New soil monitoring findings raise questions about the long-term consequences of keeping vineyard soils bare through repeated herbicide applications.
A report from Marlborough District Council has identified significant differences in soil health between the bare ground beneath vineyard vines and the grassed areas between rows, with the council recommending that vineyard managers reduce herbicide use to help restore soil organic matter.
The findings raise an important question: Are vineyard weed-control practices designed to protect production gradually undermining the health of the very soil on which that production depends?
The council’s Soil Quality in the Marlborough Region 2025 report [PDF], released in 2026, draws on a soil monitoring programme that has operated for 25 years. Its findings point to persistent problems with soil compaction, declining organic matter and reduced soil function across some of the region’s productive land.
For vineyards, the evidence is particularly revealing.
What Marlborough’s Soil Testing Found Beneath Vineyard Vines
In many conventional vineyards, vegetation beneath the vines is controlled using herbicides, leaving a strip of bare soil along each row. Meanwhile, grass and other vegetation are maintained between the rows, often through mowing.
These two areas can be only a short distance apart, yet they experience substantially different management practices.
Marlborough District Council has been examining those differences by separately sampling soil beneath vines, in vehicle wheel tracks and in the grassed inter-row areas.
The results reveal a consistent pattern.
Soils beneath the vines have significantly lower levels of total carbon, an important indicator of organic matter, than soils in the grassed areas between rows.
They also have significantly less plant-available water — the water that soil can retain and make available for plant growth.
The council’s statistical analysis confirmed that the differences in soil carbon and plant-available water between the two areas were highly significant and unlikely to be explained by random sampling variation alone.
This is not simply a matter of soil appearance or whether vegetation is present. The differences extend to measurable properties that influence soil fertility, moisture retention and the ability of soil to support healthy plant growth.
The report concludes that vineyard floor management has a clear effect on soil function.
How Herbicide Use May Be Affecting Vineyard Soil Organic Matter
One of the most significant passages appears in the report’s discussion of anaerobically mineralisable nitrogen, a measure used to assess the soil’s capacity to supply nitrogen through the decomposition of organic matter.
The council found noticeably lower values beneath vineyard vines than in the wheel tracks and inter-row areas.
Its explanation is direct:
“The regular use of herbicide in this area is probably limiting organic matter input.”
The report goes on to warn that the long-term consequences could include reduced nitrogen availability, potentially leading to increased fertiliser use or declining crop growth.
This raises an uncomfortable possibility.
A management practice intended to reduce competition from unwanted vegetation may also be reducing the natural replenishment of organic matter that helps maintain soil fertility.
If less organic matter enters the soil, the processes responsible for releasing nutrients can be affected. Over time, growers may face greater dependence on external fertiliser inputs to compensate for declining natural soil function.
That is hardly an encouraging outcome for a practice intended to support efficient vineyard production.
Lower Soil Carbon Linked to Less Plant-Available Water
The council also investigated the relationship between soil carbon and plant-available water using 277 vineyard soil observations collected across its monitoring programme.
The results showed that soils with higher carbon content generally retained more water available to plants.
Under-vine soils tended to have both lower carbon and lower plant-available water than grassed inter-row soils.
This matters in a region where access to water, irrigation requirements and resilience during dry conditions are important considerations for growers.
Organic matter contributes to soil structure, supports biological activity and helps create the pore spaces that allow soil to store and release water.
When organic matter declines, the consequences can extend beyond nutrient availability.
The council’s findings show that lower soil carbon beneath vineyard vines is associated with a measurable reduction in the soil’s capacity to hold water available for plant growth.
The council’s findings demonstrate that bare under-vine areas have significantly lower soil carbon and plant-available water than neighbouring grassed areas. These differences are measurable, even when they are not immediately visible above ground.
How often are these longer-term costs considered when decisions are made about routine herbicide applications?
Why Marlborough Council Recommends Reducing Herbicide Use
Perhaps the most important aspect of the report is that Marlborough District Council does not stop at identifying the problem.
It recommends changes to vineyard management.
In its soil quality recommendations, the council states:
“Vineyard under-vine areas show soil degradation from reduced organic matter levels.”
It then recommends reducing herbicide use and considering a combination of minimal under-vine cultivation and mowing.
Other suggestions include allowing weeds to grow over winter, using grazing to manage excessive vegetation, and applying compost or other carbon sources to improve soil organic matter.
Elsewhere, the report states that reductions in vineyard herbicide use may be necessary to increase organic matter and improve nitrogen cycling.
These are significant recommendations from a council responsible for monitoring the condition of the region’s soils.
They acknowledge that maintaining bare ground beneath vines is not necessarily a neutral management practice.
It may have consequences for soil fertility, water retention and the long-term sustainability of vineyard production.
