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Providing evidence to improve practice

Action: Soil: Plant or maintain ground cover in orchards or vineyards Mediterranean Farmland

Key messages

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Organic matter (12 studies): Ten studies (eight replicated, randomized, and controlled, and two site comparisons) from Chile, France, Spain, and the USA found more organic matter in soils with ground cover, compared to soils without ground cover, in some or all comparisons. Two meta-analyses of studies from Mediterranean climates also found more organic matter in plots with ground cover.

  • Implementation options (4 studies): One study from France found more organic matter in soils with permanent ground cover, compared to temporary ground cover, in one of three comparisons. Two studies from the USA found similar amounts of organic matter in soils with resident vegetation or seeded cover crops. One study from Spain found more organic matter where cover crops were incorporated into the soil.

Nutrients (12 studies)

  • Nitrogen (9 studies): Five studies (four replicated, randomized, and controlled, and one site comparison) from Chile and Spain found more nitrogen in soils with ground cover, compared to soils without ground cover, in some or all comparisons. One replicated, randomized, controlled study from the USA found less nitrogen in soils with ground cover, in some comparisons. Two replicated, randomized, controlled studies from Spain and the USA found inconsistent differences in nitrogen between soils with or without ground cover. One replicated site comparison from France found similar amounts of nitrogen in soils with or without ground cover.
    • Implementation options (5 studies): Two studies from Spain and the USA found more nitrogen in soils that were cover cropped with legumes, compared to non-legumes, in some or all comparisons. Two studies from vineyards in the USA found similar amounts of nitrogen in soils with resident vegetation or seeded cover crops. One of these studies also found similar amounts of nitrogen in soils with different types of seeded cover crops, and in soils with or without tillage (both with ground cover). One study from Spain found more nitrogen where cover crops were incorporated into the soil.
  • Phosphorus (4 studies): One replicated site comparison from France found more phosphorus in soils with ground cover, compared to bare soils, in one of six comparisons. Two studies (one replicated, randomized, and controlled, and one site comparison) from Spain and the USA found less phosphorus in soils with seeded cover crops, compared to tilled soils, in some comparisons. One replicated, randomized, controlled study from Chile found similar amounts of phosphorus in soils with seeded cover crops and bare soils.
    • Implementation options (3 studies): One study from France found more phosphorus in soils with permanent ground cover, compared to temporary ground cover, in one of three comparisons. One study from the USA found similar amounts of phosphorus in soils with resident vegetation or seeded cover crops. One study from Spain found different amounts of phosphorus in soils with different types of seeded cover crops.
  • Potassium (3 studies): One replicated, randomized, controlled study from Chile found more potassium in soils with seeded cover crops, compared to bare soils. Two site comparisons (one replicated) from France and Spain found similar amounts of potassium in soils with ground cover, compared to tilled or bare soil.
    • Implementation options (1 study): One study from the USA found similar amounts of potassium in soils with resident vegetation or seeded cover crops.
  • pH (4 studies): Two studies (one replicated, randomized, and controlled, and one site comparison) from Spain and the USA found lower pH levels in soils with ground cover, compared to soils without ground cover. One replicated, randomized, controlled study from Chile found higher pH levels in soils with ground cover. One replicated site comparison from France found similar pH levels in soils with or without ground cover.

Soil organisms (6 studies)

  • Microbial biomass (4 studies): Four replicated studies (three randomized and controlled, one site comparison) from France and the USA found more microbial biomass in soils with ground cover, compared to bare or tilled soils, in some or all comparisons.
    • Implementation options (1 study): One study from France found more microbial biomass in soils with permanent ground cover, compared to temporary ground cover, in some comparisons.
  • Fungi (2 studies): One replicated, controlled study from the USA found more symbiotic fungi (mycorrhizae) in soils with seeded cover crops, compared to tilled soils, in some comparisons, but found similar numbers of roots that were colonized by mycorrhizae. One replicated, randomized, controlled study from the USA found inconsistent differences in mycorrhizae in soils with seeded cover crops or tilled soils.
  • Bacteria (1 study): One replicated, randomized, controlled study from Spain found more bacteria, but similar levels of bacterial diversity, in soils with ground cover, compared to bare soils.
  • Nematodes (1 study): One replicated site comparison from France found more nematodes in soils with ground cover, compared to bare soils.
    • Implementation options (1 study): One study from France found more nematodes in soils with permanent ground cover, compared to temporary ground cover, in one of three comparisons.

