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Second-Use of Horticultural Rock Wool in NZ

Article written by W.T. Bussell, School of Natural Sciences, Unitec New Zealand, Private Bag 92 025, Auckland and S. McKennie, Veg-Gro Supplies, 10 Firth Street, Drury.

Article reproduced with permission of David Peterson and Peter Cooper, Vegfed NZ

1. INTRODUCTION
About 1000 cubic metres of horticultural rockwool, mostly in the form of slabs, were imported into New Zealand in the year to March 2004 and this volume is estimated to rise to 2,000 – 3,000 cubic metres in five years.  Single use slabs, which comprised 95% of the slabs imported, rather than more year slabs are likely to continue to be the most widely used slabs in New Zealand.  Thus alternatives to the current unsustainable methods of disposal of used slabs being sent to the local tip or disposed of locally require urgent consideration.  A MAF Sustainable Farming Fund Grant, approved in June 2002, and financial and in kind contributions from Tomato Sector, Veg-Gro Supplies Ltd and Grodan A/S have supported two project aims: -

2. POSSIBILITIES FOR SECOND USE OF HORTICULTURAL ROCKWOOL
Our review of overseas literature was published in March 2004 as a paper titled ‘Rockwool in horticulture, and its importance and sustainable use in New Zealand’ by W T Bussell and S McKennie in the New Zealand Journal of Crop and Horticultural Science, Vol 32, p29-37.  A pdf file of this paper is obtainable from either author.
The possible second uses for horticultural rockwool, which provide options both within and outside horticulture, include:

• Recycling to rockwool factory
• Soil amendment
• Mixing into other substrate e.g. peat
• Mixing into compost
• Brick Production
• Landfill cover
• Mining cover/recultivation
• Mushroom casing
• Fibrous mats for pressing
• Manufacture of artificial soil from waste rock wool
• Mineral wool waste cement
• Asbestos-free, fibre reinforced composite cement products
• Fillers for bituminous mixtures, especially asphalt mastic
• Adhesive Mastic

All of these options, with the exception of recycling to a rockwool factory and brick production, could be considered to ensure the sustainable second use of horticultural rockwool in New Zealand.  Further options are possible, for example placing used slabs (with the plastic sleeves removed) as mulch around avocado trees.  In a pilot study, this mulch has successfully suppressed weed growth for two years and crop growth has not been affected.  New tree roots have grown into the slabs causing breakdown of the slabs.  Mulching of avocado and other tree crops (e.g., citrus) with surface roots may be a worthwhile and appropriate second use of rockwool in New Zealand.

3. PREPARATION OF ROCKWOOL SLABS FOR SECOND USE IN NEW ZEALAND
When preparing slabs for second use for our experimental work described below, we made some observations and had some experiences that are relevant for larger scale preparation of slabs for second use in New Zealand.  We did all our testing on slabs which has plastic sleeves removed.  It is likely that sleeve removal should always precede slab grinding in New Zealand situations because small pieces of plastic sleeve could be a considerable hindrance to second use options with well decomposed roots to the required size (largest pieces about 10mm diameter), with either a hammermill or a mulcher.  Organic material may need to be separated from ground rockwool for some second use options.  Using mulchers on-site to grind used slabs at a grower’s property could be good practice for dealing with the approximately 30% of their original volume when they are ground.  On-site grinding of used slabs would minimise transportation costs and the overall economic impact of rockwool slabs prepared for second use.

4. TRIAL WITH HEBE GROWN IN MIXTURES OF PLANTING MIX AND GROUND ROCKWOOL
A trial, with four replicates of treatments in a randomised block design, to study how Hebe plants grow in Mace planting mix containing 0%, 10%, 20% and 30% of recycled used rockwool was planted at Unitec on 11 November 2003.  The potting mix/rockwool mixtures were 300mm high potagers with four plants 500mm apart in each plot (figure 1).

 

Figure 1
The trial area in early May 2004.  The plot plants are cv Inspiration
 
 
 

During the trial irrigation was done and additional fertiliser applied as required.  Plant height, plant width (two measurements at right angles to each other and averaged before graphing) and stem diameter at ground level were measured first on 12 November 2003 and then fortnightly until the end of April 2004.  All plants grew well throughout the trial (figure 2).  
Plant height, plant width and stem diameter was similar for all treatments at most recording days.  The few occasions when differences between treatments were statistically significant are given in Table 1.  The most notable significant differences are control plant heights or widths being significantly smaller than some or all of the rockwool treatments at the last two measurements.  This was at the end of the driest spell of weather during the course of the trial and may suggest that rockwool incorporation is improving waterholding capacity of the planting mix.  These results also suggest that mixing ground used rockwool with planting mixes is unlikely to harm plant growth.

 
 
 
 

Table 1
Plant Height (mm) and plant width measurements showing significant differences.
 
 
 
 
 
 
 
 
 
 
 
 
 

 

Treatment	Height 11/12/2003	Height 15/04/2004	Height 15/04/2004	Width 30/04/2004
Control	193	260	418	450
10% rockwool	186	281	455	473
20% rockwool	169	266	455	466
30% rockwool	185	266	455	480
LSD 5%	19	14	26	20

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