Phosphorus content in soil, uptake by plants and balance in three European long-term field experiments

The fate of phosphorus (P) derived from mineral fertilisers and organic manures, and the effective P balance, have been assessed in three long-term field experiments at Rothamsted (UK), Bad Lauchstaedt (Germany) and Skierniewice (Poland). This paper discusses the plant availability, uptake and overall utilisation of P over the last 30 years, based on soil test P availability indices and crop analyses determined by the standard methods used in each of the three countries. The data suggest that differences in soil type significantly influence the dynamics of P at the three locations, but most significantly between a loess Chernozem at Bad Lauchstaedt with a high organic matter content and the soils at the other two locations which have a low organic matter content. The application of P either as inorganic fertiliser or organic manure had a considerable influence on the availablity, uptake, leaching or fixing of P, but the crop recovery rate of P from mineral fertiliser did not exceed 35% with the smallest recovery (average 18%) occurring in the soil with the highest clay content at Rothamsted. At Bad Lauchstaedt and Rothamsted the most efficient utilisation of P (averages of 47% and 37%, respectively) was from soils treated with farmyard manure (FYM), with the greater quantity of P either leached or fixed (8 and 25 kg ha^-1 y^-1, respectively) occurring in soils treated with superphosphate. At Skierniewice, however, the reverse was true. Overall, the most efficient crop utilisation from mineral P (30% average) was from the loamy sand at Skierniewice. P balances for the three locations show that quantitatively, for the same P input, the amount of P either leached from or fixed in the plough layer of Broadbalk field, Rothamsted, was 2–3 times greater than at Skierniewice and 3–6 times greater than at Bad Lauchstaedt. The results suggest that differences in the soil physico-chemical properties, climate, the availability of other major nutrients, and the form in which P is applied, all influence the effectiveness of P fertilisation and P balance. The investigation highlights the importance of maintaining long-term field experiments and archived soil and crop samples on a world-wide basis for understanding nutrient cycling and fertility dynamics.     

Interactions of aerobically decomposed cattle manure and nitrogen fertilizer applied to soil

Inorganic N fertiliser may be applied to soil in addition to cattle manure by smallholder farmers in developing countries: (a) to complement fertilization; (b) to control a possible immobilisation of N by the manure; and (c) to eliminate the risk of yield depression due to lack of plant available N. The aim of this study was to find out if and how much N was immobilised by cattle manure, if and when remineralisation of N will take place and, if added N has an effect on decomposition of cattle manure in soil. A laboratory study was conducted applying inorganic N fertiliser to soil (NH4NO3 equivalent to 30, 60 and 120 kg N ha-1) together with four cattle manures with different C/N ratios (9–18). CO2–C mineralisation and changes of inorganic N in soil were determined over 60 d. Immobilisation of fertiliser N occurred with manure having the lowest C/N ratio but not with the manures having a higher C/N ratios. Maximum immobilization of fertiliser N (23–36%) occurred within 21 d and thereafter N was mineralised. Carbon dioxide evolution decreased in cattle manure-amended soil at increasing rates of N fertiliser, but decomposition was still higher than from the unamended control. None of the manure treated soils had significantly different contents of inorganic N after 2 months of incubation. It was not possible to use the C/N ratio of aerobically decomposed cattle manure as a tool to predict mineralization or immobilization of N. It was concluded that aerobically decomposed solid cattle manures do not contribute to the N supply of crops in the short term but can immobilize fertiliser N applied at the same time.     

Development and evaluation of an improved soil test for phosphorus, 3: field comparison of Olsen, Colwell and Resin soil P tests for New Zealand pasture soils

