Introduction
A shift from conventional to organic agriculture should promote the conservation of natural resources and the development and use of sustainable farming practices, not just the prohibition of synthetic fertilizers and pesticidesReference El-Hage Scialabba and Hattam1. Soil and water are the world's most important natural resources, and their careful management should be paramount in organic agriculture. In the erosion-prone Mediterranean region, prioritizing soil conservation is key to sustainable farming, where soil organic matter (SOM) levels are inherently lowReference Zalidis, Stamatiadis, Takavakoglou, Eskridge and Misopolinos2, and the loss of existing organic matter contributes to a rapid decline of soil fertility, degradation of soil structure, and an increased risk of erosionReference Stanners and Bourdeau3.
The severe problems of soil erosion become evident on a drive along any road in the olive-producing areas of Andalusia, southern Spain, where olive trees dominate the landscape in an impressive monoculture that occupies 1.5 Mha, or 17% of the total surface area of Andalusia4. In a region often subjected to torrential rainstorms, the most common soil management practice among olive farmers over the past few decades has been to maintain bare soil year-round, underneath and between olive trees, by means of frequent tillage and/or use of herbicides. These practices have led to serious problems of erosion, soil compaction, loss of soil fertility, sedimentation of waterways and contamination of potable water sourcesReference Gómez, Battany, Renschler and Fereres5–Reference van Wesemael, Cammeraat, Mulligan and Burke7. Soil degradation problems are expected to worsen as the total area of land devoted to olive production in Andalusia has rapidly expanded and cultivation has intensified in response to farm mechanization, new harvesting technologies and European Commission policies promoting productivity increasesReference Zalidis, Stamatiadis, Takavakoglou, Eskridge and Misopolinos2,Reference de Graaff and Eppink8. Small farmers on marginal lands have found themselves unable to compete with the new mechanized production systems because of their higher production costs and lower yields. Bleak economic prospects, among other factors, have led these farmers to adopt alternatives to conventional olive growing, such as organic productionReference Alonso Mielgo, Sevilla Guzmán, Romera Jiménez and Guzmán Casado9.
Regulations for the certification of organic agriculture prohibit the use of herbicides or chemical fertilizers; therefore the change in the type of inputs used on the farm is inherent to the shift to organic farming. However, the changes in soil management practices and the effects on soil fertility and soil erosion risk are not as evident or well understoodReference Siegrist, Schaub, Pfiffner and Mader10, nor are soil management practices regulated under organic certification. In conventional agriculture, degradation of soil structure and function due to tillage or erosion is often combated by further cultivation and application of additional inputs rather than remedial or preventative measuresReference Holland11. In organic farming, however, the dependence on additional inputs to make up for the loss of the natural fertility of the soil is unsustainable, and is more costly than in conventional farming, given the limited access to and laborious application of manure and high costs of organic fertilizer productsReference Sánchez12. Soil management practices on organic farms should encompass maintenance of the natural fertility of the soil and prevention of soil degradationReference El-Hage Scialabba and Hattam1.
Soil erodibility, defined as soil susceptibility to detachment and transport by agents of erosion, is highly influenced by soil managementReference Morgan13. Farmers' management decisions affect soil surface dynamics, such as soil compaction caused by machinery or overgrazing, or protection of the soil by a vegetative coverReference Lal14. The state of the soil surface in turn affects the hydrology of the orchard, determining water balance, runoff generation and soil erosionReference Lal14. Surface compaction and surface sealing decrease rainwater infiltrationReference van Wesemael, Cammeraat, Mulligan and Burke7, causing increased surface runoff and elevated risk of erosionReference Stanners and Bourdeau3. The presence of a vegetative cover not only protects the soil surface, but also significantly increases the formation of SOM and the biological activity in soil, thereby improving soil aggregate stability and preventing the formation of a surface sealReference Lado, Paz and Ben-Hur15. Vegetation also increases infiltration due to the increase of soil macroporosity, causing a decrease in surface runoffReference Rawls, Brakensiek and Morel-Seytoux16, and consequently a decrease in erosion risk.
