Potential impact of agroforestry on soil nutrient balances at the farm scale in the East African Highlands

There is much current interest in the potential role of agroforestry in the mitigation of nutrient depletion in Sub-Saharan Africa. Using data from farm surveys and trials, a static model of N and P flows was constructed for a standard farm system, representative of typical subsistence farms in humid parts of the East African Highlands. The model was used to explore the possible impact of improved agroforestry systems on nutrient budgets, to identify priorities for research.Soil nutrient balances in the standard farm system were - 107 kg N and - 8 kg P ha^–1 yr^–1. Agroforestry systems did not significantly reduce the N deficits except when a high proportion of the total biomass was returned to the soil, rather than removed from the farm. Agroforestry increased N input through biological N fixation and deep N uptake, but this was offset by a larger nutrient removal from the farm in harvested products, which increased from 38 kg N in the standard system to 169 kg N ha^–1 yr^–1 in an intensive dairy-agroforestry system. Agroforestry did not increase P inputs, and harvested P increased from 6 kg P in the standard farm system to 29 kg P ha^–1 yr^–1 in the dairy-agroforestry system. Thus, moderate P inputs, of 20 kg P ha^–1 yr^–1 were required to maintain soil P stocks.N leaching from the field was the most significant nutrient loss from the farm system, with a range of 68 to 139 kg N ha^–1 yr^–1. The capture of subsoil N by deep-rooted trees in agroforestry systems substantially increased N-use efficiency, providing 60 kg N ha^–1 yr^–1 in the dairy-agroforestry system. The budgets were sensitive to N mineralization rates in subsoils, N losses from soils and manures, and effectiveness of deep-rooted plants in subsoil N capture, for which there is little data from the region. Therefore, high priority should be given to research in these areas.The current model can not account for important feedback mechanisms that would allow analysis of the long-term effects of nutrient budgets on nutrient availability and plant productivity. Dynamic models of farm nutrient budgets that include such interactions are needed to further assess the sustainability of farming systems.     

Nitrogen budgets and environmental capacity in farm systems in a large-scale karst region, southern China

Field surplus nitrogen (N) and farm disposal N are major sources of water pollution in farming systems. These sources are estimated from N budgets in field and whole farms, which are associated with the production and consumption of food. This study was conducted to evaluate these two pollution sources in the steep mountainous karst region of Quibainong, Guangxi Province, southern China. The region is, characterized as an area of upland farms, due to the shallow soils and rapid water drainage through cracks in the limestone. Although field surplus N in 1960 was only 4.1 kg N ha^–1, current field surplus N ranged from 10.1 to 463 kg N ha^–1, with values above 50 kg N ha^–1 in farms along roads and less than 40 kg N ha^–1 in the farms away from roads. The results obtained in near-road farms were similar to those in a previous study of N budgets in China. There was a significant positive correlation between the field surplus N and N application rate, including when the previous data were incorporated. The proportion of manure to total N application decreased with increase of N application. Chemical fertilizer was applied in greater quantity in economically rich farms. Therefore, the increase of field surplus N in Quibainong may be caused by economic improvement. Although livestock and human excreta were stocked in manure barns, unused excreta N increased with the increase of N excreted. The unused excreta N also increased with the decrease of feed self-sufficiency, but was not related to N application rate. These facts indicate that livestock husbandry in Quibainong is related to economic status of farms, but independently of crop production.The N application rate of more than 160 kg N ha^–1 increased field surplus N to an extent greater than crop uptake N, and a N application rate of more than 185 kg N ha^–1 increased the potential nitrate-N concentration to more than 10 mg L^–1. Therefore, 160–185 kg N ha^–1 is suggested to be the environmental capacity to sustain optimal N cycling in Quibainong. The average value of excreta N produced on near-road farms in Quibainong was 171 kg N ha^–1. If excreta N was used evenly for crop cultivation without chemical fertilizer in whole fields, the optimal N cycling would be maintained.The survey conducted here using a questionnaire was effective in evaluating all kind of N flows in the farming systems.     

