Cheryl A. Palm , Ken E. Giller , Paramu L. Mafongoya and M.J. Swift. 1. 2. Tropical Soil Biology and Fertility Programme (TSBF), PO Box 30592, Nairobi, Kenya; ...
Nutrient Cycling in Agroecosystems 61: 63–75, 2001. 2001 Kluwer Academic Publishers. Printed in the Netherlands.
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Management of organic matter in the tropics: translating theory into practice 1, 2 3 1 Cheryl A. Palm *, Ken E. Giller , Paramu L. Mafongoya and M.J. Swift 1
Tropical Soil Biology and Fertility Programme ( TSBF), PO Box 30592, Nairobi, Kenya; 2 Department of Soil Science and Agricultural Engineering, University of Zimbabwe, MP167, Mount Pleasant, Harare, Zimbabwe; 3 International Centre for Research in Agroforestry ( ICRAF), Masekera, Chipata, Zambia; * Author for correspondence (e-mail: cheryl.palm@ tsbf.unon.org) Received 21 September 1999; accepted in revised form 18 February 2000
Key words: Decision trees, Organic input quality, SOM fractions, Synchrony
Abstract Inputs of organic materials play a central role in the productivity of many tropical farming systems by providing nutrients through decomposition and substrate for synthesis of soil organic matter (SOM). The organic inputs in many tropical farming systems such as crop residues, manures, and natural fallows are currently of low quality and insufficient quantity to maintain soil fertility hence there is need to find alternative or supplementary sources of nutrients. Knowledge gained over the past decade on the role of organic resource quality in influencing soil nutrient availability patterns (Synchrony Principle) and SOM maintenance (SOM Principle) provides a strong scientific basis on which to develop management tools. This scientific information must be linked with farmer knowledge and circumstances to provide a realistic approach to soil fertility and SOM management in the tropics. A decision tree has been developed for testing hypotheses about the resource quality parameters that affect nitrogen release patterns and rates. The decision tree is linked to an Organic Resource Database (ORD) with detailed information on the resource quality of agroforestry trees and leguminous cover crops providing a systematic means of selecting organic resources for soil fertility management. The decision tree has also been translated into a practical field guide for use with farmers in evaluating organic materials. The longer-term effects of organic inputs on SOM might also be addressed through the decision tree and database. It is generally believed that materials good for short-term soil fertility will not build or maintain SOM; if true then it is difficult to imagine practical means of maintaining SOM in the African context where short-term fertility issues will take precedence over longer-term maintenance of SOM.
Introduction The management of organic matter for nutrient supply and soil improvement is as old as the history of arable agriculture itself. Yet science has been slow to provide the predictive understanding that will assist farmers to move beyond their own traditional knowledge based on centuries of empirical trial and error. This hiatus might be attributed to the success in application of mineral fertilizers which has dominated northern agriculture. The basic understanding of the principles governing the decomposition and nutrient release from organic materials were laid down by Selman
Waksman and others during the 1920s (Waksman and Stevens 1928; Tenney and Waksman 1929). Longterm experiments have also demonstrated the relative effects of different organic and mineral supplements to crop production, but both the scientific principles and experimental results were subsequently largely ignored (see an historical review by Heal et al. (1997)). Increased interest during the nineteen sixties in decomposition processes in natural ecosystems and further refinement and synthesis of the principles laid out by Waksman (see Box 1) provided the opportunity for research targeted at the development of improved practices of organic matter management.
