Neutron Probe, access tube
(click for larger image)
Research Instrumentation
Monitoring Soil Moisture in
(Carrots, Potatoes, Beans, Onions):
Water balance and actual soil moisture measurements are made at the field throughout the entire growing season. Rain gauges provide information about irrigation and rainfall events, which are necessary when a budget approach is used to schedule irrigation. The checkbook method is one such irrigation scheduling method that North Dakota State University endorses. Another way to monitor and schedule irrigation is with tensiometers. Producers set an upper limit to how dry the soil can get and when the tensiometer gauge reads above that limit it is time to irrigate. Watermark blocks are also used to measure soil moisture, they rely on electrical resistance to indirectly measure soil moisture. All of the above mentioned instrumentation is used to evaluate soil moisture during the growing season, but the graphs represent soil moisture measurements taken from neutron probes.
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A neutron probe contains a source of fast neutrons and slow neutron detector. Neutrons are emitted from a radioactive source (e.g. Radium or Americium-beryllium) at a very high speed. When these fast neutrons collide with a small atom such as the hydrogen contained in soil water, their direction of movement is changed and they lose part of their energy. These “slowed” neutrons are measured by a detector tube and are calibrated to indicate the amount of water present in the soil. The neutron probe is lowered down a tube to take readings at different depths in the soil profile. Neutron probe counts are taken at 6 depths in the soil profile. Readings are taken at 6, 12, 18, 24, 36 and 48 inches. |
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Collection Funnel Rain Gauge |
We use cylindrical rain gauges that have a 4" diameter collection funnel and a capacity of 10". The gauges measure to the nearest 0.01". The gauges are read manually after every event, whether rain or irrigation. |
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Tipping Bucket Rain Gauge |
We use a "tipping bucket" rain gauge for automated measurement of rain and irrigation. These gauges have two small water-holding cups on each side of a teeter-totter type of device. The cups pivot so that the cup on the high side is under the rain collection funnel. When enough rain has fallen, the teeter-totter reverses and dumps the full cup, positioning the other cup under the funnel to continue the collection of rain. Each tip is calibrated to 0.01" of rain. The date and time of every tip is recorded by a switch and data logger. Periodically during the season, these data loggers will need to be read to determine the timing and amount of rain and irrigation. |
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Tensiometer
Gauge Tensiometer
in Field
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Tensiometers measure attraction of water by the soil. The tensiometer is filled with water and then placed in the soil. Water in the tensiometer is drawn through the fine porous cup into the adjacent soil until equilibrium is reached. The tension at which the water is being pulled by the soil can be measured directly from a vacuum gauge. The drier the soil the more tension is present and the higher the reading on the gage. Tensiometers are similar to the root of a plant and this is why they are some times called a “mechanical root”. Tensiometers are used successfully in determining the need for irrigation in sandy soils. |
| A Watermark block is a soil moisture block that operates on the same principle as a gypsum block. Watermark blocks measure the electrical resistance in a small cylinder of prepacked porous media. The block is installed in the soil like a tensiometer, with soil moisture moving in and out of the block depending on the dryness or wetness of the surrounding soil. The wetter the porous media in the block, the lower its resistance. By having a known medium in the block, the electrical resistance can be calibrated to read out the soil moisture tension. Thus the Watermark block gives the same in formation as a tensiometer–the soil moisture tension, a measure of how hard plants must "pull" on the soil to remove water. Both Watermark blocks and tensiometers are useful tools for irrigation scheduling. |
