Billion-dollar soils washing into rivers
We’re losing soil at an alarming rate, but there are some gaping holes in our knowledge about why and where from, reports Eloise Gibson
You can't grow a lot of food without soil, certainly not in a dairy-, wine- and vege-hungry nation like New Zealand.
Yet dirt – the foundation of our food supply – is being washed down our rivers and into the ocean at a worrying rate.
The latest report from the Ministry for the Environment and Statistics NZ, Our Land 2018, says New Zealand is losing about 192 million tonnes of soil a year.
At that rate, we’re contributing about 1.7 percent of all the sediment lost globally, despite having just 0.2 percent of the world’s land area.
Much of that is coming from underneath grassy farm paddocks, which are shedding 44 percent or 84 million tons of the lost soil into rivers.
That hurts river quality and farm productivity – but it's not as simple as saying that farming always causes erosion.
As the Ministry for the Environment explained to Newsroom, New Zealand has naturally high rates of erosion, due to a combination of steep terrain, rock and soil types, high rainfall and storms.
What farming is doing is worsening an already-bad situation in our hill country, where a naturally-high erosion rate is worsened by removing trees and shrubs to plant pasture for animals to eat.
Having large amounts of pasture in areas at high risk of erosion is a problem because soils are more vulnerable once people remove woody vegetation such as trees and shrubs.
It’s likely the 44 percent of soil lost from under pastures is coming from a much smaller proportion of farms – the hilly, vulnerable ones.
That's reflected in the astonishing figures for Gisborne, where the region's soils are shedding 4844 tonnes per square kilometer a year, mostly from under pasture, compared to the national average of 720 tonnes.
The report points to worsening future problems as heavy rain and drought – enemies of soil quality and quantity – increase with climate change.
However there are several yawning gaps, as the report itself acknowledges. For example, the Ministries can’t compare soil losses from pastoral farming with how much soil is being lost from conservation land, which makes up a whopping 44 percent of the South Island.
Nor can it say how much precious dirt is being washed or blown away from cropping land or plantation pine forests.
There’s no good information on soil and erosion trends over time, because there is no national monitoring programme.
Many trends, it confirms, are ones that people who are interested in land probably know about: New Zealand’s rates of soil loss are being worsened by land-hungry cities, which are eating into productive land, as well as an undiminished appetite for lifestyle blocks on our city fringes.
Added to that are major earthquakes causing landslips, such as the Kaikoura earthquake.
Risks to quality, and quantity
The report is the first from the two ministries to specifically explore New Zealand’s land use. It contains interesting tidbits about how people have shaped the land over the past two decades. For example, the food we produce from soil is shifting from vegetables and other horticulture to berries and fruit (including wine grapes) and from meat to dairying, changing both the exports we sell and the nutrients we add and subtract from the land.
The report reiterates that soil is economically valuable, as well as being crucial to human life: there were $35.4 billion in exports from what we produced on the land in 2016, while tourists keen on our natural beauty spent $14.7 billion in New Zealand. All up that’s 70 percent of New Zealand’s total export earnings.
The same year, land-based primary production’s share of gross domestic product was 3.7 percent, while tourism’s share was 5.7 percent of GDP.
It's clear that we don't always treat soil like the taonga it is.
It isn't just soil quantity that's suffering, but quality – although the report provides only a snapshot of quality issues, not the trends.
The main issues identified were too much potentially water-polluting phosphorous and compaction of ground from intense farming activities aboveground.
The authors used soil monitoring data from 11 of New Zealand’s regions (out of 17) between 2014 and 2017.
The other five councils either don’t monitor soil or didn’t supply their results for the report, inevitably skewing the national figures.
The monitoring showed 83 percent of tested sites were within the Ministry’s guidelines for five of the seven soil health indicators (incuding pH, carbon and nitrogen).
But the remaining two health indicators were concerning: high and low phosphorus content (an indicator of soil fertility) and low macroporosity (a measure of how many pore spaces there are in the soil, an indicator of physical robustness).
More than 48 percent of the sites councils tested were outside the target range for one of those measures. A third of sites had high soil phosphorus levels, meaning excess phosphorus might travel into waterways through erosion and run-off and trigger growth of unwanted plants and reduce water quality.
At 44 percent of sites the soil was too compact, making it less productive, less biodiverse and restricting plant growth. Squished-down soil also doesn’t drain as well, which can raise the greenhouse gas emissions from cow urine. That’s because the potent greenhouse warmer nitrous oxide tends to spike when soil is wet.
Soil under dairy, cropping, horticulture, and meat farms was more frequently outside the healthy range than other soil: for example 51 percent of dairy sites had too much phosphorus and 65 percent were too compacted.
And while soil was obviously being degraded by some (though definitely not all) farming practices, soil that was being converted from farming to housing was perhaps a bigger worry. The report highlights how urban expansion is eating some of our most versatile productive land. Studies based on changes in land cover showed that between 1990 and 2008, 29 percent of new urban areas were on soil that could be valuable for food-growing. In 2013, lifestyle blocks occupied 10 percent of New Zealand’s most versatile land, fragmenting it and making to harder to use for large-scale farming.
Among the major gaps in the data are what is happening in the five out of 16 regional councils that did not provide any soil monitoring data for the study, and answers to broader soil productivity questions, such as how much food lifestyle blocks are growing when they replace farms.
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