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Action: Soil: Grow cover crops in arable fields Mediterranean Farmland

Key messages

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Organic matter (12 studies): One meta-analysis of studies from Mediterranean-type climates and ten replicated, controlled studies (nine randomized, two before-and-after) from Italy, Spain, and the USA found more organic matter (mostly measured as carbon) in soils with winter cover crops, compared to soils without them, in some or all comparisons. One replicated, randomized, controlled, before-and-after study from Italy found inconsistent differences in organic matter in soils with or without winter cover crops (sometimes more, sometimes less).

Nutrients (22 studies)

  • Nitrogen (21 studies): Ten replicated, randomized, controlled studies (two before-and-after) from Italy, Spain, and the USA found more nitrogen in soils with winter cover crops, compared to soils without them, in some comparisons. One replicated, randomized, controlled study from the USA found less nitrogen in soils with winter cover crops, compared to soils without them. Ten replicated, controlled studies (nine randomized, two before-and-after) from Italy, Spain, and the USA found inconsistent differences in nitrogen (sometimes more, sometimes less) between soils with or without winter cover crops (but see the paragraphs, below, for distinctions between different forms of nitrogen).
  • Phosphorus (1 study): One replicated, randomized, controlled study from the USA found similar amounts of phosphorus in soils with or without winter cover crops.
  • Potassium (1 study): One replicated, randomized, controlled, before-and-after study from the USA found an increase in potassium in soils with winter cover crops, and no increase in soils without them.

Soil organisms (12 studies)

  • Microbial biomass (6 studies): Five replicated, randomized, controlled studies from the USA found more microbial biomass in soils with cover crops, compared to soils without them, in some or all comparisons. One replicated, randomized, controlled, before-and-after study from Italy found inconsistent differences in microbial biomass (sometimes more, sometimes less) between soils with or without winter cover crops.
  • Nematodes (2 studies): Two replicated, randomized, controlled studies from the USA found more nematodes in soils with cover crops, compared to soils without them, in some comparisons. One of these studies also found a higher ratio of bacteria-feeding nematodes to fungus-feeding nematodes in soils with cover crops, compared to soils without them.
  • Earthworms (2 studies): One replicated, controlled study from the USA found more earthworms in soils with winter cover crops, compared to soils without them. One replicated site comparison from the USA found similar numbers of earthworms in soils with or without winter cover crops.
  • Bacteria and fungi (2 studies): One replicated, randomized, controlled study from Spain found more bacteria and fungi in soils with winter cover crops, compared to soils without them, in some comparisons. One replicated, controlled study from Italy found more spores and species of beneficial fungi (mycorrhizae) in soils with winter cover crops, compared to soils without them, in some comparisons.

Soil erosion and aggregation (4 studies)

  • Soil erosion (2 studies): Two controlled studies (one replicated and randomized) from Israel and the USA found less erosion of soils with cover crops, compared to soils with fallows or bare soils.
  • Soil aggregation (2 studies): Two replicated, randomized, controlled studies from Spain and the USA found more water-stable soil aggregates in plots with winter cover crops, compared to plots without them, in some or all comparisons.

Greenhouse gases (5 studies)

  • Carbon dioxide (5 studies): Three controlled studies (two replicated and randomized) from Italy and the USA found similar amounts of carbon dioxide in soils with or without cover crops. Two replicated, randomized, controlled studies from the USA found more carbon dioxide in soils with cover crops, compared to soils without them, in some comparisons.
  • Carbon storage (1 study): One replicated, randomized, controlled study from Italy found more carbon accumulation in soils with cover crops, compared to soils without them, in some comparisons.
  • Nitrous oxide (2 studies): One replicated, randomized, controlled study from the USA found more nitrous oxide in soils with cover crops, compared to soils without them, in some comparisons. One controlled study from the USA found similar amounts of nitrous oxide in soils with cover crops or fallows.

Implementation options (9 studies): Five studies from Italy, Spain, and the USA found more nitrogen in soils that were cover cropped with legumes, compared to non-legumes. One study from the USA found inconsistent differences in nitrogen (sometimes more, sometimes less) between soils with different cover crops. One study from the USA found no differences in phosphorus or microbial biomass between soils with different cover crops. One study from Italy found differences in beneficial fungi (mycorrhizae) between plots with different cover crops. One study from Spain found higher soil quality in plots with long-term cover crops, compared to short-term.

Supporting evidence from individual studies

1 

A replicated, randomized, controlled study in 1986–1988 in an irrigated lettuce field in the Salinas Valley, California, USA, found less ammonium in plots with winter cover crops, compared to winter fallows. Nutrients: Less ammonium was found in soils with cover crops, compared to fallows, in at least one of eight comparisons (after harvesting the spring crop, in plots that were side-dressed with fertilizer: 4.4 vs 5.2 ppm NH4-N). Similar amounts of nitrate were found in soils with or without cover crops (in March 1998: 11–27 vs 29 ppm NO3-N). Methods: There were six plots (10.7 x 1.1 m raised beds) for each of two winter cover crops (broad beans or rye) and six control plots (bare fallow, maintained with herbicide). The cover crops were seeded in November 1986–1987, irrigated until emergence, and chopped, disked, and chisel ploughed in spring (25–30 cm depth). Lettuces were planted in May and July 1987 and March and August 1988, and they were harvested in July and October 1987 and June and October 1988. The lettuces were irrigated (1–2 cm every 2–3 days until emergence, then 2 cm/week), and some lettuce plots were fertilized (110–220 kg N/ha in total; up to 110 kg N/ha as side-dressing). Soil samples were collected in March, June, August, and September 1988 (0–22 cm depth, 6 cm diameter, four samples/plot).

