Organic farming has become increasing popular in all areas of agriculture, including the field of viticulture. Between 2001 and 2008, the area of organically farm vineyards in France increased by 110%. In organic viticulture, organic fertilizers and some non-synthetic pesticides are allowed, whereas in conventional farming, manufactured inorganic fertilizers and chemical/synthetic pesticides are frequently used. In lieu of pesticides (though sometimes non-synthetic pesticides are used), organic farming utilizes tillage or grass-cutting for weed management. It is because of these and some other differences, it is claimed that organic farming is better for the environment by reducing the intensity of disturbance on the soil.

Organic farming aims to increase soil biological function by improving upon its physical, chemical, and biological properties; including water circulation and aeration, availability of nutrients, and biodiversity. By improving upon these properties, soil quality is thus enhanced. In viticulture, the following parameters are measured when determining soil quality: soil bulk density, pH, nutrient availability, organic matter content, and soil water holding capacity. Soil scientists and viticulturists consider soil microorganisms to be very important indicators of soil health, as these microorganisms form very close relationships with their surroundings.

Microorganisms in the soil are important for many reasons, including the decomposition of organic matter, humus formation, nutrient cycling, and symbiotic relationships with other organisms in the soil. Macroorganisms, such as nematodes and earthworms, are also important indicators of soil health, since they are present in all types of soil and frequently alter the physical properties of it, which indicates the fate of the organic matter of the soil.

To date, there has been very little research on organic viticulture and its’ effects on soil function and health. The study presented today aimed to measure the long-term effects of organic viticulture by physical and chemical indicators, as well as the availability of micronutrients, contaminants (such as copper), and bioindicators (such as nematodes and earthworms).

Methods

The experiment was conducted in May of 2009 in Cruscades (South of France, in the Languedoc-Roussillon region). There were no slopes in any of the plots. The soil was silt-clay (42 +/-2% silt, 36 +/-1% clay, 22 +/-2% sand). The soil was calcerous, with a pH of 8.3. The soil-water holding capacity was 20.6+/-0.5%(w/w).

The experiment was conducted on a 24 commercial wine grape vineyard with a mean area of 1.5ha. Grape varieties included Cabernet Sauvignon, Carignan N, Chardonnay, Cinsault, Grenache N, Merlot, Mourvèdre, Pinot N, and Syrah. The year of plantation ranged from 1932 to 2003. Plantation density was between 3300 and 5000 vines per hectare. Ten plots were managed conventionally, and all other were managed organically.

Five of the plots were managed organically since 2001 and certified in 2004, four plots were managed organically since 1997, and 5 plots were managed organically since 1991. The conventional management practices were identical for each treatment before the switch to organic practices, and the organic practices were identical after the switch. Four subplots per plot were sampled (5 vines per 4 inter-rows). In total, there were 96 subplots sampled.

Sampling occurred in the spring, a few days after mild rain events. For the earthworm sampling (between the 4th and 15th of May), soil water content was 14.7+/-0.3% (w/w). For the soil sampling (between the 22nd and 28th of May), soil water content was 11.2+/-0.2% (w/w).

Soil and earthworms were sampled in the 0-15cm of topsoil of the center of the inter-row, with one soil and one earthworm sample per subplot. Each soil sample contained four subsamples. Soil samples were sieved at 1cm before biological analysis, and at 2mm before chemical and physical analysis. Soil density was measured using the cylinder method. For earthworm sampling, monoliths of soil of 45cmx45cm at 15cm depth were taken per subplot. Earthworms were sampled using the hand-sorting method.

The following were measured for the physical and chemical analysis: bulk density, total organic carbon (TOC), total nitrogen (N), effective cation exchange capacity (CEC), phosphorous (P), potassium (K), and copper (Cu). For the biological analysis, soil microbial biomass carbon (MB) was measured. An average of 150 nematodes per sample were counted and identified and grouped into different trophic levels (plant feeders, bacterial feeders, fungal feeders, omnivores, and predators). Earthworms were counted and weighed, and identified as adults or juveniles, and grouped into two different ecological levels (endogeics and anecics).

Results

Physical and Chemical

Organic plots trended toward higher bulk density than conventional plots.

o Only organically managed plots from 1997 had bulk densities significantly higher than conventional plots.

TOC content significantly increased from the conventional plots to the organically managed plots from 1991 (32% increase).

o There was a big increase between organically managed plots from 2001 and organically managed plots from 1997 (15% increase).

Conventional plots and organically managed plots from 2001 had significantly lower N content than organically managed plots from 1997 and 1991.

There was a large decrease in available P content between the conventional plots and organically managed plots from 2001 (58% decrease).

There was an increase in available P content between organically managed plots from 2001 to 1997 (43% increase) and from 1997 to 1991 (65% increase).

