Effects of improved drainage and nitrogen source on yields, nutrient uptake and utilization efficiencies by maize (Zea mays L.) on Vertisols in sub-humid environments

Nitrogen is the most limiting plant nutrient in Vertisols in Kenya. Soil properties, climatic conditions and management factors as well as fertilizer characteristics can influence fertilizer nitrogen (N) use efficiency by crops. Vertisols, characterized by low-basic water infiltration rate, are prone to waterlogging under sub-humid and humid conditions. We determined effects of drainage, N source and time of application on yields, nutrient uptake and utilization efficiencies by maize grown on Vertisols in sub-humid environments. Treatments comprised two furrows (40 cm and 60 cm deep) and a check (i.e., no furrow), calcium nitrate to furnish NO₃-N, ammonium sulphate to supply NH₄-N at 100 kg N ha⁻¹, a control (i.e., no fertilizer N), and fertilizer N application at sowing, 40 days after sowing, and split (i.e., half the rate at sowing and half 40 days after sowing). A split-plot design was used in which drainage formed the main plots and N source × time of N application formed the sub-plots. Higher grain and total dry matter yields, harvest index, leaf N content, uptake of N, P and K, as well as N agronomic (NAE) and recovery (NRE) efficiencies were obtained from drained compared to undrained plots. The increase ingrain yields as a result of drainage varied from 31 to 45% for control, 35 to 43% for NO₃-N, and 16 to 21% for NH₄-N treatments. Drainage resulted in total N uptake increases from 50 to 80 kg N ha⁻¹ in control plots, 80 to 130 kg N ha⁻¹ in NO₃-N treated plots, and 90 to 130kg N ha⁻¹ in NH₄-N treated plots. Ammonium-N source was superior to NO₃-N source in terms of higher yields, NAE, and NRE in undrained plots, but the two N sources behaved similarly in drained plots. Delayed or split NO₃-N application gave higher yields, NAE and NRE than when all N was applied at sowing in undrained plots. There was no difference between 40 cm and 60 cm deep furrows in terms of crop yields and nutrient use efficiencies. Thus, draining excess water with furrows at least 40 cm deep is essential for successful crop production in these Vertisols under sub-humid conditions. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of planting technique and amendment type on pearl millet yield, nutrient uptake, and water use on degraded land in Niger

Due to increased population pressure and limited availability of fertile land, farmers on desert fringes increasingly rely on marginal land for agricultural production, which they have learned to rehabilitate with different technologies for soils and water conservation. One such method is the indigenous zai technique used in the Sahel. It combines water harvesting and targeted application of organic amendments by the use of small pits dug into the hardened soil. To study the resource use efficiency of this technique, experiments were conducted 1999–2000, on-station at ICRISAT in Niger, and on-farm at two locations on degraded lands. On-station, the effect of application rate of millet straw and cattle manure on millet dry matter production was studied. On-farm, the effects of organic amendment type (millet straw and cattle manure, at the rate of 300 g per plant) and water harvesting (with and without water harvesting) on millet grain yield, dry matter production, and water use were studied. First, the comparison of zai vs. flat planting, both unamended, resulted in a 3- to 4-fold (in one case, even 19-fold) increase in grain yield on-farm in both years, which points to the yield effects of improved water harvesting in the zai alone. Zai improved the water use efficiency by a factor of about 2. The yields increased further with the application of organic amendments. Manure resulted in 2–68 times better grain yields than no amendment and 2–7 times better grain yields than millet straw (higher on the more degraded soils). Millet dry matter produced per unit of manure N or K was higher than that of millet straw, a tendency that was similar for all rates of application. Zai improved nutrient uptake in the range of 43–64% for N, 50–87% for P and 58–66% for K. Zai increased grain yield produced per unit N (8 vs. 5 kg kg⁻¹) and K (10 vs. 6 kg kg⁻¹) compared to flat; so is the effect of cattle manure compared to millet straw (9 vs. 4 kg kg⁻¹, and 14 vs. 3 kg kg⁻¹), respectively, Therefore zai shows a good potential for increasing agronomic efficiency and nutrient use efficiency. Increasing the rate of cattle manure application from 1 to 3 t ha⁻¹ increased the yield by 115% TDM, but increasing the manure application rate further from 3 to 5 t ha⁻¹ only gave an additional 12% yield increase, which shows that optimum application rates are around 3t ha⁻¹.     

Effect of Continuous use of Sodic Irrigation Water with and without Gypsum, Farmyard manure, Pressmud and Fertilizer on Soil Properties and Yields of Rice and Wheat in a Long term Experiment

