Soil Science

Soil Science
Soil Science encompasses many aspects of soils study such as the disciplines of soil physics, soil chemistry, soil classification, soil microbiology, etc. One of these studies is pedology. Pedology is the study of soils in a three dimensional context in their natural setting across the landscape in terms of origins, morphology (forms), classifications, characteristics or attributes, profile or cross-section, surface and internal water relations, geology, plant ecology, etc. and interpretations for use and management.

Most soils develop from weathered mineral geologic materials and usually include some organic materials in their upper part. This is called the O horizon, and it is composed of organic material, generally not considered topsoil, but leaf litter and muck. The rock that the mineral part of the soil originates from is called the parent material. Through the further influence of the soil-forming processes of additions, removals, transfers and transformations the nature of the parent materials are altered to the extent that they become soils. The soil-forming processes are controlled by the soil forming factors of parent material, climate, living organisms, relief, and time. Thus the similarity or contrast of soils from place to place reflects the similarity or contrasts of the soil-forming processes and factors.

The primary convention for naming soils is to select a named geographic feature in the vicinity where the soil was first recognized and identified as a new soil, e.g: the Dunkirk soil series for the village of Dunkirk near Lake Erie, NY. All soils found to have closely similar attributes are classified as Dunkirk wherever they are found. That distribution can be more localized or it can be somewhat more extensive.

The description of the soil usually identifies attributes such as pH (degree of alkalinity or acidity), maturity (stage of pedogenic development), texture, consistence, color, gravel content and type, drainage class, and other soil features.

The United States and other nations have soil maps that show the soils in relation to one another as they occur across the landscape. On these maps each soil is characteristically related to a unique landscape position. Thus, the pattern of distribution and the location and extent of the soil type can be comprehended. Soil maps are a prime requisite for many users who need site information to make informed decisions on many aspects of use and management. Comparison of soil resources and their respective suitabilities and/or limitations can also be used to evaluate alternate sites for intended uses.

In the United States the Cooperative Soil Survey has more than a hundred year history. The various surveys were published with text and maps as soft covered books. Now they are published through the internet Web Soil Survey at: http://websoilsurvey.nrcs.usda.gov/app/

Soil Equations
The Bulk Density of a soil is its dry mass divided by the total original volume of the soil. The porosity of a soil is the volume of its pores divided by the total volume of the soil. The particle density of a soil is the mass of its solids divided by the volume of its solids.

Soil Water
Field Capacity When a soil has drained freely for several days after rain or irrigation, the water content is the field capacity of the soil.

The field capacity is a measure of the soils ability to store water. A sandy soil freely drains and the field capacity is low. A clayey soil holds water and has a high field capacity. A clayey soil can store more water than a sandy soil.

Soil structure effects the field capacity. A structureless clay with no large pores has a very high field capacity. A well structured clay with large pores will drain freely and has a lower field capacity.

Organic matter can absorb water and increases field capacity.

The field capacity of a soil is mainly determined by the pore size distribution. Small pores hold water by capillary forces. Large pores freely drain and do not hold water at field capacity.

Wilting Point When a soil is dry and plants suffer from permanent wilting because they are unable to absorb water, the moisture content is the wilting point of the soil.

When the moisture content is above the wilting point, plants can absorb water from the soil. Below the wilting point, water is tightly adsorbed to clay particles and is unavailable to plants.

Sandy soils have very low wilting points while clays have high wilting points. A clay contains a significant amount of water unavailable to plants.

Available Water The amount of water stored by a soil that can be absorbed by a plant is the available water and is equal to the water held at the field capacity minus the wilting point.

The available water is the water stored by a soil and is useful to plants. Texture, structure and organic matter have an effect on the available water. Clayey soils generally have a higher available water than sandy soils. A well developed structure can increase the available water. Organic matter absorbs water and increases the available water.

