Fertility Building

Nitrogen (N) is a key nutrient in helping to achieve acceptable yields and crop quality. Through an uptake of mineral N in the soil and fixation of N₂ from the atmosphere, legumes can provide a source of ‘free nitrogen’

Nitrogen fixation explained

Through a symbiotic relationship with rhizobacteria, legumes are able to fix aerial N and provide it in a form that is readily available to plants.

Growing legumes in rotation allows for plant N to be built up in above and below-ground residues. It is broken down by microbial activity following incorporation and released for uptake by the following crop. Legume-based leys provide additional benefits through soil cover and reduced soil disturbance. Greater N fixation has been demonstrated in minimum tillage systems.

For most legume species, yield increases as the quantity of fixed N increases. However different factors affect the amount of N that is fixed, which include:

• the crop’s growing conditions (e.g. soil, climate, disease);
• the capacity of the crop / rhizobium symbiosis to fix N;
• crop management;
• the length of time that the crop is grown.

Factors affecting N Fixation

Because fixation tends to decrease with legume age, there are some contradictions which can make management decisions difficult.

In most organic rotations more N is likely to be fixed where legumes follow crops that have previously depleted soil N levels. The soil mineral N pool can be increased by manure application, grazing, and cutting and mulching.

Factors affecting N fixation include:

• The position of the crop in the cropping rotation, the duration of cropping and methods of cultivation.
• Harvesting and removal of other crop residues from the field will remove much of the fixed N and therefore reduce the benefit to the following crops.
• During the establishment period, weeds can compete with a ley for light, nutrients and water.
• Adding manure to fertility building leys, whilst potentially increasing N availability, is likely to reduce the proportion that is fixed by the legumes in the ley.

How can I maximise N fixation?

Growing a legume in a mixture with a non-N fixing species can increase the proportion of N obtained from the atmosphere. In grass / clover leys, the grass utilises the soil N; the build-up of N that may otherwise inhibit fixation is therefore avoided.

The use of permanent beds of legumes grown alongside a cash crop can allow both cash and fertility building crops to be grown every year. The companion crop can be mown or grazed to control competition and encourage nutrient transfer. Results from a Defra funded study at the North Wyke (Rothamsted) research site in Devon, conducted in collaboration with Duchy College, show that the proportion of N that is fixed from legumes is greater when the crop is cut and removed rather than cut and directly mulched. For the full report, click here

Whilst for livestock farms, cutting and removing is common practice in the form of silage-making, in an all crop situation such as a vegetable rotation, the most effective use of a fertility building crop would be to remove regularly cut material and apply it as a mulch to other crops. This would provide additional organic matter and encourage the fertility building legumes to maximise N fixation.

To view the nitrogen fixing potential and nitrogen release rate of different green manure species please click here: Sort Out Your Soil; A Practical Guide to Green Manures

Before N from legumes can be used by the crop it must be mineralised in to plant available form (nitrate and ammonium). Once in this mineralised form it is susceptible to loss. It is important therefore that as much as possible is retained for use by the crop.

The rate of mineralisation depends on environmental conditions (e.g. soil moisture and temperature), soil texture (potentially slower in clays than sands) and the composition of the crop materials (fresh green residues decompose more rapidly than old “stemmy” material).

• Losses occur mainly through nitrate leaching and sometimes as ammonia volatilisation. Plough the ley therefore as late as possible in autumn or in the spring to minimise leaching loss. Leaching post ploughing can cause the greatest N loss from a rotation.
• The main loss of ammonia is from manure during grazing or after application. Rapid incorporation of manure after application will reduce volatilisation losses.

