Soil phosphorus distribution in sequentially extracted fractions in tropical coffee-agroecosystems in the Atlantic Forest biome,Southeastern Brazil

Phosphorus (P) deficiency is one of the most important constraints to food production, particularly in tropical Oxisols. This study aimed to characterize the inorganic and organic P (Pi and Po) fractions in the soil in three smallholding coffee cultivation fields managed under either agroforestry (AGF) or full sun (FSC) agroecosystems. The work was carried out in areas situated at the municipalities of Divino and Araponga in the Zona da Mata in the state of Minas Gerais, southeastern Brazil. Soil P forms including H₂O–Pi; NaHCO₃–Pi,Po; NaOH–Pi,Po; diluted HCl–Pi; concentrate HCl–Pi,Po and Residual-P were sequentially extracted in samples from 0 to 5 and 5 to 10-cm soil layers. Together, H₂O–Pi and NaHCO₃–Pi accounted on average for only 4% of soil total P in the cultivated soils. HClconc.-Pi ranged from 142.8 to 372.4 mg kg⁻¹ being the predominant Pi fraction. AGF systems promoted an increment of 8% in the NaHCO₃–Po fraction in relation to the FSC systems in the upper soil layer. The AGF systems increased HClconc.-Po pool in relation to the FSC systems in Divino and Araponga(I) soils, indicating that agroforestry is an important management strategy to increase bioavailable P and for the maintenance of organic P pool. The distribution of inorganic and organic P pools varied among the different study sites, showing that P cycling depends on the inherent characteristic of each agroecosystem. The availability of P to plants in coffee-agroforestry fields is directly associated with the cycling of the organic P pool.     

The distribution of phosphorus fractions and desorption characteristics of some soils in the moist savanna zone of West Africa

The fractionation of soil P into various organic and inorganic pools with differing levels of bioavailability, coupled with knowledge of the P adsorption and desorption characteristics of the soils, provides insights into management strategies that enhance P availability to crops. Sequential soil P fractionation was conducted on samples from 11 soil profiles and different experimental fields selected from the derived savanna (DS) and northern Guinea savanna (NGS) zones of the West African moist savanna to assess the influence of soil characteristics and management on soil P pools. Phosphorus adsorption and desorption studies were conducted on samples from the surface horizon of the soil profiles. The total P content varied within and among the soil profiles and tended generally to decrease as depth increased. The total P content in topsoil varied from 90 to 198 mg kg^–1 of which about 30% was organically bound P. The resin P fraction was generally low (mean = 5 mg kg^–1, topsoil) and decreased with depth. These low resin P levels indicate low P availability. Within the DS, where the organic resource (OM) was Senna siamea residues, the effects on soil P fractions of OM and soluble P fertilizer (PF), whether sole or in combination, were site-specific. While resin P was significantly increased by OM in some sites, no significant differences were observed in others. In the NGS fields, farmyard manure (organic resource, OM) combined with PF and PF applied alone increased the inorganic P (Pi) fractions extractable with resin, bicarbonate, and NaOH by about 400% but had no significant effect on the organic P (Po) pools and the more stable Pi forms. The P sorption capacities were low, with the adsorption maximum deduced from the Langmuir equation ranging from 36 to 230 mg kg^–1. The amount of P sorbed to maintain 0.2 mg l^–1 in solution ranged between 0.6 and 16 mg kg^–1. Phosphorus desorption with anion exchange resin differed among the soils, with the recovery of added P ranging from 17 to 66% after 96 h. On average, more of the applied P was recovered in the DS soils than in the NGS soils. Because of the relatively low sorption capacity and the relatively high percentage recovery, small additions of P to most of the soils studied might be adequate for crop growth. In essence, quantities of P fertilizer needed in these soils might be estimated based on considerations of P uptake by crops rather than on sorption characteristics.     