 

2 

A replicated, randomized, controlled before-and-after study in 1989–1991 in an irrigated lettuce field in Salinas, California, USA, found less nitrate, and nitrate depletion, in soils with winter cover crops, compared to bare fallows. Nutrients: At the beginning of spring, less nitrate was found in soils with cover crops, compared to bare fallows, in some comparisons (all cover crops in 1990: 2–6 vs 18–21 µg NO3-N/g dry soil; one of two cover crops in 1991: 66–79 vs 85–112). After the first rainfall in spring, more nitrate was found in soils with winter cover crops, compared to bare fallows (amounts of nitrate not clearly reported). The inference was that more nitrate was depleted by cover crops over winter, and more nitrate was leached from bare fallows in spring. In early spring, more ammonium was found in soils with winter cover crops, compared to bare fallows (0–15 cm: 2–6 vs 0–1 µg NH4-N/g soil), but similar amounts were found later in the spring (0–15 cm: 0.5 µg), in 1991. In the lettuce-growing season, similar amounts of nitrate, ammonium, and mineralizable nitrogen were found in plots with winter cover crops or bare fallows (0–60 cm: 9–60 µg NO3-N/g dry soil; 0–15 cm: 0.2–0.8 µg NH4-N/g dry soil; 0–15 cm: 3–6 µg mineralizable N/g dry soil). Methods: In 1989–1990, six winter cover crops (Raphanus sativus oilseed radish, Brassica hirta white senf mustard, Brassica alba white mustard, Lolium multiflorum annual ryegrass, Secale cerale Merced rye, and Phacelia tanacetifolia) were grown on three plots each (two 12 m rows/plot), and bare fallows were maintained (with herbicide and hand cultivation) on three plots. In 1990–1991, two winter cover crops (Secale cerale Merced rye and Phacelia tanacetifolia) were grown on six plots each (two 8 m rows/plot), and bare fallows were maintained on six plots. Cover crops were tilled into the soil (15–20 cm depth in March 1990, depth not reported in February 1991). Lettuce was sown in April 1990–1991. All plots were irrigated and fertilized (56–85 kg N/ha, before sowing lettuce). Soil samples were collected in November 1989–1990, January 1990–1991, February 1991, and March 1990 (0–60 cm depth, 4 cm diameter, two cores/plot), weekly from late March to the end of June 1990 (0–15 cm depth), and every 2–7 days from mid-February to the end of March 1991 (0–15 cm depth).

 

3 

A replicated, randomized, controlled study in 1991–1992 in an irrigated lettuce field in the Salinas Valley, California, USA, found less nitrate, but more ammonium, mineralizable nitrogen, and microbial biomass, in soils with winter cover crops, compared to bare soils. Nutrients: Less nitrate, more ammonium, and more mineralizable nitrogen were found in soils with winter cover crops, compared to bare soils, before the cover crops were incorporated into the soil (4.3 vs 8.6 g NO3-N/m2, 0–60 cm depth; 1.8 vs 1.1 g NH4-N/m2, net mineralizable nitrogen, 0–30 cm depth; 0.26 vs 0.24 g NH4-N/m2, 0–60 cm depth), and also in some comparisons after the cover crops were incorporated (1–6 vs 3–24 µg NO3-N/g dry soil, 0–15 cm depth; 3–38 vs 1–21 µg NH4-N/g dry soil, net mineralizable nitrogen, 0–15 cm depth; 0.17–0.98 vs 0.10–0.68 µg NH4-N/g dry soil; 1–15 cm depth; number of significantly different comparisons not clearly reported). Soil organisms: More microbial biomass (measured as nitrogen) was found in soils with winter cover crops, compared to bare soils, in some comparisons (14–29 vs 6–14 µg N/g dry soil). Methods: Three plots had winter cover crops (Merced rye Secale cereale, sown on 19 December 1991) and three plots had bare soils over winter. The plots (raised beds) were 8 x 4 m each. All plots were disked on 8 April (incorporating the cover crops). Soil samples were collected 6 days before the cover crops were incorporated, and on 7–9 days between cover-crop incorporation and lettuce harvesting. Lettuce was sown on 8 May and harvested on 8 July 1992.

 

4 

A replicated, randomized, controlled study in 1992–1993 in an irrigated broccoli field in the Salinas Valley, California, USA, found more mineralizable nitrogen and more microbial biomass in soils with winter cover crops, compared to soils without winter cover crops, but found inconsistent effects on nitrate. Nutrients: More mineralizable nitrogen was found in soils with cover crops, compared to bare soils, in five of 14 comparisons (8.5–17 vs 3–9 µg NH4-N/g dry soil, 0–15 cm depth), but there were similar amounts of ammonium in soils with or without cover crops (0.5–4.2 µg NH4-N/g dry soil, 0–15 cm depth). Less nitrate was found in soils with cover crops, in three of nine comparisons at the end of the cover-cropping season (March–April: 0–7 vs 3–10 µg NO3-N/g dry soil, 0–15 cm depth), but more was found in one of nine comparisons (4 vs 2 µg). Soil organisms: More microbial biomass (measured as carbon) was found in soils with cover crops, compared to bare soils, in seven of 14 comparisons (150–500 vs 70–150 µg C/g dry soil, 0–15 cm depth), and more microbial biomass (measured as nitrogen) was found in two of 14 comparisons (11–30 vs 5–12 µg N/g dry soil). Methods: There were three plots for winter cover crops (half Phacelia tanacetifolia phacelia and half Secale cereale Merced rye, sown in November 1992 and mown in March 1993) and three control plots with bare soil in winter. All plots (252 x 24 m) were tilled in March 1993 (15 cm depth), and the cover crops were incorporated into the soil. Two broccoli crops were grown on raised beds (first crop: April–August 1993; second crop: August–November 1993). All plots were irrigated (440–450 mm/crop, subsurface drip irrigation) and fertilized (41–42 g N/m2/crop). Soil samples were collected 16 times in November 1992–August 1993, including nine samples in March–April, when the cover crops were incorporated (0–75 cm depth, 6 cm diameter, four cores/plot).

 

5 

A replicated, randomized, controlled study in 1991–1994 in an irrigated tomato field in the San Joaquin Valley, California, USA, found more organic matter and nitrogen, and higher soil stability, in soils with winter cover crops, compared to winter fallows. Organic matter: More organic matter was found in soils with cover crops, compared to fallows, in some comparisons in the spring (e.g., spring 1994: 1.05–1.15% vs 0.70%, 0–15 cm depth; number of significantly different comparisons not clearly reported). Nutrients: More nitrogen was found in soils with cover crops, compared to fallows, in some comparisons in the spring (e.g., spring 1993: 0.75% vs 0.90% total nitrogen, 0–15 cm depth; number of significantly different comparisons not clearly reported). Soil erosion and aggregation: More stable soils were found in plots with cover crops, compared to fallows (data on percentage of water-stable aggregates reported as model results). Methods: There were four plots (93 x 7 m plots) for each of three winter cover crops and one control (winter fallow). The cover crops were Hordeum vulgare barley, Vicia dasycarpa Lana woollypod vetch, or a barley-vetch mixture, seeded in October 1991–1993 and incorporated into the soil in March 1992–1994 (15–20 cm depth, rotary tiller). Soil samples were collected in spring and autumn (0–15 cm depth).

