Nitrogen fertigation of trickle-irrigated potato

This three-year field study, on Pellic Vertisol, was designed to investigate the response of trickle-irrigated potato (Solanum tuberosum L.) to four nitrogen levels continually applied with the irrigation stream. Waters containing 70, 130, 190, and 250 mg Nl^–1 and uniformly supplied with 50 and 120 mgl^–1 of P and K, respectively, were applied when the soil water potential was between 0.03 and 0.04 MPa. The amount of water applied at each irrigation was equivalent to 0.8 of pan evaporation from a screened USWB Class A pan. The resulting N application totals ranged from 205 to 735 kg ha^–1. Significant buildup of soil NO₃-N occurred below 45 cm depth with the two higher amounts of N but not with the 70 or 130 mg Nl^–1. A concentration of 130 mg Nl^–1 was adequate for maintaining petiole NO₃-N above the critical value throughout the growing period. The highest yield of good quality (58130 kg ha^–1; specific gravity 1.071) was obtained with 130 mg Nl^–1. It was concluded that fertigation (combined irrigation with fertilization) is a promising means for maintaining N concentration in the soil throughout the growing period at desirable levels, without undue losses by leaching.     

Effect of P fertilizer on alleviating chloride toxicity in wheat

A pot study with four levels each P (control, 6.5, 13.0, 19.5 mg kg^–1) and Cl^– in irrigation water (control, 30, 60, 90me l^–1) was carried out to test Cl^– and P interaction on wheat. It has been found that optimum P nutrition alleviates the toxic effect of excess Cl^–. Highest P rate resulted in a significant yield increase upto a Cl^– level of 60me l^–1. Cl^– depressed P content in the plant only at a Cl^– level of 90me l^–1, while P rates had no major impact on the Cl^– concentration in the plant. Lower Cl^– concentration at higher P rates are rather a dilution effect than an antagonistic one. With optimum P supply higher Cl^– contents in the plants were tolerated than with a low P supply.     

Yield,water use and nitrogen uptake for different water and N levels in winter wheat

In an effort to establish an optimum combination of water and nitrogen for winter wheat a field investigation was carried out on a coarse loamy sand soil during 1984–85 and 1985–86 to assess effects of irrigation regime (IR) and N application on yield, water use and N uptake. The treatments compromised all combinations of three irrigation regimes (IR) based on ratios of irrigation water to cumulative pan evaporation viz.1.2 (I-1), 0.9 (I-2) and 0.6 (I-3) and four rates of N, viz. 0, 60, 120 and 180 kg ha^–1. Grain yield increased with increase in frequency of irrigation. In spite of wide differences in weather during the two years, scheduling of irrigation at IW/CPE = 1.2 gave the highest wheat yield on the coarse-textured soil. During 1984–85, the rainless year, grain yield under I-1 was 20 and 32 per cent higher than I-2 and I-3, respectively. With increasing N rate the yield and water use efficiency increased progressively upto 180 kg N under I-1 and upto 120 kg N ha^–1 under I-2 and I-3 regimes. During 1985–86, the wet year, grain yield response to IR was relatively low. Irrespective of IR, yield increased progressively upto 180 kg N ha^–1 during the wet year. Irrigation water regimes and N application also influenced leaf area index and root growth of wheat. The yield of unfertilized wheat was relatively less affected by seasonal rainfall and IR.Both N uptake and grain yield of wheat were found to increase linearly with increase in water use. Water use efficiency was highest under I-1 regime at all levels of N in the dry season of 1984–85 and under I-3 regime in the wet season of 1985–86. Increase in N uptake with increasing N rates was significantly higher under I-1 than I-2 and I-3 regimes. The N use efficiency being maximum at 60 kg N ha^–1, decreased at higher N levels irrespective of IR.     

