- Author: Ben Faber
. A rule of thumb is that irrigation should be done when about 50% of the water has been depleted from the soil in the plant's root zone. This 50% value, however, allows a buffer of water in the soil in case the weather suddenly turns hot and windy. Sandy soils hold less water than clay soils and must be irrigated more frequently. A common misperception is that it takes more water to grow plants in sandy soil than in clay soil; however, the total amount required for the whole year is the same for both soil types. The amount of sunlight, wind, temperature, and humidity control how much water a plant needs, and the soil is only the reservoir.
Determining water content by soil texture
To check the water content in the soil based on the soil texture, dig 8 to 16 inches down into the soil with a trowel, shovel, or soil tube and feel the soil. At about 50% available water:
- coarse soil appears almost dry and form a ball that does not hold shape
- loamy soil forms a dark ball that is somewhat moldable and can form a weak ribbon when squeezed between the fingers
- clayey soil forms a good, dark ball, makes a ribbon an inch or so long, and slightly sticky
This method, however, gives only an approximate water content; instruments can give more precise readings.
For more information, refer to the USDA guidelines at:
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_051845.pdf
- Author: Ben Faber
Irrigation timing can be determined more precisely using a tensiometer. These water-filled tubes with a pressure gauge accurately reflect the amount of energy a plant needs to extract water from the soil. The pressure gauge measures tension values in centibar units (cbars). For citrus, when the gauge reads 30 centibars, it is a good time to irrigate. Tensiometers must be placed in the root zone between the emitter and the tree trunk. Having two tensiometers next to each other can be helpful in deciding when to turn the system on and off. For example, a tensiometer at a depth of 1 foot would indicate when to turn the water on, and a second at 3 feet would indicate when to turn the water off. Prevent tensiometers from being damaged during harvesting and other grove operations by placing a plastic milk crate or some other structure over each device.
Other devices can also be used to measure soil moisture. Gypsum blocks are very effective; the part in the ground is inexpensive but the reading device costs about $250, so a relatively large acreage is required to spread out the cost of the system. Portable meters rely on an electrical current carried by water in the soil. Even cheap $10 meters can give a rough estimate of the soil water content, but they are not very effective in rocky ground, because their sensitive tips break easily.
Soil-based methods monitor an aspect of soil moisture that, depending on the method, requires a correlation to plant water use. Some methods are well understood and inexpensive, others are expensive, inaccurate, inappropriate, or not well researched. Some methods allow multiple site readings, while others require a device to be left in place. Some measure soil water directly (e.g., oven-drying), and others measure another parameter, such as electrical conductance. Some methods are affected by salts or soil iron content, and others have limited value in the desired soil moisture range. Some, like tensiometers and gypsum blocks, give a reading from a porous material that comes to equilibrium with soil moisture, while many others use the soil directly as the measured medium—an important distinction, since discontinuities in the soil caused by rocks or gopher holes can affect readings. Also, some of the older techniques have been improved. For example, gravimetric oven-drying can now be done by microwave, considerably speeding up the process, and tensiometers and gypsum blocks can now be found with digital readouts and connections to data loggers that make data easier to manage. Many types of monitoring devices are available; the table below describes their characteristics.
As with any tool, the value of these devices increases with use and familiarity. Even though several are stationary devices, by placing them in representative positions in the grove, they can accurately reflect the entire grove. Some types of device can be stationary or portable, depending on the model.