 

6 

A replicated, controlled study in 1996–1998 in an irrigated tomato field in the San Joaquin Valley, California, USA (same study as (7)), found more soil carbon and earthworms in plots with winter cover crops (and no tillage), compared to plots with bare fallows (and tillage in spring). Organic matter: More soil carbon was found in plots with cover crops, compared to fallows (0.66–0.72% vs 0.62% carbon, 0–0.6 inches depth). Soil organisms: More earthworms were found in plots with cover crops, compared to fallows (2.1 vs 0.6 earthworms/square foot). Methods: There were 12 plots (4.5 x 27.5 m plots) for each of two treatments (two grass-legume mixtures as winter cover crops, sown in October 1996–1997, killed and retained as mulch, with no tillage, in March 1997–1998) and there were 12 control plots (bare fallow in winter, with herbicide, and conventional tillage in spring). Soil carbon was sampled in September 1998 (eight subsamples/plot, 0–0.6 inches depth). Earthworms were sampled in March 1998 (two cylinders/plot, 16.5 inches diameter, 6 inches depth, sprinkled with mustard powder so that earthworms would come to the surface). It was not clear whether these results were a direct effect of cover crops or tillage.

 

7 

A replicated, controlled study in 1996–1998 in an irrigated tomato field in the San Joaquin Valley, California, USA (same study as (6)), found less nitrate in winter and spring, but more nitrate in summer, in plots with winter cover crops (and no tillage in spring), compared to plots with bare fallows (and tillage in spring). Nutrients: Less nitrate was found in plots with cover crops, compared to fallows, when measured in winter or spring (19 of 32 comparisons: 0.9–4.1 vs 3.8–7.9 ppm, 0–30 cm depth), but more nitrate was found when measured in summer (27 of 32 comparisons: 21–41 vs 8–14 ppm, 0–30 cm depth). Methods: There were 12 plots (4.5 x 27.5 m plots) for each of four treatments (two grass-legume mixtures, or two legumes without grasses, as winter cover crops, sown in October 1996–1997, killed and retained as mulch, with no tillage, in March 1997–1998) and each of two controls (bare fallows in winter, with or without herbicide, and conventional tillage in spring). Tomato seedlings were transplanted in April 1997–1998. The tomatoes were irrigated (two inches/week) and fertilized (0, 100, or 200 lb N/acre, in March 1997 and May 1998). Soil nitrate was sampled four times in 1998 (0–30 cm depth, three samples/plot). It was not clear whether these results were a direct effect of cover crops or tillage.

 

8 

A replicated, randomized, controlled study in 1995–1998 in an irrigated tomato field in Davis, California, USA, found more nitrogen, more nematodes, and a higher proportion of bacteria-feeding nematodes, in soils with cover crops, compared to soils without cover crops. Nutrients: More nitrogen was found in soils with winter cover crops, compared to plots without cover crops, in two of three comparisons (10–31 vs 7–16 µg N/g dry soil, cumulative). Soil organisms: A higher proportion of bacteria-feeding nematodes, compared to fungus-feeding nematodes, were found in soils with cover crops, compared to soils without cover crops, in four of six comparisons (data reported as the Channel Index). Similar numbers of nematodes were found in soils with or without winter cover crops, in 1995–1996 (bacterial or fungal feeders: 3,100–9,800 vs 3,800–8,300 nematodes/litre soil). More nematodes were found in soils with summer cover crops, compared to soils without cover crops, in one of two comparisons in 1995–1996 (bacterial feeders: 12,000 vs 3,900 nematodes/litre soil). More nematodes were found in soils with summer and/or winter cover crops, compared to soils without cover crops, in four of six comparisons in 1996–1997 (bacterial and fungal feeders, in plots with irrigation: 2,300–3,600 vs 400–500 nematodes/litre soil), but similar numbers were found in soils with or without winter cover crops in 1997–1998 (bacterial and fungal feeders: 1,200–3,800). Similar numbers of other nematodes (omnivores and predators) were found in soils with or without cover crops (data not reported). Greenhouse gases: Similar amounts of carbon dioxide were found in soils with or without cover crops (soil basal respiration in 1995–1996: 10–13 µg CO2/g dry soil/hour). Methods: Cover crops were planted in different numbers of plots in different years (1995–1996: 16 plots with winter cover crops, eight plots with summer cover crops, 16 control plots without cover crops; 1996–1997: 12 winter, four summer, eight controls; 1997–1998: 28 summer and/or winter, four controls). Plots were 3–4 beds wide and 10 m long. Some summer cover crops were retained over winter, and some were mown and replaced with winter cover crops. Summer cover crops were mixtures of oats and legumes, planted in August–September. Winter cover crops were legumes (Vicia sativa common vetch), planted in November. In spring, cover crop residues were mown and either removed or evenly distributed among all plots and incorporated into the soil. Some plots were irrigated during the cover-cropping seasons. All plots were irrigated during the tomato-growing season. Herbicide was used on all plots, but no inorganic fertilizer was used. Soil samples (16 soil cores/plot, 30 cm depth, 2.5 cm diameter) were collected at different times for nutrients (once per week, 1–7, 11, and 14 weeks after incorporating cover crop residues, in spring), greenhouse gases (after 1, 4, and 7 weeks), or soil organisms (four times in summer/autumn, and 1, 4, 7, 11, 14, and 18 weeks after incorporating residues), in 1995–1996. In 1996–1998, soil samples for nutrients and soil organisms were collected less frequently (1996–1997: four times in spring; 1997–1998: once in autumn, once in spring, and once when tomatoes were harvested).

