Liming and Its Soil Plant Relationship
Liming is the application (to soil) of calcium- and magnesium-rich materials in various forms, including marl, chalk, limestone, burnt lime or hydrated lime.
In acid soils, these materials react as a base and neutralize soil acidity. This often improves plant growth and increases the activity of soil bacteria, but oversupply may result in harm to plant life.
By the end of this unit, you will be able to discuss what liming is all about and explain causes of soil acidity
What is liming soil?
Liming is a traditional procedure in preparing soil for planting. It is the application of calcium- and magnesium-rich materials to soil in various forms, including marl, chalk, limestone, or hydrated lime.
Lime used on farm land is also called agricultural lime. The primary reason to apply agricultural lime is to correct the high levels of acidity in the soil.
Acid soils reduce plant growth by inhibiting the intake of major plant nutrients -nitrogen, phosphorus and potassium. Some plants, for example legumes, will not grow in highly acidic soils.
Causes of Soil Acidity
·Leaching of land caused by high rainfall levels.
·Application of modern chemical fertilizers, which are the major contributors of acidified soil.
· Acidic precipitation in its true sense that is H+ ions in precipitation.
· The deposition from the atmosphere of acidifying gases or particles such as sulphur dioxide (SO2). ammonia (NH3) and nitric and hydrochloric acids (HNO3; HCl).
· Nutrient uptake by crops and root exudates.
· The mineralization of organic matter.
1. Leaching of basic cations
Over time, soils also become acidic because calcium and magnesium leach out, because hydrogen is added to soils by decomposition of plant residues and organic matter, or because nitrification of ammonium occurs when fertilizer (UAN solutions, urea, ammonium nitrate, ammonium sulfate, anhydrous ammonia), manure, or plant residues are added to the soil.
Lime will neutralize this acidity by dissolving, whereupon it releases a base into the soil solution that reacts with the acidic components, hydrogen and aluminum.
2. Acidic precipitation
‘Pure’ rain is usually slightly acid, with a pH of between 5 and 5.6 because of the dissolution of carbon dioxide (CO2) and the dissociation of the resulting carbonic acid (H2CO3).
A soil exposed to such rain, but no other acidifying inputs and receiving no lime, would attain the same equilibrium pH as that of the rain.
There are, however, very strong localized effects because human activity has increased the acidity of precipitation through emissions of acidifying compounds such as SO2 and nitrogen oxides (NOx) from industry and motor vehicles, and NH3 volatilized from manures and fertilizers
3. Acidifying fertilizers and legumes
The most important causes of soil acidification on agricultural land are the application of ammonium‐based fertilizers and urea, elemental S fertilizer and the growth of legumes (Bolan & Hedley, 2003). Ammonium salts strongly acidify soils through the process of nitrification.
NH+ 4+2O2 =NO− 3+2H+ +H2O
4. Nutrient uptake by crops and root exudates
Plant growth and nutrient uptake result in some localized acidification around plant roots through the exudation of acids from the roots. Excluding the particular case of legumes, the contribution of this to bulk soil acidification is small (<10%) when compared with N and S fertilizer inputs but it has an important influence on the bioavailability of plant nutrients in the rhizosphere.
5. Mineralization
When microorganisms decompose soil organic matter they produce CO2, which dissolves in soil water to form H2CO3 in the same way as in rain. Thus, soil and root respiration can result in a large concentration of CO2 in soil air, but because acidic soil solutions hold very little CO2, the process is unlikely to cause soil pH to decline below 5 (Bolan et al., 2003).
Soil with pH below 5.5 and below 70% of saturation requires liming. The best time is when plowing stubble, when there are no crops in the field. Effect of liming takes on average 6-7 years.
Agricultural lime has good effects on soil:
·Increases the pH of acidic soil
·Provides a source of calcium and magnesium for plants
·Permits improved water penetration for acidic soils
·Improves the uptake of major plant nutrients (nitrogen, phosphorus, and potassium) of plants growing on acid soils Most of farming crops require neutral soil, with pH around 6-7, but there are also cultures that need expressly acidic or alkaline soil.
6. Soil pH and Soil Acidity
Soil pH is an indicator of “soil acidity”. A pH of 7.0 is defined as neutral. Values below 7.0 are acidic, and values above 7.0 are basic or alkaline. Small changes in numbers indicate large changes in soil acidity. A soil with a pH of 5 is 10 times more acidic than a soil with a pH of 6 and 100 times more acidic than a soil with a pH of 7.
Most plants can grow in slightly acidic soils, so the goal of liming is not to raise the pH to neutral (7.0), but to avoid crop problems related to excessive acidity.
Benefits of Proper Lime Use
Proper liming provides a number of benefits:
·Plants develop healthier roots because they are exposed to less potentially toxic aluminum. Better root growth may enhance drought tolerance.
·Lime is a source of calcium (as well as magnesium, if dolomitic limestone is applied).
·Nutrient solubility is improved by a higher pH, so plants have a better nutrient supply. (The optimum pH for most crops is 5.8 to 6.2 when grown on mineral soils in North Carolina.)
·Increased soil CEC occurs, as well as reduced leaching of basic cations, particularly potassium.
·Nodulation of legumes is enhanced, which improves nitrogen fixation.
·Triazine herbicides, such as atrazine and simazine, work better.
·Optimal pH allows the breakdown of some herbicides, preventing damage to rotational crops.
·Some nematicides work better.
In conclusion, Liming is the application (to soil) of calcium- and magnesium-rich materials in various forms, including marl, chalk, limestone, burnt lime or hydrated lime.
“Soil acidity” is the term used to express the quantity of hydrogen (H) and aluminum (Al) cations (positively charged ions) in soils.
When levels of hydrogen or aluminum become too high—and the soil becomes too acid—the soil’s negatively charged cation exchange capacity (CEC) becomes “clogged” with the positively charged hydrogen and aluminum, and the nutrients needed for plant growth are pushed out. Application of lime neutralizes the acidity due to hydrogen and aluminum.
In this article, you have learnt that: Liming is the application of calcium and magnesium rich materials which may be in various forms to the soil to reduce acidity.
Also, you have also learnt that some of the causes of acidity may include leaching of basic cations, acid rains, application of acidic fertilizers, mineralisation of organic matter and nutrient uptake by crops as well as exudates of acidic substances from roots of certain plants.
