Hobbies And Interests
Gravimetric Technique
The gravimetric method is a direct way of measuring the water content of the soil, whether it's plant-available or plant-unavailable moisture. According to Irrigation Automation, the estimator dries a sample of soil in an oven, heating it there for 24 hours at a temperature of 221 degrees Fahrenheit. The tester then expresses the amount of soil content in terms of inches of water for every foot of soil. The website cites this method as inexpensive and accurate, yet can require much labor and be time-consuming. Also, if the soil has rocks in it, it could complicate the process.
Hand-Feel Method
The hand-feel technique is faster than the gravimetric method. However, as the Colorado State Extension website says, a scientist should only use this system if he is an experienced soil moisture estimator. He may employ it with the gravimetric method, because while the hand-feel technique will lead to the sampling of a wider sampling of material, he can use the gravimetric method to corroborate the results of the hand-feel test.
Di-Electric
The di-electric approach to soil research involves time domain reflectometry (TDR) and frequency domain reflectometry (FDR), also called capacitance, probes. Its aim is to measure the difference in the capacity of the soil to transmit high frequency magnetic pulses, or waves. The key difference is that while the TDR measures the time a wave requires for travel through the soil -- the FDR gauges soil capacitance through the use of radio frequency waves. According to Irrigation Automation, these probes are highly accurate, with a plus or minus 1 to 2 percent margin of error. However, the FDR and TDR both have drawbacks. For example, the FDR is subject to salinity errors and its probes are subject to damage on rocky soil; the electronics of the TDR are quite complex and expensive.
Neutron Probe Method
The neutron probe method involves an instrument, a neutron probe, which measures the slowdown of any rapid neutrons that a collision between the soil water and molecular hydrogen may emit into the soil. It gauges the total amount of water in a volume of soil-based on the slowdown of the neutrons. According to Irrigation Automation, these probes are quite expensive: As of August 2011, they can run anywhere between $3,500 and $4,000.
Tensiometer Technique
A tensiometer is an airtight, hollow tube full of water. The user attaches a porous ceramic cup to the end of the tube. The scientist places the tube in the soil. At the other end is a vaucum gauge. The tensiometer measures soil moisture tension, which Irrigation Automation describes as an index of how tightly a given soil sample can hold water. An advantage to this system is that salt dissolution in water is no object to the functioning of a tensiometer. The device measures soil moisture with reasonable accuracy in the wet range. However, it does not function well in measuring dry soil.
Other Methods
A scientist may employ a variety of other methods for collecting soil for moisture testing. For example, he may use moisture blocks, which, according to the Colorado State Extension, are of two types: gypsum and watermark. These work best on coarse, textured soils. Also, sampling a portion of soil and sending it to a lab is an option. But it's not free. For example, as of 2010, the Colorado State University lab charged between $5 and $10 for such tests.