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Providing evidence to improve practice

Action: Soil: Use organic fertilizer instead of inorganic Mediterranean Farmland

Key messages

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Organic matter (13 studies): Eight replicated studies (including one meta-analysis) from France, Italy, Spain, Turkey, and Mediterranean countries found more organic matter in soils with organic fertilizer, compared to inorganic fertilizer, in some comparisons. Five replicated, randomized, controlled studies from Greece, Spain, and the USA found similar amounts of organic matter in soils with organic or inorganic fertilizer.

Nutrients (14 studies)

  • Nitrogen (9 studies): Four replicated studies (three controlled, two randomized; one site comparison) from France, Italy, and Spain found more nitrogen in soils with organic fertilizers, compared to inorganic fertilizer, in some comparisons. Five replicated, randomized, controlled studies from Greece, Spain, and the USA found similar amounts of nitrogen in soils with organic or inorganic fertilizer.
  • Ammonium (3 studies): Two replicated, randomized, controlled studies from Italy and Spain found more ammonium in soils with organic fertilizer, compared inorganic fertilizer, in some comparisons. One replicated, randomized, controlled study from Spain found similar amounts of ammonium in soils with organic or inorganic fertilizer.
  • Nitrate (3 studies): One replicated, randomized, controlled study from Spain found less nitrate in soils with organic fertilizer, compared to inorganic fertilizer, in some comparisons. Two replicated, randomized, controlled studies from Portugal and Spain found similar amounts of nitrate in soils with organic or inorganic fertilizer.
  • Phosphorus (5 studies): Three replicated, randomized, controlled studies from Italy and Spain found more phosphorus in soils with organic fertilizer, compared to inorganic fertilizer, in some or all comparisons. One replicated site comparison from France found less phosphorous in soils with organic fertilizer, in some comparisons. One replicated, randomized, controlled study from Spain found similar amounts of phosphorous in soils with organic or inorganic fertilizer.
  • Potassium (6 studies): Three replicated, randomized, controlled studies from Italy and Spain found more potassium in soils with organic fertilizer, compared to inorganic fertilizer, in some comparisons. Three replicated studies (two controlled, one site comparison) from France and Spain found similar amounts of potassium in soils with organic or inorganic fertilizer.
  • pH (6 studies): Four replicated studies (three randomized and controlled, one site comparison) from France, Italy, and Spain found similar pH levels in soils with organic or inorganic fertilizer. One replicated, controlled study from Italy found higher pH levels in soils with organic fertilizer, in some comparisons. One replicated, randomized, controlled study from Spain found lower pH levels in soils with organic fertilizer, in some comparisons.

Soil organisms (7 studies)

  • Microbial biomass (4 studies): Four replicated studies (three randomized and controlled, one site comparison) from France, Italy, and Spain found more microbial biomass in soils with organic fertilizer, compared to inorganic fertilizer, in some comparisons.
  • Other soil organisms (4 studies): One replicated, randomized, controlled study from Spain found fewer bacteria in soils with organic fertilizer, compared to inorganic fertilizer, in one comparison. One replicated site comparison from France found fewer nematodes in plots with organic fertilizer, compared to inorganic fertilizer, in some comparisons. One replicated, randomized, controlled study from Spain found fewer mites in plots with organic fertilizer, compared to inorganic fertilizer. One replicated, randomized, controlled study from Italy found inconsistent differences in microbes between plots with organic or inorganic fertilizer.

Soil erosion and aggregation (5 studies): Three replicated, randomized, controlled studies from Turkey and Spain found greater aggregation in soils with organic fertilizer, compared to inorganic fertilizer, in some or all comparisons. Two replicated, randomized, controlled studies from Spain and the USA found no difference in aggregation between soils with organic or inorganic fertilizer.

Greenhouse gases (11 studies)

  • Carbon dioxide (5 studies): Four replicated, randomized, controlled studies from Italy and Spain found higher carbon dioxide emissions from plots with organic fertilizer, compared to inorganic fertilizer, in some comparisons. One replicated, randomized, controlled study from Spain found similar carbon dioxide emissions from plots with organic or inorganic fertilizer.
  • Methane (4 studies): Two replicated, randomized, controlled studies from Spain found that more methane was absorbed by soils with organic fertilizer, compared to inorganic fertilizer, in some comparisons. Two replicated, randomized, controlled studies from Spain found that similar amounts of methane were absorbed by soils with organic or inorganic fertilizer.
  • Nitrous oxide (8 studies): Five replicated, randomized, controlled studies from Spain found similar nitrous oxide emissions from plots with organic or inorganic fertilizer. Three studies (including one meta-analysis and two replicated, randomized, controlled studies) from Spain, the USA, and Mediterranean countries found lower nitrous oxide emissions from plots with organic fertilizer, compared to inorganic fertilizer, in some comparisons.