Balanced pasture fertilization in the Basque Country: I. Phosphorus and potassium budgets on dairy farms

Dairy farming is the main agricultural activity of the Basque Country. A dairy farm is characterized as a system with soils and crops, forage, cattle and manure as main components, and in such a system, nutrient cycling is very important to maintain soil fertility and optimize forage production. To quantify nutrient transfers in the cycle, a simple system was developed and has been applied to seventeen farms to examine its ability to achieve a balanced P and K fertilization. These farms have provided data on inputs (fertilizer, feeds, concentrates), pasture and manure management, and outputs (milk production), and soil samples have been taken from farm pastures. Phosphorus and K in excreta and uneaten pasture is used with a relatively high efficiency as suggested by the relatively high efficiency of P and K utilization by the pasture that usually ranges from 70 to 90%. Concentrate feeding (3000 kg cow^–1 yr^–1) represents one of the main P and K inputs in Basque Country dairy farms, averaging 26 and 66 kg ha^–1, respectively. Besides, release of K in the soil through slow liberation from non-exchangeable sites was estimated as 30 kg ha^–1. Thus, a high efficiency in excreta recycling would diminish substantially P and K mineral fertilizer needs. Farm nutrient budgets appear to be a convenient tool for determining nutrient shortages and surpluses at farm level, and thus they are considered as a first step to support a better management of maintenance fertilization of permanent pastures.     

Cadmium concentration in vegetable crops grown in a sandy soil as affected by Cd levels in fertilizer and soil pH

Field trials were conducted over a three-year period with chinese cabbage (Brassica pekinensis Rupr.) and carrots (Daucus carota L.) grown in a sandy soil with pH adjusted to 5.5 and 6.5. The NPK fertilizers containing 1, 30, 90, and 400 mg Cd kg^–1 P were applied at the rate of 0.07, 2.1, 6.3 and 28 g Cd ha^–1 yr^–1. The amounts of Cd added through phosphate rock also ranged between 0.1 and 28 g ha^–1 yr^–1. The increased Cd application rates through NPK fertilizers increased the Cd concentration in both vegetables but the differences among treatments were not found to be significant. The Cd uptake by both crops was significantly (p<0.01) higher at pH 5.5 than at pH 6.5. Chinese cabbage exhibited lower Cd concentration than carrots. Carrot leaves contained higher Cd than its roots. Cadmium removals by chinese cabbage and carrot were about 0.7 and 1.3 g ha^–1 yr^–1, respectively. At pH 5.5, Cd concentrations in the two crops, based on a three-year average, were 23 and 46% higher than at pH 6.5. Cadmium uptake by chinese cabbage from different sources of phosphate rock was affected to a very limited extent. Cadmium concentration generally increased over the years. Cadmium extracted by ammonium nitrate after harvest of the crops was closely related with soil pH and Cd concentration in the plants.     

Response to fertilization and nutrient deficiency diagnostics in peach palm in Central Amazonia

Peach palm (Bactris gasipaes Kunth) is increasingly grown in the tropics for its heart-of-palm and fruit. Determining fertilization response and diagnosing nutrient status in peach palm may require methods that consider the particularities in nutrient acquisition and recycling of perennial crops. Responses to nutrient additions, and the diagnostic value of soil and foliar analyses were examined in three field experiments with three-year old peach palm stands on Oxisols in Central Amazonia. To diagnose P-deficiency levels in soils, samples from 0–5 cm and 5–20 cm depth were analyzed for available P by different methods (Mehlich-1, Mehlich-3 and Modified Olsen). The second and fifth leaves were analyzed to assess N, P and K deficiencies. Field experiments involved several combinations of N (from 0 to 225 kg ha^–1 yr^–1), K (from 0 to 225 kg ha^–1 yr^–1) and P (from 0 to 59 kg ha^–1 yr^–1). Palms on control plots (unfertilized) and those receiving 225 kg ha^–1 yr^–1 N and 2 Mg ha^–1 of lime yielded between 4 and 19% of the maximum growth which was obtained with N, P and K applications. In one of the experiments, yield of heart-of-palm was positively related to N additions at the lowest levels of P (8.6 kg ha^–1 yr^–1) and K (60 kg ha^–1 yr^–1) additions. In one experiment, critical leaf N level was 2.5% for the second leaf and 2.2% for the fifth leaf. Some growth responses to P additions at constant N and K levels were observed (e.g., 797 kg ha^–1 yr^–1 of heart-of-palm with 39.3 kg ha^–1 yr^–1 of applied P, and 632 kg ha^–1 yr^–1 of heart-of-palm with 10.9 kg ha^–1 yr^–1 of applied P in one experiment, and 2334 kg ha^–1 yr^–1 of heart-of-palm with 39.3 kg ha^–1 yr^–1 of P and 1257 kg ha^–1 yr^–1 of heart-of-palm with 19.7 kg ha^–1 yr^–1 of P in another trial). In the experiment for fruit production from peach palm, total plant height did not respond to P additions between 19.7 and 59 kg ha^–1 yr^–1 and K additions between 75 and 225 kg ha^–1 yr^–1. Leaf P levels were found to be above the proposed critical levels of 0.23% for the third leaf and 0.16% for the fifth leaf. Plants in this experiment, however, showed evident symptoms of Mg deficiency, which was associated with a steep gradient of increasing Mg concentration from the fifth leaf to the second leaf. Standard leaf diagnostic methods in most cases proved less useful to show plant N and P status and growth responses to N and P additions. Soil P determined by common extractions was in general too variable for prediction of growth.     

