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: Jim Wolpert - UC Davis
Soil Moisture Sensors
Jim Wolpert, University of California, Davis
Soil Moisture Content
The quantity of water in soil is called the soil moisture content. After rainfall or irrigation, some water drains from the soil by the force of gravity. The remaining water is held in the soil by a complex force known as surface tension and varies depending on the amount of sand, silt, and clay. Sands, with larger particles and smaller total surface area, will hold less water than clays, which have much smaller particles and larger total surface area. The drier the soil, the greater the surface tension, and the more energy it will take for a plant to extract water.
Vineyard managers often measure soil water content as a guide to determine their irrigation timings and amounts. There are several methods for monitoring soil water content. Correlating these methods with actual inches of moisture per foot of soil is very complicated (see Recommended Links) but at the very least can help a grower to identify patterns of water use, depth of irrigation, and soil water content trends over time.
A tensiometer, as its name implies, is a device for measuring soil moisture tension. The design is a simple tube with a porous cup at the lower end and a vacuum gauge on top. The tube is filled with water, sealed airtight, and placed in soil. As soil dries, water is pulled from the porous cup into the soil, creating a vacuum and causing the gauge to move. As soil continues to dry, more water is pulled out and the suction increases. As soil re-wets after a rain or irrigation, water moves back into the cup and the suction decreases. Installing tensiometers in soil requires attention to detail to obtain accurate readings (see Recommended Links for installation downloads).
Tensiometers are usually placed as a pair with the shorter tube positioned in the middle of the rooting zone (e.g., 18 inches deep) and a longer tube positioned near the bottom of the rooting zone (3 to 4 feet deep). Growers can use the difference between the two tubes to monitor water usage and determine the effective depth of irrigation. At least two stations (two tubes per station) are recommended per field, or more depending on soil variability.
Tensiometers have the advantage of being inexpensive, and easy to install, maintain, and read. They are better in fine-textured soils where good contact can be made between the porous cup and the soil. They do not work well in coarse sands where good contact may not be possible. Because the gauges are aboveground, the units are prone to damage by vineyard equipment.
Electrical Resistance Blocks
Electrical resistance blocks are also known as gypsum blocks or soil moisture blocks. They are simple devices with two electrodes embedded in a block of gypsum or other similar material. When blocks are buried in soil, water moves into or out of the block, depending on the moisture of the soil, changing the resistance between the two electrodes. Like tensiometers, gypsum blocks are cheap and easy to install. They are usually installed in at least two stations per field, at two depths, and must be installed correctly to provide accurate readings. Some block designs perform better under wet soil conditions and some correct for soil temperature. The meter used to read the blocks can be moved from field to field, but is specific to the block design (i.e., it is not a simple ohm meter). The wires aboveground are much less prone to damage by equipment compared to tensiometers.
A neutron probe uses a radioactive source for measuring soil moisture. A tube, usually made of PVC or aluminum, is installed in soil to a depth of interest and the radioactive probe is lowered into soil to measure soil moisture at as many depths as desired. The probe emits fast neutrons that are slowed by water in the soil in a way that can be calibrated to the soil water content. The probe has a significant advantage, especially for perennial crops, because access tubes are easy to install and relatively permanent. Another advantage is the reading accounts for a spherical area about 10 inches in diameter, much greater than other methods. The major limitation to this method is the probe itself; it is expensive and the presence of a radioactive source triggers requirements for operators to be trained and licensed in handling, storage, and use. In some production regions, service providers are available, usually at a fixed cost per access tube for a growing season.
Di-electric sensors measure the di-electric constant of soil, a characteristic that changes with changing soil moisture. A common method is called time domain reflectometry, or TDR. The theory behind how this method works is too complicated to be discussed here. The advantage of these types of systems is that they are designed to be left in place and provide continuous readings of soil moisture. The disadvantages are that the units are expensive and read soil moisture only a very small distance from the unit.
All measures of soil moisture suffer from the same limitation — the value of the information is dependent on the extent to which the soil where the measurements are taken reflects the rest of the field. Where soil variability is high, growers must exercise caution in relying too heavily on relatively few measurements.
Irrigation of Winegrapes, University of California
Irrigation Basics for Eastern Washington Vineyards, Washington State University
Reviewed by Ed Hellman, Texas AgriLife Extension and Eric Stafne, Mississippi State University
Historically if you wanted to know what soil type and the description for it, it was necessary to go to the library, NRCS office or Coop Extension office to find the soil survey. It is a book out of print now, but it has maps with outlines of the soils of that county and the descriptions that characterize those soils. The USDA put the Soil Survey online about 10 years ago
and it is great unless the internet goes down.
Toby O'Geen at UC Davis has taken the same information and repackaged it so that it is easier to use
I was recently in an orchard looking at what appeared to be avocado root rot. I was checking on the symptoms and quizzing the grower on the other cultural practices. The grower was prepared with soil, leaf and water reports. I asked how much nitrogen was being applied and it was something in the 50 pounds nitrogen per acre. When I saw the water report it listed nitrate in the water at the level of 84 ppm. Rarely do you see a yield response in plants when soil nitrogen exceeds 90 ppm nitrate. The soil nitrogen would pretty rapidly take on the nitrogen level of the water. So this grower was very close to the level at which no yield improvement would occur, and in fact would be increasing vegetative growth and hence increasing pruning problems. I looked at the leaf nitrogen levels and they were quite high, as well. I suggested that it would be a good idea to cut back significantly the amount of fertilizer applied, if not stopping application all together. There are many areas along the coast where avocados are grown where there are high nitrates in the water. Water can be a significant source of nutrients, as well as toxics, such as boron, chloride, sodium and total salts. Learn to read all your reports – soil/leaf/water – and figure out the puzzle of fertilizing appropriately.