A long term field experiment was conducted for 8 years during 1994–2001 to evaluate the effect of N, P, K and Zn fertilizer use alone and in combination with gypsum, farmyard manure (FYM) and pressmud on changes in soil properties and yields of rice and wheat under continuous use of sodic irrigation water (residual sodium carbonate (RSC) 8.5 meq l⁻¹, and sodium adsorption ratio (SAR) 8.8 (m mol/l)^1/2 at Bhaini Majra experimental farm of Central Soil Salinity Research Institute, Karnal, India. Continuous use of fertilizer N alone (120 kg ha⁻¹) or in combination with P and K significantly improved rice and wheat yields over control (no fertilizer). Phosphorus applied at the rate of 26 kg P ha⁻¹ each to rice and wheat significantly improved the yields and led to a considerable build up in available soil P. When N alone was applied, available soil P and K declined from the initial level of 14.8 and 275 kg ha⁻¹ to 8.5 and 250 kg ha⁻¹ respectively. Potassium applied at a rate of 42 kg K ha⁻¹ to both crops had no effect on yields. Response of rice to Zinc application occurred since 1997 when DTPA extractable Zn declined to 1.48 kg ha⁻¹ from the initial level of 1.99 kg ha⁻¹. Farmyard manure 10 Mg ha⁻¹, gypsum 5 Mg ha⁻¹ and pressmud 10 Mg ha⁻¹ along with NPK fertilizer use significantly enhanced yields over NPK treatment alone. Continuous cropping with sodic water and inorganic fertilizer use for 8 years slightly decreased the soil pH_e and SAR from the initial value of 8.6 and 29.0 to 8.50 and 18.7 respectively. However, treatments involving the use of gypsum, FYM and pressmud significantly decreased the soil pH and SAR over inorganic fertilizer treatments and control. Nitrogen, phosphorus and zinc uptake were far less than additions made by fertilizer. The actual soil N balance was much lower than the expected balance thereby indicating large losses of N from the soil. There was a negative potassium balance due to greater removal by the crops when compared to K additions. The results suggest that either gypsum or FYM/pressmud along with recommended dose of fertilizers must be used to sustain the productivity of rice – wheat system in areas having sodic ground water for irrigation.     

Millet nutrient use efficiency as affected by natural soil fertility,mineral fertilizer use and rainfall in the West African Sahel

Field experiments were designed to investigate the effectiveness of integrated soil fertility management (ISFM), comparing fertilizer use efficiency and its impact on millet, cultivated close to the homestead (“infields”) and away from the homestead (“outfields”). Millet yields and response to N (0, 30, and 60 kg ha⁻¹) and P (0, 15, and 30 kg ha⁻¹) were determined on nine infields and nine outfields over a period of 3 years (from 1999 to 2001) in the southern Sahel of Niger. Rainfall was 650, 470, and 370 mm during the three successive years, interaction between decreasing rainfall and millet yield performance was also analyzed. While soil organic carbon (1.5 g kg⁻¹ on outfields and 1.6 g kg⁻¹ on infields) and pH-H₂O (4.8 on outfields and 5.1 on infields) were comparable, total-N, plant available P (measured as P-Olsen and P-Bray), and exchangeable Ca, K, and Mg levels were higher on infields as compared to outfields. Without fertilizer, average grain yield (GY) and stover yield obtained on infields were three times as high as on outfields. GY across years and fertilizer treatments was higher on infields as compared to outfields (P < 0.001). Average yield was 800 kg ha⁻¹ on outfields and 1,360 kg ha⁻¹ on infields (P < 0.001). On outfields, average GY was stagnant over the 3-year experimental period. Despite declining rainfall, millet GY across all treatments gradually increased over time on infields (P < 0.001). P fertilization alone resulted on both field types to steadily and substantial yield increases while yield response to N fertilization was only obvious when fertilizer P was applied. With no fertilizer applied, N uptake on infields (19 kg N ha⁻¹) was more than twice as high as on outfields (7 kg ha⁻¹), and P uptake was four times higher on infields (3 kg ha⁻¹) than on outfields (0.8 kg ha⁻¹). Indigenous soil N supply was on average 24 kg N ha⁻¹ on outfields and 46 kg N ha⁻¹ on infields. Average value for indigenous soil P supply was 4 kg P ha⁻¹ on infields and 2 kg ha⁻¹ on outfields. Apparent recovery of fertilizer N applied varied considerably among treatments and ranged from 17 to 23% on outfields and 34 to 37% on infields (P < 0.001). Average apparent recovery of fertilizer P applied was significantly higher (P < 0.001) on infields (31%) than on outfields (18%) over the 3-year growing period, illustrating ISFM-induced positive effect on millet nutrient N and P use. Results indicate higher inherent soil fertility, underline ISFM-induced drought tolerance of soils on infields as compared to outfields, and highlight the crucial role of fertilizer P (especially on outfields) for millet production. These call for site-specific nutrient management and support, even under low rainfall conditions, the potential value of fertile infields for efficient and productive external input use and sustainable millet production in West African Sahel.     

Raising the productivity of smallholder farms under semi-arid conditions by use of small doses of manure and nitrogen: a case of participatory research

Participatory on-farm trials were conducted for three seasons to assess the benefits of small rates of manure and nitrogen fertilizer on maize grain yield in semi-arid Tsholotsho, Zimbabwe. Two farmer resource groups conducted trials based on available amounts of manure, 3 t ha⁻¹ (low resource group) and 6 t ha⁻¹ (high resource group). Maize yields varied between 0.15 t ha⁻¹ and 4.28 t ha⁻¹ and both absolute yields and response to manure were strongly related to rainfall received across seasons (P < 0.001). The first two seasons were dry while the third season received above average rainfall. Maize yields within the seasons were strongly related to N applied (R ² = 0.77 in season 1, and R ² = 0.88 and 0.83 in season 3) and other beneficial effects of manure, possibly availability of cations and P. In the 2001–2002 season (total rainfall 478 mm), application of 3 and 6 t ha⁻¹ of manure in combination with N fertilizer increased grain yield by about 0.14 and 0.18 t ha⁻¹, respectively. The trend was similar for the high resource group in 2002–2003 although the season was very dry (334 mm). In 2003–2004, with good rainfall (672 mm), grain yields were high even for the control plots (average 1.2 and 2.7 t ha⁻¹). Maize yields due to manure applications at 3 and 6 t ha⁻¹ were 1.96 and 3.44 t ha⁻¹, respectively. Application of 8.5 kg N ha⁻¹ increased yields to 2.5 t ha⁻¹ with 3 t ha⁻¹ of manure, and to 4.28 t ha⁻¹ with 6 t ha⁻¹ of manure. In this area farmers do not traditionally use either manure or fertilizer on their crops, but they actively participated in this research during three consecutive seasons and were positive about using the outcomes of the research in future. The results showed that there is potential to improve livelihoods of smallholder farmers through the use of small rates of manure and N under semi-arid conditions.     

