Nursery Crop Science
Clay Substrate Amendment Saves and Pays
James S. Owen, Jr.
North Carolina State University
Department of Horticulture Science
Soilless substrate is the ‘green’ industry standard for growing
ornamental plants, yet soilless substrates do not offer the nutrient retention
or water buffering capacity provided by soil. Clay minerals and humic matter
are the dominant soil components that provide these beneficial attributes. With
little nutrient or water buffering capacity, high water and nutrient inputs
are required to yield a salable plant quickly.
The majority of containerized nursery crops in the United States are grown in
bark based container substrate (West Coast and Eastern United States). Pine
bark, common in the Eastern United States, when combined with frequent irrigation
and high fertility levels produces rapid plant growth. However, environmental
concerns and water restrictions are forcing growers to rethink production practices,
particularly, in regards to water and fertilizer usage. Due to pine bark’s
limited water and nutrient buffer capacity, growers cannot simply reduce water
or fertilizer usage without sacrificing plant growth and quality. Modifying
container substrate to increase water and nutrient buffering capacity might
increase water and nutrient efficiency.
Prior to 2002, there were seven research studies focused on adding clay to soilless
container substrate in the United States. Of those only two looked at clay and
bark combinations. The remaining five were with peat based substrate. In addition,
the research done with clay in Europe was with peat. Therefore, it was not surprising
that little was known about using clay in bark. Prior to 2002, the most detailed
study had been conducted with arcillite (a calcined montmorillonite and illite
clay) by Warren and Bilderback in 1992. Arcillite was incorporated into pine
bark with rates from 0 to 136 lbs/yd3. Container capacity, available water,
and bulk density increased with increasing rate of arcillite. Substrate NH4,
P, and K concentrations increased with increasing arcillite rate suggesting
that arcillite improved retention within the container substrate. Plant growth
increased curvilinearly with arcillite rate, with maximum dry weight occurring
at 112 lbs/yd3.
The value of adding clay to soilless substrates has been debated for years.
Even though amending soilless substrates with clay has many potential benefits,
there was little empirical evidence to answer this question. ‘Clay’
is often used generically to describe soils that have high water and nutrient
holding capacity. Clays, like soils, are not the same due to differences in
physical and chemical properties as a result of handling, source, and packaging.
The effectiveness of clay will differ with type (2:1 versus 1:1), handling (temperature
treatment, particle size), and source or location mined (chemical composition).
The type of clay and heat treatment (pasteurized or calcined) are important
factors affecting water holding capacity and available water content, thus determining
water buffering capacity. These factors, in addition to the chemical composition
and particle size, determine phosphorus retention in the substrate and availability
to the plant. Phosphorus retention is a function of the abundance of exposed
aluminum and iron oxides which can be a result of handling, source, and type
of clay. Therefore, it is misleading to simply talk about ‘clay’
soils. Will any clay soil improve the water and nutrient capacity of a soilless
substrate? That led us to begin looking at the differing types of clay, particles
sizes, and heat treatments to see how these factors might affect the response
of clay amended pine bark substrates.
An international research conference in 2001 re-ignited our interest in using
clay as an amendment with pine bark substrates. Research conducted in the Netherlands
suggested that clay as a peat substrate amendment was worth the additional cost
and that particle size along with the type (2:1 and 1:1) of clay had an impact
on the effect of clay.
In 2002, we examined several types of clay in combination with several particle
sizes and heat treatments [calcined (~800 F) or pasteurized (~250 F)] as a pine
bark amendment. Most 2:1 clays treatments increased water and nutrient buffering
capacity compared to the 8 pine bark: 1 sand substrate. However, there was a
wide range of results. Cumulative water applied to the substrate amended with
small (24-48 mesh) or large (4-20 mesh size) particle size clay was ~ 6 and
3 gallons less, respectively, than the 8 pine bark : 1 sand substrate. This
translates to 100,000 to 200,000 gallons per growing acre per season. Even though
the clay amended substrates required less water, plant growth was not affected
since Cotoneaster dammeri ‘Skogholm’ root and shoot dry weight were
similar in all treatments. In addition to water savings, the 2:1 clays gave
us increased phosphorous retention, with maximum retention being obtained from
the small particle size, calcined clay amended pine bark substrate.
The increased water and nutrient buffering capacity of a clay amended pine bark
substrate could allow growers more flexibility in cultural management. This
could allow nurseries to incorporate best management practices (BMP's) while
maintaining maximum plant growth, making the BMP's a more attainable goal.