Soil tests suitable for estimating the phosphorus (P) status of soils fertilised with soluble or sparingly soluble P fertilisers (reactive phosphate rock) were evaluated using the New Zealand Ministry of Agriculture Technology (NZMAFTech) National Series forms of phosphate trials on permanent pastures located throughout NZ. This included a common core of treatments comparing Sechura phosphate rock (SPR) with triple superphosphate (TSP). At each site, a re-application of twice maintenance TSP was superimposed on one-half plots that previously had received six annual applications of increasing amounts of P (0, 0.5, 0.75, 1.0 and 2.0 times the maintenance rate) in the form of TSP or SPR. Before the re-application of TSP, soil samples (0–30 and 0–75 mm depths) were collected from each plot. All the trials were run for 1 year during which seven to ten harvests were taken. Pasture response was expressed as percent increase in yield obtained with re-application over the previous treatment.The 0.5 NaHCO₃ based (Olsen P) extractant with different combinations i.e. soil volume (Olsen (v)), soil weight (Olsen (w)), shaking time variations (Olsen (16 h)) and soil:solution ratio (Colwell), and Resin P soil tests were conducted on soils taken from the plots prior to re-application of TSP. The Olsen (v), Olsen (16 h) and Colwell P values increased with increasing rates of P applied in all soils with values for sparingly soluble P materials being less than where soluble P fertiliser had been previously applied. The Resin P values showed similar increases with P applied regardless of the solubility of previously applied fertiliser. When the yield increases caused by TSP application to all treatments (irrespective of fertiliser source) were regressed against soil test values, Resin P explained 76% of the variation in yield response, compared to 50% by Olsen (v), 42% by Olsen (w), 39% by Olsen (16 h) and 40% by Colwell P. Partitioning the data according to fertiliser source slightly improved the coefficient of determination for Resin P for both the soluble (R²=0.81) and sparingly soluble (R²= 0.80) P fertilisers. With 0.5 M NaHCO₃ (Olsen) extractants, R² values consistently indicated a poorer prediction for the SPR treatments. A Resin P model was able to account for more variance in yield response to re-applied TSP, than an Olsen P model because the Olsen model underestimated the yield response to re-applied TSP on the PR treatments. The Resin test is more suitable than the current Olsen test for assessing the plant available P status of soils previously fertilised with fertilisers of varying solubility.Dr. A.G. Sinclair died on 3 December 1996 whilst this paper was in preparation.     

Crop residue management to sustain soil fertility and irrigated rice yields

Field experiments were conducted on a sandy clay loam soil (deep aquic ustorthent) for five consecutive seasons from wet season (WS) 1998 to WS 2000 with a permanent layout at the Directorate of Rice Research (DRR) farm, ICRISAT campus, India, to study the influence of incorporation of rice straw residues alone or in combination with in situ grown green manure (GM) and straw burning on soil fertility, irrigated rice productivity and pest incidence in comparison with only fertiliser application (control). The residue treatments received uniform doses of N, K, Zn at the same level as that in control plots. The crop residue treatments favourably influenced some of the soil parameters over control. Recycling of crop residues by incorporation or burning increased soil available K and organic carbon significantly over control, while total N content increased by residue incorporation. Bulk density decreased with residue incorporation as compared to control and burning treatments. Yellow stem borer was the only pest observed, with higher white ear damage recorded during wet seasons ranging from 14.2–31.3% in 1999 and 16.8–29.7% in 2000. The damage was higher with straw + green manure, apparently reflecting the quantum of N applied through crop residues and fertilisers. The influence of crop residue treatments on yield parameters like panicle and spikelet number was more apparent after two cycles of residue incorporation, recording significant effects on rice productivity in the dry and wet seasons of 2000. Rice yield increased by 1.0 to 1.2 t/ha in DS and 0.4 to 0.8 t/ha in WS.     

Crop production,soil carbon and nutrient balances as affected by fertilisation in a Mollisol agroecosystem

A 19-year field experiment on a Mollisol agroecosystem was carried out to study the productivity of a wheat-maize-soybean rotation and the changes in soil carbon and nutrient status in response to different fertiliser applications in Northeast China. The experiment consisted of seven fertiliser treatments: (1) unfertilised control, (2) annual application of P and K fertilisers, (3) N and K fertilisers, (4) N and P fertilisers, (5) N, P and K fertilisers, (6) N, K and second level P fertilisers, and (7) N, P and second level K fertilisers. Without fertiliser, the Mollisols could support an average yield of 1.88 t ha⁻¹ for wheat, 3.89 t ha⁻¹ for maize and 2.12 t ha⁻¹ for soybean, compared to yields of 3.20, 9.30 and 2.45 t ha⁻¹ respectively for wheat, maize and soybean if the crop nutrient demands were met. At the potential yield level, the N, P and K removal by wheat are 79 kg N ha⁻¹, 15 kg P ha⁻¹, and 53 kg K ha⁻¹, by maize are 207 kg N ha⁻¹, 47 kg P ha⁻¹, and 180 kg K ha⁻¹, by soybean are 174 kg N ha⁻¹, 18 kg P ha⁻¹, and 55 kg K ha⁻¹. Crop yield, change in soil organic carbon (SOC), and the total and available nutrient status were used to evaluate the fertility of this soil over different time periods. This study showed that a fertiliser strategy that was able to maintain yields in the short term (19 years) would not maintain the long term fertility of these soils. Although organic carbon levels did not rise to the level of virgin soil in any treatment, a combination of N, P and K fertiliser that approximated crop export was required to stabilise SOC and prevent a decline in the total store of soil nutrients.     

