Integrated soil management for the savanna zone of W. Africa: legume rotation and fertilizer N

Integrated soil management with leguminous cover crops was studied at two sites in the northern Guinea savanna zone of northern Nigeria, Kaduna (190 day growing season) and Bauchi (150 days). One-year planted fallows of mucuna, lablab, and crotalaria were compared with natural grass fallow and cowpea controls. All treatments were followed by a maize test crop in the second year with 0, 30, or 60 kg N ha^–1 as urea. Above ground legume residues were not incorporated into the soil and most residues were burned early in the dry season at the Kaduna site. Legume rotation increased soil total N, maize growth in greenhouse pots, and dry matter and N accumulation of maize. Response of maize grain yield to 30 kg N ha^–1 as urea was highly significant at both sites and much greater than the response to legume rotation. The mean N fertilizer replacement value from legume rotation was 14 kg N ha^–1 at Kaduna and 6 kg N ha^–1 at Bauchi. W ith no N applied to the maize test crop, maize grain yield following legume fallow was 365 kg ha^–1 higher than natural fallow at Bauchi and 235 kg ha^–1 higher at Kaduna. The benefit of specific legume fallows to subsequent maize was mostly related to above ground N of the previous legume at Bauchi, where residues were protected from fire and grazing. At Kaduna, where fallow vegetation was burned, maize yield was related to estimated below ground N. The results show that legume rotation alone results in small maize yield increases in the dry savanna zone.     

Use of nitrification inhibitors (neem and DCD) to increase N efficiency in maize-wheat cropping system

A 2-year field experiment was conducted to study the effects of the nitrification inhibitors dicyandiamide (DCD) and neem cake on the efficiency of applied prilled urea nitrogen in a maize-wheat cropping system. Prilled urea (PU), neem cake coated urea (NCU) and DCD blended urea (DCDU) were applied to maize at two levels (60 and 120 Kg N ha^–1) and two methods (all preplant and split) of N application along with a no-nitrogen control and their relative residual effect was studied on succeeding wheat grown with three levels of N as PU.In 1990 maize responded well to N up to 60 kg N ha^–1; at this level PU increased maize yield by 1.03 t ha^–1, whereas NCU and DCDU increased maize yield by 1.55 and 1.18 t ha^–1 over the control, which was equivalent to an application of 127 and 94 kg N ha^–1 as PU, respectively. Furthermore, when the results were averaged over two years of study, residual N from the application of NCU and DCDU at 60 kg N ha^–1 left after maize cropping increased the grain yield of the succeeding wheat crop grown with 60 kg N ha^–1 as PU by 1.97 and 1.68 t ha^–1, respectively, over a no nitrogen control or 60 kg N ha^–1 as PU applied to the maize. This was equal to an application of 96 and 82 kg N ha^–1 as PU to wheat.Thus, neem cake increased the efficiency of urea N applied to maize and benefits were also seen in the succeeding wheat yield in the maize-wheat cropping system.     

Direct and residual effect of liming on yield and nutrient uptake of maize (Zea mays L.) in moderately acid soils in the savanna zone of Nigeria

Field experiments were conducted during wet season of 1980, 1981 and 1982 to determine the direct and residual effect of liming on yield and nutrient uptake of maize in moderately acid soils (pH -H₂O; 5.0–5.4) at three locations viz Kontagora, Tumu and Yandev in the savanna zone of Nigeria. Maize crop was grown at five lime rates 0, 0.5, 1.0, 2.0 and 4.0 t ha^–1 and two N sources (calcium ammonium nitrate and urea). Liming at a rate of 2 t ha^–1 maintained high maize yield for three years after application at Kontagora and Yandev. At Tumu 1 t ha^–1 was sufficient to get high yield of maize for three years. Higher rates of lime significantly depressed yield. Uptake of N, P and K was increased significantly with lime application upto 2 t ha^–1 lime at Kontagora and Yandev but at Tumu it increased only upto 1 t ha^–1. The response of P uptake to liming was higher in comparison to N and K uptake. Calcium and magnesium uptake responded upto 4 t ha^–1 lime at Kontagora & Yandev and upto 2 t ha^–1 at Tumu. The residual effect of liming lasted longer than 2 years. High lime rates reduced maize yields and crop nutrient uptake.     