Soil erosion and aggregation (10 studies)

  • Soil erosion (7 studies): Six replicated, randomized, controlled studies from Chile, Italy, Spain, and the USA found less erosion of soils with ground cover, compared to bare or tilled soils, in some comparisons or all comparisons. One replicated, controlled study from France found similar amounts of erosion in plots with or without ground cover.
    • Implementation options (1 study): One study from Italy found the least erosion with permanent cover crops, and the most erosion with temporary cover crops.
  • Soil aggregation (5 studies): Four replicated, randomized, controlled studies from Chile and Spain found that soil aggregates were more water-stable in plots with seeded cover crops, compared to tilled or bare soils, in some or all comparisons. One site comparison from Spain found inconsistent differences in water stability between soils with seeded cover crops and bare soils.

Greenhouse gases (3 studies): Two replicated, randomized, controlled studies from a vineyard in the USA found more carbon dioxide or nitrous oxide in soils with cover crops, compared to tilled soils. One replicated, randomized, controlled study from an olive orchard in Spain found similar amounts of carbon dioxide in soils with cover crops, compared to tilled soils.

Implementation options (1 study): One study from the USA found similar amounts of carbon dioxide in soils with different types of ground cover.

 

Supporting evidence from individual studies

1 

A replicated, randomized, controlled study in 2000–2003 in a rainfed olive orchard near Cordoba, Spain (partly the same study as (10)), found more organic matter, less erosion, and higher soil stability in plots with cover crops, compared to conventional tillage or bare fallows. Organic matter: More organic matter was found in soils with cover crops (1.5%), compared to conventional tillage (1.2%) or bare fallows (0.9%). Soil erosion and aggregation: Less soil was lost in runoff from plots with cover crops, compared to bare fallows (1.2 vs 8.5 t/ha/year), but similar amounts were lost from plots with cover crops or conventional tillage (1.2 vs 4.0). Higher stability was found in soils with cover crops, compared to bare fallows (83% vs 60% of aggregates were water-stable), but similar stability was found in soils with cover crops or conventional tillage (82% vs 72%). Methods: There were three plots (6 x 12 m plots, with two olive trees each, on a 13% slope) for each of three treatments: cover crops (2 x 12 m barley strips, sown in October), conventional tillage (15 cm depth, 3–4 passes from September), or bare fallows (with herbicide, weed-free). Plots with cover crops were tilled before the barley was sown (10 cm depth). Runoff was collected with tipping-bucket gauges, and sediment was collected in barrels, from autumn 2000. Soil samples were collected in summer 2003 (0–5 cm depth).

 

2 

A replicated, controlled study in 2001–2003 in an irrigated vineyard in the Salinas Valley, California, USA (same study as (6)), found more fungal spores in soils between vine rows with cover crops, compared to those without cover crops. Soil organisms: More fungal spores (mycorrhizae) were found in soils with cover crops, in at least one of three seasons (spring: 110–130 vs 70 spores/g soil). Similar numbers of vine roots were colonized by mycorrhizae in vine rows with or without cover crops (data not reported). Methods: There were nine plots (0.045 ha) for each of two cover crops (Secale cereale Merced rye or Triticosecale triticale, in the central 80 cm of the 240 cm between vine rows, which were disked every year in November, before they were planted, and were mown every year in spring), and there were nine control plots (bare soil between the vine rows, which were disked every month). Soil and vine roots (8 cm root and 10 g soil from 20 vines/plot, 0–30 cm depth) were collected in summer (July 2002), winter (February 2003), and spring (April 2003). Cover-crop roots were collected in winter and spring. Spores and fungal colonies were measured in soil and roots.

 

3 

A study in 1998–2002 in an irrigated vineyard in the Sacramento Valley, California, USA, found more nitrogen in soil that was cover cropped with legumes, compared to grasses. Implementation options: More nitrogen was found in soil that was cover cropped with legumes, compared to grasses (0.26% vs 0.22% total nitrogen). Methods: A leguminous cover crop (Trifolium fragiferum perennial strawberry clover) was planted in the southern half of the vineyard, and three native Californian, perennial, summer-dormant grasses (Elymus glaucus blue wildrye, Hordeum brachyantherum meadow barley, and Bromus carinatus California brome) were planted in the northern half. These cover crops were planted between every other vine row. They were mown 4–5 times/year and their residues were retained. The vineyard was fertigated with drip lines. Soil samples were collected in five sub-plots, in one 10 x 15 m plot, in each cover crop (0–10 cm depth, 3 cm diameter, nine times in July 2001–October 2002).