Effect of phosphate fertiliser type on the accumulation and plant availability of cadmium in grassland soils

Cadmium (Cd), a potentially toxic heavy metal for humans and animals, accumulates in the liver and kidneys of older animals grazing New Zealand and Australian pastoral soils. Phosphorus (P) fertiliser is the major input of Cd into these farming systems. A study was conducted to evaluate the effects, over 10 years, of annual application (30 kg P ha^–1 yr^–1) of four forms of P fertilisers having different solubilities and Cd contents [41, 32, 10 and 5 g Cd g^–1 for North Carolina phosphate rock (NCPR), single superphosphate (SSP), diammonium phosphate (DAP) made from low Cd phosphate rocks and Jordan phosphate rock (JPR) respectively] on soil and herbage Cd concentrations. Ten years of fertiliser application caused a marked increase in surface soil Cd concentrations. Total soil Cd was significantly higher in SSP and NCPR treatments compared to control (no P fertiliser), JPR and DAP treatments in the 0–30 and 30–75 mm soil depths. Plant-available Cd (0.01 M CaCl₂ extractable Cd) was higher in SSP treatments than in control and other fertiliser treatments. Chemical analysis of herbage samples showed that there was no significant difference in Cd concentration in pasture grasses between treatments in the second year of the trial but in the eighth and tenth year, plots fertilised with SSP and NCPR had significantly higher Cd in pasture grasses in most of the seasonal cuts compared to control, JPR and DAP. Cadmium recovery by both grasses and clover was less than 5% of Cd applied in fertiliser. Clover Cd concentration and yield were much lower than those for grass and therefore its contribution to pasture Cd uptake was very low (< 7%). A strong seasonal effect on grass Cd concentration, which is inversely related to pasture growth rate, was observed in all three sampling years — Cd concentration was highest during autumn and lowest in spring. Total Cd contents of the fertilisers and their rate of dissolution rather than soil pH [pH (H₂O) at 30–75 mm depth of 5.39, 5.20, 5.11 and 5.36 for NCPR, SSP, DAP and JPR treatments respectively]influenced soil and herbage Cd. These results showed that the use of P fertilisers with low Cd contents will reduce herbage Cd levels and has the potential of reducing Cd levels in grazing animals and their products.     

Stratification of soil phosphorus,pH, and macro-cations under intensively cropped grass ley

The large amounts of nutrients applied to and removed from soil by intensive grass production may cause quick changes in the nutrient pools available to plants and exposed to leaching and runoff losses. Stratification of applied nutrients is especially important for phosphorus (P), which moves slowly in soil. To study the vertical distribution of extractable nutrients and soil pH in different types of ley soil, P fertilisers were incorporated or placed prior to sowing or broadcast annually at ten sites for 3 years. Then the soils were sampled in several layers 2.5 or 5 cm deep and analysed for pH and the concentrations of phosphorus, potassium (K), calcium (Ca), and magnesium (Mg) extractable with acid ammonium acetate. In mineral soils, broadcast P mainly remained within the uppermost 2.5 cm of soil, in which the concentrations of extractable P more than doubled during the study period. When commonly used NK and NPK fertilisers were applied, the uppermost 5 cm of soil was acidified by about 0.5 pH units and its Ca value decreased by about 25%. Broadcast K enriched a thin surface layer even if the K balance was negative. Estimation of the concentration of dissolved phosphate in runoff suggested that the high P losses that are possible at excessive levels of soil test P can be diminished by perennial grasses supplied with abundant water and other nutrients. Surface-applied P appeared to increase the losses, but even a shallow placement seemed to prevent them efficiently if all fertiliser granules become covered with soil.     

