Soil Chemical Properties

Soil Nutrients - Soils supply virtually all of the essential elements or nutrients required for plant growth. For example, most of the plant -available nitrogen as nitrate and ammonium N comes from microbial decomposition and processing of nitrogenous substances in organic matter.
Nitrogen is used by organisms for protein synthesis and is essential for chlorophyll synthesis in green plants. A nitrogen deficiency in plants is observed as yellowing in older leaves, or in the entire plant in very severe deficiencies. The nitrogen cycle "in a nutshell" consists of a stepwise process of organic matter decomposition or mineralization. This also releases other important nutrients (P, P, Ca, Mg, S, Fe, Mn, Cu, Zn, B, and Mo) back into the environment. Mineralization involves microbial conversion of proteinaceous substances and other N-containing organic substances into ammonium N (NH4N), which is either used by plants directly, fixed on cation exchange sites of clays and organic matter, or oxidized into nitrite and nitrate (nitrification). Ammonium oxidation releases hydrogen ions into the soil, which is manifested as a drop in soil pH. Nitrate N can be converted into nitrogen gas and returned to the atmosphere through the process of denitrification. Nitrogen gas itself can be "fixed" as protein nitrogen by soil bacteria and cyanobacteria, thereby returning gaseous N to the soil and water.
Some nutrients, cationic or positively-charged ions such as calcium, magnesium and potassium, are released into the soil solution from soils and parent materials during normal weathering processes (and also, via decomposition of organic matter). Such ions can be sorbed onto negatively charge sites on surfaces and within clay minerals and organic matter. This property of clays and organic matter is termed cation exchange capacity or CEC. Generally, the higher the clay and/or organic matter content, the higher the CEC for that soil. Thus, CEC is a measure of the nutrient holding (and buffering) capacity of a soil, and in a sense, is an index of soil fertility.

Soil pH & Acidity - One of the more important properties of soils in Mississippi for growing plants, pH is a measure of the hydrogen ion concentration of soil water. Soil acidity (pH < 7) controls the availability to plants of most of the micronutrients, and also affects plant utilization of nitrogen, phosphorus, sulfur, calcium, and magnesium.
Chemically, pH is the negative log of the hydrogen ion activity in the water or a soil solution. Simply put, when there is a high hydrogen ion level, a soil will be "acidic." If the hydrogen ion concentration is low, the soil will be slightly acidic, neutral or even alkaline.
The pH is measured on a scale of 0 to 14. However, for the vast majority of soils in natural environments in Mississippi, pH ranges from as low as 4.0 to as high as 8.4. A pH of 7 is considered neutral, below 7 is acid, and above 7 is alkaline. The lower the pH value, the more acid the soil. Soil pH is measured on a logarithmic scale, so that a change of pH of 1 unit (from 6 to 5 for example) is actually a 10x change in hydrogen ion activity. A change of 2 units (from 6 to 4) results in a 100x change!
If a soil is acidic, for many cultivated plants, the pH must be adjusted upward by adding liming materials to the soil. Some plants (e.g., pines, azaleas, blueberries) actually prefer acidic conditions, and some materials can be added to reduce pH. Therefore, a solid knowledge of plant environmental conditions is crucial when you're planting anything in the landscape.
Application of liming materials should ideally be based on the results of a soil test. Soils have various buffering capacities due to variable amounts of clays and organic matter, and liming materials are added based upon (1) the soil pH, and (2) the buffering capacity. An old USDA "yearbook of agriculture" on soils (Yearbook, 1957) has a convenient table for liming soils (pounds limestone per 1000 square feet) based upon soil texture (southern coastal states) and pH. The table below is modified from the USDA table:

Texture pH 4.5~5.8
Sands, Loamy Sands add 12 -15 lbs
Sandy Loams add 20 - 25 lbs
Loams add 30 - 40 lbs
Silt Loams add 45 - 60 lbs
Clay Loams add 60 - 65 lbs

Application of this amount of lime will increase soil pH to levels that will support economic yield increases. In the table, notice that the lime rate increases, as the buffering capacity increases based upon soil texture. The rates have been reduced by about 75% of the old USDA figures to reflect modern realities about soil pH and liming.