 

4 

A replicated, randomized, controlled study in 2002–2005 in an irrigated vineyard in the Napa Valley, California, USA, found similar amounts of organic matter and nutrients in soils with seeded cover crops or resident vegetation. Implementation options: Similar amounts of organic carbon were found in soils with seeded cover crops or resident vegetation (21–24 mg organic matter/g dry soil). Similar amounts of nitrogen (1.6–1.8 mg total N/g dry soil), phosphorus (17–22 µg Olsen P/g dry soil), and potassium (7.3–7.7 µmol exchangeable K/g dry soil) were found in soils with seeded cover crops or resident vegetation. Methods: No tillage or conventional tillage was used on eight plots each, between the vine rows (three vine rows/plot). A disk plough was used for conventional tillage (15 cm depth, once/year in April–June). Four plots with conventional tillage had annual cover crops (seeded in October 2002–2004) and four plots had no seeded cover crops. Four plots with no tillage had annual cover crops (seeded in October 2002–2004), and four had perennial cover crops (seeded in October 2002). All plots were drip irrigated in July–October (85 kl/ha/week). Soil samples were collected under grape vines and between the rows (0–15 cm depth, 4.6 cm diameter, four samples/plot in each location).

 

5 

A replicated, randomized, controlled study in 1976–2004 a rainfed olive orchard in southeast Spain (same study as (11)) found more organic matter and nitrogen in soils with cover crops, compared to soils without cover crops. Organic matter: More organic carbon was found in soils with cover crops, in two of four comparisons (23 vs 39–42 Mg C/ha, 0–30 cm depth). Nutrients: More nitrogen was found in soils with cover crops, in two of four comparisons (2.9 vs 4.4–6.5 Mg total N/ha, 0–30 cm depth). Implementation options: More organic carbon and nitrogen were found in plots with cover crops that were incorporated into the soil in spring, compared to cover crops that were suppressed with herbicides or mown in spring and retained on the surface (39–42 vs 26–30 Mg C/ha, 4.4–6.5 vs 3.4–3.9 Mg total N/ha). Methods: Herbicide was used on seven plots in autumn, but not on 28 other plots, which had resident vegetation over winter. The resident vegetation was controlled in spring with herbicide (seven plots), tillage (seven plots, 0–25 cm depth), mowing (seven plots), or mowing and tillage (seven plots, 0–25 cm depth). Plots had 16 olive trees each. Foliar fertilizer was used. Two soil samples were collected in each plot (0–30 cm depth, in February 2004, before spring tillage).

 

6 

A replicated, randomized, controlled study in 2000–2005 in an irrigated vineyard in the Salinas Valley, California, USA (same study as (2)), found more organic matter and microbial biomass, less nitrate, phosphorus, and soil erosion, and lower pH levels in soils with cover crops, compared to bare soils, between vine rows. Organic matter: More organic matter was found in soils with cover crops, compared to bare soils (1.15–1.55% vs 0.95–1.10%). Nutrients: Less nitrate and phosphorus was found in soils with cover crops, compared to bare soils, in five of six comparisons (3–11 vs 17–28 ppm nitrate-N; 20–22 vs 24–25 ppm Olsen-P). Lower pH was found in soils with cover crops, compared to bare soils (data not reported). Soil organisms: More microbial biomass (measured as carbon) was found in soils with cover crops, compared to bare soils, in one of two comparisons (plots that were cover cropped with rye: 105 vs 83 µg C, in vine rows; 190 vs 100 µg C, between vine rows; 0–12 inches depth). More beneficial fungus colonies (mycorrhizae) were found on vine roots in plots with cover crops, compared to bare soils, in two of six comparisons (with rye as the cover crop, and with pre-emergence herbicide or cultivation under the vines: 26–27% vs 21–21% of root length was colonized), but fewer colonies were found in one of six comparisons (with rye, and with post-emergence herbicide under the vines: 17% vs 26%). Soil erosion and aggregation: In winter, less sediment was lost in runoff from plots with cover crops, compared to bare soils, in one of two comparisons (with triticale as the cover crop: 508 vs 1,735 mg/litre). Methods: There were nine plots for each of two treatments and one control. The treatments were triticale (X Triticosecale) or Secale cereale Merced rye, planted in November 2000–2004 as cover crops (32 inches width) between the vine rows (8 feet width), mown in spring, and disked into the soil in the following November. Bare soils were maintained in the controls through disking in spring and summer (depth not reported). Each plot had 100 vines and the adjacent areas between the vine rows. All plots were drip-irrigated in April–October. Soil samples were collected when the vines were flowering (May 2003–2005, 10 samples/plot, 0–12 inches depth, between the vine rows). Vine roots were collected in April 2003, May 2004, and June 2005 (for mycorrhiza measurements). Runoff was measured with sumps (16 inches diameter, 5 feet depth) at the lower end of each plot. It was not clear whether these results were a direct effect of cover crops or tillage.

