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Action: Soil: Add compost to the soil Mediterranean Farmland

Key messages

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Organic matter (12 studies): Twelve replicated, controlled studies (ten randomized) from Italy, Spain, Syria, Turkey, and the USA found more organic matter in soils with added compost, compared to soils without added compost, in some or all comparisons.

Nutrients (10 studies): Six replicated, controlled studies (five randomized) from Italy, Portugal, Spain, and Syria found more nutrients in soils with added compost, compared to soils without added compost, in some or all comparisons. Three replicated, randomized, controlled studies from Italy and the USA found inconsistent differences in nitrogen between soils with or without added compost. One replicated, randomized, controlled study from the USA found no differences in phosphorus between soils with or without added compost. Three replicated, randomized, controlled studies from Italy and Spain found similar pH levels in soils with or without added compost.

Soil organisms (10 studies): Six replicated, controlled studies (five randomized) from Italy, Spain, and the USA found more microbial biomass in soils with added compost, compared to soils without added compost, in some or all comparisons. Two replicated, controlled studies from Italy and the USA found similar amounts of microbial biomass in soils with or without added compost. One replicated, randomized, controlled study from Italy found inconsistent differences in bacterial abundance between plots with or without added compost. Two replicated, randomized, controlled studies from Italy and Spain found differences in bacteria communities, in some comparisons.

Soil erosion and aggregation (5 studies): Two replicated, controlled studies (one randomized) from Spain found less erosion of soils with added compost, compared to soils without added compost, in some or all comparisons. Four replicated, randomized, controlled studies from Spain and Turkey found that soils with added compost were more stable than soils without added compost, in some or all comparisons.

Greenhouse gases (10 studies): Six replicated, controlled studies (five randomized) from Italy, Spain, and the USA found more greenhouse gas in soils with added compost, compared to soils without added compost, in some or all comparisons. Four replicated, randomized, controlled studies from Spain found no differences in greenhouse gas between soils with or without added compost.

Implementation options (2 studies): One replicated, randomized, controlled study from Syria found more nitrogen in soils with compost added every two years, compared to soils with compost added every four years. One replicated, randomized, controlled study from Italy found inconsistent differences in bacteria abundance between soils with different amounts of added compost.

Supporting evidence from individual studies

1 

A replicated, randomized, controlled, study in 1995 in a broccoli field in the Salinas Valley, California, USA, found similar amounts of nitrate, pH levels, and carbon dioxide emissions in soils with or without added compost. Nutrients: Similar amounts of nitrate, and similar pH levels, were found in soils with or without added compost (3–10 vs 2 kg NO3-N/ha; pH 8.1–8.3 vs 8.1). Greenhouse gases: Similar carbon dioxide emissions were found in soils with or without added compost (soil respiration: 17–32 vs 63 kg CO2-C/ha/day). Methods: There were four plots for each of three compost treatments (0, 22, or 44 Mg/ha). Fertilizer (165 kg ammonium nitrate/ha) was added to half (6.1 x 7.7 m) of each plot. The compost was made from green wastes (>30%), cow manure (>20%), spoiled hay (>15%), clay soil (>5%), and crop processing residues. Soil samples were collected on 11 October 1995 (0–7.6 cm depth).

 

2 

A replicated, controlled study (year not reported) on a slope in Murcia, Spain, found less soil erosion in plots with added compost, compared to plots without added compost. Soil erosion and aggregation: Less soil was lost in runoff from plots with added compost, compared to plots without added compost, after rainfall events (eight initial events: 17 vs 299 g soil/m2; later events: 5 vs 62). Methods: Composted municipal waste was added to three treatment plots, but not three control plots (10 x 3 m plots, 15% slope). Soil loss was measured in runoff water, collected from the lower edge of each plot, after each rainfall event. Enough compost was added to the soil to increase its organic carbon content by 2%. The soil was rotovated to a depth of 20 cm, to incorporate the compost.

