Organic matter management (OMM) refers to various agricultural practices to maintain and increase the organic matter status of soils with the aim of enhancing soil fertility, in particular by ameliorating the provision, storage and release of nutrients as well as by improving the soil structure, which in turn increases the infiltration and retention capacity of the water in the soil.
Significance of OMM in relation to soil fertility, water management and climate risk adaptation
OMM as a measure to enhance soil fertility is an integral part of devising farming- and cropping systems. The design principle aims at maintaining a cycle of organic matter between soil, plants and animals, creating equilibrium between in- and outputs at the field and farm level.
In tropical smallholder agriculture many useful practices of OMM can be found, but they are often not sufficiently adapted to respond to current changes like shrinking farm sizes, degraded soils etc. So many farmers practise continuous cropping, often of monocultures without sufficient restitution of organic matter, resulting in soil exhaustion and declining productivity, which in turn worsens the water-holding capacity of soils. The key of OMM is maximisation of biomass production, keeping a nutrient balance on field and farm-level and slowing down the decomposition of OM. Both the production of OM as well as the decomposition are accelerated in the warm and humid conditions of tropical climates.
By increasing soil fertility OMM contributes to food security and higher economic returns, which is mainly a result of mobilising local farm resources. In most cases, mineral fertilisers show a better response level when combined with organic fertilisers (saving on costly external inputs). OMM provides smallholders with the most important locally available means of maintaining and generating soil fertility.
The significance of OM for agricultural water management lies not only in improved soil properties like infiltration-rate and water-holding capacities but also in a number of OMM practices which increase water harvest potentials (like hedgerows, mulching) and reduce unproductive water losses (evaporation, etc.).
OMM provides a number of effective responses to the current challenge of adapting farming practices to the extremes caused by climate change (rainstorms, flooding, drought). Humus-rich soils are less vulnerable to erosive rains and drought. Farms designed with the aim of creating maximum biomass production and storage (agroforestry, multiple cropping) add to the protection of soils and create carbon-rich agro-ecosystems (carbon sinks of living and dead organic matter). In irrigation, agricultural watershed management is important in order to reduce flooding or drought. Hillsides managed with the principles of OMM can contribute to this.
Impact on soil parameters
The properties of tropical soils are greatly variable and the production and decomposition of OM highly dependent on climate. These two factors have an influence on how far soil parameters are affected through OMM. The main effects are as follows:
- OM affects the cation exchange capacity (CEC) of a soil. In very sandy or highly-weathered and acid soils humus provides almost the entire CEC.
- OM provides a slow-flowing source of plant nutrients that are contained in the plant tissue and are released through its decomposition (mineralisation).
- OM contributes to improvement of soil structure (enhancement of soil life, increasing aggregate size and with it the macro-pore-space), reducing erodibility of soils.
- Through its impact on soil structure higher amounts of rainwater can infiltrate into the humus-rich soils (infiltration-rate)
- The retention capacity of water (field capacity) and the available water capacity usable by plants are influenced by the OM content of the soil.
- In case OM is used to cover the soil instead of leaving it bare, soil temperatures and evaporation are kept at lower levels thus slowing biomass decomposition in the soil.
Optimal C-contents of soils are generally higher in humid climates than dry ones (6% - 1,5% C ).
Farm-sources of organic matter
The first source to be found are the residues left by crops and weeds on the field. They can stay on the field or be removed and transformed into compost or animal manure. In most smallholder farms the amount of crop residues produced on the farm is not sufficient for maintaining soil fertility. When this is the case, additional biomass can be produced in various ways:
- Cultivating green manure and/or cover crops as part of regular crop rotations
- Agroforestry and hedgerow cropping practices
- Production on specific plots may be destined to grow biomass for another field (for example for mulching of coffee plantations)
- Fields with fodder crops are also a source of organic matter (via the animal stomach producing excrements). Grazing outside the farmed area may benefit soils inside the farm when animals are kept inside for part of the day or night (dung left during stabling, kraaling)
- In very poor soils the external restitution of deficient nutrients through mineral fertilisers needs to be considered to boost biomass production
There are manifold practices and ways to return the produced biomass back to the field depending on the farming system and site conditions. A general rule for most tropical smallholdings is as follows: produce the maximum amount of organic matter which can be recycled to the soil to preserve soil fertility and increase the productivity of the farm.
