26 Chemical properties of soil

Meenakshi Nandal

epgp books

 

 

27.1 Introduction

 

Soil chemistry is the collaboration of various chemical components that exists among soil particles as well as in the soil solution the water sustained by soil. The chemical reactions that occur in soil are highly complicated, but their understanding will better help in managing turf and ornamentals. Soils act as storehouses for plant nutrients. Many nutrients, such as calcium and magnesium, may be supplied to plants entirely from reserves retained in the soil. Others like potassium are added regularly to soils as fertilizer with the purpose of being withdrawn when needed by crops. The relative capability of soils to store one particular group of nutrients, the cations, is referred to as cation exchange capacity or CEC. Soils are composed of a mixture of sand, silt, clay and organic matter. Both the clay and organic matter particles have a net negative charge. These negatively charged soil particles will attract and hold positively-charged particles, much like the opposite poles of a magnet attract each other. By the same demonstration, they will repel other negatively-charged particles, similarly as like poles of a magnet repel each other.

 

27.2 Cation Exchange Capacity

 

Most chemical interactions occurring in soil on colloidal surfaces are because of their charged surfaces. Colloids have charged surfaces because of their chemical make-up and large surface area, which attract, ‘ions’ (charged particles) present in the soil solution. Depending on the ionic charge, size and concentration in the soil, ions can be sorbed by the colloid surface or exchanged with other ions and released to the soil solution. (Fig 27.1)

complex 27.3 Factors affecting Cation Exchange Capacity

 

Soil texture: The negatively charged clay colloids attract positively charged cations and retain them. Therefore, the cation exchange capacity of soils rises with rise in percentage of clay content.

 

Clay soils with high CEC can hold large quantity of cations and reduces the loss of cations by leaching. Sandy soils, with low CEC, hold lesser amount of cations and thus cations are evacuated from soil by leaching (Fig 27.2).

 

Soil organic matter: High organic matter content increases the CEC. The CEC of clay

 

minerals varies from 10 to 150 [cmol (p+) kg-1] whereas that of organic matter ranges from 200 to 400 [cmol (p+) kg-1].

 

Nature of clay minerals: The CEC and specific area of the clay minerals are in the order: smectite > fine mica > kaolinite. Thus the CEC of a soil dominated by smectite type of clay minerals is much higher than kaolinite type dominated soils

 

Soil Reaction: As the pH is increased, the hydrogen held by the organic colloids and silicate clays (Kaolinite) becomes ionized and replaceable. The net result is enhancement of negative charge on the colloids and in turn an increase in CEC.

 

27.4 Importance of Cation Exchange

 

 

Cation exchange is an important parameter in determination of soil fertility, soil acidity and basicity that results in altering soil physical properties as well as in mechanism for purifying or altering percolating waters.

 

Plant nutrients like calcium, magnesium, and potassium are supplied to plants in large amount from exchangeable forms.

 

  • The exchangeable K is a major source of plant K.
  • The exchangeable Mg is often a major source of plant Mg.
  • The amount of lime required to raise the pH of an acidic soil is greater as the CEC is greater.
  • Cation exchange sites hold Ca+, Mg+, K+, Na+, and NH4+ ions and slow down their losses by leaching.
  • Cation exchange sites hold fertilizer K+ and NH4+ and greatly reduce their mobility in soils.
  • Cation exchange sites adsorb various metals (Cd2+ , Zn2+, Ni2+, and Pb2+) which are present in wastewater adsorption, removes them from the percolating water, thus purifying the water which drains into groundwater.

Fig 27.2 Range of Cation Exchange Capacity Source:Brady. The Nature and Properties of Soils. MacMillan

 

The soil’s ability to absorb and exchange ions is known as ‘exchange capacity’. Although both positive and negative charges are present on colloid surfaces, soils of this region are dominated by negative charges and have an overall (net) negative charge. Therefore, more cations (positive ions) are attracted to exchange sites than anions (negative ions), and soils tend to have greater cation exchange capacities (CEC) in comparison to anion exchange capacities (AEC). Fine-textured soils usually have a greater exchange capacity than coarse soils because of a higher proportion of colloids.

 

27.5 Soil pH

 

Soil pH is associated with the soil’s acidity or alkalinity and is the measure of hydrogen ions (H+) in the soil. A large amount of H+ relates to the low pH value and contrary. The pH scale varies from 0 to 14 with 7 being neutral, below 7 acidic, and above 7 alkaline or basic. Soil pH can influence CEC and AEC by changing the surface charge of colloids. A higher concentration of H+ (lower pH) will nullify the negative charge on colloids, thus decreasing CEC and increasing AEC. The opposite occurs when pH increases. (Fig 27.3)

 

Importance of soil pH in crop production-

 

  • 1) It is useful in determining the availability of plant nutrients e.g. P is fixed by Al and Fe oxides at low pH, at high pH it is fixed by Ca. Therefore, P is available maximally at near neutral pH.
  • 2) pH effects the availability of toxic amounts of minerals and elements that can diminish the crop growth
  • 3) It manipulates the population and activities of beneficial microbe.

 

27.6 Salt-Affected Soils

 

The presence and concentration of salts in soil can have conflicting impact on soil function as well as management. Salt affected soils mostly occur in arid and semiarid regions where evaporation surpasses precipitation and dissolved salts are left behind to accumulate, or in areas where vegetation or irrigation changes have resulted into leaching of salts and accumulation in low-lying places (saline seeps). The three main categories of salt-affected soils are saline, sodic and saline-sodic. Saline soils comprises of large amount of soluble salts, primarily calcium (Ca2+), magnesium (Mg2+), and potassium (K+), whereas sodic soils are dominated by sodium ions (Na+). Saline-sodic soils have both high salt and Na+ content. Concentration of salts in soil influences the structure, porosity and plant water relations which ultimately leads to reduced productivity (Table 1).

