The soil, a complex, dynamic and changing structure

Soil texture

Soil texture is the percentage of the weight of the total mineral soil and it corresponds to several classes of particle sizes: gravel, sand, silt and clay.

  • Particles larger than 2 mm in diameter are classified as gravel .
  • The sand is easily distinguishable, has little capacity to absorb and retain nutrient cations.
  • The silt is finer than sand, is grainy to the touch and retains better water and nutrient ions.
  • Clay particles are difficult to see with the naked eye and are like dust to the touch, they are colloidal so they can form a suspension in water and they are also active sites for the adhesion of nutritive ions or water molecules. As a result, clays control the most important properties of the soil, including plasticity and ion exchange between soil particles and the water present in the soil. However, soil with a high clay content can have problems with water drainage and when it dries it will show cracks.

Most soils are a mix of texture classes.

From an agricultural perspective, sand provides the soil with good drainage and helps facilitate cultivation, but sandy soil also dries easily and loses nutrients through leaching. On the contrary, clay tends to not have good drainage and can easily become compacted and make work difficult, however, it is good at retaining moisture and nutrients.

Whether the texture of a soil is good depends on the crops that are growing in it. For example, potatoes grow best in well-drained, sandy soils, which helps prevent tubers from breaking and makes harvesting easier. Palay rice grows best in heavy soils, with high clay content due to the adaptations of the crop to humid environments. In general, a clay loam soil may be better especially in dry environments, while a sandy loam is better in humid environments. But in addition, the addition of organic matter changes the relationships of the particles in the mixtures.

The information provided by the color of the soil

Soil color plays a more important role in identifying soil types , but at the same time it can tell us a lot about the history of soil management and development.

  • The dark colors are usually an indication of high organic content, especially in temperate regions.
  • The red and yellow soil generally indicate high levels of iron oxide formed under conditions of good ventilation and draining.
  • The gray or brown yellowish colors may indicate poor drainage, these colors are formed when iron is reduced to ferrous form more ferric oxidation wing forms by the abundance of oxygen.
  • The whitish or light colors often indicate the presence of quartz, carbonates or plaster.

Standardized color charts are used to determine the color of the soil.

Soil color can be an indicator of certain kinds of soil conditions, although more specific analysis of soil chemistry and structure would be required to complete the information, but color is a good start.

Catonic exchange capacity

Plants obtain mineral nutrients from the soil in the form of dissolved catons, the solubility of which is determined by the electrostatic attraction of water molecules. Some important mineral nutrients, such as potassium and calcium, are in the form of positive ions, others, such as nitrates and phosphates, are in the form of negative ions. If these dissolved ions are not immediately absorbed by plant roots or fungi, they run the risk of being washed out of the soil solution.

The clay and humus particles, separately or together, form sheet-like structures (micelles) that have negatively charged surfaces and attract positive ions that are smaller and more mobile. The amount of sites available in the micelles to bind positive ions (cations) determine what is called the cation exchange capacity of  the soil, which is measured in milliequivalents of cations per 100 g of dry soil or in cmol (+) kg -1 of soil. The higher the cation exchange capacity, the more capacity the soil has to retain and exchange cations, preventing the leaching of nutrients and allowing plants to have adequate nutrition.

The cation exchange capacity varies from soil to soil, depending on the structure of the clay / humus complex, the type of micelle present, and the amount of organic matter incorporated into the soil.

Humus increases the cation exchange capacity due to its colloidal nature, being of great help in soils used for agriculture.

Soil acidity and its pH

Soil pH and acid-base balance is of great importance for agriculture and gardening .

The pH range of soils is between very acidic (pH 3) and strongly alkaline (pH 8). Any soil close to 7 (neutral) is considered basic and those below 6.6 are considered acidic. Few plants, especially agricultural ones, do well outside the pH range of 5 to 8.

Legumes are particularly sensitive to low pH, due to the impact that soil acidity has on symbiotic microbes in nitrogen fixation. Bacteria, in general, are adversely affected by low pH.

The acidity of the soil is known for its effects on the availability of nutrients, however, these effects are not so much due to direct toxicity on the plant but rather to the difficulty of the plant to absorb specific nutrients both at very low pH and to very high. That is why it is important to keep the soil pH in the optimal range. Many soils increase their acidity through natural processes. The acidification of the soil is the result of loss of base by leaching caused by water seeping into the ground, by the extraction of nutrient ions by plants and production of organic acids by plant roots and microorganisms.

Salinity and alkalinity of soils

In the soils of arid and semi-arid regions of the world, the accumulation of salts is common, both in soluble and insoluble forms. In areas of low precipitation and high evaporation, dissolved salts such as Na + and Cl- are common, combined with others such as Ca2 +, Mg2 +, K +, HCO3- and NO3-. Irrigation often adds more salts to the soil, especially in areas with high evaporation potential where the added salts reach the soil surface by capillary movement during evaporation. In addition, many inorganic fertilizers, such as ammonium nitrate, can increase salinity because they are in the form of salts.

Soils with a high concentration of neutral salts are called saline, which generally have a pH greater than 8.5. These soils constitute a problem for plants due to the osmotic imbalance.

Alkaline soils are a problem due to the excess of OH- ions, due to the difficulty for the extraction of nutrients and for the development of plants. In some regions, saline-alkaline conditions occur when both forms of salts are present.

The proper management of soil water and irrigation becomes the key aspect to face these conditions.


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