Forms and exchangeability of inorganic phosphate in composted solid organic wastes

Switzerland yearly produces more than 260,000 Mg of compost, two thirds of which is recycled in agriculture and horticulture. This research was undertaken to examine the forms and availability of inorganic P (Pi) in Swiss composts made from solid kitchen and garden wastes using the isotopic exchange kinetic technique, a sequential Pi extraction and magic angle spinning (MAS) solid-state ³¹P nuclear magnetic resonance (NMR) spectroscopy. The different approaches described in this paper demonstrate the presence of a complex mixture of Pi species in the studied composts. Isotopic exchange experiments and sequential extraction showed that these composts contained relatively large concentrations of rapidly available Pi. Significant correlations were observed between the concentration of water-soluble Pi (Cp), and the total N, C and P content of composts suggesting that organic substances partly controlled the amount of rapidly available Pi. Significant correlations were observed in alkaline composts between the amount of Pi which can not be exchanged within 3 months and the total P and Ca content. In alkaline composts solid-state MAS ³¹P NMR results suggested the presence of a range of slightly soluble and poorly crystallized Ca-P compounds such as apatites or octacalcium phosphates and of organic P compounds. The slowly or non-exchangeable Pi present in these composts could therefore be bound to Ca in the form of apatites or octacalcium phosphates.     

Co‐composting an animal fatty‐proteinaceous waste with a solid lignocellulosic by‐product from the olive oil industry (‘alperujo’)

BACKGROUND: The production of biodegradable wastes and their disposal cause a major financial problem in many industrial activities. Co‐composting was thought to be a feasible alternative for disposing of a strongly alkaline waste from the pharmaceutical industry (AW), mainly consisting of animal fats and partially hydrolysed proteins in a stable emulsion. The AW was added gradually, during the early phase of the composting process, to a substrate made up of ‘alperujo’ (AL), the wet, lignocellulosic, solid by‐product of the olive oil industry, and fresh horse manure, which was added to improve the physical structure of the composting substrate. RESULTS: The addition of AW reduced organic matter degradation during composting, enriched the amount of organic compounds in the water‐ and alkali‐soluble fractions and increased mineral salt contents. Thus, significantly higher electrical conductivity, humification indices and contents of organic matter, P, K and Na were recorded in the end‐composts resulting from AW treatments. However, the application of one AW‐based compost led to soil N immobilisation, as revealed by an incubation experiment, which must be considered in order to avoid potential N starvation in the short term. CONCLUSION: According to these results, composting can be used as a disposal alternative for AW, leading to end‐products with potential uses as organic amendments or fertilisers for agricultural systems. In addition, these composts could be used to produce alternative liquid organic fertilisers, based on the extraction of their humic‐like fraction. Copyright © 2009 Society of Chemical Industry     

Predicting N mineralized in a Georgia Coastal Plain field

The N mineralized from soil organic matter provides an important portion of N available for crop production. The objective of this study was to determine the amount of spatial variability in N mineralization potential in a field and to evaluate three different methods that might be used to estimate this variability. The three methods tested included predicting the N mineralized from surface soil properties as well as from a biological and a chemical procedure. Three soils varying in N mineralization potential were selected for the study from a field in the Georgia Coastal Plain. The N mineralized from these soils was determined by an N balance of unfertilized and cropped plots. The amount of N mineralized could not be reliably predicted from surface soil organic C, although surface soil clay concentration was positively correlated with the N mineralized. The N mineralized that was predicted using mineralization parameters determined by aerobic incubation, adjusted daily for soil water content and temperature, was approximately 50% of the field measurements of N mineralized. The values of NH₄-N extracted with hot 2 M KCl were related significantly to N mineralized in the field (r²= 0.60) and also to the zero order rate constant of mineralization, k₀ (r²= 0.77), determined from the N mineralized in the aerobic laboratory incubation.     