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Box 1. History of the Synchrony Principle 1979–1982: Synthesis of the decomposition paradigm • Review of decomposition research; environment by quality regulatory paradigm (Swift et al. 1979) • Elaboration of resource quality controls–from C:N ratios to confirmation of the importance of lignin and polyphenols (Berg and Staff 1980; Aber and Mellilo 1982) • Development of the Synchrony concept (Swift et al. 1980; Swift 1981; Anderson and Swift 1983) 1985–1987: Introducing the Synchrony Hypotheses (TSBF 1985; 1987) • Increasing the proportion of low-quality (e.g. low N and P, high lignin content) litter inputs at the onset of rains extends the time period of availability of nutrient to the plant, • The availability of nutrients to plants (e.g., from mineral fertilizers) can be delayed through microbial immobilization on low quality litters, thence decreasing the risk of losses through leaching, • Below-ground litter is a more important source of immediately available nutrient than above-ground litter. 1986–1997: Testing and refining the Synchrony hypotheses (Palm and Sanchez 1991; Tian et al. 1993; 1995; Constantinides and Fownes 1994; Mafongoya et al. 1997c; Myers et al. 1994; 1997) • Residues high in lignin (low quality) will result in a low net mineralization and plant uptake in the first cropping season, but will produce a greater residual effect in subsequent seasons. • Residues high in tannins (intermediate quality) exhibit delayed nutrient release, but will after a lag period release nutrients rapidly. • In environments where significant leaching or denitrification occurs, plant uptake of mineral N applied at planting can be increased by simultaneous application of a low N organic materials (low quality) which temporarily immobilizes N early in the crop growth cycle and remineralizes N later. Current understanding–New synthesis of resource quality control of decomposition (Cadisch and Giller 1997; Handayanto et al. 1997a; 1997b; Mafongoya et al. 1998; Palm et al. 1997) • High quality organic materials can be equally effective as mineral N sources • Organic materials high in lignin do result in low mineralization but this is not followed by any substantial residual mineralization. • Organic materials high in tannins (particularly condensed tannins or proantocyanidins) exhibit delayed N release or immobilization, immobilization can be quite prolonged and the subsequent mineralization phase is rarely at rates or levels high enough to meet crop demand. • Application of low N content materials can reduce losses of N through leaching and denitrification compared to high N materials but the subsequent remineralization is at rates and levels insufficient to meet crop demand. • Mixing high and low quality nutrient sources generally results in an intermediate N release, not a synchronous release pattern.
Inputs of organic matter still play a central role in the maintenance of productivity in a wide variety of cropping systems in the tropics. Organic matter inputs such as tree leaves and litters, green and farmyard manures and the stover and roots from crop residues provide both a short-term supply of nutrients through decomposition and substrate for the synthesis of soil organic matter (SOM). Crop-fallow rotations under shifting cultivation serve as the classic example of these dynamic processes (Nye and Greenland 1960). SOM decline during the cropping phase is a major reason for crop yield decline, organic inputs increase during the vegetative fallow period resulting in the replenishment of SOM. Historically, as fallow periods shortened, farmers developed alternative means of maintaining or building fertility. Nonetheless, the economic circumstances in many areas of the tropics,
from semi-arid to humid regions, force farmers to still rely largely on inputs of organic matter, soil organic matter and biological processes for managing soil fertility, with or without mineral fertilizers. In many cases, the farming systems that developed relied on the concentration of nutrients from large areas of land to sustain the fertility of smaller areas of land for crops. Mixed crop / livestock systems are typical examples, where cattle graze extensively and the manure is collected from the pens where animals are stalled overnight and applied on fields close to the homestead (e.g. Prudencio (1993)). Other traditional systems developed which relied on inputs from nitrogen fixation to maintain fertility of their soils (Sturdy 1939). Inevitably, the quantity and nutrient supplying capacity of traditional organic inputs such as crop residues, manures, and natural fallows gradually de-
65 creases in the absence of inputs to systems, creating a need to find alternative or supplementary sources of nutrients for soil fertility management. Organic matter management, within or outside a framework of integrated nutrient management, thus remains a keystone agricultural practice. The objective of the research of the Tropical Soil Biology and Fertility Programme (TSBF), ‘to determine the management options for improving tropical soil fertility through biological processes’, was formulated to promote the application of the principles of decomposition ecology to this demand (TSBF 1984). The approach that was formulated was based on the general paradigm that the time course of decomposition of organic materials and the release of nutrients from them is regulated by three main groups of factors – the physico-chemical environment, the chemical composition (resource quality) of the material, and the decomposer community. The potential for management of decomposition processes largely resides in the manipulation of these regulatory factors i.e., through the quality, quantity, timing and location of inputs of organic resources to the soil. The programme of research advocated by TSBF is encapsulated in two guiding principles; Synchrony and Soil Organic Matter (TSBF 1985; 1987): • SYNCHRONY: The release of nutrients (N,P) from above and below-ground litter can be synchronized with plant growth demands; • SOIL ORGANIC MATTER: Soil organic matter constitutes both a sink and a source of plant nutrients (N, P) and hence acts as a regulator of temporal and spatial patterns of nutrient availability. The quantity and quality of soil organic matter is influenced by the nature of the above- and below-ground litter inputs. These principles were designed to promote the aim of TSBF to produce a scientifically-based, predictive understanding of decomposition, nutrient availability, and soil organic matter formation following the application of organic materials. It was also recognised that the strategic research must be supplemented by a strong applied and practical focus, working with smallholder farmers to identify and develop approaches that ensure this understanding can be translated into adoptable management options. In this paper we review the current status of these two principles in relation to scientific understanding, predictive ability, and practical application to management of soil fertility in the tropics.