- Author: Ben Faber
- Author: Jim Downer
Leaf analysis is the preferred method of guiding a fertilizer program for fruit tree crops. Soil testing is less important, since the tree has the capacity to store nutrients in its various parts – roots, trunk, stems and leaves. However, soil testing is a component of a plant nutrient management program and has been standard practice for growers to aid in adjusting fertilizer applications. Soil testing is performed not only to improve plant growth, but also to reduce over-application of fertilizers that may lead to nutrient toxicities, excessive leaching and consequent economic losses.
For maximum accuracy and benefit, soil testing must be conducted using reliable methods on correctly-sampled soils (if the user is not trained in obtaining representative soil samples, test results even from the same soil can vary greatly). Test results must also be properly interpreted for a specific crop. Interpretative guidelines are readily obtainable for many agronomic and horticultural crops, as well as landscape trees. Cost for laboratory analysis for pH, NO3-N, P2O5 (Olsen), and extractable K2O are typically under $20 per analysis, but frequently results take from 1-4 weeks to get back to the grower.
By contrast, many retail garden centers offer commercial test kits, ranging in cost from $10 to $50 for multiple tests, so that the cost per test can be relatively low. These commercial kits are also advantageous because results can be obtained within one to two days. Commercial kits typically use a colorimetric method for indicating macronutrient and pH levels. Soil is measured into a sample container, extractant is added, and after a specified time for the reaction, the user compares the color obtained to a color card corresponding to categorical nutrient and pH levels.
We have always wondered how well these kits performed, so we purchased five commercially-available test kits and compared their results to standard laboratory analysis of NO3-N, P2O5 (Olsen), extractable K2O and pH from the same soil type with three distinct cropping histories (Soils 1, 2, and 3). The objectives were to identify differences in accuracy, if any, among test kits and to suggest a kit that most closely corresponds to analytical lab results.
Four of the kits, “La Motte Soil Test Kit” (La Motte Co., Chesteron, MD); “Rapitest®” (Luster Leaf Products, Woodstock, IL); “Quick Soiltest” (Hanna,Woonsocket, RI); and “NittyGritty” (La Motte Co. Chesteron, MD) measured nitrate-N, P2O5, K2O and pH. “Soil Kit” (La Motte Co., Chesteron, MD) measured only nitrate-N, P2O5 and K2O. The kit results for macronutrients were categorical (high, medium, and low); pH results were numeric, rounding to half pH units for the Rapitest® and one pH unit for the other three kits. The manufacturers’ instructions for each kit were followed for soil testing.
Results show that pH measures from LaMotte Soil Test Kit and Rapitest consistently matched lab results. Soils 1 and 3 proved to be in the pH 6.5 range, but the pH of Soil 2 was 7.8, technically beyond the capacity of Rapitest (pH 4.5-7.5). NittyGritty did not match lab results at all. Quick SoiltTest generally indicated lower pH values than the analytical lab. Results from LaMotte Soil Test Kit, Rapitest, and Quick Soiltest consistently matched the analytical lab results for nitrate-N and P2O5, while Soil Kit and NittyGritty did not. Soil Kit and NittyGritty analyzed K2O content with greater accuracy than for the other nutrients; the commercial tests in total corresponded with the analytical lab 82% of the time for this test. For Soil 3; all the commercial test results matched the analytical lab results 100%.
Precautionary measures for these commercial kits may increase their accuracy. For Soil Kit and Nitty Gritty, the extracting powders that came with the kits dissolved poorly; these kits generally yielded inaccurate results, but pulverizing the tablets or powders may increase extraction potential. Interpretation of color development should be made only within the time specified by the kit instructions because color intensity could vary within minutes. Also, interpretation can occasionally vary depending on the user. In this study, the observers independently interpreted the same result for 91% of the tests; this would probably be an acceptable proportion for a home gardener or farmer individually conducting tests, but occasional independent interpretation by another source may change the result.
La Motte Soil Test Kit results corresponded to those from the analytical lab for pH and all nutrients (86% of the tests matched). This kit is suitable for growers because it proved to be very accurate even over a range of pH values and is housed in a hard-sided, padded container. Rapitest yielded accurate results 92% of the time for all nutrients and pH less than 7.5, and was comparatively easy to use and interpret. Quick Soiltest matched the analytical lab results only 64% of the time because pH and K2O values were inaccurate. Interpretation of values from this kit may have resulted in application of potassium in excess of the needs of Soils 1 and 2.
An important limitation of all commercial test kits is the approximate or categorical value of nutrient content (i.e., low, medium, high). Analytical labs must be used when precise values are required. Nevertheless, commercially-available kits such as Rapitest and La Motte Soil Test Kit have shown to provide accurate, fast, and economical results and can help growers improve nutrient management.