Fly ash—a potential source of soil amendment and a component of integrated plant nutrient supply system

In sub-tropical climate the high rainfall and high temperature is responsible for low soil productivity due to losses of bases and low organic matter content in soil. In acid lateritic soil low availability of P and high content of Al and Fe posses nutritional imbalance which is generally corrected by lime materials. Alkaline fly ash can be used in such problematic soil as an amended material and also it acts as source of plant nutrition for crop production. An attempt was made to develop an integrated plant nutrient supply system utilizing the fly ash along with other organic wastes like paper factory sludge, farm yard manure, crop residue and chemical fertilizers for rice-peanut cropping system. Direct and residual effects of fly ash were assessed based on crop yield, nutrient uptake and changes in soil characteristics. The application 10 t ha⁻¹ of fly ash in combination with organic sources and chemical fertilizer increased the grain yield and nutrient uptake of rice, and pod yield of peanut compared to chemical fertilizers alone. The heavy metal contents in plant and soil system was analyzed and found to remain below the permissible level. The results indicated that fly ash could be applied safely to tropical agro eco-systems for retaining productivity of acid lateritic soil.     

Thermal analysis in the evaluation of compost stability: a comparison with humification parameters

Characterization of organic matter of six composts from agroindustrial wastes was carried out by both chemical analysis and thermal analysis in order to assess their level of stability. Degree of humification (DH%) and index of humification (HI) were calculated after extraction, fractionation and analysis of the organic carbon from composts. Differential Scanning Calorimetry (DSC) and thermogravimetry (TG) were simultaneously performed in oxidizing conditions on whole ground composts. Thermoanalitical data resulted to be useful in integrating quantitative information coming from chemical analysis of humified fraction of compost organic matter. Particularly, DSC curves allow to distinguish between well and poor stabilized composts, and information deriving from weight losses, registered by TG curves, permits to individuate a thermoanalytical parameter (R1) that resulted to be well correlated to humification parameters DH% and HI.     

Quality of soil organic matter and C storage as influenced by cropping systems in northwestern Alberta,Canada

Crop rotations and reduction in tillage are commonly recommended for sustained crop production and enhancing soil quality. Our objective was to evaluate the long-term effects of cropping systems (1968–1992) on soil structure, carbon storage and the quality of soil organic matter. The study was conducted on a silt clay loam soil (Typic Cryoboralf) near Beaverlodge, Alberta, The cropping systems were: (a) continuous barley (Hordeum vulgare L.) (CB); (b) continuous bromegrass (Bromus inermiss Leyess.) (CG); (c) continuous forage legume (Medicago sativa L. until 1977, and Trifolium pratense L. since 1978) (CL); and (d) 3 years of bromegrass-legume forage alternating with 3 years of barley (RF). Our data showed that the CG and CL treatments had more stable aggregates with greater mean weight diameter (MWD) than soil under continuous barley. Organic C, total N and the light fraction in soil under CG and CL were higher than those of the other two treatments. Soil under CG had the highest and CB the lowest amounts of acid-hydrolyzable monosacchrides (comprising glucose, arabinose, xylose, mannose and galactose). Higher galactose + mannose concentration in soil under CG indicated a higher soil microbiological activity. Microbial biomass C and N followed the trend among treatments in whole and light fraction organic matter, and total extracted sugars. Soil organic matter ¹³C-NMR spectroscopy showed that: (i) soil under CB contained the highest amounts of aromatic and the lowest content of aliphatic-C, (ii) soil under CL had the lowest phenolic-C and the least aromaticity, and (iii) soil under CG and RF had the highest amounts of aliphatic-C which includes labile substances such as amino acids and carbohydrates, indicating an improvement in the quality of organic matter. It is concluded that perennial forage crops can improve soil structure and soil organic matter quality and quantity as compared with cereal monoculture.     