Simulating phosphorus responses in annual crops using APSIM: model evaluation on contrasting soil types

Crop simulation models have been used successfully to evaluate many systems and the impact of change on these systems, e.g. for climatic risk and the use of alternative management options, including the use of nitrogen fertilisers. However, for low input systems in tropical and subtropical regions where organic inputs rather than fertilisers are the predominant nutrient management option and other nutrients besides nitrogen (particular phosphorus) constrain crop growth, these models are not up to the task. This paper describes progress towards developing a capability to simulate response to phosphorus (P) within the APSIM (Agricultural Production Systems Simulator) framework. It reports the development of the P routines based on maize crops grown in semi-arid eastern Kenya, and validation in contrasting soils in western Kenya and South-western Colombia to demonstrate the robustness of the routines. The creation of this capability required: (1) a new module (APSIM SoilP) that simulates the dynamics of P in soil and is able to account for effectiveness of alternative fertiliser management (i.e. water-soluble versus rock phosphate sources, placement effects); (2) a link to the modules simulating the dynamics of carbon and nitrogen in soil organic matter, crop residues, etc., in order that the P present in such materials can be accounted for; and (3) modification to crop modules to represent the P uptake process, estimation of the P stress in the crop, and consequent restrictions to the plant growth processes of photosynthesis, leaf expansion, phenology and grain filling. Modelling results show that the P routines in APSIM can be specified to produce output that matches multi-season rotations of different crops, on a contrasting soil type to previous evaluations, with very few changes to the parameterization files. Model performance in predicting the growth of maize and bean crops grown in rotation on an Andisol with different sources and rates of P was good (75–87% of variance could be explained). This is the first published example of extending APSIM P routines to another crop (beans) from maize. Dr R. J. Delve has recently left CIAT and joined Catholic Relief Services, Kenya.     

Controlling soil water content in fertiliser dissolution experiments

Phosphorus lost in runoff from agricultural land leads to the enrichment of surface waters and contributes to algal blooms. Fertilisers are one source of this P. To compare the water available P of different fertiliser formulations in the laboratory it is necessary to control environmental conditions, temperature, relative humidity and soil water content, prior to simulating rainfall. Two chambers were designed in which relative humidity and soil water content were controlled using salt solutions. An initial design comprising a sealed chamber with three layers of soil samples over a salt bath was found to be inferior to a single layer design. The changes in water content of soil samples were used to test the single layer chamber in a constant temperature environment (15 °C) using a saturated KCl solution (90% relative humidity). Based on the final soil water content of the samples, the spatial variation within the chamber was within tolerable limits. The single layer chamber was used for a simulation experiment comparing the water available P of two commercial fertilisers. Using a saturated resorcinol solution (95% relative humidity) soil samples were equilibrated at 15 °C for 21 days, fertiliser added, and the water available P measured up to 600 h after fertiliser application. The results indicate that the amount of water available P was related to the fertiliser compound and exponentially related to the time since fertiliser application. It was concluded that the single layer chamber is suitable for controlling relative humidity and soil water content in trials such as these where the water available P of fertilisers are being compared.     

Influence of different manuring systems with and without biogas digestion on soil organic matter and nitrogen inputs,flows and budgets in organic cropping systems

