Soil properties

Soil Structure

The term soil structure refers to the arrangement and organization of the particles which make up our soil. These particles can be arranged in a loose and haphazard manner, or they can form a distinct, uniformly structured pattern.

Basically, there are three broad categories of soil structure:

• Single Grained
  Single grained particles are those which are totally unattached to each other. They are entirely loose.
• Massive
  when soil is tightly packed, such as in the case of dried clay, it is said to be massive.
• Aggregated
  Aggregated soil is located somewhere between the two extremes. It is an intermediate condition in which the particles are relatively alike and arranged in small clods.

Soil Consistency

Soil consistency is the strength with which soil materials are held together or the resistance of soils to deformation and rupture. Soil consistency is measured for wet, moist and dry soil samples.

Kinds of soil on the basis of consistency:

1)      Loose, if the soil is non-coherent (single-grain structure)

2)      Hard, if the soil resists moderate pressure, can barely be broken between the thumb and forefinger, but can be broken in the hands without difficulty.

3)       Friable, if the soil crushes easily under gentle to moderate pressure.

4)      Firm, if the soil crushes under moderate pressure but resistance is noticeable

5)      Plastic, if a wire can be formed but, when it is broken and returned to its former state, it cannot be formed again

6)      Sticky, if the soil sticks to both the thumb and forefinger and tends to stretch a little and pull apart rather than pulling free from your fingers.

Soil Pores

Any open space within the soil framework. The porosity of a soil Is judged by the percentage of pore space. Water will not drain freely through the fine capillary pores, with an average pore space of less than 0.03 mm, and which retain water through surface tension, but drains freely through the larger non-capillary pores.

Pores are the sine qua non of soil. Soil without pores we call rock. Life enters the soil through the pores, and further it is sustained by them. Pores allow roots, plus other soil flora and fauna, to penetrate the soil. Pores act both as conduits for, and reservoirs of the necessities of life. Water infiltration and storage, gaseous entry and exhaust, plus chemical transport and exchange are all facilitated by the network of pores that in sum often accounts for about half of the soil's total volume.

Bulk Density:

The oven dry weight of a unit volume of soil inclusive of pore spaces is called bulk density. The bulk density of a soil is always smaller than its particle density. The bulk density of sandy soil is about 1.6 g / cm3, whereas that of organic matter is about 0.5. Bulk density normally decreases, as mineral soils become finer in texture. The bulk density varies indirectly with the total pore space present in the soil and gives a good estimate of the porosity of the soil. Bulk density is of greater importance than particle density in understanding the physical behavior of the soil. Generally soils with low bulk densities have favorable physical conditions.

Factors affecting bulk density
 
1. Pore space: Since bulk density relates to the combined volume of the solids and pore spaces, soils with high proportion of pore space to solids have lower bulk densities than those that are more compact and have less pore space. Consequently, any factor that influences soil pore space will affect bulk density.

2. Texture: Fine textured surface soils such as silt loams, clays and clay loams generally have lower bulk densities than sandy soils. This is because the fine textured soils tend to organize in porous grains especially because of adequate organic matter content. This results in high pore space and low bulk density. However, in sandy soils, organic matter content is generally low, the solid particles lie close together and the bulk density is commonly higher than in fine textured soils.

3. Organic matter content: More the organic matter content in soil results in high pore space there by shows lower bulk density of soil and vice-versa.

Soil organic matter:

Soil organic matter consists of a variety of components. These include, in varying proportions and many Intermediate stages:

• raw plant residues and microorganisms
  (1 to 10 per cent)
• "active" organic traction (10 to 40 per cent)
• resistant or stable organic matter (40 to 60 per cent) also referred to as humus.

Raw plant residues, on the surface, help reduce surface wind speed and water runoff. Removal, incorporation or burning of residues predisposes the soil to serious erosion.