 

3 

A replicated, controlled study in 1997–1998 in farmland in Murcia, Spain, found more organic matter, soil organisms, and greenhouse gas in soils with added municipal waste compost, compared to soils without compost. Organic matter: More organic carbon was found in soils with added compost, compared to soils without added compost (14–33 vs 2–4 g C/kg soil). Soil organisms: More microbial biomass (measured as carbon) was found in soils with added compost, compared to soils without added compost (360–760 vs 130–320 µg C/g soil). Greenhouse gases: More carbon dioxide was found in soils with added compost, compared to soils without added compost (40–150 vs 5–15 mg CO2/kg soil/day). Methods: Municipal waste compost was added to three treatment plots (25–30 kg compost/m2), but not three control plots (no compost). The plots were 5 x 3 m. Soil samples were collected 0–24 months after adding the compost (eight time points, four samples/plot, 0–15 cm depth).

 

4 

A replicated, randomized, controlled study in 1995–1999 in farmland in southern Turkey found more organic matter and greater soil stability in soils with added compost, compared to soils without added compost, at some depths. Organic matter: More organic matter was found in soils with added compost, compared to soils without added compost, at one of two depths (0–15 cm: 1.8% vs 1.6%). Soil erosion and aggregation: Larger soil particles were found in plots with added compost, compared to plots without added compost, at one of two depths (15–30 cm: 0.31 vs 0.18 mm mean weight diameter). Methods: Compost (25 t/ha) was added to three treatment plots (10 x 20 m), but not three control plots. The compost was made of grass, stubble, and leaves. Wheat, sweet peppers, maize, and wheat were grown in rotation. Soils were sampled in 1999, after harvesting the last wheat crop (0–30 cm depth). Wet sieving was used to determine mean weight diameter.

 

5 

A replicated, randomized, controlled study in 1998–2000 in an irrigated vegetable field in the Salinas Valley, California, USA, found more organic matter, and sometimes found more microbial biomass, in soils with added compost, compared to soils without added compost. Adding compost had inconsistent effects on nutrients. Organic matter: More carbon was found in soils with added compost, compared to soils without added compost (15 vs 14 g total C/kg soil, 0–15 cm depth). Nutrients: More nitrogen was found in soils with added compost, compared to soils without added compost (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 added compost, compared to soils without added compost, in 13 of 14 comparisons (4–54 vs 5–64 g NO3-N/g soil), but more nitrate was found in one of 14 comparisons (34 vs 24). At depths of 0–15 cm, less nitrate was found in soils with added compost, compared to soils without added compost, in six of 16 comparisons (2–17 vs 3–64 μg NO3-N/g soil), but more nitrate was found in one of 16 comparisons (31 vs 12). At depths of 0–15 cm, less ammonium was found in soils with added compost, compared to soils without added compost, in eight of 16 comparisons (1–6 vs 5–9 μg NH4-N/g soil), but more ammonium was found in two of 16 comparisons (4–7 vs 1–4). Soil organisms: More microbial biomass (measured as carbon) was found in soils with added compost, compared to soils without added compost, nine of 16 comparisons (120–220 vs 80–130 μg C/g soil). More microbial biomass (measured as nitrogen) was found in soils with added compost, compared to soils without added compost, in 10 of 16 comparisons (14–26 vs 5–17 μg N/g soil). Methods: There were four plots (0.52 ha), for each of four treatments (minimum 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 (Merced rye) was grown every autumn or winter. The compost was made from municipal yard waste, salad packing plant waste, horse manure, clay, straw, and other compost. Lettuce or broccoli crops were grown in raised beds. Sprinklers and drip irrigation were used in all plots. Soils were disturbed to different depths (conventional tillage: 50 cm with disking, cultivating with a liliston, sub-soiling, bed re-making, and bed-shaping; minimum tillage: 20 cm with a liliston, rollers, 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.

 

6 

A replicated, randomized, controlled study in 2004 in a maize field in the Jarama river basin, Spain, found no difference in greenhouse-gas emissions between soils with or without added compost. Greenhouse gases: No difference in nitrous oxide emissions was found between soils with or without added compost (composted pig slurry: 9.3 vs 8.6 kg N/ha; composted municipal solid waste 7.1 vs 8.6). Methods: There were three plots (40 m2) for each of two treatments (composted solid fraction of pig slurry or composted municipal solid waste, both with urea) and one control (urea only). Urea was applied at a rate of 50 kg N/ha. Both composts were applied at a rate of 175 kg available N/ha. Soils were cultivated to a depth of 5 cm to incorporate the fertilizers. Nitrous oxide was measured in closed chambers (two chambers/plot, one within a maize row, one between rows, 35 cm diameter, 23 cm height, one sample/week, April–September).