Phosphorus fractions and adsorption characteristics in grassland soils of varied soil phosphorus status

Characterization of phosphorus (P) in soils is important both agronomically and environmentally, although the outcome may depend on the technique applied. Consequently, we evaluated fractionation and adsorption, individually and jointly, and relevant ancillary soil attributes, to determine the dominant functional characteristics of soil P in 32 fertilized temperate grassland Inceptisols classified by eight soil series, and by two soil-P index and parent material groups. Residual P was low (30.7%) and organic P (Po) prominent, 42.0% vs. 17.5% for equivalent soils in unfertilized natural ecosystems. Labile fractions comprised 6.8% inorganic P (Pi) and 9.1% Po. The proportional increase in high vs. low index soils (Morgan P > 6.0 mg l⁻¹ vs. ≤ 6.0 mg l⁻¹) was higher for Pi, and highest for labile and moderately labile fractions. Only moderately labile Pi and Po differed significantly between soils of limestone and non-limestone origin. Oxalate extractable Fe (Fe_ox) and buffering (EBC) were higher in the latter. The equilibrium P concentration (EPC) was substantially higher in the high index group, and EBC and binding energy (k) substantially lower, with no significant difference in sorption maximum (Pmax). EBC equated with weak to strong buffering in different soil series, and conformed better than k to ancillary attributes. Pmax correlated in order Al_ox > clay > OC > Fe_ox, and more broadly reflected sorption attributes than oxalate-based sorption capacity (PSC). Principal component (PC) analysis showed consistent differentiation of P fractions, mostly labile and moderately labile, in PC 1 vs. adsorption and ancillary attributes in PC 2. However, scatterplots of PC scores showed that adsorption characteristics provided better functional differentiation than P fractions for distinguishing individual soil series, which may have implications in selection and interpretation of extractants not only for environmental but also for agronomic soil-tests.     

Soil order and management practices control soil phosphorus fractions in managed wetland ecosystems

Phosphorus, an essential element for plant and animal growth, can also impair water quality. Understanding management effects on P dynamics can aid in the management of these systems to reduce nonpoint source pollution and improve fertilizer use efficiency. A sequential P fractionation procedure was used to evaluate labile to recalcitrant inorganic (Pi) and organic (Po) fractions in wetland rice soils. In this study we evaluated 71 wetland soils in the Sacramento Valley, California, consisting of different soil orders (Alfisols, Entisols, Mollisols and Vertisols) and different management systems (conventional rice, organic rice and natural wetlands). Total soil P ranged from 165 to 784 μg g⁻¹ and averaged 415 μg g⁻¹. Mollisols contained significantly more Pi and Po than all other soils; especially more HCl-Pi and Po. Although most studies ignore Po in the HCl fraction, 9% of total P was recovered in this fraction, suggesting that this fraction should not be ignored in studies aimed at quantifying and understanding organic P. The HCl-Po fraction was closely correlated with HCl-Pi, suggesting that it may be Ca bound P. Soils managed under organic rice production had higher NaHCO₃-Pi and NaOH-Pi levels than conventional rice systems; while the natural wetland systems showed intermediate amounts. Organic or synthetic P fertilizers applied in excess of the amount of P that is removed during harvest was recovered as NaHCO₃-Pi and NaOH-Pi.     

Effects of organic matter on the physical and the physicochemical properties of an illitic soil

The main thrust of this study is to investigate the effects of organic matter on the physical and the physicochemical properties of illitic soils. For this purpose, organic matter (peat) was added to inorganic illitic clayey soil at eight levels (0%, 5%, 10%, 12.5%, 15%, 17.5%, 20%, and 30% by weight). The physicochemical properties of the resulting soils were determined using a Grain Size Analyzer (GSA) with specific surface area measurement, Scanning Electron Microscopy (SEM), and Infrared Spectroscopy (IR). The physical properties of the mixtures were determined by conducting a series of laboratory tests including Atterberg limits, compaction, unconfined compressive strength, and swell characteristics tests. The results showed that at low organic contents (in general less than 15%) the soil particles tend to aggregate, whereas at higher organic contents the soil particles tend to disperse. Also, the IR tests showed that direct chemical interactions took place between the organic and the inorganic fractions of the mixtures. The tests on the physical properties showed that at low organic content (up to 10%) the plasticity index slightly increased then after, the plasticity index decreased with increasing organic content. Organic matter has shown to decrease the maximum dry density and increase the optimum water content, nevertheless, although organic matter decreased the soil's compactability, the feasibility of compaction of slightly organic content soils still exists. Moreover, it was shown that organic matter decreased the peak strength values and increased the water contents at these peak strengths. The final free swell for illitic soils increased with increasing levels of added organic matter.     

