Alice Ambler and Dr Chris Warburton Brown, October 2019
Mulch is a layer of material applied on the soil surface in a cultivated area. Mulch is usually organic in nature. Hopkins (1954) defined mulch as any dead vegetation lying on the soil, free of the parent plant and easily distinguishable as of organic origin, although today non-vegetative materials are often used. Commonly used mulch materials are straw, bark chips, cardboard, rotted manure, compost, or the organic residue of a previous crop. Mulch may be applied to bare soil or around established plants. Organic mulches will be incorporated into the soil over time by natural decomposition. Mulching is used both in commercial crop production and in domestic gardening, and in countries and climates throughout the world.
Claims made for mulch include conservation of soil moisture, improved soil properties, enriched soil biology, reduced weed growth and less soil erosion. The evidence for each of these claims will be explored in turn, and consideration given to the known limitations of mulching.
Effect on soil moisture
Headline: Experimental studies show that mulch reduces evaporation, increases soil moisture and moderates soil temperatures. This is particularly effective in very hot weather. 0.5 inch depth of mulch seems sufficient to give significant soil moisture benefits. Combining mulch with no till cultivation increases the benefits.
Hopkins (1954) conducted one of the first experimental studies of mulching in small plots, looking at optimum mulch depth, soil evaporation and soil warmth. The study was conducted in Kansas, USA, a dry areas subject to drought. Under unusually high temps and deficient rainfall, available moisture in the top 2 feet of the mulched area was 6%, but less than 1% on the bare soil area. Mulch of 0.5 inch thickness reduced evaporation by 41% from that of bare soil, but an additional 2.5 inches of mulch reduced evaporation only 67%. It was concluded that mulch prevents high surface temps, and thus retards evaporation and improves the soil as a habitat for organisms. Conversely, deep mulch may retard initiation of growth by causing soil to warm more slowly.
Watson and Kupkowski (1991) used 18 inches of course, uniform, fresh wood chip mulch to measure the effect on soil and tree growth. This is an exceptionally deep mulch. They measured Soil oxygen diffusion rate, temperature, pH, moisture, and nitrate levels. Measurements were taken from next to the mulched area - not under the mulch. They found no significant reduction in oxygen content under the mulch, compared to unmulched soil. Soil temperatures were nearly identical under the mulch and in adjacent unmulched soil, however, under the mulch, temperatures fluctuated only half as much between cool and hot sampling days. There was higher soil moisture under the mulch.
Hobbs et al (2008) reported on their Mexican case study of conservation agriculture. Crop residues from previous crops were retained on the soil surface and minimal tillage was introduced in the hope of increasing soil moisture, a limiting condition on local agriculture. Yield, organic matter, nitrogen levels, surface soil aggregate size and soil microbial biomass were measured. It was found that mulch with minimal till was very suitable for this climate - they increased soil moisture, organic matter, nitrogen levels, surface soil aggregate size, soil microbial biomass, and yield.
He and colleagues (2009) studied the long term effects on soil, yields and irrigation of long term mulching in a no-till wheat/oats system in Inner Mongolia, China. They compared no-tillage with straw cover, sub-soiling with straw cover, rototilling with straw cover and traditional tillage. A randomized 10x100m block layout was used. The largest yield improvements coupled with the greatest water use efficiency were achieved by no-tillage with straw cover. Ten-year mean crop yields increased by 14.0% and water use efficiency improved by 13.5% compared to traditional tillage due to greater soil moisture and improved soil physical and chemical status.
Kader et al (2017) conducted a useful review of recent literature on the effects mulching technology and techniques have on the soil. They looked at mulching materials and methods, application of mulching materials, effects of mulching materials, impacts of mulching, economic aspects of mulching, and directions for future research. Experimental studies show that various mulching materials affect the soil hydrothermal regime, altering moisture and temperature of the soil. These alterations influence soil microbiology - essential to create a favorable soil environment for plant growth. Mulching materials modify the microclimate and reduce soil evaporation, significantly impacting water saving in agriculture.