Plant residues for tillage and no-tillage systems
Residues of crops and weeds (leaves, stems, fruits) can be used for incorporation into the soil or for mulching. The effect will depend largely on the amount of biomass left after harvest (above ground biomass, but also roots, microorganisms). The amount is determined by the type of cropping system, by the harvest-index of crop varieties (ratio of total above ground dry matter and dry matter of harvested crop organ) and by yield and fertility status of the soil. In case, for example, of a rich maize harvest, the residues might be sufficient to provide enough mulch cover of the soil to protect it until the moment the subsequent crop has covered the soil. The new crop can be directly sown into the flattened crop residue layer (shredded / chopped or not) after having treated the weeds (hoeing or herbicides). Specific tools and machines are available for this purpose. The technique is widely used in intensive, mechanised farming and has the advantage of not having to till the soil. The soil is continuously protected against erosive rains as well as from the accelerated breakdown of OM that goes along with soil tillage. (For further benefits see mulching).
The alternative to the no-tillage method is to incorporate the residues into the upper layer of soil after harvest or before sowing for the following growing season. In mechanised agriculture bulky plant residues like maize straw are shredded and then integrated into the soil by shallow ploughing or with a rotary tiller. The crop type impacts on the organic matter status of the soil. When incorporating material like straw with wide C:N-ratios an application of small amounts of N-fertilizers is needed to avoid the fixation of nitrogen in the rapidly developing microorganisms (N- immobilisation). Crop residues are often integrated into the soil with other fertilisers like manure or mineral. (For effects of these measures see green manure).
Limiting factors: Smallholder farmers are often eager to remove plant residues from the field as these are valued for various purposes: animal fodder, bedding, thatching of roofs, etc. During workload peak times the residues might also be burned.
Compost is the end product of the composting process through which organic materials, mainly plant residues, are decomposed and transformed by micro-organisms into humus. It is used as an effective fertiliser and soil conditioner in intensive gardening and in farming.
There are a number of conditions for successful composting. An optimal environment for the activity of micro-organisms has to be created by providing materials with sufficient amounts of carbon, nitrogen (a C/N ratio > 35:1 slows the process down), water, as well as a good supply of oxygen through sufficient aeration. After 1 -3 days the decomposition process starts with a thermic phase, where temperatures may sharply rise above 60°C and kill pathogens as well as the germination capacity of seeds. In the following cooling phase, temperatures will slowly decline and allow other micro-organisms like fungi to continue the process of decomposition and transformation. At the final stage, small animals like earthworms, centipedes etc. will colonise the compost and contribute to a homogenous mixture of final produce.
There are various methods of composting recommended for small-scale farming and gardening. Most common is the use of compost heaps, inspired by the ‘Indore method’ developed by A. Howard (1943) in India. Bigger quantities of organic materials are loosely piled up in heaps large enough to allow for build-up of heat (1,00 length x 1,20 width x 1,00 m height). If necessary, stalky stuff has to be chopped to a size of 10 – 15 cm and be well mixed in. If materials with a wide C/N ratio are composted, layers of animal manure, fresh leaves or urine are added to increase nitrogen content. The material should be moist (15% - 50%) and aerated (mixing in small twigs allows for loose layering and use of aeration poles, which are pulled out when the pile is finished). The hotter the heap the quicker the material breaks down. It might be necessary to aerate the heap by turning it over and to water it in order to keep the rotting process going. The final produce is a loose humid mixture of aggregates of dark brown or black colour with a smell like forest soils. To prevent the compost heap drying out, it has to be protected against winds and direct sunshine. Excessive rainfall may also harm the rotting process and leach nutrients.
The use of composting pits beside the homestead is common amongst farmers. Daily kitchen and household waste goes into the pit. It provides protection against drying, but if there is no roofing or drainage the pits might be flooded during abundant rains. Aeration is limited in the pits. In many cases, compost pits suffer from a decomposition process which is cold, slow and uneven, and this can limit its usefulness The absence of heat in the process can lead to the propagation of unwanted plants or weeds (through seeds of propagation material).
