Composted Sewage Sludge - A Media Component For Rhododendron Production
R.L. Ticknor, D.D. Hemphill, Jr., and D.J. Flower 1
North Willamette Agricultural Experiment Station, Aurora, OR
Oregon State University Agricultural Experiment Station Technical Paper No. 7408
Two rates of composted sewage sludge were combined with pumice and either bark, peat moss, or sawdust to form 6 potting mixes. Increasing compost content from 25 to 50% increased initial pH, electrical conductivity, air-filled pore space, and nutrient levels of the media, and increased growth of Rhododendron 'Vulcan'. Plant growth in 25% compost mixes was equal to that in two commercial mixes while growth in 50% compost mixes was equal to a fertilized bark mix.
Azaleas and rhododendrons grow best in well aerated, moisture-retentive media. Peat moss has traditionally been a major component of potting mixes for these plants and has been mixed with the soil when setting out plants in the landscape. The variable quality and relatively high price of peat moss has caused nurserymen to use other products such as ground bark, wood chips and straw for azalea and rhododendron culture.
Another source of organic material for potting mixes is sewage sludge or municipal waste composts (3, 4, 5, 8, 12). Not all sludge composts are suitable for rhododendrons because of high levels of heavy metals (1, 2). Lime used to remove solids from waste water in some systems results in a high pH compost (5). Composting in a temperature controlled closed vessel system kills weed seeds, pasteurizes the compost, and eliminates undesirable odors (5).
Testing of sewage sludge-conifer sawdust compost from a closed vessel system demonstration plant in Portland, Oregon was started in January 1983. Portland produces sewage sludge which contains plant nutrients and trace elements at levels near or slightly below the national median (2). Composted sludge became available in the Portland area in bulk for commercial growers and in plastic bags for home owners with the completion of the full scale plant in October 1984. Similar composts will be available in other parts of the U.S. in the future. But the nutrient element content could differ from compost made in Portland.
In January 1983, three bedding plant species were planted in 19 mixes based on this compost and in five commercial mixes. Plant growth in the mixes was at least equal to that obtained in the commercial mixes (3). Starting in March 1983, six of the most promising compost based mixes from the bedding plant trial were evaluated for growth and foliar chemical analysis of Photinia and Thuja. Results of the Photinia and Thuja trial are to be published elsewhere (11).
Methods And Materials
Sewage sludge from Portland, Oregon composted with conifer sawdust in a demonstration closed vessel system was used to prepare six potting mixes with either 25 or 50% by volume compost (Comp), 50 or 25% hammer milled conifer bark (Bark), conifer sawdust (Saw), or sphagnum peat moss (Peat), and 25% pumice plus gypsum (3 lbs/yd 3 ). Growth of plants in the compost mixes was compared to that in three commercially used mixes: (1) bark mix consisting of conifer bark plus 11 lbs of 18N-2.6P-10K (18-6-12) Osmocote® fertilizer, 3.5 lbs concentrated superphosphate, and 1.5 lbs each of dolomite, gypsum, limestone, and MicroMax® trace elements/yd 3 ; (2) commercial mix consisting of 4 peat moss, 4 pumice, 1 conifer bark, and 1 sand supplemented with 3.0 lbs each of dolomite, and limestone, 1.3 lbs urea formaldehyde, 1.0 lb concentrated superphosphate, 0.7 lb iron sulfate, 0.5 lb each of calcium and potassium nitrates, and 2 oz. FTE 503/yd 3 ; (3) Amfac nursery mix consisting of peat moss with undisclosed fertilizer additives. Particle size distribution of the components of the compost and bark mixes has been reported (3).
'Vulcan' rhododendron cuttings rooted in 2.25" x 3.5" square pots were potted into 6" x 7" one gallon containers on March 24, 1983, and were grown at 55°F minimum night and 90°F maximum day temperatures in a double polyethylene covered house. Daylength was extended with fluorescent lights from 10 p.m. to 4 a.m. from March 24 through May 1. The plastic was removed on June 1 and the plants were grown in full sun. Five replications of five plants were grown in each media.
Overhead irrigation was applied daily. Peters® 30N-4.3P-8.3K (3O-10-10) at 400 ppm N was applied with a hose at weekly intervals from April 19 until June 15 when Osmocote 18-6-12 was applied at 1 teaspoon per pot.
Height, maximum width, and the number of flowering and vegetative branches were recorded on November 30, 1983. In January 1984, plants were removed from the pots and depth of root penetration was evaluated.
