For centuries, agricultural practices have been shaping and influencing the environment both ecologically and morphologically. Since many vineyards are planted on hill slopes, the environmental impact is often greater than those planted on flat terrain. The main contributor to altered terrain morphology and environmental impact of vineyards planted on hill slopes is soil erosion and run-off. One of the main defenses against weed control in vineyards, soil tillage, often increases soil erosion and decreases soil fertility by speeding up the mineralization of organic compounds, and by simple hydrological properties.
Problems with these erosion and fertility issues are often ameliorated by different types of permanent or temporary cover crops. With cover crops, comes an increase in biological activity including increases in populations of natural enemies to grapevine pests. Traditionally, during higher precipitation times of year, vineyards are planted with fast-growing plants, and when intense grapevine growth is occurring, soil tillage is employed instead. These soil tilling practices leave vineyards more vulnerable to erosion in high rainfall events and often greatly increase surface runoff.
The choice of more permanent cover crops is important, requiring plants that have strong root structures, yet do not complete with the grapevines for resources. Comparing periodically tilling the soil to permanent cover crops (the former of which is the practice in traditional wine making regions), the use of permanent cover crops is advantageous, as it increases the organic matter in the soil while intensifying the biological activities of many macro and microorganisms. Some disadvantages of permanent cover crops are impaired grapevine growth and reduced grape yield as a result of competition for resources between the grapevine and the cover crop.
Occasional heavy rains are responsible for great amounts of soil erosion in certain environments. As a result of global warming, more frequent and longer dry seasons are expected, as well as heavier occasional soil-eroding rain events. In Slovenia, the location where today’s study was performed, the estimated increase in temperature between 2001 and 2030 is 0.5 to 2.5oC. As a result of this changing climate, vineyard management practices need to be adopted that do not significantly contribute to soil erosion in occasional heavy rainfall events, yet that can still hold up to longer and more severe droughts without the negative implications of permanent cover crops.
One vineyard management practice suggestion, which is the experimental treatment of the study presented today, considers periodic soil tillage during the growing season, which is later filled in by naturally occurring permanent vegetative growth. In a nutshell, the overall objective of the study presented today examines the effect of a permanent cover crop on soil erosion and nutrient loss compared with periodic soil tillage between rows.
This experiment was conducted at the University Centre for Viticulture and Enology at Meranovo, Faculty of Agriculture and Life Sciences, Maribor in northeast Slovenia. Experiments were performed on a 6 year old Sauvignon blanc vines. Central European climate conditions were the norm.
The slope of the vineyard was at a 34% incline, length of the rows were 82m, distance between rows was 2.4m, and average area of each replicate was 197m2. The soil was a medium deep loam, with a pH of 5.3. The soil contained 35.9mg soluble P2O5, 45.7mg soluble K2O, and 25.9mg soluble MgO per 100g dried soil from a soil layer of 0-20cm. No fertilizers were applied during the experiment.
Every spring before the experiment began; an herbicide (glyphosate) was applied at the soil surface near the grapevines. Five years prior to the experiment, the space between rows had been grown over by natural vegetation and mulched 6 times per year. For the experiment, the two treatments were 1) treatment with a permanent cover crop and 2) treatment with periodic soil tillage of each second area between rows. For the permanent cover treatment, mulch was applied 6 times per year. For the periodic soil tillage treatment, soil was tilled with a rotary tiller when the sward reached 30cm, with the remaining areas between the rows receiving mulch at the same time. Treatments were carried out between the 3rd week in May and the 1st week in August. In the periodic soil tillage treatment, the central areas between the rows were tilled. In the permanent cover treatment, all areas were mulched regularly.
To estimate the weight of the soil eroded, pits (240x30x40 cm) were dug at the foot of the hill to catch any soil eroded from further up the slope. Eroded soil samples were taken once per month, and were dried and weighed. Chemical analyses were performed to measure concentrations of P2O5, K2O, and MgO. Macroorganism activity was measured by counting the number of earthworms per m2 in soil layers of 0-60cm deep. To estimate climate change, data regarding the ripening on Sauvignon blanc between 1980 and 2008 in the same region of Slovenia were analyzed. This analysis indicated trends toward a shorter growing season. Harvest of the grapes occurred when the sugar content reached 18-20oBrix.