The contribution of dairy farming on peat soil to N and P loading of surface water

In agriculturally used peat land areas, surface water quality standards for nitrogen (N) and phosphorus (P) are frequently exceeded, but it is unclear to what extent agriculture is responsible for nutrient loading of the surface water. We quantified the contribution of different sources to the N and P loading of a ditch draining a grassland on peat soil (Terric Histosol) used for dairy farming in the Netherlands. Measurements were performed on N and P discharge at the end of the ditch, supply of N and P via inlet water, mineralization of soil organic matter, slush application, composition of the soil solution, and on N losses through denitrification in the ditch for 2 years (September 2000 to September 2002). Discharge rates at the end of the ditch were 32 kg N ha^–1y^–1 and 4.7 kg P ha^–1y^–1. For N, 43 to 50% of the discharge was accounted for by applications of fertilizers, manure and cattle droppings, 17 to 31% by mineralization of soil organic matter, 8 to 27% by nutrient-rich deeper peat layers, 8 to 9% by atmospheric deposition and 3 to 4% by inlet water. For P, these numbers were 10 to 48% for applications of fertilizers, manure and cattle droppings, 2 to 14% mineralization of soil organic matter, 33 to 82% nutrient-rich peat layers and 5 to 6% inlet water. The results of this paper demonstrate that nutrient loading of surface water in peat land areas involves several sources of nutrients, and therefore, reducing one source to reduce nutrient inputs to surface water is likely to result in modest effectiveness.     

Suitability of different organic amendments from agro-industrial wastes in organic lettuce crops

An inadequate replacement of organic matter in agricultural lands progressively leads to soil fertility reduction and therefore, soil application of organic fertilizers and amendments should be promoted. The objectives of this 3-year research project on organic lettuce were to investigate the agronomic performance of experimental organic amendments obtained by using agro-industrial wastes in comparison with a commercial organic fertilizer, and to evaluate their short-term effects on soil mineral-N and soil organic carbon changes. Two types of olive pomace mixtures, with different initial C/N ratios, were composted and either stopped at the active phase (A1 and B1) or processed until maturation (A2 and B2). Also an anaerobic digestate (DA) and the B2 in combination with mineral fertilizer (B2-MIN) were studied. The four composts, DA and B2-MIN were compared with a commercial organic fertilizer (Org), and an unfertilized control (N0). Results suggested that the best compromise for organic lettuce yield and soil fertility could be obtained with the B2 compost, thus highlighting the need for choosing a good stage of maturity of the compost, along with an appropriate C/N ratio of composting mixture, to improve the fertilizing efficiency of agro-food residues in organic farming. Furthermore, the application of immature amendments did not increase (B1) or significantly reduce (A1) marketable lettuce yield, indicating that the choice of organic fertilizer is an important concern in sustainable agriculture, especially in organic vegetable production.     

Effects of drying and rewetting on carbon and nitrogen mineralization in soils and incorporated residues

An understanding of nitrogen mineralization from residues and soil organic matter is important to understand the quantity of N available from the soil for crop production. The objective of this study was to determine effects of repeated wetting and drying of soils on rates of N mineralization. The study compared mineralization rates in three kaolinitic, low organic matter soils, utilizing cotton leaves or compost as residues. One set of treatments was subjected to repeated drying and rewetting, whereas the other was kept at constant moisture content. Mineralized N was measured by leaching with 0.01 M CaCl₂ periodically, for 185 days. Rates of C mineralization were measured in the treatment containers by periodic measurement of CO₂ respiration rates. In constant moisture conditions, soils with cotton leaf residue mineralized between 25% and 40% of N applied as residue, whereas soils with compost mineralized between 3.8% and 9.3%. In fluctuating moisture conditions, soils with cotton leaf residue mineralized between –1.3% and 6.9%, whereas soils with compost mineralized from 1.6% to 3.3%. Moisture effect was not significant in soils without residue, with soils mineralizing between 16 and 47 mg N kg^–1. Carbon mineralization rates were not significantly affected by moisture. Both residue and soil type affected rates of C mineralization.     