Sixteen Rhododendron 'Holden', 32 'Virginia Richards', and 32 'Vulcan' rooted cuttings in 2.25" square by 3.5" deep pots were potted into 6" x 7" one gallon pots on April 6, 1984. Half were potted in conifer bark with 11 lbs of Osmocote 18-6-12, 3.5 lbs concentrated superphosphate, and 1.5 lbs each of dolomite, gypsum, limestone, and MicroMax trace elements added per yd 3 . The other half was potted into a compost: bark: pumice (2:1:1) mix containing 5.5 lbs Osmocote 18-6-12/yd 3 . The plants were held in an unheated polyethylene house until cover removal on June 12, 1984. Irrigation and fertilization practices were the same as in 1983. Growth measurements were recorded on November 5, 1984.
|Table 1. Physical characteristics on March 24, 1983 and elemental content of potting mixes on August 4, 1983.|
|Mix||AS||WS||EC||pH||Percent Dry Weight||Parts Per Million Dry Weight|
|Comp 1 Peat 2 1||17.9||54.1||0.37||4.5||7.7||.50||08||.06||.45||.14||.10||77||3311||59||14||83||2281||437||0.7||8|
|Comp 2 Peat 1||23.3||49.3||0.93||4.9||7.4||.68||.24||.07||.40||.11||.10||86||4692||113||12||221||3895||549||1.8||15|
|Comp 1 Bark 2||12.5||50.5||0.53||4.9||7.1||.42||.15||.05||.37||.08||.08||126||3781||65||7||140||2534||398||1.0||10|
|Comp 2 Bark 1||16.1||47.8||0.95||5.1||6.9||.50||.22||.06||.36||.09||.08||132||5367||107||7||300||3515||403||1.9||16|
|Comp 1 Saw 2||22.3||50.0||0.56||5.6||7.3||.41||.12||.06||.26||.08||.06||132||2859||55||19||111||1752||462||0.8||7|
|Comp 2 Saw 1||23.3||48.1||1.15||5.8||7.4||.58||.27||.09||.40||.10||.08||154||4185||107||12||245||3592||683||1.7||14|
All compost-based mixes contained 1 part (25.%) by volume pumice, Comp 1 = 25% composted sewage sludge, Comp 2 = 50% Peat = peat moss
Bark = Hammer milled conifer bark
Saw = Conifer sawdust.
AS = Air space
WS = Water space
EC = Soluble salts
Results And Discussion
Mixes containing 50 percent compost had more air space (AS) lower water space (WS) as well as higher soluble salts (EC) and higher pH than did mixes with 25 percent compost (Table 1). The bark mix had the highest AS and EC and the lowest pH. The commercial mix had relatively low AS, EC, and the highest pH. The nursery mix had the lowest EC but a relatively high pH. Compost mixes made with bark had lower AS than did those containing peat moss or sawdust.
No consistent change in the amounts of nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, or nickel was measured in the potting mixes between August 4 and December 19, 1983 samplings. With few exceptions, the levels of manganese, iron, copper, boron, zinc, aluminum, and sodium were lower in December than in the August sampling. Only results of the August sampling are reported (Table 1).
|Table 2. Growth of 'Vulcan' rhododendron in compost-based and comparison mixes. Planted March 14, 1983; results recorded November 30, 1983.|
|# vegetative shoots||# flower shoots||Rating of root growth 1|
|Comp 1 Peat 2||14.6||18.8||1.9||5.4||2.5|
|Comp 2 Peat 1||14.8||19.8||2.7||6.4||2.4|
|Comp 1 Bark 2||13.1||17.3||1.0||5.6||2.1|
|Comp 2 Bark 1||13.4||19.1||1.6||6.7||2.1|
|Comp 1 Saw 2||13.8||16.1||2.2||4.5||1.8|
|Comp 2 Saw 1||14.2||18.1||2.2||6.2||1.9|
|LSD 5% level||N.S. 2||1.2||N.S.||1.2||0.3|
1 = Root ball depth to 3.5 inches, 2 = 3.5 to 5.0 inches, 3 = roots to bottom of the pot.
2 No significant differences.
Height, width, and number of flowering and vegetative shoots of 'Vulcan' were greater in mixes containing 50 rather than 25% compost although these differences were not always statistically significant (Table 2). Fifty percent compost mixes contained greater amounts of nitrogen (N), phosphorus (P), and minor elements except boron (B) (Table 1). Plants in the bark mix had the most flower shoots at 7.8 per plant but all of the 50% compost mixes had over 6 flower shoots per plant which is very acceptable for a one year 1 gallon plant. Plants with the best overall appearance and size were grown either in the bark mix or in the compost: peat mixes which also had the most extensive root growth.
The observed differences in plant size and type of growth of 'Vulcan' do not appear to be the result of the level of any single nutrient element in the leaves (Table 3). A Ca:Mg ratio of 4:1 was associated with increased flowering when compared to lower ratios (10). Nursery, commercial and compost 1 :sawdust 2 produced plants with the fewest flower shoots and had Ca:Mg ratios less than 4:1.