Improved understanding of the relationship between farmers' decision-making about soil management practices and the risk of soil erosion can facilitate the development of appropriate soil conservation policies. This relationship needs to be examined from two perspectives: the rationale behind soil management decisions and the effect those decisions have on soil erosion risk. Little research has been done on the relationship between organic production systems and soil erosionReference Siegrist, Schaub, Pfiffner and Mader10, especially in the Mediterranean basin, and no previous studies have been conducted about soil erosion in the organic olive orchards of Andalusia, where the organic movement is relatively young, founded officially in 1991. There are over 32,000 ha devoted to organic olive production in Andalusia at present, accounting for 38% of the total area devoted to organic agriculture17. The majority of organic olive orchards in Andalusia, more than 17,000 ha, are located in Córdoba4, mostly in the north of the province (Fig. 1).
Figure 1. Location of all organic farms and the two study areas in the province of Cordoba.
We conducted a field study with three objectives: (1) to describe the change in soil management practices that have accompanied the shift from conventional to organic farming; (2) to analyze the soil management practices of organic olive farms in the Córdoba province as influenced by farm size, economic dependence on farm and geographic location; and (3) to investigate the relationship between soil management practices and erosion risk on organic olive orchards in the province of Córdoba.
Materials and Methods
Study sites
Two geographic areas in the province of Córdoba were identified for our study, the olive growing area in the north of the province that we refer to as ‘la Sierra’, and the olive growing area in the south that we refer to as ‘la Campiña’ (Fig. 1). These two areas were selected to best represent the agricultural systems in the province due to their differing management systems, agro-economy, soil type and topography.
La Sierra is a mountainous area characterized by steep slopes of up to 70% and shallow soils of dominant associations Xerochrept/Haploxeralf and Xerochrept/Xerorthent/Xerumbrept18 developed on slates of the Precambrian and of the Carboniferous periods19. Farmers in this area have traditionally carried out low-input management systems, using animal traction for tillage, and livestock in the olive orchards as a sylvopastoral system that offers natural weed control and soil fertilization. These orchards are very old (more than 100 years), planted approximately 8 m×8 m to 10 m×10 m, generally with only one trunk per tree, and are almost entirely rain-fed4. It is not common for farmers to plant a cover crop in addition to the natural vegetation. Farmers till on average once a year with either animal traction or chisel or disk plows. Tillage operations are generally carried out once after the first rains in the fall (October) and/or in February–March after the harvest and tree pruning is finished. When tillage is carried out by animal traction, contour plowing is almost universal. When mechanical traction is used, farmers till in the direction of the slope or in a crossed pattern as to avoid tipping the tractor. This low-tech management system is due to the steep slopes and inherent low productivity of the orchards. Conversion to organic farming has been successful in this area in part due to the easy transition from the traditional management system to an organic system and the relative importance of the subsidies available for organic production compared to the income derived from the sale of olivesReference Sánchez12. In 2003, agricultural subsidies for organic farming in the province of Córdoba reached 3.7 million euros, almost entirely dedicated to olive farmers. This was equivalent to approximately an average of 150 euros per ha of olive orchards17, in addition to the standard subsidies per kg of oil given to all producers. Without these subsidies, net farm profit would be negative in many areas of la Sierra 4.
The second study area, la Campiña, is characterized by rolling plains with moderate slopes generally below 25%, and deeper soils of dominant association Xerochrept/Xerorthent/Chromoxerert18 developed mainly on marls and limestone–marl alternations of the Neogene period19. Farmers in this area generally carry out an intensive, high-input, high-tech management system on more productive farms compared to their northern counterparts. Orchards in la Campiña are newer than in la Sierra, and the surface area devoted to olive orchards is increasing4. Trees are planted from 12 m×12 m in the most traditional orchards with three trunks per tree, to 6 m×8 m (or 6 m) in the newer orchards with only one trunk per tree, and a significantly larger percentage of orchards are irrigated4. Farmers till on average three times a year with disk or chisel plows. Tillage operations are carried out after the first fall rains (October), to prepare the ground before the harvest (November–December), after the harvest and tree pruning (February–March) and sometimes again in mid-spring (April–May). Contour plowing is not commonly implemented, and on sloped farms tillage is often performed in the direction of maximum slope. The adoption of organic agriculture has been much less widespread in this area due to the relative unimportance of the subsidies compared to the income derived from the sale of olives4.
