Over the past several years, we have been working with growers to integrate sensor-automated irrigation into greenhouse production. Many of our grower collaborators have found success using these systems, and there is an overview about that project in a related article in this issue (see “Precision Irrigation: How And Why?”). One goal for our future research has been to integrate both irrigation and fertilization.
In a recent project at the University of Georgia, we worked to achieve that goal. We developed a system that precisely provides both water and fertilizer to plants. We tested this system on a long-term crop, Lenten Rose, which our grower-collaborator at Evergreen Nurseries in Statham, Ga. indicated are challenging to grow due to issues with crown rot.
Automated Fertigation With Real-Time EC Sensors
This system uses sensors to measure substrate moisture, EC and temperature (GS3 sensors, Decagon Devices, Figure 1, slideshow). Our system automates both fertigation and irrigation using the substrate moisture and EC measurements from these sensors. If you are accustomed to monitoring EC using the pour-thru method, using real-time EC sensors will be similar. They measure bulk EC (the combined EC of substrate particles, air spaces and the solution in the substrate) and use that to estimate solution EC, which is similar to what is measured using the pour-thru.
However, one important difference in our approach is that fertility recommendations for crops may differ. This is because you are maintaining a constant EC in the substrate, rather than using a conventional fertility approach where substrate EC would fluctuate. Another important difference is that this system results in little leaching. The positive impact of reduced leaching is that there is almost no fertilizer waste. However, similar to using a subirrigation system, there may be different fertilizer recommendations for plants than in traditional irrigation and fertilization systems. Buildup of fertilizer salts in the substrate needs to be prevented.
In this system, water and fertilizer are provided in different lines, based on sensor measurements (Figure 2, slideshow). We maintained stable substrate moisture and EC levels in substrates. We grew Lenten Rose in this system for six months last spring and summer in Georgia at two relatively high substrate moisture contents (40 and 50 percent) and a wide variety of ECs (0.25 to 2.0 mS/cm) in 1-gallon pots. We used a 15-5-15 Cal-Mg fertilizer for this study; however, any water-soluble fertilizer could be used in this sort of system.
We were able to control both EC and substrate moisture using this system. Since we were only applying water and fertilizer as needed, we used surprisingly little of both to grow our crop. Only 1.3 to 2.6 gallons of water was needed to grow Lenten Rose. Using less water would represent an environmental benefit, and would be a great way for growers in dry areas of the country to save water.
Surprisingly, some of our plants received absolutely no fertilizer for more than six months. Since this is a zero-leach system, starter fertilizers alone provided enough fertility to maintain ECs above 0.75 mS/cm all summer. Consequently, these plants were never fertilized and didn’t grow well. Other plants that were maintained at higher ECs still received relatively little fertilizer. Each plant received the equivalent to 0.1 to 1.13 gallons of a 200 ppm N solution.
Growing Plants With Less Fertilizer
The appearance of our plants varied greatly when we grew Lenten Rose with so little fertilizer; however, water did not impact plant growth (Figure 3, slideshow). The plants that received no fertilizer (EC of 0.25 to 0.75 mS/cm) were unsalable. Although they had some fertilizer from the starter charge in our substrate, that was not enough to support plant growth.
These plants had small, chlorotic leaves. In general, the overall size of these plants was smaller compared to those grown at higher substrate ECs. By comparison, all plants that received fertilizer at some point during our study (1.0 to 2.0 mS/cm) were, not surprisingly, darker green and larger in size. All plants that received any fertilizer were saleable. Generally, plants were larger when they were grown at higher ECs.
We observed the plants for crown rot throughout the summer, which is when Lenten Rose typically has problems with this disease in Georgia. None of the plants had disease symptoms. Better control of both water and fertilizer may have enabled us to prevent disease. In a commercial setting, this would provide growers with the ability to prevent shrinkage and loss of sales.
Integrating Automated Fertigation Into Your Greenhouse
If you are interested in using this approach in your greenhouse, you have a few options. It is possible to measure substrate moisture and automate irrigation using a variety of greenhouse control systems. These include most advanced greenhouse environmental control systems or a soon to be released, stand-alone system from our collaborators at Decagon Devices. We have also worked to develop low-cost, custom-built systems, using inexpensive, open-source microcontrollers (Arduino, Raspberry Pi), which can be a good option for small to mid-sized greenhouses. Growers may monitor EC and make decisions regarding fertilizer applications based on measurements from the sensors.
We hope EC automation will become commercially available in much the same way that irrigation automation has in recent years. However, even the ability to only monitor EC in real time will provide greater decision-making capability than is currently available for growers doing routine pour-thru measurements. This information will provide growers with the ability to reduce fertilizer applications and time them for when they are most needed for crops.
For growers using controlled release fertilizers, there is great potential to monitor EC and determine how temperature and other environmental factors impact release rate. For example, in warm climates, such as Georgia, we have worked with nursery growers who have sudden, high controlled fertilizer release rates in the summer due to warm temperatures. Having greater ability to monitor the relationship between water, EC and temperature will help growers decide when additional fertilizer is needed, which will result in higher plant quality and fertilizer savings.
The Capes Foundation (Ministry of Education, Brazil; postdoctoral researcher scholarship, BEX 2620/13-8) and a USDA-NIFA Specialty Crops Research Initiative Award (#2009-51181-05768) funded this research. The authors thank James Greenhouse and the Fafard Corporation for providing plants and substrate to support this work.