The Synchrony Hypotheses: Quantity, quality, timing and placement of organic inputs Since the initial statement of the Synchrony principle (TSBF 1985), considerable advances have been made that improve our ability to predict nutrient release patterns from organic materials, the outcome of mixing high and low quality organic materials, and the quantity of organic materials needed to replace a given quantity of mineral fertilizer. The original approach to the Synchrony concept, that nutrient losses could be minimized by matching nutrient availability patterns with crop demand, focused on reducing the initial, excess nutrients from mineral fertilizers and high quality organic materials by mixing with poorer quality materials that immobilize nutrients (Box 1, Figure 1). The immobilization would be followed by release of the nutrients, when they were in more demand by the growing plants. Synchrony research has focused primarily on nitro-
Figure 1. Schematics of original concept of the Synchrony principle (A) and current understanding (B). (Q 5 resource quality of the organic material).
66 gen since it is often the nutrient most limiting to plant production. Organic materials can also affect plant production through several other mechanisms including the addition of other macro- or micronutrients, the priming effect, reduction in P-adsorption capacity, and others (Palm et al. 1997). This review focuses primarily on the nitrogen effects. Experimental approaches for achieving Synchrony have included comparing the N release patterns of organic materials of differing quality (Constantinides and Fownes 1994; Handayanto et al. 1994; Palm and Sanchez 1991; Tian et al. 1993), mixing organic materials of different quality (Becker and Ladha 1997; Handayanto et al. 1997a; 1997b; Mafongoya et al. 1997b), combining organic and mineral nutrient sources (Jones et al. 1997), and timing and placement of application (Mafongoya et al. 1997a; 1997c; Mulongoy et al. 1993). Predictions of the nitrogen release patterns of organic materials are based on the resource quality of the materials (Cadisch and Giller 1997; Mafongoya et al. 1998), the resource quality being defined by the nutrient content and the type and proportions of carbon compounds in the organic material (Melillo et al. 1989; Palm and Rowland 1997; Swift et al. 1979). Results from laboratory incubation studies and field trials relating N release patterns to organic resource quality have led to the elaboration of a variety of multi-variable equations for predicting the release of N (Constantinides and Fownes 1994; Palm and Sanchez 1991; Tian et al. 1995). Although differing in details all these relationships are based on a hierarchical set of critical values of N, lignin and polyphenol content for predicting N release patterns (Mafongoya et al. 1998). The overall conclusion from these studies is that there is no single organic material that releases N in perfect Synchrony to plant demand, giving slow initial mineralization or immobilization followed by a large, rapid mineralization. High quality materials (high N, low lignin, low polyphenol) have similar nitrogen availability patterns as mineral fertilizers with a large proportion of the N available in advance of the main period of N-uptake by the growing plant. Materials of ‘poorer quality’ (high lignin or high polyphenol) release a smaller total proportion of their N, and, although sometimes subject to a lag, most of it is released at a slow continuous rate, not followed by a period of rapid mineralization (Figure 1). It has also been recognized that achieving synchrony between nutrient supply and crop demand is important primari-
ly in areas where water supply is abundant and excess nutrients are susceptible to gaseous losses and leaching (Myers et al. 1997). Organic materials can be ranked into categories of relative N supplying capacity, but we are not yet able to predict how much of a specific material or its combination with mineral fertilizer must be applied to produce the same yield as the mineral N fertilizer applied alone. The proportion of N recovered by the first crop following addition of legume residues tends to be between 10 and 20%, whereas recoveries measured for mineral N fertilizers in Africa are around 20–30% (Giller and Cadisch 1995). This indicates that situations exist where recoveries from organics and mineral fertilizers could be equivalent. Network trials, designed according to recommendations in (Palm et al. 1997), currently underway in East and Southern Africa do indicate that high quality organic inputs (N concentrations . 35 g kg 21 ) can be equally effective sources of N as mineral fertilizers (P. Mutuo, personal communication). Mixing low quality and high quality organic inputs generally results in a mineralization pattern equal to the weighted average of the patterns of the two separate materials. Although there may be a reduction in available N immediately after application of the materials (presumably due to immobilization of N by the lower quality material) this is not followed by a period of rapid N release, as was predicted from some of the original Synchrony hypotheses. In some cases there have been non-additive nutrient availability patterns from mixes of low and high quality materials but the results are difficult to predict and again do not necessarily result in a synchronous curve of nutrient availability (Mafongoya et al. 1998; Palm et al. 1997). Two reasons for the unpredicted behavior in N mineralization of mixtures have been identified. In cases where a large amount of C is available from a poor quality residue the capacity for N immobilization may be much greater than the amounts of N available from the soil. In such a case mixing with an N-rich residue may result in all the N being immobilized. Such an effect was found when poor quality maize residues were mixed with leaves of pigeonpea, the N mineralization from the mixture was much less than that which would have been predicted from the individual treatments (Figure 2, Sakala et al. 2000). Unpredicted results have also been found when high quality prunings of Gliricidia sepium were mixed with prunings of Peltophorum dasyrrachis, which contains large concentrations of soluble polyphenolics (Handayanto
67 effects of the organic inputs in terms of nutrient supply or soil organic matter formation.
