Cover crops - a review
Dr Naomi van der Velden, December 2017 (updated October 2019)
Cover crops are planted to support the main food crop either directly (e.g. by providing nutrients or reducing weed competition) or indirectly (e.g. by reducing soil erosion or improving soil fertility). There are many different outcomes to be gained from including cover crops and there are many plants to choose from. The broadest category of plant choice is between nitrogen fixing legumes and non-legumes. Below is a quick summary of how they can be used followed by a more in-depth review of the scientific evidence for the suggested benefits.
How to use?
The choice of cover crop species (and mixes) and the timing of planting and removal depend on your goals for using them. Chose a cover crop that fits your food crop harvesting and planting plans and that will tolerate your conditions. Frost-susceptible species like Phacelia, mustard and radish are not suitable for colder climates, whereas rye and oats may be hardier. You can mix species to obtain multiple benefits.
Winter cover crops
Winter cover crops need sufficient time to grow before winter, and require adequate spring moisture and time to die back and release nutrients before your spring sowing. Winter cover crops are of particular benefit on irrigated, sandy soils where bare soil is readily erodible if left unprotected during the colder months (Snapp et al., 2005). Winter cover crops use water and lower water reserves in the soil for the next crop. In semiarid climates, water use by cover crops can reduce water reserves for the next crop, although surface residues can alleviate losses through evaporation. This may necessitate irrigation or early spring killing to allow for timely water recharge for germination. Sow winter cover crops after harvesting your food crop in late summer or early autumn. Remove any weeds prior to sowing. In spring, dig into the soil (green manure to add nutrients), or cut and leave as a residue (mulch to prohibit weeds) on the surface a few weeks before planting the food crop. Check for regrowth a week before planting your food crop and dig in again if necessary.
Cover crops during growing season
You can follow the above steps to cover bare soil during the growing season too, e.g. planting after removing an early crop like spring cabbages or salads and before planting a later crop like beans or squashes. It is usual to dig in or cut down cover crops before they set seeds, but some patches may be kept to encourage pollinators and to self-seed for next year. To protect from less-desirable weeds, plant low-growing cover crops like white clover, birdsfoot trefoil or hairy vetch (Vicia villosa Roth.) after your food crop has become established.
Does it work?
The main claimed benefits of cover crops are:
- Protects soil (less erosion, run-off, moisture loss)
- Improves soil moisture
- Improves soil fertility
- Increases soil carbon
- Improves soil life
- Reduces pests
- Reduces weeds
Protects soil (less erosion, run-off, moisture loss)
There is strong evidence that continuous plant cover reduces soil erosion. The exact choice of cover crops will depend on the season and soil type where cover is needed.
Bare soil, particularly in agricultural land, is prone to erosion from wind and water (e.g. Le Bissonnais et al., 2005; Blanco and Lal, 2010). Plant roots can help to bind soil and improve water penetration, and above-ground plant parts can reduce the direct impact of wind and water on the soil, although the effectiveness depends on the amount of plant cover and the intensity of the precipitation (Morgan, 2005). A study in Spanish orchards by Keesstra et al, (2016) showed that the use of soil cover, including plants and mulch (from chipped prunings), significantly decreased soil erosion (just 0.02 Mg ha-1 h-1) compared to tillage (0.51 Mg ha-1 h-1) and the application of herbicides (0.91 Mg ha-1 h-1). In the covered soils, the proportion of run off was also reduced and soil organic matter increased. This shows the importance of maintaining vegetation cover.
De Baets et al. (2011) examined how well six common cover crops reduced soil erosion in Belgium. They found that all soil with cover crops was less prone to erosion than bare soil. Plants with fine-branched root systems, such as ryegrass (Lolium perenne L.) rye (Secale cereale L.) and oats (Avena sativa L.) were very effective at controlling the amount of erosion from concentrated water flow. Plants with thicker roots and tap roots, like fodder radish (Raphanus sativus subsp. oleiferus(L.) Sazonova & Stank.) and white mustard (Sinapis alba L.) are less effective at reducing erosion and may even increase soil loss around the plant due to upstream turbulence. After frosts, the above ground parts of radish, oats, white mustard and phacelia (Phacelia tanacetifolia Benth.) die off and their roots become less effective at controlling erosion (but still better than bare soil). Therefore, crops like rye and rye grass may be more effective over winter in colder areas with higher rainfall and surface runoff. It should be noted that this study is intended to inform mechanised farming. Perennial rye grass (Lolium perenne) can be fast-growing and is widely used to establish pasture and lawns but it is highly competitve and can become a weed, especially in smaller plots. It is an environmental weed in places including New Zealand and Australia (Popay, 2013; University of Queensland, 2017).