Organic matter serves two main functions:

• Since soil organic matter is derived mainly from plant residues, it contains all of the essential plant nutrients. Accumulated organic matter, therefore, is a storehouse of plant nutrients. Upon decomposition, the nutrients are released in a plant-available form.
• The stable organic fraction (humus) adsorbs and holds nutrients in a plant available form.

Organic matter does not add any "new' plant nutrients but releases nutrients in a plant available form through the process of decomposition. In order to maintain this nutrient cycling system, the rate of addition from crop residues and manure must equal the rate of decomposition.

Soil color:

Soil color does not affect the behavior and use of soil, however it can indicate the composition of the soil and give clues to the conditions that the soil is subjected to. Soil can exhibit a wide range of color; gray, black, white, reds, browns, yellows and under the right conditions green. The development and distribution of color in soil results from chemical and biological weathering, especially redox reactions. As the primary minerals in soil parent material weather, the elements combine into new and colorful compounds. Aerobic conditions produce uniform or gradual color changes, while reducing environments result in disrupted color flow with complex, mottled patterns and points of color concentration.

Causes of Soil Color

Soil color is influenced by the amount of proteins present in the soil. Yellow or red soil indicates the presence of oxides Dark brown or black color in soil indicates that the soil has a high organic matter content. Wet soil will appear darker than dry soil. However the presence of water also affects soil color by affecting the oxidation rate. Soil that has a high water content will have less air in the soil, specifically less oxygen. In well drained (and therefore oxygen rich soils) red and brown colors caused by oxidation are more common, as opposed to in wet (low oxygen) soils where the soil usually appears grey.

Significance

Drainage refers to a soil's ability to get rid of excess water, or water in the macropores, through downward movement by gravity. It is affected by topography, texture, and tilth.  With few exceptions, one would be a crop like rice; most plants need fairly good drainage. Without good drainage, plant roots would lack oxygen, nitrogen would be lost, and certain elements like iron and manganese may become soluble enough to injure plant roots. Although clay soils are more likely to have drainage problems, drainage problems also occur on other soils where the water table is close to the surface. The water table is the upper surface of the ground water below which the soil is completely saturated with water.  Soil color can be affected by drainage. Soil color can be a tool to check if your soil is having drainage problems.

Soil depth refers to how deep, top to bottom, the topsoil plus the subsoil is. Depth can be easily determined by digging a hole. Soils are classified as being deep or shallow as follows: Deep = 1 meter; Moderately Deep 0.5 to 1.0 meters; Shallow 0.25 to 0.50 meters; and Very Shallow less than 0.25 centimeters.  Soil depth is important for plants because deeper rooting means more soil to explore for nutrients and water. Greater soil depth can also mean better drainage, as long as there are no restrictive layers in the subsoil.

A field's slope has a marked influence on the amount of water runoff and soil erosion caused by flowing water. Slope is usually measured in terms of percentages. A ten percent slope has ten meter of vertical drop per 100-meter horizontal distance. Soil conservation measures become necessary on land      with as little of a slope as 1-2% to avoid erosion problems.

Soil temperature:

Soil temperature plays an important role in many processes, which take place in the soil such as chemical reactions and biological interactions. Soil temperature varies in response to exchange processes that take place primarily through the soil surface. These effects are propagated into the soil profile by transport processes and are influenced by such things as the specific heat capacity, thermal conductivity and thermal diffusivity.

Factors Affecting The Soil Temperature And Its Control

1. Solar radiation:

The amount of heat from the Sun that reaches the earth is 2.0 cal/cm2 min -1 the amount of radiation received by the soil depends on angles with which the soil faces the Sun.

2. Condensation:

Whenever water vapor from soil depths or atmosphere condenses in the soil, its heat increases noticeably.

3. Evaporation:

The greater the rate of evaporation, the more the soil is cooled.

4. Rainfall:

Rainfall cools down the soil.

5. Vegetation:

A bare soil quickly absorbs heat and becomes very hot during the summer and become very cold during the winter.  Vegetation acts as a insulating agent.  It does not allow the soil to become either too hot during the summer and two cold during the winter.

