Providing evidence to improve practice

Action: Grow cover crops when the field is empty Soil Fertility

Key messages

Biodiversity: One controlled, randomized, replicated experiment in Martinique found that growing cover crops resulted in more diverse nematode communities. One replicated trial from the USA found greater microbial biomass under ryegrass compared to a ryegrass/vetch cover crop mix.

Soil structure: Three randomized, replicated studies from Denmark, Turkey and the UK found that growing cover crops improved soil structure and nutrient retention. One trial found higher soil porosity, interconnectivity and lower resistance in soil under cover crops, and one found reduced nitrate leaching.

Soil organic carbon: One replicated study from Denmark and one review based mainly in Japan found increased soil carbon levels under cover crops. One study also found soil carbon levels increased further when legumes were included in cover crops.

Soil organic matter: One replicated study from Denmark and three controlled, randomized, replicated studies from Australia and the USA measured the effect of growing cover crops. Three found increased nitrogen levels under cover crops, three found increased carbon, and one found increased nitrates. One trial showed that they increased regardless of whether those crops were legumes or not. Two studies from Europe (including one controlled, replicated trial) found no marked effect on soil organic matter levels.

Yield: One replicated trial from the USA found higher tomato yield from soils which had been under a ryegrass cover crop.

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

Supporting evidence from individual studies

1 

A controlled, randomized, replicated experiment in 1991-1992 on fine loam in California, USA (Wyland et al. 1995) found reduced soil nitrate under cover-cropped (4.3 g nitrate/m2) compared to bare soil (8.6 g nitrate/m2), prior to rye Secale cereale incorporation. Available nitrogen was also higher under cover-cropped (1.8 g N/m2) compared to bare soil (1.1 g N/m2). There were two winter treatments: a rye Secale cereale crop sown in December then incorporated into the soil after16 weeks using a disc plough, and bare-fallow. Both treatments received the same tillage treatment (regular passes with a disc plough). Plots were 8 x 4 m. There were three replicates. Soil was sampled prior to rye incorporation (60 cm depth).

 

 

2 

A controlled, randomized, replicated site comparison study in 1990-1994 on a sandy loam in the UK (Beckwith et al. 1998) found 79% less nitrate leaching at site A and 42% less at site B when a winter cover crop was grown, compared with the fallow (104 and 50 kg N/ha for sites A and B respectively). There were two manure treatments at site A (Shropshire): pig/cattle slurry; cattle farmyard manure (FYM), and two manure treatments at site B (Nottinghamshire): broiler (poultry) litter; FYM. Manures were applied at 200 kg N/ha monthly between September and January to overwinter fallow or onto winter rye Secale cereale. An extra treatment was included to test the nitrification inhibitor DCD, which was applied at 20 l/ha. All treatments were replicated three times at both sites. Plots were 12 × 4 m and 15 × 4 m at sites A and B respectively. The total amount of nitrate lost through leaching and total soil mineral nitrogen was measured.

 

3 

A replicated experiment from 1981 to 2002 on sandy loam in Denmark (Thomsen and Christensen, 2004) found ryegrass Lolium perenne catch cropping increased soil carbon (14.2 mg C/g) and nitrogen levels (13.3 mg N/g) compared to the control (12.7 mg C/g and 12.4 mg N/g respectively). Four straw management treatments were applied to barley Hordeum vulgare crops: straw removed (control: 0 t/ha), low (4 t/ha), medium (8 t/ha) and high straw application (12 t/ha). From 1981 to 1988 35 t/ha of pig slurry was applied to the straw treatments. After, a ryegrass catch crop was grown. Plot sizes were not specified. In 1999-2002, wheat Triticum aestivum was sown into the straw treatments. Each treatment was divided into 21.25 m2 plots and received 0, 60, 120 or 180 kg N/ha. There were three replicates. Soil was sampled to 20 cm depth.