Improves soil moisture
As well as direct erosion control, cover crops, especially deep-rooted plants like alfalfa (Medicago spp) and brassicas including forage radish (Raphanus sativus var. longipinnatus Bailey) and rapeseed (Brassica napus L.), can improve soil porosity and the infiltration of water and air into soils, and reduce soil compaction (Groenevelt et al., 1984; Chen et al., 2014). Again, these studies are all focused on mechanised farming where heavy equipment can cause compaction issues. These crops have greater impact in soils with higher clay content e.g. clay-loam compared to more sandy soils e.g. sandy loam (Chen et al., 2014), which might make them suitable for smaller-scale plots more prone to compaction.
Cover crops grown over winter can also reduce soil moisture through increased evapotranspiration. A winter cover crop producing about 2250 kg per hectare could reduce soil moisture by about 260 cubic metres per hectare (Melsinger et al., 1991). This can reduce losses of water and nutrients through leaching in water rich environments. It is likely that the landscape topography will also influence water retention with depressions having significantly higher water retention compared to slopes or summits regardless of whether or not a cover crop is used (Beehler et al., 2017). It should be noted that water use by cover crops in semi-arid regions can limit the water available for the main crop and therefore limit growth and yields (Ramos et al., 2010), although using the correct cover crop in drier areas over summer can help retain soil moisture (Delgado et al., 2007). Thus most cover crops are more suited to regions where water scarcity is not an issue for crop growth.
Retention of water in the soil or within the root zone, rather than being lost to infiltration, could reduce the amount of leaching from soils (Melsinger et al., 1991).
Improves soil fertility (reduces the need for chemical fertilisers. Reduces nutrient leaching. Increases soil carbon).
Cover crops can improve soil fertility by helping to build up stores of organic matter. This can help to sequester carbon and improve nutrient availability to future crops. Legumes can improve poor soil fertility, whereas non-legumes might be better where there is a risk of surplus nitrogen fertiliser being lost from the soil and entering watercourses.
Harvesting crops leads to overall loss of nutrients from a site. These need to be replenished to ensure longer-term productivity. Activities leading to soil erosion and leaching can exacerbate nutrient loss. Crop residues (leaving plant material after harvesting) are important in maintaining soil organic carbon and nutrients in agricultural soils. Cover crops can reduce losses of soil carbon and nitrogen from conventionally tilled grains and legumes (Plaza-Bonilla et al., 2016), further improve soil organic matter content, and soil carbon and nitrogen levels compared to frequent tillage for weed control (Ramos et al., 2010; Mitchell et al., 2017) and enhance carbon storage in soils when combined with no till or conservation till approaches (Lal, 2004).
Major improvements in soil fertility come from the use of nitrogen-fixing "legume" cover crops.
Legumes like vetch (Vicia species), clover (Trifolium species), broad beans (Vicia faba L.), peas (Pisum sativum L.) and alfalfa (Medicago sativa L.) have been shown to provide both soil nitrogen (for plants) and carbon (for soil organisms) (Tonitto et al., 2006). Appropriate use of biological nitrogen fixation through legumes can reduce the total nitrogen that needs to be added to crops (Tonitto et al., 2006). Where there are nitrogen surpluses, for example from excessive fertiliser added to a main crop, cover crops, especially non-legumes, were shown to take up a significant proportion of the additional inorganic nitrogen left after crop harvests (Wyland et al., 1996; Weinart et al., 2002). Non-legume cover crops are more effective at reducing nitrate leaching from soils with a 70% reduction compared to a bare fallow, and 40% reduction under legumes compared to bare soil (Tonitto et al., 2006).
The longer-term (15 years in this study) accumulation of nitrogen and carbon in soils can be higher where legumes are used compared to conventional management (with no loss in yields), but the actual amounts of nutrients retained in soils depends on the types of plants grown (Drinkwater et al., 1998). There’s a lack of evidence about how the quantity and quality of different crop residues can affect soil nutrients and organisms (Snapp et al., 2005), although Ghimire et al. (2017) found that in general higher quantity of cover crop residue correlated with more soil organic carbon for rapeseed, pea and oats, where the amount of residue was more important than the crop species.
Soil organic carbon has been found to be increased through using cover crops by many studies in different areas and with different crops and soil types (see Lal, 2004). More recent studies into the influence of topography (the shape of the land) on soil carbon and soil water with and without cover crops show that the accumulation of carbon in soils is uneven. Cover crops are more effective on slopes and the summits whereas the presence of a cover crop makes no difference where nutrients might naturally accumulate in depressions (Beehler et al., 2017).