Implementation options (4 studies): One study from Spain found that plots with slurry absorbed methane, but plots with manure emitted methane. One study from Italy found more organic matter, nutrients, and microbial biomass in plots fertilized with compost, compared to manure. One meta-analysis found lower nitrous oxide emissions after adding solid organic fertilizer, but not liquid organic fertilizer, compared to inorganic fertilizer. One study found inconsistent differences in soil bacteria with a single or double application of organic fertilizer.

Supporting evidence from individual studies

1 

A replicated, randomized, controlled study in 1995–1999 in arable farmland in southern Turkey found more organic matter and greater stability in soils with organic fertilizer, compared to inorganic fertilizer. Organic matter: More organic matter was found in soils with compost, compared to mineral fertilizer, at one of two depths (0–15 cm: 1.8% vs 1.7%). Similar amounts of organic matter were found in soils with manure, compared to mineral fertilizer (1.6–1.8% vs 1.6%). Soil erosion and aggregation: Larger particles were found in soils with compost, compared to mineral fertilizer, at one of two depths (0–15 cm: 0.38 vs 0.27 mm mean weight diameter). Larger particles were found in soils with manure, compared to mineral fertilizer (0.38–0.43 vs 0.19–0.29 mm mean weight diameter). Methods: There were three plots (10 x 20 m) for each of three treatments: cattle manure (25 t/ha), compost (25 t/ha), or mineral fertilizer (160 kg N/ha, 26 kg P/ha, 83 kg P/ha). 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.

 

2 

A replicated, randomized, controlled study in 2003–2004 in a vegetable field in Murcia, Spain, found lower pH levels in plots with organic fertilizer, compared to inorganic fertilizer. Organic matter: Similar amounts of carbon were found in plots with organic or inorganic fertilizer (5–8 vs 3–5 g/kg). Nutrients: Lower pH was found in plots with organic fertilizer, compared to inorganic fertilizer, in one of four comparisons (pH 7.7 vs 8). Methods: Plots (6 m2) growing Swiss chard Beta vulgaris followed by saltwort Beta maritima either had organic fertilizer (51 t/ha cow manure) or inorganic fertilizer (200 kg/ha). Soil was sampled four times, at sowing and sampling of each species (0–20 cm depth).

 

3 

A replicated, randomized, controlled study in 2002–2005 in an irrigated maize field in Greece found similar amounts of carbon and nitrogen in plots with organic or inorganic fertilizer. Organic matter: Similar amounts of organic carbon were found in soils with organic or inorganic fertilizer (5.7 vs 5.5 g C/kg). Nutrients: Similar amounts of nitrogen were found in soils with organic or inorganic fertilizer (0.81 vs 0.79 g Kjeldahl N/kg, 0–30 cm depth). Methods: Plots (5.6 x 8 m) had organic fertilizer (liquid cow manure 80 Mg/ha/year, before sowing) or inorganic fertilizer (260 kg N/ha/year and 57 kg P/ha/year, before sowing) (six plots each). Fertilizers were incorporated with a disk harrow (12–15 cm depth) within two hours of application. Soil samples were collected at the end of the growing season in 2005 (three samples/plot, 0–30 cm depth).

 

4 

A replicated, randomized, controlled study in 2004 in a maize field in the Jarama river basin, Spain, found similar greenhouse-gas emissions in soils with organic or inorganic fertilizer. Greenhouse gases: No difference in nitrous oxide emissions was found in soils fertilized with pig slurry, compared to soils fertilized with urea (untreated pig slurry: 8.3 vs 8.6 kg N/ha; digested pig slurry: 7.7 vs 8.6). Methods: There were three plots (40 m2) for each of two organic fertilizers (anaerobically digested thin fraction of pig slurry or untreated pig slurry) and one mineral fertilizer (urea, which was a mineral fertilizer in this study, but urea is also produced from animal waste). Slurries were applied at a rate of 175 kg available N/ha. Urea was applied at a rate of 50 kg 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).