 

7 

A replicated, controlled study in 2001–2003 in a degraded wood pasture in Catalonia, Spain, found more nitrate in soils with added compost, compared to soils without it. Nutrients: More nitrate was found in soils with added compost, compared to soils without it (24 vs 3 kg N-NO3/ha). Methods: Composted sewage sludge was added to five treatment plots (10 t dry matter/ha), but not five control plots (no compost). Each plot was 20 x 5 m. To restore the wood pasture, shrubs and small trees were crushed and scattered on the soil, and grass seeds were sown. Soil was collected in soil cores (10 cores/plot, 0–20 cm depth).

 

8 

A replicated, randomized, controlled study in 2001–2005 in the Guadalquivir Valley, Andalusia, Spain, found more soil organisms, more carbon dioxide, and greater stability in soils with four years of added compost, compared to soils without added compost. Soil organisms: More microbial biomass (measured as carbon) was found in soils with added compost, after 3–4 years of adding it, compared to soils without it, in five of six comparisons (351–501 vs 118–120 µg C/g dry soil), but no differences were found after 1–2 years of adding compost (171–317 vs 119–128). Greenhouse gases: More carbon dioxide was found in soils with added compost, after four years of adding it, compared to soils without added compost (1,596–2,004 vs 859 mg/kg soil). Soil erosion and aggregation: More stable soils were found in plots with added compost, after 3–4 years of adding it, compared to plots without it (data reported as log instability index), and more stable soils were also found after two years of adding compost, in two of three comparisons, but no differences were found after one year of adding it. Methods: There were three plots (10 x 7 m) for each of three treatments (5, 7.5, or 10 t/ha of organic matter, added as composted beet vinasse and crushed cotton gin waste) and one control (no compost). The compost was added in October 2001–2004. Soil samples were collected one day before the compost was added (four subsamples/plot, 0–25 cm depth).

 

9 

A replicated, randomized, controlled study in 2001–2005 in the Guadalquivir Valley, Andalusia, Spain, found greater stability and less erosion in soils with added compost, compared to soils without added compost. Soil erosion and aggregation: More stable soils were found in plots with added compost, compared to plots without added compost, in two of four years (data reported as log instability index). Less soil was lost from plots with added compost, compared to plots without added compost, after rainfall (60 mm rainfall/hour, in two of four years: 171–182 vs 210–211 kg/ha; 140 mm rainfall/hour, in three of four years: 366–430 vs 508–520). Methods: There were four plots (9 x 9 m) for each of two treatments (10 or 20 t compost/ha) and one control (no compost). The compost was added in October 2001–2004, and soils were ploughed to a depth of 25 cm. Soil samples were collected one day before the compost was added, in 2002–2004, and also in October 2005 (gauge augers, 30 mm diameter, 25 cm depth). Soils were watered to simulate rainfall in October 2002–2005 (60 or 140 mm rainfall/hour), and soil loss was measured in plots (1 x 1 m) that overlapped the borders of the treatment and control plots by 0.5 m.

 

10 

A replicated, randomized, controlled study in 2002–2005 in a barley field in Toledo, Spain, found more soil organisms in plots with added compost, compared to plots without added compost, in two of four comparisons. Soil organisms: More microbial biomass (measured as carbon) was found in soils with low amounts of added compost, compared to plots without added compost (one application: 157 vs 76 mg C/ha; one application/year: 265 vs 76), but significant differences were not found between plots with high amounts of added compost and plots without added compost (one application: 95 vs 76; one application/year: 136 vs 76). Methods: The compost was made from sewage sludge. There were four plots (10 x 3 m) for each of four fertilizer treatments (20 or 80 t compost/ha, applied once in three years or once/year) and one control (no fertilizer). Plots were fertilized in mid-September and planted in mid-October.