The fertilising value of compost varies according to its composition. According to a literature review by Müller-Sämann average values are 0.5 % N, 0.2% P and 0.5% K in fresh mass (50% H2O). These values can be considerably increased when adding manure, urine or other additives. Nitrogen recovery by crops represents only a small amount of the total nitrogen applied by the compost (3 – 10%), whereas P and K uptake by plants was high.
There is a wide range of use of compost. In farming of annual crops, it is directly tilled into the soil (10 cm deep) before sowing or planting with amounts from 10 to 35t/ha. It can be combined with mineral fertilisers. On poor soils and when compost is scarce, it can be applied directly to the planting furrows or put into the planting hole. In horticulture it is used for preparing growing media for nurseries (mixed with steamed sand, soil etc.) as it provides nutrients in a form available to plants and its porous structure holds moisture and air well. In small-scale intensive horticulture it is used in much higher amounts. When planting bananas or fruit-trees it will be mixed into the soil for filling the planting hole.
Composts, like other organic manuring have an impact on soil physical properties. They are a means to improving water retention (field capacity) in soils. This is of particular importance in sandy, low-humus soils. In non-mechanised farming the high labour demand of compost-making and transport to the field is an obstacle to its use. However, composting has good potential for small-scale intensive gardening around the homestead, where household waste and dung from animals can be used and the compost can be applied in horticultural crops in the house garden.
Animal manure is made from animal excrements like droppings and urine with the addition of bedding material and fodder residue in different combinations. The excrements can also be used in liquid state (slurry) for fertilising. There are many practices to produce and use manure. They are determined by the type of livestock, the way the animals are kept (from collection of droppings of grazing animals to permanent stabling), the process of producing the manure and the sources of fodder and bedding material. In certain farming systems it is possible to use animal manure as the sole source for maintaining soil fertility.
The effect of manure on soil fertility and plant growth is highly variable and determined by several factors:
- (i) the quality of manure,
- (ii) the available quantity and
- (iii) the application and spreading techniques
Quality: The type and proportions of raw material from which the manure is made and the practise of preparing the manure determine its content of nutrients and organic matter. The nutrient content of solid and liquid excrement varies considerably according to the type of animal. During storage the fresh manure undergoes a rotting process, which narrows the C/N-ratio, builds up humic substances, mineralises OM. The process of collecting and storing the manure can result in a higher concentration of nutrients in the manure, and determines the level of loss (leaching through rains, loss of OM through mineralisation). Animal manures usually possess a higher concentration of nutrients than compost from plants.
Available quantity: This largely depends on the amount of fodder available and the livestock number that can be sustained from it on the farm. An extensive keeping system of grazing and collection of droppings may only produce a tenth of what can be produced from the same number of animals kept permanently in a stable with sufficient bedding material. The highest quantities are obtained in deep-litter stabling where the animals stand on their manure (needing high quantities of bedding) thus providing good storage conditions and protection of manure against rains.
Deep-litter stabling in Rwanda
Application and spreading techniques (valid also for composting): Quantities and intervals of application depend on the crop rotation. Manure should be given to those crops in a rotation which show the highest response. Subsequent crops will feed on the carry-over effect from the previous season. In arable crops one often finds 15 – 30 t/ha of manure every third year, whereas in intensive horticulture manure is applied every growing season in much higher quantities. Manure should be spread evenly over the field (no clumps) and get a shallow working-in. There are also practices to apply manure to single plants for a localised impact. This helps to save manure while getting a good return.
The systematic integration of animal husbandry into agriculture through fodder production, stabling and efficient storage and application techniques provides a good opportunity to intensify production in smallholder agriculture with the help of larger quantities of an effective organic fertiliser. At the same time one has to take into account that the amount of work related to the transport of manure and fodder production is a limiting factor for many farmers.
Mulching consists in complete or partial soil-coverage mainly using layers of plant material, such as crop residues (see above). (Mulch with plastic sheets is not considered here). Mulching is usually applied to intensive crop production, in gardens and orchards and with commercial crops such as coffee, pineapples and bananas, as it is labour-intensive when not mechanised.