The role of soil microorganisms in soil organic matter conservation in the tropics

Soil is a large sink for organic carbon within the terrestrial biosphere. Practices which cause a decline in soil organic matter cause CO₂ release, in addition to damaging soil resilience and, often, agricultural productivity. The soil micro-organisms (collectively the soil microbial biomass) are the agents of transformation of soil organic matter, nutrients and of most key soil processes. Their activities are much influenced by soil physico-chemical and ecological interactions. This paper addresses two key issues. Firstly, ways of managing, and the extent to which it is possible to manage, soil biological functions. Secondly, the methodologies currently available for studying soil micro-organisms, and the functions they mediate, are discussed. It is concluded that, as the world population develops in this new millennium, there will be an increased dependence upon biological processes in soil to provide adequate crop nutrition for the majority of the world's farmers. Although a major increase in the use of artificial fertilisers will be necessary on a global scale, this will not be an option for large numbers of farmers due to their poverty. Instead they will rely on recycling of nutrients from animal and vegetable composts and urban wastes, and biological cycling from nitrogen fixation and mycorrhizae. The challenge is to select the most appropriate topics for further research. Not all aspects are likely to lead to significantly improved agricultural productivity, or sustainability within the foreseeable future. This revised version was published online in June 2006 with corrections to the Cover Date.     

Assessing nitrogen mineralization from soil organic matter using anion exchange membranes

A simple method to assess differences in potential contribution of organic nitrogen mineralization to plant available N among soils may be useful in fertility research as well as routine soil testing. We deployed a method to assess mineralizable soil organic N using anion exchange membrane (AEM) burial. The method is based on a simple closed incubation system in which strips of AEM are buried directly in soil to adsorb NO ₃ ^- released from organic matter. An index of mineralization was obtained using the amount of NO ₃ ^- adsorbed on an AEM strip removed at the end of each incubation. The same incubation system but using 0.001M CaCl₂ solution to extract NO_3-N was used as the reference method. The mineralization indices obtained from both methods were compared with each other and with plant uptake. A total of 74 soils from across Saskatchewan were used in the study to provide a range of soil properties. Correlations between test values and N uptake by plants in two separate experiments showed the 2 week AEM incubation to be more closely correlated with plant N uptake (r² = 0.86^**** and 0.57^****, respectively) than the reference method (r² = 0.60^**** and 0.48^****, respectively).With this method, we were able to determine the influence of different tillage systems and landscape positions on mineralizable N. The results showed that the NO ₃ ^- released from soil organic matter and accumulated on the AEM reflected the expected effect of three different tillage systems and two landscape positions on mineralizable N. Cropping systems with continuous alfalfa (Medicago sativa) showed higher N release from soil organic matter than a canola (Brassica napus)-lentil (Lens culinaris)-barley (Hordeum vulgare) rotation did. Higher N mineralization was found in the lower slope positions of the landscape where organic matter contents are highest. Direct burial of AEM appears to be a simple and effective method of including a measure of N mineralization in a soil test.     

Crop residue,manure and fertilizer in dryland maize under reduced tillage in northern China: II nutrient balances and soil fertility

A long-term experiment was carried out in the dryland of northern China to assess the effects of applications of maize stover, cattle manure and NP (1:0.44) fertilizer on partial nitrogen (N), phosphorus (P) and potassium (K) balances, extractable soil N (SEN), P and K, and soil organic matter (SOM) in a spring maize cropping system, under reduced tillage conditions. The experiment was set-up according to an incomplete, optimal design, with three factors at five levels and 12 treatments, including a control with two replications. Statistical analyses using multiple regression models showed that the partial N, P and K balances were strongly influenced by annual variations in the amounts of soil water at seeding (SWS) and growing season rainfall (GSR). Most treatments had positive P but negative N and K balances. Cumulative P and K balances were reflected in extractable soil P (P-Olsen) and K (exchangeable K), but the weak relationships indicated that the sorption of P and buffering of K were strong. Cumulative balances of effective organic carbon (C) were weakly related to soil organic C (SOC) content after 12 years. Negative C balances were related to decreases in SOC, but positive C balances were not translated into increases in SOC. The analysis of nutrient balances and soil fertility indices revealed that nutrient inputs in most treatments were far from balanced. It is concluded that the concepts of ‘ideal soil fertility level’ and ‘response nutrient management’ provide practical guidelines for improving nutrient management under the variable rainfall conditions of dry land areas in northern China.     

