Agro/Hort 100 Intro to Plant Science



Soil Morphogenesis

Physical Properties of Soil

Chemical Properties of Soil

Soil Organisms


Chemical Properties of Soil

Cation Exchange Capacity (CEC) is the soils capacity to hold cations. Soil particles are composed of silicate and aluminosilicate clay. These particles are negatively charged colloids. Cations are bound ionically to the surface of these colloid particles. A cation is a positively charged ion, for example H+, Ca++, Mg++, K+, NH4+, Na+are all cations.

CEC is expressed as meq/100 g of soil.

Typical CEC values by soil type
meq/100g soil
sand 2-4
loam 7-16
clay 4-60
organic 50-300

CEC increases as the clay content increases and as the organic matter increases in a soil.



All cations are not created equal!

Cations with higher charge densities, ie, smaller cations, will replace larger cations. For example, H+ will displace Ca++; Ca++ will displace Mg++.

As organic matter decomposes in a soil, protons are produced. The bacteria in the soil causing the decomposition also respire protons (H+). As the proton content goes up, the protons displace the other bound ions on the surface of the soil particles, and these cations become available for absorption by the plant roots.



All living organisms are sensitive to pH. The plant roots will not function optimally in soils outside a specific pH range unique to that organism. If the pH of the soil is extreme either alkaline or acid, the plant will die. Soil microorganisms, insects, and other animals present in the rhizosphere are equally sensitive to pH.

Alkaline soils have pH 7.5 - 8.5.

Acidic soils have pH 4 - 6.5.

Soils with pH values outside these ranges are usually toxic to most plants.

Alkaline components are more readily leached from soils.

Why do you think this is so?



Soil pH can be altered by ammendments. Increasing organic matter will decrease pH (increase acidity). Lime can be added to increase pH (increase alkalinity). Certain fertilizers are delivered as acidic or basic solutions. These will also alter soil pH.


Soils have Buffering Capacity!!

This means that within their normal range of pH values, they can absorb lots of protons or lots of hydroxyl ions before the pH of the soil water changes. But once the buffering capacity of the soil is reached, then the pH of the soil water will change rapidly to toxic extremes. It will also take a lot of new buffering activity to repair the soil to its original pH.


Saline soils are soils with lots of soluble salts.

Sodic soils are saline soils with lots of Na+, <15% Na+.

[Soils] [Agro/Hort 100] [Agro/Hort 100 Syllabus]

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Last Updated: February 9, 2001 Error: Unable to read footer file.