The liquid phase properties of a soil, such as water content, and how water passes through the solum, are very important factors that need to be considered for plant growth and other management strategies, as they affect the mechanical properties of the soil. Hydraulic properties depend on the soil type, and land use, and for that reason vary from site to site.
If the hydraulic properties are not taken into account, the results
can be very detrimental and destructive to the soil. Loss of structure,
slaking, surface smearing and reduced infiltration from compaction are
all directly interlinked results of mismanagement of hydraulic properties.
WETNESS
The need to determine the amount of water in the soil arises frequently in many agronomic, ecological and hydrological investigations aimed at understanding the soils chemical, mechanical, hydrological and biological relationships (Hillel, 1982).
Wetness is defined as the mass of water relative to the mass of dry soil particles, and is often referred to as the gravimetric water content. Wetness depends largely on the porosity of a soil, and for that reason clayey soils, which have a high porosity generally have larger water content than do sandy soils. Also, if a soil has been compacted the result will be that the water content will decrease, as the inter-basal spacing between the pores will have been decreased.
In Narrabri the wetness was measured by way of Core Samples. This is the traditional method (Hillel, 1982) of measuring gravimetric moisture, and involves the removal of a soil core and the subsequent determination of the moist and dry weights. The moist weight is determined by weighing the sample as it is at the time of sampling, and the dry weight is obtained after drying the sample at 105o C in an oven. The mass of water lost per mass of of oven-dry soil is commonly known as wetness.
The core samples are fairly impractical as they are so time consuming with taking samples, sealing these to prevent loss of initial moisture, transporting from the field to an oven, many hours in the oven and repeated weighing all take too much time to be used as a realistic, regular method. The standard method of drying in itself is arbitrary, as some clays may still contain perceivable amounts of water even at 105o C. On the other hand, some organic matter may oxidise and decompose at this temperature so that the weight loss may not be entirely due to evaporation of water (Hillel, 1982). The errors of this method may be reduced by increasing the number of samples, but this then has to be balanced with the destruction and disturbance that taking so many cores can cause the site.
The advantages of this method are that no initial high cost is involved, and the fact that although time consuming, the concept behind this method is easy to understand and carry out.
From analysis of the results it became very clear that there was a significant difference between the tilled and the pasture in terms of wetness. The tilled section of the site was far moister, with a mean wetness of 0.21 g/g, while the pasture only had a mean of 0.08 g/g.
From the core samples that were used to measure the gravimetric moisture content, we can also work out the bulk density, by calculating the exact volume of the core.
When bulk density is plotted against land management, there is once
again a significant difference between the tilled and the pasture regimes.
The pasture has a much higher bulk density, with an average of 1.4 g/cm3,
while the tilled bulk density average is 1.2 g/cm3 . The significant
variations in bulk density are due to water content, which was greater
in the tilled area. Therefore, on drying, the ratio of solid mass to total
volume was greater in the pasture sites.
VOLUMETRIC WATER CONTENT
Another way to express the moisture content of a soil is the volumetric water content and is defined as the volume of water relative to the total volume of soil. The use of volumetric water content rather than wetness is often more convenient because it is more directly adaptable to the computation of fluxes and water quantities added or subtracted to a soil. Also, volumetric water content represents the depth ratio of soil water, i.e., the depth of water per depth of soil.
In Narrabri the volumetric water content was tested by two different methods, these being the Neutron Probe and by Time Domain Reflectometry (TDR). The Neutron Probe was first developed in the 1950’s, and since then has gained widespread acceptance as an efficient and reliable technique for monitoring soil moisture in the field (Hillel, 1982). The instrument consists of two main components:
The neutron probes principle advantages over the gravimetric core sample method are that it allows less laborious, more rapid, non-destructive, and periodically repeatable measurements, in the same locations and depths, of the volumetric wetness of a volume of soil. The method is practically independent of temperature and pressure.
Its main disadvantages, however, are the high initial cost of the instrument, low degree of spatial resolution, difficulty of measuring moisture in the surface zone (within about 15 cm of the surface, fast neutrons escape through the surface), and the health hazard associated with exposure to neutron and gamma radiation. Another disadvantage is that it is fairly laborious to move the neutron probe from one site of measurement to another, as it involves the creation of a deep narrow hole down which to pass the neutron probe. In Narrabri a mechanical soil auger was used for this, which made the task fairly simple, but this kind of equipment also involves a large financial setback.
At the survey site in Narrabri, the neutron probe was only used at four of the ten reference pegs (two in the tilled section T1 and T2 and two in the pasture section P1 and P2) due to the difficulty of augering the hole through which to pass the probe. At these four sites, the moisture content readings were taken every morning and afternoon for four days. After analysing the results it became clear that the neutron probe is not ideal for measuring very small variations in moisture content, as was the case at the survey site. This is because seems to average out the readings, which make it hard to detect small fluctuations. For practical purposes they are used extensively for irrigation scheduling, such as in the case of the Narrabri cotton fields, where there is a wide range of moisture contents and the results are more accurate.
As already mentioned, the results are not very good, as the neutron
probe finds it difficult to detect small variation in moisture content.
The results show that the average water lost per day from each of the four
sites are:
TABLE 1: Neutron Probe average daily moisture loss.
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In theory there are two reasons why one would expect the tilled section
to be loosing less water per day than the pasture section, although the
results in Table 1 do not convey this. The first of these, is that the
pasture has plants that are extracting moisture out of the soil, and the
second reason is that the tilled sites would have a lower evaporation rate
due to the surface crust that has formed, which acts like dry mulch that
reduces water loss in the lower layers.