Fate of phosphorus applied in slurry and mineral fertilizer: accumulation in soil and release into surface runoff water

Phosphorus (P) accumulation on the soil surface and its effect on the concentration of dissolved orthophosphate P (PO₄-P) in surface runoff water were studied after three years of surface application of slurry and mineral fertilizer to grass ley on a sandy soil, poor in P. The total amount of P applied was 107–143 kg ha^–1>, of which 72–119 kg ha^–1> was applied on the soil surface during two or three years without incorporation or mixing. The addition of slurry and mineral fertilizer resulted in an increase in inorganic P in the 0–5 cm but not the 5–25 cm soil layer, but organic P was not affected. The measured changes in inorganic P deviated only by 4–6 kg ha^–1> from the values derived from inputs and outputs of P (crop uptake + losses in surface runoff and drainage water). The increase in inorganic-P was accompanied by increases in the degree of P saturation (DPS) and in P extracted with acid am monium acetate (P_Ac ), sodium bicarbonate (P_Olsen) and anion-exchange resin (P_Resin). In surface runoff, 10–18 months after the last surface application of P, the mean flow-weighted concentration of PO₄-P was linearly increased with the values of DPS, P_Ac, P_Olson and P_Resin in the 0–5 cm soil layer. PO₄-P was lowest (0.033 mg l^–1> ) in the control plots and highest (0.62 mg l^–1>) in the plot where 143 kg ha^–1> P had been applied in slurry and fertilizer. On that plot, the corresponding values of DPS, P_Ac, P_Olson and P_Resin were 16%, 13 mg kg^–1>, 85 mg kg^–1> and 71 mg kg^–1 , even within a few years, and multiply the P loading to surface runoff from the site. A very shallow soil sampling (< 5 cm) is needed to assess P loading potential in a soil where P has been surface-applied.     

Response of sorghum to fertilizer phosphorus and its residual value in a Vertisol

The response of crops to added P in Vertisols is generally less predictable than in other soil types under similar agroclimatic conditions. Very few studies have considered the residual effects of P while studying responses to fresh P applications. Field experiments were conducted for three years to study the response of sorghum to fertilizer P applied at 0, 10, 20 and 40 kg P ha^–1, and its residual value in a Vertisol, very low in extractable P (0.4 mg P kg^–1 soil), at the ICRISAT Center, Patancheru (near Hyderbad), India. In order to compare the response to fresh and residual P directly in each season, a split-plot design was adopted. One crop of sorghum (cv CSH6) was grown each year during the rainy season (June-September).The phenology of the sorghum crop and its harvest index were greatly affected by P application. The days to 50% flowering and physiological maturity were significantly reduced by P application as well as by the residues of fertilizer P applied in the previous season. In the first year of the experiment, sorghum grain yield increased from 0.14 t (no P added) to 3.48 t ha^–1 with P added at the rate of 40 kg P ha^–1. Phosphorus applied in the previous year was 58% as effective as fresh P but P applied two years earlier was only 18% as effective as fresh P.     

Response of rice cultivars to phosphorus supply on an oxisol

Genotypic differences in absorption or utilization of P might be exploited to improve efficiency of fertilizer use or to obtain higher productivity on P-deficient soils. The objective of this study was to evaluate responses by 75 genotypes of upland rice (Oryza sativa L.) to two soil P levels in two field experiments. In the first experiment, soil P levels (Mehlich 1) were 1.5 mg kg^–1 and 5 mg kg^–1, and in the second experiment, 3 mg kg^–1 and 4.7 mg kg^–1 of soil, respectively. Rice cultivars differed significantly in shoot dry matter production at flowering, grain yield, and plant P status. Based on a grain yield efficiency index, cultivars were classified as P-efficient or P-inefficient. Shoot dry matter was more sensitive to P-deficiency but was not related to grain yield. Phosphorus use efficiency was higher under the low P treatment. Phosphorus uptake was significantly correlated with dry matter, P concentration and P-efficiency ratio. Results of this study indicate that genetic differences in P-use efficiency exist among upland rice cultivars and may be exploited in breeding programs.Contribution from National Rice and Bean Research Center of EMBRAPA, Goiania, Goias, Brazil and Appalachian Soil and Water Conservation Research Laboratoy, Beckley, WV, USA.     

Methods and rates of application of Mn and its critical levels for wheat following rice on coarse textured soils