 

7 

A replicated, randomized, controlled study in 2001–2006 in an irrigated vineyard in the Central Coast, California, USA (same study as (8)), found more organic matter, soil organisms, and greenhouse-gas emissions in soils with cover crops, compared to tilled soils, between the vine rows. Organic matter: More carbon was found in soils with cover crops, compared to tilled soils (9.5–11 vs 7.2 mg total C/kg soil, 0–15 cm depth). Soil organisms: More microbial biomass (measured as carbon) was found in soils with cover crops, compared to tilled soils (150–330 vs 50–190 µg C/g soil, 0–15 cm depth). Greenhouse gases: Higher carbon dioxide emissions were found in soils with cover crops, compared to tilled soils (268–291 vs 153 g CO2-C/m2/year). Methods: There were six plots for each of two cover crops (Secale cereale rye or Triticale x Triosecale Trios, sown between the vine rows in autumn, mown in spring), and there were six control plots (tilled between the vine rows every two months; depth not reported). All plots were tilled in autumn. The plots were each 84 x 1.8 m, between two vine rows. Soil samples were collected every 2–3 weeks in November 2005–2006 (two samples/plot, 0–15 cm depth).

 

8 

A replicated, randomized, controlled study in 2001–2006 in a vineyard in the Central Coast, California, USA (same study as (7)), found more soil organisms and higher greenhouse-gas emissions in plots with cover crops between the vine rows, compared to tilled soils without cover crops, but found inconsistent effects on nitrogen. Nutrients: Less nitrate was found in soils with cover crops, compared to tilled soils, between vine rows, in 12 of 19 comparisons (0–1 vs 1.4–5.7 µg NO3-N/g dry soil). In contrast, more ammonium was found in soils with cover crops, in six of 19 comparisons (during the spring rains: 1.3–3 vs 0.7–1.7 µg NH4-N/g dry soil), and more available nitrogen was found in 18 of 19 comparisons (potentially mineralizable nitrogen: 21–55 vs 2–15 µg NH4-N/g dry soil). Soil organisms: More microbial biomass (measured as nitrogen) was found in soils with cover crops, compared to tilled soils, in 15 of 38 comparisons (during the spring and autumn rains: 21–61 vs 4–39 µg N/g dry soil). Greenhouse gases: Higher nitrous oxide emissions were found in soils with cover crops, compared to tilled soils (1.9–2.3 vs 1.6 g N2O-N/ha/day). Methods: There were six plots (84.3 x 2.4 m interrows between vines) for each of two cover crops, and there were six control plots (cultivated every two months to control weeds). The cover crops (1.8 m width) were Triticale x Triticosecale Trios or Secale cereale rye, seeded in November 2001–2005 (interrows disked before seeding), and mown in April 2002–2006. Soil samples were collected every 2–3 weeks in December 2005–November 2006 (19 samples/plot, two cores/sample, 0–15 cm depth). Nitrous oxide was measured in 5.2 litre chambers (13 mL samples, every 30 minutes from solar noon, for 1.5 hours). It was not clear whether these results were a direct effect of cover crops or tillage.

