Soils at risk of perfect storm as forestry and cities grow
New Zealand is trying to grow more food for less environmental impact – but soil loss, housing pressure, the need to expand forestry and a changing climate are combining to make the task harder.
Urban sprawl and the creation of about 5800 new lifestyle blocks a year are munching our best soils and pushing food-growing operations further from their customers in cities and onto less fertile land.
And the trend's been happening right when New Zealand needs to grow more food, rein in its road transport emissions and slim its fertiliser needs.
Farming of vegetables and other foods risks being ousted onto more marginal land if housing and lifestyle blocks keep taking over the fertile band bordering cities such as Auckland – meaning more emissions-producing fertilisers may be needed to grow the same food, and requiring a longer trip by road to get produce where it can be eaten.
The information is in a new comprehensive environment report Environment Aotearoa 2019. Much of the data in the report's combined soil, climate, water, fisheries, biodiversity and other categories is not new – and there remain significant, worrying gaps – but pulling it all together as a whole every few years allows the Ministry for the Environment and policy-makers to draw useful connections.
This time, the sections on climate, water and soil show the multi-headed problem facing our food growing.
First, there’s our growing population and a sharp increase in lifestyle blocks, which have been growing at an average of 5800 new blocks a year since 1998. One 2013 study found 35 percent of Auckland’s most fertile and versatile land was used as lifestyle blocks, and, unfortunately, much of New Zealand's population happens to live right on or next to the country’s best soil, which makes up only 5 percent of New Zealand land but is the most versatile and best for growing food.
At the same time, historical forest clearance to make way for farmland has let our food economy grow, but has also made our soils more erosion-prone. Of the estimated 192 million tonnes of soil lost annually into waterways, modelling suggests 44 percent of the soil is likely to come from land covered in pasture. Soil loss estimates rely on modelling because the ministry does not know how much erosion is actually happening - no-one measures it, and nor do we have a good picture of how well soil conservation efforts or native forestation are working.
A new risk is looming as the country switches vast swathes of land to forestry – as it will need to do under virtually any scenario to meet greenhouse gas-cutting commitments – because there can also be a heightened risk of soil loss and slips during a period of up to six years when land is left vulnerable and bare between tree felling and re-planting.
The change to our land use will come at a time when weather patterns are also changing with global warming – so, for example, more intense storms and increased rain will affect parts of the country and may raise the slip risk.
The report makes no recommendations about how New Zealand can best grow food, turn a profit and retain its fertile soils while radically cutting emissions and adapting to climate change. It's "by its nature a negative report" designed to lay out the challenges, officials said in a briefing to journalists yesterday. Its role is to serve as a base for policy-makers who are grappling with those decisions, they said. There are efforts underway -- Scion has been looking into forestry practices to protect soil, farmers are experimenting with low-till and no-till pasture renewal in windy locations, some cities have attempted to use planning rules to rein in sprawl – though those efforts have come up against desperate calls to free up land to curb house prices, particularly in Auckland.
The report notes the conversion of native forest to pasture and plantation forestry has been a boon in that it's supported our economy It notes that in 2016, agriculture contributed 4.2 percent of our gross domestic product and employed more than 122,000 people, while forestry contributed over $1.7 billion to our economy and employed over 6000 people.
Yet lost soil also has a price - to agriculture, ecosystems and the wider economy. The economic losses associated with soil erosion and landslides were estimated to be at least $250–300 million a year, according to one piece of research cited.
There's an environmental cost, too. For rivers, lakes and beaches, the added soil flowing in means sedimentation problems in harbours, estuaries and river beds, where it can smother native habitat and increase the flood risk to towns and cities.
There are less obvious costs too - to paua farmers, for example. The report describes what happens in the Marlborough Sounds, where large areas of native forest have been replaced with plantation forests. When these trees mature and are harvested, the bare soil is exposed to wind and rain for several years. Sediment enters streams and rivers and, eventually, reaches the marine area where it harms kelp, smothers young pāua, hinders the growth of adult pāua, and makes it diffcult for larvae to settle, the report says. Sediment can also make pāua easier to dislodge from rocks and more vulnerable to being eaten by (non-human) predators.
The new report cites a 2018 study that estimated the impacts of sedimentation were a factor in roughly $20m lost quota value for the pāua industry nationwide in the 13 years to 2014.
These pressures are affecting food growing just as our food system is coming under greater pressure to increase production without raising its effect on the environment, officials noted yesterday. So when growers are forced onto more marginal land that is naturally less productive they require more inputs, like fertiliser, right when there's pressure to be parsimonious with fertiliser because of its flow-on impacts on the climate, waterways and farmer's wallets. Road transport is another thorny problem - New Zealand's emissions from that sector rose 82 percent from 1990 to 2016, making it our fastest-growing source of emissions, so trucking food further as farms move further from cities would be unhelpful.
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