Providing evidence to improve practice

Action: Convert to organic farming Soil Fertility

Key messages

Biodiversity: Four studies in Asia, Europe, and the USA (including two site comparison studies and three replicated trials) found higher numbers, diversity, functional diversity (see background) or activity of soil organisms under organic management.

Soil organic carbon: Two replicated trials in Italy and the USA showed that organically managed orchards had higher soil carbon levels compared to conventionally managed orchards. One randomized, replicated trial in the USA found soil carbon was lower under organic management compared to alley cropping.

Soil organic matter: One replicated trial in Canada found that soil nutrients were lower in organically managed soils.

Yields: One replicated trial in Canada found lower yields in organically managed soils. Two replicated trials in the USA (one also randomized) found that fruit was of a higher quality and more resistant to disease, though smaller or that organic management had mixed effects on yield.

SOIL TYPES COVERED: clay, clay loam, fine sandy-loam, loam, sandy loam, sandy-clay loam, silt, silty-clay, silt-loam.

 

Supporting evidence from individual studies

1 

A replicated site comparison study in 2001-2003 on a range of soil types in North Carolina, USA (Liu et al. 2007) found higher functional diversity in soil bacterial communities on organic farms(average Shannon diversity index of 2.63) compared to sustainable  (index of 2.44) and conventional (index of 2.39) farms. Of the 10 arable farms sampled, three were organic (no synthetic pesticides or fertilizers), three were sustainable (synthetic fertilizers but no pesticides) and four were conventional (synthetic fertilizers and pesticides). Three locations on each farm were sampled taking multiple soil cores. Each of the farm types encompassed a range of soils but all included farms with loamy sand soils. Additional soil types were clay loam with sand (on one organic farm), silt loam (one conventional and one sustainable farm) and clay with rock (one sustainable farm).

 

 

2 

A replicated experiment in 2009 on gravelly silt loam and sandy-clay soils in Sicily, Italy (Canali et al. 2009) found higher carbon levels in the soil (11,000 mg C/kg soil) in organic compared to conventionally managed orchards (8,750 mg C/kg soil). Greater numbers and activity of soil microorganisms occurred in organic (110 mg/kg and 8 mg/kg respectively) than conventional orchards (60 mg/kg and 8 mg/kg respectively). Maximum yield was lower in organic orchards (20 t/ha) compared to conventional orchards (35 t/ha). There were 13 replications of paired organically and conventionally managed citrus orchards (crop species not specified). The study measured soil organic carbon and microbial biomass and activity.

 

3 

A randomized, replicated experiment in 2004-2007 on sandy-clay loam in Georgia, USA (Jacobsen & Jordan 2009) found soil carbon was lower under organic management with strip tillage (8.0 MgC/ha) than under alley crop management (13.4 MgC/ha).  Soil nitrogen followed a similar pattern. Alley cropping stored more soil carbon than conventional tillage (10.7 MgC/ha). Soil microbial biomass was not affected. Crop yields were highest under alley cropping (3932 and 5060 kg/ha okra Abelmoschus esculentus and hot pepper Capsicum annum respectively) except the corn Zea mays/squash Curcurbita moschata intercrop, which was highest under organic management with strip tillage (4087 kg/ha). Four treatments included: alley cropping with strip tillage, organic management with conservation tillage, conventional tillage and fertilizer, and mowed fallow (4, 4, 8 and 6 replicates respectively). Mimosa Albizia julibrissin hedges were 5 m apart with crops grown in between in 15 cm wide by 15 cm deep furrows. Compost and straw mulch were added to each treatment. Vegetable crops were grown in rotation with winter cover crops: okra, hot pepper, corn, squash, crimson clover Trifolium incarnatum, pea Pisum sativum and rye Secale cereale. Soils were sampled each year to 15 cm depth, and measured soil carbon, nitrogen and microbial biomass.