SOM Hypotheses: Maintaining soil fertility through manipulation of different SOM fractions.
Figure 2. Example of non-additive effects of N dynamics from a mixture of pigeon pea leaves and maize stover from Sakala et al. (2000). Dashed line represents the predicted mineralization calculated as the weighted mean of the pigeon pea and maize stover curves. Bars represent standard errors.
et al. 1997b). The complexation of proteins in the Gliricidia materials by the soluble polyphenols led to unexpectedly small amounts of N release and subsequent recovery of N by maize. Although we are now able to predict the relative, short-term N supplying capacity of sole organic materials based on their resource quality in the majority of cases, we do not know the longer- term residual
There has been much less advance in our scientific understanding and predictive ability within the Soil Organic Matter (SOM) principle. The SOM hypotheses (Box 2) on the formation and function of SOM are fundamentally opposites to those of Synchrony – organic materials that provide a short-term nutrient source are not those that provide the substrate for soil organic matter. However, it is hypothesized that the maintenance of a larger pool of SOM, through inputs of large amounts of organic residues will ensure greater rates of mineralization in the longer term once the amount of SOM has increased significantly. The current approach to SOM is to view it in terms of its components parts or fractions, the fractions having different composition and turnover time (Parton et al. 1983; Stevenson and Elliott 1989). It is also hypothesized that the different SOM fractions have
Box 2. History of the Soil Organic Matter Principle 1960–1984: Developing the principles of SOM fractionation • Measurement of the light fraction (Greenland and Ford 1964) • Concept of microbial biomass (Jenkinson and Powlson 1976) • Development of multi-compartment SOM dynamic models (Jenkinson and Rayner 1977; Parton et al. 1983) 1985: Introducing the SOM Hypotheses (TSBF 1985; 1987) • Increasing the proportion of low-Q litter results in increased SOM pools, particularly of the more recalcitrant fractions. • Increased availability of nutrients to the plant (e.g., from mineral fertilizers or high quality organics) results in increased quality of the litter input and reduces the recalcitrant pool of SOM. • Below-ground litter contributes a relatively greater component to SOM formation than above-ground litter. 1986–1995: Refining the SOM hypotheses (Ingram and Swift 1989; Stevenson and Elliott 1989; Woomer and Swift 1994) • Soil organic matter (SOM) can be separated into functional pools each of which plays a particular role in nutrient release, cation exchange, and soil aggregation (Ingram and Swift 1989). • The active or labile SOM pool, although small, is relatively more important that the slow and passive pool in terms of nutrient release. • The active SOM pool increases with additions of high quality organic inputs but the slow pool decreases Current understanding • Attempts to physically isolate ‘active’ fractions may be futile as the active pool is a subfraction of all size classes (Magid et al. 1986). • The chemical oxidation approaches may be more appropriate since they include the labile carbon, no matter the soil fraction or location (Blair et al. 1997). • Need to recognize the importance of the quantities of inputs and timing in relation to sampling time • Polyphenol-rich materials may be a ‘fast route’ to enhancing SOM contents (Giller and Cadisch 1997)
68 different functions or services that vary from nutrient supply and buffering, water retention, soil structure, to carbon sequestration (van Noordwijk et al. (1997), Figure 3). A biologically active or labile fraction with short turnover time is often considered to be most important for nutrient supply while more stable fractions with longer turnover times, often referred to as slow and passive fractions, may be more important for soil physical properties. To maintain total SOM or fractions and their component or specific services, we must be able to prescribe the amount, type or quality of the organic materials to add, and the management required to achieve this goal yet there is little information to guide us. The framework presented in Figure 3 provides hypotheses for establishing the links between organic input quality, SOM fractions, and SOM functions. Progress in SOM research has been, in part, methodological. It has been difficult to physically isolate from soils fractions that represent the different functional fractions of SOM (Magid et al. 1986), despite a