Poeplau and Don (2015) sought to estimate the potential of cover crops to sequester carbon globally. They determined that if 25% of crop lands globally practiced winter cover crops, then carbon accumulation in soil would be 0.12 +/- 0.03 Pg C yr-1 (annual change rate for about 50 years) or in total 6.7 +/- 0.6 Pg C (under a future steady- state scenario). This is almost as effective as land-use changes like afforestation of croplands. This off-set would compensate for 8% of the annual direct greenhouse gas emissions from agriculture (IPCC, 2007) or about 70% of the global greenhouse gas emissions from aviation. Carbon sequestration into soils is relatively quick with half of the total effect in the first 20 years, but could last more than 100 years.
Soil carbon inputs through crop residue and cover crops increase soil microbial biomass and activity.
Improves soil life and wider biodiversity
Cover crops grown in times between main crops can increase the overall diversity of agroecosystems and potentially offer related benefits. Growing leguminous cover crops, especially when combined with conservation tillage, can not only enhance soil carbon but also enhance biodiversity (Lal, 2004). It is well established that more biodiverse ecosystems can sequester more carbon. Higher organic matter and carbon content in soils offers more food for soil organisms. Mixtures of cover crops have been found to yield more overall biomass (Finey and Kaye, 2017; Elhakeem et al., 2019) thus potentially contributing more to soil organic matter and thus supporting larger communities of soil organisms and those that feed on them.
Phaeceila is an effective green manure and, if left to flower, attractive to bees and other pollinators.
Reduce weeds and supress pests
The use of cover crops has been shown to decrease weed competition and to increase natural pest predators therefore strengthening natural pest control as well as breaking pest cycles. This can reduce the need for herbicides, pesticides or other interventions.
Winter grown cover crops can be cut and left on the soil surface to block weed germination prior to planting the food crop. Living mulches, including perennials like white clover (Trifolium repens L.) or birdsfoot trefoil (Lotus corniculatus L.), may be grown alongside the crop. Time spent hand weeding and/or the use of herbicides can be reduced using this approach. Using perennials reduces the need to resow each year. Cover crops combinations of pea, oat and hairy vetch have been shown to supress weeds in potato crops in the USA with little or no loss of yield compared to using herbicides (Gallandt et al., 1998). Intercropping of cover crops (growing in rows between the main crop) has also be found to aid weed suppression, particularly for clovers (Trifolium spp) and with ryegrass and alfalfa, and is often at least as effective as hand weeding and applying herbicides, except in years when there are a lot of weeds (Abdin et al., 2000). Spring seeded cover crops have been shown to suppress weeds in a maize (Zea mays L.) crop by up to 80% without impacting yields (DeHaan et al., 1993), and hairy vetch up to 96% weed reduction (Hoffman et al., 1993). Teasdale (1996) showed that higher levels of cover crops (as living mulch or residue) have more impact on weed control with 50-75% of weeds affected under normal densities (and over 90% with four times normal seeding rates). Weed control was found to be largely through blocking light, but also physical and chemical (allelopathy) impedance. It was most effective against smaller light-seeded annual weeds like fat hen (Chenopodium album L.) and amaranth (Amaranthus retrojlexus L.) than perennial weeds like dandelion (Taraxacum agg.).
Brassicas (cabbage family), particularly mustards and oilseed rape, can be particularly useful in supressing both weeds and soil organisms as they contain toxic compounds that can inhibit germination or decrease seedling growth for some (but not all) weed species (Harammoto and Gallandt, 2004). However, these results are not always consistent and the choice of some cover crops can promote the growth of certain weeds. For example, oilseed rape can reduce the growth of hairy nightshade (Solanum sarrachoides Sendtner) but promotes the growth of amaranth (Amaranthus hybridus L., Harammoto and Gallandt, 2004). Careful management is important to ensure that the germination and growth of the main plants is not affected by cover plants.
Compounds from brassica leaves have also be shown to be toxic to nematodes (Potter et al., 1998). Alfalfa, hairy vetch, oat and white lupin (Lupinus albus L.) were all found to reduce incidence of Rhizoctonia solani (a fungal infection) on potatoes in the USA (Honeycutt et al., 1996). Many other cover crops can also control pests or support populations of their natural predators (Lewis et al., 1997)
Improve main crop yields
Many studies have shown that cover crops, particularly legumes, can increase main crop yields. Some of this effect is likely to be for the reasons discussed above and in the information on legumes. Sometimes crop and weather combinations may lead to decreased crop yields. In general where selection of cover crops increases or does not affect yields, then their use is recommended for the wider benefits they can have on soils and ecosystems.
The GROW Observatory has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 690199.
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