Nitrogen cycling in organic farming systems with rotational grass–clover and arable crops

Organic farming is considered an effective means of reducing nitrogen losses compared with more intensive conventional farming systems. However, under certain conditions, organic farming may also be susceptible to large nitrogen (N) losses. This is especially the case for organic dairy farms on sandy soils that use grazed grass–clover in rotation with cereals. A study was conducted on two commercial organic farms on sand and loamy sand soils in Denmark. On each farm, a 3‐year‐old grass–clover field was selected. Half of the field was ploughed the first year and the other half was ploughed the following year. Spring barley (Hordeum vulgare L.) was sown after ploughing in spring. Measurements showed moderate N leaching during the pasture period (9–64 kg N ha^?1 year^?1) but large amounts of leaching in the first (63–216 kg N ha^?1) and second (61–235 kg N ha^?1) year after ploughing. There was a small yield response to manure application on the sandy soil in both the first and second year after ploughing. To investigate the underlying processes affecting the residual effects of pasture and N leaching, the dynamic whole farm model farm assessment tool (FASSET) was used to simulate the treatments on both farms. The simulations agreed with the observed barley N‐uptake. However, for the sandy soil, the simulation of nitrate leaching and mineral nitrogen in the soil deviated considerably from the measurements. Three scenarios with changes in model parameters were constructed to investigate this discrepancy. These scenarios suggested that the organic matter turnover model should include an intermediate pool with a half‐life of about 2–3 years. There might also be a need to include effects of soil disturbance (tillage) on the soil organic matter turnover.     

Effects of different potassium fertilizers suitable for use in organic farming systems on grass/clover yields and nutrient offtakes and interactions with nitrogen supply

Abstract. In organic farming systems, fertilizing materials can be used when potassium (K) deficiency is shown, but such systems are dominantly nitrogen (N) limited and this is likely to affect crop utilization of K. The supply of K to grass/clover from a range of mineral and organically based K fertilizers and its interaction with N supply were studied in a greenhouse experiment. Sequential plant cuts were taken for yield and nutrient content determinations in crop and soil. Crop yields were limited by N: where N supply was increased either through the mineralization of N from organic materials (rapemeal, farmyard manure) or inorganic fertilizer, plant yields increased significantly. Grass/clover responded better to additional K where sufficient N was available. However, yield responses to K were generally small, even in the presence of adequate N. Of the different fertilizers, kali and MSL-K increased yields above those of the control by less than 5%, sylvinite, DKSI and farmyard manure by 10–20%, and rapemeal and potassium sulphate by more than 25%. In all treatments, K offtakes in the grass/clover were considerably greater than fertilizer K inputs. The grass/clover showed an increased uptake of Na where insufficient K was available. However, the Mg content of the grass/clover was not adversely affected by K fertilizer application. Organic farmers need to consider the soil K status, the rotational nutrient budget, the supply of all nutrients in fertilizing materials and nutrient interactions to achieve effective K management in organic farming systems.     

Response of organically managed grassland to available phosphorus and potassium in the soil and supplementary fertilization: field trials using grass–clover leys cut for silage

Abstract. Effective use and recycling of manures together with occasional and judicious use of supplementary fertilizing materials forms the basis for management of phosphorus (P) and potassium (K) within organic farming systems. Replicated field trials were established at three sites across the UK to compare the supply of P and K to grass–clover swards cut for silage from a range of fertilizing materials, and to assess the usefulness of routine soil tests for P and K in organic farming systems. None of the fertilizing materials (farmyard manure, rock phosphate, Kali vinasse, volcanic tuff) significantly increased silage yields, nor was P offtake increased. However, farmyard manure and Kali vinasse proved effective sources of K to grass and clover in the short to medium term. Available P (measured as Olsen-P) showed no clear relationship with crop P offtake in these trials. In contrast, available K (measured by ammonium nitrate extraction) proved a useful measurement to predict K availability to crops and support K management decisions.     

Impact of organic manures with and without mineral fertilizers on soil chemical and biological properties under tropical conditions

Soils receiving organic manures with and without chemical fertilizers for the last 7 yr with pearlmillet–wheat cropping sequence were compared for soil chemical and biological properties. The application of farmyard manure, poultry manure, and sugarcane filter cake alone or in combination with chemical fertilizers improved the soil organic C, total N, P, and K status. The increase in soil microbial‐biomass C and N was observed in soils receiving organic manures only or with the combined application of organic manures and chemical fertilizers compared to soils receiving chemical fertilizers only. Basal and glucose‐induced respiration, potentially mineralizable N, and arginine ammonification were higher in soils amended with organic manures with or without chemical fertilizers, indicating that more active microflora is associated with organic and integrated system using organic manures and chemical fertilizers together which is important for nutrient cycling.     

