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Soil pH

pH” is a measure of a substance’s acidity or alkalinity, determined by the concentration of Hydrogen ions in a water or salt solution.

Acid soils are low in fertility because too much of the cation exchange capacity is occupied by either Hydrogen, which is not a plant nutrient, or aluminum, which is toxic to plants. Some Hydrogen ions are necessary to make most nutrients available, with the ideal pH being at 6.4 with 12% hydrogen on the exchangeable sites. No more than 12% of the soil’s CEC should be occupied by hydrogen ions. The addition of Calcium, (a cation), to soil substitutes for and replaces the excess Hydrogen on the exchangeable sites, neutralizing acids in the soil.

                                                                 A pH Scale

(acid)                                                    (neutral)                                                     (alkaline)

0       1      2       3       4      5       6       7      8       9       10      11       12       13      14

Vinegar                            H20                                    Ammonia

Many of the soil microorganisms involved in making nutrients available to plants are more active at 6.4 pH. At 5.6 pH, nearly all antibiotic production by soil bacteria stops, rendering the plant’s root system susceptible to fungi pathogen attack. Some problems associated with acid soils below 6.4 pH:

  • Interference with the availability of nutrients to plants.
  • Increased solubility of Fe, Mn Zn, Cu, and especially Aluminum to undesirable levels.
  • Reduced bacterial activity, especially the antibiotic production and nitrogen-fixing     rhizobia, and slower release nutrients from organic matter.
  • Lower total CEC, which further increases the leachability of nutrients.

Alkaline soils are even more troublesome because they are more difficult to correct. The addition of acid forming materials like Sulfur is expensive and only a temporary correction. Usually, alkaline soils reflect over fertilization, wet seep soils or naturally salty soils.   Some problems associated with alkaline soils above 6.4 pH:

  • Unavailability of many nutrients, and especially micro-nutrients.
  • Saline seep, causing soil to crust, leading to low soil Oxygen levels and increased    anaerobic bacteria populations. This condition increases microbial fermentation of  organic materials which results in toxic levels of alcohol products.