Nitrogen (N) and carbon (C) cycles are closely linked in organic farming systems. Use of residues for biogas digestion may reduce N-losses and lead to higher farmland productivity. However, digestion is connected to large losses of organic C. It is the purpose of this paper (1) to compare farming systems based on liquid slurry and solid farmyard manure regarding the N, C and organic dry matter (ODM) inputs and flows, (2) to analyse the effect of digestion on soil N, C and ODM inputs and flows within the cropping system, (3) to assess the effects of organic manure management on biological N₂ fixation (BNF), and (4) to assess the effect of biogas digestion on the sustainability of the cropping systems in terms of N and C budgets. The BNF by clover/grass-leys was the most important single N input, followed by the BNF supplied by legume cover cropping. Growth of crops in organic farming systems is very often N limited, and not limited by the soil C inputs. However, balances of N inputs showed that the implemented organic farming systems have the potential to supply high amounts of N to meet crop N demand. The level of plant available N to non-legume main crops was much lower, in comparison to the total N inputs. Reasons were the non-synchronized timing of N mineralization and crop N demand, the high unproductive gaseous N losses and an unfocussed allocation in space and time of the circulating N within the crop rotation (e.g. allocation of immobile manures to legumes or of mobile manures to cover crops). Simultaneously, organic cropping systems very often showed large C surpluses, which may be potentially increased the N shortage due to the immobilization of N. Soil organic matter supply and soil humus balance (a balance sheet calculated from factors describing the cultivation effects on humus increasing and humus depleting crops, and organic manure application) were higher in cropping systems based on liquid slurry than in those based on solid farmyard manure (+19%). Simultaneously, soil N surplus was higher due to lower gaseous N losses (+14%). Biogas digestion of slurry had only a very slight effect on both the soil N and the soil C budget. The effect on the N budget was also slight if the liquid slurry was stored in closed repositories. Digestion of residues like slurry, crop residues and cover crops reduced in a mixed farming system the soil C supply unilaterally (approximately −33%), and increased the amounts of readily available N (approximately +70–75%). The long-term challenge for organic farming systems is to find instruments that reduce N losses to a minimum, to keep the most limiting fraction of N (ammonia-N) within the system, and to enhance the direct manuring effect of the available manures to non-legume main crops.     

Effect of incorporation of crop residues and organic manures on adsorption/desorption and bio-availability of phosphate

Transformations of applied phosphorus (P) to unavailable residual soil P is the major cause of limited P supply in most of the P-deficient soils. The effect of the incorporation of crop residues (rice straw [RS] and wheat straw [WS]) and organic manures (farmyard manure [FYM] and green manure [GM]) on P release in soil and its bio-availability to various summer and winter crops was investigated in laboratory and screen house experiments. Surface (0–0.15 m) soil samples collected after 32 years of differential fertilization to maize–wheat–cowpea fodder crop rotation, were examined for adsorption/desorption behavior of P, after incubating with organics of varying C:P ratios. Incorporation of crop residues increased P adsorption maxima as well as resistance to P release in soils. Increased buffering capacities in crop residue-incorporated treatments decreased P desorption in soil, whereas the incorporation of organic manures decreased P sorption, maximum buffering capacity (MBC), bonding energy, and increased P concentration in soil solution. Although the incorporation of crop residues decreased P release in soil its bio-availability in the soil–plant system was crop-specific and varied with the time of incorporation of organics. Raya showed increased P uptake with incorporation of both RS and WS, whereas in the case of berseem increased P uptake occurred only with wheat straw. Phosphorus uptake in rice, maize, and soybean decreased with the incorporation of both RS and WS. Incorporation of crop residues 1 day before the sowing of summer crops decreased P uptake, whereas incorporation 3 weeks prior to the sowing of winter crops improved P bio-availability. Incorporation of organic manures with a narrow C:P ratio, however, improved P uptake in all the crops under investigation, in both the seasons. The results thus emphasized that adsorption parameters calculated from the examination of soil samples should not be used independently for making fertilizer P recommendations. Crop effects (root exudates) and their interaction with P reaction products in soil and synchronization in P release from organics and crop uptake need to be considered to understand the virtual behavior of P bio-availability in the soil–plant system.     

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.     

Soil organic carbon management for sustainable land use in Sudano-Sahelian West Africa

Judged by their negative nutrient balances, low soil cover and low productivity, the predominant agro-pastoral farming systems in the Sudano-Sahelian zone of West Africa are highly unsustainable for crop production intensification. With kaolinite as the main clay type, the cation exchange capacity of the soils in this region, often less than 1 cmol_c kg⁻¹soil, depends heavily on the organic carbon (Corg) content. However, due to low carbon sequestration and to the microbe, termite and temperature-induced rapid turnover rates of organic material in the present land-use systems, Corg contents of the topsoil are very low, ranging between 1 and 8 g kg⁻¹ in most soils. For sustainable food production, the availability of phosphorus (P) and nitrogen (N) has to be increased considerably in combination with an improvement in soil physical properties. Therefore, the adoption of innovative management options that help to stop or even reverse the decline in Corg typically observed after cultivating bush or rangeland is of utmost importance. To maintain food production for a rapidly growing population, targeted applications of mineral fertilisers and the effective recycling of organic amendments as crop residues and manure are essential. Any increase in soil cover has large effects in reducing topsoil erosion by wind and water and favours the accumulation of wind-blown dust high in bases which in turn improves P availability. In the future decision support systems, based on GIS, modelling and simulation should be used to combine (i) available fertiliser response data from on-station and on-farm research, (ii) results on soil productivity restoration with the application of mineral and organic amendments and (iii) our present understanding of the cause-effect relationships governing the prevailing soil degradation processes. This will help to predict the effectiveness of regionally differentiated soil fertility management approaches to maintain or even increase soil Corg levels. This revised version was published online in June 2006 with corrections to the Cover Date.     