The potential of Velvet bean (Mucuna pruriens) and N fertilizers in maize production on contrasting soils and agro-ecological zones of East Uganda

Research was conducted at two sites located in medium and low altitude zones in eastern Uganda. The aim of the study was to evaluate the benefit of Velvet bean (Mucuna pruriens) and inorganic N fertilizer in improving maize production in contrasting agro-ecological zones over two seasons. The medium altitude zone (Bulegeni) is a high-potential agricultural zone, with much more reliable rainfall and soils with high-productivity rating. The opposite is true for the low-altitude zone (Kibale). The soils were fertile for the site in the high-potential zone and poor in the low-potential zone. Over 22 weeks of fallow or relay with maize, Mucuna produced on average 8.2 t ha^–1 dry matter, accumulating 170 kg N ha^–1, with 57% of the N derived from the atmosphere in the low-potential zone, compared to 11.6 t ha^–1 dry matter, 350 kg N ha^–1, with 43% of the N derived from air, in the high-potential zone. Between 77 and 97% of the Mucuna-accumulated N was released over a period of 25 weeks, at a rate of 0.081 and 0.118 week^–1 in the high- and low-potential zones, respectively. The N-balance study shows that 93% of the applied N was accounted for in the high-potential zone, compared to 61% in the low-potential zone, due to differences in soil texture, soil fertility and maize biomass production at the two sites. As much as 44–73% of the N remained in the soil in the high-potential zone, compared to 39–53% in the low-potential zone, which might benefit the subsequent crops. There was a significant increase in maize yield in response to the added N, both from urea or Mucuna. The average increment above the control (continuous maize) was 3.2 t ha^–1 in the high-potential zone and 1.0 t ha^–1 in the low-potential zone. The maize yield increase over two seasons added up to 3.1 t ha^–1 with the application of inorganic fertilizers, and 1.9 t ha^–1 with a preceding Mucuna–maize relay in the high-potential zone, compared to an average of, 1.7 t ha^–1 with application of inorganic fertilizers and with Mucuna–maize relay in the low-potential zone. Application of P fertilizers with either N supply strategy significantly increased maize yield in the low-potential zone only, resulting in an additional 0.8 t ha^–1 for the inorganic N fertilizers and 1.3 t ha^–1 for a preceding Mucuna–maize relay. Apparently, P fertilizers are needed on poor soils. Clearly farmers stand to gain in terms of maize production from fertilizers as well as from the use of Mucuna, with more benefits from inorganic fertilizers in the high-potential zone.     

Nitrogen response of maize under temporary flooding

The adverse effect of temporary flooding on maize (Zea mays L.) yields and the nitrogen management required to mitigate the effect of flooding were studied for five years in field experiments on Choa sandy loam soil.Maize yields decreased with increase in duration of flooding and with decrease in the age of the crop at the time of flooding. Flooding periods exceeding 48 hours caused significant crop damage. The loss in yield on account of flooding was, however, less in 40-day old crops. A 24 hours flooding decreased grain yield by 17.7 and 3.9 per cent in 20-day and 40-day old crops respectively. Maximum yield loss amounted to 1.23 t ha^–1 of grains with 72 hours of flooding of 20-day old crop indicating that a younger crop is more prone to the deleterious effect of flooding.The nitrogen content of grains decreased significantly with increase in flooding period. A supplemental dose of 7 kg N ha^–1 as urea spray significantly increased grain yield. Soil application of supplemental nitrogen at the rate of 14 or 20 kg N ha^–1 enhanced the maize yield by 0.7 to 0.9 t ha^–1 under temporary flooded conditions. Spraying with urea solution increased nitrogen removed by the crop.Successive increments of 60 kg N ha^–1 gave an additional yields of 1.23, 1.01 and 0.41 t ha^–1 over the crop that received no nitrogen. Flooded maize responded to even higher rates of N fertilization than the dose of 120 kg N ha^–1 which is recommended in this region.     