 

9 

A replicated, randomized, controlled study in 1998–2000 in an irrigated vegetable field in the Salinas Valley, California, USA, found more organic matter, more microbial biomass, less nitrate, and/or less ammonium in soils with winter cover crops, compared to soils without cover crops, in most comparisons. More ammonium was found in two of 12 comparisons. Organic matter: More carbon was found in soils with cover crops (15 vs 14 g total C/kg soil; 0–15 cm depth). Nutrients: More total nitrogen was found in soils with cover crops (1.6 vs 1.5 g total N/kg soil; 0–15 cm depth). At depths of 0–90 cm, less nitrate was found in soils with cover crops (4–54 vs 5–64 g NO3–N/g soil), and less nitrate was also found at depths of 0–15 cm, in seven of 12 comparisons (2–18 vs 3–64 μg NO3-N/g soil). At depths of 0–15 cm, less ammonium was found in soils with cover crops, in six of 12 comparisons (1–4 vs 5–7 μg NH4–N/g soil), but more ammonium was found in two of 12 comparisons (4–7 vs 1–4). Soil organisms: More microbial biomass (measured as carbon) was found in soils with cover crops, in 10 of 12 comparisons (120–220 vs 80–130 μg C/g soil). More microbial biomass (measured as nitrogen) was found in soils with cover crops, in 11 of 12 comparisons (14–27 vs 5–17 μg N/g soil). Methods: There were four plots (0.52 ha), for each of four treatments (reduced tillage or conventional tillage, with or without added organic matter). In plots with added organic matter, compost was added two times/year, and a cover crop (Secale cereale Merced rye) was grown every autumn or winter. Lettuce or broccoli crops were grown on raised beds. Sprinklers and drip irrigation were used in all plots. Soils were disturbed to different depths (conventional tillage: disking to 50 cm depth, cultivating, sub-soiling, bed re-making, and bed-shaping; reduced tillage: cultivating to 20 cm depth, rolling, and bed-shaping). Soils were collected, along the planting line, with 6 cm soil cores. It was not clear whether these results were a direct effect of adding compost or growing cover crops.

 

10 

A replicated, randomized, controlled, before-and-after study in 1999–2004 in an irrigated tomato-cotton field in the San Joaquin Valley, California, USA (same study as (27)), found more carbon and potassium in soils after four years with winter cover crops, but found less carbon and no changes in potassium after four years without cover crops. Cover crops had inconsistent effects on nitrogen in soils. Organic matter: Carbon increased in soils with cover crops, after four years (before: 10,000 lb/acre; after: 12,000), and decreased in soils without cover crops, after four years (before: 10,000; after: 9,000). Nutrients: After four years, nitrogen increased in soils with cover crops (before: 1,300 lb/acre; after: 1,400–1,600), but decreased in soils without cover crops, in one of two comparisons (before: 1,400; after: 1,300), and increased in one of two comparisons (before: 1,300; after: 1,600). After four years, nitrate did not change in soils with cover crops (before: 16–19 ppm; after: 10–16), but increased in soils without cover crops, in one of two comparisons (before: 18; after: 25). After four years, potassium increased in soils with cover crops (before: 258–271 ppm; after: 314–319), but did not change in soils without cover crops (before: 271–278; after: 300–303). Methods: Rainfed winter cover crops (triticale, rye, and vetch) were planted on 16 treatment plots, but not on 16 control plots, in October 1999–2003. Crop residues were chopped in March. The plots (9 x 82 m) had six raised beds each. Tomatoes were grown in rotation with cotton. Fertilizer and herbicide was used in all plots, and tomatoes and cotton were irrigated. Soil samples were collected in spring (before planting) and in autumn (after harvest), in 2000–2004 (0–30 cm depth; number and volume of samples not reported).

 

11 

A replicated, randomized, controlled study in 2001–2004 in an irrigated maize field in southwest Spain found more organic matter, nitrogen, and microorganisms, and higher soil stability, in soils with winter cover crops, compared to soils without cover crops. Organic matter: Similar amounts of organic carbon were found in soils with short-term cover crops, compared to soils without cover crops (0–30 cm depth: 6–13 g C/kg soil), but more organic carbon was found in soils with long-term cover crops, compared to soils without cover crops, in eight of nine comparisons (8–32 vs 6–13). Nutrients: Similar amounts of nitrogen were found in soils with short-term cover crops, compared to soils without cover crops (0–30 cm depth: 0.07–0.15 g total N/kg soil), but more nitrogen was found in soils with long-term cover crops, compared to soils without cover crops, in eight of nine comparisons (0.08–0.25 vs 0.07–0.13). Soil erosion and aggregation: Higher soil stability was found in plots with short-term cover crops, compared to plots without cover crops, in two of nine comparisons (0–5 cm depth, in 2002–2003: 57%–79% vs 44–69% of aggregates were water-stable), and higher soil stability was also found in plots with long-term cover crops, compared to plots without cover crops, in eight of nine comparisons (57–88% vs 26–69%). Soil organisms: More microorganisms were found in soils with short-term cover crops, compared to soils without cover crops, in one of three years (2003: 662 vs 470 colony forming units/g dry soil), and more microorganisms were also found in soils with long-term cover crops, compared to soils without cover crops, in two of three years (576–694 vs 350–470). Implementation options: More organic carbon in eight of nine comparisons (8–32 vs 6–13 g C/kg soil), more nitrogen in seven of nine comparisons (0.08–0.23 vs 0.07–0.15 g total N/kg soil), higher stability in six of nine comparisons (58–75% vs 29–48% of aggregates were stable), and more microorganisms in one of three years (2002: 576 vs 389 colony forming units/g dry soil) were found in soils with long-term cover cropping, compared to short-term. Methods: Cover crops (Avena strigosa lopsided oats) were sown on eight plots in September 2001–2003. Four of these plots had winter cover crops for six years before this (long-term cover crops), and four plots did not (short-term cover crops). Four other plots did not have winter cover crops from 2001–2004 or before. All plots were 20 x 10 m. Cover crops were suppressed with herbicide in April 2002–2004. For organic carbon, nitrogen, and aggregate stability, soil samples were collected in March, June, and October 2002–2004 (three samples/plot, 0–30 cm depth). For microorganisms (bacteria and fungi), soils samples were collected every two months (0–5 cm depth).

