Where Does Your Olive Oil Come From? Part 2 of 5

In Part 1, we followed a chain: living soil supports a resilient tree, a resilient tree builds protective compounds in its fruit, and careful processing carries those compounds into the bottle. Many modern farming practices break that chain at its very first link. Understanding how helps explain why so much olive oil never reaches the quality its trees were capable of.

The Tillage Problem

Tillage – the mechanical breaking up of soil by plowing, disking or cultivating – has long been the default in Mediterranean olive groves. It keeps weeds down, but it also tears through the fungal threads that extend a tree’s reach, exposes organic matter to the air, and leaves the soil bare.

The long-term cost shows up in the soil itself. A 20-year study of 114 soil profiles in olive groves in southern Spain’s Los Pedroches Valley compared conventional tillage with organic management. Soil organic carbon stocks under organic management rose by 72% and 66% in two of the soil types studied. [1]

Tillage can even defeat its own purpose. Farmers often till to reduce water loss, but in a mature olive grove in southern Italy, soil tilled two to three times a year stored less water than soil covered with mowed grass left as mulch – despite the grass using water itself. Tillage created surface crusts and a compacted “plough pan,” and the rate at which water moved through the soil dropped roughly a hundredfold. [2]

Recovery is possible, but fragile. A permanent grass cover tends to rebuild organic matter and soil structure within two to three years – yet a single tillage pass can immediately undo much of that structural strength. [2]

What Happens to the Olive

Soil management reaches all the way into the fruit. In a Spanish study of olive trees under plowing versus a conserved herbaceous layer, plowed areas produced olives with lower oil extraction yield, lower maturity and poorer health. Under plowing combined with drought, total phenolic content was markedly lower. [6]

Ground cover and the mycorrhizal partnership move together. In another Spanish trial, annual cover crops raised mycorrhizal colonization of olive roots, and removing the cover reduced fruit ripeness and polyphenol levels. Bare soil did raise productivity – there is a trade-off – but cover-crop groves produced oil with higher polyphenol content while protecting the soil from erosion. [3]

Cover must be chosen and managed with care. A three-year trial in central Spain found that all groundcovers increased fungi, bacteria and other life on olive roots. A permanent grass cover built the most soil carbon but competed with the trees for nitrogen and reduced yield, while an annual legume cover improved soil structure without significantly reducing yield. [4] Living soil isn’t a matter of letting anything grow. It is a practice.

Erosion: Losing the Foundation

Mediterranean olive groves, often on slopes and exposed to intense autumn rains, are especially vulnerable to erosion. A 15-year study in Andalusia measured soil loss of about 10 tonnes per hectare per year under conventional tillage – and nearly 14 tonnes where tillage was replaced by bare, herbicide-treated soil. Adding a cover crop cut losses to about 2 tonnes. [5]

The lesson is important: simply stopping the plough is not enough. The soil needs living cover. Every tonne lost carries away the organic matter, nutrients and soil life that took generations to build.

Chemical Interference with Biology

Herbicides remove more than weeds. Research reviewed across olive-growing regions found that eliminating ground vegetation with herbicides reduced the functional diversity of soil microbes, and that glyphosate applied to soils with no prior pesticide history disrupted the balance of their bacterial communities. [3]

Crop-protection sprays matter too. As we saw in Part 1, copper – widely used against olive diseases – tracked with lower phenolic levels in olive leaves.

The Compaction Problem

Heavy machinery compacts the soil between the rows, especially where the ground is tilled and bare. Compacted soil takes in water more slowly, restricts root growth, and supports less soil life – earthworms in particular. Recovery from compaction has been estimated at 5 to 18 years, depending on the soil and climate. [2]

The Input Dependency Cycle

When tillage, bare ground and chemicals weaken the soil’s living systems, growers replace what those systems once provided with fertilizers and sprays. The logic of the cycle is simple: the less the soil does for the tree, the more must be supplied from outside. And as Part 1 showed, trees fed generously with nitrogen and phosphorus may invest less in the very protective compounds that make olive oil healthful.

Breaking the Cycle

The damage is not permanent. Soil has a remarkable capacity to recover when given the chance – but recovery means actively rebuilding its living systems, not simply avoiding harm.

This is the first link in the chain. A tree can only build protective compounds in its fruit if the soil beneath it is alive. In Part 3, we’ll look at why even organic farming, while avoiding synthetic chemicals, doesn’t always restore that foundation.

Part 1: The Hidden World Beneath Olive Trees
Part 3: The Organic Paradox – Why Certification Isn’t Enough
Part 4: Regenerative Agriculture – Working with Nature’s Intelligence
Part 5: The Future of Olive Oil

References

  1. Parras-Alcántara L, Lozano-García B. (2014). Conventional tillage versus organic farming in relation to soil organic carbon stock in olive groves in Mediterranean rangelands (southern Spain). Solid Earth 5: 299–311. https://doi.org/10.5194/se-5-299-2014
  2. Priori S, Pellegrini S, Vignozzi N, Costantini EAC. (2021). Soil Physical-Hydrological Degradation in the Root-Zone of Tree Crops: Problems and Solutions. Agronomy 11(1): 68. https://doi.org/10.3390/agronomy11010068
  3. Melloni R, Cardoso EJBN. (2023). Microbiome Associated with Olive Cultivation: A Review. Plants 12(4): 897. https://doi.org/10.3390/plants12040897
  4. Sastre B, Álvarez B, Antón O, Pérez MÁ, Marques MJ, Bienes R, García-Díaz A. (2020). Groundcovers in Olive Groves in Semiarid Climates: Are They Always Beneficial? Water 12(8): 2230. https://doi.org/10.3390/w12082230
  5. Soil Productivity Degradation in a Long-Term Eroded Olive Orchard under Semiarid Mediterranean Conditions. https://www.researchgate.net/publication/351014492
  6. López-Sánchez A, et al. Plowing vs. Herbaceous Layer Conservation Under Different Drought Stress Levels in Olive Groves: Interactions Between Tree Yield-Quality and Their Microsite.