Average annual precipitation is around 700 mm in la Sierra and 600 mm in la Campiña, in a typical Mediterranean distribution, with the rainfall concentrated mostly between November and March19.
Farmer interviews
Semi-structured interviews were conducted with 107 randomly chosen organic olive farmers within the province of Córdoba to determine soil management practices, farm size and economic dependence on the farm. These 107 farms represent 9% of all of the organic olive farmers, in both geographic areas.
Soil management practices were divided into five categories covering all major soil management practices existing in organic olive orchards in the study areas (n=number of farms): (1) mowing (M), a mechanical form of weed management requiring mowing on average twice in the spring (n=10); (2) intensive tillage (IT), tillage more than once a year (n=7); (3) tillage (T), once or less than once a year (n=22); (4) mixed system of tillage and livestock (TL), which implies tillage once or less than once a year and pasture animals as a form of weed control (n=37); and (5) livestock (L), pasture animals as the sole form of weed control (n=31). In this case, a livestock grazing system implies either owning the animals or leasing out grazing rights for other farmers' animals. The most common grazing animals are sheep.
We defined economic dependence on the farm in terms of the importance of income from olive cultivation (including subsidies) to overall family income. Three levels of economic dependence and three options for farmers not economically dependent on their olive orchards were defined, based on the information gathered in a preliminary study to best represent the most important groups within the local olive farmers. The levels of economic dependence are: (1) total dependence on olives as the only source of income; (2) dependence on olive farming together with another source of income from an agricultural activity such as animal products (meat, cheese and milk) and other crops, or from providing agricultural services such as a tractor service, or olive milling; and (3) dependence on the farm together with another source of income from a non-agricultural source, such as a job in a city or town. The three options for farmers not economically dependent on olives reflect their motivations for maintaining olive production: (1) for cultural reasons; (2) for the extra income it provides from the harvest and the subsidies; and (3) both for cultural reasons and for the extra income. Farm size was divided into five groups: less than 5 ha, 5–20 ha, 20–50 ha, 50–100 ha and greater than 100 ha.
Measurement of soil characteristics
Fieldwork was conducted during the rainy season (mid-January to mid-March) over 2 years (2003–2004). Twenty-five farms were sampled, ten in la Sierra and 15 in la Campiña. More farms were sampled in la Campiña to represent better the greater heterogeneity (soil, socio-economic factors, etc.) in organic olive farms compared to farms in la Sierra 4,19. To compare the condition of the soils under cultivation with undisturbed soil, measurements were taken in two locations on each farm: on a slope between two rows of trees within the orchard and on an adjacent slope in a nearby, undisturbed area whenever possible. In each location, the sample plot considered was a rectangular area in the middle of the slope (250 m2); sample plots in the undisturbed locations were of equal size. While conducting fieldwork, it became evident that apart from the five soil management categories mentioned above, it was necessary to create another category given that some farmers claimed to carry out a mowing (M) system, but their farms had reached such a state of soil degradation that little or no vegetation grew. We refer to this group of farms as ‘bare soil’ (BS). We refer to the results from the undisturbed areas as ‘natural soil’ (NS) (n=14). In total, the soil measurements are divided into seven categories.