Managing Legume Cover Crops and their Residues to Enhance Productivity of Degraded Soils in the Humid Tropics: A Case Study in Bukoba District,Tanzania

In degraded soils, establishment of soil-improving legumes can be problematic and requires investment of labour and other resources. We investigated various aspects of managing herbaceous legumes in farmers’ fields in Bukoba District, Tanzania. Biomass and N accumulation by Crotalaria grahamiana was 1.1 Mg ha⁻¹ and 34 kg N ha⁻¹ when established without farmyard manure (FYM) and 3.0 Mg ha⁻¹ and 95 kg N ha⁻¹ when established with 2 Mg FYM ha⁻¹, and incorporation of the biomass gave an increment of 700 kg ha⁻¹ of grain in the subsequent maize crop. Maize grain yield at different application rates of Tephrosia candida residues ranged from 1.4 to 3.3 Mg ha⁻¹ and from 2.0 to 2.8 Mg ha⁻¹ in the high and low rainfall zone, respectively. Application of tephrosia biomass at a rate of 2 Mg ha⁻¹ had no significant effect on maize yield whereas rates of 4, 6 and 8 Mg ha⁻¹ gave comparable yields. Apparent N recovery efficiencies at all rates of tephrosia residues were maximally 27 and 13% for the high and low rainfall zones, respectively. Mulching with Mucuna pruriens suppressed weeds by 49 and 68% and increased maize yield by 57 and 103% compared with the weedy fallow in the respective zones. Incorporated residues had a weaker effect on suppressing weeds and poor labour productivity (2 l and 36 kg grain person-day⁻¹) compared with mulched residues (32 and 52 kg grain person-day⁻¹) in the high and low rainfall zone, respectively. These results indicate that if well managed, legume residues have the potential to increase yields of subsequent maize crops on degraded soils.     

Nutrient flows in smallholder production systems in the humid forest zone of southern Cameroon

The flows and balances of N, P and K were studied in 20 farms in the Campo Ma’an area in Cameroon between March and August 2002 to assess the nutrient dynamics in smallholder farms. Data were collected through farmer interviews, field measurements and estimates from transfer functions. Nutrient input from mineral (IN1), animal feed (IN2a) and inorganic amendments (IN2b) was absent. Major outputs were through crop (OUT1a) and animal (OUT1b) products sold. Partial budgets for farmer managed flows were negative: −65 kg N, −5.5 kg P and −30.8 kg K ha⁻¹ year⁻¹. For inflows not managed by farmers, deep capture (IN6) was the major source: 16.6, 1.4 and 6.6 kg ha⁻¹ year⁻¹ of N, P and K, respectively. Atmospheric deposition (IN3) was estimated at 4.3 kg N, 1.0 kg P and 3.9 kg K ha⁻¹ year⁻¹, and biological nitrogen fixation (IN4) at 6.9 kg N ha⁻¹ year⁻¹. Major losses were leaching (OUT 3a): 26.4 kg N, and 0.88 kg K ha⁻¹ year⁻¹. Gaseous losses from the soil (OUT 4a) were estimated at 6.34 kg N, and human faeces (OUT 6) were estimated at 4 kg N, 0.64 kg P and 4.8 kg K ha⁻¹ year⁻¹. The highest losses were from burning (OUT 4c), i.e. 47.8 kg N, 1.8 kg P and 14.3 kg K ha⁻¹ year⁻¹. Partial budgets of environmentally controlled flows were negative only for N −4.8 kg N, +2.4 kg P and +9.6 kg K ha⁻¹ year⁻¹. The overall farm budgets were negative, with annual losses of 69 kg N, 3 kg P and 21 kg K ha⁻¹. Only cocoa had a positive nutrient balance: +9.3 kg N, +1.4 kg P and +7.6 kg K ha⁻¹ year⁻¹. Nutrients reaching the household waste (1.9 kg N, 2.8 kg P and 18.8 kg K ha⁻¹ year⁻¹), animal manure (4.9 kg N, 0.4 kg P and 1.6 kg K), and human faeces (4 kg N, 0.64 kg P and 4.8 kg K ha⁻¹ year⁻¹) were not recycled. Five alternative management scenarios were envisaged to improve the nutrient balances. Recycling animal manure, household waste and human faeces will bring the balance at −62.6 kg N, 0 kg P and +1 kg K ha⁻¹ year⁻¹. If, additionally, burning could be avoided, positive nutrient balances could be expected.     