 

12 

A replicated, randomized, controlled study in 2005–2007 in an irrigated tomato-maize field in Davis, California, USA, found more carbon, ammonium, and nematodes in soils with winter cover crops, compared to bare fallows. Organic matter: More carbon was found in soils with cover crops, compared to bare fallows (1.04% vs 0.94% total carbon). Nutrients: Similar amounts of nitrate and total nitrogen were found in soils with cover crops or bare fallows (7 vs 5–8 NO3-N ppm; 0.11% vs 0.10% total nitrogen), but more ammonium was found in soils with cover crops, for one of three mixtures of cover crops (legumes: 6 vs 5 NH4-N ppm). Soil organisms: More nematodes were found in soils with cover crops, compared to bare fallows, for two of three mixtures of cover crops (mixtures with legumes: 588–617 vs 435 nematodes/100 g soil). Implementation options: Similar amounts of carbon, total nitrogen, and nitrate, and similar numbers of nematodes, were found in soils with different mixtures of cover crops (0.99–1.04% total carbon; 0.11% total nitrogen; 5–8 NO3-N ppm; 512–617 nematodes/100 g soil), but more ammonium was found in soils that were cover cropped with legumes, compared to grains or a mixture of legumes and grains (6 vs 5 NH4-N ppm). Methods: Three mixtures of winter cover crops (legumes only, legumes and grains, or grains only) were grown on five plots each, and five control plots were bare fallows on which weeds were controlled by burning (111 m2 plots; six raised beds/plot). Tomatoes were grown in 2006, and maize was grown in 2007, without fertilizer. Soil samples were collected in May and September 2006–2007 (four times in the spring of 2007), with soil cores (12 cores/plot, 15 cm depth, 2.5 cm width).

 

13 

A replicated site comparison in 2004–2005 in nine irrigated tomato fields in the Sacramento Valley, California, USA, found similar numbers of earthworms in fields with winter cover crops or bare fallows. Soil organisms: Similar numbers of earthworms were found in fields with cover crops or fallows (26 vs 19 g earthworms/m2). Methods: Earthworms were collected from nine tomato fields (five fields with cover crops, four with bare fallows; three 30 cm3 soil pits/field), in February–April 2005. Organic matter and nutrients were measured in horizontal soil cores, collected from the walls of the soil pits (0–15 cm length). All fields were tilled in 2004, after the tomatoes were harvested, and before the cover crops were planted. The cover crops were legumes. All fields were fertilized and irrigated.

 

14 

A replicated, randomized, controlled study in 2006–2007 in an irrigated tomato field near Davis, California, USA, found more nitrate, higher greenhouse-gas emissions, and more carbon in soils with winter cover crops, compared to soils without cover crops. Organic matter: More carbon was found in soils with cover crops (maximum: 1.3% of soil was carbon), compared to those without cover crops (minimum: 1.1%). Nutrients: More nitrate was found in soils with cover crops, in five of seven comparisons (March–September: 20–70 vs 10–60 µg nitrate/g soil). Similar amounts of total nitrogen were found in soils with or without cover crops (0.1% of soil was nitrogen). Greenhouse gases: Higher nitrous oxide emissions were found in soils with cover crops, in two of four comparisons (80–150 vs 25–45 µg N2O/m2/hour), and higher carbon dioxide emissions were found in one of four comparisons (350 vs 215 mg CO2/m2/hour). Methods: Legume cover crops (Vicia villosa hairy vetch and Lathyrus hirsutus Australian winter peas) were grown on eight treatment plots, but not on eight control plots (0.075 ha plots). Cover crops were mown in late April, and mulched and incorporated into the soil in early May. All plots were irrigated and fertilized. Greenhouse gases were measured at least every 10 days in the growing season and every 2–3 weeks in the rainy season (three chambers/plot). Soil samples were collected every three weeks in the growing season, but less frequently in the rainy season (0–30 cm depth, 2.54 cm diameter soil cores).

 

15 

A replicated, randomized, controlled study in 2006–2008 in an irrigated maize field in the Ebro river valley, Spain, found less nitrogen in soils with winter cover crops, compared to bare soils, in spring, but found more nitrogen in autumn. Nutrients: In spring (after the cover crops), less nitrogen was found in soils with cover crops, compared to bare soils, in 19 of 20 comparisons (1–11 vs 3–43 mg inorganic N/kg soil, 0–120 cm depth). However, in autumn (after the maize was harvested), more nitrogen was found in plots with cover crops, compared to bare soils, in two of 20 comparisons (barley as the cover crop, 0–30 cm depth: 14–15 vs 4–7 mg inorganic N/kg soil). Methods: There were three plots (5.2 m2) for each of three winter cover crops (Hordeum vulgare barley, Brassica rapa winter rape, or Vicia sativa common vetch, sown in October 2006–2007), and three control plots with bare soil in winter. Similar amounts of nitrogen were added to all plots (300 kg N/ha), but less of it came from mineral fertilizer in plots with cover crops, to compensate for the organic nitrogen that was added to these plots when the cover crop residues were tilled into the soil. All plots were tilled in spring (March 2007–2008) and autumn (October 2006–2007). All plots were irrigated twice/week (drip irrigation, based on evapotranspiration). Maize was planted in April and harvested in October 2007–2008. Soil samples were collected before the cover crops were incorporated and after the maize was harvested (two soil cores/plot, 5 cm diameter, 0–120 cm depth). It was not clear whether these results were a direct effect of cover cropping or adding fertilizer.

 

16 

A replicated, randomized, controlled study in 2005–2006 in an irrigated, organic tomato field in Yolo County, California, USA, found less nitrate, more ammonium and microbial biomass, and higher carbon dioxide emissions in soils with winter cover crops, compared to winter fallows. Nutrients: Less nitrate was found in soils with cover crops, compared to fallows, in two of five comparisons (7 days after planting tomatoes: 3.3 vs 5.3 g N/m2; 32 days before: 0.2 vs 0.5). More ammonium was found in soils with cover crops, compared to fallows, in one of five comparisons (7 days after planting: 2.1 vs 1.7 g N/m2). Similar amounts of potentially mineralizable nitrogen were found in soils with cover crops or fallows (4.1–8.8 g N/m2). Soil organisms: More microbial biomass (measured as carbon) was found in soils with cover crops, compared to fallows, in one of four comparisons (7 days after planting: 95 vs 75 g C/m2). Greenhouse gases: More carbon dioxide was emitted from plots with cover crops, compared to fallows (28 days after planting: 223 vs 140 mg CO2-C/m2/hour). Methods: The field was levelled and fertilized (17 Mg compost/ha). Eight plots had winter cover crops (mustard Brassica nigra, planted on 3 November 2005) and eight plots had winter fallows. Each plot was 16 x 9 m. Cover crops were mown on 26 April 2006, sprinkler irrigated, and tilled into the soil (10 cm depth) after 19 days, when fallow plots were also tilled. Plots were weeded and sulfur was used against mites and diseases. Tomatoes were furrow irrigated (approximately every 11 days: 88 mm/event). Soil samples were collected on a total of five dates, before and after planting tomatoes (nutrients: 0–60 cm depth; microbial biomass: 0–30 cm depth). Greenhouse gas samples were collected after irrigation events, 28, 77, and 100 days after planting (closed chambers, for 30 minutes).

