Soil moisture characteristic

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Soil Water Measurement


The amount of moisture in the soil is an important soil characteristic in atmospheric water cycle for both small and large agricultural activities. Plants rely on soil moisture at the roots compared with the actual precipitation. The understanding of the level of moisture content of the soil is a factor that enables forecasting the risk of flash floods. In this study, the aim is to illustrate the procedure followed during soil moisture content estimation. It also illustrates the manner in which laboratory outcomes can be analyzed to understand the moisture characteristics of the soil. The main activities during this study involved laboratory procedures such as sucking the water from a burette containing sand particles and measuring the water column. This was followed by creating a graph of water content against the suction force. The downward force on the capillary was determined by multiplying the density of water, gravitational acceleration and the radius of the capillary. Another parameter that was measured during the experiment is the surface tension in the interface between water and air. In order to determine the height of water column during the suction process, the formula used was h=3/d (cm). This implies that the resulting suction by the column of water with the meniscus in the burette 10cm below the soil was likely to drain pores larger than 0.015 cm. when a difference of 100 cm exists, pores greater than 15 µm are likely to drain.

The first step involved the preparation of 4 sintered glass funnels connected to the entry of the burette. The funnel was ¾ filled with water and the burette was also filled with water. The zero mark in the burette was kept in the same level as the funnel. 200g of sand was weighed into a beaker. The suction at which significantly high amount of water is produced was estimated. The common radius of sand was determined using the formula: r=2s/rgh. Water content as determined for each suction value. The experiment started from water content (w %) at field capacity and proceeded until the values of saturation was obtained. A curve was plotted showing the relationship between w% and the corresponding suction pressure. Volumetric water content (q %) was calculated on the same graph. Another graph was plotted illustrating the relationship between q and suction. At each specific suction, h, it was assumed that all pores with a radius smaller than r=2s/rgh was filled with water. The percentage (%) of water-filled pores at the suction of 100% was plotted to determine the cumulative pore size distribution curve.

The following is the graph of volume of water drained versus the suction (cm)

soil moisture characteristic

Figure 1. A graph of Cumulative Volume versus Suction

The following is a grapgh of of mL of water per cm scution versus the suction applied

soil moisture characteristic 1

Figure 2. mL per centimeter suction vs suction applied

The folowing is the graph of water content of sand versus suction

soil moisture characteristic 2

Figure 3. Water content of sand versus suction

The following is the graph of water-filled pores versus suction during the experiment

soil moisture characteristic 3

Figure 4. Graph of water-filled pores versus suction


The experiment illustrates the laboratory procedures that were followed in determining soil moisture content. The findings were recorded and analyzed graphically and the answers to the research questions obtained from the results.

The suction at which the highest amount of water was removed is 31.2cm and the most common pore size was determined by

The filed capacity is the amount of water in the soil after draining excess water. The main factor that affects filed capacity of the size of grains of soil used. In the experiment, the maximum filed capacity of the sand was 13.2 mL. From this experiment, the wilting point can be deduced to be at a capacity of 11.3 mL/

The maximum applied suction is 103.7 cm. the corresponding pressure can be determined using the equation, pressure = ρgh = 1000*9.81*103.2/100 = 10123.92 n/m2

When the pressure is expressed in rascals, we get = 10123.92 Pascal

Porosity was calculated as follows

Weight of the sand sample used= 15 gms

Weigh of the pipette filled with water = 64 gms

When sand is added to the water = 74 grams

Weight of the displaced water = 15+64-74 = 5 grams

Grain volume =6/2.65 = 2.264 cc

The porosity of the original sample was obtained by finding the original volume of sand used. This was done as follows:

Vg=Wdry/ρgr = 20g/2.65 = 7.55cc

The porosity wad determined using the formula (Vb-Vg)/Vb = (9.8-7.55)/7.55 = 23%

If the profile of sand was 0.7m deep, at field capacity, the amount of water likely to be stored in the profile is 7*0.67 = 4.69 cm.

From this experiment, it is observed that soil materials that exhibit swelling and shrinkage would not be suitable because it would undergo shrinking while the suction is done. This will prevent the suction to be done in other stages. Thus, it would not be possible to achieve the objectives of the experiment.


This paper involved an investigation of the porosity of sand particles using various kinds of laboratory equipment and estimating the amount of water sucked from the burette. This was followed by graphical representation of various parameters and calculation of porosity of the sand. The results were discussed and the experiment concluded. The experiment is an important insight to the understanding of various characteristics of the soil that enables its use in a number of areas of application such as farming and other activities whose accomplishment is determined by the amount of water that can be retained by a soil sample.


Starr, J. L., and I. C. Paltineau. «Methods for measurement of soil water content: capacitance devices.» Methods of soil analysis: part 4 (2002).


Initial reading on Burette

Burette reading after removing 5 mL (mL)

Burette reading after equilibrium

Water removed from sand

Suction applied (cm) (height of water in burette)

Cumulative volume drained from sand

Volume per cm suction

Water filled pores

(mL) in whole sand sample