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

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

Crop response to manure and fertilizer in Burkina Faso and Niger

Farmyard manure (FYM) is valuable for soil management, especially for soils with <?10 g kg^?1 organic C in semi-arid West Africa. This study determined short-term FYM effects on yield and on response to N, P and K fertilizer for 20 trials in Niger and 28 trials in Burkina Faso involving six crops. The comparisons were of 0 and 2.5 Mg ha^?1 yr^?1 FYM applied in Niger, and of 0 and 5 Mg ha^?1 FYM applied once in 2 years in Burkina Faso. Fertilizer and FYM application alone had little effect on yield in Niger but there was a synergistic effect of fertilizer P with FYM which included increased mean responses to P of, respectively: 0.22 and 0.43 Mg ha^?1 for sorghum grain and fodder (Sorghum bicolor L.); 0.15 and 0.27 Mg ha^?1 for cowpea grain and fodder; 0.16 Mg ha^?1 grain for pearl millet (Pennisetum glaucum L.) when intercropped with cowpea (Vigna unguiculata L.); and 0.39 Mg ha^?1 for groundnut fodder (Arachis hypogea L.). Application of FYM increased pearl millet response to N but decreased legume response to K fertilizer. In Burkina Faso, there was a mean grain yield increase of 0.29 Mg ha^?1 yr^?1 due to FYM and the effect of applying both FYM and fertilizer was additive except for a synergy of N fertilizer plus manure application for maize (Zea mays L.). Therefore, farmers should apply FYM and fertilizer together in Niger but these can be applied alone or together in Burkina Faso with mostly similar effects.     

Relative efficiency of ammonium polyphosphate and orthophosphates for wheat and their residual effects on succeeding cowpea fodder

The relative efficiency of ortho and polyphosphates as P sources for wheat were studied in a field experiment with five sources—TSP, DAP, NP, APP (solid) and APP (Liquid) at three levels—13, 26 and 39 kg P ha^–1 and the residual effect was studied on a succeeding cowpea (fodder). Both the crops were grown each year at same location. Wheat responded upto 39 kg P ha^–1 in the first year but only upto 26 kg P ha^–1 in the second year. The growth parameters—plant height and dry matter production and yield attributes-ears m^–1, grains ear^–1 and test weight were favourably influenced by P application. The rate of P uptake initially exceeded the dry matter production. Wheat when fertilized with 26 or 39 kg P ha^–1 left behind a significant residual effect to raise the soil P availability and dry matter yield of cowpea. The grain yield of wheat was higher with APP's than with NP or DAP but the residual effect was the highest with NP. The magnitude of build up of soil available P with NP and APP's were similar and higher than DAP or TSP.     

Fertilization of tropical rice: A case study on Bangkok plain

This study consisted of a survey on the nutritional status of rice plants in relation to nutrient application and yield in 70 farmers' fields in four provinces of Bangkok plain during the 1977 wet season. In addition a series of fertilizer experiments were carried out on rice experimental stations in the same provinces to study yield response to N and P fertilization and to develop a fertilizer recommendation system based on plant analysis.The average grain yield in the survey was 3.2 t ha^–1 and the early (high yielding varieties), medium (local) and late maturity (local) types yielded 3.3, 2.8 and 3.0 t ha^–1, respectively. The average amount of fertilizers applied to these maturity types were 33, 15 and 7 kg N ha^–1 and 15, 8 and 6 kg P ha^–1, respectively. Regression analysis indicated only a slight correlation between yield and any level of fertilizer application. On experimental stations yields over 6 t ha^–1 were obtained with applications of N over 100 kg ha^–1 and P over 22 kg ha^–1. Evaluation of nutritional status of plants based on plant analysis showed that in all provinces there were strong and widespread nutrient deficiences primarily of N and secondarily of P, and possibly of some other nutrients. Fertilizer application based on plant analysis gave high yield responses. It was concluded that the major constraints of yield on Bangkok plain are too low fertilizer application especially of N, and unbalanced fertilization of N and P.     