 

9 

A replicated, controlled study in 1999 in a vineyard in southern France found similar amounts of erosion in plots with grass or bare soil between the vine rows. Soil erosion and aggregation: Similar amounts of soil were lost in runoff water from plots with grass or bare soil between the vine rows (26–112 vs 45–207 g soil/m2). Methods: One interrow was cultivated (10 cm depth) and planted with grasses, and one interrow was managed conventionally (with herbicide), for four months each. Rainfall was simulated in three plots, in each interrow, in June 1999 (1 x 1 m plots, 60 mm water/hour, for 60 minutes). Soil loss was measured in each plot (200 observation points/m2).

 

10 

A replicated, randomized, controlled study in 2000–2006 in a rainfed olive orchard near Cordoba, Spain (partly the same study as (1)), found more organic matter and nitrogen, less erosion, and higher soil stability in plots with cover crops, compared to soils with no tillage or conventional tillage. Organic matter: More organic matter was found in soils with cover crops, compared to bare fallows (1.2–2% vs 0.8–1%). Nutrients: More nitrogen was found in soils with cover crops, compared to bare fallows (0.08–0.11% vs 0.06–0.08% organic nitrogen). Soil erosion and aggregation: Less soil was lost in runoff from plots with cover crops, compared to bare fallows, in five of seven years (0–5 vs 1–19 t/ha/year), or compared to plots with conventional tillage, in two of seven years (0.1–5 vs 0.4–14). Higher stability was found in soils with cover crops, compared to bare fallows, in one of two comparisons (macroaggregates: 452–524 vs 258–333 g water-stable macroaggregates/g soil). Greenhouse gases: Similar amounts of carbon dioxide were found in soils with cover crops, bare fallows, or conventional tillage (soil respiration: 0.5–1.1 kg CO2/kg soil). Methods: There were three plots (6 x 12 m plots, with two olive trees each, on a 13% slope) for each of three treatments: cover crops (2 x 12 m barley strips, sown in October), conventional tillage (15 cm depth, 3–4 passes from September), or bare fallows (no tillage, with herbicide). Plots with cover crops were tilled before the barley was sown (10 cm depth). Runoff was collected with tipping-bucket gauges, and sediment was collected in barrels, from autumn 2000. Soil samples were collected in summer 2006 (0–10 cm depth, two samples/plot).

 

11 

A replicated, randomized, controlled study in a rainfed olive orchard in southeast Spain (years of study not reported, but same study as (5)) found more organic matter and soil organisms in soils with cover crops, compared to soils without cover crops, under olive trees. Organic matter: More organic carbon was found in soils with cover crops, compared to soils without cover crops (8.3–9.9 vs 5.4 g C/kg soil). Soil organisms: More bacteria were found in soils with cover crops, compared to soils without cover crops (950–1,400 vs 32–230 million 16S rRNA copies/g soil). Bacterial diversity was similar in soils with or without cover crops (data reported as Shannon diversity index). Methods: Herbicide was used on four control plots in autumn, but not on eight treatment plots, which had resident vegetation. The resident vegetation was controlled in spring with herbicide (four plots) or mowing (four plots). Plots had 16 olive trees each. Plots were not tilled. Foliar fertilizer was used. Two soil samples were collected in each plot (0–30 cm depth, sampling date not reported).

 

12 

A replicated, randomized, controlled study in 2004–2008 in a vineyard in northern Spain found more organic matter and nitrogen, and higher stability, in soils with cover crops, compared to conventional tillage, between the vine rows. Organic matter: More organic carbon was found in soils with cover crops, compared to conventional tillage, in three of eight comparisons (three of four comparisons at 0–5 cm depth: 8–20 vs 6 g C/kg soil). Nutrients: More nitrogen was found in soils with cover crops, compared to conventional tillage, in three of eight comparisons (0–2.5 cm: 99–107 vs 21 mg N-NH4/kg soil; 15–25 cm: 25 vs 12). Soil erosion and aggregation: Higher stability was found in soils with cover crops, compared to conventional tillage, in two of eight comparisons (0–2.5 cm depth: 37–42% vs 12% of aggregates were water-stable). Methods: There were three plots (three vine rows/plot) for each of two cover crops (sown Festuca longifolia grass or resident vegetation between the vine rows), and there were three control plots (conventional tillage between the vine rows: cultivator, 0–15 cm depth, every 4–6 weeks). No plots were fertilized, but herbicide was used under the vine rows. Soil samples were collected in June 2008 (six augers/plot, 0–25 cm depth).