concerted research effort in recent years (reviewed by Feller and Beare (1997)). Both microbial biomass and light fraction have been used to estimate an active, labile, nutrient supplying fraction of SOM. Microbial biomass measurements at times correlate with mineralization or crop yields but the measured value is much too small to account for the total amounts of nutrients mineralized. The light fraction, also referred to as particulate organic matter (POM) can also correlate with the nutrient supplying capacity (Barrios et al. 1997) but as with microbial biomass the size of the measured fraction is far too small. The light fraction can also vary considerably in quantity and quality according to the time of sampling following addition of organic inputs. Samples taken shortly after application of organic inputs include a mixture of the high and low quality organic materials that have not yet decomposed whereas later measurements will include primarily the undecomposed, recalcitrant materials – not a nutrient supplying fraction. Thus, the lack of a suitable measurement for estimating the
Figure 3. Proposed relationships among SOM fractions, functions, and organic input quality (adapted from van Noordwijk et al. (1997)).
69 labile SOM is not surprising when we consider that the fresh organic material added to soil (light fraction) is in the coarse-sized fraction of the soil yet the microbial biomass is located largely in the finest fractions of the soil. These two fractions, each of which contributes to the labile SOM fraction, will not be isolated together using current methods but perhaps are both captured in chemical oxidation approaches (Blair et al. 1997). Although methodological limitations have hindered our ability to predict the formation and maintenance of SOM and its varied fractions and functions, examples comparing systems with differing inputs may indicate the relative effects of the quantity, quality and management of organic inputs and the resulting SOM. The maximum SOM content of a soil, within a climatic zone, is determined largely by the silt 1 clay contents (Feller and Beare 1997; van Noordwijk et al. 1997). This limit is largely due to the physical protection of SOM that is mediated by two mechanisms which are largely effects of texture and structure (Feller et al. 1996). SOM contents in tropical agricultural systems, however, rarely reach those of the natural systems from which they were derived, the content being 20–50% that of the corresponding natural system (Detwiler 1986). The decline is due to a combination of decreased organic inputs, increased decomposition (from increased temperatures), and often tillage. The quantity of the aboveground (and belowground) inputs in agricultural systems is less than in natural systems and traditional shifting cultivation practices (Table 1). The inputs of natural
systems and fallows are generally of low quality, with N less than 20 g kg 21 and lignin greater than 200 g kg 21 ; whereas the inputs of many agricultural systems are of high quality with N greater than 30 or 40 g kg 21 and lignin less than 100 g kg 21 . If the SOM hypotheses are correct, those systems with the lower quality inputs may have a higher SOM content than the agricultural systems but a larger percentage of the SOM could be in the more recalcitrant fractions. The net effect on nutrient availability would then depend on the differences in the total SOM contents and relative proportions of the different SOM fractions between the systems. How do these differences in the quality and quantity of organic inputs between natural and agricultural systems affect the overall SOM content, composition and function? The extent to which the lower SOM content of the agricultural systems affects the various functions of the component parts of SOM is not known. Are there critical amounts of total SOM or its component parts below which the desired functions of SOM are ineffective? To manage SOM effectively, these basic questions must be addressed. The relative effects of the quantity or quality of organic inputs in building or maintaining SOM have not yet been well established so it is difficult to evaluate the possible tradeoffs between SOM and Synchrony. During the decomposition process organic matter undergoes chemical stabilization into more stable forms (Jenkinson 1981), but whether more recalcitrant forms of organic inputs can be used within agricultural systems to raise SOM contents and main-
Table 1. Quantity (dry matter, DM) and quality of organic inputs in natural and derived agricultural systems for the humid and subhumid tropics (modified from (Palm et al. 1996)) SYSTEM Forest Shifting cultivation Cropping phase Fallow phase (1–5 yr) Fallow phase (5–10 yr) Legume tree fallows (, 5 yr) Continuous cropping Cereal crop residues Farm yard manure Biomass transfer Legume cover crop a
Aboveground, DM Mg ha 21 y 21
Quality a N g kg 21
lignin g kg 21
8–11
, 20
. 300
3 2–6 5–8 1–6 b
, 25 15–25 15–20 20–30
50–100 200–400 . 300 . 200
3–9 2–10 2–8 4–10 d
, 25 7–23 c 30–40 30–50
50–100 100–200 50–200 50–100
TSBF Organic Resource Database. Mafongoya, unpublished data; Schroth et al. (1995). c Mugwira and Mukurumbira (1984). d Drechsel et al. (1996). b