Phosphorus requirement of a maize-cowpea sequential cropping on a paleudult

A field experiment was conducted for 6 growing seasons on a Paleudult to assess the influence of P rate and frequency on crop yield and P fractions in a maize-cowpea sequential cropping system. Although cropping depleted soil organic P by up to 42.1% over the cropping seasons perhaps through mineralisation, the major portion of added P and mineralised P were not utilised by the crops. Transformations were probably more to the insoluble Fe-P than Al.P. Cumulative recovery of P in maize and cowpea was never above 75% at a single application of 30kg P ha^–1 over a period of 6 seasons while cumulative recovery of annually applied P ranged from 17.1 to 31.8% over the same period. Lower values were recorded for seasonally applied P plots. Annual application of P at 30kg ha^–1 kept the available P above the critical level and maintained maize and cowpea yields at near maximum over the cropping periods.     

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.     

Effect of nitrogen,phosphorus and molybdenum application on growth and symbiotic N2-fixation of groundnut in an acid sandy soil in Niger

Two field experiments were conducted in 1988 and 1989 on an acid sandy soil in Niger, West Africa, to assess the effect of phosphorus (P), nitrogen (N) and micronutrient (MN) application on growth and symbiotic N₂-fixation of groundnut (Arachis hypogaea L.). Phosphorus fertilizer (16 kg P ha^–1) did not affect pod yields. Addition of MN fertilizer (100 kg Fetrilon Combi 1 ha^–1; P + MN) containing 0.1% molybdenum (Mo) increased pod yield by 37–86%. Nitrogen concentration in shoots at mid pod filling (72 days after planting) were higher in P + MN than in P – MN fertilizer treatment. Total N uptake increased from 53 (only P) to 108 kg N ha^–1 by additional MN application. Seed pelleting (P + MoSP) with 100 g Mo ha^–1 (MoO₃) increased nitrogenase activity (NA) by a factor of 2–4 compared to P treatment only. The increase in NA was mainly due to increase in nodule dry weight and to a lesser extent to increase in specific nitrogenase activity (SNA) per unit nodule dry weight. The higher NA of the P + MoSP treatment was associated with a higher total N uptake (55%) and pod yield (24%). Compared to P + MoSP or P + MN treatments application of N by mineral fertilizer (60 kg N ha^–1) or farmyard manure (130 kg N ha^–1) increased only yield of shoot dry matter but not pod dry matter. Plants supplied with N decreased soil water content more and were less drought tolerant than plants supplied with Mo. The data suggest that on the acid sandy soils in Niger N deficiency was a major constraint for groundnut production, and Mo availability in soils was insufficient to meet the Mo requirement for symbiotic N₂-fixation of groundnut.     

Fertility management and landscape position: farmers' use of nutrient sources in western Niger and possible improvements

Poor millet growth and yields in Niger are commonly attributed to rainfall deficits and low soil nutrient content. Land management by local farmers is done as a function of soil types, crops, and available resources. Farmer management practices in millet fields located on four different landscape positions were studied in a village in western Niger located near the 600 mm isohyet. Average distance from homestead to field was 980 m, with fields in the valley bottom much closer (average 225 m) and fields on the plateau much further (average 2300 m). Farmers considered the valley and plateau fields slightly more fertile than the other fields, but rainfall infiltration on plateau fields is often relatively poor. Nitrogen and phosphorus contents in the soil were highest on the less intensively cropped plateaus. More than 50% of the fields did not receive any applied nutrients other than during livestock grazing of leftover stover. Manure application was done through corralling in only four of the fields studied (20%), out of which three were farmed by Fulani using their own herds for manuring. There was no significant effect of landscape unit on yield, though yields in the valley and on the upper slope were slightly higher than average. Millet grain yields, soil carbon and soil phosphorus decreased significantly with distance from the living quarters. This may be because manuring usually takes place close to home (average distance in 1997 <200 m). Manure application increased millet grain production from 126 kg ha^–1 to 316 kg ha^–1 in 1997. Manuring yielded more than 1000 kg ha^–1 in 1996, when rainfall was much more favourable. Fallowed fields yielded an average 143 kg ha^–1 of millet grain in 1997, with fallow taking place an average of 1640 m from the homestead. Another soil fertility management practice included use of millet threshing residues in fields adjacent to the village. There was no chemical fertilizer application. Any improvement to the system will require the solution of existing constraints limiting the integration of livestock and crops and/or limiting the input of external sources of nutrients in Niger. These limitations can include lack of land to allow fallowing practices and/or grazing; local non-availability of mineral fertiliser; lack of capital to buy fertiliser, due in part to low millet prices; lack of means of transport for inputs; but also lack of means for pest control and lack of labour for sowing, weeding and thinning. Initial improvements may be made by making more efficient use of the available manure, through much lighter and slightly more frequent manuring of much larger areas.     