 

13 

A site comparison in 2006 in two rainfed almond orchards near Granada, Spain, found more organic matter and nitrogen, less phosphorus, and lower pH in soils with cover crops (without tillage), compared to soils with conventional tillage (without cover crops). Organic matter: More organic carbon was found in soils with cover crops (without tillage), compared to soils with conventional tillage (without cover crops) (8.4–9 vs 5.4 g total organic C/kg soil). Nutrients: More nitrogen was found in soils with cover crops (without tillage), compared to soils with conventional tillage (without cover crops) (1.1 vs 0.83 g total N/kg soil), but less phosphorus was found in one of two comparisons (oat-vetch cover crop: 1.6 vs 2.1 mg available P/kg soil), and lower pH was found (8.3 vs 8.5), but similar amounts of potassium were found (148–162 vs 186 mg available K/kg soil). Soil erosion and aggregation: Higher soil stability was found in plots with cover crops (without tillage), compared to plots with conventional tillage (without cover crops), in one of two comparisons (61–62% vs 44% of aggregates were water-stable), but lower soil stability was found in two of four comparisons (15% vs 13% change in the mean weight diameter of soil aggregates after sieving). Implementation options: More phosphorus was found in soils that were cover cropped with oats, compared to oats and vetch (2.1 vs 1.6 mg available P/kg soil). Methods: Conventional tillage (chisel plough, 20–25 cm depth, 3–4 times/year in 2001–2005, October 2005, and April and June 2006) was used in one orchard, and no tillage was used in another orchard with two cover crops (oats and vetch or oats only, sown in January 2006 on one 1 ha plot each). Both orchards were fertilized (30 t compost/ha), but the orchard with cover crops got more fertilizer (1,500 kg organic fertilizer/ha on one-third of each plot, 250 kg mineral fertilizer/ha on one-third). The orchard with cover crops had cereal-fallow rotations before the cover crops, and it was tilled in November. Soil samples were collected on 18 July 2006 (0–20 cm depth). It was not clear whether these results were a direct effect of cover crops, tillage, fertilizer, or site.

 

14 

A replicated, randomized, controlled study in 2003–2005 in a vineyard in Napa Valley, California, USA, found similar amounts of carbon and carbon dioxide in soils with cover crops or resident vegetation. Implementation options: Similar amounts of carbon were found in soils with cover crops or resident vegetation (1.64 g total C/g dry soil). Similar amounts of carbon dioxide were found in soils with cover crops or resident vegetation (9.02–10.11 vs 8.40–10.99 Mg CO2-C/ha). Methods: Short-stature barley was grown as a winter cover crop on three treatment plots (518 m2 each, four vine alleys each), but not on three control plots. All plots were disked (5 cm depth) and rolled in November 2003–2004, before the cover crops were planted. Resident vegetation regrew on control plots.

 

15 

A replicated, randomized, controlled study in 2005–2007 in irrigated vineyards in Sicily, Italy, found less erosion in plots with cover crops, compared to conventional tillage (without cover crops), between the vine rows. Soil erosion and aggregation: Less erosion was found in plots with cover crops, compared to plots with conventional tillage (0–61 vs 31–89 Mg soil loss/ha). Implementation options: In plots with cover crops, the least erosion was found in plots with permanent cover crops (Trifolium clover and Festuca grass species: 0–40 Mg soil loss/ha), and the most erosion was found in plots with temporary Vicia faba cover crops (12–61 Mg/ha). Methods: There were three plots (three vine interrows/plot; 2.2 x 3 m interrows) for each of four temporary cover-crop treatments (V. faba; V. faba and V. sativa; Triticum durum; or T. durum and V. sativa), two permanent cover-crop treatments (T. subterraneum, F. rubra, and Lolium perenne, or T. subterraneum, F. rubra and F. ovina), and three control plots (conventional tillage in the interrows, 3–4 times/year, 15 cm depth). Cover crops were sown in October. Temporary cover crops were tilled into the soil in April, but permanent cover crops were not tilled. The slope of the vineyard was 16%. Erosion was measured after each significant rainfall event (15 events in November 2005–October 2007) with sediment traps (Gerlach traps: 1 m diameter, 40 litres).