What Do the Findings Mean for Glyphosate Use in Vineyards?
Glyphosate-based herbicides are widely used for vegetation control, including in vineyards.
However, Marlborough District Council’s report does not identify the particular herbicides applied at the monitored vineyard sites, nor does it report measurements of glyphosate or its breakdown product AMPA in the soil.
The findings therefore cannot be attributed specifically to glyphosate exposure.
But that does not make them irrelevant to the glyphosate debate.
Much of the public discussion about glyphosate concentrates on its direct chemical effects — whether residues are present, how exposure is assessed and what toxicological studies reveal.
There is another question that deserves equal attention.
What happens to soil when vegetation is repeatedly removed or suppressed through herbicide-based management?
Even setting aside questions about the direct effects of herbicide formulations on soil organisms, removing vegetation can reduce the organic material returned to the soil.
Living plants contribute to soil processes through their roots, root exudates and the organic matter left behind as vegetation decomposes.
Repeatedly maintaining bare ground can interrupt those inputs.
The Marlborough findings provide a practical example of why the effects of weed-control practices should be considered more broadly than the chemical safety of an individual product.
A herbicide application may successfully eliminate unwanted vegetation while contributing to a management system that, over time, compromises other aspects of soil health.
Success at killing weeds is not necessarily the same as success at maintaining healthy soil.
Are Current Vineyard Weed-Control Practices Sustainable?
Marlborough’s vineyards are an important part of New Zealand’s wine industry. Their productivity depends not only on vines, irrigation and fertiliser, but also on the condition of the soil supporting them.
The council’s monitoring programme is particularly valuable because soil degradation can develop gradually.
A vineyard may continue producing grapes while changes in organic matter, nutrient cycling and water-holding capacity accumulate beneath the surface.
Those changes may not become obvious until additional inputs are needed to maintain productivity, or until drought and other environmental pressures expose weaknesses in the system.
The council’s report identifies problems that deserve attention now, rather than waiting for more serious consequences.
It also raises a wider question about how agricultural and horticultural management practices are evaluated.
If herbicide use reduces the need for mechanical weed control or saves labour in the short term, are those benefits being weighed against possible longer-term losses in soil function?
And if those losses increase the need for irrigation, fertiliser or soil restoration, how much of the original saving remains?
These are not questions confined to vineyards.
The same principles are relevant wherever repeated vegetation suppression leaves soil exposed and reduces the return of organic matter.
What the Marlborough Soil Health Report Reveals
Marlborough District Council has documented significantly lower soil carbon and plant-available water beneath vineyard vines compared with grassed inter-row areas.
It has identified regular herbicide use as a probable contributor to reduced organic matter inputs and warned of possible consequences for nitrogen availability.
And it has recommended reducing herbicide use as part of improving vineyard soil health.
The report does not establish that any particular herbicide chemical directly caused the observed changes. Vineyard soil conditions are influenced by several management practices, including cultivation, irrigation, fertiliser application, vehicle traffic and vegetation cover.
Nevertheless, the council’s findings challenge the assumption that routinely maintaining bare soil beneath vines is without longer-term consequences.
For No More Glyphosate NZ, the broader issue is straightforward.
Herbicide use should not be judged solely by how effectively it controls weeds or whether chemical residues fall within regulatory limits.
Its role within land-management systems — and the consequences of those systems for soil health — deserves scrutiny too.
When an official soil monitoring programme identifies degradation beneath vineyard vines and recommends reducing herbicide use, surely that is a finding the wine industry, growers and agricultural regulators should be taking seriously.
Source: Marlborough District Council, Soil Quality in the Marlborough Region 2025 [PDF], MDC Technical Report No. 26-006, August 2026.
Relevant sections: 2.1.2 (Viticulture sampling sites); 3.3.1 (Anaerobically Mineralisable Nitrogen); 3.3.2 (Total Carbon); and 4.2 (Correlation of Total Carbon to Plant Available Water).
Further Reading
How Long Does Glyphosate Stay in Soil? What Studies Show
Examines how glyphosate persists, binds to soil particles and breaks down, and what research reveals about its potential accumulation following repeated applications.
Glyphosate and Soil Health: Microbes and Earthworms
Explores research into the effects of glyphosate-based herbicides on earthworms, soil microorganisms and the biological processes essential for healthy, productive soils.
Glyphosate, Urea and Soil Nitrogen Cycling
Investigates how glyphosate and nitrogen fertilisers may interact with soil microbial activity and nutrient cycling, raising questions about long-term soil fertility.
Glyphosate in New Zealand Soils: Massey Research
Looks at New Zealand research into glyphosate behaviour in soils and the factors influencing its persistence and environmental fate.
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