Zhang and colleagues (2009) studied the effects of mulch on soil temperature, moisture and wheat yield in China. In Shaanxi province they created 5m x 5m no till experimental plots and covered them with air dried wheat straw, reapplied to keep at constant levels. Temperature was generally lower under mulch in the warm season, and maximum soil temperature was much decreased because straw has higher albedo and lower conductivity than bare soil. During cold times the soil was generally slightly warmer under the mulch. Soil water storage under the mulch was considerably higher than in the control due to the surface cover and the lower soil temperature depressing evaporation. This was more pronounced in upland sites. There were increased biomass yields under mulch - however the higher soil water enhanced vegetative growth more than grain formation.
Effect on soil chemistry and physical properties
Headline: Results of experiments are inconclusive. Mulch may increase soil potassium and phosphorous. Studies don’t agree on its effect on soil pH or nitrogen, probably because of differences in soil type and climate. Mulch between 5 and 10 cm seems most effective. Deeper mulches have less effect or even negative effects.
Billeaud and Zajicek (1989) tested different kinds of mulch and different mulch depths for their effect on weed control, soil health and plant growth. They used screened pine bark at depths of 0, 5, 10, & 15 cm, with and without weed barrier fabric. Coarser mulches out-performed finer-textured materials. As the depth of the mulch increased, soil pH, soil nitrogen content, and visual rating of plant growth decreased.
Watson and Kupkowski’s study (1991) of very deep wood chip mulch found soil pH was unaffected by the mulch. Nitrates were lower in the soil under the mulch, but values for both mulched and non-mulched soil were very low.
Greenly and Rakow (1995) Compared different kinds of wood mulch and different depths on soil, weeds and tree growth on heavy silt loam. They created small experimental plots and mulched them with chipped pine 7.5cm, 15cm, and 25cm deep, applied in May. The most effective depth was 7.5 cm. Soil oxygen levels, temps, and moisture levels were all within acceptable ranges under a mulch of this depth. Deeper mulches led to soil oxygen decline (non-significantly), temperature decline, and moisture level increase. Soil parameters (pH, nitrates, and salt levels) were unaffected.
Broschat’s 2007 experimental study considered the use of mulch on small growing plots for weed control, nutrients and water conservation. pH and Nitrogen were unaffected by the mulch treatment, although this may be due to the short experimental period. Mulch generally had little effect on soil nutrients in the dry season, but cypress mulch increased soil potassium and pine bark and eucalyptus mulch increased soil magnesium in the wet season.
He et al (2009), working in Inner Mongolia, China, found that long-term conservation tillage (combined no-till and mulch) increased soil organic matter in the top 20 cm by 21.4% and total N by 31.8% in the top 5cm compared to traditional tillage. Macro-aggregates and macroporosity also improved significantly in the 0–30 cm soil layer (P< 0.05). These improvements in soil properties are of considerable importance for the seriously degraded soils in semi arid Inner Mongolia.
Jodaugiené and colleagues (2010) conducted an experimental study of the effect of organic mulches on soil biology at the Lithuania University of Agriculture. They laid 5cm and 10cm straw, sawdust, peat and grass mulches on a medium clay loam soil before the growing season and then planted a bean crop, then repeated this annually. In plots mulched with grass the yield of bean seeds was 1.8 times higher compared with that of control plot and 26.7 times higher compared to sawdust mulched plots. In these plots crops were better supplied with nitrogen, potassium, and phosphorus.
Effect on soil biology
Headline: Studies show mulch has positive effects on soil biology, especially when combined with no till cultivation. Mulch protects soil biology from extremes of heat and moisture and from erosion. It also provides a rich source of food.
Brevault and colleagues (2007) looked at the impact of no till and mulch on soil biology in cotton fields in the Zuana region of the Sudan. The climate in this region is dry and warm (23-32 deg C) with a single rainy season from May to October. Grass and legume mulches were applied to test plots at the end of the previous growing season. Macrofauna was sampled at the seedling stage of cotton, and 30 days later. Mulching and no till cultivation improved environmental conditions for soil organisms by protecting the habitat against water and wind erosion and variations in humidity and temperature, and by increasing organic matter as a food source. Mulch provided a favourable habitat for arthropods - abundance and diversity of soil arthropods were significantly higher in no till grass and no till legumes than in control plots (+103 and 79% respectively). More earthworms were found in the no till soils. No till grass mulch and no till legume mulch favoured the establishment of diverse macrofaunal communities.