Two field experiments were conducted on Mn-deficient soils to evaluate the efficiency of rates, methods and time of MnSO₄.H₂ O application for wheat. Manganese sulphate was broadcast and mixed in soils at the rate of 5 to 50kg Mn ha^–1 before seeding and 10 to 40 kg Mn ha^–1 as top dress at 28 days — just before first irrigation. Three sprays of 1% MnSO₄·H₂O unneutralised solutions were applied, the first at 26 days — 2 days before first irrigation and the others afterward at weekly intervals. Both the methods caused a significant and marked increase in grain yield. Three foliar sprays were as effective as soil applications of 20 to 40 Kg Mn ha^–1 before seeding. The difference in grain yield resulting from soil applications of Mn before seeding and applications at the first irrigation was not significant. The DTPA-Mn status of 20 fields, selected on the basis of varying degree of Mn deficiency, was related to grain yield (r = 0.77^**). Also grain yield of all the experiments had a significant correlation with Mn content of grain (r = 0.55^** to 0.82^**) and straw (r = 0.77^** to 0.82^**). The critical limits calculated by statistical method were 1.25, 2.18 and 3.5 mg Mn kg^–1 soil for severe deficiency, deficiency and latent deficiency respectively for wheat.     

Nitrogen fertigation of greenhouse-grown strawberries

Two greenhouse experiments were conducted with strawberries (Fragaria ananassa) grown in plastic pots filled with 12 kg of soil, and irrigated by drip to evaluate the effect of 3 N levels and 3 N sources. The N levels were 3.6, 7.2 or 10.8 mmol Nl^–1 and the N sources were urea, ammonium nitrate and potassium nitrate for supplying NH₄/NO₃ in mmol Nl^–1 ratios of 7/0, 3.5/3.5 or 0/7, respectively. Both experiments were uniformly supplied with micronutrients and 1.7 and 5.0 mmoll^–1 of P and K, respectively. The fertilizers were supplied through the irrigation stream with every irrigation. The highest yield was obtained with the 7.2 mmol Nl^–1 due to increase in both weight and number of fruits per plant. With this N concentration soil ECe and NO₃-N concentration were kept at low levels. Total N and NO₃-N in laminae and petioles increased with increasing N level. With the N sources the highest yield was obtained with urea due to better fruit setting. The N source had no effect on soil salinity and residual soil NO₃-N; residual NH₄-N in the soils receiving urea and ammonium nitrate were at low levels.     

The residual effectiveness of previously applied copper fertiliser for grain yield of wheat grown on soils of south-west Australia

The residual effectiveness of copper (Cu) applied 18 to 21 years previously was estimated for grain yield of wheat. In one field experiment, current levels of Cu fertiliser were applied and its effectiveness was compared to that of the same level of Cu applied previously. The effects of nitrogen (N) fertiliser on the Cu concentration in the youngest emerged blade and in the grain, as well as the effects of N levels on the grain yield of wheat, were also studied.Where the recommended level of Cu fertiliser had been applied previously, its residual effectiveness depended on the soil type. On the grey sands over clay and gravelly sands over clay, the residual Cu would last approximately 20 years where wheat is grown in rotation with a legume crop (Lupinus augustifolius L.) and where N fertiliser is applied at high levels (92 kg N ha^–1). On the yellow brown sandy earths of the Newdegate district, the residual value was in excess of 30 years.When Cu levels in the soil are marginal, high levels of N applied to wheat crops grown on stubbles of legume crops (high soil N) could suffer from induce Cu deficiency which could reduce grain production.Critical concentrations of Cu in the youngest emerged blade of less than 1.2 mg Cu kg^–1 at Gs50–59 would indicate Cu deficiency. Cu concentrations of less than 1.1–1.2 mg Cu kg^–1 in the grain suggest that the wheat crop is marginally supplied with Cu. In both situations, Cu fertiliser needs to be applied before the next crop.     

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.     

Critical levels for soil pH,available P,K, Zn and Mn and maize ear-leaf content of P,Cu and Mn in sedimentary soils of South-Western Nigeria

In the sedimentary soils of South-western Nigeria, actual and expected relative yields of maize were plotted against soil physical factors, soil avalilable nutrients and ear-leaf content of maize. These were used to set critical ranges of these factors for optimum production. Regression equations were obtained for each of the soil and plant factors for predicting yield, thereby making possible yield prediction with levels of each of these factors in these soils if all other factors are constant.The critical range concept combined with the soil physical and chemical properties and plant nutrient content could be a useful diagnostic tool for soil ammendment in crop production. Critical ranges were set as follows:pH, 6–6.5; available P (Bray's Pl), 10–16 mg Kg^–1; Exchangeable K, 0.6–0.8 me K100g^–1; available Zn, 5–10mg kg^–1; available Mn, about 25 mg Kg^–1; Ear-leaf P, 2.5–3.0%; Ear-leaf Cu, 10–20 mg Kg^–1; Earleaf Mn, about 50 mg Kg^–1.     