 

4 

A replicated experiment in 2001-2006 on loamy soil in Saskatchewan, Canada (Malhi et al. 2009) found less nitrate (74 kg N/ha) and phosphorus (19 kg P/ha) in soil under organic inputs than under high or reduced inputs (85 kg N/ha, 24 kg P/ha respectively). Nitrate was usually higher in treatments with fewer crop types. Lower yields were recorded in organic compared to high or reduced input treatments (amounts not specified). Three input (tillage/management) levels (organic, reduced, high) were replicated four times. Within these input levels were three crop diversities: low (fallow/wheat Triticum aestivum/oilseed Brassica juncea); cereal (wheat / mustard Brassica juncea or canola Brassica napus/ lentil Len culinaris rotations; or grain (perennial forage crop (sweet clover Melilotus officinalis, pea Pisum sativum, flax Linum usitatissimum or alfalfa Medicago sativa)/ barley Hordeum vulgare) rotations. Within these were six crop phases, rotating the above species with green manure and fallow phases, which were tested in 40 x 12.8 m plots. Fertilizers and pesticides were not applied to the organic treatment. Crop rotations were six years long. Each year, two soil samples were taken from each crop phase (with a third also taken in 2006) to measure nitrate-N, carbon, nitrogen, and phosphorus.

 

5 

A randomized, replicated experiment in 2004 on a fine sandy-loam soil in North Carolina, USA (Overstreet et al. 2010) found more than four times more nematodes in organic strip tillage plots than in conventional tillage plots with synthetic chemical inputs. Nematode numbers were 41% higher under organic inputs, and 48% higher under organic with strip tillage, compared to plots with synthetic inputs and conventional tillage. Earthworm numbers were 31 times higher under strip compared to conventional tillage, and higher under organic rather than conventional inputs in spring only. Combining strip tillage and organic inputs resulted in the highest numbers of nematodes and earthworms. There were four treatments: strip tillage with organic inputs, strip tillage with synthetic inputs (pesticides and fertilizers), conventional tillage with organic inputs and conventional tillage with synthetic inputs. Within each treatment were 12.2 x 24.4 m plots which had vegetable rotations including: wheat Triticum aestivum, crimson clover Trifolium incarnatum, sweet corn Zea mays, cabbage and broccoli Brassica oleracea, tomato Solanum lycopersicum, squash Curcurbita pepo, cucumbers Cucumis sativus and peppers Capsicum annuum. There were four replications of the tillage and input combinations. The study took nematode samples and earthworm extractions (species not specified for either group).

 

6 

A replicated, paired experiment in 2004-2005 on sandy-loam and silty-clay loam soils in California, USA (Reganold et al. 2010) found 159.4% more microorganisms, 33.3% more microorganism activity and a higher genetic diversity of soil organisms (656 genes/group of organisms) on organic farms compared to conventional farms (504 genes/group of organisms). There was 22% more carbon and 30% more nitrogen in organically managed soils. Higher quality strawberry Fragaria ananassa fruit was produced on the organic farms (8.5% more antioxidants (substance which prevents a chemical reaction causing food to deteriorate)) and strawberries were more resistant to disease (strawberries survived 4.54 days on average when mould present) than on conventional farms (strawberries survived 4.15 days). Fruit from organic farms was 13.4% smaller than from conventional farms. The experimental areas included 13 replications of paired commercial organic and conventional strawberry farms. The study took repeated samples of strawberries and soils (to 30 cm depth) to measure strawberry quality, soil biological and chemical properties, and numbers of soil microorganisms.

 

7 

A replicated, randomized site comparison study in 2010 on sandy loam soils in Maharashtra, India, (Chaudhry et al. 2012) found greater functional diversity of soil microorganism communities in organically managed land (Simpson diversity index of 0.0022) compared to  chemically managed land (index of 0.0018) and fallow grassland (index of 0.0015). Organically managed land was fertilized with composted cow manure while fallow grassland was left uncultivated and chemically managed land was fertilized with a mix of nitrogen, phosphorus and potassium in a ratio of 60:30:30 kg/ha. All fertilizers were applied annually for 16 years before 2010. Within each management type, soil samples were taken from four 10 × 10 m plots, at a depth of 15 cm. Soil samples from three of the plots were then randomly selected and mixed together, providing the final sample for analysis.

 

Referenced papers

Please cite as:

Key, G., Whitfield, M., Dicks, L.V., Sutherland, W.J. & Bardgett, R.D. (2017) Enhancing Soil Fertility. Pages 383-404 in: W.J. Sutherland, L.V. Dicks, N. Ockendon & R.K. Smith (eds) What Works in Conservation 2017. Open Book Publishers, Cambridge, UK.