Use of the partial nutrient budget as an indicator of nutrient depletion in the highlands of southwestern Uganda

Agricultural management has its roots in the manipulation of the system to optimise conditions for crop production. It is now widely recognised that this could result in land degradation with subsequent serious impact on crop productivity if the nutrient losses to the agricultural system are not replaced. A nutrient budget is an account of gains and losses of nutrients in an agricultural system, a tool that could be used to develop sound nutrient management and sustainable agriculture. This tool was applied to the annual crop farming system in the highlands of southwestern Uganda to demonstrate (i) within farm nutrient depletion and accumulating zones, and (ii) the extent of nutrient losses at farm and district levels through marketing pathways. Partial nutrient budgets were constructed at field and farm levels using farmer-recorded resource inputs and outputs over a period of one year, and at the district level using annual inventory data of agricultural imports and exports. The computed nutrient balances were highly variable at field and farm levels, but predominantly negative. Nitrogen (N) gains and losses averaged 30.6 and 72.3 kg ha^–1 yr^–1, respectively in the homestead fields; 10.8 and 33.4 kg ha^–1 yr^–1 in the outfields; 15.8 and 17.4 kg ha^–1 yr^–1 at the farm level; and only losses of 5.6 kg ha^–1 yr^–1 at the district level. Potassium (K) gains and losses followed a similar trend, although less in magnitude. The phosphorus (P) balance was positive but only in the homestead fields and at the farm level. Where agricultural produce were marketed, nutrient losses were reflected more at the higher scales (e.g. district level) and became tied up in pools from which recycling back to agriculture was barely feasible, and with quite alarming monetary implications. Such results can be used to influence policies at different scales on nutrient management.     

Using a Crop/Soil Simulation Model and GIS Techniques to Assess Methane Emissions from Rice Fields in Asia. IV. Upscaling to National Levels

The process-based crop/soil model MERES (Methane Emissions from Rice EcoSystems) was used together with daily weather data, spatial soil data, and rice-growing statistics to estimate the annual methane (CH₄) emissions from China, India, Indonesia, Philippines, and Thailand under various crop management scenarios. Four crop management scenarios were considered: (a) a 'baseline' scenario assuming no addition of organic amendments or field drainage during the growing season, (b) addition of 3,000 kg DM ha^–1 of green manure at the start of the season but no field drainage, (c) no organic amendments but drainage of the field for a 14-d period in the middle of the season and again at the end of the season, and (d) addition of 3,000 kg DM ha^–1 of green manure and field drainage in the middle and end of the season. For each scenario, simulations were made at each location for irrigated and rainfed rice ecosystems in the main rice-growing season, and for irrigated rice in the second (or 'dry') season. Overall annual emissions (Tg CH₄ yr^–1) for a province/district were calculated by multiplying the rates of CH₄ emission (kg CH₄ ha^–1 yr^–1) by the area of rice grown in each ecosystem and in each season obtained from the Huke and Huke (1997) database of rice production. Using the baseline scenario, annual CH₄ emissions for China, India, Indonesia, Philippines, and Thailand were calculated to be 3.73, 2.14, 1.65, 0.14, and 0.18 Tg CH₄ yr^–1, respectively. Addition of 3,000 kg DM ha^–1 green manure at the start of the season increased emissions by an average of 128% across the five countries, with a range of 74–259%. Drainage of the field in the middle and at the end of the season reduced emissions by an average of 13% across the five countries, with a range of –10% to –39%. The combination of organic amendments and field drainage resulted in an increase in emissions by an average of 86% across the five countries, with a range of 15–176%. The sum of CH₄ emissions from these five countries, comprising about 70% of the global rice area, ranged from 6.49 to 17.42 Tg CH₄ yr^–1, depending on the crop management scenario.     

