Composts as Rooting Mix Components?

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

Plants from more than 100 genera are produced as ornamental foliage plants. Tissue culture has been used to propagate many genera in foliage plant production, but a large number of genera are commonly or exclusively propagated from cuttings. Because rooting requires high-quality media, composts have not been rigorously evaluated as components of propagation media. However, modern composting facilities can now generate composts of consistently high quality. We wondered if those composts could be used to replace some of the sphagnum peat or other expensive materials commonly used in propagation 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 household garbage, and one part of 16 percent polymer-dewatered BS, primarily sewage sludge, based on weight. After three days of aerobic digestion, the MSW/BS was windrow-cured -- when composted materials go through a curing process during which uniform and well-digested compost is produced -- for 21 days and then screened as finished compost. The YT feedstock included grasses, leaves and tree debris, which was screened and windrow-composted, one method of composting, for 90 days. The YT/BS consisted of three parts YT and two parts 16-percent lime-dewatered BS based on weight. The final YT/BS product was derived from 90-days of in-vessel composting.

The three composts described above were mixed in volumetric combinations with sphagnum peat (SP) and pine bark (PB). Twelve mixes were produced, and a common industry propagation mix, UF-2 (University of Florida container mix 2), was used as a control (See figure 1, right). There were no biohazards in handling these composts, and no odor was detected from compost-based media.

Before the experiment

Before experiment setup, physical and chemical properties of the 13 mixes were tested for bulk density, total porosity, container capacity, moisture content, air space, pH, EC, carbon to nitrogen ratio, cation exchange capacity (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 those changes were generally within desired ranges, except air space in mixes 3, 6 and 9 where it was below 10 percent. Generally speaking, mixes with bulk density ranging from 0.15-0.8 g/cc (dry weight), total porosity of 50-75 percent, container capacity of 20-60 percent by volume, moisture content of 50-75 percent and air space of 10-20 percent are considered acceptable for rooting or growing containerized plants. Air space less than 10 percent may affect root respiration and is not considered suitable for rooting.

Chemically, as compost percentages increased, EC, pH and CEC increased (See figure 2, right). The increase in EC occurred because the composts contained water-extractable calcium, potassium, magnesium, sodium and other elements. An EC reading of bulk solution extracted by the pour-though method above 3.0 dS/m is generally considered to be the upper limit for the production of foliage plants. Among the 13 mixes, EC readings higher than 3.0 dS/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 of heavy metals such as copper, cadmium, cobalt, chromium, nickel and lead were all below the EPA's permissible heavy metal ranges. Concentrations of sulfur in mixes 2, 3, 8, 9, 11 and 12 were greater than 100 ppm and boron in mix 9 was 10.5 ppm, which were considered too high for healthy growth of some foliage plants. The carbon to nitrogen ratio ranged from 15.2-26.9, suggesting that most mixes were within maturity range since composts with carbon to nitrogen ratio of 25 or less are considered to be mature.

Experiment process

After the determination of the physical and chemical properties, cupric hydroxide-treated 4-inch containers were filled with these mixes and placed on shaded glasshouse benches enclosed by polyethylene tents and misted for 10 seconds every 10 minutes from 7 a.m. to 8 p.m. daily. Two days 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 pot without rooting hormones. Each treatment had 15 replicates or 15 pots. Temperatures of the mixes ranged from 75-90º F, and the maximum light level was 800 foot-candles.

Fourteen days after sticking, cuttings from five pots were removed, and all roots longer than 0.04 inches were counted. Twenty-one days after sticking, cuttings from another five pots were removed, and individual root lengths per cuttings were measured. Total root lengths per cutting were calculated 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. Root ball 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 percent root ball coverage with white, healthy roots.

Results

Results showed that initial differences in physical and chemical 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 after sticking, root lengths were affected by the propagation mixes. Total root lengths produced in mixes 1, 4, 5, 7 and 10 were either slightly longer than or equal to those of the control for all test plants (See figure 3, below), and the differences were still evident when root ball coverage ratings were made 45 days after sticking (See figure 4, below).

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

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

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