 

5 

A replicated, randomized, controlled study in 2002–2005 in a barley field in Toledo, Spain, found more microbial biomass in soils with organic fertilizer, compared to inorganic fertilizer. Soil organisms: More microbial biomass (measured as carbon) was found in soils with organic fertilizer, compared to inorganic fertilizer, in two of eight comparisons (composted sewage sludge: 157–266 vs 83 mg C/ha). Methods: There were four plots (10 x 3 m) for each of eight organic fertilizers (20 or 80 t thermally dried sewage sludge/ha, applied once in three years or once/year; 20 or 80 t composted sewage sludge/ha, applied once in three years or once/year) and one mineral fertilizer (400 kg NPK/ha/year; 15-15-15 NPK). Plots were fertilized in mid-September and planted in mid-October.

 

6 

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 organically fertilized soils, compared to inorganically fertilized soils. Organic matter: More organic matter was found in organically fertilized soils, compared to inorganically fertilized soils, in five of 18 comparisons (greenhouse, 45 t compost/ha: 30 vs 26 mg organic C/kg dry soil; open field, 30–45 t compost/ha: 10–12 vs 8). Greenhouse gases: More carbon dioxide was found in organically fertilized soils, compared to inorganically fertilized soils, in a greenhouse (0.9–1.2 vs 0.7 μg CO2/g dry soil/hour), but there were no significant differences in an open field (0.8–1.4 vs 0.9). Methods: 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 four treatments (15, 30, or 45 t compost/ha, in March–April each year, or NPK fertilizer with 260–325 kg N/ha, 160–320 kg P2O5/ha, 140–310 kg K2O/ha). The compost was made from municipal food waste and yard trimmings. 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.

 

7 

A replicated, randomized, controlled study in 2003–2004 in three maize-tomato fields near Davis, California, USA, found lower greenhouse-gas emissions in soils with organic fertilizer, compared to inorganic fertilizer. Organic matter: Similar amounts of organic carbon were found in soils with organic or inorganic fertilizer (18 vs 19 Mg C/ha). Nutrients: Similar amounts of nitrogen were found in soils with organic or inorganic fertilizer (1.8–1.9 vs 2.0 Mg N/ha). Soil erosion and aggregation: Similar amounts of aggregation were found in soils with organic or inorganic fertilizer (1.2 vs 1.4 mm mean weight diameter). Greenhouse gases: Lower nitrous oxide emissions were found in soils with organic fertilizer, compared to inorganic fertilizer, in two of seven comparisons (emissions not reported for all comparisons; the highest emissions were found in plots with conventional tillage: 40 g N2O–N/ha/day). Methods: Organic or inorganic fertilizer was used on six plots each (1.5 x 1.0 m plots). Urea was added to inorganically-fertilized plots (April: 60 kg N/ha; May: 200 kg N/ha). On organically-fertilized plots, inorganic fertilizer was replaced, every other year, with the residues of legume cover crops (100 kg N/ha). Soil samples were collected with soil cores (two cores/plot, 4 cm diameter, 0–15 cm depth), when the maize was harvested (September). Greenhouse-gas emissions were measured with closed chambers (March–September, every three week). Maize was sown at different times (organically-fertilized plots: March; inorganically-fertilized plots: May), and different amounts of nitrogen were applied. It was not clear whether these results were direct effects of differences in the type of fertilizer (organic or inorganic), the amount of fertilizer, or the planting date.

 

8 

A replicated, randomized, controlled study in 2009 in a rainfed barley field in Spain found similar nitrous oxide emissions in plots with organic or inorganic fertilizers. Greenhouse gases: Similar nitrous oxide emissions were found in plots with organic or inorganic fertilizers (266–373 vs 345 g/ha). Lower nitric oxide emissions were found from plots with organic fertilizer, compared to inorganic fertilizers, in three of four comparisons (29–45 vs 62 g/ha). Methods: Plots (30 m2) had organic fertilizer (pig slurry, anaerobically-digested pig slurry, municipal solid waste, or composted crop residue with sludge) or inorganic fertilizer (urea), applied in January 2006 (125 kg N/ha; three plots for each fertilizer) and incorporated into the soil using a roto-cultivator (0–5 cm depth). Phosphate and potassium (75 and 40 kg/ha, respectively) were added to all plots. Greenhouse gases were measured in manual chambers (35 cm diameter, 20 cm height), four times in the first week after fertilizer application, 2–3 times/week in the first month, and once/week until the end of the cropping season or until emissions were close to zero.