 

17 

A replicated, randomized, controlled before-and-after study in 1993–2008 in a rainfed wheat-maize-wheat-sunflower field in central Italy found more organic matter and nitrogen in soils with winter cover crops, compared to soils without cover crops. Organic matter: After 15 years, more carbon was found in soils with cover crops, compared to soils without cover crops, in four of six comparisons (legume cover crops, organic carbon concentration, 0–30 cm depth, in 2008: 11–14 vs 10–12 g/kg soil), and carbon increased more over time, in two of three comparisons (legume cover crops: 6–6.5% vs 1.5% increase in Mg organic C/ha, 0–30 cm depth). Nutrients: After 15 years, more nitrogen was found in soils with cover crops, compared to soils without cover crops, in five of six comparisons (total nitrogen concentration, 0–30 cm depth, in 2008: 1.2–1.5 vs 1.1–1.3 g/kg soil), and nitrogen increased over time, rather than decreased, in two of three comparisons (0.14–0.3% increase vs 0.7% decrease in Mg total N/ha, 0–30 cm depth). Implementation options: More carbon was found in soils that were cover cropped with high-nitrogen-supply legumes, compared to non-legumes (organic carbon concentration, 0–30 cm depth: 11–14 vs 10–13 g/kg soil), and more nitrogen was found at one of two depths (total nitrogen, 0–10 cm depth: 1.5 vs 1.4 g/kg soil). Methods: There were 32 plots (21 x 11 m sub-sub-plots) for each of three treatments (non-legumes, low-nitrogen-supply legumes, or high-nitrogen-supply legumes as winter cover crops) and one control (no cover crops: crop residues and weeds). Different species of cover crops were used in different years. Half of the plots were tilled, and half were not tilled (but pre-emergence herbicide was used). Post-emergence herbicide and fertilizer were used on all plots. Soil cores were collected in 1993, 1998, and 2008 (0–30 cm depth; two samples/plot in September).

 

18 

A replicated, randomized, controlled study in 2006–2009 in an irrigated maize field in the Tajo river basin, near Madrid, Spain, found that winter cover crops had inconsistent effects on nitrogen. Nutrients: Less nitrogen was found in soils that were cover cropped with barley, compared to fallows, in one of four comparisons (31 vs 156 kg N/ha). More nitrogen was found in soils that were cover cropped with vetch, compared to fallows, in one of four comparisons (113 vs 43 kg N/ha). Implementation options: Less nitrogen was found in soils that were cover cropped with barley, compared to vetch, in two of four comparisons during the cover-cropping seasons (45–49 vs 113–184 kg N/ha), and one of three comparisons during the maize-growing seasons (99 vs 253). Methods: There were four plots (12 x 12 m plots) for each of two treatments (barley or vetch, as winter cover crops) and there were four control plots (fallow). Cover crops were sown in October 2006–2009 and maize was sown in April 2007–2009. The maize was irrigated (sprinklers) and fertilized (210 kg N/ha, split into two applications, 120 kg P/ha, and 120 kg K/ha). Soil water content was measured every hour with capacitance probes (10–130 cm depth, three probes/plot, after the cover crops and after the harvest), and nitrate in soil water was measured with ceramic suction cups (buried at 122–124 cm depth, 1 µm pore size).

 

19 

A controlled study in 2005–2006 in an irrigated tomato field in the Sacramento Valley, California, USA, found less erosion of the part of the field that was cover cropped, compared to the part that was fallow. Soil erosion and aggregation: Less sediment was lost in runoff from the cover-cropped part, compared to the fallow part, in two of four comparisons (concentrations, in winter: 0.1 vs 0.7 g total suspended solids/litre; loads, in winter: 0.9 vs 5 kg/ha/rainfall event). Greenhouse gases: Similar amounts of greenhouse gas were emitted from each part of the field (<5 g N2O-N/ha/day; 35–440 mg CO2-C/m2/hour). Methods: A field was divided into two parts: one part with a winter cover crop (mustard Brassica nigra, planted in autumn 2005, and disked into the soil in spring 2006), and one part fallow. Greenhouse gases were measured one day/month (in chambers) in randomly located 16 m2 plots (three plots in each part of the field). Runoff water was collected in autosamplers (250 mL samples, every four hours, if there was >5 cm of water in the flow meter).

 

20 

A meta-analysis from 2013 of studies in multiple countries with Mediterranean-type climates 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). Methods: 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.

 

21 

A replicated, randomized, controlled study in 2003–2005 on an irrigated vegetable farm in the Salinas Valley, California, USA, found more nitrogen in plots with winter cover crops, compared to bare fallows. Nutrients: More nitrogen (ammonium and nitrate) was found in plots with cover crops, compared to bare fallows, in some comparisons, for some cover crops (data not clearly reported; in 2005, plots that were cover cropped with legumes and rye consistently had more nitrogen than bare fallows: 5–15 vs 4–5 µg mineral N/g dry soil; in 2004, all cover cropped plots had more nitrogen in one of five comparisons: 10–17 vs 5). Implementation options: In 2005, less nitrogen was found in plots that were cover cropped with oats (3–7 µg mineral N/g dry soil), compared to legumes and rye (5–13 µg) or mustard (6–13 µg, in five of seven comparisons). In 2004, there were inconsistent differences between cover crops. Methods: Twenty-four 12 x 20 m plots were planted with winter cover crops in October 2003–2004. Each plot had one of three cover crops: Secale cereale Merced rye, mustard (Sinapis alba and Brassica juncea), or legumes and rye (Merced rye, Vicia faba, Pisum sativum, Vicia sativa, and Vicia benghalensis). The number and size of the control plots (fallows) was not clearly reported. After the cover crops were incorporated into the soil (March), soil cores were collected every 7–10 days, for six weeks (30 cm depth, 1.9 cm width, 20 bulked samples/plot).

