Slice into water savings

The LF procedure described in this article is applicable to large containers that are micro-irrigated. Micro-irrigation systems are well-suited for large containers in wide spacing arrangements compared to smaller container-grown plants in dense arrangements that are typically irrigated with overhead sprinklers. A typical micro-irrigation system delivers water directly to the container, often via a spray-stake placed near the container wall spraying inward (Fig. 1). Flow rates for spray stakes can range from 5-16 gallons/hour. For a trade 15-gallon container that has a diameter of 17 inches, this is equal to applying 5-16 inches/hour. When compared with sprinkler or overhead systems that apply water at 0.3-0.5 inch/hour, you can see why it might be easier to over-water a large, micro-irrigated container than one that is sprinkler-irrigated. This is why the LF test can play a critical role in scheduling irrigation for micro-irrigated crops.

Producers know plants need water to grow but how can water managers objectively determine how much irrigation water to apply? When a plant is grown in a container, the amount of water that is available to the plant is limited by the container volume and the container substrate’s capacity to retain water. The capacity of the container substrate to retain water is dependent on its components and their proportion. Ideally, a substrate will exhibit good water-holding properties while simultaneously providing adequate aeration for root growth. In the southeast United States, pine bark mixes are often the most popular substrates for producing plants in outdoor container nurseries. The confined volume of the container, even with a good substrate, typically requires daily irrigation if not several applications per day to ensure adequate moisture for the plant. The goal of the water manager is to apply enough water in these frequent applications to resupply the amount taken up by the plant without applying so much that excessive leachate results.

LF test and pizza pan setup

LF is defined as the amount of leachate divided by the amount of irrigation water applied to the container. Two measurements are needed for a LF test: the amount of leachate drained out of the container and the amount of water applied to the container.

The pizza pan comes into play on the first measurement. The pizza pan can be used to collect leachate from large containers (= trade 7-gallon) with micro-irrigation. A pizza pan is placed under the container and a hole punched near the rim of the pizza pan allows leachate to drain into a pan (e.g. lasagna pan) for measurement by weighing (Fig. 2). We recommend an aluminum pizza pan with a 1-inch high rim and a diameter at least as large as the top diameter of the container. Fortunately, restaurants serve up a variety of pizza sizes, so finding a 14-inch pan for trade 7-gallon containers and a 17-inch pan for trade 15-gal containers is not a problem (e.g. American Metalcraft ADEP17, 17 inches x 1 inch Standard Weight Aluminum Tapered / Nesting;www.webstaurantstore.com ). We make the drainage hole in the pizza pan by first drilling a one-eighth inch pilot hole about one-half inch from the pizza pan’s rim. We then insert the pointed end of a plumb bob into the pilot hole and hammer it to create a one-half inch hole. The punch creates a small funnel underneath the pizza pan which helps with draining. The pizza pan is placed on top of two 1-foot-long pieces of lumber (4x4 inches) to allow leachate to be collected underneath (Fig. 3). If needed, slope to improve drainage can be created with shims or small pieces of wood or bamboo. The pizza pan set up is left in the field so that leachate monitoring can be routinely conducted.

The second measurement needed for the LF test is the amount of water applied. For this, we insert an adjacent spray stake emitter into a 5-gallon pail with a one-half inch wide notch cut into the rim of the pail so that the spray stake tubing will not be crimped with the lid. After irrigation, the amount of water in the pail is determined by weighing. The spray stake emitter can be closed and left in the pail between LF tests to facilitate routine testing.

Routine LF testing and irrigation adjustment

We recommend at least three or four pizza pan setups per irrigation zone to account for plant and irrigation variability. Keep in mind the results from these three or four LF test containers will be determining the irrigation needs of the entire irrigation zone. To be conservative, include larger plants and/or border plants that may have relatively high irrigation requirements. Pizza pan setups placed near the borders of the irrigated area will decrease testing time. Because the LF test is simply a snapshot in time, LF testing should be conducted on days with normal weather and as frequently as possible. We found that once every 2-4 weeks is fine once you have established a good pattern of LF results. In general, frequency of LF testing should be higher during spring and summer months when potential evapotranspiration rates are increasing with warmer and longer days than in late fall and winter months when growth and water use slow down. If water is applied more than once a day, then leachate can be collected over all irrigation cycles during a 24-hour period. It is noteworthy that measuring leachate and amount applied for each cycle, while more work, gives an indication of how uniform the irrigation needs are from one cycle to the other.

Table 1 shows the results of a hypothetical LF test. The average LF of the four test plants was 42%. If the target LF is 25%, we know that the 6-minute run time was too long for that particular day. Using the equation that appears on this page, we can calculate an adjusted run time that would theoretically give us an LF of 25% under the same environmental conditions.

For the example in Table 1, the adjusted run time was 4.6 minutes. If your irrigation controller can only be programmed to the nearest minute, it would generally be preferable to round up and be more conservative. For example, if you rounded 4.6 minutes down to 4 minutes, the expected LF would be 13% instead of 25%. On the other hand, if you rounded up to 5 minutes, the expected LF would be 31%. Our experience is that a target of 13% LF is too low for many micro-irrigation systems so that the adjusted run time of 5 minutes would be preferable despite a slightly higher target LF. In general, we suggest a target LF of 25% for micro-irrigation production until experience indicates otherwise.

The pizza pan procedure for LF testing can help container nurseries monitor and maintain irrigation efficiency when producing plants in large containers with micro-irrigation. Based on results from seven trials conducted at three nurseries, average water use was decreased by 39% when using LF-directed irrigation versus the nursery’s traditional irrigation practice. We believe implementing a LF testing program using the pizza pan procedure can promote profitable plant growth and quality with less water. (Part one of a two-part series.)