Nursery Crop Science
Pot Luck!
Ted Bilderback
Nursery Extension Specialist
Department of Horticultural Science
North Carolina State University
Pot luck nursery production is the equivalent to filling pots with the nursery “special recipe” potting mix, incorporating a nutrient package or topdressing with a controlled release fertilizer and willing plants to grow well with little additional attention to watering or chemical properties during the growing season. Most “ pot luck” dinners provide a good selection of food groups, but there is always the risk of only having salads to fill your plate. Likewise, potting, adding fertilizer and overhead irrigating of crops may be an adequate nursery production strategy. However, a little planning and management of irrigation and nutrient monitoring can improve palatable results.
Determining how long to irrigate crops has always been an intuitive practice for container nurseries. Conservative irrigation application may not wet the potting mix thoroughly, and new crops can experience moist stress and wilting. Multiple applications frequencies provided by cyclic irrigation can sustain newly potted crops, but how long should irrigation cycles run? As plants grow and require more irrigation, how much should the irrigation run time be increased? Some crops as they grow outside the diameter of the pot seem to collect water beyond the pot and don’t need additional irrigation. Other plants in the same irrigation zone need more water to grow well. Do you know how to manage irrigation in these zones?
The answer is to monitor leaching fractions! Is monitoring leaching fractions beyond management practices of most nurseries? Maybe not! How do you monitor leaching fractions? (See Figure 1). Simply put empty pots and pots with plants in tightly fitting buckets scattered throughout an irrigation zone. How often should leaching fractions be monitored? Once a month may be adequate for irrigation planning! Paint buckets can be procured that fit many sizes of nursery containers. Water collected in buckets below an empty container represents the irrigation application volume applied during an irrigation event. Water collected in buckets below containers with plants represent the leaching fraction applied during the irrigation event. In pine bark or pine bark and sand potting substrates, the target leaching fraction is approximately 20% or 0.2. To thoroughly wet the potting mix, at least 20% more irrigation than is held by the substrate is required. Fine bark, compost, clay additions or peat moss containing substrates need only a 15% or 0.15 leaching fraction. Calculate the leaching fraction by dividing the volume in plant container buckets by the volume in buckets below the empty containers. As plants grow, irrigation run time needs to be increased to maintain a leaching fraction. Plants with various canopy architecture collect different amounts of water. Management strategies should attempt to maintain some leaching from plants that require more water, and limit excess amounts from plants that gather water. Essentially, the strategy is to average the irrigation within a block that has variable plant irrigation requirements. Making notes about variation in leaching fractions will assist you in planning for future years in deciding what crops will be placed within the same irrigation zones.
Also, irrigation volume applied by overhead sprinklers is variable between risers, as well as being variable over the distance from a nozzle on a riser. Placing buckets under empty and plant containers diagonally across an irrigation zone can provide growers with valuable details related to variability in irrigation patterns within an irrigation zone. How can growers compensate for poor uniformity in an irrigation zone? Add risers in dry areas or reduce orifice sizes in wet areas to increase uniformity. Leaching fractions can assist in adjustments needed to improve uniformity. A less standard practice is to place crops that gather water at the edges of irrigation patterns. Place plants closer to irrigation risers that repel or do not effect irrigation entrance to container surfaces.
Measuring leaching fractions provides nursery professionals with a tool that can be used for many situations. Monitoring leaching fractions can save electrical costs for running pumps, and limit the amount of run off volume from production beds. In drought years, limiting irrigation to conserve water can be managed by limiting leaching fractions to minimal leaching from crop containers. The conflicting issue with limited irrigation fractions is dealing with increasing EC (electrical conductivity / soluble salts). Also, variation in EC reading across an irrigation block may be an indicator of variable irrigation uniformity, thus differences in leaching fractions across an irrigation zone.
Leaching fraction water collected in buckets below container crops can be used to check EC and pH. However if large volumes of leachate over the 20% leaching fraction is collected, a separate leachate collection from other plant containers in the block approximately 30 minutes after irrigation provides better accuracy of EC levels. For EC monitoring, drainage can be collected from containers 30 minutes to 2 hours after irrigation, or collecting approximately 50 ml (2 fl.oz.) by pouring enough water over the surface of the container would be preferable for accurate EC and pH readings. Irrigation water is acceptable unless bicarbonates or other water impurities confuse results; if so use distilled water (See Figure 2). If EC is found to be high above 2.0 mmhos (2.0 mS/cm) a higher leaching fraction may be required to reduce high soluble salts. Frankly, run the irrigation long enough to thoroughly leach the containers. If EC levels are low (> 0.5 mmhos or -.5 mS.cm), cross check with leaching fractions. High leaching fractions leach nutrients out of containers.
In summary, monitoring leaching fractions and container leachate EC and pH levels are tools that nurseries can use to improve irrigation and nutrient efficiency. “Pot Luck” becomes a planned menu!
Figure 1: Measuring irrigation leaching fractions. Water volume collected under pots with plants is divided by the water volume collected below empty pots. The target leaching fractions for pine bark substrates is 20%.
Figure 2: The PourThru procedure can be used to collect leachate from containers for monitoring Electrical Conductivity (EC) and pH. Usually only about 50 mls (2 fl. Oz.) is needed for analysis.