70 tain nutrient supply remains an open question. Few experiments have been designed specifically to separate the effects of the organic input quality from the quantities added. Only experiments in which both the quality and quantity of materials are controlled, and not confounded, will provide sufficiently robust tests. In the few cases where similar amounts of different quality organic inputs have been applied the results are inconsistent. In some cases, materials with higher C-to-N ratios and higher lignin contents resulted in more SOM (Janzen et al. 1988; Paustian et al. 1992), while other studies have shown no effect of organic input quality on differences in SOM content (Larson et al. 1972). An example from India showed no significant differences in total soil carbon in plots receiving four years of additions of 2.7, 3.8, and 12 t / ha of Sesbania, FYM, or wheat straw, respectively (Table 2, Goyal et al. (1992)). There was, however, a tendency for higher SOM contents in the plots with FYM and sesbania, materials with higher lignin content, even though considerably smaller amounts of these materials were applied compared to wheat straw. The wheat straw plus fertilizer treatment had lower total SOM yet higher microbial C than the other treatments, which was related to the higher C input from wheat straw. The wheat straw treatment also had the lowest crop yields. The decrease in yields with wheat straw even after four years is related to net N immobilization that would be expected from a material with a C-to-N ratio above 90. In the longer term, the effects of these different treatments may equilibrate so that the soil organic matter is increased at no expense to the immediate nutrient supplying capacity of the soil. Inputs that have both a high N and lignin content may provide the opportunity for attaining both SOM and Synchrony. Treatments that include farmyard manure (FYM) often exhibit the higher SOM contents
and yields, compared to other organic inputs (Goyal et al. 1992; Woomer et al. 1997). One reason proposed for this is that farmyard manure generally has relatively high lignin contents (20%) and high N contents (W. J. Parton, personal communication), both considered necessary for forming stabilized soil organic matter. Another potential mechanism by which chemical stabilization may result in larger amounts of SOM is through the use of materials rich in reactive polyphenolic compounds. The precipitation of proteins and carbohydrates by condensed tannins has marked similarities to the ‘polyphenol theory’ of SOM formation (Schnitzer 1978), but whether this is a mechanism whereby SOM contents can be maintained above that expected for a given soil type and management is not known. The N bound through this process, however, may be unavailable for extended periods and thus may compromise the Synchrony objective of organic matter management. To summarize, few experiments have been designed properly to test the longer-term effects of the addition of different quality organics on SOM content and composition. Of particular value would be a comparison of high quality organics with an immediate nutrient value but possibly no effect on total SOM; low quality organics (high lignin) that are known to reduce nutrient availability in the short term but increase SOM in the long term; and intermediate quality farmyard manure, that contains both N and lignin, and may provide an immediate nutrient value and longer term SOM build up. The predictions of the Synchrony and SOM hypotheses present a challenge in that, if they are correct, materials that are good for short-term soil fertility management will not build or maintain soil organic matter and vice versa. It is then difficult to imagine practical means of building and maintaining soil organic matter in any context where short-term
Table 2. Amount and quality of organic additions, soil organic matter levels, and pearlmillet yields following four years of applications of inorganic fertilizer (urea) compared to the combination with organic materials of differing qualities (Adapted from Goyal et al. (1992)) Treatment
Organic input (t ha 21 y 21 )
Organic input quality C / N, lignin * (g kg 21 )a
SOM – C (g kg 21 )
Microbial C (mgC / g)
Pearlmillet (Mg / ha)
N0P0 N120P40 N60P20 1 N60 (FYM) N60P20 1 N60 (wheat straw) N60P20 1 N60 (Sesbania) LSD (P 5 0.05)
0 0 3.8 12.0 2.7
na na C / N 5 27; lignin 5 200 C / N 5 90; lignin 5 50 C / N 5 20; lignin 5 100
4.0 4.3 5.0 4.5 4.8 0.6
180 290 330 355 315 19
1.51 2.76 2.81 2.33 3.15 0.27
a
estimated from TSBF Organic Resource Database. N 5 kg of N added as fertilizer or organic material; P 5 kg of P added as inorganic fertilizer
71 soil fertility issues take precedence over longer-term maintenance of soil organic matter, even if the latter option is more sustainable. The combination of low quality organic materials with mineral fertilizers (or high quality organics) to offset the negative nutrient effects of the low quality materials may be one solution. This, however, is a solution with an added cost – a cost that smallholder farmers in the tropics often cannot meet.