Vegetative ground cover (VGC), SOM, cation exchange capacity (CEC), texture, aggregate stability, infiltration and slope were measured on each sample area of the 25 farms studied. In order to determine SOM, CEC, texture and aggregate stability, a composite sample of 30 sub-samples was taken (0–10 cm depth) in each sample plot. VGC was measured by estimating percentage of the soil covered by vegetation or vegetative residues in a 0.5 m×0.5 m square, divided into four equal sections, in a total of 16 random locations in each sample plot, based on a percentage ground cover diagram from HerwegReference Herweg20. SOM was determined using the Walkley–Black methodReference Nelson, Sommers, Page, Miller and Keeney21. CEC was measured using the ammonium replacement methodReference Rhoades, Page, Miller and Keeney (22. Texture was measured with the hydrometer methodReference Bouyoucos23. Aggregate stability was measured as percentage water-stable aggregates (WSA) of aggregates between 0.25 and 2 mm diameter24. Slope was measured with a pocket inclinometer. Olive tree canopy cover was determined by measuring the area projected onto the ground by the canopy (ten trees in each farm) in relation to the plantation density.
In 2003, infiltration was measured using the US Department of Agriculture (USDA) infiltration test24 based on a simplified single ring test. In each sample plot, five rings (22 cm in diameter) were inserted 5 cm into the ground, 17 mm of water was applied and infiltration rate was measured when it became constant, following the USDA methodology24. In 2004, we modified the methodology for use on farms with steep slopes where water did not cover the whole soil surface area inside the ring. In this case, water was applied using perforated buckets as simplified rainfall simulators at a rate of 236 mm h−1. Each ring had a tube built in at ground level that allowed for collection of the runoff. The results of the 2004 tests were standardized to the 2003 tests.
Erosion risk of the farms was assessed using a visual assessment derived from a methodology developed by MorganReference Morgan13, based on erosion symptoms. The assessment is scaled from 0 to 5, with 0 for no visual symptoms of erosion and 5 for a farm with serious rills, gullies, compaction, surface sealing or large areas with bare soil.
During the two study years (September 2002 to August 2004), the annual precipitation recorded on farms evaluated was 545 and 726 mm in la Sierra, and 534 and 568 mm in la Campiña.
Results
Changes in soil management from conventional to organic farming
Accompanying the shift from conventional to organic farming, there was a 46% decrease in farms managed with tillage (T and IT) and a 74% increase in the use of a sylvopastoral system (L and TL) despite the fact that these changes were not required by the regulations of organic certification (Fig. 2). Approximately two-thirds of the farmers who tilled intensively (IT) changed management practices when they converted to organic farming; one-half of the farmers that previously tilled intensively changed to a mowing system, and all of the farmers who previously mowed maintained their management system (Table 1). The only farmers that changed over to an IT system were those who previously used herbicides (Table 1).
Figure 2. Soil management system before and after converting to organic farming (as % over total of farmers). H=herbicides, M=mowing, IT=intensive tillage, T=tillage once or less than once a year, TL=tillage and livestock, L=livestock.
Table 1. Number of farmers (%) that changed soil management practices after the conversion from conventional to organic agriculture (for each management before converting to organic farming) (n=107).
n=total number of farmers.
The numbers in bold represent the number of farmers who have not changed management practices.
Factors influencing soil management decisions in organic farming
All of the farmers who managed their farm with mowing (M), the majority of those who tilled intensively (IT) (86%) and those who tilled and had livestock (TL) (70%) depended economically on their farm (Fig. 3). The majority of the farmers who tilled less than once a year (67%) or managed their farm with livestock (L) (65%) did not depend economically on their farm (Fig. 3). Of all farmers interviewed, 43% did not depend economically on their farm, and of the farmers who did depend economically on their farm, 58% also depended on income from non-agricultural jobs (Table 2). Only 24% of all organic olive farmers depended solely on income from the olive farm and other agricultural activities. All of the farmers who depend solely on olive farming, mowed (M) (Table 2).
Figure 3. Relationship between soil management system and farmer's economic dependence on the farm, in la Campiña and la Sierra (as % over total of farmers for each management type). H=herbicides, M=mowing, IT=intensive tillage, T=tillage once or less than once a year, TL=tillage and livestock, L=livestock.