Quantifying the effect of soil organic matter on indigenous soil N supply and wheat productivity in semiarid sub-tropical India

A field experiment was conducted on a loamy sand soil for six years to quantify the effect of soil organic matter on indigenous soil N supply and productivity of irrigated wheat in semiarid sub-tropical India. The experiment was conducted by applying different combinations of fertilizer N (0–180 kg N ha⁻¹), P (0–39 kg P ha⁻¹) and K (0–60 kg K ha⁻¹) to wheat each year. For the data pooled over years, fertilizer N together with soil organic carbon (SOC) and their interaction accounted for 75% variation in wheat yield. The amount of fertilizer N required to attain a yield goal decreased as the SOC concentration increased indicating enhanced indigenous soil N supply with an increase in SOC concentration. Besides SOC concentration, the soil N supply also depended on yield goal. For a yield goal of 4 tons ha⁻¹, each ton of SOC in the 15 cm plough layer contributed 4.75 kg N ha⁻¹ towards indigenous soil N supply. An increase in the soil N supply with increase in SOC resulted in enhanced wheat productivity. The contribution of 1 ton SOC ha⁻¹ to wheat productivity ranged from 15 to 33 kg ha⁻¹ across SOC concentration ranging from 3 to 9 g kg^-1 soil. The wheat productivity per ton of organic carbon declined curvilinearly as the native SOC concentration increased. The change in wheat productivity with SOC concentration shows that the effect of additional C sequestration on wheat productivity will depend on the existing SOC concentration, being higher in low SOC soils. Therefore, it will be more beneficial to sequester C in soils with low SOC than with relatively greater SOC concentration. In situations where the availability of organic resources for recycling is limited, their application may be preferred in soils with low SOC concentration. The results show that an increase in C sequestration will result in enhanced wheat productivity but the increase will depend on the amount of fertilizer applied and the existing fertility level of the soil.     

On-farm evaluation of leaf color chart for need-based nitrogen management in irrigated transplanted rice in northwestern India

Nitrogen use efficiency (NUE) in rice is low due to the inefficient management of fertilizer N by farmers. We evaluated a leaf color chart (LCC) as a simple tool for improving the time and rate of N fertilizer use in farmers’ fields for 4 years (2000–2003) in irrigated rice in northwestern India. Application of N fertilizer whenever leaf greenness was less than shade 4 on the LCC (the critical LCC value) produced rice grain yields on a par with blanket recommendation of applying 120 kg N ha⁻¹ in three equal splits in different years, but it resulted in an average saving of 26% fertilizer N across villages and seasons. In most situations, there was no significant advantage of applying 20 kg N ha⁻¹ as basal N at transplanting on grain yield and NUE of rice compared with no basal N. Use efficiencies of fertilizer N were higher when N was applied using LCC with a critical value of 4 than the recommended practice of applying 120 kg N ha⁻¹ in three equal split doses on all sites and in all years. The LCC with a critical value of 4 for real-time N management can be efficiently used to increase NUE in all types of inbred rice cultivars presently popular with the farmers of the Indian Punjab. The LCC is a cheap and easy-to-use tool that allows real-time N management by farmers on a large area leading to improved fertilizer N use efficiency, and reduced risks associated with fertilizer N application.     

Potential nutrient supply, nutrient utilization efficiencies, fertilizer recovery rates and maize yield in northern Nigeria

Potential N (SN) and P (SP) supplies, N and P utilization efficiencies and fertilizer recovery rates for the northern Guinea Savanna (NGS) agro-ecological zone of Nigeria were derived from data collected on farmers’ fields, and used as input in the QUantitative Evaluation of the Fertility of Tropical Soils (QUEFTS) model. The potential N supply ranged from 7 to 56 kg N ha⁻¹, with a mean of 25 kg N ha⁻¹, while SP ranged from 2 to 12 kg P ha⁻¹ with a mean of 5 kg P ha⁻¹. Both SN (CV = 42%) and SP (CV = 57%) were highly variable between farmers’ fields. Deriving potential nutrient supply from ‘a’ values gives lower estimates. The empirical equation in QUEFTS that estimates SN ( ) sufficiently predicted the SN of soils in the NGS (RMSE = 8.0 kg N ha⁻¹ index of agreement (IOA) = 0.81). The SP equation () predicted moderately potential P supply (RMSE = 6.80 kg P ha⁻¹, IOA = 0.54). When N or P is maximally accumulated in the plant (i.e., least efficiently utilized), the utilization efficiency was 21 kg grain kg⁻¹ N taken up and 97 kg grain kg⁻¹ P taken up. When these nutrients were maximally diluted in the plant (i.e., most efficiently utilized), the utilization efficiency was 70 kg grain kg⁻¹ N taken up and 600 kg grain kg⁻¹ P taken up. The range in N recovery fraction (NRF) of N fertilizer applied was from 0.30 to 0.57, with a mean of 0.39, while the P recovery fraction (PRF) ranged from 0.10 to 0.66 with a mean of 0.24. Although SP was moderately predicted, when QUEFTS model input parameters were adjusted for the NGS, the model sufficiently (IOA = 0.83, RMSE = 607 kg DM ha⁻¹) estimated maize grain yield in the NGS of Nigeria. The original QUEFTS model however, gave better predictions of maize grain yield as reflected by the lower RMSE (IOA = 0.84, RMSE = 549 kg DM ha⁻¹). Consequently, QUEFTS is a simple and efficient tool for making yield predictions in the NGS of northern Nigeria.     