Compound fertilizer placement for potatoes

Sixteen experiments were carried out on maincrop potatoes (Solanum tuberosum) in the main growing areas of the United Kingdom to compare broadcast, sideband placed and split applications of compound fertilizer. In experiments without irrigation, yield increased up to about 1250 kg ha^–1 of compound fertilizer (N:P:K 15:6.6:15.8 or 15:8.3:15.8), while with full irrigation there was a response to at least 1875 kg ha^–1. Placement gave a higher yield than broadcast at 625 kg ha^–1, while at 1250 kg ha^–1 and 1875 kg ha^–1 broadcast, placed and split applications gave similar yields.     

Relationship between extracted phosphorus and sorghum yield in a Vertisol and an Alfisol under rainfed cropping

Little attention has been devoted to calibrating the soil tests for P in the field for crops grown under rainfed conditions in different soil types. Field experiments were conducted during the 1990 rainy season (June-September) at the ICRISAT Center, Patancheru (near Hyderabad), India on nearby Vertisol and Alfisol sites having a range in extractable P, for establishing relationships between extractable P and sorghum yield.In the Vertisol, 90% relative grain yield of sorghum was obtained at 2.8 mg kg^–1 Olsen extractable P while in the Alfisol, 90% relative grain yield was achieved at 5.0 mg P kg^–1 soil. These results suggest that a single critical limit of available P does not hold true for grain sorghum in the two soil types under similar agroclimatic conditions and that the critical limit is lower for the clayey Vertisol than the sandy Alfisol.     

Uptake of fertilizer and soil nitrogen by ryegrass swards during spring and mid-season

Double-labelled¹⁵N ammonium nitrate was used to determine the uptake of fertilizer and soil N by ryegrass swards during spring and mid-season. The effects of water stress (40% of mean rainfall v 25 mm irrigation per 25 mm soil water deficit) and the rate of application of N in the spring (40 v 130 kg ha^–1) on the recovery of 130 kg N ha^–1 applied in mid-season were also evaluated. Apparent recovery of fertilizer N (uptake of N in the fertilized plot minus that in the control expressed as a percentage of the N applied) was 95 and 79% for fertilizer N applied in the spring at rates of 40 and 130 kg ha^–1, respectively. Actual recovery of the fertilizer N assessed from the uptake of¹⁵N was only 31 and 48%, respectively. The uptake of soil N by the fertilized swards was substantially greater than that by the control. However, the increased uptake of soil N was always less than the amount of fertilizer N retained in or lost from the soil. Broadly similar patterns for the uptake of fertilizer and soil N were observed during mid-season. Uptake of N in mid-season was highest for swards which received 40 kg N ha^–1 in the spring and suffered minimal water stress during this period. Application of 130 kg N ha^–1 in spring reduced the uptake of N in mid-season to an extent similar to that arising from water stress. Only 1.8 to 4.2 kg ha^–1 (3 to 10%) of the N residual from fertilizer applied in the spring was recovered during mid-season. Laboratory incubation studies suggested that only a small part of the increased uptake of soil N by fertilized swards could be attributed to increased mineralisation of soil N induced by addition of fertilizer. It is considered that the increased uptake of soil N is partly real but mostly apparent, the latter arising from microbially mediated exchange of inorganic¹⁵N in the soil.     

Identifying critical sources of phosphorus export from agricultural watersheds

Surface runoff accounts for much of the phosphorus (P) input to and accelerated eutrophication of the fresh waters. Several states have tried to establish general threshold soil P levels above which the enrichment of surface runoff P becomes unacceptable. However, little information is available on the relationship between soil and surface runoff P, particularly for the northeastern United States. Further, threshold soil P criteria will be of limited value unless they are integrated with site potential for runoff and erosion. In response, the Natural Resource Conservation Service (NRCS) developed a P Index (PI), which ranks the vulnerability of fields as sources of P loss in runoff, based on soil P, hydrology, and land use. This study evaluated the relationship between soil and surface runoff P in a study watershed in central Pennsylvania. The relationship was then incorporated into the (PI), and its impact on the identification of critical source areas within the watershed was examined. Using simulated rainfall (6.5 cm h^–1 for 30 min), the concentration of dissolved P in surface runoff (0.2–2.1 mg l^–1) from soils was related (r ²=0.67) to Mehlich-3 extractable soil P (30–750 mg kg^–1). Using an environmentally based soil P threshold level of 450 mg kg^–1 determined from the soil-runoff P relationship, the PI identified and ranked areas of the watershed vulnerable to P loss. The vulnerable areas were located along the stream channel, where areas of runoff generation and areas of high soil P coincide, and where careful management of P fertilizers and manure should be targeted.     

