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Soil pH and Organic Matter: Changing Soil pH

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Soil pH and organic matter effect functions and nutrients of the soil. Specifically pH effect performance of pesticides, influence solubility of nutrients and organic matter decomposition. To understand the nutrient availability and growing conditions for normal growth of specific crops it’s important to understand the factors which affect the soil pH and pH on the availability of nutrients.

Soil Organic Matter serves different functions in the soil, which includes retention of nutrients, water holding capacity of soil and soil aggregation. It is a key indicator of soil quality. Soil Organic Matter levels declined over the last century in most of the soils due to overgrazing grasslands and conversion of grasslands to tilled from land.

Soil ph: pH of the soil is determined through the concentration of hydrogen ions (H+). Soil pH is a measure of soil solutions (soil water with its dissolved substances) alkalinity and acidity, on a scale having reading 0 to 14. Acidic soils have a pH less than 7 and basic solutions have pH greater than 7. By definition pH measured on the negative logarithm of hydrogen ion concentration. As the hydrogen ion concentration increase, pH goes down. pH of soil influenced by both base forming and acid-forming cations in the soil. Common acid-forming cations are iron, aluminum, and hydrogen, where on other side base-forming cations are potassium, magnesium, calcium, and sodium. Optimum pH of many plants lies between 5.5 to 7.5. However, some plants adapt on pH values outside this range.

Methods for determining pH are as follows

  • Observation of soil profile: Certain characters of the profile can maybe are indicators of acids, alkaline or saline conditions.
  • Use of inexpensive testing kit in which a small sample mixes with solutions having indicators which change its colors on the basis of acidity.
  • Use of litmus paper small sample mixed in distilled water, litmus paper strip inserted in it. If the soil is acidic, the paper turns to red and if it is basic, it will turn to blue.
  • Using the electronic pH meter, solid state electrodes or glass in soil having moisture or in soil having water mixture, pH read on a screen having a digital display.

Factors affecting soil pH

The natural soil pH depends upon the composition of minerals in parent material of soil and weathering reactions which the parent material undergoes. In warm environments acidification of soil occurs as the product of weathering leached by water flow through the soil. Whereas in dry environments leaching and weathering are less and pH of the soil is neutral or alkaline.

Nutrient availability in relation to Soil pH: pH of soil affects the availability of plant nutrients:

Aluminum toxicity effects plant growth. However, through limiting root growth, it reduces the availability of nutrients. Due to roots damage, uptake of nutrients is reduced and macronutrients deficiency encountered in strongly acidic to ultra acidic soils.

Availability of molybdenum is increased when pH is high because molybdate ion is more effectively absorbed by particles of clay at low pH.

In general nitrogen, magnesium, potassium, sulfur, and calcium are available at pH 6.5 – 8 while iron, copper, boron, nickel, and zinc are available at soil pH 5 – 7. pH less than 5.5, high concentration of hydrogen ions manganese and aluminum can reach to toxic level and can limit production of crop. Phosphorus is available at pH 5.5 – 7.5.

Changing Soil pH

Increasing pH of Acidic Soil

Agricultural lime is applied to increase pH. The amount of lime is determined by the mesh size of a lime and buffering capacity of the soil. Soils with high buffering capacity require a high amount of lime to change in pH.

Other amendments are wood ash, magnesium oxide, calcium silicate, calcium oxide (burnt lime) and oyster shells. Product increase soil pH through various reactions calcium silicate neutralizes acidic activity by reacting with H+ and form a neutral solute called monosilicic acid.

Decreasing pH of Alkaline Soil

Can be reduced by using acidifying agents or acidic organic materials. Elemental sulfur can be used at a rate of 300 – 500 Kg/ha. It oxidizes in soil slowly and found sulfuric acid. Fertilizers such as ammonium nitrate, ammonium sulfate, and urea reduce pH because ammonium oxidizes and form nitric acid. Acidifying organic materials are peat and sphagnum peat moss.

Water pH

pH is a measure of the amount of free hydroxyl and hydrogen ions in water. Water with free hydrogen ions is acidic and the water having free hydroxyl ions is basic. pH can be affected due to chemicals in water, pH of water is important in a way that it tells about that the water is changing chemically. pH is reported in “logarithmic units”. Each number shows a change of 10 folds in acidity or alkalinity of water. Water having pH 5 is more acidic than 6 ten times.


pH determines the solubility and biological availability of chemical constituents like nutrients and heavy metals. For Example, it shows that how much and of which form phosphorus is abundant in water. pH also determines the water for aquatic life. In case of heavy metals, determines its toxicity. Metal is more toxic at low pH due to their increased solubility.


The water sample is placed in the cup and the glass probe at the end of the retractable arm is placed in the water. Inside the thin glass bulb at the end of the probe, there are two electrodes that measure voltage. One electrode is contained in a liquid that has a fixed acidity, or pH. The other electrode responds to the acidity of the water sample. A voltmeter in the probe measures the difference between the voltages of the two electrodes. The meter then translates the voltage difference into pH and displays it on the little screen on the main box.

Before taking a pH measurement, the meter must be “calibrated.” The probe is immersed in a solution that has a known pH, such as pure water with a neutral pH of 7.0. The knobs on the box are used to adjust the displayed pH value to the known pH of the solution.

Water and pH Quality

Exclusive high and low pH can be detrimental to the use of water. High pH causes a bitter taste, water pipes and water-using appliances become encrusted with deposits, and it depresses the effectiveness of the disinfection of chlorine, thereby causing the need for additional chlorine when pH is high. Low-pH water will corrode or dissolve metals and other substances.

Pollution can change a water pH, which in turn can harm animals and plants living in the water. For instance, water coming out of an abandoned coal mine can have a pH of 2, which is very acidic and would definitely affect any fish crazy enough to try to live in it! By using the logarithm scale, this mine-drainage water would be 100,000 times more acidic than neutral water, so stay out of abandoned mines.

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Soil pH and Organic Matter: Changing Soil pH. (2018, October 22). GradesFixer. Retrieved November 28, 2021, from
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