Association of soil-test phosphorus with phosphorus fractions and adsorption characteristics

There is incomplete understanding, based on a single comprehensive study, of the relationship between empirical extractants of soil-test P (STP) and fundamental measurements of soil-P status such as inorganic (Pi) and organic (Po) fractions, P adsorption and relevant ancillary soil attributes. Consequently, we assessed these relationships for the extractants Morgan, Mehlich-3, Olsen, Bray-1, lactate–acetate, CaCl₂ (1:2 and 1:10 soil:solution) and resin. Multiple regression analysis indicated that STP extracted with Morgan and CaCl₂ related dominantly with the most labile resin Pi fraction, Mehlich-3 and Olsen with labile NaHCO₃ Pi, Bray-1 with moderately labile NaOH Pi and lactate-acetate with relatively stable Ca-bound HCl Pi, for example. Especially for Morgan and CaCl₂ (1:2), and except for Bray-1, the best relationship of STP with adsorption characteristics was with equilibrium P concentration in solution (EPC). Buffering capacity (EBC) and binding energy (k) did not have significant effects, as indicated by regression, whereas the effects of clay and oxalate-extractable Fe (Fe_ox) were generally negative and Al_ox and organic carbon (OC) positive. Principal component analysis (PCA) highlighted many similarities in the extractants. However, regression of STP against soil-P pools, integrated as principal component scores, inadequately revealed relationships, which were better facilitated by PCA ordinations. For ordinations of STP with P fractions, eigenvalues for the first two axes explained 88.6% of the variance. Closest associations were residual Pi with HCl Pi, CaCl₂ (1:10) and to a lesser extent Morgan with OC and clay, and Mehlich-3 and Olsen with NaHCO₃ Pi and resin Pi. For ordinations of STP with P adsorption, eigenvalues for the first two axes explained 97.8% of the variance. The STP extractants grouped in specific, but distinctly different, combinations. For example, strongest inverse relationships were EBC to EPC and Morgan, and k to resin and Olsen, and to OC and clay, indicative of weak P bonding on these surfaces. These distinctions are consistent with, and provide a rationale for, the relevance of Morgan as an environmental P test vis-a-vis other extractants of STP.     

Pathways of native and fertilizer phosphorus in Argentine soils

Management of soil phosphate fertility in sustainable agriculture depends on transformations of applied fertilizers as an input to correct soil defficiencies. This research investigated the changes of P pools of different extractability with fertilization in an incubation experiment. Sequential fractionation was used in 5 native argentine soils: Entisol, Andisol, Vertisol, Mollisol and Ultisol, with 0 and 45 kg P ha^-1 added as KH₂PO₄ and incubated for 90 days. In our experimental conditions, labile inorganic P (LIP) content of control soils increased for Entisol and Mollisol (75% and 35% respectively), while moderately resistant inorganic fractions (MRIP) were greater in Andisol (95%) and Ultisol (39%) following incubation. This increase was related to a decrease in labile organic fractions (LOP) in Andisol (-73%), Mollisol (-36%) and Ultisol (-36%). Moderately resistant organic forms (MROP) were significatively lower for all soils except Mollisol.As a consequence of P fertilization, LIP increased for Entisol (146%), Vertisol (23%) and Mollisol (39%), and MRIP showed the same tendence in Ultisol (57%) and Andisol (97%). LOP was signifcatively lower for all soils, except Andisol. MROP decreased in all soils except Mollisol, with the greatest variation in Andisol (-56%). In this experiment, labile P, the agronomically important pool, showed a similar pathway for native and fertilizer P for each taxonomic Order, with a significative increase in Mollisol and Entisol. P for each taxonomic Order, with a significative increase in Mollisol and Entisol.The main reservoir for fertilizer P was IP, mainly LIP in Mollisol and Entisol, and MRIP in Vertisol, Andisol and Ultisol.Organic P tended to decrease with incubation, and the highest values of organic fractions were found in younger soils (Entisol and Andisol), followed by Ultisol. Residual effect of fertilizer could be higher in Andisol and Ultisol due to transformation into non-labile forms.     

