Composts as Rooting Mix Components?

June 12, 2003 - 09:57

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 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 75-90ยบ 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 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

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.

About The Author

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

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