Influence of farm yard manure,poultry manure and forest litter on copper solubility in soil and uptake by Phaseolus vulgaris

Long term and intensive use of copper‐based fungicides on coffee farms may contaminate soils with copper. The legacy of copper pollution may pose the risk of contaminating food crops cultivated on these soils. A randomized block design field experiment at Kilimanjaro, Tanzania was designed to investigate the effects of different application rates of cattle manure, poultry manure and forest litter on aqua regia, EDTA and CaCl₂ extractable copper in soils and copper uptake by bean plants grown on this long‐term copper‐contaminated soil (more than 50 yr of copper application). It was important to examine the potential of the organic amendments in mobilizing or immobilizing copper and assess the risks of contaminating bean crops at a farm, where the application of organic amendments was common practice. At harvest, rhizosphere soils were collected and analysed. The soils were found to have large concentrations of copper, greatly exceeding international standard levels. Poultry manure applied at 40 tons/ha significantly increased CaCl₂ extractable copper compared with the control treatment. Organic carbon, cation exchange capacity, EDTA extractable copper, aqua regia extractable copper and copper concentrations in bean seeds or leaves were not significantly changed by the organic matter treatments. The concentration of copper was significantly less in bean seeds than in bean leaves (P < 0.01). The bean plants did not take up excessive quantities of copper, and therefore, the risk of copper contamination of bean crops in this farm appears to be small.     

长期施用有机肥对瘠薄红壤有效碳库及碳库管理指数的影响

主要分析了长期施用有机肥对瘠薄红壤有效碳库(微生物量碳,易氧化碳,矿化碳)及碳库管理指数(CPMI)的影响.结果表明:长期施用有机肥对土壤有效碳库和碳素有效率有很大影响,绿肥,稻草秸秆肥和厩肥处理的土壤有机碳,微生物量碳、易氧化碳、矿化碳的数量、碳素有效率明显高于对照处理的土壤。施肥对全碳、矿化碳的影响为:厩肥>绿肥>秸秆稻草肥>本田还田>对照,对微生物量碳的影响为:绿肥>厩肥>稻草秸秆肥>本田还田>对照,对活性碳(CA)、CPMI,碳素有效率A、B、C的影响为:绿肥>稻草秸秆肥>厩肥>本田还田>对照。在提高CA、CPMI方面,绿肥和稻草秸秆肥优于厩肥。相关分析表明:土壤有效碳库和碳素有效率与土壤化学性质相关或极相关,CPMI与土壤养分因子相关或极相关,反映了农业生产措施对土壤碳库的影响,可以运用CPMI来评估土壤碳库的变化。     

Microbial biomass turnover and enzyme activities following the application of farmyard manure to field soils with and without previous long-term applications

Summary We studied the build-up and turnover of microbial biomass following the addition of farmyard manure to an unmanured soil and to soils from a long-term experiment in which different levels of farmyard manure had been applied for the last 23 years. The application of farmyard manure at 15–90 t ha^-1 to previously unmanured soil increased the microbial biomass during the first 3 months of incubation but a gradual decline occurred with further incubation for up to 12 months. Microbial biomass C was positively correlated with soil organic C and ranged from 1.8% to 2.2% of organic C after 12 months of farmyard manure applications. Biomass turnover increased with the application of farmyard manure, ranging from 0.81 to 0.87 year^-1 with various levels of manure. Amendment of soils from the long-term manure experiment with various levels of farmyard manure led to a build-up and decline in biomass C as seen in the unmanured soils, but biomass C was higher in all treatments compared to the corresponding unmanured soil treatments. Biomass turnover was greater compared to the unmanured soil treatments and it decreased with increasing levels of farmyard manure. The average soil respiratory activity increased with increasing levels of farmyard manure, but respiratory activity per unit of biomass C decreased with increasing levels of manure. Enzyme activities were greater in long-term manured soils compared to unmanured soils amended with various levels of manure. There was a significant correlation between biomass C and enzyme activities.     