The experimental study conducted in Lithuania by Jodaugiené and colleagues (2010) sampled soil urease and saccarase levels and earthworm density twice annually under different kinds of mulch. Mulching with sawdust and grass positively influenced the activity of urease. Grass significantly increased saccarase activity. Other mulches had no significant influence. Urease activity was lowest in plots mulched with peat because decomposition was slow. The thickness of the mulch had no significant effect on the activity of urease. The highest density of earthworms was in the plots mulched with grass (2.1 x higher than control) and straw mulch (1.8 x). Sawdust had no significant influence. Peat mulch decreased the number of earthworms by 29.3%. Lower density and biomass of earthworms was seen in the plots mulched with thicker layers of organic mulches. Earthworm numbers had a positive effect on the amount of plant nutrients in the soil (especially phosphorus and potassium).
Effect on weeds
Headline: Mulch is an effective way of suppressing weeds. Shallower depths (up to 8cm) seem most effective. It is best when applied with a weed barrier.
Billeaud and Zajicek (1989) found that mulch applied at shallower depths in combination with a weed barrier provided optimal weed suppression without tying up soil nitrogen or reducing plant growth. They concluded that mulch applied at shallower depths in combination with a weed barrier provided optimal weed suppression without tying up soil nitrogen or reducing plant growth. Coarser mulches out-performed finer-textured materials. Mulch over 10cm deep inhibited plant growth, although optimum depth was dependent on the mulch material used.
Greenly and Rakow (1995) found that weed density and diversity declined significantly with increasing mulch depth. However, for the tree species they were growing, stem growth was greater with the 7.5cm mulch depth than with other mulch depths or the control. The authors concluded that thicker mulches may reduce lateral root growth and slow warming too much in the spring, and therefore suggested 7.5 cm as the optimum mulch depth.
Broschat’s experiment (2007) concluded that dicot weeds were significantly less for mulched plots than for the unmulched control. However, the number of grass weeds did not differ. Mulch type had no effect on the numbers of any type of weed.
Effect on soil erosion
Headline: The two studies covering this area both concluded that mulch significantly reduces soil erosion on steep slopes.
Gholami et al (2013) measured the effect of straw mulching on soil erosion and runoff in the Aborz Mountains of Northern Iran. They carried out a laboratory analysis of the sandy loam soil from the area in various simulated weather conditions. They set up 6m by 1m erosion plots under an 8cm deep straw mulch. Runoff and soil erosion characteristics were measured. They found that straw mulch can have an effect in a short time after application. Straw mulch significantly reduced splash erosion by absorbing the raindrops' force. It also prevented soil detachment and reduced raindrop effects on the soil surface. Straw mulch increased the runoff commencement time twofold compared with controls. It also reduced the sediment yield of the water, indicating that flow could not get enough power to detach aggregate particles trapped by the mulch.
Miyata and colleagues (2009) looked at the impact of mulch on reducing soil erosion on steep slopes in Mie Prefecture, Japan. They considered a hillslope cypress plantation with 2094 mm mean annual precip, 14.4 deg C average temp and two rainy seasons. On small experimental plots they compared naturally deposited leaf litter (natural conditions) with 5cm of organic mulch. For most storm events, overland flow was less in the mulch plots. There was increased infiltration rates on the mulched soil as the soils became wet. Water repelency of the soil was more severe when soil was dry. It was estimated that erosion in plots without mulch was 5.1 times that of plots with mulch coverage.
Limits and drawbacks of mulch
Headline: Excessively deep mulch (over 8 cm) can cause problems. Excessive use of mulch around shallow rooted plants can suffocate roots. Mulches should not be used as the exclusive weed control method. The effects of mulching are not universal, and vary according to soil and climatic conditions.
Gouin (1983) explored the best ways to use mulch; the optimal depth, best kinds of mulch, when to mulch, and the benefits. The author discouraged excessive use of mulch, arguing that 1 to 2 in of mulch annually is adequate to keep soil cool, reduce water loss by evaporation, and increase water penetration into the soil. Fresh mulch should not be applied until the existing mulch is nearly decomposed. Excessive use of mulch around shallow rooted plants and large coniferous trees can be detrimental, suffocating roots. Shallow rooted plants growing on sandy well aerated soils can tolerate more mulching than plants growing on heavy clay or silt loam soils. Mulches should not be used as the exclusive weed control method, because as organic mulches decompose they enrich the soil making conditions more favourable for weed seed germination.