 

 

The degree of acidity or alkalinity is an important parameter that affects various other chemical, physical and biological properties of soil. Soil acidity is determined as the total amount of acid present in the soil. The soil reaction is expressed as the soil pH; this is the measure of the relative acidity and alkalinity of the soil.

 

Active acidity is that measured by the soil pH.

Reserve acidity is that left within the soil microcell, it is usually measured by titrating the soil solution with a base (Fig 27.4).

Causes of soil acidity:-

 

  • 1) Leaching loss of bases like Ca, Mg, etc.
  • 2) Application of acid-forming fertilizers e.g. urea, NH4+ based fertilizers
  • 3) Acid rains.
  • 4) Decomposition of organic matter, CO2 is evolved; it mixed with soil water to form weak carbonic acid (H2CO3)
  • 5) Hydrolysis of Al. Al3+ + 3H2O Al (OH) 3 + 3H+

 

27.7 Soil Nutrients

 

Sixteen  elements  or  nutrients  are  essential  for  growth  and  reproduction  in  plants.  Such  as carbon C, hydrogen H, oxygen O, nitrogen N, boron B, phosphorus P, potassium K, sulfur S,cal cium Ca, magnesium Mg, iron Fe, manganese Mn, copper Cu, zinc Zn, molybdenum MO, and chlorine Cl. Nutrients required for plants to complete their life cycle are called  as essential nutrients. Nutrients that increase the plant growth but are not important to complete the plant’s life cycle are called non-essential. With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, the nutrients originate from the mineral component of the soil. Uptake of nutrients by plants can only occur when they are present in a plant-available form. In a variety of situations, nutrients are absorbed in an ionic form from soil water. The bulk of  most  nutrient  elements  in  the  soil  are  retained  in  crystalline  form  within  primary  and secondary minerals, to support rapid plant growth. For example, the application of finely ground minerals, feldspar and apatite, to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for proper plant growth, as most of  nutrients remain bound in the crystals of those minerals. Plant growth will be hampered if a particular nutrient availability is in limited supply for  example if there  is  deficiency of  phosphorous then the supply of other nutrients  will  be  ineffective  until  the  deficiency of  phosphorous  is  removed.  The  nutrients adsorbed onto the surfaces of clay colloids and soil organic matter provide a more accessible reservoir of many plant nutrients such as K, Ca, Mg, P, and Zn. As plants absorb the nutrients from the soil water, the soluble pool is replaced from the surface bound pool. The decomposition of soil organic matter with the help of microbes is another mechanism whereby the soluble pool of nutrients is replenished this is important for the regular supply of plant-available nutrients from soil.

 

27.7.1 Pathways of mineral nutrient transport in roots.

 

Minerals are absorbed at the surface of the root, by the root hairs. While passing through the cortex, they either follow the cell walls and the spaces between them or go directly through the plasma membranes and the protoplasts of the cells, passing from one cell to the next by the plasmodesmata. When they reach the endodermis, their further passage through the cell walls is blocked by the Casparian strips, and they must pass through the membrane and protoplast of an endodermal cell before they can reach the xylem. Minerals are absorbed at the surface of the root, mainly by the root hairs. (Fig 27.5).

 

 

Movement of nutrient from soil to root

 

 

There are three basic mechanisms by which nutrients make contact with the root surface for plant uptake. They are root interception, mass flow, and diffusion.

 

 

Root interception: Root interception occurs when a nutrient comes into physical contact with the root surface. As a general rule, the occurrence of root interception increases as the root surface area and mass increases, thus enabling the plant to explore a greater amount of soil. Root interception increases by mycorrhizal fungi, which colonize rootsand enhances root exploration into the soil. Root interception is responsible for an appropriate amount of calcium uptake, and some amounts of magnesium, zinc and manganese.

 

Mass flow: Mass flow occurs when nutrients are transported to the surface of roots by the water movement in the soil (i.e. percolation, transpiration, or evaporation). The rate of water flow determines the amount of nutrients that are transported to the root surface. Therefore, mass flow decreases. Most of the nitrogen, calcium, magnesium, sulfur, copper, boron, manganese and molybdenum move to the root by mass flow.

 

Diffusion: Diffusion is the process of movement of a particular nutrient along a concentration gradient. When there is a difference in concentration of a particular nutrient within the soil solution, the nutrient will move from an area of higher to lower concentration. As the sugar dissolves, it moves through parts of the water with lower sugar concentration until it is evenly distributed, or uniformly concentrated. Diffusion delivers significant amounts of phosphorus, potassium, zinc, and iron to the root surface. Diffusion is a relatively slow process in comparison to the mass flow of nutrients with water movement toward the root. Mobility of a nutrient within the soil is closely related to the chemical properties of the soil, such as CEC and AEC, as well as the soil conditions, such as moisture. When there is enough moisture in the soil for leaching to occur, the percolating water can carry dissolved nutrients which will be subsequently lost from the soil profile. The nutrients which are easily leached are usually those nutrients that are less strongly held by soil particles. For instance, in a soil with a high CEC and low AEC, nitrate (an anion) will leach much more readily than calcium (a cation). Additionally, in such a soil, potassium will leach more readily than calcium (divalent cation) since calcium is more strongly held to the soil particles than potassium.

 

 

Summary:

 

Soil chemical properties influence various processes in the soil which make it applicable for agricultural practices as well as various other purposes. Texture, structure, and porosity effect the movement and retaining of water, air and solutes in the soil, which subsequently have impact on plant growth and microbial activity.

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