Soil health

Getting to know what is underground

It pays to know your soils when you’re a gardener, and when you're a community gardener that becomes all the more important as the land we inhabit has generally had some sort of significant prior use that may have left traces of the past in the soil, some of those traces, such as heavy metals and pesticides, can be a concern.

When Innermost began it’s life back in 2006/7 Wellington City Council commissioned a number of soil tests to determine that. The test results found negligible heavy metals which was a relief as they’re very hard to remove, but also low levels of DDT.

DDT or Dichlorodiphenyltrichloroethane, is a colorless, tasteless, and almost odorless crystalline chemical compound, an organochloride, and is a pesticide that is now banned. It is however an organic compound that can be broken down by a soil teeming with microbes.

Results were divided in two between our southern field and northern field. The northern field which had a number of raised beds had levels of 1.35 mg/kg and the southern field 0.652 mg/kg.

These levels were in a range that is acceptable for most commercial growing environments so we were good to go. But the Innermost team decided, as good stewards of the land, that we wanted to to aim for levels that were closer to the then bio gro organic certification level of 0.2 mg/kg.

So a comprehensive bio remediation plan was put in place. All of the soil in the raised beds came from off site and was fed with nutrients, compost and inoculated corrugated cardboard (see bioremediation below). And the worms and associated microbes began their work on soil restoration.

On the southern field we used an organic bioremediation process that involved using mushrooms and their outstanding role as primary decomposers of organic material. At a very high level this involved corrugated cardboard inoculated with oyster mushroom spawn supplemented with tonnes and tonnes of mulch as a growing medium for the mushroom mycelium networks. Various organic teas and preparations were added to supply the right energy and nutrients for the mushrooms so they could thrive and happily consume DDT and any other organic compounds in their way.

Over a period of 8 months more mulch, in the order of thousands of KG’s, was extended across the entire field, all of which fueled the bioremediation process that would break down the DDT to it’s most common safe elements.

One of our dilemmas was the lack of drainage in the southern field which is not good for mushrooms or an aerobic soil community in general. The field also had a clay pan about one metre below the surface and impermeable concrete sides. This meant that over the winter the field retained water and became like a ‘swimming pool’, in summer the soil became incredibly hard.

So we built some paths through the field with a draincoil underneath that directed excess water into a corner of the field now dedicated to native swamp plants. This worked extremely well and gave the soil under remediation the opportunity to be aerobic all year round.

The whole process took some time, ~18 months. Successive soil tests over that time, 2010-2012, demonstrated a steady reduction in DDT levels (average 60% reduction), closer to the organic certification level, demonstrating that the remediation process was indeed working.

Soon after, in 2013, the consensus was that the southern field soil was healthy enough to start opening it up for some long term production purposes, the first of which was a food forest. And that by our ongoing practice of mulching and compost application remaining traces of DDT would be addressed to by our thriving soil community over time.

On an annual basis we estimate that approximately 2500 kg of compost have gone into both the northern and southern fields. In recent years annual beds have been added to the southern field using compost and mulch from our own composting system, fed by Mount Victoria's kitchen waste.

As more and more of our land is utilised we will be conducting further soil tests that will look for heavy metals, toxic substances and also nutrient levels so we can tweak our nutrient management plans. A series of these tests is scheduled to occur in 2021.

If you're interested in the details of our bioremediation process we will soon be posting an article on that.