Fertilizer responses of groundnuts (Arachis hypogae L.) under continuous intensive cultivation in the Nigerian savannah

Up until now, potash fertilization has not been part of the recommended practices for groundnut production in Nigeria and only low levels of P are recommended in line with the level of agricultural technology available to the farmer. The change from the traditional non-intensive farming practice to continuous intensive cultivation coupled with the introduction of better yielding/more-nutrient demanding crop varieties have led to a deficiency of a number of nutrients.Field experiments were carried out for 5 years (repeated on the same sites) at 8 locations in northern Nigeria to evaluate the effect of four levels of phosphorus (0, 8, 16 and 24 kg P ha^–1) and three levels of potassium (0, 20, and 40 kg K ha^–1) on yield and related parameters in groundnut. Soils at the various sites were essentially loamy sands with low levels of organic carbon and cation exchange capacity.The yield levels in the Guinea savannah were generally higher than those in the Sudan savannah but the response pattern was the same. There was significant response to applied P up to 24 kg P ha^–1. Potash applied at 20 kg K ha^–1 produced significantly higher pod yields than the control plots, but higher rates of applied K did not result in any further significant yield increase, although there was a clear consistent trend towards higher yields as the K rates increased. Phosphate X potash interaction had no significant effect on yield.Except for K, the uptake of all nutrients were significantly increased by P levels. However, only the K content of haulms and the N content of kernels were significantly increased by K application. On the average, about 58% of N, 68% of P, 19% of K, 5% of Ca and 22% of Mg were contained in the kernels while 27% of N, 23% of P, 64% of K, 83% of Ca and 69% of Mg were contained in the haulms. The implication of this is that the complete removal of groundnut residue will deplete the soil cation reverves rapidly unless these losses are compensated through fertilizer use.     

Residual effects of growing mungbean and uridbean on the yield and nitrogen uptake of a succeeding wheat crop

A two year field experiment was carried out at the Indian Agricutural Research Institute, New Delhi - 110012, India to assess the effect of mungbean (Vigna radiata L.) and uridbean (Vigna mungo L.) residues on the yield and N uptake of a succeeding wheat crop as compared to sorghum fodder. Sorghum produced 3.5–7.5 times more dry matter and removed 2–3 times more nitrogen than mungbean or uridbean during same duration (80 ± 10 days) of their growth. Without N application the grain yield of wheat following mungbean and uridbean (without residue incorporation) was 0.45 and 0.48 t ha^–1 more than the yield of wheat following sorghum fodder. These yields were equivalent to that predicted when 36 and 38 kg urea-N ha^–1, respectively, was directly applied to wheat. The residual effects of these grain legumes were higher when succeeding wheat was fertilized with 60 kg urea-N ha^–1; at this level mungbean and uridbean spared 52 and 43 kg urea-N ha^–1, respectively, in succeeding wheat. The residual effect of mungbean and uridbean further increased when their residue was incorporated in soil; with this practice they spared 94 and 115 kg urea-N ha^–1, respectively, without N application to wheat and 74 and 82 kg urea-N ha^–1, respectively, with an application of 60 kg urea-N ha^–1 to wheat.Mungbean and uridbean, without residue incorporation, increased aboveground plant-N uptake of succeeding wheat by 11.5–34.9 and 10.8–34.0 kg N ha^–1, respectively; whereas with residue incorporation, they increased aboveground plant-N content of succeeding wheat by 26.1–45.8 and 32.7–47.7 kg N ha^–1, respectively.The results of the present study indicate that there is both an indirect sparing effect and a direct residual effect of mungbean and uridbean on the nitrogen needs of succeeding wheat, more so when their residues are incorporated in soil.     

Yield response and nitrogen uptake of pearl millet (Pennisetum americanum (L.) Leeke) cultivars to nitrogen application in the savanna region of Nigeria

Field experiments were carried out during the wet seasons (May to September) of 1980 and 1981 in order to determine the response of five pearl millet cultivars to nitrogen fertilization in savanna region of Nigeria. There were varietal differences in yield and nitrogen uptake in response to nitrogen rates. Most cultivars responded significantly up to 75 kg N ha^–1. Hybrid outyielded other cultivars at both locations each year. Nitrogen use efficiency was highest with the Hybrid, compared to other cultivars.     