Spatial and sectorial disaggregation of N2O emissions from agriculture in Belgium

In this study N₂O emissions from agriculture in Belgium have been split up per agro-pedological region and calculated per farm type. The N₂O emissions were calculated according to the `Revised 1996 IPCC guidelines for national greenhouse gas inventories'. Input data were weighed averages of the N balance of a large number of farms per agro-pedological region and per farm type. As such, the input data represent a theoretical farm in each agro-pedological region and for each distinguished farm type. In a first part, N₂O emissions were calculated for 10 agro-pedological regions in Belgium. The yearly N₂O emissions varied between 225 and 462 kg N₂O-N. The highest N₂O emissions (around 400 kg N₂O-N yr⁻¹) were found in regions with fertile soils, dominated by crop production or a combination of crop production and cattle breeding. The lowest emissions (around 250 kg N₂O-N yr⁻¹) were found in regions with extensive cattle breeding. N₂O emissions of 300 ± 15 kg N₂O-N yr⁻¹ were found in regions with less extensive cattle breeding or in regions with combinations of cattle, pig and poultry breeding. The N₂O emission per ha varied between 6 and 14 kg N₂O-N yr⁻¹. In a second part, N₂O emissions were calculated for 12 different farm types. The yearly N₂O emissions varied between 273 and 512 kg N₂O-N. The highest emissions were found on farms with crop production or a combination of crop production and cattle breeding. The lowest emissions were found on farms specialised in only one activity of animal breeding. Specialised pig farms and farms with combinations of cattle caused the greatest threat with respect to N₂O releases from agriculture. Their N₂O emission per ha was 18–40 kg N₂O-N yr⁻¹, which was significantly higher than the average N₂O release (10 kg N₂O-N yr⁻¹ ha⁻¹) for the other farm types. This revised version was published online in August 2006 with corrections to the Cover Date.     

A phosphorus budget of a poultry farm and a dairy farm in the southeastern U.S., and the potential impacts of diet alterations

Current and potential environmental problems associated with P transport from lands receiving high application rates of animal waste are a major concern. Phosphorus management strategies are needed to reduce P loading on land. This study was conducted to compare on-farm P budgets for a modern broiler farm and a dairy farm under traditional diets and management practices. Phosphorus inputs, recycling and outputs were assessed for both farms. A typical broiler and a dairy farmer from North Carolina were interviewed and pertinent information for the study was obtained, in cooperation with extension agents, and other professionals associated with the farms. The annual on-farm P surplus for the broiler farm was 6,380 kg, while that for the dairy farm was 1,141 kg. This corresponds to an annual application of 65 kg P ha^–1 for the broiler farm and 20 kg P ha^–1 for the dairy farm in excess of removal. The potential for reducing P surpluses by the addition of phytase enzymes and/or the use of low phytic acid corn (Zea mays L.) feed in the broiler farm diet was also assessed. Estimates by animal nutritionists indicate that feed supplementation with phytase enzyme can reduce the broiler farm's P surplus by 33%. The use of low phytic acid corn can reduce the surplus by 49% and a combination of the two can reduce the surplus by 58%. In this study, the incorporation of soybean (Glycine max (L.) Merr.) and alfalfa (Medicago sativa) land into the waste utilization plan of the dairy farm decreases the annual P surplus from 20 to 9 kg P ha^–1. The use of new feed technology and expanding waste application to a larger land base can significantly alter the P budgets of broiler and dairy farms and reduce P surpluses, minimizing the risk of environmental problems.     