Phosphorus status of soil and leaching losses: results from operating and dismantled lysimeters after 15 experimental years

The objectives of the present study were: (1) to evaluate the predicting value of the most important European soil P tests for P leaching losses; and (2) to investigate how these soil P tests reflect the development of P depth profiles in original homogeneous soils of lysimeters. The study included more than 100 lysimeters, located at the Lysimeter Station Falkenberg/Saxony-Anhalt, UFZ-Centre for Environmental Research Leipzig-Halle GmbH, Germany. Soil textures were sand, sandy loam, loam and silt. The management forms were arable land, grassland and fallow with various variation in fertilisation, crop rotation and irrigation. Samples were collected from the A-horizons and from the whole profiles of eight set-aside and dismantled lysimeters at 10-cm sections. The concentrations of total P were determined monthly in the leachates and evaluated for a three-year period. The concentrations of P extracted by ammonium acetate lactate (AL-P), double lactate (DL-P), sodium bicarbonate (Olsen-P) and ammonium oxalate (OX-P) as well as P_t were significantly correlated with each other (P<0.05–P<0.001) for arable soils. The relevant regression coefficients were strongly influenced by soil texture, soil use and management. The mean annual P concentrations of the leachates were in the range 0.4–1.2 mg l^–1 for sands and <0.001–0.1 mg l^–1 for the textures sandy loam, loam and silt. These corresponded to P leaching losses of 0.001–2846 g ha^–1 yr^–1. Mean annual and maximum P concentrations and leaching losses were significantly (r>0.954, P<0.001) predicted by the OX-P concentrations of arable topsoils in lysimeters filled with sand. For sandy loam under grass the agronomic soil P tests (AL-P, DL-P and Olsen-P) enabled reasonable predictions of P in leachate. Under arable use, factors such as fertilisation, management intensity, depth of tillage and irrigation resulted in weak correlations between soil P concentrations and P in leachate. It was shown for the first time that all P extractants reflected P enrichments in topsoils and subsoils and the development of distinct depth profiles. Influence of soil use on the depth distribution of P was more pronounced in the 0–20 cm layer than in the subsoils. Here, the original homogeneous substrate had oscillating P concentrations at 10-cm increments under all soil uses. These could not be explained by Al_ox and Fe_ox but were significantly correlated with the C_t contents and bulk density. This indicates that vertical movement of P containing organic matter along with differences in porosity contributed to the heterogeneous P distribution in the lysimeter subsoils. This new evidence must be considered if data sets from long-term lysimeter experiments are used to calibrate and validate P leaching models.     

Phosphate sorption isotherms in relation to P nutrition of wheat (Triticum aestivum)

The phosphate sorption isotherms are needed to explain differential plant responses to P fertilization in soils. Laboratory and greenhouse experiments investigated the use of phosphorus sorption isotherms in relation to P fertilizer requirement of wheat in ten benchmark soils of Punjab, India. The modified Mitscherlich Equation (3) was used to describe plant response observed in different soils. Maximum obtainable yield (MOY) ranged from 11.6 g pot^–1 in Gurdaspur (I) sandy clay loam to 7.0 g pot^–1 in Nabha sandy clay loam. Response to P applied @ 25 mg P kg^–1 soil was maximum (77%) in Bathinda sand and minimum in Chuharpur clay loam (33%). The response curvature varied from 3.74 × 10^–2 in Nabha sandy clay loam to 4.43 × 10^–2 in Kanjli sandy loam. The soil solution P required to produce optimum yield (90% MOY) varied from 1.61 µg ml^–1 in Bathinda sand to 0.10 µg ml^–1 in Sadhugarh clay. Dry matter yield obtained at 0.2 µg ml^–1 solution P concentration ranged from 55% in Bathinda sand to 85% of MOY in Gurdaspur (II) clay loam. At the same solution P concentration (0.1 µg P ml^–1), dry matter yield was 91% in Sadhugarh clay, 80% in Gurdaspur (II) clay loam and, 43% of MOY in Bathinda sand and eventually coincided with the decreasing maximum buffer capacity (MBC) in these soils. At the same level of sorbed P (100 mg P kg^–1 soil) the yield was observed to be inversely proportional to MBC. The study, therefore, concludes that, soils should be grouped according to their P sorption characteristics and MBC before using critical soil solution P as a criterion for obtaining optimum yields.     

