Acidic Soil Causes, Affects, Measure of Acidity, Testing soil pH for Agricultural Exam

Acidic Soil

Soil acidity is a potentially major issue in terms of land degradation. When soil gets overly acidic, it can cause the following problems:

  • a reduction in the availability of vital nutrients
  • Toxic components’ impact will be amplified.
  • Reduce plant output and water use.
  • Biological activities in the soil, such as nitrogen fixation, are harmed.
  • make the soil more susceptible to erosion and deterioration of the soil structure

Soil acidification, if left untreated, can have a negative influence on agricultural output and long-term farming systems. Acidification can also reach the subsurface layers, providing major challenges for plant root growth and remediation.

What causes soil acidity

Although soil acidity is a natural occurrence, it can be accelerated by certain agricultural methods.

Acidification happens in agricultural soils due to the following factors:

  • Plant and animal products are removed.
  • excess nitrate leaching
  • the use of nitrogen-based fertilisers
  • Organic materials primarily derived from plants has accumulated.

Soil acidity is a natural occurrence in locations with increased rainfall, and it varies depending on:

  • geology of the terrain
  • mineralogy of clay
  • texture of the soil
  • capability for buffering

How acidity affects plant growth

Acidity isn’t the only factor that limits plant growth. Acidity, on the other hand, can have a detrimental impact on biological activities that are beneficial to plant development.

The following are the impacts of acidity on soil:

  • It reduces the availability of plant nutrients like phosphorus and molybdenum while increasing the availability of certain elements, notably aluminium and manganese, to hazardous levels.
  • Below the rooting zone, essential plant nutrients can also be leached.
  • Acidity may harm bacteria, earthworms, and other soil organisms by reducing their ability to thrive.
  • Highly acidic soils can make it difficult for beneficial bacteria, such the rhizobia bacteria that fix nitrogen for legumes, to survive.

Soil pH as a measure of acidity

The pH of the soil is a measure of its acidity or alkalinity. A pH of 7 is neutral, while a pH of more than 7 is alkaline and less than 7 is acidic. A pH of 6 is 10 times more acid than a pH of 7. This is because pH is measured on a logarithmic scale.

The pH of the soil may be tested in two ways: in water (pHw) or in calcium chloride (pHCa), and the pH will differ depending on which method is chosen. pH measured in calcium chloride is, on average, 0.7 pH units lower than pH measured in water.

A pH of 4.3 that was measured in calcium chloride could be around 5pH measured in water

Between pHCa and pHw, there are a few differences:

  • In Australia, soil pHCa readings range from 3.6 to 8 for a variety of soil types (sandy loams to heavy clays). The pHw of soil varies between pHw 4 and pHw 9.
  • Low salinity soils may have a pHw of 0.6 to 1.2, whereas high salinity soils may have a pHw of 0.1 to 0.5. For a wide variety of soils, research has revealed a difference of 0.7.
  • Alkali mineral soils containing sodium carbonates and bicarbonates may have higher pHw values of approximately 10.
  • According to research, the seasonal variance in pHw can vary by up to 0.6 pH units in a single year. Seasonal effects on soil pHCa readings are less pronounced.

Make sure a laboratory specifies whether method (water or calcium chloride) was used to measure the pH of your soil.

Soil pH levels

For most plants, a pHCa range of 5 to 6 is considered optimum. Once the pHCa of the soil goes below 5, acid soils have a significant impact on plant productivity:

  • pH 6.5 — close to neutral — For many acid-sensitive plants, this is ideal. It’s possible that certain trace elements will become inaccessible.
  • pH 5.5 — slightly acid — The best possible balance of key nutrients and trace elements for plant absorption
  • pH 5.0 — moderately acid — Depending on the soil type, aluminium (Al) can be harmful to plants below pH 4.8. Because phosphorus reacts with aluminium, it may be less accessible to plants.
  • pH 4.5 — strongly acid — In hazardous amounts, aluminium becomes soluble. Depending on temperature and moisture, manganese (Mn) becomes soluble and poisonous to plants in some soils. Molybdenum (Mo) is a rare metal. Bacterial activity in the soil is reduced.
  • pH 4.0 — extremely acid — It is possible for irreversible soil structural disintegration to develop.

The availability of soil nutrients to plants as well as how the nutrients interact will be influenced by soil pH.

Consider the following scenario:

  • Many elements, such as iron, aluminium, and manganese, become less accessible to plants at low pH, while others, such as iron, aluminium, and manganese, become hazardous to plants. Insoluble compounds are also formed when aluminium, iron, and phosphorus interact.
  • Calcium binds phosphorus at high pH, rendering it inaccessible to plants, while molybdenum becomes poisonous in certain soils. In some soils, boron may be poisonous.

The level of pHCa impacts both the availability of soil elements and the way they interact with each other. A pH between 5 and 6 usually has the ideal combination of elements.

Testing soil pH

One of the most often studied soil characteristics is pH. Because of the following reasons:

  • Testing is a simple process.
  • pH measurement equipment may be found at a reasonable price in the field.

Don’t rely on field test kits to make conclusions about lime application rates. Only test kits can tell you if your soil is acidic or alkaline. If you don’t have enough other nutrients, you’re not going to receive much of a reaction from lime.

The most accurate findings will come from a professional soil sample analysis by a recognised laboratory.

Parent Material:

Acid rocks such as granite and rhyolite disintegrate and decompose, resulting in the buildup of acid soil debris.


Continuous leaching eliminates the majority of CaCO3 and gypsum from soil in locations with high rainfall, resulting in increasing soil acidity.

Organic Matter:

The carboxylic, enolic, and phenolic groups in organic matter detritus dissolve and release H+, contributing to soil acidity.

Root Biomass and Soil Organism:

Soil acidity is mostly caused by the respiration of plant roots and soil microorganisms.

During breathing, CO2 combines with water to form carbolic acid, which then degrades to release H+.

CO2 + H2O—-> H2CO3 + H+

Plants, on the other hand, exchange cations with external medium and produce an equal amount of H+, contributing to soil acidity.

Liming to correct soil pH

The most feasible management strategy for most acid soils is to apply lime to preserve existing soil pH or improve surface soil pH.

Consider liming whenever the pH falls below pHCa 5.0 for a higher possibility of successfully growing acid-sensitive species.

If acidic paddocks are not limed, the soil pH will continue to decline until it reaches pHCa 3.8 to 4.2.

Lime application for permanent pasture

Spreading lime on the surface and letting it to work its way into the soil is suitable in permanent pasture conditions. Surface application is preferable than none at all.

Lime reactions are often observed in the first and second years for cropping systems, but can take up to 5 years for permanent grazing systems depending on soil type, rainfall, and lime quality.

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