Effects of cropping history and phosphorus source on yield and nitrogen fixation in sole and intercropped cowpea–maize systems

Symbiotic N₂-fixation, N uptake efficiency, biomass- and crop production of cowpea and maize as affected by P source, sole- and intercropped, and introduction of break crops were studied on a farmer’s fields in semi-arid Tanzania. Cowpea fixed around 60% of its N from the atmosphere amounting to 70 kg N ha⁻¹ under sole and 36 kg N ha⁻¹ under intercropping as estimated by the ¹⁵N isotope dilution method around peak biomass production. The amount of N₂-fixed was 30–40% higher when P was applied as either TSP or MRP whereas cowpea yield were unaffected. Intercropped maize with 19,000 plant ha⁻¹ accumulated the same amount of N as 38,000 sole cropped maize plants although intercropping reduced the dry matter accumulation by 25%. The N uptake efficiency of the applied ¹⁵N labelled fertiliser was 26%, which equal a total pool of early available plant N of 158 kg N ha⁻¹. Under the N deficient conditions, P application did not increase the grain yield of maize. The LER indicate that sole cropping required 18% more area than intercropping in order to produce the same grain yield, and 35% more land when LER was based on N uptakes. Introduction of break crops in the maize systems, more than doubled accumulation of dry matter and N in the grain compared to continuous maize cropping. During maturation sole crop cowpea shedded leaves containing 41 kg N ha⁻¹. The current findings underline the importance of crop diversity in Sub Saharan Africa agriculture and emphasise the need for including all residues, including shedded leaves, in nutrient balance studies.     

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.     

The effect of crop residue and fertilizer use on pearl millet yields in Niger

A field study was conducted over a 4-year period in Niger, West Africa, to determine the effects of crop residue (CR), fertilizer, or a combination of crop residue and fertilizer (CRF) on yields of pearl millet (Pennisetum glaucum [L.] R. Br.). Despite a decline in yields of control plots (initial yields were 280 kg grain ha^–1 declining to 75 kg grain ha^–1 over 4 years), yields of fertilizer plots were maintained at 800–1,000 kg grain ha^–1. Continued application of CR slowly augmented yields to levels similar to those of the fertilized plots. The effects of CR and fertilizer were approximately additive in the CRF plots. Addition of CR and fertilizer increased soil water use over the control by 57 mm to 268 mm in an average season and helped trap wind-blown soil. These plots tended to exhibit slightly higher soil pH and lower Al saturation than did the fertilized treatments. Return of CR to the soil resulted in significantly reduced export of most plant nutrients, especially Ca, Mg, and K.     

Uptake of NPK and yield of rainfed groundnut (Arachis hypogaea L.)

Experiments were conducted on sandy loam soils of Tirupati campus of Andhra Pradesh Agricultural University for two rainy seaons of 1980 and 1981 to study the effect of split application of NPK fertilizers on Spanish bunch groundnut. The fertilizer doses were 40 N, 20 P and 40 K kg ha^–1 in 1980 and 30 N, 10 P and 25 K kg ha^–1 in 1981.In 1980, uptake of N (48 kg ha^–1), P (7 kg ha^–1) and K (37 kg ha^–1) was maximum with the application of 10 N, 5 P and entire 40 K kg ha^–1 as basal and 30 N and 15 P kg ha^–1 at 30 days after sowing, leading to highest pod yield (0.76 t ha^–1). In 1981, application of 20 N, 10 P and 25 K kg ha^–1 as basal dose and 20 N kg ha^–1 at 30 days after seeding resulted in highest uptake of N (114 kg ha^–1), P (17 kg ha^–1) and K (58 kg ha^–1) and hence the pod yield (2.36 t ha^–1).Differences in the uptake of NPK and pod yield in 1980 and 1981 was due to variation in total rainfall and its distribution during the crop period. Rainfall was equally distributed throughout the crop period in 1981, whereas there were two prolonged dry spells of more than 40 days in 1980.     

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.     