The use of farmgate balances and soil surface balances as estimator for nitrogen leaching to surface water

Farmgate balances (FGBs), defined as the difference between nutrient input and nutrient output at farm level, are currently used as a tool to monitor changes in nitrogen (N) and phosphorus (P) leaching to groundwater and surface water. We postulate that the estimator value of FGBs for N and P leaching to groundwater and surface water depends on (1) the distribution of N and P surpluses over fields within farms, and (2) the partitioning of the surplus over the various nutrient loss pathways. In this study, we assessed intra-farm variability of N and P surpluses and its possible consequences on N leaching to surface waters. Furthermore, we investigated the effect of policies to decrease N and P surpluses at farm level on N and P surpluses at field level. FGBs were derived for six dairy farms in a hydrologically rather isolated polder with grassland on peat soil for three years (1999, 2000 and 2001). Soil surface balances (SSBs), defined as the differences between nutrient input and nutrient output at field level, were derived for the accompanying 65 fields for the same years. On average, FGB surpluses decreased from 271 kg N ha^–1 y^–1 and 22 kg P ha^–1 y^–1 in 1999 to 213 kg N ha^–1 y^–1 and 13 kg P ha^–1 y^–1 in 2001. Variances in N and P surpluses between fields per farm were compared with variances between farms. For N, variances between fields per farm exceeded variances between farms for all years. A non-linear model was fitted on the measured N loading of the surface water. This model showed that N leaching to surface water was underestimated by 5–46% if the variability in N surpluses between fields per farm was not taken into account. We concluded that estimation of N leaching to surface water, based on data at farm level, can lead to underestimation of the N leaching due to the large variability in N surpluses between fields per farm. The extent of this bias by a given distribution of N surpluses within farms was largely controlled by the partitioning of the N surplus over the various nutrient loss pathways, notably denitrification.     

Phosphate fertilization in intensive annual cropping with wheatgreen gram (or cowpea) — pearl millet

In a four year study on a wheat-green gram (or cowpea) — pearl millet intensive cropping system a total production of 9–10 tonnes of wheat equivalents per year removed 29–30kg P ha^–1. If only 26 kg P ha^–1 was used then total grain production as well as P uptake, was highest when all the P was applied to wheat. Only when amounts larger than 26 kg P ha^–1 were applied was it justified to apply P to pearl millet and green gram (or cowpea). Productivity of the cropping system increased up to 58.5 kg P ha^–1 and at this level two thirds of P was applied to wheat, while pearl millet and green gram or cowpea received the remaining one-third. A positive P balance in soil was observed only when 26 k P ha^–1 yr^–1 or more was applied.Pressure of growing population and per capita diminution or arable land has focussed attention on multiple cropping systems in many Asian countries [1, 2]. In North-Western India the cropping system changed from a single rainy (July–October) or winter (November–April) crop a year prior to the 1960's to two-crops-a-year (both a rainy season and winter crop) in the 1970.s and then in the late 1970's a third summer (May–June) crop was also included. Wheat — green gram (or cowpea) — pearl millet is such a three-crops-a-year multiple cropping system.Phosphate is the costliest major plant nutrient in India and farmers following multiple cropping systems are keen to know the way the phosphate should be apportioned to different crops in a cropping system particularly when small amounts of P are applied. Such information can come only from long-term P fertilization experiments [3, 4]. The objective of the present experiment on a wheat-green gram (or cowpea) — pearl millet multiple cropping system was to study the direct and residual effects of P applied to one crop on the other crops grown in succession and to find the best possible way in which a limited amount of P could be apportioned between the different crops in the rotation. An attempt has also been made to work out the P balance in soil.     

Calculated rates of soil acidification of intensively used grassland in the Netherlands