Comparative short-term effects of different quality organic resources on maize productivity under two different environments in Zimbabwe

Major challenges for combined use of organic and mineral nutrient sources in smallholder agriculture include variable type and quality of the resources, their limited availability, timing of their relative application and the proportions at which the two should be combined. Short-term nutrient supply capacity of five different quality organic resources ranging from high to low quality, namely Crotalaria juncea, Calliandra calothyrsus, cattle manure, maize stover and Pinus patula sawdust were tested in the field using maize as a test crop. The study was conducted on two contrasting soil types at Makoholi and Domboshawa, which fall under different agro-ecological regions of Zimbabwe. Makoholi is a semi-arid area (<650 mm yr⁻¹) with predominantly coarse sandy soils containing approximately 90 g kg⁻¹ clay while Domboshawa (>750 mm yr⁻¹) soils are sandy-clay loams with 220 g kg⁻¹ clay. Each organic resource treatment was applied at low (2.5 t C ha⁻¹) and high (7.5 t C ha⁻¹) biomass rates at each site. Each plot was sub-divided into two with one half receiving 120 kg N ha⁻¹ against zero in the other. At Makoholi, there was a nine-fold increase in maize grain yield under high application rates of C. juncea over the unfertilized control, which yielded only 0.4 t ha⁻¹. Combinations of mineral N fertilizer with the leguminous resources and manure resulted in between 24% and 104% increase in grain yield against sole fertilizer, implying an increased nutrient recovery by maize under organic–mineral combinations. Maize biomass measured at 2 weeks after crop emergence already showed treatment differences, with biomass yields increasing linearly with soil mineral N availability (R ² = 0.75). This 2-week maize biomass in turn gave a positive linear relationship (R ² = 0.82) with grain yield suggesting that early season soil mineral N availability largely determined final yield. For low quality resources of maize stover and sawdust, application of mineral N fertilizer resulted in at least a seven-fold grain yield increase compared with sole application of the organic resources. Such nutrient combinations resulted in grain harvest indices of between 44% and 48%, up from a mean of 35% for sole application, suggesting the potential of increasing maize productivity from combinations of low quality resources with mineral fertilizer under depleted sandy soils. At Domboshawa, grain yields averaged 7 t ha⁻¹ and did not show any significant treatment differences. This was attributed to relatively high levels of fertility under the sandy-clay loams during this first year of the trial implementation. Differences in N supply by different resources were only revealed in grain and stover uptake. Grain N concentration from the high quality leguminous resources averaged 2% against 1.5% from sawdust treatments. We conclude that early season soil mineral N availability is the primary regulatory factor for maize productivity obtainable under poor sandy soils. Maize biomass at 2 weeks is a potential tool for early season assessment of potential yields under constrained environments. However, the likely impact on system productivity following repeated application of high N-containing organic materials on different soil types remains poorly understood.     

Effects of timing of nitrogen and sulphur fertilizers on yield, nitrogen, and sulphur contents of Tef (Eragrostis tef (Zucc.) Trotter)

Nitrogen use efficiency (NUE) of tef, a major staple crop in Ethiopia, is very low, either caused by untimely use of nitrogen (N) fertilizers or lack of other essential nutrients like sulphur (S). The average grain yield of this crop is low, averaging <0.8 Mg ha⁻¹ in farmer’s fields of the semi-arid conditions. Therefore, the present study was conducted to see the effect of the timing of combined N and S fertilization on the yield, yield components, and N and S concentration in the plant parts of the crop. A factorial combination of three rates of N (0, 70, and 105 kg ha⁻¹) with four rates of S (0, 16, 32, and 48 kg ha⁻¹) was applied in randomized complete blocks in three replications. The experiment was carried out in the 2004 and 2005 cropping seasons in the Cambisols of the semi-arid area of Ethiopia. The crop responded significantly (P < 0.05) to both split (one-third at planting and two-thirds at late tillering) and whole (all at planting) N and S applications and years. Combined N and S fertilization increased the dry matter (DM) and grain yields on average by 1.7 and 0.3 Mg ha⁻¹, compared with the control. Similarly, S fertilization increased the NUE of the tef crop by 36%. Nitrogen concentration of shoots was found to significantly increase with S application (P < 0.05), with strong positive interactions both in the split and whole applications. The sulphur increase in grains was significant with N rates for both applications, with significant interaction effects observed for the split application in both cropping seasons. Split application resulted in 0.9 and 0.3 Mg ha⁻¹ significant increase in DM and grain yields, averaged for both years and treatments compared with the whole application. Similar significant increases were observed for panicle yield, NUE, and shoot and grain N and S concentrations. The average N:S ratio in grains was 10.6:1. Significant (P < 0.05) yearly variations were also observed. Dry matter and grain yields of 2005 were higher on average by 2.10 and 0.32 Mg ha⁻¹ than those of the 2004 cropping season. The percentage of N and S concentrations of grains, averaged for both applications, were higher by 13 and 9% in 2004; even though the N and S uptakes of 2005 were higher on average by 5.0 and 0.5 kg ha⁻¹ than those of the 2004 cropping season. This work showed that the yield response and NUE of the tef crop could be improved with split N and S fertilizer applications, with tef-producing farmers benefitting from the application of S-containing N fertilizers to soils deficient in these nutrients.     