The value of poultry manure for wetland rice grown in rotation with wheat

Poultry manure applied alone or in combination with urea at different N levels was evaluated as a N source for wetland rice grown in a Fatehpur loamy sand soil. Residual effects were studied on wheat which followed rice every year during the three cropping cycles. In the first year, poultry manure did not perform better than urea but by the third year, when applied in quantities sufficient to supply 120 and 180 kg N ha^–1, it produced significantly more rice grain yield than the same rates of N as urea. Poultry manure sustained the grain yield of rice during the three years while the yield decreased with urea. Apparent N recovery by rice decreased from 45 to 28% during 1987 to 1989 in the case of urea, but it remained almost the same (35, 33 and 37%) for poultry manure. Thus, urea N values of poultry manure calculated from yield or N uptake data following two different approaches averaged 80, 112 and 127% in 1987, 1988 and 1989, respectively. Poultry manure and urea applied in 1:1 ratio on N basis produced yields in between the yields from the two sources applied alone. After three cycles of rice-wheat rotation, the organic matter in the soil increased with the amount of manure applied to a plot. Olsen available P increased in soils amended with poultry manure. A residual effect of poultry manure applied to rice to supply 120 or 180 kg N ha^–1 was observed in the wheat which followed rice and it was equivalent to 40 kg N ha^–1 plus some P applied directly to wheat.     

Effects of different levels of chemical Nitrogen (urea) onAzolla and Blue-green algae intercropping with rice

Application of higher levels (60 and 90 kg N ha^–1) of nitrogen fertilizer (Urea) inhibited the growth ofAzolla pinnata (Bangkok) and blue-green algae (BGA) though the reduction was more in BGA thanAzolla. Inoculation of 500 kg ha^–1 of freshAzolla 10 days after transplanting (DAT) in the rice fields receiving 30, 60 and 90 kg N ha^–1 as urea produced an average of 16.5, 15.0 and 13.0 t ha^–1 fresh biomass ofAzolla at 30 DAT, which contained 31, 31 and 27 kg N ha^–1, respectively. The dry mixture of BGA (60%Aulosira, 35%Gloeotrichia and 5% other BGA on fresh weight basis) inoculated in rice field 3 DAT at a rate of 10 kg ha^–1 showed a mat formation at 80 DAT with an average fresh biomass of 8.0, 5.8 and 4.2 t ha^–1 containing 22, 17 and 12 kg N ha^–1, respectively with those N fertilizer doses.Application ofAzolla showed positive responses to rice crop by increasing the panicle number and weight, grain and straw yields and nitrogen uptake in rice significantly at all the levels of chemical nitrogen. But, the BGA inoculation had a significant effect on the grain and straw yields only during the dry season in the treatment where 30 kg N was applied. During the wet season and in the other treatments performed during the dry season no significant increase in yields, yield components and N uptake were observed with BGA.The intercropping ofAzolla and rice in combination with 30, 60 and 90 kg N ha^–1 as urea showed the yields, yield attributes and nitrogen uptake in rice at par with those obtained by applying 60, 90 and 120 kg N ha^–1 as urea, respectively but, the BGA did not. The analysis of soil from rice field after harvest showed thatAzolla and BGA intercropping with rice in combination with chemical fertilizer significantly increased the organic carbon, available phosphorus and total nitrogen of soil.     

