Monitoring for profit

If you grow crops in containers, monitoring source water quality and substrate fertility are keystones to producing high-quality plants. Your soilless substrate, irrigation water and fertilizer can be compared to the tires of your car, oil in the engine and fuel to spark combustion — all of which require monitoring and routine maintenance.

Many growers go to great lengths to select and fine-tune their substrates and fertilizers. Growers also may treat their water to remove diseases, provide fertility, target specific pH and alkalinity ranges, or a combination thereof. Often, the performance of the integrated (substrate: fertilizer: water) container system is only checked when problems arise, such as the fertilizer running out or releasing too fast, substrate pH goes above or below target, water treatment efficacy decreases, or an unexpected crop mineral nutrient deficiency or toxicity appear. Catching such a problem after plant vigor and aesthetics are diminished is often too late, resulting in increased time to market, or unsalable plants.

For many growers, records of diagnostic monitoring or routine measurements (i.e., just checking — no problems noted) are not maintained because data collection, analysis, and interpretation require additional effort, time and expertise. Regular monitoring can help you better understand how your fertilizer, lime, substrate, or water treatment system performs in your unique production system, helping you further refine your production inputs to yield high quality crops in the most cost-effective manner.

Monitoring tools

Cooperative Extension and allied suppliers have provided proven methods and tools to monitor substrate fertility via extract or direct methods to measure pH and electrical conductivity (EC), a proxy for fertilizer salts. Irrigation water quality can also be monitored via limited in-house analysis or outsourcing full-analysis to laboratories. Meters that measure pH and EC are readily available from numerous suppliers. An accurate meter and probe cost approximately $200. Pocket colorimeters require an initial investment of $50 or less and can provide the concentration of alkalinity or chlorine with < 5% error. Laboratories can provide a detailed water analysis including alkalinity, sodium adsorption ratio, and mineral nutrient concentrations for $75 or less.

To aid in intuitive routine monitoring of water and fertility, extension specialists recently created GroZoneTracker.com. GroZoneTracker.com is a mobile website that enables growers to record, track, and share water quality, substrate pH, and substrate EC values. Each data point is geotagged and time-stamped to easily track data throughout the production cycle. GroZoneTracker.com provides instant feedback, letting you know if recorded values are within the self-determined desired range, nearly outside the desired range, or need immediate action to correct. The mobile website can be added as an icon on your mobile device via the default web browser to behave like a native app.

Substrate fertility

Use the pour-through method to check the pH and EC of substrate solution of actively growing containerized plants. In brief, the pour-through method works by displacing a small amount of water (leachate) from the pores of the substrate approximately 45 minutes after an irrigation event — either by (1) slowly pouring water evenly over the surface or (2) tipping the container to cause drainage of stored water. The captured leachate is placed in a small sampling container, and the EC and pH are then measured using a meter.

The target EC readings of substrate solution often correlate with their nutrient requirements and subsequent plant growth rate (i.e., low, medium, and high — see Table 1). Controlled release fertilizer longevity impacts achievable EC levels. Controlled release fertilizers with longevities exceeding 8 months may exhibit decreasing EC values while still providing fertility for optimal plant growth. Higher EC levels can be accomplished with shorter-duration CRFs or supplemental liquid fertilization. Suggested EC levels only need to be achieved leading up to and during active growth and not throughout the production period.

Specific mineral nutrient plant availability is directly related to substrate solution pH. Nitrogen and potassium availability remains relatively constant to plants within a pH range of 4.3 to 7.3., In general, calcium and magnesium availability increases as pH increases. Phosphorus and micronutrient (iron, manganese, zinc, copper, and boron) availability decreases with increasing pH. The majority of woody ornamental crops are produced with substrate solution pH 5.3 to 6.3 and EC ˜ 0.8 mS/cm (Table 1).

Water quality

Growers can use the same tools to make inferences about water quality. Collect a water sample (1) in the pond, away from the shore at the intake depth of the pump, (2) in the pump house pre- or post- treatment, or (3) at the irrigation nozzle (best reflects the water the plant receives). The latter two should be captured after running water for adequate time to ensure stagnant water lines are clear ensuring the sample is representative of a normal pumping or treatment event. A clean sample bottle should be used and preferably rinsed three times with the sample water. You can measure pH, EC, total dissolved solids, alkalinity and chlorine at the nursery or send the sample to a laboratory for full analysis. Prior to shipping, seal the bottle with minimal air or headspace and refrigerate. Collecting water at each of the three locations provides valuable information to aid in decision making.

Monitoring water EC can allow one to quantify nutrients being applied when irrigating with recaptured water or to identify salinity issues. A complete laboratory analysis provides the concentrations at parts-per-million or pounds-per-acre-inch of each mineral nutrient.

Table 2 presents desired ranges for water quality components at the pump and at the nozzle. Quantifying alkalinity, carbonate or bicarbonate concentrations, and sodium adsorption ratio of source water (well or pump) can help you determine if you need to acidify water to neutralize alkalinity. Measuring alkalinity at the nozzle will let you track how irrigation water influences substrate pH over time. You may be liming when you irrigate. Additionally, if chlorine is used to sanitize source water prior to irrigation, pH should be adjusted to fall within a range of 6.0 to 7.5 to maximize chlorine efficacy. Chlorine dioxide is less affected by pH than other forms of chlorine and may not require water acidification.

Understanding how your irrigation water and fertility interact helps you control your integrated production system to prevent complications when quickly producing high quality, saleable crops – thus increasing profits. Both substrates and water sources are dynamic systems that constantly change based on temperature and agrochemical input.

Therefore, a singular measurement only provides a reference for a specific point in time, not a complete picture. Routine monitoring over time helps you understand lime and fertilizer longevity, pH maintenance in substrates, and seasonal changes in water pH in wells or surface waters.

Use existing methods and tools to perform routine monitoring of both the substrate solution and water quality. Record data using GroZoneTracker.com and evaluate the data using the built-in visualization tools to see how using routine monitoring data can help you make decisions to enhance timing and quality of crop production.

Dr. Jim Owen (jsowen@vt.edu) is an Extension Specialist and Assistant Professor of Horticulture at Virginia Tech and Dr. Sarah White (swhite4@clemson.edu) is an Extension Specialist and Associate Professor in the Department of Plant and Environmental Sciences at Clemson University. GroZoneTracker.com was made possible by funding from the Horticulture Research Institute, the American Floral Endowment, and USDA National Institute of Food and Agriculture Specialty Crops Research Initiative.