Using Biosolids as Growing Media in Greenhouse Production of Chrysanthemums
Rita Hummel, Michele Krucker, and Craig Cogger
Background
Greenhouse and nursery production of high-value plants for landscape and interior use is a specialized segment of horticulture. According to the 2003 USDA statistics, 44 million container-grown plants with a wholesale value of $292.2 million were sold by nursery operations in Oregon and Washington. The wholesale value of plants grown in containers by Washington and Oregon's greenhouse producers was $133.1 million. The USDA statistics apply only to operations with sales of $100,000 or more.
Root systems of container-grown plants are restricted to small volumes of media that must provide physical support while acting as a reservoir for nutrients, water and oxygen. The growing media are typically soilless and composed of 70 to 80% organic materials. Sphagnum peat moss has been a standard organic component for container production since the 1950's. But peat is a natural resource harvested from wetlands and concern about possible ecosystem damage due to its harvest is increasing. With rising costs and potential for scarcity, the search is on for other types of organic materials that can replace peat in container growing media.
Objectives
Using both overhead-sprinkler and water-conserving subirrigation systems we evaluated biosolids as a substitute for sphagnum peat in container media and determined the potential for biosolids to reduce the nitrogen fertilizer requirements of container-grown plants.
Methods
An experiment was conducted in the greenhouse at Puyallup to compare growth and quality of 'Shasta' chrysanthemum plants grown in biosolids/bark media to plants grown in a standard commercial peat-perlite medium (ppc). The biosolids were a Class A compost (Groco) and a Class A blend (Tagro). These were mixed 50:50 (by volume) with Douglas-fir bark to produce the experimental media: grob and tagb. Air-filled porosity (AP), water-holding capacity (WHC), pH, electrical conductivity (EC), NO3, NH4 and C:N ratio were determined on the media prior to planting (Table 1).
Liquid fertilizer was applied at 200 ppm nitrogen every other day (highN rate) or 200 PPM nitrogen every fourth day (lowN rate), while P and K were kept constant at 100 PPM and 200 PPM at each application. Irrigation treatments consisted of overhead irrigation applied by pressure-compensating sprinklers and subirrigation with a Bottom Up™ mat that combined the properties of ebb and flow and capillary action. Rooted mum cuttings were potted into 4-inch containers, there were 8 replications per treatment combination arranged in randomized complete block design on the greenhouse benches. Plant growth, consumer quality, leaf color and root growth were measured, as well as flower number and diameter. Leachate was collected from containers at intervals during production and nitrate levels were measured. Soluble salt levels were also measured because high salt concentrations can be a problem with subirrigation.
Results
Air-filled porosity and water-holding capacity are the media physical properties of primary interest to growers because they directly influence plant growth and cultural practices like irrigation. AP and WHC of the biosolids media were similar to the control and within the recommended ranges of 10-20% AP and 20-60% WHC for container media (Table 1). The biosolids media had pH, EC and C:N ratios similar to the control and within the recommended ranges for container media.
Figure 1 shows a plant chosen to represent each irrigation/fertilizer/growing media treatment combination. All plants in this experiment were considered of salable quality (rating of 3).
Figure 1.
The highN ppc, highN grob, highN tagb and lowN tagb were rated outstanding (rating of 5) while the lowN grob was rated similar to the lowN ppc in both irrigation treatments (Figure 2a).
The flower buds and dry weights of subirrigated lowN tagb plants were greater than the highN ppc controls while the overhead irrigate highN tagb plants had more buds and were larger than the highN ppc controls (Figure 2b and c). In both irrigation treatments, flower bud number and size of lowN tagb and highN grob were similar to the highN controls.
Results of this experiment indicate that high quality biosolids products can replace peat as a component of container media. The increase in plant dry weight and number of flower buds in tagb was most likely due to the greater concentration of N in this medium (Table 1). For subirrigated plants, the N in tagb compensated for reduced N fertilization while overhead-irrigated lowN tagb plants were nearly equal in size to the highN ppc controls. Measurements of EC in leachate from subirrigated plants during production indicated soluble salts levels did not exceed 0.9 (dS/m), well below levels considered detrimental to plant growth. Growth and quality of subirrigated plants equaled or was better than overhead irrigated plants in this experiment.
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