Providing evidence to improve practice

Action: Grow cover crops beneath the main crop (living mulches) or between crop rows Soil Fertility

Key messages

Biodiversity: One randomized, replicated study from Spain found that cover crops increased bacterial numbers and activity.

Erosion: Two studies from France and the USA showed reduced erosion under cover crops. One controlled study showed that soil stability was highest under a grass cover, and one randomized replicated study found that cover crops reduced soil loss.

Soil organic matter: Two controlled trials from India and South Africa (one also randomized and replicated) found that soil organic matter increased under cover crops, and one trial from Germany found no effect on soil organic matter levels.

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

 

Supporting evidence from individual studies

1 

A randomized, replicated, controlled study in 1988-1989 in Oklahoma, USA (Roberts & Cartwright 1991) found that cabbage Brassica oleracea plots with rye Secale cereal cover crops retained more soil (25% drop in bed height over nine months) than bare ground controls (35% drop). Beds with hairy vetch Vicia villosa cover crops dropped in height by 29% over nine months but mean bed height was similar to beds in rye cover-cropped plots and bare controls. Above ground, rye produced more dry vegetation (4,754 kg/ha after six months) than hairy vetch (1,213 kg/ha). Raised cabbage beds (90 cm wide, 20 cm tall) were planted with cover crops (rye, hairy vetch or left bare) in mid-October 1988. Plots were sprayed with glyphosate on 5 April 1989 and cabbages were planted into cover crops on 17 April. Cover crop treatments were replicated three times in plots of 1.8 x 6.1 m. Bed height was measured on 3 November 1988, 27 March and 12 July 1989.

 

2 

A controlled experiment in 1991-2000 on a sandy loam in Champagne, France (Goulet et al. 2004) found that soil particle stability was highest in the topsoil under a grass cover (index rating (K) = 21.7), followed by coniferous bark mulch (rating = 15.2) and poplar bark mulch (rating = 13.6), compared to the control (rating = 10.5). The conifer bark layer also increased stability in soils to 20 cm depth. Three treatments and a control were tested, comprising: a bluegrass Poa pratensis cover between vine rows only; organic mixed mulch of coniferous silver fir Abies alba, Norway spruce Picea excelsa and Scots pine Pinus sylvestri bark between and in vine rows (61 t/ha applied every three years); organic mulch of poplar Populus spp. bark (67 t/ha applied every three years); and bare soil between rows (control). Treatments and controls were tested in 35 x 8 m and 15 x 8 m plots, respectively. Soil under the grass cover was sampled in and between vine rows while the mulch and control treatments were sampled between vine rows only. All soils were sampled to 20 cm depth.

3 

A controlled experiment from 1993 to 2003 on sandy soil in Lutzville, South Africa (Fourie et al. 2007) found that after five years, soil organic matter to 30 cm depth was 60% higher in grazing vetch, pink Seradella Ornithopus sativus and ‘Saia’ oats Avena strigosa cover crops compared to the controls. Soil organic matter to 60 cm depth was higher under grazing vetch Vicia dayscarpa (52% greater) compared to either control treatment. Seven cover crop species were tested in 108 m2 plots of grape Vitis vinifera vine in a vineyard, including: rye Secale cereale, oats A. sativa, ‘Saia’ oats, Parabinga medic Medicago truncatula, pink Seradella and grazing vetch. All cover crops were sown once or twice annually. All treatments received chemical control, applied before buds opened. There were two controls with no cover crop, one mechanically weeded between vines with chemical control within vines, and the other receiving full surface chemical control. Soil samples were taken from rows between vines and measured soil organic matter. This summary reports overall soil organic matter only, but the study also reported on separate components of organic matter.

 

4 

An experiment in 2001-2005 on silty loam soil in Villmar-Aumenau, Germany (Möller 2009) found no changes in soil carbon or nitrogen when wheat Triticum aestivum was undersown with clover Trifolium spp. and grass (species not specified). Manuring and cover cropping treatments with various crops also had no effect. 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 cereale 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 (wheat sown in February/March, cover crop sown at same time). 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. 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.

 

5 

A randomized, replicated experiment in 2008 on fine sandy loam soil in Spain (Moreno et al. 2009) found that bacteria counts and activity were highest in the mowed cover crop (0.95 billion/g soil and 1,087 μg PNP/g/h) and lowest when herbicides were added (1.37 billion/g soil and 519 μg PNP/g/h). All treatments had a higher microbial diversity and soil organic carbon levels than the treatment with no cover. There were four long-term treatments in an olive Oleo europaea orchard: tillage (3-4 passes with disk harrow to 30 cm depth and tine harrowing (remove small weeds and smooth the soil surface ready for sowing) in summer), no-till, no cover crop (treated with glyphosate herbicide), cover crop (weeds left to grow) treated with herbicides (in March), and mown cover crop (herbicide-free). Each plot was 11 x 11 m and consisted of 16 olive trees. Each treatment was replicated four times. Two soil samples were taken from the centre of each plot. Soil bacterial numbers and community structure were measured. PNP (purine nucleoside phosphorylase) an enzyme which breaks down proteins, was used as an indicator of bacterial numbers.

 

6 

A controlled, randomized, replicated experiment in 2009 on gravelly-sandy-loams in South Andaman Islands, India (Pandey & Begum 2010) found 41% more soil carbon and 46% more soil nitrogen in coconut palm Cocos nucifera plots with cover crops than in the control. Adding phosphorus to the cover crop increased nitrogen levels by 16%. Nitrogen mineralization (breakdown of organic matter, e.g. leaves, into mineral nitrogen) was 39% and 73% higher in soils with a cover crop, and a cover crop plus phosphorus respectively, compared to the control. There were six replicates of four treatments in a coconut plantation: no cover crop (control), no cover crop plus phosphorus (16% P at 24 kg/ha), cover crop (kudzu Pueraria phaseoloides), and cover crop plus phosphorus. Each plot was 40 x 40 m and contained 28 coconut palms, 7.5 m apart. Ten soil samples were taken monthly to 15 cm depth from each plot. Soil carbon, nitrogen and nitrogen mineralization were measured.

 

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.