The role and function of organic matter in tropical soils

Soil organic matter (SOM) has many functions, the relative importance of which differ with soil type, climate, and land use. Commonly the most importantfunction of OM in soil is as a reserve of the nitrogen and other nutrients required by plants, and ultimately by the human population. Other important functions include: the formation of stable aggregates and soil surface protection; maintenance of the vast array of biological functions, including the immobilization and release of nutrients; provision of ion exchange capacity; and storage of terrestrial carbon (C). This paper considers the quantity and quality of SOM of soils in the tropics, which are estimated to contain one quarter of the C in the global pool in terrestrial soils, and supports strongly the use of analytical methods to characterizing labile SOM to develop valuable insights into C dynamics. As in other regions, the transformation of tropical lands for agriculture exploits SOM, and in particular nutrient reserves. The process of exploitation is accelerated in the tropics by the necessity to increase agricultural production, largely through agricultural intensification, to overcome inadequate nutrition, to satisfy population growth, and to cope with the limited reserves of arable land. Poverty has an overriding influence on the exploitation and degradation processes. Areas at greatest risk of land degradation are the infertile acid soils of the tropics, which, invariably, are cultivated by the poor. Soil organic matter has a central role in sustainable land management, but perspectives on the roles of SOM differ widely between farmers, consumers, scientists and policy-makers. Some consider SOM as a source of nutrients to be exploited, whereas others can afford to utilize it as a key component in the management of the chemical, biological, and physical fertility of soils. Still others see SOM as a dumping ground for excess nutrients and toxins, or as a convenient store for fossil fuel emissions, particularly CO₂. Farmers need sustainable land management systems that maintain OM and nutrient reserves. Nevertheless, many available practices, whether based on indigenous or scientific knowledge, do not meet social and economic criteria that govern farmer behaviour. Much scientific knowledge about the various roles of SOM does not reach farmers and other decision-makers in a form that can be used easily. The biggest challenge to researchers is to engage with clients to pinpoint gaps in knowledge and utilize new and existing information to devise decision support Systems tailored to their needs. This revised version was published online in June 2006 with corrections to the Cover Date.     

Combined effect of water and organic matter on phosphorus availability in calcareous soils

Phosphorus removal from soil solution is mainly due to adsorption and precipitation. For calcareous soils, with a large reservoir of exchangeable calcium, precipitation of insoluble Ca-P phases is the predominant process that reduces P availability to plants. Soil water content positively affects P-precipitation, while the addition of organic matter (OM) has an opposite effect. Little information on the effect of soil organic matter on P-insolubilisation as a function of soil water contents has prompted this study of the variation of extractable P, after addition of mineral P fertiliser. Columns packed with a calcareous soil were enriched with different levels of OM, extracted from Irish peat, and subjected to different rainfall simulations. After 102 days of experimentation and 171 mm of accumulated rainfall, the Olsen-P was 53% of the initially applied amount in 6.2% OM-enriched soil, 37% in 4.1% OM-enriched soil, and 20% in untreated soil (1.9% of OM). While the curve describing Olsen-P decrease as a function of accumulated rainfall was clearly exponential for untreated soil, the curves for OM-enriched samples were flatter, evidence that OM addition modified P-insolubilisation. The P-insolubilisation, after P-fertilisation, at several constant values of soil moisture for (i) calcareous soil, (ii) calcareous soil after removing carbonates and saturating the exchange complex with Ca, and (iii) calcareous soil after addition of different levels of OM followed first-order kinetics. The K_obss followed the order: Ca-saturated soil > untreated soil > OM-enriched samples. Results from rainfall simulation experiments and kinetics of Olsen-P decrease at several constant soil moisture contents indicated that the soil water amount was the main factor in reducing extractable P after P fertilisation and that the soil OM content was the main factor in keeping P in extractable forms. On the other hand, the addition of OM to calcareous soil increased the extractable P at each soil moisture regime, decreasing P-insolubilisation more effectively at lower soil water contents. P-sorption isotherms of calcareous soil after addition of different levels of OM showed that the presence of OM mainly influences P-insolubilisation, but not the adsorption process.     

