Moisture sensors are used with all methods of irrigation providing very beneficial results to growers. However, in microirrigation they are a great tool if a grower expects to optimize the irrigation system to achieve full benefits and avoid problems. Why are moisture sensors so important in microirrigation? The objective in microirrigation is to continuously supply each plant with readily available moisture to meet evapotranspiration as it occurs, and to replace the moisture claimed by this process from the soil reservoir shortly after it has occurred. This is accomplished by supplying filtered water in frequent, slow applications through mechanical devices known as emitters, drippers or micro-sprinklers. One of the major problems is poor soil water distribution because such a small surface area is wetted. Distribution of the water into the effective plant root zone is dependent on the soil and its ability to transmit water laterally. The lighter the soil, the bigger the problem. A designer must not only keep in mind the fact that the engineering may have to provide for future addition of more emission devices in permanent crops, but that the number and placement of these must match the ability of the soil to move water laterally into the effective root zone desired. Roots will not grow into dry soil! In certain areas where salinity is a factor, the outward and downward wetting front serves to push salts away from the effective root zone. If this process is not continued, salts can readily move back into the root zone resulting in some rather severe damage. Another fact about microirrigation is that the frequent, light applications of water are intended to keep the soil moisture reservoir at close to field capacity. This optimizes the amount of readily available water. Moisture sensors to monitor water distribution Moisture sensors help you know if your system is accomp-lishing adequate distribution by their proper placement in areas where normal root development must take place. In trees, the most active roots will be located at or near the dripline of the tree so moisture sensors must be located there. In shrubs, locate the instruments in between plants. A key factor to keep in mind on placement is that the sensing tips must be in the area wetted by the system. With a true drip or trickle system, the emitter wets a very small area, with the subsurface moisture forming a ?EUR??,,????'??wetted onion?EUR??,,????'?? as the soil moves the water downward and outward. If the Irrometer sensing tips are either too close or too far away from the emitter, you could be getting false readings. The instruments should be placed 12''-18'' from an emitter. With micro sprinklers or sprays, a much larger surface area is wetted. Irrometers should be placed about 24''-36'' from the micro sprinkler or spray. With subsurface drip irrigation (sdi), the water source could be 9''-18'' below the soil surface, but you still may require adequate moisture to be maintained towards the soil surface in the most active portion of the root system. Placing the sensors so the tips are closer to the soil surface can monitor this upward movement of water. With proper placement, moisture sensors will allow you to accurately keep track of the lateral movement of water. You are looking for a nice continuous band of moisture down in the soil of the plant?EUR??,,????'???s root zone and Irrometers can help you accomplish this. Moisture sensors to maintain field capacity This is probably the most important aspect of why moisture sensors are so critical in microirrigation since we really do not use the soil as a ?EUR??,,????'??reservoir?EUR??,,????'?? for water, as is the case with sprinkler and surface irrigation. The management objective is to never let the soil dry out very far beyond field capacity, which, in most soils, is somewhere in the 10-20 centibar range. We don?EUR??,,????'???t want to keep the soil saturated at all times (below 10 cb), because this deprives the root system of needed oxygen. But, if we let the soil dry too much (i.e. 40-60 cb), we may never ?EUR??,,????'??catch up?EUR??,,????'?? with a micro system, since water is applied at such a low rate. Because the tensiometer measures soil water suction directly, and because it is most accurate in the ?EUR??,,????'??wet?EUR??,,????'?? end of the soil water range (10-50 cb), it is by far the best choice for keeping track of available soil water in the ?EUR??,,????'??range?EUR??,,????'?? we are trying to maintain. But the major factor is that there is no practical way of keeping soil moisture in the range we want to maintain, without doing soil moisture measurement. There is no way you can ?EUR??,,????'??feel?EUR??,,????'?? the difference between 10 and 25 centibars. This is why moisture sensors are so important to good management of microirrigation. Irrometers to prevent salt accumulation By placing tensiometers near the edges of the ?EUR??,,????'??wetted onion?EUR??,,????'?? area, applied water can be managed so as to force salts outward and downward away from the roots. This keeps salts from accumulating where damage can be done. Automation of micro systems Since micro systems are typically low volume, rather small solenoid valves (1''-3'') can be used to irrigate very large blocks. Controllers, or time clocks, can be very simple but must be capable of being programmed with micro time (hours) rather than sprinkler time (minutes). However, controllers are very valuable and in most cases imperative. For one thing, controllers ensure that irrigation does not interfere with the cultural practices at your site. But, the essence of using a controller is that it gives you the opportunity to carefully control the length of running time on a given valve and thus put water at the exact depth desired. This is done by running short but repeated cycles and letting the Irrometer permit as many of these cycles as needed to get the water to a specific depth. Once that has been achieved, the sensor prevents any additional cycles not needed, even though they have been programmed. Thus, only needed irrigation will take place with unnecessary irrigation being eliminated. Frequency of monitoring locations The variables that determine monitoring locations are: soil type, topography, and sun exposure. Remember that you are monitoring the soil moisture status for a given tree or plant and using that as an ?EUR??,,????'??indictor?EUR??,,????'?? for a given area. Don?EUR??,,????'???t make a mistake by spreading your instruments over too large an area. Different soil types hold water differently and should be monitored separately. Slopes should be monitored at the top, bottom and sometimes in the middle as well to accommodate for distribution differences from runoff. Areas with full sun exposure will require more water than shady areas, and thus should be monitored separately. Typical monitoring location It is important to note where in the root system a plant actually takes its moisture. The top quarter of the root system extracts 40% of the total moisture, second quarter extracts 30%, third quarter extracts 20% and the bottom quarter extracts 10%. Thus 70% of the total moisture is extracted in the upper half of the root system. It is for this reason that it is imperative to monitor soil moisture in at least two depths in the root horizon. The typical monitoring location with a micro system utilizes a 12'' and 24'' instrument. Deeper instruments can be helpful in verifying that a ?EUR??,,????'??leaching?EUR??,,????'?? type irrigation has driven any accumulated salts down below the active root system. Fertigation These days, most microirrigation systems are used to apply nutrients (principally nitrogen) in soluble form directly with irrigation events. We ?EUR??,,????'??spoon-feed?EUR??,,????'?? water and we ?EUR??,,????'??spoon-feed?EUR??,,????'?? nutrients. We can use an Irrometer to keep track of our water, so why not measure our nutrients in the soil? There are units specifically designed to extract soil water samples from the root zone, which are then tested for concentration of the applied nutrient. Summary Whether you automate or not, if you microirrigate, you really need to consider measuring your soil moisture if you expect to achieve top results from your irrigation system. Your ?EUR??,,????'??investment?EUR??,,????'?? in a moisture sensor is one of the least expensive insurance policy you could ever purchase.