 

16 

A meta-analysis from 2013 of studies from multiple Mediterranean countries found a higher percentage of organic matter in soils with cover crops, compared to bare soils. Organic matter: A higher percentage of organic carbon was found in soils with cover crops, compared to bare soils (10% higher). The Web of Knowledge database was searched, using the keywords, “Mediterranean”, “soil”, and “conventional”, and 13 data sets from 10 studies of cover cropping were found and meta-analysed. The most recent studies included in this meta-analysis were published in 2011. It was not clear how many of these studies were from arable fields, orchards, or vineyards.

 

17 

A replicated, randomized, controlled study in 2008–2011 in an irrigated avocado orchard in Chile found more organic matter and nitrogen, higher pH, less erosion, and higher stability in soils with cover crops, compared to bare soils, between avocado rows. Organic matter: More organic matter was found in soils with cover crops, compared to bare soils (2.6–2.8% vs 2.1–2.2%). Nutrients: More nitrogen (1.0–1.1 vs 0.8–0.9 mg total N/g soil), but similar amounts of nitrate (11–59 vs 8–67 mg NO3/kg soil), and more potassium (162–197 vs 100–122 mg K/kg soil), but similar amounts of phosphorus (11–15 vs 9–10 mg P/kg soil), were found in soils with cover crops, compared to bare soils. Higher pH was found in soils with cover crops, compared to bare soils (6.8–7.2 vs 6.6–6.9). Soil erosion and aggregation: Less soil was lost in runoff water from plots with cover crops, compared to bare soil (0 vs 1,000–3,400 vs kg/ha/year). More stable soils were found in plots with cover crops, compared to bare soil (32–39% vs 22–29% of soil aggregates were water-stable). Methods: Cover crops were grown in five treatment plots, and bare soil was maintained with herbicide in five control plots, in an avocado orchard, on a 47% slope (10 x 50 m plots). The groundcover (Lolium rigidum ryegrass and a legume, Medicago polymorpha) was sown in August 2008 and mown in February 2009–2010 (residues were retained). All plots were fertilized and irrigated. Soil samples were collected along the tree rows in winter 2009–2011 (0–10 cm depth, 2 cm diameter soil cores). Soil loss was measured in runoff water, in buried barrels downslope of each plot.

 

18 

A replicated, randomized, controlled study in 2003–2005 in eight rainfed olive orchards in southern Spain found less erosion in plots with cover crops, compared to tilled plots. Soil erosion and aggregation: Less soil was lost in runoff from plots with cover crops, compared to tilled plots, on seven of eight farms (63–89% less soil). Methods: On each of eight farms, cover crops were grown (two of eight farms) or weeds were not controlled (six of eight farms) on three plots, but weeds were controlled by conventional tillage (depths not reported) on three plots (1 m2 microplots). Plots were surrounded by steel sheets, which routed the runoff into plastic containers. Soil loss was measured in water samples, after each rainfall event.

 

19 

A replicated, randomized, controlled study in 2008–2010 in an irrigated vineyard in the San Joaquin Valley, California, USA, found similar amounts of nitrogen in different treatments. Implementation options: Similar amounts of nitrogen were found in soils with cover crops or resident vegetation (amounts of nitrogen not reported), in soils with different types of cover crops (oats only, or oats and legumes: amounts of nitrogen not reported), and in soils with or without tillage (amounts of nitrogen not reported). Methods: Either seeded cover crops or resident vegetation was grown between the vine rows on 16 plots each (two vine rows/plot, 190 vines/row). The cover crops were either oats or oats and legumes, on eight plots each, seeded in November. The plots were mown in spring, before tillage. No tillage was used on half of the plots, and conventional tillage was used on the other half. A disk plough (15–20 cm depth) was used for conventional tillage, in spring, summer (three times), and autumn. Herbicide was used to control weeds in the vine rows (50 cm width). Soil samples were collected in spring, before mowing and tillage (five soil cores/plot, on 40 m transects; depths not reported).