The significance of available nutrient fluxes in N and P budgets for maize cropping on a Rhodic Kandiustox: a study with compost,NP fertilizer and stubble removal

Nutrient budgets may be useful tools for nutrient management of crops especially if they estimate the nutrient fluxes available from a variety of sources including organic and inorganic fertilizer, crop residues and soil organic matter. The aim of the present study was to develop a budget of available nutrients by determining the contribution of mineralized nutrient fluxes and fertilizer input relative to nutrient losses and removal in harvested products in the overall N and P balances. N and P inputs and outputs and available N and P fluxes in the soil were estimated for 3 consecutive maize crops where inputs and outputs were altered by NP fertilizer, compost and stubble removal on a Rhodic Kandiustox. A sensitivity analysis of calculated and measured nutrient budget items was conducted to identify the main factors affecting the accuracy of the nutrient balance calculations. Mineral fertilizer rate was the major factor for maize nutrient budgets as shown by its contribution to N and P balances. Without mineral fertilizer application, soil organic matter (SOM) mineralization was the most important within-season nutrient input. In the case of N, shoot uptake was the main output followed by denitrification. Phosphorus adsorption by the soil was the major P output from the available pools followed by shoot uptake. SOM mineralization maintained the pools of available N and P if stubble of the previous crop was returned. Mineral fertilizer application, which produced surplus balances of N and P, would however, be needed to attain high yield, even with stubble return. The available N and P from compost were not significant inputs in the nutrient balances until year 3. Total N and resin extractable P in soil after five crops supported the calculated nutrient balances indicating the importance of available nutrient fluxes in calculating N and P balances.     

Unconventional feeds for small ruminants in dry areas have a minor effect on manure nitrogen flow in the soil–plant system

In dry areas, unconventional feeds are increasingly used for mitigating feed shortages and rangeland degradation. We evaluated how feeding sheep diets containing olive leaves, saltbush leaves and olive cake affects manure quality compared to a barley straw based diet. Soil incubation and plant growth experiments were carried out to measure soil nitrogen (N) mineralization and N uptake by barley plants and to calculate N flow through the feed-animal-soil–plant system. Fresh feces, composts consisting of feces, urine and straw, and ammonium sulfate fertilizer were mixed with soil at rate of 90 mg N kg^?1 soil dry matter. Comparisons were made with non-amended soils (control) and soils amended with fresh olive cake applied at 90 and 22.5 mg N kg^?1 soil dry matter, respectively. The latter treatment enabled investigation of the effect of passage of olive cake through the digestive tract of sheep on N availability and phenol transformation. Applying fresh olive cake and feces, except the saltbush leaf derived feces, resulted in a net N immobilization. All composts resulted in net N mineralization, although not significantly different from the 0N control soil. Barley growing in soils with amendment that caused N immobilization took up less N than barley growing on the 0N treatment. Reduction in N uptake was most pronounced after amendment with fresh-olive cake. Treatments with net mineralization increased barley N uptake over the 0N treatment with 2–16 % of N applied being taken up. Dietary composition had a minor effect on N fertilizer value of either feces or compost, but feces N alone was not an efficient N source.     

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.     

Process-based modelling of nitrous oxide emissions from different nitrogen sources in mown grassland

The process-based Pasture Simulation Model (PaSim 2.5) has been extended to simulate N₂O production and emission from grassland caused by nitrogen inputs from different sources. The model was used to assess the influence of management on N₂O emissions, such as the effect of shifts in the amount and timing of fertilizer application. Model performance has been tested against season-long field measurements at two different field sites. Simulation results agreed favourably with measured N₂O emission and soil air concentrations, except during an extremely wet period at one site when grass growth was very poor. The results of short-term and long-term simulation runs demonstrated the potential of the model to estimate N₂O emission factors under various conditions. During the first growing season, simulated emissions from organic fertilizers were lower than from synthetic fertilizers because more of the nitrogen was used to build up soil organic matter. The relative difference between the fertilizer types became larger with increasing application rate. The difference between fertilizer types was smaller at steady-state when higher soil organic matter content from repeated application of organic fertilizer over time led to enhanced mineralization and N₂O emissions. The dependence of simulated N₂O emissions on N input was close to linear at low, but non-linear at high fertilization rates. Emission factors calculated from the linear part of the curve suggested that the factors used in the current IPCC method underestimate the long-term effects of changes in fertilizer management. Furthermore the simulations show that N₂O emissions caused by nitrogen inputs from the decomposition of harvest losses and from biological fixation in grassland can be considerable and should not be neglected in national emission inventories. This revised version was published online in August 2006 with corrections to the Cover Date.     