 

 

4 

A controlled, randomized, replicated experiment in 2005 on  fine sandy loam  in Massachusetts, USA (Ding et al. 2006) found higher soil organic carbon and nitrogen levels under vetch Vicia villosa/rye Secale cereale cover crops (14.1 kg C/m3 soil and 1.7 kg N/m3) or rye cover alone (15 kg C/m3 soil and 1.7 kg N/m3), compared to soil with no cover crop (12.2 kg C/m3 soil and 1.4 kg N/m3 soil respectively), regardless of nitrogen fertilizer rate. There were three cover crop treatments, including: vetch /rye, rye alone, no cover crops (control). Each 3 × 7.5 m replicate was treated with nitrogen fertilizer at a rate of 0, 67, 135 or 202 kg N/ha. There were four replicates. Plots were seeded in September and cut at the end of May.  The main crop corn Zea mays was seeded in June and harvested in August.

 

5 

A randomized, replicated, controlled experiment between October 1998 and July 2001 on silty-clay soil in Samsun, eastern Turkey (Gülser 2006) found improved soil structure, increased soil organic carbon content between 1 and 37%, and reduced soil penetration resistance between 15 and 36% under forage (animal feed) cropping, relative to unplanted fallow controls. Bromegrass Bromus inermis was the most effective forage and perennial ryegrass Lolium perenne the least. After autumn ploughing (to 15 cm depth) and rototilling, perennial ryegrass, bromegrass, alfalfa Medicago sativa, small burnet Sanguisorba minor, subterranean clover Trifolium subterraneum and purple crownvetch Coronilla varia forage treatments were established and compared with unplanted fallow controls. Forages were grown on three 2 × 5 m plots and seeded in rows 40 cm apart. Soil characteristics were measured on samples taken from the top 15 cm of soil.

 

6 

A review of 120 papers testing interventions on a range of soils largely in Japan, found enhanced soil organic carbon storage under cover crop management, no-tillage practices and manure (and other organic by-products) application. Longer-term cover crops resulted in considerable increases in soil organic carbon. In the review, research by Wagger (1988 & 1989) found that carbon input to the soil was greater under legumes due to their lower lignin and cellulose (substances which form cell walls in plants) content, and because legume growth is not limited by nitrogen availability in the soil. Balanced and integrated increases in the soil organic carbon pool, lessening of non-carbon dioxide emissions, and control of soil nutrients based on location-specific recommendations are also needed. No review methods were specified. Tillage systems reviewed included: no-tillage, conservation tillage (surface residues retained), conventional tillage (mouldboard plough, rotary tillage, disced). Cover crops reviewed included a mix of leguminous and grass covers: rye Secale cereale, hairy vetch Vicia villosa, and crimson clover Trifolium incarnatum.

 

7 

An experiment in 2001-2005 on silty loam soil in Villmar-Aumenau, Germany (Möller, 2009) found no obvious changes in soil carbon or nitrogen despite different cover crop and manure management. There were two trials. Trial 1 had eight treatments: (1-2) Clover/grass ley; (3) wheat Triticum aestivum plus cover crops receiving farmyard manure (FYM) as slurry or effluents; (4) potatoes Solanum tuberosum receiving FYM and solid effluents, or silage maize Zea mays receiving FYM; (5) rye Secale cereal plus cover crops plus FYM; (6) Peas Pisum sativum plus cover crops; (7) Spelt T. aestivum ssp. Spelta plus cover crops plus FYM, and (8) wheat undersown with clover/grass ley plus FYM and solid effluents. Trial 2 included: (1) clover/grass ley; (2) potatoes plus solid effluents; (3) winter wheat plus liquid effluents; (4) peas; (5) winter wheat plus liquid effluents; (6) spring wheat plus solid effluents. Which applications of slurry were digested or not were not specified. All manuring treatments were applied before ploughing. Five soil samples were taken from each plot to 30 cm depth and measured soil nitrogen and carbon.

 

8 

A randomized, replicated study in 2008-2009 on a sandy-loam soil in Foulum, Denmark (Kadžienė et al. 2011), found a higher number of air spaces in the soil and how connected they are under fodder (animal feed) radish Raphanum sativus, compared to dyer’s woad Isatis tinctoria.. Root growth in the 12-16 cm layer was limited by high soil resistance in directly drilled (-100 kPa) or harrowed (-30 kPa) soil, compared to ploughed soil. A spring barley Hordeum vulgare crop was the main experimental area, replicated three times. It was part of a longer term rotation (not specified). The cover crops were dyer’s woad and fodder radish. Tillage treatments consisted of two 3 × 72.2 m plots within the crop, which were divided into smaller 3 × 13.7 m sub-plots. Tillage was direct drilling, harrowing or ploughing. Soil samples were takento16 cm depth from each sub-plot.

