Once the tube is filled, a small hand-held suction pump is used to remove air bubbles from the tube. The lid of the reservoir is then retightened, sealing the lower tube. It is important to follow all of the manufacturer’s recommendations for installation and maintenance, including the use of an additive to minimize algal contamination of the water in the tensiometer. When used properly, tensiometers will provide accurate “soil/water tension” readings on a range of crops. These readings provide the irrigation manager with critical information that can be used to establish irrigation schedules adequate to maintain soil moisture at levels conducive to good crop growth and productivity.In many ways electrical resistance sensing devices are similar to tensiometers—the main difference is the method used to measure soil moisture. ERSDs utilize two “electrodes” cast into a porous material . The two electrodes in the “block” are attached to wires that run from the ERSD to the surface. These wires are often protected within a ½-inch PVC tube that is attached to the ERSD. The ESRDs are buried in the soil at various depths and locations, similar to tensiometers, and like the tensiometer, a soil/water slurry is used when the ERSD is installed to establish good soil contact with the instrument. To get a reading from the ERSD the irrigator uses a small, inexpensive, growing cannabis indoors hand-held electrical resistance meter that is temporarily connected to the wire leads from the buried ERSD. The meter allows a very low electrical current to flow between the two electrodes in the ERSD and displays an electrical resistance reading.
This reading reflects the amount of moisture within the porous material, since the buried ERSD takes on the moisture properties of the surrounding soil. Due to the electrical conductivity potential of water, the higher the concentration of moisture within the porous block the lower the resistance and, conversely, the lower the concentration of moisture within the block the higher the resistance. At field capacity the block is wet; as the growing plants start to extract moisture from the soil, the moisture is also pulled from the ERSD and the conductivity reading will reflect this change in soil moisture. Note that high salt concentrations in the soil solution will affect the accuracy of the reading, since salts increase electrical conductivity. This potential salt impact needs to be taken into account when deciding which monitoring tool is best suited to your farm. Electrical resistance sensing devices are relatively inexpensive and easy to install and monitor. Like tensiometers, they are left in the field for the duration of the cropping cycle and provide critical irrigation scheduling information that enables the irrigation manager to make informed decisions about irrigation frequency and quantity based on site-specific data.Central California’s Mediterranean climate creates the conditions that make dry farming possible. In normal years Central Coast rainfall is generated by storms that develop in the Gulf of Alaska and sweep south and then east, moving from the Pacific Ocean across the region from November through February and into March. High pressure then dominates the region from April through September and often into October, pushing rainfall to the north during the Central Coast’s long “summer drought.” Thus the region rarely receives significant rainfall from May through September.
Rainfall amounts vary considerably across the Central Coast, influenced in large part by the location, height, and orientation of the area’s numerous mountain ranges. Steeper ranges parallel to the coast can cause significant orographic lifting of moisture-laden air, resulting in high rainfall amounts on the west side of these slopes. These ranges also create rain shadows on the east sides, reducing rainfall in these areas. From San Luis Obispo County in the south to San Mateo County in the north, rainfall amounts vary from approximately 8 inches up to approximately 35 inches per year depending on the effects of the mountain ranges and specific storm dynamics.Higher afternoon temperatures and ET rates in the range of .33 inches per day, typically encountered in the more inland valleys with less marine influence, are much less suited to dry farming, especially of tomatoes, since it can be difficult for the plants to access deeper moisture quickly enough to maintain turgidity during periods of high evapotranspiration. However, some crops can be successfully dry farmed in inland valleys: although not within the scope of this article, wine grapes, olives, and apricots are successfully dry farmed in California on small acreages in areas with little or no maritime influence.The best soils for dry farming have relatively high clay content. Sandy loam soils or loam soils that overlay deeper clay soils also work well for dry farming. Soils higher in sand content do not hold soil moisture as well as clay and clay loam soils and therefore are typically not used for dry farming. And because organic matter increases the soil’s porosity, it does not improve conditions for dry farming. A grower considering dry farming should bore numerous holes up to 4 feet deep throughout the production area using a 2-inch slide hammer and soil probe to obtain soil “plugs”: soils suitable for dry farming will exhibit continuity within the different horizons and a loam or sandy loam upper horizon going directly to clay.