The major processes involved in acidification of soils under intensively managed grassland are the transformation and subsequent leaching of applied nitrogen (N), assimilation of excess cations in herbage and acidic atmospheric deposition. Carbonates from fertilizers and excess cations in purchased concentrates are the most important proton (H⁺) neutralizing agents applied to grassland. In this study, the effects of grazing, cutting and N application on the net proton loading from each of the main processes were calculated, using a simple model.On mown swards, simulated excess cation uptake by the sward released 4.5–9.3 kmol_c H⁺ ha^–1 yr^–1. The total proton loading on mown grassland decreased from about 8.0 to 5.3 kmol_c ha^–1 yr^–1 when fertilizer N input as CAN-27 increased from 0 to about 400 kg ha^–1 yr^–1. Contributions from atmospheric deposition ranged from 2.2 kmol_c ha^–1 yr^–1 when herbage yield exceeded 10 Mg ha^–1 yr^–1 to 3.0 kmol_c ha^–1 yr^–1 when herbage production was only 5.5 Mg ha^–1 yr^–1.On grazed swards, transformation of organically bound N from urine and dung to nitrate (NO ₃ ^- ) and the subsequent leaching of excess NO ₃ ^- was the main source of protons. Application of 400 kg N ha^–1 yr^–1 to grazed swards increased the proton loading from transformed N from 3.9 to 16.9 kmol_c ha^–1 yr^–1. The total proton loading on grazed swards exceeded that of mown swards when the input of fertilizer N exceeded 150 kg ha^–1 yr^–1.Underestimation of the amount of N immobilized in the soil biomass and lost by denitrification may have resulted in a slight overestimation of the amount of N lost by leaching and thereby also the simulated total proton loading.     

Phosphorus feeding and manure nutrient recycling on Wisconsin dairy farms

Recently approved nutrient management regulations for livestock operations focus on a farm’s ability to recycle the phosphorus (P) contained in manure. Most efforts to improve dairy manure management emphasize manure handling, storage, and land application techniques. Little is known about relationships between dairy feeding practices and manure P levels under farm conditions, or between herd size, cropland area and a farm’s ability to recycle manure P through crops. A survey of 98 representative dairy farms in Wisconsin showed that most farms were self-sufficient in forage (alfalfa, corn silage) and grain production. Lactating dairy cows derived 90% of their feed dry matter (DM) and 78% of their P intake from these homegrown feeds. The P content (DM basis) of the dairy diet ranged from 2.3 to 8.5 with an average of 4.0g P kg⁻¹. Approximately 85% of the surveyed dairy farms fed P in excess of the recently updated National Research Council (NRC) requirements. On these farms, amounts of P in manure were related to dietary P. Of the annual manure P excreted by cows fed a diet supplement, approximately two-thirds is derived from homegrown feeds and one-third from imported mineral and protein supplements. Stocking rates ranged from 0.19 to 1.68 AU ha⁻¹. Farms having stocking rates of less than 0.70 AU ha⁻¹ are self-sufficient in feed production. Approximately half of the farms are self-sufficient in feed production, 68% produce 90%, and 80% produce 80% of their annual feed requirement. Approximately 40% of the farms have a positive P balance (manure P exceeds harvested crop P). On these farms, lowering dietary P to the levels recommended by NRC would reduce the number of farms having a positive P balance by 67%, and the land area in positive P balance by 60%. For farms having a high animal stocking rate, manure export, the addition of cropland for manure spreading, and/or reductions in livestock (cow and/or heifer) numbers may be the only feasible strategies for achieving P balance on a farm. This revised version was published online in November 2006 with corrections to the Cover Date.     

Phosphorus (P) management in the ‘De Marke’ dairy farming system

In the sandy regions of the Netherlands water quality is threatened by high losses of nutrients from intensive dairy farms. About 67% (32 kg ha^-1yr^-1) of farm inputs of P in purchased feeds and fertilisers do not leave in milk or cattle. The Dutch government defined decreasing maximum permitted nutrient surplusses for the period 1998–2008, at 9 kg ha^-1yr^-1 for P. Farmers suppose that reducing the surplusses will be costly, because it limits application of slurry, which then has to be either exported or additional land has to be purchased. Moreover, farmers are worried about the impact on soil fertility and crop growth. To explore the possibilities of reducing surplusses by improved management, farming systems research is carried out at prototype farm De Marke. Results indicate that average intensive dairy farms can reduce P surplus sufficiently, without the need to buy land or to export slurry. Key factors are reductions in purchased feeds (by reduced needs per kg milk as a result of a higher milk yield per cow, less young stock and judicious feeding) and fertilisers (by improved management of home-made manure and an increased maize area). Initially, P fertility status of the fields of De Marke decreased, but stabilised in the seventh year at a level not restrictive to crop production.     

