preventing them from entering the ground water.

Cation Exchange Capacity (CEC)… Definition: “The sum total of exchangeable cations that a soil can adsorb. This soil property is due to the negative electrical charge of the colloidal (both organic and inorganic) fraction of most soils. The negative charge is balanced by adsorbed cations so that the soil system as a whole is electrically neutral. The balancing cations represent a definite quantity referred to as the cation exchange capacity (CEC).” – Soil Science Society of SA

Soil pH… This gives a measure of the acidity or basicity of a soil. 0-7 = acidic; 7-14 = basic. Acidity is measured by determining the concentration of Hydrogen (H + ) ions in the soil. Higher concentration of H + ions = high acidity, higher concentration of OH - ions = high basicity. In general, the ideal pH for plant growth is about 5.5 in organic soils and about 6.5 in mineral soils.

Where do H + and OH - ions come from?

Why is soil pH important? Affects solubility of minerals. Affects type, numbers and activity of microorganisms. Fungi tolerate acidity better than bacteria. Bacteria often negatively affected by high acidity (i.e. low pH). Indirectly affects aggregate stability. Determines what happens to many soil pollutants. CEC increases with soil pH.

Soil Buffering Capacity… The tendency of soils is to resist changes of the pH of the soil solution. This resistance is termed “buffering”. Soils have different buffering capacities. Generally, higher CEC = greater buffering capacity. Buffering capacity indicates dynamic equilibrium of soil solution. Changes of all types tend to be resisted by the system.