(Tropical) soil organic matter modelling: problems and prospects

Soil organic matter plays an important role in many physical, chemical and biological processes. However, the quantitative relations between the mineral and organic components of the soil and the relations with the vegetation are poorly understood. In such situations, the use of models is an important research tool to explore the relations between the various components, to increase insight into processes, to examine the consequences of management, and to explore the possibilities for modification. An overview is given of the state-of-the-art in modelling of soil organic matter dynamics, with special emphasis on the processes in tropical regions. Major points identified as insufficiently developed include: Methodology is lacking to quantitatively describe the impact of soil texture and soil structure on the activity of soil biota. Effects of the microfauna on modelled organic matter transformations in the soil appear to be lacking. Techniques for direct measurement of pool sizes and characterisation of the relevant properties of the variety of organic substances would represent a major step towards verification of models and the revision of inherent concepts. The non-nutrient functions of organic matter, such as structural stability, water holding capacity and cation exchange capacity, need far more attention in modelling of soil organic matter. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chemical Nature and Diversity of Phosphorus in New Zealand Pasture Soils Using <Superscript>31</Superscript>P Nuclear Magnetic Resonance Spectroscopy and Sequential Fractionation

Information on the phosphorus (P) forms of pasture soils is central to agricultural and environmental sustainability. We used a combination of ³¹P nuclear magnetic resonance (NMR) spectroscopy of NaOH–EDTA extracts and sequential fractionation to investigate P forms, with an emphasis on organic P in relation to environmental and biophysical variables, in 24 diverse pasture soils taken from around New Zealand (organic C 19–102 g kg⁻¹, total P 116–2746 mg kg⁻¹, pH 5.2–7.0). Soils were separated by cluster analysis of soil physicochemical properties and sequentially extracted P pools into those either derived from volcanic-ash materials or not. This separation was also evident for organic P species in NaOH–EDTA extracts, which removed about 75% of total soil organic P. The major organic P compound class was monoesters (24–60% of extracted P), made up of 14 to 91% myo-inositol hexakisphosphate. The next largest organic P class was diesters (0–4% of P extracted), which were enriched in volcanic-ash soils (monoester to diester ratio = 14) compared to non volcanic-ash soils (ratio = 30). Correlation analysis indicated that mean annual temperature had a significant negative and positive effect on monoester and diester concentrations, respectively. This was attributed to better physical protection of monoesters (especially inositol phosphates) from mineralization, and increased diesters from microbial activity and biological turnover. The anomalous behaviour of volcanic-ash soils was attributed to slow mineralization and the sampling of soils at different times of year without full knowledge of the large and highly dynamic microbial biomass P pool which is predominantly diesters.     