Land application of poultry litter and water quality in Oklahoma,U.S.A.

With the rapid growth of the poultry industry in Oklahoma, U.S.A., more litter is applied to farm land. Thus, information is required on the impact of applications on regional soil and water resources. The effect of soil and poultry litter management on nitrogen (N) and phosphorus (P) loss in runoff and subsurface flow from four 16 m² plots (Ruston fine sandy loam, 6 to 8% slope) was investigated under natural rainfall. Plots under Bermudagrass (Cynodon dactylon) received 11 Mg litter ha^–1, which amounts to contributions of approximately 410 kg N and 140 kg P ha^–1 yr^–1. In spring, litter was broadcast on 3 of the plots; the upper half of one and total area of the other two. One of the total-area broadcast plots was tilled to 6 cm, the other remained as no till. The fourth plot served as a control. Relative to the control, litter application increased mean concentrations of total N and total P in runoff during the 16-week study for no-till (15.4 and 5.8 mg L^–1) and tilled treatments (16.7 and 6.1 mg L^–1). However, values for the half-area application (5.6 and 2.0 mg L^–1) were similar to the control (5.7 and 1.3 mg L^–1). Interflow (subsurface lateral flow at 70 cm depth) P was not affected by litter application; however, nitrate-N concentrations increased from 0.6 (control) to 2.9 mg L^–1 (no till). In all cases, < 2 % litter N and P was lost in runoff and interflow, maintaining acceptable water quality concentrations. Although litter increased grass yield (8518 kg ha^–1) compared to the control (3501 kg ha^–1), yields were not affected by litter management. An 8-fold increase in the plant available P content of surface soil indicates long-term litter management and application rates will be critical to the environmentally sound use of this nutrient resource.     

Response surface analysis of the effects of seeding rates,N-rates and irrigation frequencies on durum wheat I. Grain yield and yield components

Interactive effects of nitrogen (N) rates, seeding (S) rates and irrigation frequencies on grain yield and yield components of durum wheat were studied for four years under field conditions at Tulelake, California. Each year the experiment was conducted using a split-plot design with 4 irrigation frequencies as main plots and combinations of 5 N-rates (0 to 360 kg/ha) and 5 S-rates (50 to 250 kg/ha) as subplot treatments replicated 4 times. A quadratic response surface model (RSM) was used to study the effects of these treatments on grain yield and yield components (tillers/area, kernel number/spike, kernel weight/spike and 100-seed weight). The RSM was very effective for analysis and data reduction for estimating the optimum combinations of N and S for maximizing the grain yield and yield components. The N utilization and uptake efficiency increased with each irrigation treatment and peaked at irrigation treatment C. Both N and uptake utilization efficiency decreased with each increment of N-rate.In most cases, the effect of irrigation was independent of N and S. One irrigation at tillering increased grain yield and yield components significantly over only a preplant irrigation. The response of additional irrigations were comparatively small and significant only in some cases. Both N and S had significant effects on grain yield and yield components, however, the response of N was larger than that of S. With increasing N-rate, grain yield and tiller number increased with the expected peak beyond 360kg N ha^–1 but the increments beyond 180 kg N ha^–1 were of progressively smaller magnitude. The kernel number and kernel weight per spike also increased with N-rate giving a peak between 270 and 360 kg N ha^–1. With increasing S grain yield and tiller number/area increased while kernel number and kernel weight per spike decreased progressively. It was impossible to maximize yield and yield components at a given combination of N, S, and irrigation. According to the model, grain yield and tiller number were maximized at the highest level of N and S, while kernel number and kernel weight/spike were maximized at the lowest S (50 kg ha^–1) and about 314 kg N ha^–1 under adequate water supply. On the basis of the findings of this study and output of the model, 180–360 kg N ha^–1, 150–250 kg S ha^–1 and two post-sowing irrigations (at tillering and at boot stage) in addition to a preplant irrigation was recommended for optimum yield. An additional irrigation might be required depending on the weather conditions during the grain filling period.     

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

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