Agriculture Reference
In-Depth Information
The aggregate component of an engineered media provides pore space for air,
water and gas exchange (necessary functions to support plant life), and ensures
rapid drainage (to prevent ponding and thus excessive loading). Most LWA are
inert, have coarse texture and low organic content, which means that key func-
tions for plant growth must be supplemented. For example, coarse inert materials
usually have low cation exchange capacity (CEC), a chemical property describing
the ability of a material to hold positively charged ions. In this case, CEC is a useful
indicator of the potential to store macronutrients required for plant growth such as
calcium (Ca 2+ ), magnesium (Mg 2+ ), potassium (K + ), and sodium (Na + ). In New
Zealand, the recommended LWA for extensive living roof growing media is a com-
bination of pumice and zeolite; the former has low CEC, and so it is supplemented
by a small addition of higher-cost (but equally lightweight) zeolite (Fassman-Beck
and Simcock 2013). CEC may also provide some chemical buffering, for example
moderation of pH where acid rain is a concern (Berghage et al. 2007). Recycled
aggregates or other reused materials must be considered for the potential to con-
tribute contaminants (Molineux et al. 2009; Ye et al. 2013). Bricks or clay roof tiles
can contribute to water-holding capacity (depending on the clay and iring process)
(Graceson et al. 2013), but may contain elevated concentrations of heavy metals,
which could compromise the potential for urban agriculture (Ye et al. 2013), but
stormwater implications were not deliberately tested. Crushed concrete may
elevate pH (Vijayaraghavan et al. 2012). Despite effective drainage potential
(Vijayaraghavan et al. 2012), tire crumb has been shown to contribute zinc in lea-
chate (Solano et al. 2012), an undesirable outcome for water quality.
The challenges posed by the aggregate component for plant growth from low
organic content and low water-holding capacity are supplemented by an addition
of less than 20 percent v/v organic matter. In many cases, 10 percent v/v or less
of organic matter is desirable. The FLL's maximum limit of 20 percent organic
matter was originally speciied by German ire regulations, to prevent smoulder-
ing burns in living roofs with low plant density or during dry periods in summer.
Limiting organic content of the growing medium to the level that can be sus-
tained by plant inputs (once established) helps to limit the potential for nutrient
leaching. Limiting organic content also limits the types of plants to those able to
grow adequately under nitrogen-stressed conditions (Nagase and Dunnett 2011).
Encouraging hardier plants may reduce the maintenance burden by reducing
biomass and growth of adventive (weedy) plants. Nagase and Dunnett (2011)
concluded that 10 percent v/v organic matter (as greenwaste compost) was
optimal for the growth of four, non-sedum living roof species as this produced
stable plant growth regardless of water availability. Inadequate organic supply
may result in nitrogen stress, with longer vegetation establishment times required
with decreasing organic content without supplemental irrigation or fertilization.
Nonetheless, the low organic content should provide adequate moisture storage
potential for stormwater control and sustaining plants during normal periods of
dry weather, ine particles for root development, and nutrient supply for a
thoughtfully speciied plant community.
 
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