 

9 

A replicated, randomized, controlled study in 2001–2009 in an irrigated nectarine orchard in Italy found more organic matter, nutrients, and microbial biomass in soils with organic fertilizer, compared to inorganic fertilizer. Organic matter: More organic matter was found in soils with organic fertilizer, compared to inorganic fertilizer, in 11 of 32 comparisons (2–5% vs 1–2%). Nutrients: More nitrogen (ammonium), phosphorus, and potassium was found in soils with organic fertilizer, compared to inorganic (ammonium, in two of 28 comparisons: 10–15 vs 6–7 mg/kg; phosphorus, in one of four comparisons: 24 vs 14 mg/kg; potassium, in three of four comparisons (299–350 vs 234 mg/kg). Similar pH levels were found in plots with organic or inorganic fertilizer (pH 7.7–7.8 vs 7.8). Soil organisms: More microbial biomass (measured as carbon) was found in soils with organic fertilizer, compared to inorganic fertilizer, in 11 of 76 comparisons (5–22 vs 4–11 mg/g). Implementation options: More organic matter, nutrients, and microbial biomass was found in plots with compost, compared to manure (organic matter, in seven of 21 comparisons: 2–5% vs 2–3%; ammonium, in two of 21 comparisons: 10–14 vs 6 mg/kg; phosphorus, in one of three comparisons: 24 vs 20 mg/kg; potassium, in one of three comparisons: 350 vs 312 mg/kg; microbial biomass, in nine of 60 comparisons: 5–22 vs 2–13 mg/g). Similar pH levels were found in plots with compost or manure (pH 7.7 vs 7.8). Methods: There were four plots for each of four organic-fertilizer treatments (5 t compost/ha in May; 5 t/ha split into two applications, in May and September; 10 t/ha split into two; or 5–10 kg dry cow manure/ha), and there were four plots for inorganic fertilizer (70–130 kg N/ha, 100 kg P/ha, 200 kg K/ha; plot size not reported). The compost was made from domestic organic waste and urban pruning material (50% each). Fertilizers were 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) and four times in spring and summer in 2008–2009 (0–80 cm depth for nitrogen, and 4–20 cm depth for microbial biomass).

 

10 

A replicated, randomized, controlled study in 2006 in a barley field in the Henares river basin, Spain, found that more methane was absorbed by soils with organic fertilizer, compared to inorganic fertilizer. Greenhouse gases: More methane was absorbed by soils with organic fertilizer, compared to inorganic fertilizer, in one of four comparisons (digested slurry: –286 vs –115 mg C/m2). No differences in carbon dioxide emissions were found between soils with organic fertilizers or urea (334–466 vs 458 kg C/ha). Methods: There were three plots (30 m2) for each of four organic fertilizers (anaerobically digested thin fraction of pig slurry, untreated pig slurry, composted municipal solid waste, or sewage sludge and composted crop residues) and one mineral fertilizer (urea), applied in January (125 kg available N/ha). Plots were cultivated (0–5 cm depth) to incorporate the fertilizers. 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).

 

11 

A replicated, randomized, controlled study in 2006 in a rainfed almond orchard near Granada, Spain, found no differences in organic matter, nutrients, or soil stability between plots with organic or inorganic fertilizer. Organic matter: Similar amounts of organic carbon were found in soils with organic or inorganic fertilizer (8.8 vs 8.6 g C/kg soil). Nutrients: Similar amounts of nitrogen (1.1 g N/kg soil), phosphorus (1.7 vs 2 mg P/kg soil), and potassium (156 vs 153 mg K/kg soil), and similar pH levels (pH 8.3), were found in soils with organic or inorganic fertilizer. Soil erosion and aggregation: Similar soil stability was found in plots with organic or inorganic fertilizer (62% of soil aggregates were water-stable; 16% vs 15% change in the mean weight diameter of soil aggregates after sieving). Methods: Organic fertilizer (1,500 kg compost/ha, made from sheep manure and turf) or mineral fertilizer (250 kg/ha, 4.6% N, 1.2% P, 1.5% K) was used on 18 plots each (588 m2). Some organic fertilizer was used on all plots (30 t manure/ha), and one-third of the plots were grazed by sheep (7 kg organic C/ha from excrement). All plots had cover crops. Soil samples were collected on 18 July 2006 (0–20 cm depth). It was not clear whether these results were a direct effect of the type or amount of fertilizer.