Levels of ATP in different organic wastes under composting conditions

BACKGROUND: The concentration of ATP in selected samples from the composting process of several organic wastes (municipal solid wastes, wastewater sludge, animal by‐products and cow manure) has been determined in order to characterize the aerobic biological activity of such wastes. RESULTS: The values obtained ranged from 0 (in old stable compost from cow manure) to 0.07 µmol ATP g^?1 dry matter in thermophilic samples of municipal solid wastes composting. In general, it was found that ATP levels were in agreement with the stage of the composting process (initial stage, thermophilic period and maturation). On the other hand, ATP concentration correlated well (P < 0.05) with the overall respiration activity during a complete composting process of municipal solid waste at full scale. CONCLUSION: ATP concentration can be used to determine the biological activity of organic solid wastes in different stages of their biological treatment and to predict compost stability prior to soil application. Copyright © 2008 Society of Chemical Industry     

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.     

Effect of compost and soil properties on the availability of compost phosphate for white clover (Trifolium repens L.)

Wide variation in results exists in the literature on the effectiveness of composts to sustain the phosphorus (P) nutrition of crops. The aim of this work was to assess the importance of some soil and composts properties on the utilization of compost-P by white clover (Trifolium repens L.). This study was carried out with samples collected from four composts made from solid kitchen and garden wastes, and with two soil samples taken from the A horizon of a P-rich sandy acidic Dystrochrept and of a P-limited clayey calcareous Eutrochrept. Changes in the amount of inorganic P (Pi) isotopically exchangeable within 1 min (E_1min) were measured during 32 weeks in incubated soil-composts or soil-KH₂PO₄mixtures where P sources had been added at the rate of 50 mg P kg^–1 soil. Uptake of compost-P or KH₂PO₄-P by white clover was measured on the same amended soils during 16 weeks. In both soils, the application of composts resulted after 32 weeks of incubation in E_1min values ranging between those observed in the control without P and those observed in the KH₂PO₄treatment, i.e., in values ranging between 4.2 and 5.9 mg P kg^–1 in the sandy acidic soil and between from 1.6 to 4.3 mg P kg^–1 in the clayey calcareous soil. The total coefficient of utilization of compost-P (CU-P) by white clover reached values in both soils for the four composts ranging between 6.5% and 11.6% of the added P while in the presence of KH₂PO₄ the CU-P reached values ranging between 14.5% in the clayey calcareous soil and 18.5% in the sandy acidic soil. Results obtained in the sandy acidic soil suggest, that white clover initially used a fraction of the rapidly exchangeable compost P, while at a latter stage plant roots enhanced the mineralisation of compost organic P and took up a fraction of the mineralized P. These relations were not observed in the clayey calcareous soil probably because of its high sorbing capacity for P. In the sandy acidic soil, composts application increased the uptake of soil P by the plant from 31.4 mg P kg^–1 soil in the control without P to values ranging between 37.9 to 42.7 mg P kg^–1 soil in the presence of composts. This indirect effect was related to a general improvement of plant growth conditions in this soil induced by compost addition (from 9.9 g DM kg^–1 soil in the control without P to values ranging between 14.0 to 16.1 g DM kg^–1 soil in the presence of composts) and/or to the release of Al- or Fe bound soil P to the solution due to soil pH increase following compost application. Finally the total coefficient of utilization of P (CU-P) derived from KH₂PO₄ and composts was related to the total amount of N exported by white clover in the P-limited clayey calcareous soil but not in the P-rich sandy acidic soil. This suggests that in a soil where N₂ biological fixation is limited by low P availability, the CU-P of a compost by white clover is not only related to the forms of P present in the compost but also to its effect on N nutrition. However, it is not clear whether this improved N nutrition was due to compost mineralisation, or to an indirect compost effect on the N₂ biological fixation.