Managing soil fertility diversity to enhance resource use efficiencies in smallholder farming systems: a case from Murewa District,Zimbabwe

Smallholder farms in sub-Saharan African exhibit substantial heterogeneity in soil fertility, and nutrient resource allocation strategies that address this variability are required to increase nutrient use efficiencies. We applied the Field-scale resource Interactions, use Efficiencies and Long-term soil fertility Development (FIELD) model to explore consequences of various manure and fertilizer application strategies on crop productivity and soil organic carbon (SOC) dynamics on farms varying in resource endowment in a case study village in Murewa District, Zimbabwe. FIELD simulated a rapid decline in SOC and maize yields when native woodlands were cleared for maize cultivation without fertilizer inputs coupled with removal of crop residues. Applications of 10 t manure ha⁻¹ year⁻¹ for 10 years were required to restore maize productivity to the yields attainable under native woodland. Long-term application of manure at 5 and 3 t ha⁻¹ resulted in SOC contents comparable to zones of high and medium soil fertility observed on farms of wealthy cattle owners. Targeting manure application to restore SOC to 50–60% of contents under native woodlands was sufficient to increase productivity to 90% of attainable yields. Short-term increases in crop productivity achieved by reallocating manure to less fertile fields were short-lived on sandy soils. Preventing degradation of the soils under intensive cultivation is difficult, particularly in low input farming systems, and attention should be paid to judicious use of the limited nutrient resources to maintain a degree of soil fertility that supports good crop response to fertilizer application.     

Growth dynamics of reed canarygrass ( Phalaris arundinacea L.) and its allocation of biomass and nitrogen below ground in a field receiving daily irrigation and fertilisation

Biomass and nitrogen in the roots, rhizomes, stem bases and litter of reed canarygrass (Phalaris arundinacea L.) were repeatedly estimated by soil coring, and root growth dynamics of this potential energy crop was studied for two years using minirhizotrons. Results are discussed in relation to above-ground biomass and nitrogen fertilisation. Five treatments were used: C₀, unfertilised control; C1, fertilised with solid N fertiliser in spring; I1, irrigated daily, fertilised as in C1; IF1 , irrigated as I1 and fertilised daily through a drip-tube system; IF2, as in IF1 but with higher N fertiliser rates. Biomass of below-ground plant parts of reed canarygrass increased between the first and second years. Up to 50% of total plant biomass and nitrogen were recovered below-ground. The highest proportions were found in C₀. The calculated annual input via root turnover ranged between 80 and 235 g m-2. In absolute terms, up to 1 kg and 10 g m-2 of biomass and nitrogen, respectively, were found in below-ground plant fractions. High inputs of stubble and accumulated below-ground biomass will occur when the ley is ploughed, which will result in a highly positive soil carbon balance for this crop in comparison with that of conventional crops such as cereals.     