Plant response to the management of fluid and solid N fertilizers applied to furrow-irrigated corn

The use of fluid fertilizers has increased in recent years. Plant response to field management practices of fluid and solid N fertilizers in furrow-irrigated field studies has not been well-documented. This research studied the response of corn (Zea mays L.) to several field management practices of fluid and solid N fertilizers applied at several rates. Corn grown with sidedressed applications of the fluid fertilizers, urea ammonium nitrate (UAN) and 18-0-0+7Ca, generally had higher grain yields, higher yield efficiencies, higher ear populations, larger seed size, more kernels per ear, and a higher ear leaf N concentration than corn grown with preplant broadcast treatments of urea, ammonium nitrate (AN), and UAN. In 1988, corn grown with 280 kg N ha^–1 of AN applied preplant broadcast had a lower grain yield, yield efficiency, kernels per ear, and ear leaf N concentration, while ear population and kernel size were unchanged, in comparison to split applications of UAN at 224 kg N ha^–1. In 1989, corn grown with three split applications of UAN at 280 kg N ha^–1 had a higher grain yield and produced more kernels per ear without affecting yield efficiency, ear population, kernel size, or ear leaf N concentration compared with treatments at the 224 kg N ha^–1 rate. Use of split, side-dressed N management practices in furrow-irrigated corn should eliminate the need to use excessive N rates while maintaining grain yields and other plant responses, resulting in more efficient N use than traditionally achieved.     

Fertilizer requirement of rice-wheat and maize-wheat rotations on coarse-textured soils amended with farmyard manure

Field experiments with rice-wheat rotation were conducted during five consecutive years on a coarse-textured low organic matter soil. By amending the soil with 12t FYM ha^–1, the yield of wetland rice in the absence of fertilizers was increased by 32 per cent. Application of 80 kg N ha^–1 as urea could increase the grain yield of rice equivalent to 120 kg N ha^–1 on the unamended soil. Although the soil under test was low in Olsen's P, rice did not respond to the application of phosphorus on both amended and unamended soils. For producing equivalent grain yield, fertilizer requirement of maize grown on soils amended with 6 and 12 t FYM ha^–1 could be reduced, respectively to 50 and 25 per cent of the dose recommended for unamended soil (120 kg N + 26.2 kg P + 25 kg K ha^–1). Grain yield of wheat grown after rice on soils amended with FYM was significantly higher than that obtained on unamended soil. In contrast, grain yield of wheat which followed maize did not differ significantly on amended or unamended soils.     

Effect of rates and sources of nitrogen application on yield and nutrient uptake of Citronella Java (Cymbopogon winterianus Jowitt)

Two field experiments were conducted for two crop cycles each of two years (1985–87 and 1986–88) on an entisols to study the effect of rate and sources of N application on yield and nutrient uptake of Citronella Java (Cymbopogon winterianus Jowitt). Fresh herbage and essential oil yields were significantly influenced by application of N up to 200 kg ha^–1 yr^–1, while tissue N concentration and N uptake increased only to 150 kg N ha^–1. The oil yields with Neem cake coated urea (urea granules coated with Neem cake) and urea super granules were 22 and 9% higher over that with prilled urea and urea supergranules were significantly increased up to 200 kg N ha^–1 while with Neem cake coated urea, response was observed only to 150 kg N ha^–1! Estimated recovery of N during two years from Neem cake coated urea, urea supergranules and prilled urea were 38, 31 and 21%, respectively.     

Initial and residual effects of nitrogen fertilizers on grain yield of a maize/bean intercrop grown on a Humic Nitosol and the fate and efficiency of the applied nitrogen