Changes in quantity of mineral nitrogen in three grassland soils as affected by intensity of nitrogen fertilization

Changes in quantity of soil mineral nitrogen down to a depth of 1 m in cloverfree grassland were monitored within one growing season and over successive growing seasons. Accumulation of mineral nitrogen in the soil occurred on permanent grassland with split application of nitrogen totalling more than 400 kg N ha^–1 yr^–1 and on young grassland, sown after arable crops, with applications of more than 480 kg N ha^–1 yr^–1. The relationship between the rate of nitrogen application minus nitrogen uptake, and accumulation of mineral nitrogen in the upper 50 cm of soil during each growing season is described.     

Response of lowland rice and common bean grown in rotation to soil fertility levels on a Varzea soil

Brazil has approximately 30 million hectares of lowland areas, known locally as Varzea, but very little is known about their fertility and crop production potential. A field experiment was conducted for three consecutive years to evaluate response of lowland rice (Oryza sativa L.) grown in rotation with common bean (Phaseolus vulgaris L.) on a Varzea (low, Humic Gley) soil. Rice was grown at low (no fertilizer), medium (100 kg N ha^–1, 44 kg P ha^–1, 50 kg K ha^–1, 40 kg FTE-BR 12 ha^–1), and high (200 kg N ha^–1, 88 kg P ha^–1, 100 kg K ha^–1, 80 kg FTE-BR 12 ha^–1 fritted trace element-Brazil 12 as a source of micronutrients) soil fertility levels. Green manure with medium fertility was also included as an additional treatment. Average dry matter and grain yields of rice and common bean were significantly (P < 0.01) increased with increasing fertilization. Across the three years, rice yield was 4327 kg ha^–1 at low fertility, 5523 kg ha^–1 at medium fertility, 5465 kg ha^–1 at high fertility, and 6332 kg ha^–1 at medium fertility with green manure treatment. Similarly, average common bean yield was 294 kg ha^–1 at low soil fertility, 663 kg ha^–1 at medium soil fertility, 851 kg ha^–1 at high fertility, and 823 kg ha^–1 at medium fertility with green manure treatment. Significant differences in nutrient uptake in bean were observed for fertility, year, and their interactions; however, these factors were invariably nonsignificant in rice.     

Direct and residual effect of fertilizer zinc application on the yield and chemical composition of rice-wheat crops in an alkali soil

A field experiment was conducted on an alkali soil to evaluate the direct and the residual effect of six levels of zinc i.e. 0, 2.25, 4.5, 9.0, 18.0 and 27.0 kg Zn ha^–1 added either once to the first crop only or continuously to each crop on the growth, yield and chemical composition of plants grown in a rice-wheat cropping sequence. The soils were amended with gypsum applied at the uniform rate of 14 t ha^–1. Zinc was supplied as zinc sulphate. Application of zinc at the rate of 2.25 kg ha^–1 to both rice and wheat crops or an annual application of 4.5 kg Zn ha^–1 only to rice was found optimum for rice-wheat sequence. Higher zinc applications increased the availability of zinc in the soil and its content in the plants but did not increase crop yield. DTPA extractable zinc build up was more for zinc applied at the rate of 2.25 kg ha^–1 to each crop compared to a single zinc application of equivalent amount. Results of these studies have shown that continuous Zn application up to 27 kg Zn ha^–1 to each crop did not induce nutrient imbalances and had no adverse effect on crop yield.     

Vertical distribution of heavy metals in wastewater-irrigated vegetable garden soils of three West African cities