Roots in these three mixes were the least extensive and this condition may be responsible for the low Ca:Mg ratio. The Ca:Mg ratio of the potting mixes does not appear to be related to the foliage Ca:Mg ratio.
Compared to previous foliar analysis of container-grown 'Vulcan' rhododendrons (9), nitrogen and phosphorus were in the normal range. Leaf potassium levels were lower than average for container grown plants but higher than the average for field-grown plants (9, 10). Calcium levels were intermediate between levels found in good field and container grown plants (9, 10). A magnesium level of 0.23-0.26 percent was found in good container and field grown plants (9, 10); plants grown in compost mixes were in this range. Leaf Mg levels of 0.41 and 0.30 percent respectively in the nursery and commercial mixes are probably too high.
Sulfur was not determined in previous analyses so a level associated with good growth has not been established. Manganese levels found in this experiment were in the normal range (9, 1 0). Iron levels were higher than those found in surveys of field-grown 'Vulcan'. Plants grown at the North Willamette Station usually have high levels of iron because of the high iron content of the water.
Boron levels were very low in all treatments compared to previous analyses of container- (20 ppm) and field-grown plants (33 ppm). Zinc levels were higher than found in field-grown plants (10) but were the same or lower than in container plants. (9).
Only a limited supply of compost was available for testing in 1984 so the number of plants with each potting mix bark or compost is small. Only 8 plants were used with 'Holden' and 16 with 'Virginia Richards' and 'Vulcan'. Statistical analysis indicates that the differences in height and width shown in Table 4 are not significant. Only the difference in numbers of vegetative shoots with 'Virginia Richards' and flowering shoots with 'Holden' were significantly different.
Plants grown in a 50% compost mix with 1 lb N/yd 3 from Osmocote 18-6-12 were essentially equal to those grown in bark with 2 lbs N/yd 3 from Osmocote 18-6-12 plus calcium, magnesium, and trace elements.
Plant size in 1984 was smaller than in 1983 however the plants were potted into 1 gallon containers later and were grown without heat and supplemental light.
|Table 3. Foliar nutrient content of 'Vulcan' rhododendron grown in compost-based and comparison mixes, 1983|
|Potting Mix||Percent Dry Weight||Parts per Million Dry Weight|
|Comp 1 Peat 2||1.65||.22||0.77||0.99||0.25||.17||161||265||5.1||8.4||30.0||29|
|Comp 2 Peat 1||1.67||.27||0.77||1.03||0.24||.19||175||316||5.2||7.8||39.0||32|
|Comp 1 Bark 2||1.75||.29||0.78||1.10||0.24||.21||191||267||4.5||11.2||36.4||31|
|Comp 2 Bark 1||1.81||.26||0.74||0.74||0.25||.19||185||280||4.4||8.6||46.2||31|
|Comp 1 Saw 2||1.80||.23||0.75||0.92||0.25||.17||177||321||3.6||9.0||28.8||29|
|Comp 2 Saw 1||1.78||.23||0.76||0.99||0.23||.16||188||279||4.2||8.0||42.8||27|
|LSD 5% level||0.17||.04||0.22||NS||0.04||NS||42||NS||1.4||2.8||7.1||8|
|Table 4. Growth of three rhododendron cultivars in bark and 50% compost mixes, 1984|
|# vegetative shoots||# flowering shoots|
* Significant at 5% level for a variety and column
** Significant at 1 % level for a variety and column
Conclusions And Recommendations
Composted Portland sewage sludge used as 25 or 50 percent of a potting mix resulted in satisfactory growth of Rhododendron 'Vulcan'. Growth in 50 percent compost was equal to or better than that in non-compost mixes, particularly when peat moss or bark was the other organic component in the mix. Sawdust was less satisfactory as the other organic component.
Supplemental nitrogen was needed with all potting mixes. Leaf phosphorus levels of plants grown in compost were generally high; supplemental phosphate may not be needed with compost. We suggest that supplemental fertilizers containing a low phosphorus level be used to maintain adequate phosphate in the mix. Potassium levels in the leaves were adequate with liquid feeding with 30-1 0-1 0 and top dressing with Osmocote 18-6-12. Potassium levels in the compost-based mixes analyzed at the end of the growing season were lower than in the non-compost, indicating probable need for more potassium-containing fertilizer after potting. Irrigation water at the North Willamette Experiment Station contains high levels of calcium and magnesium (20 ppm); it may be necessary to add more of these elements at some locations.
Addition of minor elements to compost-based mixes does not appear necessary, with some possible exceptions. Compost-based mixes tended to have low boron content and leaves of plants grown in these mixes usually had lower boron content than did plants in the non-compost mixes. A spray application of boron during the growing season might be necessary to prevent a deficiency.
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1 Professor, Associate Professor, and Research Technician, respectively.
This research supported in part by Taulman-Weiss Co., Atlanta, GA.