Initial and residual effects of nitrogen (N) fertilizers on grain yield of a maize/bean intercrop grown on a deep, well-drained Humic Nitosol (66% clay, 3% organic carbon) were evaluated. Enriched (¹⁵N) N fertilizer was used to study the fate of applied N in two seasons: using urea (banded) at 50 kg N ha^–1 in one season, and¹⁵N-enriched urea (banded), calcium ammonium nitrate (CAN, banded), and urea supergranules (USG, point placement) were applied in the other season (different field) at 100 kg N ha^–1. Nitrogen fertilizer significantly (P = 0.05) increased equivalent maize grain yield in each season of application with no significant differences between N sources, i.e., urea, CAN, and USG. Profitmaximizing rates ranged from 75 to 97 kg N ha^–1 and value: cost ratios ranged from 3.0 to 4.8. Urea gave the highest value: cost ratio in each season. Most (lowest measurement 81%) of the applied N was accounted for by analyzing the soil (to 150 cm depth) and plant material. Measurements for urea, CAN, and USG were not significantly different. The high N measurements suggest low losses of applied N fertilizer under the conditions of the study. Maize plant recovery ranged from 35 to 55%; most of this N (51–65%) was in the grain. Bean plant recovery ranged from 8 to 20%. About 34–43% of the applied N fertilizer remained in the soil, and most of it (about 70%) was within the top soil layer (0–30 cm). However, there were no significant equivalent maize grain increases in seasons following N application indicating no beneficial residual effect of the applied fertilizers.     

Ammonia volatilization from grassland receiving nitrogen fertilizer and rotationally grazed by dairy cattle

The micrometeorological mass balance method was used to measure ammonia (NH₃) volatilization from rotationally grazed swards throughout the 1987 and 1988 growing seasons. In both years the swards were dressed with calcium ammonium nitrate (CAN) split over 7 dressings. In 1987 the sward received a total of 550 kg N ha^–1, in 1988 a total of 550 or 250 kg N ha^–1. For the 550 kg N ha^–1 treatments there were 8 and 9 grazing cycles, respectively, in 1987 and 1988 and 7 for the 250 kg N ha^–1 treatment. Losses from the 550 N sward were 42.2 and 39.2 kg N ha^–1 in 1987 and 1988, respectively; this was equivalent to 8.5 and 7.7% of the N returned to the sward in the excreta of the grazing cattle. The NH₃ loss from the 250N sward was 8.1 kg N ha^–1 in 1988, which was equivalent to 3.1% of the N returned to the sward in excreta during the growing season. There was a wide variation in NH₃ volatilization between the individual grazing periods. This indicates the necessity of continued measurements throughout the growing season to obtain reliable data on NH₃ volatilization. Soil humidity is suggested to be a key factor, because emissions were high from wet soil, and low from drier soil. Results of a Monte Carlo simulation study showed that the measured NH₃ loss from the 250 and 550 N swards had a standard deviation of 13 and 5% of the mean, respectively.     

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.     

A farm-level evaluation of nitrogen and phosphorus fertilizer use and planting density for pearl millet production in Niger

Mineral fertilizer use is increasing in West Africa though little information is available on yield response in farmers' fields. Farmers in this region plant at low density (average 5,000 pockets ha^–1, 3 plants pocket^–1), which can affect fertilizer use efficiency. A study was conducted with 20 farmers in Niger to assess the response of pearl millet [Pennisetum glaucum (L.) R. Br.] to phosphorus and nitrogen fertilizers under farm conditions. In each field, treatments included control, single superphosphate (SSP) only, SSP plus N (point placed near plant), and either SSP or partially acidulated phosphate rock (PAPR) plus N broadcast. N and P were applied at 30 kg N ha^–1 and 30 kg P₂O₅ ha^–1. Farmers were allowed to plant, weed, etc., as they wished and they planted at densities ranging from 2,000 to 12,000 pockets ha^–1. In the absence of fertilizer, increasing density from 2,000 to 7,000 pockets ha^–1 increased yield by 400%. A strong interaction was found between fertilizer use and density. Farmers planting at densities less than 3,500 pockets ha^–1 had average yields of 317 kg grain ha^–1 while those planting at densities higher than 6,500 pockets ha^–1 showed average yields of 977 grain ha^–1. Though phosphate alone increased yields significantly at all densities, little response to fertilizer N was found at densities below 6,000 pockets ha^–1. Significant residual responses in 1987 and 1988 were found to P applied in high-density plots in 1986. Depending on fertilizer and grain prices, analysis showed that fertilizer use must be be combined with high plant density (10,000 pockets ha^–1) or no economic benefit from fertilizer use will be realized.     