Table 2. Main economic reasons for maintaining the olive organic orchards and associated soil management practices (as percentage of farmers over total for each economic dependence level) (n=107).
n=number of total farmers.
1 Percentage of farmers in each economic dependence level.
This relationship between soil management and economic dependence on the farm was associated with the farm size. Larger farms (more than 100 ha) were associated with mowing (M) (57%), whereas the most common management practice on small farms (less than 5 ha) was tillage (T) (52%), and on farms between 5 and 100 ha, farmers tended to use a livestock grazing system (TL or L) (Fig. 4a). The larger the farm, the more economic dependence farmers tended to have on the farm. All of the farmers with farms more than 100 ha in size and 80% of those with farms between 50 and 100 ha depended economically on their farm (Fig. 4b), whereas less than 20% of farmers with small farms (less than 5 ha) depended economically on their farm. On farms between 5 and 20 ha, economic dependency on the farm and on income from non-agricultural activities prevails (50%); however, on farms of 20–50 ha, economic dependence on the farm and on income from other agricultural activities prevails (42%) (Fig. 4b).
Figure 4. Distribution of (a) soil management practices, and (b) farmer's economic dependence on the farm, in each farm size group (as % over total of farmers for each farm size group). H=herbicides, M=mowing, IT=intensive tillage, T=tillage once or less than once a year, TL=tillage and livestock, L=livestock.
Results from the two geographic areas differed significantly with respect to farm size, economic dependence and management practices. In la Campiña, farms were larger, with an average farm size of 93 ha and 28% of all the farms having more than 100 ha, whereas in la Sierra, farms were smaller with an average farm size of 18 ha and 73% of the farms having less than 20 ha. Farmers in la Campiña tended to depend economically on their farms (78% of all the farmers) but in la Sierra farmers tended to rely more heavily on external forms of income (48% did not depend on the farm). In la Campiña, the majority of farmers mow (M) and till intensively (IT) (78% of the farmers), whereas in la Sierra, livestock (L) and TL were the most common soil management practices (73%) (Figs. 2 and 3). In la Sierra, farmers have farmed organically for more years, 93% of the farms are managed by family members, and farmers are on average older than in la Campiña. Average yields in la Sierra are significantly smaller than yields in la Campiña (Table 3).
Table 3. Socio-economic characteristics of olive organic farms in the two geographic areas studied (averages): olive production per tree (kg), size of farm (ha), age of farmer, experience on farm, employees on farm, and years (average) in organic farming.
Data from farmer surveys (n=107), where n=total number of farmers.
Soil characteristics and erosion risk
The soil data taken on the 25 farms sampled are summarized in Table 4. VGC is highly related to soil management practices. The highest ground cover values are associated with L, TL, and M management systems, whereas the lowest values were found on farms with IT and BS management systems. SOM is highly associated with VGC in relation to farm management system. Despite the differences observed in SOM between management systems, these differences were not perceived in WSA, except in the BS farms with the most degraded soils that showed significantly lower WSA. However, there are important and similar differences in WSA between the natural and agricultural soils in both geographic locations (Table 4). Infiltration rates on L farms were significantly higher than the other management types, following the pattern observed in VGC and SOM results. The lowest infiltration rates were seen on farms with BS management systems due to their inherently compacted soils. Significant differences in infiltration rates were also observed between natural and agricultural soils in both geographic locations associated with higher SOM in the natural soils (Table 4).
Table 4. Soil characteristicsFootnote 1: VGC in winter (%), SOM (%), CEC (cmol kg−1), WSA (%), and infiltration rate (cm h−1) for natural and agricultural soils and for each soil management system, in the two geographic areas (n=25).
1 Mean±standard error.
n=number of farms.