The Effects of Long‐term Fertiliser Applications on Soil Organic Carbon and Hydraulic Properties of a Loess Soil in China

Based on a 28‐year in situ experiment, this paper investigated the impacts of organic and inorganic fertiliser applications on soil organic carbon (SOC) content and soil hydraulic properties of the silt loam (Eumorthic Anthrosols) soils derived from loess soil in the Guanzhong Plain of China. There were two crop (winter wheat and summer maize) rotations with conventional tillage. The treatments included control without fertiliser application, organic manure application (M), chemical fertiliser application (NP), and the application of organic manure with chemical fertiliser (MNP). The results showed that the 28‐year organic manure applications (M and MNP) significantly (p < 0·05) increased SOC content at surface layer (0–10 cm), but the effect of chemical fertilisers alone on SOC was not significant. Organic manure treatments (M and MNP) apparently improved soil hydraulic properties. Compared with control, field capacity and total porosity significantly (p < 0·05) increased while soil bulk density significantly (p < 0·05) decreased for organic manure applications. The M and MNP treatments increased soil water retentions by 3·2–10·8%, which was dependent of suction tensions. However, the NP treatment had no significantly impact on soil water retention compared with control. Neither organic nor inorganic fertiliser applications significantly changed saturated hydraulic conductivity. However, a clear difference was observed for unsaturated hydraulic conductivity between the M and the control at 0–5 cm. Overall, long‐term applications of organic manuring increased SOC content and amended soil hydraulic properties. However, the effects of chemical fertilisers on these soil properties were limited. Copyright © 2015 John Wiley & Sons, Ltd.     

Detectability of degradable organic matter in agricultural soils by thermogravimetry

Sustainable agricultural land use requires an assessment of degradable soil organic matter (SOM) because of its key function for soil fertility and plant nutrition. Such an assessment for practical land use should consider transformation processes of SOM and its sources of different origin. In this study, we combined a 120‐day incubation experiment with thermal decay dynamics of agricultural soils altered by added organic amendments. The aim was to determine the abilities and limits of thermal analysis as a rapid approach revealing differences in the degradability of SOM. The carried out experiments based on two independent sampling sets. The first sample set consisted of soil samples taken from non‐fertilized plots of three German long‐term agricultural field experiments (LTAEs), then artificially mixed with straw, farmyard manure, sheep faeces, and charcoal equal to 60 Mg ha^?1 under laboratory conditions. The second sample set based on soil samples of different treatments (e.g., crop type, fertilization, cultivation) in LTAEs at Bad Lauchstädt and Müncheberg, Germany. Before and after the incubation experiment, thermal mass losses (TML) at selected temperatures were determined by thermogravimetry indicating the degradability of organic amendments mixed in soils. The results confirmed different microbial degradability of organic amendments and SOM under laboratory conditions. Thermal decay dynamics revealed incubation‐induced changes in the artificial soil mixtures primarily at TML around 300°C in the case of applied straw and sheep faeces, whereas farmyard manure showed mainly changes in TML around 450°C. Charcoal did not show significant degradation during incubation, which was confirmed by TML. Detailed analyses of the artificial soil mixtures revealed close correlations between CO₂‐C evolution during incubation and changes in TML at 300°C with R² > 0.96. Results of the soils from LTAEs showed similar incubation‐induced changes in thermal decay dynamics for fresh plant residues and farmyard manure. We conclude that the practical assessment of SOM could be facilitated by thermal decay dynamics if modified sample preparation and evaluation algorithms are used beyond traditional peak analysis.     

Influence of different agricultural management systems on nitrogen leaching: results of lysimeter studies

The objective was to estimate the potential risk of N leaching into the groundwater under various types of agriculture by using lysimeter experiments on the nitrogen(N)‐cycle of various soil types. Results were obtained with 12 weighable, monolithic lysimeters with a surface area of 1 m², a total depth of 3 m, and free drainage. Mean annual N‐leaching losses of 5 to 44 kg ha^—1 and nitrate concentrations of the seepage water (leachate) between 80 and 200 mg l^—1 were measured during the period of intensive agricultural use. On fallow land with a well‐established grass vegetation, some nitrate was removed by the plants. As a result, the nitrate concentrations in the leachate were reduced significantly. Ecological farming measures generally reduced N leaching losses and kept the N‐concentration in the leachate below the German threshold value for drinking water with 50 mg l^—1 nitrate. However, ploughing in of clover or leguminous vegetation and the application of farmyard manure in autumn caused the nitrate concentration in the leachate to rise significantly above the mentioned threshold value.<?show $6#>     