 

9 

A replicated, randomized, controlled experiment in 2010-2011 on clay soil, in central Martinique (Djigal et al. 2012), found more diverse soil nematode communities under cover crops (0.8 Shannon index) than in the no cover crop control (0.45 Shannon index), with grass cover crops having the highest soil nematode diversity (1.2 Shannon index). The study used six treatments in an old banana Musa spp. system: control without cover crop, a self-seeded cover crop, a grass Paspalum notatum cv. common, and three legumes (perennial soybean Neonotonia wightii, tropical kudzu Pueraria phaseoloides, and Brazilian Lucerne Stylosanthes guyanensis). There were three replicate plots of each treatment, giving a total of 18 plots. Four soil samples were taken from each plot twice, once at 16 and again at 20 months after the experiment was established. Soil samples were then mixed prior to analysis in the laboratory.

 

10 

A controlled, replicated experiment in 2005-2009 on silty loam soil in eastern Spain (García-Orenes et al. 2012) found no marked difference between the soil in the ploughed then sown oats Avena sativa treatment and the control, after five years. There were three replicates of five management treatments including: residual herbicide use; ploughing (4 times a year to 20 cm depth); ploughing then sown oats (as before, then oats sown in spring); addition of oat straw mulch; land abandonment (control). Plots were 6 × 10 m. Soil under native vegetation was used as a reference. Six soil samples from each plot were taken annually to 5 cm depth. Five rainfall simulations were also conducted during the summer drought period on 1 m2 plots. Simulations lasted one hour at 55 mm/h (simulating thunderstorm rain levels). The study measured soil organic matter, arbuscular mycorrhizal proteins, aggregate stability and soil erosion.

 

11 

A controlled, replicated experiment in 2005-2009 on loam in Michigan, USA (Nair & Ngouajio 2012) found higher microbial biomass under perennial ryegrass Lolium perenne and compost (195-210 μg/g dry soil) than under ryegrass without compost, or ryegrass/vetch Vicia sativa with and without compost (145-160 μg/g dry soil). Microbial respiration was highest in soil under the ryegrass-compost combination (282 μg carbon dioxide/g dry soil), compared to ryegrass/vetch with no compost (126 μg carbon dioxide). Tomato Lycopersicon esculentum yield was higher in soils after the ryegrass-compost treatment (44 kg/ha) than in ryegrass/vetch with no compost (22 kg/ha). It was not clear whether these effects were due to the cover crop or compost treatments. Two cover crop treatments were sown into soil between crops: ryegrass and ryegrass with vetch. Within these were two compost treatments: compost (25 t/ha dairy compost, but reduced to 12.5 t/ha in 2009) and no compost. There were four replications. Cover crops were mowed and incorporated into the soil before tomato seedlings were transplanted into 7.6 x 0.6 m beds. Four soil samples were taken to 15 cm depth from each treatment during the growing season.

 

12 

A controlled, randomized, replicated experiment in 2009-2010 on sandy clay soil in south-eastern Australia (Zhou et al. 2012) found higher levels of soil carbon and nitrogen under all cover crops (vetch Vicia villosa (12.6 and 810.3), pea Pisum sativum (12.6 and 752.5), wheat Triticum aestivum (14.8 and 807.5), oat Avena strigosa (10 and 770), and  mustard Brassica juncea (10.45 and 777.5)), compared to the control (6.8 mg C/kg dry soil, 622.5 mg N/kg dry soil respectively). There were no differences in carbon or nitrogen levels between legume and non-legume cover crops. Crop residue quantity was highest in wheat (143 g/m2) compared to vetch (163 g/m2), pea (110 g/m2) and mustard (100 g/m2). Six cover crop treatments (4 × 10 m) included: two legume crops (vetch, pea); three non-legume crops (wheat, Saia oat, Indian mustard); and a no-crop control. There were three replicates of each treatment. Five soil samples were taken from each plot to 10 cm depth.

 

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.