• Management of cover crops – options and timings. To reduce N leaching risks, cover crops need to be established early in autumn to ensure good crop growth and hence N uptake. Establishment techniques should ensure that soil moisture is conserved and that there is a good seed / soil contact. Some cover crops do not release N quickly enough to be utilised by the following crop. If N is released too slowly and not fully used by the cash crop it can be leached in the following season.
• Bi-cropping – the use of permanent beds of a legume grown alongside a cash crop has potential as an alternative technique to planning a legume into the rotation by itself. It also helps to reduce pest and disease severity.
• Interactions with cultivations / seed bed preparations – cultivation practices that increase mineralisation of the soil organic matter will lead to increased levels of soil N with the result that following legume crops obtain less of their N from the atmosphere. Fertility builders which are not easily destroyed by cultivation or seedbed preparation can become weeds in the following cash crop.
• Interactions with other nutrients – where the growth of legumes is affected by nutrient deficiency (or acidity), the potential for soil N build up is reduced.
• Organic manures – nutrients applied to ensure satisfactory growth of legumes (usually as animal manures) contain N, and their use may inhibit N fixation (short term) and reduce clover content (long term). The effects will be less with FYM and composts than with slurry and poultry manure (which contains higher proportions of readily available N).
• Interactions with pests and diseases – Fertility building crops are subject to pests and diseases that can reduce their effectiveness at increasing soil N supply, and the risk of carry over to the next crop. Maintaining a green cover in the autumn to minimise N leaching losses can increase the risk of pests and diseases.
• Costs – the economic viability of conventional rotations tend to be reduced by the need to have crops in the rotation purely for fertility building. The use of marketable grain legumes in stockless rotation enables some income to be derived from the fertility builder. However the N fixed is likely to be less as some of the N will be used from the fertility builder. However the N fixed is likely to be less as some of the N will be used in the production of seed e.g. peas.
• Crop sequences – as in most organic rotations, more N is likely to be fixed where legumes follow crops that have previously depleted soil N levels.

Sources: Advisory Leaflet – Soil Fertility Building Crops in Organic Farming and Defra Project OF0316 – A review of leguminous fertility building crops with particular reference to Nitrogen fixation and utilisation.

One way to improve efficiency is to combine several legume and grass species in a mixture, incorporating a number of slower growing species. This can result in a more efficient transfer of N to the subsequent crop as a result of different species decomposing at differing rates, providing more opportunity to match N availability to N requirements. This has been investigated in the nationwide Legume LINK project LK09106 (LINK PDF), funded by DEFRA and administered by research stations (hubs) across the UK. Duchy College Rosewarne was the hub for the South West with several farms taking part. The research has shown that:

• The stability of ley performance is greater in multi-species mixtures than in single legume situations;
• mixing different legume species has been found to help suppress weeds;
• widening the range of species has increased opportunities to build short term leys into rotations on conventional farms via cover cropping or under-sowing.

Farmers who took part were asked to drill their usual grass ley mixture as a control and half an acre of the all-species-mixture (provided by the Organic Research Centre at Elm Farm). Farmers found that adding grass to legumes increased the dry matter output per hectare and that the grass grew quicker when mixed with legumes.

The species mixture contained: red clover, white clover, crimson clover, alsike clover, black medic, sainfoin, meadow pea, lucerne, large birdsfoot trefoil, birds food trefoil, timothy, meadow fescue, Italian ryegrass and perennial rye grass.

The success of fertility building crops depends on a number of factors such as soil type, species characteristics i.e. establishment rate and growth habits, timing of planting, how they are integrated with other crops and environmental conditions.

What do I need to consider?

• Specific aims and circumstances i.e. soil type, farming system, previous cropping, future cropping plans and climate.
• Fertility building crops can be subject to pests and diseases; consider therefore cropping history and previous pest and disease problems before selecting the crop, and ensure it is not of the same ‘type’ as the vegetable or arable cash crop in a rotation.
• Some legumes are more suitable than others as annual build-up crops due to differences in establishment rate and growth. Grain legumes i.e. lupins, peas and beans provide cash income but tend to fix less N, so are best suited to short-term N boosts in a rotation.

Need something long-term?

• A mix of red and white clover with perennial ryegrass can improve soil fertility and structure. The mix should be grown for 1 year before incorporation.
• Red and white clover can fix up to 300 kg N/ha which is released rapidly after incorporation. The ryegrass acts like a sponge holding the nitrogen and delaying its release.
• For leys that will be in the ground for longer than 2 years, mixing cocksfoot (which is drought resistant) and clover will help improve soil structure and add organic matter.

Winter mixes

• Combining a nitrogen holder i.e. grazing rye and a fixer i.e. vetch can help improve soil over winter. They can be sown from mid-September until mid-October, and incorporated from February until late April.

Summer mixes

• Fast growing annual green manures can add organic matter, suppress weeds, hold surplus soil nitrogen and act as a break crop by interrupting pest and disease cycles.

• Up to 4 or 5 crops can be mixed i.e. mustard, crimson, clover, phacelia and Persian clover, and should be planted from late spring onwards on bare ground or immediately after whole crop silage or an early cereal harvest. It should be ready for turning after 8-10 weeks and can be incorporated before a winter cash crop.

For a quick flowchart guide to choosing green manure mixes, click here.