Amelioration of saline‐sodic soils with organic matter and their use for wheat growth

Abstract

Few reports highlight the effect of organic matter in improving the properties of saline soils. A laboratory study was conducted to determine the effect of adding farm yard manure (manure), Egyptian clover hay (clover hay), and wheat straw, at 1 and 3% of soil weight on water stability of soil aggregates (WSA), water‐holding capacity (WHC), pH, and electrical conductivity of soil extract (ECe) of a normal, saline, and saline sodic soil. After 90 and 180 days, WSA and WHC increased, while pH and ECe decreased. Soil properties improved most by adding 3% manure to all the soils. Wheat grown in these soils indicated significant differences for various growth and yield variables, especially the root growth, number and area of green leaves, and grain yield. Organic matter added to these soils increased WSA and WHC and decreased pH and Ece. The WHC had no correlation with pH, but was negatively correlated with Ece. Increased WSA caused the leaching of excess ions and reduced their toxicity, while enhanced WHC increased the availability of water to the roots and promoted growth. It is concluded that manure ameliorated salt affected soils and promoted wheat growth better than clover hay and wheat straw.     

Einfluß organischer Düngungen auf bodenphysikalische Parameter

Influence of organic amendments on soil physical properties The effect of applying crop residues additionally to fertilzers or farmyard manure on soil structure was studied in Upper Bavaria. Crop residues were applied each year for 13 years before sampling. The Aquic Hapludalf, derived from loess underlain by Tertiary fine sand, was cropped with vegetables. Resistance to penetration, saturated hydraulic conductivity (k_f), pore size distribution, and aggregate stability (AS) were determined. The application of residues and/or manure increased the organic matter content of the top soil, resulting in increases in AS and k_f. However, both of these parameters were higher following residue application rather than that of manure. Only the application of residues improved the plant available water capacity by about 10 mm within 70 cm depth of soil profile. The other properties were not significantly affected, probably because they were also influenced by the weather and soil management practices. The values for these properties must therefore be considered as only momentary.     

Nitrogen turnover in a repeatedly manured arid subtropical soil: Incubation studies with 15N isotopes

Under the hot and moist conditions of irrigated agriculture in the arid subtropics, turnover of organic matter is high, which can lead to considerable carbon (C) and nitrogen (N) losses. Therefore, sustainable use of these soils requires regular manure application at high rates. To investigate the contribution of consecutive manure applications to an arid sandy soil to various soil N pools, goat manure was isotopically labeled by feeding ¹⁵N‐enriched Rhodes grass hay and applied to the soil during a two‐year field experiment. In the first year, soils received ¹⁵N‐labeled manure to distinguish between soil‐derived and manure‐derived N. In the second year, these plots were split for the application of either ¹⁵N‐labeled or unlabeled manure to discriminate N derived from previous (first year) and recent (second year) manure application. Soil samples (of control and ¹⁵N‐manured soil) were collected at the end of the first and the second year, and incubated in two laboratory experiments with labeled or unlabeled manure. At the beginning of Experiment 1, 7% of total N, 11% of K₂SO₄ extractable N, and 16% of microbial biomass N were derived from previously field‐applied manure. While the application of manure during incubation increased microbial biomass N by 225% and 410% in the control soil and the previously field‐manured soil, respectively, N₂O emissions were more affected on the control soil, releasing considerable amounts of the soil N‐pool (80% of total emissions). In Experiment 2, 4% of total N, 7% of K₂SO₄ extractable N, and 7% of microbial biomass N derived from previously applied manure, and 4%, 8%, and 3% from recently applied manure, respectively. Microbial biomass N and N₂O‐N derived from manure declined with time after manure application, whereas in Experiment 1 this tendency was only observed for microbial biomass N.     