Some impacts of mulching:
Mulch materials: crop residues and weeds (available in situ, less transport, but often not in sufficient quantity, except for plants with high ratio of crop residues like bananas). Nearby sources from hedgerows, field trees or plants from erosion control strips can be used without too much labour input for transport. Special mulch production plots for high-value crops like coffee are used when the crop provides high income. The properties of mulch material have to be considered: physical properties like stalky and hard material have to be chopped before applying, densely-layering material like fresh grass with more narrow C/N-ratio decompose more rapidly than bulky, loosely layering aerated materials. This means that the aim of physical protection of the soil and that of plant nutrition might be in competition.
Amount and intervals: There are only general rules. Mulching is dependent on local climate, crops and properties of mulch material. The thickness of layer affects the ground cover and with it the protection of the soil. But to allow rainfall to reach the soil the layer should not be too thick. Light rains might be absorbed by too thick a mulching layer. Material with speedy decay requires regular replacement or thicker layers if permanent cover is intended (permanent crops like banana plantation or like in Conservation Agriculture. See ‘Green manure/covercrops). Layers of 5 to 15 cm are usual. Green material in thin layers under humid and warm conditions may decay within two months. But there might be no need to replace the layer as crop stands form an effective soil cover several weeks after sowing. Attention has to be paid to weed propagation through seed or fresh cuttings when applying mulch material from outside sources.
Timing: Mulching should be applied to annual crops before or shortly after sowing / planting, depending on the technique. In the case of permanent crops it should be renewed before the mulch layers become too thin and ineffective as a result.
Spatial arrangement: Total surface cover is often used, but this needs the highest amount of mulch material. The alternative is mulch strips covering the area on both sides of seed-rows or every second row (not possible with traditional broadcasted sowing). It is also possible to mulch single but tall plants when they are widely spaced, reducing the amount of mulch needed per field.
Specific benefits: As with cover-crops, there are the particular benefits of protecting the soil against run-off loss of water and top-soil, reducing splash erosion, maximizing water infiltration through improved soil structure and physical barriers to surface run-off, reducing evaporation losses of water from soil, lower soil temperatures, control of weeds, and increase of OM content in soil. The effects on crop yield also depend on the speed of mineralisation of the mulch.
Green manure/cover crops and crop rotation
Objectives: Green manure and cover crops (GMCC) are part of crop rotations and planted for several aims:
- in tillage systems for incorporating biomass (above ground and root) into the soil to enable accumulation and recycling of nutrients and maintenance or increase of soil-OM,
- in no-tillage cropping systems to produce high amounts of above-ground biomass for soil cover (also zero-tillage or Conservation Agriculture)
- to fight weeds and reduce disease and pest infestation
- for regeneration of severely degraded soils which do not allow productive cultivation of crops anymore
- for secondary purposes like production of forage, seeds, firewood etc.
Green manure/cover crops as part of crop rotation in tillage systems (arable cropping systems)
Green manure replaces the natural fallowing practised in many parts of the tropics since the dawn of traditional shifting cultivation. GMCC allows for much faster regeneration of soil fertility. GMCCs can be grown simultaneously with the cash- or subsistence crops or in succession with them, often filling the phase between two rainy seasons. Like with crop residues (int. link), they are flattened and then ploughed under before the sowing of the subsequent crop. If they are part of a proper succession they need the usual input of tillage and sowing. Smallholder farmers often consider this a disproportionate investment if the GMCC are not providing a secondary use such as animal forage.
In many cases it is possible to grow GMCCs simultaneously with the principal crop (also helping to avoid workload peaks). In such cases they can be undersown when the principal crop is well under way, e.g. with a first or second mechanical hoeing of weeds or some weeks before harvesting. The main factors to be considered in creating equilibrium between the requirements of the crops in terms of growth factors and those of the GMCC are: timing of GMCC seeding and harvesting, spacing on the field, and choice of crops and GMCC species. Handling these factors correctly helps reduce competition for soil moisture, light and nutrients while allowing GMCC to produce an optimal amount of OM and effectively smother weeds. The overlapping of growing periods between principal crops and GMCC also reduces the time soils are lying bare and unprotected.