Adsorbed phosphorus partitioning in some benchmark soils from Northeast Brazil

The partitioning of adsorbed P between labile and non-labile pools by soils is fundamental to the residual effect of fertilizer-P added to soils. The main objective of the study was to determine the partitioning of adsorbed P between the labile and non-labile phases by some benchmark soils of northeast Brazil for which is little is known. Surface and subsurface samples of several soils: Non-Calcic Brown soil and Planosol (Haplustalfs), Cambisol (Ustropept), Lithosols (Orthents) and Alluvial soil (Tropaquept) were equilibrated with varying concentrations of KH₂PO₄. The readily exchangeable portion of the adsorbed P was determined by anion exchange resin (AER). Considerable hysteresis was observed between adsorbed P and AER-P. To quantify the extent of the hysteresis, a critical P concentration (P_crit), the amount of P adsorbed at zero desorption by AER, was defined. The P_crit of the soils averaged across the soil depths followed the order: Non-Calcic Brown soil > Planosol > Alluvial > Cambisol > Lithosol. The P_crit correlated with clay and oxalate Fe (Fe_o). The P affinity index (K) estimated by Langmuir adsorption model accounted for 66% of the variance in P_crit. A sequential extraction with 0.5M NaHCO₃, 0.1M NaOH and 11.5M HC1 to remove the labile, moderately labile and non-labile P forms respectively, indicated that between 63 and 99% of adsorbed P was in the labile pool (AER-P + HCO₃-P + OH-P), suggesting that the soils might have high potential for residual fertilizer-P responses.     

Comparison of methods for the estimation of inert carbon suitable for initialisation of the CANDY model

Almost all soil organic carbon turnover models rely on a partitioning of total organic carbon into an inert and a decomposable pool. The quantification of these pools has a large impact on modelling results. In this study several methods to estimate inert carbon in soils, based either on total soil organic matter or physical protection, were assessed with the objectives of (1) minimising errors in carbon and nitrogen dynamics and (2) ensuring usability for sites with marked differences in site conditions. CANDY simulations were carried out by varying solely the method for calculating the size of the inert carbon pool used to initialise the model. Experimental data from Bad Lauchstädt and Müncheberg were used for the simulation. The data were made available for modellers at a workshop held at Müncheberg (Germany) in 2004. The results concerning not only carbon but also nitrogen dynamics were analysed by applying selected statistical methods. It was shown that even in short-term simulations model initialisation procedure may influence the simulation results considerably. Three methods of estimating inert carbon were identified as being the most appropriate. These methods are either based on soil texture or pore-space classes and therefore account for the physical protection of soil organic matter. Thus, physical protection seems to be of major importance. By extending the scope of the investigation into nitrogen dynamics, additional support for the applicability of a selected method was obtained.     

Long-term changes in phosphorus fractions in growers’ paddocks in the northern Victorian grain belt

Increases in P fertiliser use in Australian grain production systems over the past decades have not necessarily coincided with improved nutrient-use efficiency by crops because only a small proportion of applied P has been directly used by crops, leaving large amounts of the P in soil. Information on the transformation of applied P and the residual effectiveness of P fertiliser in growers’ paddocks under their crop rotations in a wide range of soil types may help to develop improved management practises for P fertilisers. The present study examined the size and changes of P pools in soils in the major regions used for grain production across northern Victoria. Soil samples were collected from 43 sites representing all the major soil types. In addition, samples were collected from adjacent ‘reference’ areas across fence-lines where remnant native vegetation was present to examine long-term changes in soil fertility as a result of cropping. Highly positive P balance in 27 growers’ paddocks indicates potentially high rates of P fertilization in excess of that exported/lost from these paddocks. The accumulated P was transformed into both labile and non-labile pools depending on the general chemical–physical properties of a particular soil. In particular, in soils with a sandy texture and low oxalate-extractable Al and Fe such as the sandy Calcarosols, the residual P fertilizer was preferentially transformed into labile and moderately labile pools. These P fractions can be available for subsequent crops. In contrast, a large proportion of P fertilizer applied to other soil types has been transformed into the non-labile P pools where crops have difficulty in accessing. It is suggested that P application rates should be reduced to avoid the loss of P through sorption/precipitation, leaching or run-off while still meet crop demand.     