Application of untreated wastewater to irrigate urban vegetable gardens is raising serious concern about possible health risks associated with the consumption of these vegetables particularly with regard to the concentrations of heavy metals (HM) in their edible portions. The soil concentrations of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn), were investigated in seven vegetable gardens from the three West African cities of Kano (Nigeria), Bobo Dioulasso (Burkina Faso) and Sikasso (Mali). Also determined were input–output balances of Cd and Zn from five vegetable gardens under 30 years of wastewater irrigation in Kano. In these gardens Cd (2.3–4.8 mg kg⁻¹) and Zn (13–285 mg kg⁻¹) concentrations throughout the profile attained unsafe levels. The concentrations of Cu (0.8–18 mg kg⁻¹), Cr (1.8–72 mg kg⁻¹), Ni (0–17 mg kg⁻¹) and Pb (0.6–46 mg kg⁻¹) were below the safety thresholds for arable soils. Overall, concentrations of Zn, Cd, Pb and Ni were higher in Kano than in Bobo-Dioulasso and Sikasso. Input–output analyses in Kano indicated that irrigation wastewater contributed annually 400–3,700 g Cd ha⁻¹ and 7,200–22,300 g Zn ha⁻¹, fertilizer 30–2,100 g Cd ha⁻¹ 50–17,600 g Zn ha⁻¹, harmattan dust 0.02–0.4 g Cd ha⁻¹ and 40–200 g Zn ha⁻¹ while 300–500 g Cd ha⁻¹ and 2,700–4,700 g Zn ha⁻¹ came from rainwater inputs. Input–output calculations subtracting the amounts of HM taken out in vegetable biomass and that lost to leaching from total inputs yielded an annual net positive balance of 700–4,160 g Cd ha⁻¹ and 9,350–39,700 g Zn ha⁻¹. If such balances remain unchanged for another 10–20 years vegetables raised in these garden fields are likely to be unsuitable for human consumption.     

Impact of rice straw management practices on yield,nitrogen uptake and soil properties in a wheat-rice rotation in northern India

To study the long term effects of rice straw management practices in a wheat-rice rotation, experiments were started from the dry season (Nov–May) of 1984 to wet season (July–Nov) of 1989. Each year, six straw management practices, viz. control (C), straw incorporation (SI), straw mulch (SM), straw burning (SB), animal manure incorporation (AM), and straw and animal manure incorporation together (SI+AM) were imposed to wheat crop and their subsequent residual effect was studied on the following rice crop under three levels of N, viz. 0, 60 and 120 kg N ha^–1. The rate of straw and animal manure used was 5 t ha^–1 on dry weight basis.The wheat yield and N uptake did not vary significantly under control and SB throughout the experimental period. But, the production level of wheat and N uptake were consistently higher under AM and SM over these two treatments. The SI+AM which had significantly lower wheat yields and N uptake over the AM during the first crop, became equal to that of AM and SM during the second and third crops, and out yielded these two treatments from the fourth crop onward. Straw incorporation which produced wheat yield and N uptake even less than control and SB during the first two crops, resulted in wheat yield and N uptake equivalent to AM and SM from the fourth crop onward.None of the straw management practices had residual effects on the yields and N uptake during the first rice crop, except SM which reduced the rice yields and N uptake in the first two crops. However, AM and SI+AM in the second crop; AM, SI+AM and SI in the third crop; and AM, SI+AM, SI and SM from the fourth crop onward had significant and positive residual effects on rice yields and N uptake. Among these four treatments, SI+AM produced residual effects which were significantly higher than the remaining three treatments. Considering the production levels of wheat and rice, SI+AM treatment resulted in savings of 60 kg N ha^–1 each for wheat and rice.After five years of experimentation, the maximum soil build-up of organic carbon; available N, P and K; and DTPA-extractable Zn, Cu, Fe and Mn was observed under SI+AM, followed by AM and SM and it was minimal under SB and control treatments. The treatments of AM and SI+AM also resulted in a high percentage of water-stable aggregates of 70.25 mm in diameter (80.9%), larger mean weight diameter (0.82 mm), higher porosity (54.2%) and lower bulk density (1.19 Mg m^–3).     

Twenty-five years of phosphate and potassium fertilization of a crop rotation

Effects of rate and placement of phosphate and potassium fertilizers was studied using a 4-year rotation of corn (Zea mays L.), soybeans (Glycine max L.), wheat (Triticum vulgare L.) and hay (later changed to corn). Yields increased with increased P until 22 kg P ha^–1 yr^–1 was applied. Yields increased with increased K applications to 140 kg K ha^–1 yr^–1. Broadcast P applications gave high yields than row applications. Crop response to P was affected more by soil P level than by application to the specific crop. Residual effect from K fertilizer applications did not last as long as the residual effect from P application. Soil tests for available P were closely correlated with rate of P application over the 25-year period. Soil tests for P were higher where P was banded where P was broadcast indicating less tie-up of P by the soil where less mixing occurred.Journal Paper No. 7910, Purdue University Agricultural Experiment Station.