Response of dryland wheat to fertilizer nitrogen in relation to stored water,rainfall and residual farm yard manure

Yield response of dryland wheat to fertilizer N application in relation to components of seasonal water (available soil moisture and rainfall) and residual farm yard manure (FYM) was studied for five years (1983–84 to 1987–88) on a maize-wheat sequence on sandy loam soils in Hoshiarpur district of Punjab, India. Four rates of N viz. 0, 40, 60 and 80 kg ha^–1 in wheat were superimposed on two residual FYM treatments viz. no FYM (F₀) and 15 t ha^–1 (F₁₅) to preceding maize. FYM application to maize increased the residual NO₃-N content by 19–30 kg ha^–1 in the 180 cm soil profile. For a given moisture distribution, F₁₅ increased attainable yields. Over the years, F₁₅ increased wheat yield by 230 to 520 kg ha^–1. Response to fertilizer N was lower in FYM amended plots than in unamended plots. Available soil moisture at wheat seeding and amount and distribution of rainfall during the vegetative and the reproductive phases of crop development affected N use efficiency by wheat. Available soil moisture at seeding alone accounted for 50% variation in yield. The residual effect of FYM on wheat yield could be accounted for by considering NO₃-N in 180 cm soil profile at seeding. The NO₃-N and available soil moisture at wheat seeding along with split rainfall for two main phases of crop development and fertilizer N accounted for 96% variation in wheat yield across years and FYM treatments.     

Effectiveness of fall- versus spring-applied urea on barley

Field experiments were conducted in north-central and central Alberta to determine the effect of pellet size and depth of placement on yield and N uptake of barley from fall- and spring-applied urea. The application rate was 56 kg N ha^–1. Fall incorporated commercial urea (0.01 g) gave 792 kg ha^–1 lower yield and 15 kg ha^–1 less N uptake than similarly applied commercial urea in spring on the average for the five experiments. The effectiveness of fall-applied N tended to be greater with large urea pellets (2.5 g), especially when they were placed 15 cm deep. Specifically, the relative yield efficiency of fallversus spring-applied N was 77% when the larger pellets were placed 4 cm deep and 95% when placed 15 cm deep. However, large pellets were less effective than commercial urea when both were applied in spring at sowing or two weeks before.     

Effect of nitrification inhibitors on herb and essential oil yield of Japanese mint on sandy soil

Pyrethrum (Chrysanthemum cinerariefolium) flowers have been observed to have insecticidal properties and could be used as an indigenous nitrification inhibitor for increasing N-use efficiency. A field experiment was conducted at the Central Institute of Medicinal and Aromatic Plants, Lucknow, India during 1988 and 1989 to evaluate the relative performance of pyrethrum flower waste and Dicyandiamide (DCD) as nitrification inhibitors applied with prilled urea (PU) to Japanese mint (Mentha arvensis L.). The results revealed that application of the nitrification inhibitors with prilled urea significantly increased the herb and essential oil yield of the crop compared to that of prilled urea alone. Addition of Dicyandiamide and pyrethrum flower waste gave 30 and 23% more herb yield than prilled urea alone, the corresponding increase in oil yield being 27 and 22%, respectively. Application of nitrogen at 200 kg ha^–1 in dicayndiamide or pyrethrum flower waste treated soil significantly enhanced the herb and essential oil yields and N-uptake by the crop to more than that for 300 kg N ha^–1 with prilled urea. Both the materials improved the N use efficiency by one and half time as compared to that with PU at 100 kg N ha^–1. The results indicate pyrethrum flower dust can be effectively used as a potential nitrification inhibitor.     