Effects of fertilization on nutrient budget and nitrogen use efficiency of farmland soil under different precipitations in Northeastern China

Based on a consecutive 16-year field trial and meteorological data, the effects of fertilization on the nutrient budget and nitrogen use efficiency in farmland soil under different precipitation years were studied. With no fertilization treatment, the grain yield of maize was 3,520 kg ha⁻¹ (mean yield over 13 years). But the maximum yield increased to 7,470 kg ha⁻¹ when treated with mineral N, P and K fertilizers and recycled manure. The nutrient uptake also increased by twofold to threefold in NPKM treated field compared with that in the control treatment. The highest yields were obtained in years with normal precipitation, despite the different fertilization schemes. The lowest yields were obtained in drought or waterlogging years, which were 44.7–58.5% of the yields in years with normal precipitation. It also appeared that the deficits of N, P and K were greater in the years with proper precipitation than those in arid or flood years, because more production was removed from the field. Soil total N decreased significantly when treated with mineral fertilizer or recycled manure alone. The maximum deficit of soil total N was observed in control treatment (557 kg ha⁻¹) from 1990 to 2005. The N treatment resulted in a significant negative balance of P, due to the high yield of the crop in response to applied N. The application of NP or N to soils resulted in a greater negative K balance than that of the control. The greatest negative balance of total P and available P were obtained under the control and N treatment, and the highest deficit of soil total K and exchangeable K were obtained under NP treatment. We found that the rate of 150 kg N ha⁻¹ year⁻¹ was inadequate for maintaining soil N balance, and amendment of soil with organic source could not stop the loss of soil P and K. The applying rates of 150 kg N ha⁻¹ year⁻¹, 25 kg P ha⁻¹ year⁻¹, and 60 kg K ha⁻¹ year⁻¹ combined with 2–3 t ha⁻¹ organic manure were recommended to maintain soil fertility level. The nitrogen use efficiency (NUE) was greatly improved in the years with proper precipitation and balanced fertilization. Higher NUE and grain yields were achieved under NPK and NPKM treatments in years with normal precipitation. The results clearly demonstrated that both organic and mineral fertilizers were needed to increase crop production, improve NUE and maintain soil fertility level.     

Towards understanding factors that govern fertilizer response in cassava: lessons from East Africa

Information on fertilizer response in cassava in Africa is scarce. We conducted a series of on-farm and on-station trials in two consecutive years to quantify yield responses of cassava to mineral fertilizer in Kenya and Uganda and to evaluate factors governing the responses. Average unfertilized yields ranged from 4.2 to 25.7 t ha⁻¹ between sites and years. Mineral fertilizer use increased yields significantly, but response to fertilizer was highly variable (−0.2 to 15.3 t ha⁻¹). Average yield response per kg applied nutrient was 37, 168 and 45 and 106, 482 and 128 kg fresh yield per kg of applied N, P and K, respectively in 2004 and 2005. Fertilizer response was governed by soil fertility, rainfall and weed management, but was not influenced by variety, pest and disease pressure and harvest age. Relative N and K yields were positively correlated to SOC and exchangeable K, while response to fertilizer decreased on more fertile soils. Still, fertilizer response varied widely on low fertility soils (e.g. on soils with <10 g kg⁻¹ SOC, responses ranged from −8.6 to 24.4 t ha⁻¹), indicating strong interactions between factors governing fertilizer response. Response to fertilizer was reduced if total rainfall <1,500 mm or rainfall from 0 to 3 months after planting <400 mm. Fertilizer application promoted plant growth and resulted in a better soil coverage and reduced weed competition. Yields in fertilized fields were independent of weed management, unless growing conditions were unfavourable.     

Nitrogen Cycling with Respect to Environmental Load in Farm Systems in Southwest China

In Qibainong, a steep-mountainous karst region in southwestern China, self-sustaining societies have long existed, but increasing socioeconomic liberation has fuelled the recent rapid structural change of its economy. Consequently, environmental deterioration and exhaustion of resources have become problematic issues. We carried out a field survey in Qibainong in southwestern China and used both estimated and measured N flows and N balances from obtained results. Our results are summarized as follows (1) farmers used large amounts of chemical N fertilizers at intensities of 113–1124 kg N ha⁻¹; (2) substantial application of chemical fertilizer in Qibainong has contributed to an increase in potential NO₃-N leaching of 6–511 kg N ha⁻¹, followed by NH₃ volatilization; (3) crop products are largely distributed to feed livestock, the products of which are a major income source; (4) this area has a great requirement for imported food; (5) in addition, unused manure N (up to 191 kg N ha⁻¹) is generated by the increase in manure N production. Chemical fertilizer application, in addition to unused manure can be regarded as a major source of environmental damage. Based on the relationship between the N application rate and the NO₃-N leaching potential, we estimated the critical limit of the N application rate of chemical fertilizer + manure to be 297 kg N ha⁻¹. In Qibainong, unused manure, which is an important nutrient resource, was applicable within the critical limit. We recommend that all manure N produced within the village be used effectively on arable land, and that any shortages be supplemented by chemical N fertilizer up to 297 kg N ha⁻¹ to maintain water resource quality. Further improvement might be achieved through incorporating chemical fertilizers, P and K supplemented manure, and so on.     