 

11 

A replicated, randomized, controlled study in 2003–2005 in farmland in the Sele river plain, Italy, found more organic matter and more carbon dioxide in soils with added compost, compared to soils without added compost. Organic matter: More organic carbon was found in soils with added compost, compared to soils without added compost, in eight of 18 comparisons (greenhouse: 28–32 vs 23–25 mg organic C/kg soil [dry weight]; open field: 10–12 vs 8). Greenhouse gases: More carbon dioxide was found in soils with added compost, in five of six comparisons (greenhouse: 0.9–1.2 vs 0.7 μg CO2/g soil [dry weight]/hour; open field: 1.2–1.4 vs 0.6). Methods: The compost was made from municipal food waste and yard trimmings. At each of two sites (unheated tunnel greenhouse with 24 m2 plots, or open field with 70 m2 plots), there were three replicates for each of three treatments (15, 30, or 45 t compost/ha, in March–April each year) and one control (no compost). Crops were grown in rotation (greenhouse: tomatoes, beans, lettuce; open field: tomatoes or eggplants, endive and/or broccoli sprouts). Soil samples (five/plot, 0–20 cm depth) were collected three times/year before the crops were harvested (greenhouse: spring, autumn, winter; open field: summer, autumn, winter). Organic carbon was measured in winter samples (residual carbon). Carbon dioxide (soil respiration) was measured in all samples.

 

12 

A replicated, randomized, controlled study in 2001–2009 in an irrigated nectarine orchard in Italy found more organic matter, nutrients, and soil organisms in plots with added compost, compared to plots without added compost. Organic matter: More organic matter was found in plots with added compost, in 11 of 24 comparisons (2–5% vs 1.5%). Nutrients: More nitrogen was found in soils with added compost (ammonium, in four of 21 comparisons: 7–15 vs 4–6 mg/kg dry soil; nitrate, in at least eight of 84 comparisons: results not clearly reported). More phosphorus and potassium were found in plots with added compost (phosphorus, in one of three comparisons in 2006: 24 vs 13 mg P/kg dry soil; potassium, in two of three comparisons: 299–350 vs 227 mg K/kg dry soil). Similar pH levels were found in plots with or without added compost (pH 7.8 vs 7.7). Soil organisms: More microbial biomass (measured as carbon) was found in plots with added compost, in 14 of 60 comparisons (5–22 vs 4–10 mg C/g dry soil). Soil erosion and aggregation: Similar water-stability was found in soils with or without added compost (13% vs 14–17% of soil aggregates were water-stable). Methods: There were four plots for each of three compost treatments (5 t/ha in May, 5 t/ha split into two applications, in May and September, or 10 t/ha split into two), and there were four control plots (no fertilizer; plot size not reported). The compost was made from domestic organic waste and urban pruning material (50% each). Compost was tilled into the soil (25 cm depth). Soil samples were collected in September (3–40 cm depth for organic matter in 2001–2008 and phosphorus in 2006; 5–40 cm depth for aggregate stability in 2008) and four times in spring and summer in 2008–2009 (0–80 cm depth for nitrogen, and 4–20 cm depth for microbial biomass).

 

13 

A replicated, randomized, controlled study in 2006 in a barley field in the Henares river basin, Spain, found no difference in greenhouse-gas emissions between soils with or without added municipal waste compost. Greenhouse gases: No difference in greenhouse-gas emissions was found between soils with or without added compost (carbon dioxide: 466 vs 411 kg C/ha; methane: –231 vs –294 mg C/m2). Methods: Composted municipal solid waste (125 kg available N/ha) was added to three treatment plots, but not three control plots, in January. Plots were 30 m2. Plots were cultivated (0–5 cm depth) to incorporate the compost. Barley was planted in January and harvested in June. Greenhouse-gas emissions were measured with closed chambers (35 cm diameter, 25 cm height, 1–4 measurements/plot/week, 23 January–28 November).

 

14 

A replicated, randomized, controlled study in 1996–2008 in farmland near Aleppo, Syria, found more nitrogen, and sometimes found more organic matter, in soils with added compost, compared to soils without added compost. Organic matter: More organic matter was found in soils with added compost, compared to soils without added compost, in two of eight comparisons (2003, compost added every two years: 13–17 vs 10–12 g/kg soil). Nutrients: More nitrogen was found in soils with added compost, compared to soils without added compost (0.75–0.93 vs 0.71 g/kg soil). Implementation options: More nitrogen was found in soils with compost added every two years, compared to every four years (0.93 vs 0.75 g/kg soil). Methods: There were two plots (25 x 25 m) for each of eight treatments (10 Mg/ha dry weight of compost added every two or four years, with shallow or conventional tillage, and two-course or four-course crop rotations) and four controls (no compost, with shallow or conventional tillage, and two-course or four-course crop rotations). The compost was 75% plant residues, 20% sheep manure, and 5% soil. The crop rotations were vetch-barley (two-course) or vetch-barley-vetch-wheat (four-course). Soils were tilled with a mouldboard plough to a depth of 30 cm after cereal crops (conventional tillage) and/or with a cultivator to a depth of 12 cm after vetch (conventional and shallow tillage). Soils were sampled in 2003 (0–30 cm depth) and 2008 (0–20 cm depth).