Method |
Cost |
Ease of use |
Accuracy |
Reliability |
Salt-affected |
Stationary |
Gypsum block |
L |
H |
H |
H |
L |
YES |
Tensiometer |
L |
M |
H |
M |
L |
YES |
Portable tensiometer |
M |
M |
H |
M |
L |
NO |
Solid-state tensiometer |
M |
H |
H |
H |
L |
YES |
Time domain reflectometer |
H |
M |
H |
H |
M |
BOTH |
Neutron probe |
H |
L |
H |
H |
L |
YES |
Feel (soil probe) |
L |
H |
H |
H |
L |
NO |
Gravimetric (oven) |
L |
M |
H |
H |
L |
NO |
Conductance |
L |
H |
M |
M |
H |
BOTH |
Capacitance |
M |
H |
M |
H |
M |
BOTH |
H – high, M – medium, L - low
- Author: Ben Faber
When we think about space missions, we tend to look toward the stars to planets like Mars where robotic rovers roam, gathering data and sending it back to Earth. Rarely do we think about missions closer to home. But a view of Earth from 426 miles above is helping us monitor droughts, predict floods, improve weather forecasts and assist with crop productivity. This year, the National Aeronautics and Space Administration (NASA) launched a new satellite called SMAP (Soil Moisture Active-Passive) with the help of a team that included U.S. Department of Agriculture (USDA) hydrologist Susan Moran at the Agricultural Research Service's (ARS) Southwest Watershed Research Laboratory in Tucson, Arizona, and physical scientist Wade Crow and hydrologist Thomas Jackson at ARS's Hydrology and Remote Sensing Laboratory in Beltsville, Maryland. ARS scientists played a key role in designing and implementing SMAP—an orbiting observatory that measures the amount of water in the top layer of the soil everywhere on Earth. SMAP gathers soil moisture data that can help track diseases and famine, predict weather and climate patterns, assist emergency workers' response to natural disasters and let farmers know what crops to plant. “We've seen impressive advances in our ability to produce crops on a given area of soil, but have also retained susceptibility to climate events, particularly droughts that occur when there is inadequate soil water for crops,” Crow says. “The idea is to better predict and monitor droughts so they don't turn into food crises, and soil moisture is the most direct and earliest indication of drought.” SMAP provides the best global view of soil moisture to date, Crow says. Therefore, it has the potential to help monitor global food production. It's the best soil moisture sensor ever deployed due to its resolution, accuracy, global coverage and repeat time. Before the satellite was launched, volunteer users around the world could put SMAP's simulated data to use, Moran says. “We wanted to make sure our products would be as good as possible and easy to access. In return, these users provided feedback on how SMAP could help them.” Early users included those in agriculture, weather, human health, emergency response and military readiness. Data were used to monitor droughts, predict floods and even to predict the water supply in New York City, Moran says. “Some used the data to predict large regional dust storms that affect the health of millions of people in Saharan Africa and throughout the Middle East,” she adds. “In Germany, the data were used to map sea ice in hopes of improving maritime navigation, and at Texas A&M University, researchers looked at the impact of hurricanes on power outages.” SMAP will release the first data products to the public in August. To learn more, go to http://smap.jpl.nasa.gov/. - See more at: http://blogs.usda.gov/2015/04/07/usda-nasas-global-view-of-earths-soil-holds-many-benefits/#sthash.okw1NTAN.dpuf
- Author: Ben Faber
There's been a lot of salt damage to fruit trees after two years of drought using poor quality water and not doing adequate leaching. The situation could be improved by knowing when to irrigate. Here's some guidelines on selecting soil moisture measuring devices.
Soil moisture sensors fall into two broad categories, volumetric and tensiometric methods. One tells you how much water is in the soil and the other tells how difficult it is to remove water. Volumetric methods require a calibration of the sensor to the soil, whereas tensiometric is good to go when installed. The case in both methods is the grower learns to keep soil moisture within a given range of values and in theory, the plant is kept in a better condition with improved health and yields.
The most common volumetric methods rely on measuring the dielectric constant of the soil which determines the velocity of an electromagnetic wave or pulse. These sensors have become widely used because they have a good response time, do not require maintenance and can provide continuous readings, allowing for automation. There are several different methodologies used Time Domain Reflectometry, Frequency Domain Reflectometry (Capacitance), Amplitude Domain Reflectoremtry (Impedance), Phase Transmission, and Time Domain Transmission. There is quite a range in prices for these different devices, but they generally do not require maintenance once installed and not require close soil contact.
The tensiometric methods include Tensiometers, Gypsum Blocks, Granular Matrix Sensors, Heat Dissipation and Soil Psychrometer. These techniques rely the sensor to come into equilibration with the soil moisture and generally are unaffected by soil salinity. Gypsum blocks and Granular Matrix are not very responsive in sandy soils and require good soil contact.