Phosphorus availability and farm structural factors: examining scarcity and oversupply in north‐east Germany

Assessing factors influencing phosphorus (P) availability in soils is important in preventing its overexploitation and excessive application in agricultural systems. Despite high historical P applications in the federal state of Brandenburg (Germany), county data on soil test P (STP) reveal considerable disparities in soil available P. In addition, negative soil balances as a result of small mineral P and manure inputs have been observed, raising questions about the factors leading to this situation. Our work focused on identifying possible causes operating at the farm management level by conducting a letter survey in two administrative districts of Brandenburg, the counties Barnim and Uckermark, linking farm management factors (ownership type, farm size, land tenure, animal husbandry with or without grassland and its intensity, presence of a biogas plant and organic production) to farmers’ self‐indicated levels of STP. Small‐ to medium‐sized individual farms tended to have (very) high STP, while large partnership farms and companies/cooperatives were sensitive to factors resulting in low STP. Farms with low shares of land ownership, the presence of grassland, extensive cattle farming and stockless organic farming had lower STP. On the other hand, biogas plants, partly in combination with intensive livestock (cattle) farming, were associated with larger STP. It was concluded that more care should be devoted to the design of agricultural policies and that further (inter‐ and transdisciplinary) research on this topic is needed.     

Nitrogen pools and turnover in arable soils under different durations of organic farming: I: Pool sizes of total soil nitrogen,microbial biomass nitrogen,and potentially mineralizable nitrogen

Soil organic matter contents, soil microbial biomass, potentially mineralizable nitrogen (N) and soil pH values were investigated in the Ap horizons of 14 field plots at 3 sites which had been under organic farming over various periods. The objective was to test how these soil properties change with the duration of organic farming. Site effects were significant for pH values, microbial biomass C and N, and for potentially mineralizable N at 0—10 cm depth. The contents of total organic C, total soil N, and potentially mineralizable N tended to be higher in soils after 41 versus 3 years of organic farming, but the differences were not significant. Microbial biomass C and N contents were higher after 41 years than after 3 years of organic farming at 0—10 cm depth, and the pH values were increased at 10—27 cm depth. Nine years of organic farming were insufficient to affect soil microbial biomass significantly. Increased biomass N contents help improve N storage by soil micro‐organisms in soils under long‐term organic farming.     

Quantification of chemical sulphur species in bulk soil and organic sulphur fractions by S K‐edge XANES spectroscopy

A new data treatment method for fitting spectra obtained by sulphur (S) K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy was used to quantify the chemical S speciation at three experimental sites with arable soils receiving the same long‐term field treatments. Two treatments, crop residue (CR) incorporation and farmyard manure (FYM) application, with equal applications of mineral nutrients were included in the study. In the new data treatment method, internally calibrated spectra of dilute solutions (30 mm ) of model compounds were used to fit the sample spectra. This greatly enhanced the reliability of quantitative determination of contributing S species in soil samples and soil extracts. The results indicated that long‐term FYM application shifted S species composition from highly oxidized towards intermediate oxidization in two of the soils, but in the third soil the opposite trend was observed. Sulphur XANES spectroscopy of acetylacetone extracts of physically protected and unprotected organic S in two of the soils revealed that physical protection was not related to S speciation; however, intermediate forms of oxidized S species appeared to accumulate in the residual S pool (not extractable by acetylacetone).     

Land use effects on carbon quality and soil biological properties in Eutric Cambisol

Abstract

The choice of prospective type of farming requires knowledge about the specific relationships that exist between farm management practices and base environmental conditions. Nowadays the protection of soil organic carbon is one of the main tasks, because organic carbon in addition to soil fertility can act in elimination of soil contamination and carbon sequestration. Field experiments were focused on the effect of intensive farming without organic inputs versus grassland on organic carbon content. Organic carbon content (C_ox) and humic substance fractions (C-humic acids and fulvic acid fractions), hot water extractable carbon and selected microbial characteristics in Eutric Cambisol were monitored during the period 1999–2010. A priming effect of soil cultivation was detected immediately after tillage. Arable soil with ‘intensive’ crop sequences (exclusively cash crops, cereals, oil plants) and with an optimal level of chemical inputs (mineral fertilizers, pesticides), but without organic farmyard manure had lower content of all carbon forms compared with grassland. ¹³C NMR spectroscopy and thermal analysis (TGA) were applied to characterize humic acid (HA) structure and stability. More carbon, less oxygen and more aromatic compounds were detected in grassland HA. Slight differences were found in HA thermo-oxidative stability and degradability, which was probably caused by changes in elemental composition and structure. Even the land use had no significant effect on basic microbiological characteristics (basal respiration, microbial biomass and qCO₂); the physiology of the microbial community of grassland was altered by a higher ability to utilize L- and D-glutamic acid. The L/D ratio of glutamic acid mineralization indicated no occurrence of stress in soil for both types of farming. It has been demonstrated that although losses of carbon as a result of land-use conversions are generally more rapid, gains of carbon in grassland followed by changes in management practices can also occur.     

