Determinación del contenido de humedad en el suelo
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Overview
Fuente: Laboratorios del Dr. Ian Pepper y el Dr. Charles Gerba – Universidad de Arizona
Demostrando autor: Bradley Schmitz
Suelos contienen normalmente una cantidad finita de agua, que puede ser expresado como el “contenido de humedad del suelo”. Esta humedad existe dentro de los espacios de poro entre los agregados del suelo (espacio de poros entre agregados) y dentro de agregados del suelo (espacio de poros intra agregados) (figura 1). Normalmente este espacio de poros está ocupado por aire y/o agua. Si los poros están ocupados por aire, el suelo está completamente seco. Si los poros están llenos de agua, el suelo se dice que ser saturada.
Figura 1. Espacio poroso en el suelo.
Principles
En ambientes naturales al aire libre, se agrega agua al suelo por lluvia o riego deliberada de plantas. En cualquier caso, la humedad del suelo aumenta más los poros se llenan con agua a expensas de aire. Si los poros se llenan con agua, exceso de agua ahora lixiviará hacia abajo (figura 2) a través de poros de suelo continuo, hasta que el cese de la lluvia o el riego. Lixiviación continuará hasta que las películas de agua dentro de los poros son sostenidas por la tensión superficial de los coloides del suelo contra la fuerza de gravedad. Esta situación se conoce como el suelo está a capacidad de campo del”” con respecto a la humedad del suelo. Un suelo a capacidad de campo tiene poros parcialmente llenos de aire, rodeada de películas de la humedad del suelo. Normalmente un suelo a capacidad de campo es óptimo para el crecimiento de las plantas y los microorganismos del suelo aeróbico, ya que hay aire y agua. Por el contrario, un suelo saturado creará anegadas condiciones anaerobias que pueden matar las plantas y suprimir microbios de suelo aeróbico, mientras que estimular los microbios anaerobios.
Figura 2. Lixiviación en el suelo de nutrientes.
Considere una muestra de suelo húmedo en un recipiente como un vaso de precipitados. El peso del suelo húmedo consiste en el peso de las partículas del suelo seco más el peso del agua en el suelo. Si se agrega más agua al suelo, aumenta el peso húmedo del suelo. El peso seco de las partículas de suelo dentro de la muestra es fija , es decir, un peso que es el peso seco. Por el contrario, hay un número infinito de pesos húmedos, dependiendo de la cantidad de agua se agrega al suelo. Debido a esto, cuando hacer laboratorio de experimentos con el suelo, el contenido de humedad del suelo normalmente se expresa en una base de peso seco, ya que el peso seco es constante en el tiempo, mientras que el peso húmedo o mojado puede cambiar con el tiempo. Al expresar los resultados de un experimento como el contenido de nutrientes de un suelo, uso de la base de peso seco proporciona normalización del resultado final.
Procedure
Results
Calculate the soil moisture content for each of the replicate samples using the following equation:
% moisture content (MC) = 
(dry wt. basis)
Example Calculations:
M = 102 g
D = 90 g
∴ % MC = 
MC = 13.3%
With the addition of 5 g of water, new M = 107 and D still equals 90
∴ % MC = 
New MC = 18.9%
Applications and Summary
Knowledge of the moisture content of a soil on a dry weight basis is useful in a number of ways. For example, if the experiments are conducted with soil that should be amended with a known concentration of ammonium fertilizer (for example 50 μg/g), then the moisture content on a dry weight basis must be determined. If the calculation was completed on a wet weight basis, the amount of fertilizer to be added would depend on the moisture content (and therefore the moist weight) of the soil sample. Likewise, if potted plants are considered, the moisture content must be known in order to make sure that the soil isn’t too dry (not enough moisture for plant growth) or too wet (waterlogged and anaerobic). In a field situation, knowledge of the soil moisture content can prevent excess irrigation and leaching of soil nutrients.
Transcript
The amount of water held in soil is an important component of biological and ecological processes, and is used in applications such as farming, erosion prevention, flood control, and drought prediction.