 

15 

A replicated, randomized, controlled study in 2006–2008 in a cereal field in the Castelo Branco region, Portugal, found similar amounts of nitrate in soils with and without added compost. Nutrients: Similar amounts of nitrate were found in plots with and without added compost, in 30 of 32 comparisons (1–50 vs 1–10 mg NO3-N/L water). In two of 32 comparisons, more nitrate was found in plots with added compost, compared to plots without (33 days after single application: 16 vs 7 mg NO3-N/L water; 130 days after split application: 12 vs 1 mg). Methods: Water in the soil was collected in porous ceramic suction cup samplers (four/plot, 0.6–0.7 m depth, 50 kPa for 24 hours), whenever drainage occurred (October–November and April–May, 16 samples in total). There were three plots (5.6 x 8 m) for each of two treatments (single application of compost in spring, or split application in spring and autumn) and one control (no compost). The compost was made from municipal waste. Maize was grown in spring–summer, and oats were grown in autumn–winter.

 

16 

A replicated, randomized, controlled, paired study in 2008–2011 in grazed annual grasslands in California, USA, found more organic matter and carbon dioxide in soils with added compost, compared to soils without added compost. Organic matter: More organic carbon was found in soils with added compost, compared to soils without added compost, in three of seven comparisons (350–1,000 more g C/m2). Greenhouse gases: More carbon dioxide was found in soils with added compost, compared to soils without added compost, in four of six comparisons (150–250 more g CO2-C/m2/year), but no differences were found in other greenhouse gases (methane: –1.4 to –2.5 g CH4-C/ha/day; nitrous oxide: 0.1–1.0 g N2O-N/ha/day). Methods: Composted organic green waste was added to three treatment plots (129 g total N/m2), but not to six control plots, at each of two sites (coastal grassland in Nicasio and valley grassland in Browns Valley). The plots were 25 x 60 m. Greenhouse gases were measured in flux chambers, every 1–4 weeks for three years. Organic carbon was measured in soil samples that were collected at the end of the growing seasons (May or June, nine soil cores/plot, 7 cm diameter, 10 cm depth).

 

17 

A replicated, randomized, controlled study in 2009–2011 on two farms in the Salerno district, Italy, found more organic matter, and sometimes found more ammonium, but found no difference in soil organisms or pH, in soils with added compost, compared to soils without added compost. Adding compost had inconsistent effects on nitrate. Organic matter: More organic carbon was found in soils with added compost, compared to soils without added compost (5–110% more). Nutrients: More nitrate was found in soils with added compost, compared to soils without added compost, in 10 of 18 samples (20–125% more), but less nitrate was found in the first two samples (25–70% less). More ammonium was found in soils with added compost, compared to soils without added compost, on one of two farms (amounts of ammonium not reported). No difference in pH was found between soils with and without added compost (pH levels not reported). Soil organisms: No difference in microbial biomass was found between soils with and without added compost (amounts of biomass not reported). Methods: On each of two farms, there were three plots (7 x 5 m) for each of four treatments (30 or 60 Mg organic matter/ha/year, with a carbon-to-nitrogen ratio of 15:1 or 25:1) and one control (no organic matter). Organic matter was added in February 2009, February 2010, and June 2011. It was made from the composted organic fraction of municipal solid waste, and it was mixed with wood scraps to control the carbon-to-nitrogen ratio. It was not clear whether these results were a direct effect of adding composted municipal waste or wood scraps.

 

18 

A replicated, randomized, controlled study in 2013 in greenhouses in southeast Spain found more soil organisms, but similar amounts of carbon dioxide, in soils with added compost, compared to soils without added compost. Soil organisms: More microbial biomass (measured as carbon) was found in soils with added compost, compared to soils without added compost (280–330 vs 130–160 mg C/kg soil). Greenhouse gases: Similar amounts of carbon dioxide were found in soils with and without added compost (8–19 vs 7–15 mg CO2-C/kg soil/day). There were four replicates for each of four treatments (50.5 t/ha of compost R1 or 40 t/ha of compost R2, with low or medium doses of mineral fertilizer) and two controls (low or medium doses of mineral fertilizer). Mineral fertilizer (Hoagland’s solution) was added in two of three waterings (medium dose) or one of five waterings (low dose). Compost R1 was made from sheep and goat manure. Compost R2 was made from alperujo, manure, and olive prunings.