 

22 

A replicated, randomized, controlled study in 2009–2012 in two irrigated vegetable fields in central Italy found more nitrate in soils with winter cover crops, compared to bare soils. Nutrients: More nitrate was found in soils with cover crops, compared to bare soils, in two of 12 comparisons (in plots with hairy vetch as the cover crop: 6–12 vs 3–8 mg NO3-N/kg dry soil), but there were similar amounts of ammonium (0–4 mg NH4-N/kg dry soil). Implementation options: More nitrate was found in soils with hairy vetch as the cover crop, compared to oats or oilseed rape, in two of four comparisons (6–12 vs 2–6 mg NO3-N/kg dry soil), but similar amounts of ammonium were found (1–4 mg NO3-N/kg dry soil). Methods: There were nine plots (6 x 4 m plots) for each of three winter cover crops (hairy vetch, oats, or oilseed rape) and nine control plots (bare soil, maintained with herbicide). Cover crops were sown in September 2009–2010 and suppressed in May 2010–2011 (chopped and incorporated into the soil with a mouldboard plough, 30 cm depth). Pepper seedlings were transplanted into these plots in May 2010–2011 and were last harvested in October 2010 and September 2011. After the pepper harvest, endive and savoy cabbage seedlings were transplanted into these plots, and they were harvested in December 2010 and November 2011 (endive) or March 2011 and February 2012 (cabbage). No fertilizer was added while the crops were growing, but the plots were irrigated. Nitrogen was measured in soil samples (10 samples/plot, 0–30 cm depth, when the endive and cabbages were harvested).

 

23 

A replicated, randomized, controlled study in 2011–2014 in irrigated potato fields in Israel found less soil erosion in plots with cover crops, compared to bare soil. Soil erosion and aggregation: Less erosion was found in plots with cover crops, compared to bare soil (2012–2013: 0.1–0.3 vs 3.5–4.5 mm soil loss). Methods: Different plots were used in different years (2011–2012: 350 m2 plots, 20 plots with cover crops, eight plots without cover crops; 2012–2013: 695 m2 plots, 10 with, 10 without; 2013–2014: 1,800 m2 plots, four with, four without). Different mixtures of cover crops were used in different years, but oats were used in all years, and triticale was used in Years 1 and 2 (2011–2013). Plots without cover crops were weeded (tilled bare; some plots in all years) or weedy (not tilled; some plots in Year 1). Herbicide and fertilizer were used on all plots. Soil loss was measured in buckets, after each rainfall event (one 10 litre bucket/plot). Plots had a 5–7% slope.

 

24 

A replicated, randomized, controlled study in 1993–2011 in arable farmland in Davis and the Salinas Valley, California, USA, found more soil organisms, but no difference in nutrients, in soils with winter cover crops, compared to soils without cover crops. Nutrients: No difference was found in phosphorus, or the change in phosphorus over time, in soils with or without cover crops (2011: 519 vs 517 mg total phosphorus/kg soil; 1994–2011: 23 vs 21 mg less total phosphorus/kg soil; experiment in Davis). Soil organisms: More microbial biomass (measured as phosphorus) was found in soils with cover crops, compared to soils without cover crops (1.4 vs 1 mg phosphorus/kg soil; experiment in Davis). Implementation options: No differences in phosphorus or microbial biomass (measured as phosphorus) were found between soils with different species of cover crops (513–535 mg total phosphorus/kg soil; 3.1–3.7 mg microbial phosphorus/kg soil), or in soils with cover crops grown every year, compared to every four years (513 vs 497 total; 3.8 vs 2.0 microbial; experiment in the Salinas Valley). Methods: In one experiment (in Davis), nitrogen-fixing cover crops (peas, vetch, and/or fava beans) were grown in six treatment plots, but not in six control plots. Wheat was grown in rotation with cover crops (once every two years) or in rotation with fallows. In another experiment (in the Salinas Valley), there were four plots (240 m2) for each of four treatments (legume-rye, mustard, or rye cover crops grown every year, or legume-rye cover crops grown every four years). Lettuce and broccoli were grown in rotation (two crops/year). Soil samples were collected in soil cores (20 cores/plot; 0–30 cm depth) in 2011. Soil cores were also collected in 1993 (number of samples not reported).

 

25 

A replicated, randomized, controlled study in 2011–2013 in two irrigated tomato fields in central Italy found more organic matter and greater carbon accumulation in plots with winter cover crops, compared to plots without cover crops, but cover crops had inconsistent effects on nitrogen. Organic matter: When the tomatoes were harvested, more organic carbon was found in soils with winter cover crops, in 17 of 24 comparisons (1.1–1.8% vs 1–1.6% of soil was organic carbon). Nutrients: When the tomatoes were harvested, more organic nitrogen was found in soils with winter cover crops, in 14 of 24 comparisons (0.12–0.2% vs 0.11–0.15% of soil was organic nitrogen), but less was found in five of 24 comparisons (0.12–0.13% vs 0.14–0.15%). Greenhouse gases: Similar amounts of carbon dioxide were emitted from soils with or without cover crops (3.2–4.2 Mg C/ha), but more carbon accumulated in soils with cover crops, in four of six comparisons (1.1–2.1 vs 0.4–0.7 ratio of C input to output). Implementation options: More carbon accumulated in soils that were cover cropped and mulched with hairy vetch, compared to other species, in three of four comparisons (1.9–2.1 vs 1.1–1.4 ratio of C input to output). Methods: Three species of winter cover crops (Vicia villosa hairy vetch, Phacelia tanacetifolia lacy phacelia, or Sinapis alba white mustard) were sown on three plots each, in September, and winter weeds were controlled with herbicide on three control plots (18 x 6 m plots). The cover crops were mown and mulched (strips, 80 cm width) in May, and the control plots were tilled (depth not reported). Tomato seedlings were transplanted in May (transplanted into the mulch in treatment plots) and harvested in August. All plots were tilled (30 cm depth) and fertilized (100 kg P2O5/ha­, harrowed to 10 cm depth) in September. Some plots were also fertilized (100 kg N/ha) in June–July. Soil samples were collected after the tomatoes were harvested (0–20 cm depth). Carbon dioxide emissions (closed chambers, 1,334 cm3 volume, 30–180 seconds/sample) were measured weekly, or within 48 hours of rainfall, in the tomato-growing season. It was not clear whether these results were a direct effect of cover cropping, mulching, herbicide, or tillage.