Farmer's view on soil organic matter depletion and its management in Bangladesh

Bangladesh is an agricultural country. About 80% of the total population live in rural areas. The contribution of agriculture to the gross domestic product is 30%. Rice is the major food crop while jute, sugarcane and tea are the main cash crops. Other important crops are wheat, tobacco, pulses, vegetables and fruits. Overall productivity in Bangladesh is stagnating or declining. The implication of yield stagnation or declining productivity is severe, since these trends have occurred despite rapid growth in the use of chemical fertilisers. Depletion of soil organic matter is the main cause of low productivity, which is considered one of the most serious threats to the sustainability of agriculture in Bangladesh. In Bangladesh, most soils have less than 17 g/kg and some soils have less than 10 g/kg organic matter. Farmers realise that there is a problem with soil fertility related to organic matter depletion. Farmers say that organic matter increases yield, reduces the production cost, improves crop growth and the economy, increases water-holding capacity and improves the soil structure. They recognise soil with higher organic matter content by darker brownish to black in colour. Some farmers are using fast-growing trees such as Flemingia macrophyla, Ipilipil (Leucaen leucophala), Glyricidia sepium, Boga Medula (Tephrosia candida), Dhol Kolmi (Ipomoea fistulosa), etc., as living fences, which can be used as fuel, fertiliser and fodder. To increase the soil organic matter, farmers use green manure crops, compost, quick compost, cow dung, azolla, etc. However, fuel for cooking purposes is limited and cow dung and crop residues are largely used as fuel. Crop residues are also used as fodder for livestock. Farmers expressed the wish to learn more about organic fertilizer management. However, sufficient food should be produced to keep pace with population growth. To alleviate the hunger and poverty is to increase the intensity of agricultural production and maintain favorable ecological conditions. Therefore, more organic matter should be used in the farmers' fields to sustain the soil fertility in an intensive farming system. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phosphorus Mass Balances for Successive Crops of Fertilised Rainfed Rice on a Sandy Lowland Soil

Raising and sustaining rice yields in the rainfed lowlands requires an understanding of nutrient inputs and outputs. On sandy lowland rice soils, managing phosphorus (P) supply is a key factor in achieving increased yields and sustainable production. Phosphorus inputs, rice yields, and crop P uptake were used to quantify P requirements of rice: together with results on soil P fractions, P balance sheets were constructed over five consecutive cropping seasons on a sandy Plinthustalf near Phnom Penh, Cambodia. Grain yields ranged from 665 to 1557 kg ha⁻¹ with no added P. Average yields increased significantly with P fertiliser application over five consecutive crops by 117, 139 and 140% when the phosphate fertiliser was applied at 8.25, 16.5 and 33 kg P ha⁻¹, respectively. Without added P fertiliser, a net loss of 1.2 kg P ha⁻¹ per crop was estimated with straw return and 2.0 kg P ha⁻¹ per crop with straw removed from the field, whereas, with added P fertiliser, there was a net P gain in the soil of 5.6 or 9.5 kg ha⁻¹ per crop when straw was removed and returned to the soil, respectively. After one crop, the addition of P fertiliser significantly (P < 0.01) increased recovery in all soil P fractions. Across five successive crops, repeated application of 16.5 and 33 kg P ha⁻¹ rates resulted in progressive P accumulation in the soil, especially a labile NaOH–Po pool, but had no effect on yields and P uptake of rice. By contrast, 8.25 kg P ha⁻¹ per rice crop was generally adequate for grain yields of 2.5–3.0 t ha⁻¹ and to maintain soil P pools.     

Balance sheet of phosphorus,sulphur and potassium in a long-term grazed pasture supplied with superphosphate

A balance sheet of P, S and K was constructed for a long-term trial which investigates the effects of three rates of superphosphate (9% P, 11% S) on pasture production on border-strip irrigated land grazed with sheep. A balance sheet of the inputs and outputs of P, S and K to the trial over a 38 year period showed that of the nutrients applied in fertiliser, only 51–59% of the P and 15–31% of the S were retained in the soil. Small amounts were lost in animal products (4–19% of the applied nutrients) but major losses were attributed to runoff of P as particulate matter (dung and soil particles) during irrigation and leaching of sulphate-S during irrigation. Losses of K from the site were small and had no effect on total soil K content. The distribution of soil nutrients across the border-strips was also investigated. The results showed that the concentrations of total soil P and S and exchangeable K were significantly greater at the sides of the irrigation borders than in the main strip area of pasture. This was caused by deposition of a disproportionate amount of dung and urine (and therefore nutrients) on the levees where the sheep tended to camp. It was calculated that with increasing superphosphate rates greater amounts of P were transferred to the levees due to the increased amounts of P being recycled via the animals (as a result of increased herbage P concentration, pasture production and stocking rate).     