Effect of tree foliage of locust bean (Parkia biglobosa) and neem (Azadirachta indica) on soil fertility and productivity of maize in a savanna alfisol

Organic matter (OM) management is key to sustainable fertility and productivity of tropical soil systems. Short-term evaluation of the potentialsof foliage of neem and locust bean in soil fertility improvement, as measured by the productivity of maize, was carried out on a savanna Alfisol from 1995 to1997. The trees are indigenous to the savanna, and common in farms and homesteads in the region. The objective was to determine the effects of theseeasily renewable and cheap OM sources on maize response to applied inorganic fertilizer and soil chemical properties. Treatments consisted of three sourcesof organic matter: no-OM, neem and locust bean, and three rates of inorganic fertilizer, applied at 0, and of the recommended rate of120–27–50 NPK for maize, which in addition served as check. Results obtained over the three seasons were consistent, and revealed that neem wassuperior to locust bean by a factor of 2 and also to no-OM treatments, but were all inferior to the recommended fertilizer rate (with a maximum yield of 3.75t/ha). Isolated relative effects for neem and locust bean only in the third year were 138% and 86% respectively, and declined progressively withincreasing levels of applied inorganic fertilizer. This trend was consistent for the three years, during which soil productivity (relativity index) clearlyappreciated. Results also revealed that P and organic C accumulation was greater with locust bean, but neem decomposed faster and depressed pH less, a clearindication that indices of soil productivity may not always be tied to OM or absolute values of soil properties alone in savanna soil systems. The studyalso revealed that the effects of the tree foliages, the inorganic fertilizer rates and the interactions were significant at various levels of probability forgrain, stover, plant height, available P, OM and pH. There was the possibility of better crop performance with combinations of OM and inorganic fertilizer levels for the soils. Integrated fertility management systems using 3 tons of neem foliage and recommended inorganic fertilizer rate produced a goodmaize crop, and offers an economically viable option for the resource poor maize farmers of the Nigerian savanna.     

Composting of Disposal Organic Wastes: Resource Recovery for Agricultural Sustainability

One of the major problems of agricultural soils in the tropical regions of the Pacific is the low organic matter content. Because of the hot and humid environment, the soil organic matter (SOM) is minimal due to rapid decomposition. Composted organic material is being applied on agricultural fields as an amendment to provide nutrients and enhance the organic matter content for improving the physical and chemical properties of the cultivated soils. In addition land application of composted material as a fertilizer source effectively disposes of wastes that otherwise are buried in landfills. In our soil program at the University of Guam, we are evaluating the use of organic material as an alternative to synthetic fertilizers. Its goal is to develop management strategies and use available resources for improving crop production while conserving resources and preserving environmental quality. Our case study project is designed to improve soil fertility status by using composted organic wastes and assessing how the nitrogen and other essential nutrients contribute to long-term soil fertility and crop productivity without application of synthetic fertilizers. In our pilot project, compost is produced from wood chips, grinded typhoon debris mixed with animal manure, fish feed, shredded paper and other organic wastes. Mature compost is then applied on the field at the rates of 0, 5, 10 and 20 t/ha as a soil amendment on the eroded cobbly soils of southern Guam. Corn is planted and monitored for growth performance and yield. The effect of land application of composted material on the SOM content and overall soil quality indices are being evaluated in this pilot study.     

Evaluation of iron oxide impregnated filter paper method as an index of phosphorus availability in paddy soils of Tanzania

Phosphorus is a major yield limiting nutrient in rice production and yet most soil test methods for predicting P availability to plants have a number of shortcomings especially under flooded conditions. The objective of this study was to evaluate the iron oxide impregnated filter paper (Pi) method for assessing changes in P availability in soils subsequent to flooding and to determine the suitability of the method in assessing P status in paddy soils. The results indicated that available P increases considerably (between 34% and 256%) subsequent to flooding and this affected responsiveness of the soils to P application. Two versions of the Pi method namely; the Pi method used under flooded conditions (Pi-P_f) and Pi strips embedded directly in flooded soils (Pi-P_fe) were effective in assessing P availability in paddy soils. The two Pi methods were better correlated with rice dry matter yield than the traditional soil tests and are hence recommended for use in soil testing under flooded conditions. The tentative critical levels of P for the two procedures under pot conditions were 22 mg/kg for Pi-P_f and 15 mg/strip for Pi-P_ef.     