 

12 

A replicated, randomized, controlled in 2006–2008 in an irrigated alfalfa field in Spain found no differences in nutrients between soils with organic or inorganic fertilizers. Nutrients: Similar amounts of nitrogen (3%), phosphorous (0.3%), and potassium (3%) were found in plots with organic or inorganic fertilizer. Methods: Lysimeters (5 m2 and 1.5 m deep) had either organic fertilizer (pig slurry: 170 or 340 kg N/ha/year) or inorganic fertilizer (phosphorous-potassium: 200 kg/ha/year; phosphorus pentoxide and potassium oxide: 150 kg/ha/yr). Soil was sampled before sowing alfalfa (April 2006), at the start of the second growing season (February 2007), and at the end of the two growing seasons when the slurry was applied (November 2007 and 2008).

 

13 

A replicated, randomized, controlled study in 2007–2009 in an irrigated onion field near Madrid, Spain, found similar nitrous oxide emissions in plots with organic or inorganic fertilizer, but more methane was absorbed by plots with organic fertilizer, in some comparisons. Greenhouse gases: Similar nitrous oxide emissions were found in plots with organic or inorganic fertilizer (1.1–1.2 vs 1.2 kg/ha). More methane was absorbed by plots with organic fertilizer, compared inorganic fertilizer, in one of two comparisons (–0.49 vs –0.02 kg/ha). Implementation options: Plots that were fertilized with slurry absorbed methane, but plots that were fertilized with manure emitted methane (–0.5 to –0.02 vs 0.08 kg/ha). Methods: Plots (20 m2) had organic fertilizer (anaerobically digested pig slurry, or hen and goat manure) or inorganic fertilizer (urea) in 2007 and 2008 (110 kg N/ha; three plots for each). Fertilizers were immediately incorporated into the soil (10 cm depth), using a rotocultivator. Plots were irrigated 1–2 times/week (608–618 mm/year). Greenhouse-gas samples (closed chambers, 19 litre volume, 10 mL samples, 0, 30, and 60 minutes after closing) and soil samples (0–10 cm depth) were collected four times/week in the first two weeks after fertilizer was applied, twice/week during the first month, and once/week until the end of cropping season.

 

14 

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 organic fertilizer, compared to inorganic fertilizer, in most comparisons. Nutrients: Similar amounts of nitrate were found in soils with organic fertilizer, compared to inorganic fertilizer, in 78 of 80 comparisons (1–50 vs 1–31 mg NO3-N/litre water). In two of 16 comparisons, less nitrate was found in plots with cattle slurry, compared to mineral fertilizer (13 days after application: 11 vs 28 mg NO3-N/litre water; 33 days after application: 6 vs 15 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 five organic-fertilizer treatments (single application in spring, or split application in spring and autumn, of municipal waste compost or sewage sludge, or split application of cattle slurry) and one mineral-fertilizer treatment. Maize was grown in spring–summer, and oats were grown in autumn–winter.

 

15 

A replicated, controlled study in 2006 in an almond orchard in Italy found more carbon and nitrogen, and higher pH levels, in plots with organic fertilizer, compared to inorganic fertilizer. Organic matter: More carbon was found in soils with organic fertilizer, compared to inorganic fertilizer, in seven of 12 comparisons (8,173–9,420 vs 7,307–8,740 mg/kg). Nutrients: More nitrogen was found in soils with organic fertilizer, compared to inorganic fertilizer, in 10 of 12 comparisons (1,027–1,280 vs 760–1,037 mg/kg). pH: Higher pH levels were found in soils with organic fertilizer, compared to inorganic fertilizer, in six of 12 comparisons (pH 8.3–8.8 vs 7.5–8.0). Methods: Plots (495 m2) had organic fertilizer (manure pellets: 1.5 t/ha) or inorganic fertilizer (300 kg/ha in summer; three unspecified doses in spring) (three plots for each fertilizer). Plots were drip-irrigated (2,000 m³/ha/year). Soil samples were collected in 2006 (five samples/plot, 0–15 cm depth).