Matching green manures to the soil type

• All green manures will add organic matter and some species with be better suited to certain soils. This is a quick guide:
•  

Adapted from Sort Out Your Soil; A Practical Guide to Green Manures

The nationwide Legume LINK project (LK09106), funded by DEFRA, investigated combining different legume and grass species in mixtures, incorporating a number of slower growing species. Replicated field experiments were conducted over three years at multiple locations, testing the performance of 12 legume species and 4 grass species sown in monocultures, as well as in a mixture of 10 of the legumes and all 4 grasses (the All Species mix, ASM).

The ASM contained: red clover, white clover, crimson clover, alsike clover, black medic, sainfoin, meadow pea, lucerne, large birdsfoot trefoil, birdsfoot trefoil, timothy, meadow fescue, Italian ryegrass and perennial rye grass.

It revealed that:

• No single species scored highly on all of the evaluation criteria;
• white clover in particular was outscored by other legume species on parameters such as early development and resistance to decomposition;
• in comparison with the monocultures, the ASM showed increased ground cover, increased above-ground biomass and reduced weed biomass;
• benefits with regards to productivity increased over time;
• the stability of biomass production across sites was greater in the ASM than in the legume monocultures;
• and ecological modelling revealed that the three best multifunctional mixtures all contained black medic, lucerne and red clover.
• Click here to see the full report.

• For a successful clover-based ley, it needs to cover the ground quickly, establish well in a range of environmental conditions and be highly persistent until incorporation into the soil.
• The optimum build up period for white clover swards is about three years.
• Clover competes poorly with grass for soil nutrients and therefore needs a plentiful supply of nutrients when grown in a mixture.
• Different percentages of clover in a sward can release varying amounts of N depending on the age of the ley and the method by which the clover is incorporated.
• The pictures below depict the visual differences between swards with 15%, 25-30% and 50% coverage (left to right).

What are farmers doing?

Farmers are increasingly looking to include forage legumes in fertility building mixes, i.e. lucerne, which produces large quantities of high quality feed.

In Europe, organic farmers most frequently use grass and clover mixes, the most popular being white and red clover (Trifolium repens and T. pratense), and perennial and Italian ryegrass (Lolium perenne and L. multiflorum). The potential for N fixation by these leys is high but the establishment can be under-par due to the cool, moist conditions required by the clover species.

The current practice of using mixtures of red or white clover and ryegrass can lead to a lack of synchronisation between the release of nutrients following incorporation of the green manure and the demands of the following crop.

What do farmer’s think?

The comments below are from individual farmers that highlight the different considerations when using mixed crops for fertility building. All comments were recorded from Legume LINK meetings held between 24th January 2011 and 4th of May 2011.

Farmer One:

“Seeds of different sizes go through the drill at different rates. Settlement in the bag means that the bigger ones go down first and the bag has to be re-filled. There is a time/cost factor in this.”

“Low ground cover is no good for our cutting regimes.” (Referring to species such as black medic which is initially matt forming, dense and woody.)

“I would like a mix which suited the plants to the regime and management of the farm.” (Some plants react better to being cut / grazed regularly, some grow better for silage. These often out-compete the finer herb species.)

“Italian ryegrass forced itself to head under stress (due to dry weather conditions) and had seeded itself before bulking up. There is an implication for the following wheat / trit crop being infested with Italian ryegrass. Perennial ryegrass does not bolt so fast.”

“Huge variation in species maturing and bulking leads to problems with selecting a cutting date and making good quality silage.” (However the farmer was happy with the silage he made. He suggested having strips of herbs like the Sainfoin separately so that they are not outcompeted by the stronger growing herbs in the mix)

Farmer Two:

“When the field was grazed with cattle the white clover in the control recovered quicker but this may have been because the all-species-mixture was grazed more. It was noticeable that sheep grazed the all-species-mixture more.”

“During the first year 2009 the all-species-mixture performed well producing 13kg/acre. During the hot dry spell the all-species-mixture also did well.”

“All the species were there in year one, they were diminished in year 21 and non-existent in year 3.”

“The recovery rate was good but the herb species which had been there in autumn 2010 were not there by spring 2011. Two cold winters finished them off.”

Farmer Three:

“Found that red clover fixes well and gives the boost that his farm needs. White clover grazes but does not fix so well.”

“Red clover was old before it was cut and had a tendency to woody stalks which was not so good in silage.”

“I thought Italian ryegrass and red clover would dominate the sward but it didn’t. There was a good balance.”

Click here for a quick guide to help you choose the most appropriate green manure crop. (Taken from Sort Out Your Soil; A Practical Guide to Green Manures)