Horizons with a larger particle size, e.g., containing sand or gravel, will impede water’s ability to be drawn upward to the plant’s root zone, thus making dry farming less feasible. Preparing and planting a small area of the field is the best way to determine whether the site and conditions are suited to dry farming.Soil preparation that conserves or “traps” winter rainfall is critical for successful dry farming. In the spring, prior to planting, residual rain moisture is typically lost from the root zone as water percolates down through the soil horizon with the help of gravity. High clay content in the soil, and to a lesser extent soil organic matter , greatly facilitates the soil’s ability to hold water in the root zone against the pull of gravity. As the weather warms, soil moisture is also lost through surface evaporation. Evaporation occurs as water is drawn upward via small channels between soil particles; these channels can be thought of as capillaries within the soil horizon. Polar bonds between water molecules and the forces of cohesion facilitate water’s upward movement through the soil: as water near the soil surface evaporates, growing indoor cannabis water lower in the soil is pulled nearer the surface, much like liquid being drawn through a straw. Thus in fields destined for dry farming it is critical to break up the capillaries near the surface to minimize the evaporative loss of residual rain moisture during late spring and summer. This breaking of capillaries is typically accomplished with relatively shallow mechanical soil tillage. Commonly used tillage tools include rototillers and disc harrows, often followed by secondary tillage implements such as spring tooth harrows. The resultant tilled zone is called a “dust mulch.” This dust mulch provides an effective barrier to the potential evaporative loss of residual rain moisture held within the root zone of the soon-to-be-planted dry-farmed crop. When creating the initial dust mulch, timing is critical: the grower must trap as much rain moisture in the soil as possible, yet avoid working the soil when it is too wet. Wet soils, especially “heavier” soils high in clay content, are subject to clod formation and compaction caused by tractor operations. It is also important to minimize tillage depth when preparing soil for planting annual dry-farmed crops, since deeper tillage could disrupt the lower soil capillaries that are critical for soil water movement below the tilled zone. The dust mulch needs to be maintained with fairly frequent and light tillage operations from the time of initial tilling until the crops are too large to cultivate effectively. Although dry farming relies on winter rainfall, several scenarios can necessitate irrigation prior to planting. During dry springs it is sometimes necessary to pre-irrigate the beds before planting using either overhead irrigation or drip lines in order to establish an optimal stand. When a mechanical spader is used to incorporate a high residue cover crop prior to dry farming it is often necessary, in the absence of post-tillage rain events, to pre-irrigate with overhead sprinklers to facilitate the cover crop’s breakdown. On a garden scale, you may need to hand water the newly planted plants to assist in rooting and uniform establishment.In any dry farming system, variety selection is absolutely critical. Varieties that do well as dry-farmed crops typically have an aggressive root system capable of reaching deep into the soil horizon to tap the stored rain moisture. It is interesting to note that growers in the Central Coast region have trialed literally hundreds of varieties of heirloom, open pollinated and hybrid tomatoes and, to date, none have compared to ‘Early Girl’ in their ability to set roots deep and consistently produce a high yield of high quality, flavorful, and marketable fruits with no irrigation. ‘New Girl’, a recently introduced variety, is closely related to ‘Early Girl’ and appears to have many of the same favorable characteristics.Dry-farmed crops with extensive root systems can effectively extract deep residual rain moisture from a fairly large area within their roots’ grasp.
Competition from other nearby crop plants or weeds can result in water-stressed plants that produce very little fruit and remain stunted. For this reason it is critical to plant out dry-farmed crops in a much wider spacing than is typically used for irrigated crops of the same type. Good weed management in a dry farm system is also critical, since most weeds have aggressive root systems capable of outcompeting most crop plants for both water and nutrients. As an example of plant spacing, irrigated tomatoes are commonly spaced 2 feet apart within the row with rows spaced 4 feet apart, a density of roughly 5,400 plants per acre. A typical spacing for dry-farmed tomatoes would be 6 feet between rows and 6 feet between plants, for a total plant population of 1210 plants per acre. As you can see from this example a significant yield reduction can be expected from most dry-farmed crops simply based on per acre plant populations. A higher price premium for dry-farmed tomatoes will often make up for the yield loss related to wider spacing.As a rotation within a diverse irrigated cropping system, dry farming has many advantages. The lack of irrigation in a dry-farmed production block can lead to improved soil tilth, since dry surface soil is not prone to compaction or clod formation from both foot traffic associated with harvest and tractor compaction from cultivation operations. Problem weeds are much easier to deal with when irrigation is eliminated for a season and weed seed development is easily minimized in a dry-farmed block. If water is a limited resource on a farm then dry farming makes perfect sense as a means of maintaining production while eliminating the need for irrigation. Forcing deep rooting of dry-farmed crops can also facilitate the extraction of nutrients that have leached below the root zone of most irrigated crops through excessive rainfall or irrigation. This is particularly true of tomatoes, which are highly sought after by savvy consumers and the Central Coast region’s chefs. As a result, the production and sale of dry-farmed tomatoes has become an important and economically viable niche market for small-scale organic specialty crop growers on the Central Coast. Finally, although dry farming may not be appropriate for every cropping system and region, understanding the basic principles of dry farming can lead to a greater knowledge of the complexities of water and soil dynamics, tillage, weed management, and fertility management. This knowledge can in turn lead to a greater understanding of your particular production system. In regions where conserving water is critical, applying dry farming principles to irrigated systems can result in improved water use efficiencies, better weed management, and improved soil tilth and productivity.While these statistics clearly illustrate the enormous quantity of water used in agriculture, they also suggest that irrigation has far-reaching consequences on water quality. In an effort to maximize crop yields, many farmers apply nitrogen-based synthetic fertilizers. More than half of the nitrogen applied may go unused by crops, ending up in surface water runoff or leaching into groundwater and causing severe water quality and other public health concerns for rural communities, many populated by poor, immigrant farm workers.3 4 As this supplement illustrates, how farmers use irrigation and apply fertilizers affects not only their crops, but also their neighbors.