Erenstein (2002) conducted a detailed review of prior research on crop residue mulching in tropical and semi-tropical countries. The review focussed on crop residue mulch in large scale growing including field farming. The author focussed on mulching in terms of conservation. It provides many useful references finding other papers. The author concluded that the potential of crop residue mulching is site-specific, dependent on the local biophysical and socio-economic environment. Techniques and results differ between developed and developing country agricultural settings.
Conclusion
This review has shown that there is good scientific evidence for four of the claimed benefits of organic mulching; conservation of soil moisture, enriched soil biology, reduced weed growth and less soil erosion. The evidence base comes from all inhabited continents and from several different soil types and climate zones. Mulch depth below 8 cm seems to give the best results. Benefits can be enhanced when mulch is used in combination with minimal till or not ill cultivation.
The fifth claimed benefit, the effect of mulch on soil physical properties and chemistry, is unclear and seems to vary according to soil types and climate.
Acknowledgements
This text is based on an academic literature review by Alice Ambler of the James Hutton Institute as part of our collaborative GROW Observatory project. 
The GROW Observatory has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 690199.
References
Billeaud, L., and Zajicek, J., 1989. Mulching for weed control, Grounds Maintenance, 24, 10-12, 14, 120-121
Broschat, T.K., 2007. Effects of mulch type and fertilizer placement on weed growth and soil pH and nutrient content, HortTechnology, 17, 174-177
Brévault, T., Bikay, S., Maldés, J.M. and Naudin, K., 2007. Impact of a no-till with mulch soil management strategy on soil macofauna communities in a cotton cropping system, Soil and Tillage Research, 97, 140-149
Erenstein, O., 2002. Crop residue mulching in tropical and semi-tropical countries: an evaluation of residue availability and other technological implications, Soil Tillage Research, 67, 115-133
Gholami, L., Sadeghi, S.H. and Homaee, M., 2013. Straw Mulching Effect on Splash Erosion and Sediment Yield from Eroded Plots, Soil Science Society of America Journal, 77, 268–278.
Gouin, F.R., 1983. Over-mulching, a national plague. Weeds, Trees, and Turf, 22, 22-23
Greenly, K.M., and Rakow, D.A., 1995. The effect of wood mulch type and depth on weed and tree growth and certain soil parameters, Journal of Arboriculture, 21, 225-231.
He, J., Kuhn, N. J., Zhang, X. M., Zhang, X. R., & Li, H. W., 2009. Effects of 10 years of conservation tillage on soil properties and productivity in the farming-pastoral ecotone of Inner Mongolia, China. Soil Use and Management, 25(2), 201–209.
Hobbs, P.R., Sayre, K. and Gupta, R., 2008. The role of conservation agriculture in sustainable agriculture, Philosophical Transactions of the Royal Society B, 363, 543-555.
Hopkins, H.H., 1954. Effects of mulch upon certain factors of the grassland environment, Range Management, 7, 255-258
Jodaugiené, D., Pupaliené, R., Sinkevičiené, A., Marcinkevičiene, A., Žebrauskaité, K., Baltabuonyté, M. and Čepulieném, R., 2010. The influence of organic mulches on soil biological properties, Zemdirbyste-Agriculture, 97, 33-40
Kader, M.A., Senge, M., Mojid, M.A. and Ito, K., 2017. Recent advances in mulching materials and methods for modifying soil environment, Soil and Tillage Research, 168, 155-166
Miyata, S., Kosugi, K., Gomi, T., and T.Mizuyama, T., 2009. Effects of forest floor coverage on overland flow and soil erosion on hillslopes in Japanese cypress plantation forests, Water Resources Research, 45, DOI:10.1029/2008WR007270
Watson, G.W. and Kupkowski, G., 1991, Effects of a deep layer of mulch on the soil environment and tree root growth, Journal of Arboriculture, 17, 242-245. Chicago.
Zhang, S., Lövdahl, L., Grip, H., Tong, Y., Yand, X. and Wang, Q., 2009, Effects of mulching and catch cropping on soil temperature soil moisture and wheat yield on the
Loess Plateau of China, Soil and Tillage Research, 145, 111-117