Comment: Jacqueline Rowarth discusses the pros and cons of regenerative agriculture and finds that, in this case, one size does not fit all.

Regenerative agriculture is being promoted as the saviour for New Zealand.

The suggestion that it can produce the food that is needed without creating environmental impacts is perfect.

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Add an income, and it is the goal for most farmers, whatever the label of their production system.

Farmers who adopt regenerative agriculture manage the grazing impacts of their livestock by moving them around in large groups.

Instead of applying fertiliser, the animals trample their own dung and urine into the soil surface. This restores soil carbon and biodiversity.

Nobody appears to have asked where the nutrients in the dung and urine came from, but under regenerative agriculture an increase in soil organic matter to 6 per cent has been recorded in the US.

New Zealand pasture soils are generally around 8 per cent organic matter.

Certainly, there are soils with lower organic matter content, but there are also soils with higher content.

Our Land 2018, produced by the Ministry for the Environment, reported that 95 per cent of sites tested in New Zealand are within the target range for soil carbon. (Soil carbon accounts for approximately 58 per cent of organic matter).

Rotational grazing

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New Zealand farmers already do what regenerative agriculture advocates – but here it is called rotational grazing.

Although the application of fertiliser is common in New Zealand, without it the soil organic matter would decrease.

It is often overlooked that a tonne of soil carbon is associated with over 80kg of nitrogen, 20kg of phosphorus and 14kg of sulphur (as well as other nutrients).

It is the release of nutrients from organic matter that can provide nutrition for pasture or crops, but those nutrients can also be lost – through leaching, for instance.

In addition, when the soil organisms break down the organic matter to obtain energy and nutrients, carbon dioxide is released to the atmosphere.

Cropping farmers are also being offered a solution to the use of fertiliser nitrogen.

Dr Jacqueline Rowarth. Photo / Supplied
Dr Jacqueline Rowarth. Photo / Supplied

Diverse crop rotations and no-till planting are part of the advice. But again, in New Zealand, even under intensive cropping – such as in the Waikato where maize silage is followed by annual ryegrass continuously – soil organic matter can be 10 per cent.

Without the fertiliser nitrogen commonly used in cropping, yields are restricted.

Rothamsted Research in the UK began research trials in the 1840s and has shown that form of nutrient (organic from farmyard manure vs inorganic from manufactured fertiliser) makes no difference to yield: it is quantity of nutrient input that matters.

In addition, the trials showed that organic nitrogen was insufficient to create the highest yields.

While it is accepted that "highest" might not be what is required, the debate needs to be in context with the concept of sustainable land management.

Taking more land into agricultural production rather than increasing yield on current agricultural land is part of the reason that the Amazon rainforests are under threat.

Whatever the production system, changing any inputs, whether fertiliser, water, light or heat, affects the balance of the soil organisms and the soil organic matter.

If the weather becomes warmer and drier, soil organic matter will be eroded, releasing more carbon dioxide into the atmosphere.

This is because the plants that produce the residues that feed the soil organisms will stop growing before the soil organisms stop consuming the residue. If soil was included in the Emissions Trading Scheme, drought could create a liability for farmers.

Greater nutritional value?

A second aspect of the regenerative agriculture movement is the suggestion that food grown with the regenerative approach will have greater nutritional value than food grown conventionally. Evidence to support this claim is very hard to find.

Robin Marles, Senior Scientific Advisor for the Bureau of Nutritional Sciences, Health Canada, has shown that most historical reports of soil nutrient status have not accounted for "changes in data sources, crop varieties, geographic origin, ripeness, sample size, sampling methods, laboratory analysis and statistical treatment".

When archived soil samples were compared with matched current soils, soil mineral composition was not found to have changed "in locations cultivated intensively with various fertiliser treatments".

The same applies to garden fruit and vegetables – differences in the literature reflect different measurement techniques.

Changes between old and new varieties are acknowledged. The new varieties have generally been developed to meet consumer demands for juicy sweetness and reduced perishability.

The major difference in food now, in comparison with last century, is not what comes out of the ground or off the tree, but what comes out of the supermarket or off the takeaway counter – consider a potato in comparison with crisps (which became part of the Consumer Price Index in 1974) or frozen chips (added in 1993).

The proponents of regenerative agriculture have used overseas experience to make the case for New Zealand, but our soils, topography, and climate are very different from conditions in Australia or rangeland America.

Context is everything and one size very rarely fits all.

- Dr Jacqueline Rowarth CNZM CRSNZ HFNZIAHS has a PhD in Soil Science and has been analysing agri-environment interaction for several decades.