The other method of measuring volumetric moisture content was by Time
Domain Reflectometry (TDR). This method measures the bulk soil dielectric
constant and electrical conductivity for simultaneous determination of
soil moisture content and salinity. As with the Neutron Probe, the most
common use for TDR is for irrigation scheduling.
The way that TDR works is by embedding a parallel three pronged metallic probe into the soil. An electromagnetic pulse is launched along the probes, and the travel time and dissipation measured for the pulse to travel to the end of the probe and back again (Dalton, 1992). The volumetric water content is calculated over a cylindrical volume of soil whose diameter is the distance between the prongs, and whose length is the length is the length of the metal prongs.
The dielectric constant reduces and imposes the velocity of the electromagnetic pulse, and therefore the time taken to travel up and down the probe or wave guide is increased. Thus the dielectric constant is inversely proportional to the time of travel. The more water is in a soil volume the larger the dielectric of that soil and the more the time of travel.
Advantages of this method are, that unlike the Neutron Probe, TDR can measure very close to the surface by insertion of the probes horizontally. Also, once the TDR has been inserted into a soil profile, it does not require any more human labor to work out the water content, as the TDR can be connected to a data-logger which constantly monitors the progress of the site.
Disadvantages are that the TDR measures only a small area compared to Neutron Probe, only 8cm or 20 cm, depending on which length of probe is used. As the 20 cm probe measures a larger cross section of the soil, and also the time measurements are more accurate over this length, it will give a more accurate results than the more localized 8cm probe.
Analysis of the data from the TDR, in both wet and dry states correlate to the gravimetric core sample method by showing again that the pasture was drier than the tilled.
Another experiment that was carried out at P2 with the TDR, to see how saline water affects the accuracy of the measurements that are made.
Firstly a tension disk permeameter was run into the soil to achieve
steady -state infiltration. This was followed by a pulse of saline water,
and finally more water via the disk. Figure below shows how salt effects
TDR measurements at 1 cm and 5 cm depths. The TDR overestimates the water
content when in saline soils because the ionic solution absorbs some of
the electromagnetic pules, thereby increasing the time of travel.
Theoretically, due to the saturated state of the soil, the moisture
content should not have increases drastically. But the TDR at 1 cm recorded
the pulse of salt as a clearly detectable increase in moisture content,
as it moved with piston-like flow. By the time the saline solution was
recorded by the 5 cm TDR, the effect was more diffuse.
The energy state of water in the soil is an important physical factor which can affect the growth of plants. "The physiochemical condition or state of soil water is characterised in terms of its free energy per unit mass, termed the potential."(Hillel, 1980).
Differences in the potential energy of water in the soil causes water flow. In the soil, water moves from high to low potential. This is expressed as the potential energy of soil water relative to that of water in a standard reference state. Knowing the potential of soil water can help estimate how much work plants need to exert to extract water from the soil matrix. Under normal conditions in the field the soil is not saturated and hence has negative potential as the hydrostatic pressure of water in this state is less than that of the reference.
Soil water is subject to many force fields that cause its potential to differ from that of the standard state. (Hillel, 1980) These include the attraction of the soil matrix for water, presence of solutes, external gas pressure and gravitation.
Gravitational energy is independent of the chemical composition and pressure of the soil water, therefore depending only on relative elevation. Matrix potential results from the capillary and adsorption effects of the soil. It denotes the affinity of the water to the whole matrix of the soil including its pores and particle surfaces together.
Adsorption is most important in clayey soils and is influenced by the double layer and exchangeable cations. In sandy soils, adsorption is more important and the capillary effect predominates.
Wetness and matrix potential are functionally related to each other . The graphical presentation of this relationship is called the moisture characteristic curve. From this curve it is possible to acquire knowledge on the pore size distribution and structure.
The filter paper method can be used to obtain a soil matrix potential reading. It involves equilibrating filter paper with the soil to obtain a moisture content of the soil which is then converted to a matrix potential using a standard moisture characteristic of the filter paper.
The principle behind the filter paper method is that the pores in the papers exert a matric potential on soil moisture. In response to this negative potential, water moves from the soil into the paper. As water enters the filter paper, the potential of the paper decreases until equilibrium between the soil moisture potential and the filter paper moisture potential is achieved. The gravimetric moisture potential of the filter paper can be determined by the difference between the wet and dry weights of the filter paper divided by the mass of the dry filter paper. This gravimetric moisture content can be directly related back to the moisture potential of the moist filter paper through use of the known moisture characteristic. This moisture potential of the filter paper is equal to the moisture potential of the soil sample.
The other method used for obtaining soil water potentials, were gysum blocks, which are comprised of two electrodes embedded in a block of gypsum. The electrical resistance between these two electrodes is measured which is converted to give a moisture potential. The gypsum block once in the soil equilibrates with the soil water suction of its surrounding environment, through which it conducts electrical current .The gypsum block has a fixed porosity distribution that supplies a fixed environment for the electrodes regardless of the surrounding soil type.
Gypsum blocks also have characteristic electrical conductivity and resistance attributes. When they are relatively dry, the resistance of the blocks is very high. However, with increasing moisture (decreasing moisture potential) within the blocks, the resistance diminishes. As a result of this phenomenon, it is possible to find an empirical equation describing this relationship and allowing the moisture potential of a soil to be determined indirectly through measuring the electrical resistance of gypsum blocks embedded in the soil.
The gypsum block is cheap, practical and reliable with a life span of approximately five years. The addition of a data logger make this a convenient way to measure soil potential and moisture content. A disadvantage of this method, is that after the initial insertion of the blocks, a time period of several days is required for equilibration.