The farms studied in la Campiña had principally clay loam soil textures as well as some loam soils; farms in la Sierra had sandy loam soil textures. CEC was directly related primarily to SOM as well as to the clay content of the soils (Table 4). The average slope for all of the sample plots in la Campiña was 16% (max 40%) and in la Sierra was 37% (max 65%). The average canopy cover on the farms sampled was 22% and 18% in la Campiña and la Sierra, respectively.
There were large differences in erosion risk between management systems (Fig. 5). Farms managed with IT and those with sparse vegetation cover (BS) showed higher erosion risk than those with M, L and TL management systems. Lower erosion risk was associated with higher percentage of VGC.
Figure 5. Erosion risk estimations based on MorganReference Morgan13 (0=low risk, 5=high risk).
Discussion
Our study has shown that the amount and frequency of tillage has decreased with the conversion to an organic cropping system (Fig. 2 and Table 1). Tillage in traditionally rain-fed olive orchards is considered necessary to avoid competition with the natural vegetation for water and nutrients, to reduce compaction and to improve infiltration. Control of vegetation by tillage also reduces the risk of wildfire during the dry season. Additionally, there are cultural reasons for which farmers till; many farmers still till so as not to have what their community would consider a ‘dirty’ orchard with weeds. While animals were the only source of power, tillage did not represent a significant threat to soil conservation in the region because of limited frequency and intensity of tillage. However, the extension of use of the tractor exacerbated soil erosion problemsReference Gómez, Battany, Renschler and Fereres5,Reference van Wesemael, Cammeraat, Mulligan and Burke7. Careful management of weeds is necessary to avoid loss of production due to competition for water in a rain-fed system, but the presence of VGC is beneficial for the protection of the soil against erosion, and for maintaining SOM levels, and soil structureReference Lal14–Reference Rawls, Brakensiek and Morel-Seytoux16,Reference De Ploey25. There are several studies in AndalusiaReference Castro Nieto26,Reference Pastor27 showing that it may be possible to have vegetative cover without suffering a loss of production provided that the VGC is controlled adequately. Appropriate weed control in organic farming can be achieved by mowing or by grazing livestock. The shift to an organic production system has been accompanied by an increase in mowing in la Campiña and an increase in livestock in la Sierra (Figs. 2 and 3). There has been more adoption of mowing practices in la Campiña due to flatter slopes that facilitate the use of machinery and higher net profits that allow for greater economic investment in this area. The lack of a grazing tradition in la Campiña has also contributed to the increased adoption of mowing compared to la Sierra.
The two distinct groups of organic farms defined in this study (la Sierra and la Campiña) are differentiated by their location, soil type, agricultural history, socio-economic situation, farm size and yield potential. It can be seen from the results of this study that soil management practices are tied to geographic location (Fig. 3) and associated with economic dependence on the farm (Table 2 and Fig. 3) which is largely determined by farm size (Fig. 4). Sole economic dependence on olive orchards is not feasible in the mountainous areas of la Sierra, but is more common in la Campiña where yields per tree for organic olive are up to five times higher (Table 3) and the farms are almost five times larger. The farmers in la Campiña are also younger, have been managing the farms for fewer years and are more often employees of the farm and not the owners (Table 3). In accordance with our results, studies have shown that farmers with larger farmsReference Ogunlana28–Reference Rawal30 and younger farmers more readily adopt new technologiesReference Ogunlana28,Reference Feder, Just and Zilberman31, in this case associated with IT and mowing. All of these factors contribute to a more mechanized and productive cropping system in la Campiña than in la Sierra.