Translating Theory Into Application Knowledge gained over the past decade on the role of the environment and organic resource quality in influencing soil nutrient availability patterns and soil organic matter maintenance provides us with a scientific basis on which to develop management tools. Some degree of progress has also been made in linking this scientific information with farmer knowledge and circumstances to provide practicable and realistic approaches to soil fertility and soil organic matter management in the tropics. Information on the resource quality (nutrient, lignin, and soluble polyphenolic contents) of a variety of crop residues, agroforestry trees and shrubs, and leguminous cover crops has been compiled into an Organic Resource Database (ORD) (http: / / www.wye.ac.uk / BioSciences / soil / ). The database can be used to select organic materials for specific resource quality parameters and their influence on nutrient release patterns or soil organic matter formation. Through comparative review of the data, critical values for the different resource quality parameters have been proposed which regulate the shift from net N mineralization to immobilization (Palm and Rowland 1997; Palm et al. 1997). Net mineralization is
thus predicted for nitrogen concentrations greater than 25 g kg 21 , lignin values less than 150 g kg 21 and soluble polyphenols less than 40 g kg 21 . From these critical values a simple decision tree (Figure 4, Palm and Rowland 1997); this is an attempt to quantitatively define high and low quality organic materials as they relate to nitrogen release patterns. The decision tree can be used for testing these categories but at this time is applicable only for biomass transfer (cut and carry) of plant materials, not animal manures.) has been developed that proposes four quality categories of organic materials (Table 3). The focus of the critical values and decision tree has been on the short-term effects of organic additions on nutrient availability but it could also serve as a framework for testing the longer term effects of organic resource quality on soil organic matter maintenance and composition (Table 3). Whereas materials in the high quality category can be applied directly as a substitute for mineral fertilizer, those of intermediate-high quality, because of an initial and sometimes prolonged immobilization of N are not suitable for this purpose but have potential for the longer term build up of soil organic matter. The materials in the two low quality categories both have low N content, the intermediate-low material has low lignin resulting in a similar short term N immobilization to that in the intermediate-high group – but its role in SOM formation is uncertain. The lowest quality group, with low N and high lignin or polyphenolics results in longer term immobilization but may also lead to the formation of SOM, particularly if N is supplied as high quality organic materials or mineral fertilizer. Through network trials currently being conducted in East and Southern Africa the predictions of this decision tree are being tested and the fertilizer equivalency value of many commonly used organic materials is being established. The decision tree, organic resource database, and results from network trials will provide a systematic means of selecting and managing organic resources for soil fertility management. These same trials if conducted over the longer term could also provide guidelines on organic input quality and SOM formation, composition, and services.
Translating Application Into Practice Figure 4. Decision tree for biomass transfer of plant materials for soil fertility management: Translating theory into application (from Palm et al. (1997)).