Green Manure for permanent soil cover in no-tillage cropping systems
Continuous soil cover with living or dead plant material is the ultimate goal in no-tillage cropping systems, a technique rapidly spreading, mainly in the Americas. It is also known under the term of ‘Conservation Agriculture’. It can be used with both annual and perennial crops. Annual crops are planted (sown) without tillage into the soil under the layer of flattened GMCC and crop residues by opening a small hole or narrow trench with the help of special tools or machinery. The functional mechanisms of GMCC are similar to those described under. ‘mulching’(int.link). The protection of soils against erosion and weeds, as well as infiltration of rain, is effective both with living plants and dead biomass. For biological retention of nitrogen (to prevent leaching) it is good to maintain a living biomass.
Green Manure for regeneration of degraded land
In many cases green manure is a means to regenerating highly degraded soils which no longer allow any successful cultivation of crops. It may take several seasons of green manure (2 – 5 years) to re-establish a viable level of soil fertility. In cases of highly weathered, acid soils with marked nutrient deficiencies of P or other elements, green manure will only work when combined with specific fertiliser inputs. The right choice of green manure species plays a key role in succeeding in the regeneration of soil fertility.
Choice of GMCC species
There are many GMCC species with distinct characteristics, which have to be to tried out under local farm conditions to identify the best options to fit local site conditions and what the farmers choose. Selecting GMCC species depends on soil fertility status, climate and seasonal patterns, crop rotation and the main function of GMCC in the cropping system. Generally GMCCs have a high capacity of nutrient uptake from soil, fast growth and the capacity to deeply root even down to layers of subsoil, not accessible to crops, where they access reserves of nutrient and moisture. The carry-over effect on the subsequent plant depends on this. Other desirable traits of GMCCs are fast emergence and initial growth to quickly cover soil, competitive ability with weeds, ease of producing seeds, ease in managing, fair resistance to disease or pest, good carry-over effect on the following crop.
Green manure and cover crops: Dwarf mucuna, because of its nonclimbing habit, may be associated early with annual crops such as corn.
On more fertile soils GMCC species are chosen for their capacity of fixing biological nitrogen (leguminous species), nutrient cycling and high biomass production. In these conditions herbaceous species like Mucuna pruriens, which can produce up to 7 t/ha and more of above ground dry matter in only 4 to 5 months, would be a reasonable choice. On soils with medium to lower fertility Mucuna is likely to fail whereas woody perennials such as Cajanus cajan usually develop well and produce 5 – 25 t/ha dry matter within a year. On highly degraded soils the use of mixtures of different species will help to identify the ones with the best performance on the given site.
The benefits can be measured in terms of the impact of GMCC on yields of associated or subsequent subsistence and/or cash crops and on the economic return of the cropping system as a whole. Yield response can be considerable, usually ranging from 20% to 50% on more fertile soils and even more on poorer soils. This is to a large extent due to the high amounts of symbiotically fixed nitrogen accumulated by leguminous GMCC and carried forward to the next crop (often between 100 to 200 kg N/ha). The amount of other nutrient elements present in the GMCC biomass like phosphorus, potash, calcium and magnesium are taken from what is present in the soil. But due to the capacity of certain GMCC species to unlock nutrient reserves of the soil, also from deeper layers of sub-soil, they can be used to improve the nutrient stock and cycling. The residual effect of GMCC mainly works on in the following season, sometimes also in the second. This is why GMCC has to become a regular part of the crop rotation. On soils with medium to low nutrient status it is a necessary complementary measure to other fertility management measures like application of manure (int.link to manure) and restitution of deficient elements (except N) through mineral fertilisers. Using gramineous GMCC with a large C:N-ratios may lead to a temporary microbial immobilisation of N which will hamper the growth of subsequent crops. A small input of N (20 -40 kg/ha) is therefore needed before sowing the main crops.
On degraded soils good results with GMCC were reported on severely eroded shallow soils on sloping lands with sufficient nutrient reserves. Perennial shrubby GMCC loosened the soil and deepened the topsoil layer. Limits were experienced with highly weathered, depleted soils with low pH-levels where no nutrient reserves could be mobilised, not even from the subsoil.