 

16 

A meta-analysis from 2013 of studies in Mediterranean climates found a higher percentage of organic carbon in soils with added organic matter, compared to conventionally fertilized soils. Soil organic matter: A higher percentage of soil organic carbon was found with than without added organic matter (24% higher). Methods: The Web of Knowledge database was searched, using the keywords, “Mediterranean”, “soil”, and “conventional”, and 37 data sets from 26 studies of organic amendment were found and meta–analysed. The most recent studies included in this meta–analysis were published in 2011.

 

17 

A meta-analysis from 2013 of studies in Mediterranean climates found that nitrous oxide emissions from soils were lower after adding organic fertilizer, compared to synthetic fertilizer. Greenhouse gases: Nitrous oxide emissions were 23% lower after adding organic fertilizer, compared to synthetic fertilizer. Implementation options: Nitrous oxide emissions were lower after adding solid organic fertilizer, but not liquid organic fertilizer, compared to synthetic fertilizer (solid: 28% lower; liquid: 8% lower). Methods: Solid organic fertilizers included cover-crop residues, manure, composed manure, composted municipal solid waste, and composted thick fractions of digested pig slurries. Liquid organic fertilizers included raw or digested pig slurries. Synthetic fertilizers included ammonium nitrate, ammonium sulfate, urea, and NPK. Eight studies were included in the meta-analysis. These studies were found by searching the Web of Knowledge database, using the terms “nitrous oxide” or “N2O” and “emission” and “Mediterranean” or the name of a country with a Mediterranean climate, and also by searching the references in the publications that were found.

 

18 

A replicated, randomized, controlled study in 2000–2005 in an irrigated barley-maize field in Spain found more water-stable aggregates in soils with organic fertilizer, compared to inorganic fertilizer. Soil erosion and aggregation: More water-stable aggregates were found in soils with organic fertilizer, compared to inorganic fertilizer (15–17% vs 6%). Methods: Plots (3.8 x 2.5 m) had inorganic fertilizer (barley: 150 kg N/ha/year; maize: 100 kg N/ha/year) or organic fertilizer (slurry: 30, 60, 90, or 120 Mg/ha/year) in 2000–2003 (three plots for each). Phosphorus (120 kg P2O5/ha) and potassium (180 kg KCl/ha) were added to all plots in 2003 and 2004. Barley was sown in December 2003 and harvested in June 2004. Maize was sown in July 2004 and harvested in December.

 

19 

A replicated, randomized, controlled study in 2002–2012 in a rainfed cereal field in Spain found more organic matter and nutrients, but fewer mites, in soils with organic fertilizer, compared to inorganic fertilizer. Organic matter: More organic matter was found in plots with organic fertilizer, compared to inorganic fertilizer, in one of 10 comparisons (1.9% vs 1.6%). Nutrients: More nitrogen (in two of 10 comparisons: 0.14% vs 0.10–0.12%), phosphorus (in 8 of 10 comparisons: 35–78 vs 24–40 mg/kg), and potassium (in six of ten comparisons: 268–528 vs 188–294 mg/kg) was found in soils with organic fertilizer, compared to inorganic fertilizer, but similar pH levels were found. Soil organisms: Fewer oribatid mites were found in plots with organic fertilizer, compared to inorganic fertilizer, in one of ten comparisons (2,404 vs 5,440 individuals/m2). Methods: Plots (11 x 12.5 m or 7 x 12.5 m) had no fertilizer, slurry (pig: 30 or 55 t/ha/year; sow: 25, 55, or 80 t/ha/year), or mineral fertilizer (60 or 120 kg N/ha) (12 replicates of each, but three replicates with sow slurry at 25 t/ha/year). Plots had reduced tillage (disc-harrowing, 15 cm depth) or no tillage (with herbicide). Straw was removed from all plots. Soil samples were collected in October 2011, February 2012, and May 2012 from plots without fertilizer and plots with 25 t/ha/year (three cores/plot, 0–5 cm depth). The other plots were sampled in May 2012.

