A friend of my brother had a tree blow over in the very strong winds Auckland had last week. My brother knowing I need regular delivery's of mulch dropped a load round at my place which I promptly used on paths and in the chicken run. Last year I probably brought on 3 cubic meters of weeds for compost 3 cubic meters of horse manure for compost and growing potatoes and 2 cubic meters of mulch. The latest delivery prompted me to think about the sustainability of what is commonly thought of as gardening.
As my experience and those I know has shown most gardeners have to bring quite a lot of biomass onto their propertys. This biomass can be compost, materials to make compost out of, woodchips, leaves potting mix etc. This constant inflow of organic carbon is necessary to replenish the carbon lost from soil through cultivation and because most of the crops we grow are leafy nitrogen rich plants that do not store much carbon. Another major factor is that we send all our human manure off site, this very rich outflow of nutrients and carbon must be replaced every year. Obviously having to bring carbon onto your site every year to replace that which you use cannot be sustainable right? And if everyone is doing this we are going to need to put aside a lot of land to grow that carbon.
This constant inflow of Organic Matter is just one of the many reasons why home gardens require a constant inflow of resources. To get around this there are several things one can do. John Jeavons a leading proponent of biointensive gardening techniques recommends that half your garden be planted in carbon crops for compost. Similarly I have planted fast growing plants like Abyssinian bananas, reeds and wild ginger which can be used to make compost out of. I have also planted heaps of trees whose prunings will eventually provide carbon.
Overall though intensive home gardening is only sustainable when land is left in low intensity wild or semi wild states. This is permacultures zones four and five, areas which are unmanaged and which provide space to hunt and gather, selectively log certain trees, collect firewood, observe etc. These semi wild areas also help to create rainfall, harbour beneficial insects, retain soil moisture and keep thousands of important an ecological processes working. Without these areas intensive gardening becomes fragile with nutrient loss, pest problems and drought all becoming real threats.
More on resiliency in horticulture here at Jeff Vails site