 

26 

A replicated, randomized, controlled before-and-after study in 2012–2013 in two irrigated tomato fields in central Italy found more organic matter, nitrogen, and soil organisms in soils with cover crops (and no tillage), compared to soils without cover crops (with tillage), in spring. By the end of summer, less organic matter, but more nitrogen, had accumulated in soils with cover crops, and there were inconsistent effects on soil organisms. Organic matter: In May, more organic carbon was found in soils that had been cover cropped and mulched, compared to soils that had not, in two of six comparisons (lacy phacelia or white mustard, in 2013: 16 vs 12 mg C/g soil). By August, less organic carbon had accumulated in soils with mulch, compared to soils without mulch, in two of six comparisons (lacy phacelia or white mustard, in 2013: –1% to 4% vs 28% increase in organic carbon). Nutrients: In May, more nitrogen was found in soils that had been cover cropped and mulched, compared to soils that had not, in one of two years (all cover crops, in 2013: 1.3–1.5 vs 1.1 mg N/g soil). By August, more nitrogen had accumulated in soils with mulch, compared to soils without mulch, in one of six comparisons (white mustard, in 2013: 44% vs 2% increase in nitrogen). Soil organisms: In May, more microbial biomass (measured as carbon) was found in soils that had been cover cropped and mulched, compared to soils that had not (140–330 vs 100–150 µg C/g soil), and more microbial biomass was also found in two of three comparisons in August 2012 (175 vs 135 µg C/g soil), but less was found in two of three comparisons in August 2013 (175–210 vs 270 µg; 2012 was hotter and drier than 2013). Methods: Three species of winter cover crops (Vicia villosa hairy vetch, Phacelia tanacetifolia lacy phacelia, or Sinapis alba white mustard) were sown on three plots each, but not on three control plots (plot size not reported), in September. The cover crops were mulched in May, and the control plots were tilled (depth not reported). Tomato seedlings were transplanted in May (transplanted into the mulch) and harvested in August. All plots were tilled in September. Soil samples were collected at the beginning (May) and end (August) of the tomato-growing season (0–20 cm depth). It was not clear whether these results were a direct effect of cover cropping, mulching, or tillage.

 

27 

A replicated, randomized, controlled study in 1999–2009 in an irrigated tomato-cotton field in the San Joaquin Valley, California, USA (same study as (10)), found more organic matter in soils with winter cover crops, compared to soils without cover crops. Organic matter: More carbon was found in soils with cover crops (26–29 vs 23–24 t total C/ha). Methods: Rainfed winter cover crops (triticale, rye, and vetch) were planted on 16 treatment plots, but not on 16 control plots, in October 1999–2008. Crop residues were chopped in March. The plots (9 x 82 m) had six raised beds each. Tomatoes were grown in rotation with cotton. Fertilizer and herbicide were used in all plots, and tomatoes and cotton were irrigated. Soil samples were collected in autumn 2007 (0–30 cm depth, 7.6 diameter soil cores, 6–8 subsamples/plot).

 

28 

A replicated, controlled study in 2011–2012 in an irrigated tomato field near Pisa, Italy, found that similar numbers of tomato roots were colonized by mycorrhizae (beneficial fungi), but found more mycorrhizae spores, and more mycorrhizae species, in soils with planted cover crops, compared to resident (unplanted) vegetation. Soil organisms: Similar numbers of tomato roots were colonized by symbiotic fungi in plots with cover crops, compared to resident vegetation (28–42% vs 30–37% of roots were colonized). More mycorrhizae spores were found in soils with cover crops, in three of six comparisons (10.3–18.5 vs 7–8.5 spores/g soil). More mycorrhizae species were found in soils with cover crops, in two of six comparisons (when tomatoes were harvested, in plots that were cover cropped with Brassica juncea or a mixture of species: 29–30 vs 24 species). Implementation options: More tomato roots were colonized by mycorrhizae in plots that were cover cropped with Vicia villosa (42% of roots were colonized) or a mixture of species (35% of roots were colonized), compared to B. juncea (28%), in one of two comparisons (when tomatoes were flowering). More mycorrhizae spores were found in plots that were cover cropped with V. villosa (14.2–18.5 spores/g soil), compared to the other two cover crops (species mixture: 10.3–10.8; B. juncea: 7.8–9.1), and more spores were also found in plots with the species mixture, compared to B. juncea, in one of two comparisons (when tomatoes were flowering: 10.3 vs 7.8). More mycorrhizae species were found in plots that were cover cropped with B. juncea or the species mixture, compared to V. villosa (29–30 vs 25 species). Methods: There were three plots (plot size not reported) for each of three winter cover crops (B. juncea, V. villosa, or a mixture of seven species) and three control plots (without cover crops, but with resident vegetation). Cover crops were sown on 19 October 2011, and then mown and incorporated into the soil in spring 2012. Tomato seedlings were transplanted into the plots (into raised beds) on 30 May 2012. Tomatoes were drip irrigated. Soil samples were collected when the tomatoes were flowering (10 April 2012) and when they were harvested (20 September 2012) (four soil cores/plot, 0–20 cm depth). Half of the seedlings were inoculated with two species of mycorrhizae.

 

29 

A replicated, randomized, controlled study in 2009–2011 in an irrigated eggplant field in central Italy found more nitrogen in soils with winter cover crops, compared to bare soil. Nutrients: More nitrogen was found in soils with cover crops, compared to bare soil, for one of three cover crops (hairy vetch: 34 vs 23 mg inorganic N/kg dry soil). Implementation options: More nitrogen was found in soils with hairy vetch as the winter cover crop, compared to oats or oilseed rape (34 vs 20 mg inorganic N/kg dry soil), and no differences in nitrogen were found between soils with oats or oilseed rape as the winter cover crop. Methods: Three species of winter cover crops (Vicia villosa hairy vetch, Brassica napus oilseed rape, or Avena sativa oats) were sown on three plots each (6 x 12 m plots) in September 2009–2010, and no cover crops were sown on three plots (weeded, bare soil). The cover crops were mown and used as mulch (50 cm wide) in eggplant rows, in May 2010–2011. Eggplant seedlings were transplanted into the plots in May, and fruits were harvested four times/year in July–September 2010–2011. Soil samples were collected when the seedlings were transplanted and when the last fruits were harvested each year (0–30 cm depth, six samples/plot). All plots were fertilized before the cover crops were grown, but not after. All plots were irrigated.

 

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.