 

20 

A replicated, randomized, controlled study in 2010–2013 in rainfed barley fields in Spain (same study as (21)) found higher carbon dioxide emissions in plots with organic fertilizer, compared to inorganic fertilizer. Organic matter: Similar amounts of carbon were found in soils with organic and inorganic fertilizers (92–110 vs 87–101 Mg/ha). Greenhouse gases: Similar uptake of methane was found in plots with organic fertilizer compared to inorganic fertilizer (–3 to –1 vs –4 to –1 kg C/ha). Higher carbon dioxide emissions were found in plots with organic fertilizer, compared to inorganic fertilizer, in two of 12 comparisons (4,586 vs 3,575–3,802 kg C/ha). Methods: Plots (inorganic: 50 x 6 m or 40 x 6 m; organic: 40 x 12 m) had inorganic fertilizer (60, 75, 120, or 150 kg N/ha) or organic fertilizer (pig slurry: 75 or 150 kg N/ha) (three plots for each). Plots had conventional tillage (mouldboard plough: 25 cm depth; cultivator: 15 cm depth) or no tillage. Soil samples were collected at the end of the experiment (two samples/plot; 0–75 cm depth).

 

21 

A replicated, randomized, controlled study in 2010–2013 in rainfed barley fields in Spain (same study as (20)) found less nitrate and lower nitrous oxide emissions, but more ammonium, in plots with organic fertilizer, compared to inorganic fertilizer. Nutrients: Less nitrate was found in plots with organic fertilizer, compared to inorganic fertilizer, in two of four comparisons (59 vs 107–148 kg/ha). More ammonium was found in plots with organic fertilizer, compared to inorganic fertilizer, in one of four comparisons (16 vs 9 kg/ha). Greenhouse gases: Lower nitrous oxide emissions were found in plots with organic fertilizer, compared to inorganic fertilizer, in one of four comparisons (0.1 vs 0.3 mg/m/day). Methods: Plots (inorganic: 50 x 6 m or 40 x 6 m; organic: 40 x 12 m) had inorganic fertilizer (60, 75, 120, or 150 kg N/ha) or organic fertilizer (75 or 150 kg N/ha) (three plots for each). Plots had conventional tillage (mouldboard plough: 25 cm depth; cultivator: 15 cm depth) or no tillage. Soil samples (0–5 cm depth) and nitrous oxide samples (closed chambers, 15 mL samples, 0, 30, and 60 minutes after closing), were collected every 2–3 weeks in 2011–2013.

 

22 

A replicated, randomized, controlled study in 2010–2012 in a rainfed barley field in Spain found higher carbon dioxide emissions in plots with organic fertilizer, compared to inorganic fertilizer. Nutrients: Similar amounts of ammonium (3 vs 2 mg/kg) and nitrate (89 vs 85 mg/kg) were found in plots with organic or inorganic fertilizer. Soil organisms: Similar amounts of microbial biomass (measured as carbon) were found in plots with organic or inorganic fertilizer (859 vs 978 mg/kg), but more microbial biomass (measured as nitrogen) was found in plots with organic fertilizer (338 vs 183 mg/kg). Soil erosion and aggregation: More water-stable aggregates were found in plots with organic fertilizer, compared to inorganic fertilizer, in one of two comparisons (0.2 vs 0.1 g). Greenhouse gases: Higher carbon dioxide emissions were found in plots with organic fertilizer, compared to inorganic fertilizer (1,669 vs 1,199 µg/kg macroaggregates/hour). Similar methane fluxes were found in plots with organic or inorganic fertilizer (–0.1 vs 0.1 µg/kg macroaggregates/hour). Similar nitrous oxide emissions were found in plots with organic or inorganic fertilizer (1 vs 0.9 µg/kg macroaggregates/hour). Methods: Plots had organic or inorganic fertilizer (150 kg N/ha) (three plots each; plot size not clearly reported). Plots had conventional tillage (20 cm depth) or no tillage. Soil samples (0–5 cm depth) and gas samples (15 mL) were collected in March 2012.

 