Nitrogen use efficiency by maize as affected by a mucuna short fallow and P application in the coastal savanna of West Africa

Maize is the primary food crop grown by farmers in the coastal savanna region of Togo and Benin on degraded (rhodic ferralsols), low in soil K-supplying capacity, and non-degraded (plinthic acrisols) soils. Agronomic trials were conducted during 1999–2002 in southern Togo on both soil types to investigate the impact of N and P fertilization and the introduction of a mucuna short fallow (MSF) on yield, indigenous N supply of the soil, N recovery fraction and internal efficiency of maize. In all plots, an annual basal dose of 100 kg K ha^–1 was applied to the maize crop. Maize and mucuna crop residues were incorporated into the soil during land preparation. Treatment yields were primarily below 80% of CERES-MAIZE simulated weather-defined maize yield potentials, indicating that nutrients were more limiting than weather conditions. On degraded soil (DS), maize yields increased from 0.4 t ha^–1 to 2.8 t ha^–1 from 1999 to 2001, without N or P application, in the absence of MSF, with annual K application and incorporation of maize crop residues. Application of N and P mineral fertilizer resulted in yield gains of 1–1.5 t ha^–1. With MSF, additional yield gains of between 0.5 and 1.0 t ha^–1 were obtained at low N application rates. N supply of the soil increased from 10 to 42 kg ha^–1 from 1999 to 2001 and to 58 kg N ha^–1 with MSF. Application of P resulted in significant improvements in N recovery fraction, and greatest gains were obtained with MSF and P application. MSF did not significantly affect internal N efficiency, which averaged 45 kg grain (kg N uptake)^–1. On non-degraded soils (NDS) and without N or P application, in the absence of MSF, maize yields were about 3 t ha^–1 from 1999 to 2001, with N supply of the soil ranging from 55 to 110 kg N ha^–1. Application of 40 kg P ha^–1 alone resulted in significant maize yield gains of between 1.0 (1999) and 1.5 (2001) t ha^–1. Inclusion of MSF did not significantly improve maize yields and even reduced N recovery fraction as determined in the third cropping year (2001). Results illustrate the importance of site-specific integrated soil fertility management recommendations for the southern regions of Togo and Benin that consider indigenous soil nutrient-supplying capacity and yield potential. On DS, the main nutrients limiting maize growth were N and probably K. On NDS, nutrients limiting growth were mainly N and P. Even on DS rapid gains in productivity can be obtained, with MSF serving as a means to allow farmers with limited financial means to restore the fertility of such soils. MSF cannot be recommended on relatively fertile NDS.     

Effects of different manuring systems with and without biogas digestion on nitrogen cycle and crop yield in mixed organic dairy farming systems

Field trials were carried out between 2002 and 2005 to investigate the effects of biogas digestion in a mixed organic dairy farming system with arable land and grassland on nutrient cycling, nitrogen (N) uptake and crop yields within a cropping system comprising a whole crop rotation. Five treatments were carried out: (i) solid farmyard manure, (ii) undigested liquid slurry, (iii) digested liquid slurry, (iv) digestion of liquid slurry and field residues such as crop residues and cover crops, and (v) similar to iv, but with additional N inputs at the equivalent of 40 kg N ha⁻¹ farmland through digestion of purchased substrates. The term “manure” is used in the present study to mean all kind of aboveground organic residues left on the field (“immobile manures”, such as crop residues and green manures incorporated directly into the soil) or added as stable wastes or effluents of biogas digestion (“mobile manures”). The total aboveground biomass growth and the overall aboveground N uptake of non-legume maincrops were higher in the liquid slurry manure treatment than in the solid farmyard manure system (+5% and +9%, respectively). The digestion of the liquid slurry increased N uptake and crop yields only after soil incorporation of the slurry shortly after field spreading. The additional collection and digestion of field residues such as cover crops and crop residues, combined with a reallocation of the effluents, strongly increased the amounts of “mobile” manure, allowing a more focussed allocation of the available N. This led to an increase in the aboveground N uptake (+12%) and biomass yield (+4%) of the five non-legume crops, due to a better adapted allocation of nutrients in space and time. Results obtained with spring wheat showed that removal of cover crops in autumn, and their digestion, combined with subsequent use as manure in spring resulted in a better synchronisation of the crop N demand and the soil N availability, in comparison with a strategy where the biomass was left on the field as green (immobile) manure. The inclusion of external substrates led to a further increase of 8% in N uptake, but not to a significant increase in aboveground dry matter yields.