 

20 

A replicated, randomized, controlled study in 2009–2011 in a rainfed vineyard in northern Spain found more nitrogen and ammonium in soils with cover crops, compared to conventional tillage, between the vine rows, but found inconsistent differences in nitrate. Nutrients: More nitrogen was found in soils with cover crops, compared to conventional tillage, in one of 12 comparisons (clover, 0–15 cm depth: 2,050 vs 1,900 kg total N/ha), and more ammonium was found in two of 12 comparisons (clover, 0–15 cm depth: 5.1–6.3 vs 3.5–4.8 kg NH4-N/ha). Less nitrate was found in soils with cover crops, compared to conventional tillage, in some comparisons, but more nitrate was found in other comparisons (2–35 vs 2–26 kg N-NO3/ha). Implementation options: Less nitrate (0–15 cm depth: 2–5 vs 11–35 kg N-NO3/ha; 15–45 cm depth: 2–9 vs 15–50), and less ammonium at one of two depths (0–15 cm depth: 3–5 vs 5–6.3 kg NH4-N/ha), were found in soils that were cover cropped with grass (barley) compared to legumes (clover), after one year of cover cropping, but not before. Methods: There were three plots (four vine rows/plot, 20 vines/row) for each of two cover crops (Hordeum vulgare barley or Trifolium resupinatum Persian clover between the vine rows, sown in February 2009 and 2011), and there were three control plots (conventional tillage between the vine rows: disk plough, 0–15 cm depth, every 4–6 weeks in February–August). No plots were fertilized. Herbicides were used under the vine rows. Vine prunings were retained between the rows. Soil samples (0–45 cm depth, three samples/plot) were collected five times/season (April–September).

 

21 

A replicated site comparison in 2009 in rainfed vineyards in southern France found more organic matter, phosphorus, and soil organisms in soils with ground cover, compared to bare soils. Organic matter: More organic carbon was found in soils with ground cover, compared to bare soils, in three of six comparisons (permanent cover crops: 12–20 vs 6–14 g C/kg soil). Nutrients: More phosphorus was found in soils with ground cover, compared to bare soils, in one of six comparisons (permanent ground cover: 11 vs 7 mg available P/kg soil). Similar amounts of nitrogen and potassium, and similar pH levels, were found in soils with ground cover, compared to bare soils (data not reported). Soil organisms: More microbial biomass (measured as carbon) was found in soils with ground cover, compared to bare soils, in four of six comparisons (50–140 vs 30–90 mg C/kg soil), and more nematodes were found in one of three comparisons (747–1,371 vs 351 total nematodes/100 g soil). Implementation options: More organic carbon was found in soils with permanent ground cover, compared to temporary ground cover, in one of three comparisons (18 vs 13 g C/kg soil), and more phosphorus was found in one of three comparisons (11 vs 7 mg available P/kg soil). More microbial biomass (measured as carbon) was found in soils with permanent ground cover, compared to temporary, in two of three comparisons (120–150 vs 90–120 mg C/kg soil), and more nematodes were found in one of three comparisons (1,371 vs 747 total nematodes/100 g soil). Methods: In 146 plots of three soil types, there was permanent vegetation (4–22% of plots in each soil type), temporary vegetation (48–68%), or bare soil (16–42%) between the vine rows, for at least five years before soil sampling. Soil samples were collected from the interrows in March–May 2009 (10 homogenized samples/plot, 0–15 cm depth).

 

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A meta-analysis from 2016 of 24 studies in orchards and vineyards in Spain found more organic matter in soils with winter cover crops, compared to soils with conventional tillage. Organic matter: More organic carbon was found in soils with cover crops, compared to conventional tillage (data reported as the response ratio: 1.35). Methods: The Scopus database was searched for publications in January 2016, using the keywords, “olive” or “vineyard” or “almond” or the scientific names of these species, and the keywords “soil organic carbon” or “soil organic matter”. Together with publications from another meta-analysis(16), 24 replicated, controlled studies from 2005 to 2015 were meta-analysed. In these studies, soil samples were collected from depths of 0–10 to 0–90 cm in almond orchards, olive orchards, and vineyards in Mediterranean climates in Spain. Plots with cover crops mostly had resident vegetation over the winter, which was controlled by mowing, grazing, or using herbicide in the spring, or reduced tillage in spring and autumn. In plots without cover crops, resident vegetation was controlled throughout the year by using herbicide and/or conventional tillage. It was not clear whether these results were a direct effect of cover crops, tillage, herbicide, mowing, or grazing.

 

Referenced papers

Please cite as:

Shackelford, G. E., Kelsey, R., Robertson, R. J., Williams, D. R. & Dicks, L. V. (2017) Sustainable Agriculture in California and Mediterranean Climates: Evidence for the effects of selected interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.