Alterations in total microbial activity and nitrification rates in soil due to amoxicillin‐spiked pig manure

Most veterinary drugs enter the environment via manure application. However, ecotoxic effects of antibiotics are varying as a function of their physicochemical characteristics and for most antibiotics it is still unclear how these substances interact with soil biota. It was the aim of the present study to investigate effects of manure containing different concentrations of the antibiotic amoxicillin (AMX) on microbial‐community function in two different soils over an incubation time of 18 d. Therefore, soil respiration, potential nitrification, and the products of N turnover were measured. We could show that CaCl₂‐extractable amounts of AMX in soil are low, even shortly after the application of high doses together with manure. Thus, not surprisingly basal respiration in soil was not influenced by the addition of the antibiotic with manure. In contrast, mainly shortly after manure addition the kinetic of substrate‐induced respiration was clearly shifted by the treatments depending on the presence of AMX in the manure. Potential nitrification rates in the two different soils were not significantly affected when data were related to the overall incubation time by the application of AMX to the manure. However, shortly after the addition of the manure containing AMX, a tendency to lower turnover rates was visible compared to the application of pure manure.     

Effects of grass vegetation strips on soil conservation and crop yield under rainfed conditions in the Indian sub‐Himalayas

Vegetation strips (VS) along with conservation tillage, application of organic amendments and weed mulching improve crop yields by reducing run‐off and topsoil erosion. To investigate these issues, an experiment was conducted under rainfed conditions using grass VS for four and a half years (June 2007 to October 2011) at Dehradun, Uttarakhand, in the Indian Himalayan region. VS were incorporated in the experimental plots (Entisols) in a randomized complete block design in permanent 100 × 20 m (2000 m²) plots with a 2% slope to evaluate the effects of vegetation strips on run‐off, soil loss and crop yield. Three treatments were evaluated: (i) without VS with recommended NPK under conventional tillage, (ii) panicum as VS with recommended NPK under conventional tillage and (iii) palmarosa+ that consists of palmarosa as VS along with organic amendments (farmyard manure, vermicompost and poultry manure) and weed mulch under minimum tillage. The results show that soil loss and run‐off were significantly lower (P < 0.05) in plots under palmarosa+ than without VS treatment. Mean soil loss of 3.4, 5.2 and 7.1 t/ha was recorded from palmarosa+, panicum and without VS treated plots, respectively. Mean run‐off was 234, 356 and 428 mm from plots under palmarosa+, panicum and without VS, respectively. Maize yield was lower on the plots under palmarosa+ compared with panicum, but significantly higher (P < 0.05) than without a strip. The succeeding rainfed wheat yield was significantly greater in plots under palmarosa+ than in the initial years. The wheat yield equivalent was significantly higher in plots under palmarosa+ followed by panicum and without VS. Thus, vegetation strips are recommended for wider adoption to reduce run‐off and soil loss and to increase crop yield. The long‐term goal is to achieve a palmarosa+ system (palmarosa as a vegetation strip along with organic amendments, farmyard manure, vermicompost and poultry manure) and weed mulch under minimum tillage.     

The impact of land‐cover change on soil properties in northern Ghana

Effects of changes in land‐cover on soil quality parameters in an area in northern Ghana were studied. Land‐cover changes were derived from maps of the study area for 1984, 1992 and 1999. There were no significant differences between properties of soils under natural vegetation and soils put under cultivation from 1992, but permanently cultivated soils (1984–1999) showed significantly lower physical and chemical soil properties. Soils recently opened up since 1992 for cultivation in the last seven years (i.e. 1992–1999) were found to manifest significantly higher contents of organic C, N, Ca, Mg and ECEC than those under permanent cultivation, suggesting that continuous cropping is responsible for deterioration in soil quality. Minimum organic C contents necessary to meet critical levels of selected soil quality parameters were estimated. The organic C content of recently cultivated soils would need to be increased by about 7 t ha⁻ to replenish soil nutrient capital. This calls for a strategy to synchronize organic matter management with inorganic fertilizer application. Further research is also needed to develop farming systems that conserve organic matter and also improve the quality of organic matter in the study area. Copyright © 2004 John Wiley & Sons, Ltd.