Relationships between soil urease activity and other properties of some tropical wetland rice soils

Ten Philippine wetland rice soils differing widely in pH, texture and organic matter were studied to determine relationships between urease activity and other soil properties. Simple correlation analyses of urease activity with soil properties indicated that urease activity was correlated highly significantly with total N (r = 0.91)^**, and organic C (r = 0.89^**) but was not significantly correlated with CEC, Clay, pH active Fe or active Mn content. From multiple regression analyses it was observed that organic matter content of soils measured by organic C and total N accounted for most of the variation in urease activity.     

Does the Pi strip method allow assessment of the available soil P?: Comparison against the reference isotope method

Experiments have been carried out in acid soils developed under tropical climates with and without phosphate rock (PR) addition to assess the ability of the Pi strip method to extract available soil P compared with the Isotopic Exchange Kinetics method (IEK). In the Pi strip method strips of filter paper, previously impregnated with iron hydroxide acting as a sink for phosphate ions from soil components, are added to an aqueous soil suspension. The extracted phosphate ions are eluted in diluted sulfuric acid and quantified by a colorimetric method. Available soil P, defined as the amount of phosphate ions that can move from the soil particles to soil solution, is described by three factors: an intensity factor, a quantity factor and a capacity factor. These three factors were determined by the IEK-method. Following the addition of carrier-free ³²PO₄-ions to soil, the ability of the Pi strip to extract available soil P was assessed: (i) by comparing the quantity of instantaneously exchangeable P (E₁) to the quantity extracted with the Pi strip; (ii) by determining the fraction of ³²P extracted with the Pi strip, and (iii) by comparing the specific activity (SA) of P present as phosphate ions extracted by the Pi strip to the specific activity of P in the soil solution. It was observed that (i) E₁ and the amount extracted with the Pi strip are highly correlated, (ii) the recovery of ³²P extracted by the Pi strip varies between 17 and 66%, and (iii) the specific activity of P extracted by the Pi strip is of the same order of magnitude as that of P in the soil solution. In acid soils low in available P, part of the P in aqueous KCl-extracts is presumably not only present as free phosphate ions but also occluded in the form of a soluble complex, whose isotopic exchangeability is significantly lower than that of phosphate ions transferred to the Pi strip. It is concluded from the results that the Pi strip method can be recommended in routine analysis for the determination of the quantity factor. However, this method cannot provide intensity or capacity factors and therefore needs to be complemented by the IEK-method for full characterization of the available soil P status.     

Impact of planted fallows and a crop rotation on nitrogen mineralization and phosphorus and organic matter fractions on a Colombian volcanic-ash soil

Soil fertility replenishment is a critical factor that many farmers in the tropical American hillsides have to cope with to increase food crop production. The effect of three planted fallow systems (Calliandra houstoniana-CAL, Indigofera zollingeriana-IND, Tithonia diversifolia-TTH) and a crop rotation (maize/beans-ROT) on soil nitrogen mineralization, organic matter and phosphorus fractions was compared to the usual practice of allowing natural regeneration of native vegetation or natural fallow management (NAT). Studies were conducted on severely degraded Colombian volcanic-ash soils, 28 months after fallow establishment, at two on-farm experimental sites (BM1 and BM2) in the Cauca Department. Tithonia diversifolia had a significantly higher contribution to exchangeable Ca, K and Mg as well as B and Zn; the order of soil nutrient contribution was TTH > CAL > IND > NAT > ROT. On the other hand, lND had significantly higher soil NO₃⁻–N at both experimental farms as compared to all the other fallow system treatments. For the readily available P fraction, CAL and ROT had significantly higher H₂O–Po and resin-Pi, respectively, in the 0–5 cm soil layer; whereas TTH showed significantly higher values for both H₂O–Po and resin-Pi in the 5–10 cm soil layer. Significant effects were observed on the weights of the soil organic matter fractions which decreased in the order LL (Ludox light) > LM (Ludox intermediate) > LH (Ludox heavy). Indigofera zollingeriana showed greater C, N and P in the soil organic matter fractions than all the other fallow treatments, with NAT having the lowest values. It is concluded that planted fallows can restore soil fertility more rapidly than natural fallows.