Composts as Rooting Mix Components? By Jianjun Chen, Dennis McConnell, Russell Caldwell and Kelly Everitt

University of Florida research determines whether composts can be used as rooting mix components in foliage crops.

The benefits of using composts as soil amendments have beenwell documented. Composted biosolids (BS), municipal solid waste (MSW) and yardtrimmings (YT), when appropriately applied to soil, have been shown to improvevegetable, fruit and field crop production. Composts have also been used ascomponents of container media for producing bedding, landscape and foliageplants. However, composts have not been used as components of propagation mixesfor rooting foliage, or any other type of plant cuttings.

Plants from more than 100 genera are produced as ornamentalfoliage plants. Tissue culture has been used to propagate many genera infoliage plant production, but a large number of genera are commonly orexclusively propagated from cuttings. Because rooting requires high-qualitymedia, composts have not been rigorously evaluated as components of propagationmedia. However, modern composting facilities can now generate composts ofconsistently high quality. We wondered if those composts could be used toreplace some of the sphagnum peat or other expensive materials commonly used inpropagation mixes for rooting foliage or any other type of plant cuttings.

The composts used in our evaluation were composted MSW/BS,YT and YT/BS. The MSW/BS was composed of two parts of MSW, mainly householdgarbage, and one part of 16 percent polymer-dewatered BS, primarily sewagesludge, based on weight. After three days of aerobic digestion, the MSW/BS waswindrow-cured — when composted materials go through a curing process duringwhich uniform and well-digested compost is produced — for 21 days and thenscreened as finished compost. The YT feedstock included grasses, leaves andtree debris, which was screened and windrow-composted, one method ofcomposting, for 90 days. The YT/BS consisted of three parts YT and two parts16-percent lime-dewatered BS based on weight. The final YT/BS product wasderived from 90-days of in-vessel composting.

The three composts described above were mixed in volumetriccombinations with sphagnum peat (SP) and pine bark (PB). Twelve mixes wereproduced, and a common industry propagation mix, UF-2 (University of Floridacontainer mix 2), was used as a control (See figure 1, right). There were nobiohazards in handling these composts, and no odor was detected fromcompost-based media.

Before the experiment

Before experiment setup, physical and chemical properties ofthe 13 mixes were tested for bulk density, total porosity, container capacity,moisture content, air space, pH, EC, carbon to nitrogen ratio, cation exchangecapacity (CEC) and concentration of extractable mineral elements. Physically,as the percentage of composts increased in the mixes, bulk density increased,while total porosity, moisture content and air space decreased. But thosechanges were generally within desired ranges, except air space in mixes 3, 6and 9 where it was below 10 percent. Generally speaking, mixes with bulkdensity ranging from 0.15-0.8 g/cc (dry weight), total porosity of 50-75percent, container capacity of 20-60 percent by volume, moisture content of50-75 percent and air space of 10-20 percent are considered acceptable forrooting or growing containerized plants. Air space less than 10 percent mayaffect root respiration and is not considered suitable for rooting.

Chemically, as compost percentages increased, EC, pH and CECincreased (See figure 2, right). The increase in EC occurred because thecomposts contained water-extractable calcium, potassium, magnesium, sodium andother elements. An EC reading of bulk solution extracted by the pour-thoughmethod above 3.0 dS/m is generally considered to be the upper limit for theproduction of foliage plants. Among the 13 mixes, EC readings higher than 3.0dS/m were seen in solutions extracted from mixes 2, 3, 6, 8, 9, 11 and 12,which had compost percentages of 50 percent or higher. The concentrations ofheavy metals such as copper, cadmium, cobalt, chromium, nickel and lead wereall below the EPA’s permissible heavy metal ranges. Concentrations of sulfur inmixes 2, 3, 8, 9, 11 and 12 were greater than 100 ppm and boron in mix 9 was10.5 ppm, which were considered too high for healthy growth of some foliageplants. The carbon to nitrogen ratio ranged from 15.2-26.9, suggesting thatmost mixes were within maturity range since composts with carbon to nitrogenratio of 25 or less are considered to be mature.

Experiment process

After the determination of the physical and chemicalproperties, cupric hydroxide-treated 4-inch containers were filled with thesemixes and placed on shaded glasshouse benches enclosed by polyethylene tentsand misted for 10 seconds every 10 minutes from 7 a.m. to 8 p.m. daily. Twodays later, single eye cuttings of pothos (Epipremnum aureum) ‘Golden Pothos’,terminal cuttings of maranta (Maranta leuconeura) ‘Kerchoveana’ and schefflera(Schefflera arboricola) ‘Goldenfinger’ were stuck using three cuttings per potwithout rooting hormones. Each treatment had 15 replicates or 15 pots.Temperatures of the mixes ranged from 75-90º F, and the maximum lightlevel was 800 foot-candles.

Fourteen days after sticking, cuttings from five pots wereremoved, and all roots longer than 0.04 inches were counted. Twenty-one days aftersticking, cuttings from another five pots were removed, and individual rootlengths per cuttings were measured. Total root lengths per cutting werecalculated by adding the lengths of all roots. Forty-five days after sticking,root-ball coverage of the cuttings in the remaining five pots was graded. Rootball coverage rating was based on the following scales: 1 = 0-20 percent, 2 =21-40 percent, 3 = 41-60 percent, 4 = 61-80 percent, and 5 = 81-100 percentroot ball coverage with white, healthy roots.


Results showed that initial differences in physical andchemical properties of the mixes had little if any effect on root initiation,i.e., root numbers per cutting 14 days after sticking. However, 21 days aftersticking, root lengths were affected by the propagation mixes. Total rootlengths produced in mixes 1, 4, 5, 7 and 10 were either slightly longer than orequal to those of the control for all test plants (See figure 3, below), andthe differences were still evident when root ball coverage ratings were made 45days after sticking (See figure 4, below).

Root length measurements and root ball coverage ratingsreveal that mixes with at least 10 percent air space, low concentrations ofmineral elements, low initial EC readings (3.0 dS/m or less based on thepour-through method) and low pH (3.8-5.0 initially) had better root growth thanother mixes. In addition to the control, mixes possessing these characteristicswere 1, 4, 5, 7 and 10. These five mixes were formulated by combining compostedMSW/BS or YT/BS volumetrically at 20 percent or less or composted YT at 50percent or less with equal volumes of peat and bark.

Overall, this evaluation shows that any of the threecomposts, after being appropriately mixed with sphagnum peat and pine bark, canbe used to formulate propagation mixes for rooting foliage plant cuttings. Theuse of composts to formulate rooting mixes provides an additional market outletfor composts, and it reduces the amount of sphagnum peat use in propagationmixes.

The authors would like to thank AllGro, Inc., West PalmBeach, Fla. (YT/BS); Consolidated Resources Recovery, Sarasota, Fla. (YT); andSumter County Solid Waste Facility, Lake Panasoffkee, Fla. (MSW/BS), forproviding composted materials; Fafard, Inc., Apopka, Fla., for providingsphagnum peat and pine bark used in this study, and Dr. Robert J. Black for thecritical reading of this report. This study was supported in part by the Centerfor Biomass Programs, IFAS, University of Florida.

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Jianjun Chen, Dennis McConnell, Russell Caldwell and Kelly Everitt

Jianjun Chen is assistant professor, Russell Caldwell is biological scientist and Kelly Everitt is research assistant at the University of Florida, Apopka, Fla. Dennis McConnell is professor in the Department of Environmental Horticulture at the University of Florida, Gainesville, Fla. They can be reached by phone at (407) 884-2034 or E-mail at [email protected]

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