 

19 

A replicated, randomized, controlled study in 1996–2011 in a vineyard in Navarra, Spain, found more organic matter, nutrients, and greenhouse-gas emissions in soils with some types of added compost, compared to soils without added compost. Organic matter: A higher percentage of organic matter was found in plots with added compost, compared to plots without added compost, for one of three types of compost (SMC: 1.8 vs 1.2% organic matter). Nutrients: More nitrogen, phosphorus, and/or potassium were found in plots with some types of added compost, compared to plots without added compost. The largest differences were between plots with added sheep-manure compost and plots without added compost (nitrogen: 0.10% vs 0.06%; phosphorus: 81 vs 30 mg/kg; potassium oxide: 474 vs 232 mg/kg). Similar pH was found in all plots (7.34–7.41). Soil organisms: Similar bacteria communities were found in all plots, for 11 of 12 bacteria genera. However, in plots with added compost, a higher percentage of RNA sequences came from Rhizobium species, for two types of compost (SMC: 0.3%; OF-MSW: 0.5%), but not for one type (PEL: 0.2%), compared to plots without added compost (0.1%). Greenhouse gases: Higher greenhouse-gas emissions were found in plots with added compost, for one type of compost (OF-MSW: 1,745 kg CO2 equivalent/ha; cumulative over 115 days after adding compost or fertilizer), but not for two types (SMC: 1,591 kg; PEL: 1,598 kg), compared to plots without added compost (1,104 kg). Higher nitrous oxide emissions were found in plots with added compost, compared to plots without added compost (1.8–5.1 vs 1.7 g N2O–N/ha/day; 15 days after compost). Methods: Three types of compost were compared: pelletized organic compost (PEL), compost from the organic fraction of municipal solid waste (OF-MSW), and sheep-manure compost (SMC). Each of three compost treatments and one control was assigned to a plot (15 vines), and there were three blocks (the size of plots within blocks was not clearly reported). The vines were planted in 1996. Compost was added in February 1998–2011 (PEL: 3,700 kg fresh weight/ha/year; OF-MSW: 4,075 kg; SMC: 4,630 kg). For N, P, K, and pH measurements, soil samples were taken at the end 2011 (four/plot, 0–30 cm depth). For greenhouse-gas measurements, ambient air samples (20 ml, 10/plot, closed chamber technique) were taken over 115 days after adding compost. For partial prokaryotic 16S rRNA sequencing, soil samples (four/plot, 5–30 cm depth) were taken 15 days after adding compost.

 

20 

A replicated, randomized, controlled study in 2010–2011 in the Jarama river basin, Spain, found more carbon dioxide and higher soil stability, but rarely found more organic matter, in soils with added compost, compared to soils without added compost. Organic matter: Higher percentages of organic carbon were found in soils with added compost, compared to soils without added compost, in one of five comparisons (spring 2010, immediately after adding compost: 72% vs 85% unhydrolyzed carbon), but not in four of five comparisons (summer 2010–spring 2011: 63–72% vs 55–72%). Soil erosion and aggregation: Larger soil aggregates were found in plots with added compost, compared to plots without added compost (3.0–3.4 vs 0.7–1.3 mm mean weight diameter). Greenhouse gases: More carbon dioxide was found in soils with added compost, compared to soils without added compost (amounts of carbon dioxide not reported). Three plots (10 x 15 m) were fertilized with municipal solid waste compost in spring 2010, and three plots were not fertilized. Carbon dioxide was measured once every two weeks (three open chambers/plot, 20 cm diameter, 5 cm deep in the soil). Soil cores were collected (three 100 cm2 soil cores/plot) for other measurements.