6. Color of the soil:

Black colored soils absorbs more heat than light closured soils Hence black color soils are warmer than light colored soils.

7. Moisture content: 

A soil with higher moisture content is cooler than dry soil.

8. Tillage:

The cultivated soil has greater temperature amplitude as compared to the uncultivated soil.

9. Soil texture:

Soil textures affect the thermal conductivity of soil. Thermal conductivity decreases with reduction in particle size.

10. Organic matter content:

Organic matter reduces the heat capacity and thermal conductivity of soil,  increases its water holding capacity and has a dark color, which increases its heat absorbability.

11. Slope of land:
Solar radiation that reaches the land surface at an angle is scattered over a wider area than the same amount of solar radiation reaching the surface of the land at right angles.  Therefore, the amount of solar radiation reaching per unit area of the land surface decreases as the slope of the land is increases.

Soil drainage:

Soil drainage refers to the soil’s natural ability to allow water to pass through it. Dense soil will hold water, while loose soil will allow water to pass through quickly. Soil drainage may determine which types of plants grow well in it.

Description

Very rapidly drained
Water is removed from the soil very rapidly in relation to supply. Excess water flows downward very rapidly if underlying material is pervious. There may be very rapid subsurface flow during heavy rainfall provided there is a steep gradient. Soils have very low available water storage capacity (usually less than 2.5 cm) within the control section and are usually coarse textured, or shallow, or both. Water source is precipitation.

Rapidly drained
Water is removed from the soil rapidly in relation to supply. Excess water flows downward if underlying material is pervious. Subsurface flow may occur on steep gradients during heavy rainfall. Soils have low available water storage capacity (2.5-4 cm) within the control section, and are usually coarse textured, or shallow, or both. Water source is precipitation.
Well drained
Water is removed from the soil readily but not rapidly. Excess water flows downward readily into underlying pervious material or laterally as subsurface flow. Soils have intermediate available water storage capacity (4-5 cm) within the control section, and are generally intermediate in texture and depth. Water source is precipitation. On slopes subsurface flow may occur for short durations but additions are, equaled by losses.
Moderately well drained
Water is removed from the soil somewhat slowly in relation to supply. Excess water is removed somewhat slowly due to low perviousness, shallow water table, lack of gradient, or some combination of these. Soils have intermediate to high water storage capacity (5-6 cm) within the control section and are usually medium to fined textured. Precipitation is the dominant water source in medium to fine textured soils; precipitation and significant additions by subsurface flow are necessary in coarse textured soils.
Imperfectly drained
Water is removed from the soil sufficiently slowly in relation to supply, to keep the soil wet for a significant part of the growing season. Excess water moves slowly downward if precipitation is the major supply. If subsurface water or groundwater, or both, is the main source, the flow rate may vary but the soil remains wet for a significant part of the growing season. Precipitation is the main source if available water storage capacity is high; contribution by subsurface flow or groundwater flow, or both, increases as available water storage capacity decreases. Soils have a wide range in available water supply, texture, and depth, and are gleyed phases of well drained subgroups.
Poorly drained
Water is removed so slowly in relation to supply that the soil remains wet for a comparatively large part of the time the soil is not frozen. Excess water is evident in the soil for a large part of the time. Subsurface flow or groundwater flow, or both, in addition to precipitation are the main water sources; there may also be a perched water table, with precipitation exceeding evapotranspiration. Soils have a wide range in available water storage capacity, texture, and depth, and are gleyed subgroups, Gleysols, and Organic soils.
Very poorly drained
Water is removed from the soil so slowly that the water table remains at or on the surface for the greater part of the time the soil is not frozen. Excess water is present in the soil for the greater part of the time. Groundwater flow and subsurface flow are the major water sources. Precipitation is less important except where there is a perched water table with precipitation exceeding evapotranspiration. Soils have a wide range in available water storage capacity, texture, and depth, and are either Gleysolic or Organic.