The available water stored in a soil is often measured as mm of water. Farmers prefer to sow a crop when there is sufficient water stored in the soil to ensure good plant growth. If the amount of stored water is low, the farmer needs to pray for rain. The amount of water available to plants also depends on the depth of the plant roots.

Soluble salts in soils reduces the availability of water to plants. Salt increases the osmotic pressure of water. High salt content in soils will kill plants. 

Soil Water Infiltration
It is well known that urban development increases flooding. In urban areas the coefficient of runoff increases, and the time of concentration decreases. The coefficient of runoff in undisturbed areas can be as low as 0.1, while in inner city suburbs, coefficient of runoff can be as high as 0.9.

During rainfall a high value of coefficient of runoff occurs when most rainfall flows over the land and little water is absorbed by soils. When all the rain flows over the surface and no rain infiltrates into the soil the coefficient of runoff equals 1. If a high volume of water enters the soil the coefficient of runoff is small and approaches 0.

The main source of fresh water is rainfall runoff, which is widely used to meet human needs. Runoff is a vital part of long term water supply and renews water resources be they rivers, lakes or reservoirs. Plants need water to grow and evapotranspiration by plants returns water from the soil into the atmosphere.

Rate of water infiltration The rate at which water enters the soil during rain or irrigation is the infiltration rate.

When rain first enters a dry soil, infiltration is rapid, then decreases as it continues to rain. During the early stages of rainfall, water fills up the dry pores. When the pores are filled with water and the soil is saturated, infiltration slows down until it reaches a constant value.

When it has stopped raining, the surface soil will be saturated with water. Free water in a saturated soil will drain under gravity. After about 2 3 days, free water will have percolated downwards and the soil moisture will reach an equilibrium. When the free water has drained downwards, the moisture is at 'field capacity'.

At field capacity, soil water is held in the soil by several forces. The major part of the water is held by capillary forces. Some water is adsorbed onto the surfaces of soil particles, especially clay particles and organic matter.

Capillary water is the main supply of water to plants. Capillary water may be removed by transpiration by plants and evaporation. If the top soil is dry and the underlying soil is wet, water can move upwards by capillary action.

Above the water table there is a capillary fringe. Water moves upwards from the water table by Capillary forces. In some. soils, especially clays which contain many fine pores, the capillary fringe can be up to one metre above the water table.

During rain the infiltration rate slows down as the soil becomes wet.

Infiltration rates vary greatly between different soils, and typical values are, 100mm/hour during the first hour of rain, decreasing to 10mm/hr after six hours of rain.

Peds are small aggregates (1 2mrn ) found in many soils. Pedal soils have a good structure and water flows easily through these soils. Apedal soils have a poor structure and apedal clayey soils have poor water infiltration. Sandy soils generally have high water infiltration and clayey soils have low infiltration rates.

If the intensity of rainfall is constant, and infiltration decreases at a typical rate, there is no runoff during the early stages of the rain event. When the infiltration rate is slower than the rainfall intensity, runoff is expected to occur.

Infiltration rate is controlled by the soil layer which has the lowest hydraulic conductivity. Often the top soil is a loam and has a high hydraulic conductivity. A clayey subsoil with a low hydraulic conductivity will act as a throttle, slowing down the final infiltration rate.

In the field, infiltration can be directly measured using a ring infiltrometer. A circular tube is forced into the soil, which is ponded with water. At specific time intervals, rate of water flow into the soil is recorded. To reduce variability, the rings need to be fairly large, generally rings should be bigger than 112 meter in diameter. Often a double ring infiltrometer is used to reduce errors from lateral water movement. Variability in these experiments is often high, so it is advisable to carry out a number of replicates.

It is common in many soils for the infiltration rate to decrease by a factor of ten, after six hours of rain. A steady state equilibrium is usually reached before six hours. Infiltration rates vary greatly between different soils, and typical values are, 10Omm/hour during the first hour of rain, decreasing to 1 mm/hr after six hours of rain.