Variable grain legume yields,responses to phosphorus and rotational effects on maize across soil fertility gradients on African smallholder farms

Promiscuous soyabean varieties have potential to contribute significantly to income generation, food security and soil N budgets on smallholder farms. One of the major factors limiting this potential is farmers’ preference to allocate nutrient resources to food security cereal crops on the most fertile fields, leaving grain legumes to grow on residual fertility on infertile fields. Two experiments were conducted to: (i) compare the current farmer practice with targeting manure and single super phosphate (SSP) to soyabean in a three-year rotation cycle on two fields with different soil fertility: an infertile sandy soil and a more fertile clay soil; and (ii) assess the effects of variability of soil fertility within and across farms on productivity of soyabean and groundnut. In the first experiment, soyabean (<0.2 t ha⁻¹) and maize yields (<0.7 t ha⁻¹) without fertilizer were poor on a degraded sandy soil. Both crops responded poorly to SSP due to deficiency of other nutrients. Manure application significantly increased soyabean and maize yields, led to yield stabilization over three seasons and also significantly increased the proportion of N₂ fixed by soyabean (measured using ¹⁵N natural abundance) from 60% to 83%. On the sandy soil, P was used more efficiently and gross margins were greater when SSP and manure were applied to maize in a maize–soyabean rotation. Soyabean and maize yields without fertilizer inputs were larger on clay soil with moderate fertility (0.4–0.7 t ha⁻¹ and 2.0–2.3 t ha⁻¹ respectively) and were significantly increased by application of SSP and manure. Within rotations, P recovery was higher when manure and SSP were applied to maize (43 and 25%) than when applied to soyabean (20 and 19%). However, application of manure to soyabean on the clay was more profitable than application to maize for individual crops and within rotations. In the second experiment, soyabean and groundnut yields were largest (∼1 and ∼0.8 t ha⁻¹ respectively) on plots closest to homesteads on wealthy farms, which were more fertile due to good past management. Yields were poor (< 0.5 t ha⁻¹) on other fields which previously had received little nutrient inputs. Soyabean and groundnut yields correlated well with available P (R ² = 0.5–0.7) and soil organic C (SOC) contents (R ² = 0.4–0.6). For smallholder farmers to maximise benefits from legume production they need to focus attention on the more fertile plots, although production should be optimized in relation to maize. Targeting nutrients to maize as currently practiced by farmers was more efficient and economic under poor soil fertility conditions, whilst potential exists to increase income by targeting manure to soyabean on the more fertile soils.     

Effects of nutrient cycling on grain yields and potassium balance

Soybean-maize rotation is a profitable cropping system and is used under rain fed conditions in north China. Since crop yields have been reported to decrease when K fertilizers are not used, we analyzed the productivity trends, soil-exchangeable and non-exchangeable K contents, and K balance in a continuous cropping experiment conducted in an area with an alfisol soil in the Liaohe River plain, China. The trial, established in early 1990 and continued till 2007, included 8 combinations of recycled manure and N, P, and K fertilizers. In the unfertilized plot, the yields of soybean and maize were 1,486 and 4,124 kg ha⁻¹ respectively (mean yield over 18 years). The yields of both soybean and maize increased to 2,195 and 7,476 kg ha⁻¹, respectively, in response to the application of inorganic N, P, and K fertilizers. The maximum yields of soybean (2,424 kg ha⁻¹) and maize (7,790 kg ha⁻¹) were obtained in the plots under treatment with N, P, and K fertilizers and recycled manure. K was one of the yield-limiting macronutrients: regular K application was required to make investments in the application of other mineral nutrients profitable. The decrease in the yields of soybean and maize owing to the absence of K application averaged 400 and 780 kg ha⁻¹, respectively. Soybean seed and maize grain yields significantly increased with the application of recycled manure. For both these crops, the variation coefficients of grain were lower with treatments that included recycled manure than without treatment. After 18 years, the soil-exchangeable and non-exchangeable K concentrations decreased; the concentrations in the case of treatments that did not include K fertilizers were not significantly different. Treatment with N, P, and K fertilizers appreciably improved the fertility level of the soil, increased the concentration of soil-exchangeable K, and decreased the non-exchangeable K concentration. In soils under treatment with N, P, and K fertilizers and recycled manure, the soil-exchangeable and non-exchangeable K levels in the 0–20 cm-deep soil layer increased by 34% and 2%, respectively, over the initial levels. Both soil-exchangeable and non-exchangeable K concentrations were the highest with on treatment with N, P, and K fertilizers and recycled manure, followed by treatment with N, P, and K fertilizers. These concentrations were lowest in unfertilized soils; the other treatments yielded intermediate results. The results showed a total removal of K by the crops, and the amount removed exceeded the amount of K added to the soil; in treatments that did not include K fertilizers, a net negative K balance was observed, from 184 to 575 kg ha⁻². The combined use of N, P, and K fertilizers and recycled manure increased the K content of the 0–20 cm-deep soil layer by 125% compared to the increase obtained with the application of N, P, and K fertilizers alone. The results clearly reveal that current mineral fertilizer applications are inadequate; instead, the annual application of recycled manure along with N, P, and K fertilizers could sustain future yields and soil productivity.     