- For the periodic soil tillage treatment: erosion totals per year ranged from 1680kg soil/ha in 1997 to 2012kg soil/ha in 2002.
o Nutrient loss: 676g P2O5, 861g K2O, and 491g MgO per ha.
- For the permanent cover treatment: erosion totals per year averaged to 92kg soil/ha (less than 5% of the soil tillage treatment).
o Nutrient loss: 33g P2O5, 42g K2O, and 24g MgO per ha (significantly less lost than the soil tillage treatment).
- Further analysis showed that a greater portion of soil erosion in the soil tillage treatment occurred during one particular period in August for all six years (between 1997 and 2002).
o In August during each year, maximum erosion occurred soon after tillage and was caused by slower grass recovering.
o Comparing August with May, the erosion was significantly less in the month of May for all 6 years.
§ In addition to both slope and the amount of precipitation, soil erosion is also strongly affected by the time of tillage and the rate of regrowth afterwards.
- The lowest soil erosion and nutrient loss were observed in the permanent cover treatment.
- The average number of earthworms in the permanent cover treatment was nearly 3 times higher than in the soil tillage treatment.
o The highest density of earthworms in the permanent cover treatment was found between 0 and 20cm into the soil.
o The highest density of earthworms in the soil tillage treatment was found between 10 and 20cm into the soil.
o Soil macroorganisms were most numerous and active in the permanent cover treatment, though establishment and maintenance of a permanent cover crop is only possible in areas that have sufficient rainfall during the growing season.
§ The number of earthworms could be considered to be a strong indicator of vineyard soil management quality.
- The periodic soil treatment significantly increased the grape yield and pruning dry weight over both years analyzed, compared with the permanent cover treatment.
o This result is speculated to be because of an increased mineralization of organic matter in the tilled soil, thereby allowing the stored nutrients (particularly, nitrogen) to become available for uptake by the plants.
- In regards to climate change, hot summers resulted in earlier ripening of Sauvginon blanc grapes and increased temperatures (by 1.1oC in 28 years)
o The growing season of Sauvignon blanc was shortened overall by 2-3 weeks in the last decade of study.
o Titratable acid content was reduced as a result of higher temperatures and earlier ripening.
The results of this study indicate that periodic soil tillage only appeared to be problematic in regards to significant soil erosion when the tillage occurred in the month of August when rainfall was at its’ greatest. Since periodic soil tillage was associated with increased grape yield and vine vigor, it may be in a vineyards’ best interest to employ the practice. It is important, however, to understand that soil tillage cannot be performed at any time during the summer, and should be systematically employed when rainfall is not going to cause significant soil erosion and nutrient loss. Examining the results of this study, it appears that in order to maximize grape yield and vine vigor without causing heavy soil erosion and nutrient loss, soil tillage should be performed only in the spring months.
It should also be noted that this recommendation only applies to those areas with similar climate and environmental conditions as the study vineyard in Slovenia. Areas with significantly more rain or even significantly drier will likely need to take different approaches to their vineyard management practices. This soil tillage practice will need to be adapted to wetter and drier environments in order to maximize grape yield and minimize soil erosion and nutrition loss, thus more studies will need to be performed.
Overall, I thought this was a very informative study that provides a significant recommendation for finding a balance between grape yield and soil health in climates similar to that of Slovenia in central Europe. More work needs to be done in other climates, however, this study provides a solid launching point for future studies that address the question of vineyard management practices involving soil health and grape yield.
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Source: Vrsic, S., Ivancic, A., Pulko, B., and Valdhuber, J. 2011. Effect of soil management systems on erosion and nutrition loss in vineyards on steep slopes. Journal of Environmental Biology 32: 289-294.
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