There was a significant increase in available K content from the conventional plots to the organically managed plots from 1991 (81% increase).

There were no significant differences in Cu content in any of the plots; however the highest values of Cu were measured in the organically managed plots.

Highest values of effective CEC (which were significant compared to other plots) were found in organically managed plots from 2001 and 1997.

Biological

Organically managed plots from 1997 and 1991 had significantly higher microbial biomass carbon than conventional plots and organically managed plots from 2001 (34% higher).

Nematode density was lowest for the conventional plots.

o Nematode density was 45% higher in the organically managed plots from 2001 than the conventional plots.

o Nematode density was 79% higher in organically managed plots from 1991 than conventional plots.

o Plant-feeder density was 126% higher and 187% higher in organically managed plots from 2001 and 1991, respectively, compared to conventional plots.

o There were no significant differences in any of the plots in regards to bacterial-feeder densities, though the lowest density was found in the conventional plots.

o Fungal-feeder density increased from the conventional plots to the organically managed plots from 1991.

§ Fungal-feeder was significantly higher in organically managed plots from 2001 (43% higher) and from 1991 (97% higher) compared to conventional plots.

o Combined densities of omnivores and predators was significantly higher in organically managed plots from 2001 (44% higher) than all other plots.

The highest biomass of earthworms was found in the conventional plots.

Authors’ Interpretations of Results

According to the authors of this study, they found no significant differences before 11 years of organic farming for TOC (total organic carbon), N (total nitrogen), K (available potassium), soil microbial biomass, and fungal-feeding nematode density, though other soil quality indicators such as available phosphorous (P) content rapidly decreases after the conversion from conventional to organic methods. The authors explained that some research attribute this result to an exhaustion of available phosphorus that was built up from conventional fertilizers rich in phosphorous. Afterwards, they noted an increase in P and K over time with organic methods, which could be from microorganisms releasing organic acids that could convert to available P and K. Since the authors found an increase in soil microbial biomass over 7 years of organic farming, this could explain why P and K were found to increase.

In organic farming, Cu is frequently used as an approved pesticide for downy mildew. In this study, the authors did not find any differences in Cu concentrations between the conventional plots and the organic plots, which was not explained other than the collection method may not have been suitable to detect any differences.

Organic farming also employs the use of more grass cover and other organic matter, which has frequently been shown to increase the population of earthworms in the soil. However, in this experiment, the authors actually found a decrease in the number of earthworms in the soil when comparing organically farmed plots to conventionally farmed plots. The authors attributed this discrepancy to the hand-collection method, as the larger earthworms were able to escape collection and retreat into deeper soil and thus not counted. This result could also be attributed to the fact that organic farming methods often employ more tillage, which studies have shown to decrease the macroorganism populations in the soil. Tillage results in greater soil compaction, due to the weight of the machinery used more often on the soil than in conventional practices. This soil compaction results in lower populations of macroorganisms, and may partially explain the results found here.

In regards to soil organic matter, it is a very important component of the soil, and is often considered the most important aspect of soil management in farming. This experiment showed that organically managed plots had higher levels of total organic carbon, which indicates high soil quality. This result was partially attributed to the abundance of grass cover in the organically managed plots, which helped increase the overall total organic carbon levels. This total organic carbon acts as a nutrient resource for microorganisms, which stimulates microbial growth and increases soil health and quality overall.

In conclusion, the authors of this study showed that there are clear differences in soil quality between conventional and organically managed vineyards over time (in a Mediterranean climate such as Southern France). During the transition period between conventional and organically managed plots, there is a decrease in available resources, due to exhaustion of the excess nutrients that were created from synthetic fertilizers used under conventional farming practices. Once the microbial community in the soil starts producing their own soil nutrients via metabolism byproducts and increased grass cover increases soil carbon supplies, microbial growth is stimulated and increases the quality of the soil overall.

Final Thoughts and blabberings….

One thing this study did not look at was how these soil differences affected the growth of the vines and quality of wines produced from the grapes of those vines. That complementary experiment is key to tie in what is known about soil quality under organic practices and the resulting effect on the wine produced from organically grown grapes. Since other studies have looked at this topic, it is possible to make inferences; however, there are almost always methodological differences between experiments that make interpretations of results more difficult. I would like to have seen these authors take this experiment that one step further, and provide chemical analysis of the resulting grapes and finished wine from the vines grown under the conventional and organically managed plots (though technically, the experiment served its’ sole purpose justly).

What do you all think of this study and the researchers’ interpretation of the results? Do you have any of your own experiences to share? Please feel free to leave your comments below.

Source: Coll, P., Le Cadre, E., Blanchart, E., Hinsinger, P., and C. Villenave. 2011. Organic viticulture and soil quality: A long-term study in Southern France. Applied Soil Ecology 50: 37-44.

DOI: 10.1016/j.apsoil.2011.07.013

I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!