 

21 

A replicated, randomized, controlled study in 2015 in a sorghum field in southern Italy found inconsistent differences in bacteria abundance between soils with or without added compost, but found higher bacteria diversity in soils with added compost. Soil organisms: Higher bacteria abundance was found in soils with added compost, in three of 32 comparisons (Actinobacteria and one other group: 111–2,017 vs 32–1,746 phylotype 16S rRNA sequences), but lower bacteria abundance was found in three of 32 comparisons (Gemmatimonadetes and Proteobacteria: 80–1,658 vs 141–1,810 sequences). Higher bacteria diversity was found in soils with added compost, compared to soils without added compost, in one of many comparisons (with double compost application: data reported as Chao 1 index). Implementation options: Higher bacteria abundance was found in plots with a double application of compost, compared to single application, in one of 16 comparisons (Proteobacteria: 1,658 vs 1,469 sequences), but lower bacteria abundance was found in one of 16 comparisons (another group: 34 vs 111 sequences). Methods: Compost was added to eight treatment plots (single application: 130 kg N/ha; double application: 260 kg N/ha), but not four control plots (5 x 8 m plots). After three years of compost addition, plants were dug up (three plants/plot) and soil that was clinging to plant roots was collected for sampling bacteria (through RNA sequencing).

 

22 

A replicated, randomized, controlled study in 2003–2011 in farmland the Salinas Valley, California, USA, found no difference in nutrients or soil organisms between soils with or without added compost. Nutrients: No difference in phosphorus was found between soils with or without added compost (497 vs 456 mg total phosphorus/kg soil). Soil organisms: No difference in microbial biomass (measured as phosphorus) was found between soils with or without added compost (2.0 vs 1.7 mg chloroform-extractable phosphorus/kg soil). Methods: Composted yard waste (15 Mg/ha/year) was added to four treatment plots (240 m2), but not four control plots. 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.

 

23 

A replicated, randomized, controlled study in 2009–2011 in farmland in the Salerno district, Italy, found more organic matter and more nutrients in soils with added compost, compared to soils without added compost, in some comparisons, but less nitrate was found in some comparisons. Organic matter: More organic matter was found in soils with added compost, compared to soils without added compost, in some comparisons (farm 1: 30–100% more organic carbon, in 19 of 28 comparisons; farm 2: up to 70% more; number of significant comparisons not reported). Nutrients: More nitrate was found in soils with added compost, compared to soils without added compost, in some comparisons (Farm 1: 55–185% higher nitrate concentration, in eight of 16 reported comparisons; Farm 2: 45–70% higher, in four of 16 reported comparisons), but less nitrate was found in some comparisons (Farm 1: 45–55% less, in two of sixteen reported comparisons). More nitrogen and/or phosphorus were found in soils with added compost, compared to soils without added compost, in some comparisons (up to 50% more nitrogen; number of significant comparisons not reported; percentage increase in phosphorus not reported). Methods: There were three plots (approximately 30 m2) for each of eight treatments (30 or 60 t/ha of added organic matter, with carbon-to-nitrogen ratios of 15:1 or 2:1, with or without mineral fertilizer) and two controls (no organic matter, with or without mineral fertilizer), on two farms. Composted municipal solid waste was mixed with poplar tree prunings to control carbon-to-nitrogen ratios in the organic matter. Crops were grown in unheated glasshouses (farm 1: lettuce and melon; farm 2: kohlrabi). Organic matter was added in early 2009 and 2010. Soil samples were collected at seven time points in two years (five subsamples/plot, 0–20 cm depth). It was not clear whether these results were a direct effect of adding compost or adding poplar prunings.

 

24 

A replicated, controlled study in 2014 in 29 organic vegetable fields on the Central Coast, California, USA, found more organic matter, phosphorus, and potassium in soils with added compost, compared to soils without added compost. Organic matter: More organic matter was found in soils with added compost, compared to soils without added compost (data reported as model coefficients). Nutrients: More phosphorus and potassium, but similar amounts of nitrate and similar pH levels, were found in soils with added compost, compared to soils without added compost (data reported as model coefficients). Methods: In each of 29 vegetable fields, compost was added to one plot, but not to one adjacent plot (5 x 5 m plots), 1–2 months before lettuces were planted (25 t compost/ha, made from cow, chicken, and green manures). Lettuces were planted in spring (5–28 March) and summer (30 May–5 July). Lettuce weights were measured at maturity in one 1 x 1 m quadrat/plot. Soil samples were collected in spring (1.25 cm diameter, 0–10 cm depth).

 

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.