Soils typically contain a finite amount of water, which can be expressed as the soil moisture content. Moisture exists in soil within the pore spaces between soil aggregates, called inter-aggregate pore space, and within pores in the soil aggregates themselves, called intra-aggregate pore space. If the pore space is occupied entirely by air, the soil is completely dry. If all of the pores are filled with water, the soil is saturated.
The measurement of the amount of water held within the soil, or the soil moisture content, is essential to the understanding of soil characteristics and the types of plants and microorganisms that reside in it.
This video will introduce the basics of soil moisture content, and demonstrate the procedure for determining moisture content in the laboratory.
In outdoor environments, water is added to soil naturally through rainfall or deliberately with the irrigation of plants. As the pores in the soil become filled with water at the expense of air, the soil moisture increases. When all of the pores are filled with water, the soil is saturated. If the soil at the surface is saturated, excess water will leach downward through pores into deeper soil. Leaching continues until there is not enough water to saturate all of the pore space. At this point pores contain some air and thin films of moisture. The water films within the pores are held by the surface tension of soil colloids, thus water stops leaching.
After leaching stops, and excess water has drained from the soil, the soil is described as being at field capacity. Soil at field capacity has pores that are partially filled with air, surrounded by films of moisture. Soil at field capacity is optimal for plant growth and aerobic soil microorganisms, since both air and water are available. In contrast, saturated soil, where all pores are filled with water, will create an anaerobic environment that can kill plants and suppress aerobic soil microbes.
The mass of moist soil consists of the mass of the dry soil particles, plus the mass of the water within the soil. The dry mass of the soil particles is fixed, whereas the amount of water within moist soil can vary. Therefore, moisture content is calculated on a dry basis, rather than a total mass basis, to ensure consistency. The moisture content of soil is described as the ratio of the mass of water held in the soil to the dry soil. The mass of water is determined by the difference before and after drying the soil.
The following experiment will demonstrate how to measure soil moisture content in the laboratory using these principles.
To begin, collect soil samples and transfer them into the laboratory. Samples of soil can be collected in the field using a soil auger, or a trowel. Use of a soil auger allows for the soil to be sampled to specific depths. Transfer them into the laboratory. Weigh two aluminum dishes, and accurately record the weight of each dish. Aliquot approximately 20 g of the moist soil into each aluminum dish, then reweigh the dish. Subtract the weight of the empty dish from the full dish to acquire the moist soil weight.
Next, dry the soil overnight in an oven set to 105 °C. On the next day, carefully remove the soil samples from the oven using tongs. Place the soil samples on the bench top to cool. When the dry soil samples are cool, reweigh them and record the total weight. Subtract the weight of the aluminum dish, and record the dry soil weight.
Calculate the moisture content of the soil by subtracting the weight of the dry soil from the weight of the moist soil, and then dividing by the weight of the dry soil.
Although the measurement is simple, it is important to determine soil moisture content in order to better understand soil characteristics.
Soil moisture content plays a large roll in environmental concerns, especially when considering soil runoff that may contain fertilizers and pesticides. In this example, soil runoff was analyzed using a simulated rainfall study in order to determine the retention of a compound in moist soil.
Soil, containing urea, was packed into soil boxes and assembled under a rainfall simulator. Soil runoff was collected, and the concentration of urea in the runoff water calculated. The amount of urea in the soil runoff was higher for soils that had higher moisture content, indicating that urea is better absorbed in drier soil, than in moist.
The fate of chemicals in soil can also be analyzed by direct pore water sampling, using a lysimeter, as shown in this example. In this experiment, lysimeters, or long metal tubing, were installed in soil with turf grass to analyze pore water in vegetative soil.
The pore water sampler was then installed, and water pumped from the lysimeter after applying chemicals to the soil. The collected water was then analyzed, and the concentration of applied chemicals correlated to soil depth and moisture content.
The results demonstrated that the concentration of the herbicide monosodium methyl arsenate, or MSMA, was the highest in the top 2 cm of soil.
You’ve just watched JoVE’s introduction to soil moisture content. You should now understand how to accurately measure soil moisture content in the laboratory. Thanks for watching!