The multiple services provided by SOM have effects on plant production, nutrient use efficiency and the
Delayed, short or long term
Low – Short term immobilization Very low and possible long term immobilization
N . 25, lignin . 150 or polyphenol . 40
N , 25, lignin , 150 and polyphenol , 40 N , 25, lignin . 150 or polyphenol . 40
Intermediate-High quality
Intermediate-Low quality (Short term) Low quality (Long term)
Little or negative effect on total SOM; increased active fraction (soil microbial biomass) Increased particulate (light) and passive fractions Little effect on total SOM Increased particulate (light) and passive fractions High and immediate N . 25, lignin , 150 and polyphenol , 40 High quality
Soil organic matter formation Nitrogen supplying capacity Resource Quality Parameters N, lignin, soluble polyphenols (g kg 21 ) Resource quality category
Table 3. Proposed categories of organic materials based on N, lignin, and polyphenol contents and their hypothesized effects on nitrogen supply and soil organic matter
72 long-term production potential of a soil system. Many farmers in the tropics are aware of the short and long-term benefits of SOM and use organic inputs in their agricultural systems. Practical issues such as the immediate production goals, amounts of organic materials needed and available, and the labor required to use them will determine what is practicable and what, in the end, farmers will do and what organic materials they will use. If faced with the choice of an organic material that provides an immediate nutrient source but will not build soil organic matter or a material that immobilizes nutrients in the short term but builds soil organic matter and nutrient availability over the longterm, farmers will most likely choose the material with the short term benefit. A challenge for us, as researchers, is to determine how we can apply the enhanced understanding which is derived from the results of this research to assist farmers in managing their organic resources (Giller 2000). The decision tree provides a potential tool for assisting in making choices between organic matter management practices. As described above the options it gives are fourfold. This is probably about the right level of sensitivity for practicality. It is interesting to note the difference between this and the continuous quantitative predictions of simulation models that have been the target in much nutrient management research. Although their predictions have much greater sensitivity and are thus more scientifically satisfying, their practical application may lack the decisiveness which farmers need. The decision tree in its original form lacks practicality at the farm level in that it is dependent on the availability of information such as that in the Organic Resource Database. We have therefore translated the decision tree into a form which can be used directly to facilitate dialogue with farmers and extension agents on the quality of organic materials and how they can best be put to use (Figure 5, Giller 2000). By using a simple set of field tests which can be used as a ‘rough and ready’ guide for assessing organic resource quality in terms of its N, lignin and reactive polyphenol contents (Giller and Cadisch 1997) the decision tree can be used in the field. The substitute for N analysis is simply leaf color as green leaves have N values of 20 g kg 21 or more whereas yellow leaves have smaller N contents as N is retranslocated from the leaves during senescence. An approximation that can be used to test lignin contents in the field is simply whether the material can readily be torn or crushed to a powder when dry. Reactive polyphenol contents can be evalu-
73 ated on the basis of astringency when tasted due to their ability to complex with salivary proteins. A group of smallholder farmers in Zimbabwe when asked to evaluate a range of multi-purpose legume trees as potential fodder for cattle readily identified all of those with large concentrations of soluble reactive polyphenols. While this decision tree will undoubtedly not always be completely accurate, it provides a tool which can be used in participatory research with farmers to assist decisions in the development of practical approaches to managing organic resources. The scientific knowledge encapsulated in this decision tree, together with other ideas relating to management of animal manures, have already been incorporated into an extension manual by a non-governmental organization which is working with farmer groups on natural resource management (IIRR 1998).
Future Challenges Although the initial steps have been taken to develop tools which can be used to manage organic resources at the farm level, we recognise that smallholder farmers in the tropics face a wide variety of serious constraints. Perhaps the most serious of these is the shortage of organic resources of suitable quality for soil amendment, and the conflicting demands for these scarce resources as animal feed and fuel. For many farmers the demands for increased productivity from smaller land areas, or the need to improve the productivity of degraded and depleted environments requires access to resources external to their farms. Extensive livestock systems in which organic resources are effectively harvested over large areas are under increased pressure in many parts of the tropics. Systems based on N 2 -fixing legumes can assist in some situations (Giller and Cadisch 1995), but prod-
Figure 5. Farmer decision tree for assessing organic matter quality and management: Translating application into practice (from Giller (2000)).
uctive and profitable systems in which sufficient organic matter can be grown without substantial costs or tradeoffs too often remain the dreams of researchers. Farming which relies purely on organic inputs is unlikely to provide the productivity desired. On many soils increases in productivity will be closely linked to the use of mineral fertilizers, though these often remain beyond the reach of smallholder farmers. The benefits of organic resources in improving the efficiency with which mineral fertilizers are utilized may be small in the short-term but higher in the longer-term because of the link to maintenance of SOM contents (Giller et al. 1997; Palm et al. 1997). Hopefully by developing tools that jointly allow a deeper understanding of their constraints we can assist farmers in optimizing their use of all available resources.
Acknowledgements The authors would like to thank the Soil, Water, and Nutrient Management Program (SWNM) of the CGIAR, DFID, and the Rockefeller Foundation for funds that have supported this work.
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