Overall, the shift to organic olive production has resulted in a net decrease in tillage (both intensive and minimal) and use of herbicides, and in an increase in mowing and use of livestock, alone or combined with tillage to control weeds (Fig. 2 and Table 1). These changes have resulted in an increase of VGC and a corresponding decrease in soil erosion risk (Table 4 and Fig. 5). Increase in ground cover has been shown to reduce soil loss dramatically in sloping olive orchardsReference Gómez, Romero, Giráldez and Fereres32–Reference Francia, Martínez Raya and Ruíz Gutiérrez34. It has also been shown that additional reduction in erosion risk in organic systems with vegetative cover is achieved through increased levels of soil biological activity, SOM, aggregate stability and infiltrationReference Siegrist, Schaub, Pfiffner and Mader10,Reference Schjonning, Elmholt, Munkholm and Debosz35,Reference Tan, Drury, Reynolds, Gaynor, Zhang and Ng36. The results from this study show that differences in SOM, infiltration rates and water aggregate stability were significant between the management systems that have low VGC (IT and BS) and the other management systems that present values closer to those shown by the natural soils (Table 4 and Fig. 5).
Even though mowing systems had a high percentage of vegetative cover, infiltration rates in la Campiña soils remained low (Table 4). These low infiltration rates are most likely due to soil compaction as a result of excessive traffic and the compounded effect of the easily compacted clay textures of the soils in la Campiña. The effects that excessive traffic has on compaction and on soil lossReference Hussain, Olson and Siemens37–Reference Raper, Reeves, Burmester and Schwab39 were not manifested in visual signs of soil erosion on the farms with mowing (M) systems evaluated in this study, due to the positive effect of the vegetative coverReference Raper, Reeves, Burmester and Schwab39 and therefore the risk of erosion for the farms with this management system was low (Fig. 5).
Contrary to what would be expected, our erosion assessment indicated that the farms located in the steeper la Sierra showed less erosion symptoms than the farms located in the flatter la Campiña. Soils in la Sierra are slightly less erodable than soils in la Campiña; average soil erodibility rates (as defined for RUSLEReference Renard, Foster, Weesies, McCool and Yoder40) are 0.03 and 0.04 t ha h (ha MJ mm)−1 for la Sierra and la Campiña, respectively41. However, the large differences in soil erosion risk between la Sierra and la Campiña are most likely due to the prevailing management systems of grazing livestock and minimal tillage in la Sierra, whereas IT and heavy use of machinery on clay loam soils exacerbate erosion risk in la Campiña. Differences in erosion risk associated with these management practices (Fig. 5 and Table 4) are of the same magnitude or larger than the differences in soil erodibilityReference Gómez, Battany, Renschler and Fereres5 published for these soils. Even though a grazing system runs the risk of being erosive, we found that in general the farms with livestock did not show signs of overgrazing.
The shift from conventional to organic farming is still ongoing in the area, and the effects that it may have on a regional level may change with time. However, all of the farms sampled in this study have been managed organically for at least 4 years, sufficient time for soil properties to be affected.
Conclusion
This study shows that the shift to organic farming in olive orchards in the province of Córdoba has been associated with increased protection of the soil and reduced erosion risks. This was due to the decrease in soil management practices that may have a negative effect on soil conservation such as IT, and an increase in practices that have a positive effect, such as grazing livestock (L and TL), and mowing (M), systems that incorporate a VGC.
The organic farming initiative in the province of Córdoba has had a more important effect in la Sierra in terms of area of land affected by the change in soil erosion risk (Fig. 1). However, in la Sierra, the majority of the farms converted to organic are the farms that already had extensive, traditional management systems. The change in management systems and the impact on erosion risk was greater in la Campiña, as can be seen in the decrease in farms managed with IT, and an increase in farms managed with mowing. However, there is still an important percentage of organic olive farmers that manage their farms with IT and have high erosion risk levels. This is important in the development of agricultural policy and certification guidelines of organic farming. With respect to soil conservation, rewarding farmers for organic farming can potentially decrease soil erosion risk if soil management practices on organic farms are monitored carefully.
Acknowledgements
The authors thank the farmers and the olive oil cooperatives that collaborated with this study, and the CAAE. J. Milgroom acknowledges a fellowship from the Fulbright Program who funded most of her stay in Cordoba during the course of this work. Financial support from the European Commission OLIVERO project QLK5-CT-2002-01841 is acknowledged.