Good yield response to GMCC is not only due to improved nutrient accumulation and cycling but also to the improvement of soil structure and biological activity. The prevention of losses of run-off water and of soil erosion also play an important role.
Through improved water household and the higher carbon stocks of cropping systems, the use of GMCC contributes to the mitigation of climate change.
Agroforestry and hedgerow cropping
Definition: Agroforestry integrates trees into agriculture for the purpose of delivering products (fruits, forage, wood, etc.) and services like windbreaks, shading for improved microclimate and less evaporation, but also in terms of supplying important quantities of organic matter through their root biomass, shedding and harvesting of leaf biomass. Agroforestry systems can be designed in such a way as to maximise benefits for soil organic matter, for controlling run-off and splash erosion and for improved infiltration of water.
Practices: Agroforestry methods were already widely practiced in traditional agriculture. Modern techniques aim at optimising agroforestry through the adequate choice and spatial arrangements of trees and management of the tree component in order to reduce competition with the field crops under the trees. A widely applied technique consists in hedgerow-cropping (alley-cropping), where hedges are massively pruned before the cropping season and leaf biomass integrated into the soil or left as mulch.
Specific benefits are the sheltering of agriculture crops, the enlargement of the root and above-ground borders of the cropping systems. Deep-rooting trees might tap nutrients and water in deeper strata of the soil than the usual crops. There is a high-production potential of biomass for all purposes of organic matter use: mulching, incorporation into the soil, composting, for animal feed and for manure production, but also for fruits. If the component of trees is not adequately managed competition with crop yields for light, water, nutrients will be higher. Agroforestry, as with green manures and cover crops, can be seen as an effective means to build carbon rich agro-ecosystems contributing to the sequestration of atmospheric carbon.
- ↑ Howard, Sir Albert (1943): An Agricultural Testament.
- ↑ GIZ/Müller-Sämann, K. M. (1986): Bodenfruchtbarkeit und standortgerechte Landwirtschaft. Maßnahmen und Methoden im Tropischen Pflanzenbau.
- ↑ GIZ/Pietrowicz, P. et al (1998): Agriculture Écologique au Rwanda. GTZ. ISBN 3-8236-1294-8.
FAO/GIZ/Florentín, M.A. et al. (2011): Green manure/cover crops and crop rotation in Conservation Agriculture on small farms. http://www.fao.org/fileadmin/user_upload/agp/icm12.pdf [accessed 19 April 2013]
GIZ/Kuchelmeister, G. (1989): Hedges for resource-poor land users in developing countries.
Pietrowicz, P, Koschi, J., Neumann, I. (1998) Agriculture Ecologique au Rwanda. Recherche et développement dans le projet Agro-Pastoral de Nyabisindu.
Further reading and external links
For basic knowledge regarding organic matter management: http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html
Blum, W. E. H. (2007): Bodenkunde in Stichworten. ISBN 3-443-03117-x
Dupriez H. and P. de Leener (1989): African Gardens and Orchards. 1989 ISBN 0-333-49076-2
Dupriez H. and P. de Leener (1992): Ways of Water. ISBN 2-87105-011-2
FAO/Corsi, S. et al. (2012): Soil Organic Carbon Accumulation and Greenhouse Gas Emission Reductions from Conservation Agriculture. Integrated Crop Management Vol.16-2012. ISBN 978-92-5-107187-8. http://www.fao.org/fileadmin/user_upload/agp/icm16.pdf [accessed 19 April 2013]
GIZ/Kotschi, J. et al. (1984): Standortgerechte Landwirtschaft zur Entwicklung kleinbäuerlicher Betriebe in den Tropen und Subtropen. ISBN 3-88085-264-2
GIZ/Müller-Sämann, K. M. (1986): Bodenfruchtbarkeit und standortgerechte Landwirtschaft. Maßnahmen und Methoden im Tropischen Pflanzenbau.
GIZ/Kotschi, J. et al.(1991): Standortgerechte Landwirtschaft in Ruanda. Zehn Jahre Forschung und Entwicklung in Nyabisindu.