Benefits of organic residues and chemical fertilizer to productivity of rain-fed lowland rice and to soil nutrient balances

Low yields and high risk characterize many rain-fed lowland rice environments, including those in Laos. Drought and fluctuating soil-water conditions (from aerobic to anaerobic states) can limit productivity and the efficient use of applied nutrients. Although addition of organic matter may improve the efficiency of fertilizer use, on-farm residues, for example farmyard manure (FYM), rice straw and rice hulls, are, currently, poorly utilized in these systems. Single and multi-year experiments were designed to evaluate the effect of these residues on rice productivity and efficiency of fertilizer use at four sites. Rice yield without fertilizer but with addition of residues ranged from 1.1 to 1.7 t ha⁻¹ across sites and years. In response to fertilizer, yields increased on average by 1.4 t ha⁻¹. For all sites and years there was a significant response of yield to organic residues applied without fertilizer, with responses ranging from 0.2 to 1.4 t ha⁻¹. In 58% of cases there was no residue×fertilizer interaction (benefits of residues when applied with fertilizer were additive). In 38 and 4% of cases the interaction was negative (no response to residues if fertilizer was already applied) or positive (synergistic), respectively. In the multi-year studies, the type of interaction varied between years, suggesting that seasonal events, rather than soil type, determine the type of interaction. The greatest benefits of applying organic and chemical fertilizers together were observed in years when soil-water conditions were unfavorable (fluctuating anaerobic–aerobic conditions). The long-term effects of these different management strategies on soil nutrient balances suggest that N, P, and K balances were maintained as a result of balanced commercial fertilizer management but that addition of residues further enhanced these balances. All residues, when applied alone, resulted in positive soil Si balances; only with FYM were long-term N, P, and K balances maintained or positive, however. For resource-poor farmers, applying on-farm residues can be a sustainable approach to increasing productivity.     

Conservation tillage,local organic resources and nitrogen fertilizer combinations affect maize productivity,soil structure and nutrient balances in semi-arid Kenya

Smallholder land productivity in drylands can be increased by optimizing locally available resources, through nutrient enhancement and water conservation. In this study, we investigated the effect of tillage system, organic resource and chemical nitrogen fertilizer application on maize productivity in a sandy soil in eastern Kenya over four seasons. The objectives were to (1) determine effects of different tillage-organic resource combinations on soil structure and crop yield, (2) determine optimum organic–inorganic nutrient combinations for arid and semi-arid environments in Kenya and, (3) assess partial nutrient budgets of different soil, water and nutrient management practices using nutrient inflows and outflows. This experiment, initiated in the short rainy season of 2005, was a split plot design with 7 treatments involving combinations of tillage (tied-ridges, conventional tillage and no-till) and organic resource (1 t ha⁻¹ manure + 1 t ha⁻¹ crop residue and; 2 t ha⁻¹ of manure (no crop residue) in the main plots. Chemical nitrogen fertilizer at 0 and 60 kg N ha⁻¹ was used in sub-plots. Although average yield in no-till was by 30–65% lower than in conventional and tied-ridges during the initial two seasons, it achieved 7–40% higher yields than these tillage systems by season four. Combined application of 1 t ha⁻¹ of crop residue and 1 t ha⁻¹ of manure increased maize yield over sole application of manure at 2 t ha⁻¹ by between 17 and 51% depending on the tillage system, for treatments without inorganic N fertilizer. Cumulative nutrients in harvested maize in the four seasons ranged from 77 to 196 kg N ha⁻¹, 12 to 27 kg P ha⁻¹ and 102 to 191 kg K ha⁻¹, representing 23 and 62% of applied N in treatments with and without mineral fertilizer N respectively, 10% of applied P and 35% of applied K. Chemical nitrogen fertilizer application increased maize yields by 17–94%; the increases were significant in the first 3 seasons (P < 0.05). Tillage had significant effect on soil macro- (>2 mm) and micro-aggregates fractions (<250 μm >53 μm: P < 0.05), with aggregation indices following the order no-till > tied-ridges > conventional tillage. Also, combining crop residue and manure increased large macro-aggregates by 1.4–4.0 g 100 g⁻¹ soil above manure only treatments. We conclude that even with modest organic resource application, and depending on the number of seasons of use, conservation tillage systems such as tied-ridges and no-till can be effective in improving crop yield, nutrient uptake and soil structure and that farmers are better off applying 1 t ha⁻¹ each of crop residue and manure rather than sole manure.     

Long-term effect of continuous cropping of irrigated rice on soil and yield trends in the Sahel of West Africa

The effects of 18 years continuous cropping of irrigated rice on soil and yields were studied in two long-term fertility experiments (LTFE) at Ndiaye and Fanaye in the Senegal River Valley (West Africa). Rice was planted twice in a year during the hot dry season (HDS) and wet season (WS) with different fertilizer treatments. Soil organic carbon (SOC) under fallow varied from 7.1 g kg⁻¹ at Fanaye to 11.0 g kg⁻¹ at Ndiaye. Rice cropping maintained and increased SOC at Ndiaye and Fanaye, respectively and fertilizer treatments did not affect SOC. Soil available P and exchangeable K were maintained or increased with long-term application of NPK fertilizers. Without any fertilizer, yields decreased by 60 kg ha⁻¹ (1.5%) and 115 kg ha⁻¹ (3%) per year at Fanaye and Ndiaye, respectively. The highest annual yield decreases of 268 kg ha⁻¹ (3.6%) and 277 kg ha⁻¹ (4.1%) were observed at Fanaye and Ndiaye, respectively when only N fertilizer was applied. Rice yields were only maintained with NPK fertilizers supplying at least 60 kg N, 26 kg P and 50 kg K ha⁻¹. It was concluded that the double cropping of irrigated rice does not decrease SOC and the application of the recommended doses of NPK fertilizer maintained rice yields for 18 years.