23 

A replicated, randomized, controlled study in 1996–2011 in a vineyard in Navarra, Spain, found more organic matter and nutrients, and higher greenhouse-gas emissions, in plots with organic fertilizer, compared to inorganic fertilizer. Organic matter: More organic matter was found in plots with organic fertilizer, compared to inorganic fertilizer, in one of three comparisons (SMC compost: 1.8% vs 1.1%). Nutrients: More nitrogen (in two of three comparisons: 0.1% vs 0.06%), phosphorus (65–81 vs 29 mg/kg), and potassium (potassium oxide, in one of three comparisons: 474 vs 253 mg/kg) were found in soils with organic fertilizer, compared to inorganic fertilizer. Soil organisms: Similar bacteria communities were found in all plots, for 11 of 12 bacteria genera. However, in plots fertilized with compost, a lower percentage of RNA sequences came from Nitrosporia or Nitrosolobus species (0.0–0.1%), compared to plots fertilized with inorganic fertilizer (0.2%). Greenhouse gases: Higher greenhouse-gas emissions were found in plots with organic fertilizer, compared to inorganic fertilizer (1,591–1,745 vs 1,053 kg CO2 equivalent/ha, cumulative over 115 days after fertilizer). Similar nitrous oxide emissions were found in plots with organic or inorganic fertilizer (1.8–5.1 g N2O-N/ha/day, 15 days after fertilizer). Methods: Three types of organic fertilizer (compost) were compared with one mineral fertilizer (MIN): pelletized organic compost (PEL), compost from the organic fraction of municipal solid waste (OF-MSW), and sheep-manure compost (SMC). Each treatment was assigned to a plot (15 vines), and there were three blocks (the size of plots within blocks was not clearly reported). Compost or fertilizer was added in February 1998–2011 (PEL: 3,700 kg fresh weight/ha/year; OF-MSW: 4,075 kg; SMC: 4,630 kg; MIN: 340 kg NPK/ha/year). For N, P, K, and pH measurements, soil samples were taken at the end of 2011 (four samples/plot, 0–30 cm depth). For greenhouse-gas measurements, air samples (20 ml, 10 samples/plot, closed chambers) were taken over 115 days after adding fertilizer. For partial prokaryotic 16S rRNA sequencing, soil samples (four/plot, 5–30 cm depth) were taken 15 days after adding fertilizer.

 

24 

A replicated, randomized, controlled study in 2015 in a sorghum field in Italy found inconsistent differences in bacteria between plots with organic or inorganic fertilizer. Soil organisms: More bacteria were found in plots with organic fertilizer, compared to inorganic fertilizer, in three of 32 comparisons (111–2,017 vs 201,690 phylotype 16S rRNA sequences), but fewer were found in three of 32 comparisons (60–1,658 vs 103–1,858 sequences). Implementation options: More bacteria were found in plots with a double application of organic fertilizer, compared to a single application, in one of 16 comparisons (1,658 vs 1,469 sequences), but less were found in one of 16 comparisons (34 vs 111 sequences). Methods: Plots (5 x 8 m) had inorganic fertilizer (130 kg urea/ha) or compost (single application: 130 kg N/ha; double application: 260 kg N/ha) (four plots for each). 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).

 

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A replicated, randomized, controlled study in 2003–2004 in an irrigated maize field in Spain found similar amounts of nitrogen in plots with organic or inorganic fertilizer. Nutrients: Similar amounts of nitrogen were found in plots with organic or inorganic fertilizer (21–80 vs 13–37 kg N/ha). Methods: Plots (30 x 40 m) had organic fertilizer (pig slurry: 30, 60, 90, or 120 Mg/ha) or inorganic fertilizer (0, 180, 240, or 300 kg N/ha) (three plots for each). Slurry was immediately covered after application. Lysimeters (2.6 x 2 m; 1.5 m depth) were installed in each plot, five years before the study. Each lysimeter was drip-irrigated, simulating flood irrigation (May to mid-September, with 7–12 intervals). Soil samples were collected after harvest (0–120 cm depth).

 

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A replicated site comparison in 2009 in rainfed vineyards in southern France found more organic matter, nitrogen, and microbial biomass, but less phosphorus and fewer nematodes, in soils with organic fertilizer, compared to inorganic fertilizer. Organic matter: More organic carbon was found in soils with organic fertilizer, compared to inorganic fertilizer, in one of three comparisons (11 vs 7 g C/kg soil). Nutrients: More nitrogen was found in soils with organic fertilizer, compared to inorganic fertilizer, in one of three comparisons (1.1 vs 0.7 g N/kg soil), but less phosphorus was found in one of three comparisons (6 vs 8 mg P/kg soil). Similar amounts of potassium and similar pH levels were found in soils with organic or inorganic fertilizer (data not reported). Soil organisms: More microbial biomass (measured as carbon) was found in soils with organic fertilizer, compared to inorganic fertilizer, in one of three comparisons (49 vs 23 mg C/kg soil), and fewer nematodes were found in one of three comparisons (616 vs 860 total nematodes/100 g soil). Methods: In 146 plots of three soil types, inorganic fertilizer only (37–69% of plots in each soil type) or at least some organic fertilizer (31–63%) was used 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).

 

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.