在土壤中的水分含量的测定
English
Share
Overview
资料来源： 实验室的博士伊恩胡椒和博士查尔斯称-亚利桑那大学
演示作者： 布拉德利施密茨
土壤通常含有有限数量的水，可以表示为”土壤水分含量”。这水分存在和土壤团聚体 （内集料孔隙空间） 内土壤团聚体 （间集料孔隙空间） 之间的孔隙空间 (图 1)。通常这个孔隙空间是由空气和/或水占用。如果所有的毛孔都被空气，土壤是完全干燥的。如果所有的毛孔都充满了水，土壤是说饱和。
图 1。在土壤中的孔隙空间。
Principles
在室外的自然环境中，水被添加到通过降雨或蓄意灌溉的植物的土壤。在任一情况下，土壤水分增加更多的毛孔变得充满水以空气为代价。如果所有的毛孔变得充满水，多余的水会现在浸下降 (图 2) 通过连续土壤孔隙，直到雨或灌溉停止。浸会继续直到水膜的孔隙内由表面张力的土壤胶体对重力的作用。这种情况称为处于”外地”的能力，对土壤水分的土壤。在田间持水量的土壤具有孔隙部分充满空气，土壤水分膜所包围。通常在田间持水量的土壤是最优为植物的生长和有氧土壤中的微生物，因为空气和水是可用。相比之下，饱和的土将创建淹水的厌氧条件可以杀死植物和抑制有氧土壤中的微生物，同时刺激的厌氧微生物。
图 2。浸在土壤中的养分。
考虑一个烧杯等容器内潮湿的土壤样品。潮湿的土壤中的重量包括干燥土壤颗粒的重量再加上土壤中的水的重量。如果更多的水添加到土壤中，土壤的湿的重量增加。干重的土壤颗粒内部的样品即是干重的一个重量被固定。相比之下，有无限多的湿重，取决于土壤加多少水。正因为如此，当做实验室实验与土壤，土壤的水分含量是通常表示干重量在基础上，因为干燥重量是不断随着时间的推移而潮湿或湿重量可以更改随着时间的推移。当表达等土壤养分含量的实验的结果，使用干重基础提供标准化的最终结果。
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!