Response of three sorghum varieties to N supply and plant density in a tropical environment

Field trials were conducted at Samaru, Nigeria to investigate the growth, yield and grain quality response of three grain sorghum (Sorghum bicolor L. Moench) varieties (L. 187, SK 5912 and FFBL) to N fertilization under varying plant densities (33300, 50000 or 66600 plants ha^–1). Year × N interactions were significant for yield components and so were variety × N and variety × plant density interactions. Grain yield increased 41, 42, and 126% with application of 60 kg N ha^–1, the optimum N rate, a response which was associated with variations in grain weight per panicle, panicle weight and grain number. Varieties SK 5912 and FFBL produced more straw in response to added N than did var. L. 187 while yield components in var. SK 5912 and L.187 responded better than those in var. FFBL. Yield components declined in var. SK 5912 and L. 187 as plant density was increased to either 50000 or 66600 plants ha^–1. Grain crude protein (CP) content and protein yield were increased 8 and 52% respectively by 60 kg N ha^–1 but CP content declined as plant density was increased. Grain tannin content was virtually unaffected by increasing N supply. Optimum plant density for grain sorghum production in this environment is in the range of 50000 plants ha^–1.     

Effect of urea management on yield and N use efficiency of lowland rice on an acid sulfate soil

Field experiments were conducted during 1988–1989 at two adjacent sites on an acid sulfate soil (Sulfic Tropaquept) in Thailand to determine the influence of urea fertilization practices on lowland rice yield and N use efficiency. Almost all the unhydrolyzed urea completely disappeared from the floodwater within 8 to 10 d following urea application. A maximum partial pressure of ammonia (pNH₃) value of 0.14 Pa and an elevation in floodwater pH to about 7.5 following urea application suggest that appreciable loss of NH₃ could occur from this soil if wind speeds were favorable. Grain yields and N uptake were significantly increased with applied N over the control and affected by urea fertilization practices (4.7–5.7 Mg ha^–1 in dry season and 3.0–4.1 Mg ha^–1 in wet season). In terms of both grain yield and N uptake, incorporation treatments of urea as well as urea broadcasting onto drained soil followed by flooding 2 d later were more effective than the treatments in which the same fertilizer was broadcast directly into the floodwater either shortly or 10 d after transplanting (DT). The¹⁵N balance studies conducted in the wet season showed that N losses could be reduced to 31% of applied N by broadcasting of urea onto drained soil and flooding 2 d later compared with 52% loss by broadcasting of urea into floodwater at 10 DT. Gaseous N loss via NH₃ volatilization was probably responsible for the poor efficiency of broadcast urea in this study.     

The effect of fertilizer placement on nitrogen uptake and yield of wheat and maize in Chinese loess soils

Field trials were carried out to study the fate of¹⁵N-labelled urea applied to summer maize and winter wheat in loess soils in Shaanxi Province, north-west China. In the maize experiment, nitrogen was applied at rates of 0 or 210 kg N ha^–1, either as a surface application, mixed uniformly with the top 0.15 m of soil, or placed in holes 0.1 m deep adjacent to each plant and then covered with soil. In the wheat experiment, nitrogen was applied at rates of 0, 75 or 150 kg N ha^–1, either to the surface, or incorporated by mixing with the top 0.15 m, or placed in a band at 0.15 m depth. Measurements were made of crop N uptake, residual fertilizer N and soil mineral N. The total above-ground dry matter yield of maize varied between 7.6 and 11.9 t ha^–1. The crop recovery of fertilizer N following point placement was 25% of that applied, which was higher than that from the surface application (18%) or incorporation by mixing (18%). The total grain yield of wheat varied between 4.3 and 4.7 t ha^–1. In the surface applications, the recovery of fertilizer-derived nitrogen (25%) was considerably lower than that from the mixing treatments and banded placements (33 and 36%). The fertilizer N application rate had a significant effect on grain and total dry matter yield, as well as on total N uptake and grain N contents. The main mechanism for loss of N appeared to be by ammonia volatilization, rather than leaching. High mineral N concentrations remained in the soil at harvest, following both crops, demonstrating a potential for significant reductions in N application rates without associated loss in yield.