Volumetric Sensors
|
TDR |
FDR |
ADR |
PT |
TDT |
Cost (including logger/reader) |
$400- 20,000 |
$100-3,500 |
$500-700 |
$200-400 |
$400-1,300 |
Field Maintenance |
No |
No |
No |
No |
No |
Affected by salts |
High levels |
Minimal |
No |
>3dS/m |
High levels |
Installation method |
Buri9ed |
Buried |
Buried |
Buried |
Buried |
Soil type not recommended |
Organic, salt, high cay |
None |
None |
None |
Organic, salt high clay |
Tensiometric Sensors
|
Tensiometer |
GB |
GMS |
HD |
SP |
Cost (including logger/reader) |
$50-75 |
$400-700 |
$200-500 |
$300=-500 |
$500-1000 |
Field Maintenance |
Yes |
No |
No |
No |
No |
Affected by Salts |
No |
>6 dS/m |
>6dS/m |
No |
maybe |
Installation method |
Buried |
Buried |
Buried |
Buried |
Buried |
Soil Type not recommended |
Sandy |
Sandy, high clay |
Sandy, high clay |
Sandy |
Sandy, high clay |
- Author: Ben Faber
Soil moisture sensors fall into two broad categories, volumetric and tensiometric methods. One tells you how much water is in the soil and the other tells you how tightly the soil holds on to the water. Volumetric methods require a calibration of the sensor to the soil, whereas tensiometric is good to go when installed. For both methods, the grower learns to keep soil moisture within a given range of values and, in theory, the plant is kept in a better condition with improved health and yields and potential improved irrigation efficiency.
The most common volumetric methods rely on utilizing the dielectric constants of the soil and water, with water’s dielectric constant being much greater than soils’. The velocity of an electromagnetic wave or pulse depends on soil moisture content. These sensors have become widely used because they have a good response time, do not require maintenance and can provide continuous readings, allowing for automation. There are several different methodologies used: Time Domain Reflectometry, Frequency Domain Reflectometry (Capacitance), Amplitude Domain Reflectometry (Impedance), Phase Transmission, and Time Domain Transmission. There is quite a range in: (1) prices for these different devices, (2) requirements for calibration with soil moisture content, and (3) requirements for close soil contact. Some devices are affected by the chemical nature of the soil. Even if they are not calibrated they can be used as relative change in moisture content over time.
The tensiometric methods include: Tensiometers, Gypsum Blocks, Granular Matrix Sensors, Heat Dissipation and Soil Psychrometer. These techniques require the sensor to come into equilibration with the soil moisture and generally are unaffected by soil salinity. Gypsum blocks and Granular Matrix are not very responsive in sandy soils and require good soil contact. These methods are less affected by salinity and do not require soil calibration, because they are reflecting the soil moisture tension the roots are seeing.
All soil moisture sensors need to be placed in a position that represents the irrigated area. They need to be placed in the active root zone where water is applied and taken up and must be near trees representative of the whole irrigated area. They should not be next to a sick tree, a smaller tree compared to the other trees or be in an area that obstructs applied water (such as under a canopy that intercepts applied water). It is always best to reinforce sensor values with manual field measurements with a soil probe to ensure that sensor placement and response is truly reflective of what is going on in the field, before completely relying on the sensor values. As with all field equipment, occasional visual inspection of the field and sensor readings should be made to make sure the situation has not changed, such as an emitter has clogged or broken near the sensor.
Volumetric Sensors
|
TDR |
FDR |
ADR |
PT |
TDT |
Appx. Cost (including logger/reader)+ |
$400- 20,000 |
$100-3,500 |
$500-700 |
$200-400 |
$400-1,300 |
Field Maintenance |
No |
No |
No |
No |
No |
Affected by salts |
High levels |
Minimal |
No |
>3dS/m |
High levels |
Soil type not recommended |
Organic, salt, high cay |
None |
None |
None |
Organic, salt high clay |
+ Prices as of 2009
Tensiometric Sensors
|
Tensiometer |
GB |
GMS |
HD |
SP |
Appx. Cost (including logger/reader)+ |
$50-300 |
$400-700 |
$200-500 |
$300--500 |
$500-1000 |
Field Maintenance |
Yes |
No |
No |
No |
No |
Affected by Salts |
No |
>6 dS/m |
>6dS/m |
No |
maybe |
Soil Type not recommended |
Sandy |
Sandy, high clay |
Sandy, high clay |
Sandy |
Sandy, high clay |
These sensors can be purchased individually and installed by the grower, or increasingly there are companies that provide a monitoring station that includes soil moisture, sensors for estimating plant evapotranspiration , data logger and software that determines an irrigation schedule for the crop. Some of these systems are outright purchase and some are for lease. In the future, there will be affordable satellite imagery that can help in irrigation scheduling, showing how rapidly this technology is changing.