Studies on the substitution of inorganic fertilizers with organic manure and their effect on soil fertility in rice—wheat rotation

Field studies on the substitution of N and P fertilizers with farm yard manure (FYM) and their effect on the fertility status of a loamy sand soil in rice—wheat rotation are reported. The treatments consisted of application of 12 t FYM ha^–1 in combination with graded levels of N and P. Application of fertilizer N, FYM and their different combinations increased the rice yield significantly. There was no significant response to P application. The magnitude of response to the application of 12 t FYM and its combined use with each of 40 kg and 80 kg N ha^–1 was 0.7, 2.2 and 3.9 t ha^–1 respectively. Application of 120 kg N ha^–1 alone increased the yield by 3.9 t ha^–1, and was comparable to rice yield obtained with 80 kg N and 12 t FYM ha^–1. This indicated that 12 t FYM ha^–1 could be substituted for 40 kg N as inorganic fertilizer in rice. In addition FYM gave residual effects equivalent to 30 kg N and 13.1 kg P ha^–1 in the succeeding wheat. The effect of single or combined use of inorganic fertilizers and FYM was significantly reflected in the build up of available N, P, K and organic carbon contents of the soil. The relationship for predicting rice yield and nutrients uptake were also computed and are discussed.     

Effect of split application of phosphorus on the growth ofAzolla and low land rice

TheAzolla pinnata (Vietnam) inoculated in rice field 10 days after transplanting (DAT) at a rate of 500 kg ha^–1 fresh biomass along with phosphorus fertilizer application produced a mat on the water surface at 30 DAT. The three split application of phosphorus as 4.4, 2.2 and 2.2 kg P ha^–1 applied at 10, 15 and 20 DAT, respectively produced 67% more biomass and 57% more Nitrogen inAzolla than those obtained by applying 8.8 kg P ha^–1 at 10 DAT. Whereas, the two splits of phosphorus as 6.6 and 2.2 kg and 4.4 and 4.4 kg P ha^–1 applied 10 and 15 DAT, respectively produced 20 and 33% more biomass and 14 and 27% more Nitrogen only.The three split application of phosphorus also increased the grain and straw yields, panicle number and weight, nitrogen concentration and its uptake in rice significantly over application of the entire amount once only. An increase of 10% grain yield and 13% straw yields was observed when 8.8 kg P ha^–1 was applied in three splits rather than applied at one time. On the average an increase of 24% grain and 23% straw yields in rice were observed due toAzolla intercropping and 22% and 16%, respectively due to phosphorus application. The intercropping ofAzolla with rice along with phosphorus application also increased the fertility level of soil by increasing the total nitrogen, organic carbon and available phosphorus of soil.     

The maximum yield potential of subterranean clover swards achieved from superphosphate applications is influenced by the maturity and the plant density of the swards

In two field experiments, one conducted in 1987 and the other in 1988, the effect of maturity grading (days from emergence of seedlings to appearance of first flowers) of four clover cultivars (Trifolium subterraneum cvv. Northam, Dalkeith, Junee and Karridale) on the relationship between yield (herbage and seed) and the level of superphosphate applied was measured for dense (170 kg seed ha^–1), single-strain, undefoliated swards. In another two field experiments, one conducted in 1987 and the other in 1988, the effect of the density of clover plants, produced by sowing 1, 10, 100 and 1,000 kg seed ha^–1 ofT. subterraneum cv. Karridale in 1987 and cv. Junee in 1988, on the relationship between yield (herbage and seed) and the level of superphosphate applied was measured for single-strain, undefoliated swards. In all experiments, phosphorus concentration in dried herbage or seed (tissue test for P) were related to plant yields.For herbage production, the maturity of the clover cultivar largely affected the maximum yields (i.e. yield plateaux) achieved for the relationship between yield and the level of phosphorus (P) applied. The P requirements of the different cultivars were similar. For seed production, however, the different cultivars achieved different maximum yields and the P requirement of the clover cultivars were different. The later-maturing cultivars would have experienced greater water stress whilst producing seed which may have affected the P requirements of the different cultivars for seed production.For herbage production in the two plant density experiments, as the density of clover plants in the swards was increased, then the maximum herbage yields for the relationship between yield and the level of P applied increased markedly. The P requirements of the different density swards was in most cases not greatly affected. By contrast, the maximum seed yields tended to decrease drastically with increasing plant density. The 10 kg seed ha^–1 sowing rate produced the largest maximum seed yields. Swards with larger plant densities produced greater herbage yields which presumably increased water stress during seed production thus reducing the maximum seed yields for the relationship between yield and the level of P applied.When tissue test for P values were related to absolute yields, foreach harvest, different relationship between yield and tissue test for P values were required for different clover cultivars or for swards with different plant densities. However, expressing yield as a percentage of the maximum yield for the relationship between yield and tissue test for P values reduced differences foreach harvest due to different clover cultivars or different plant densities. But regardless of whether absolute yield or percentage of the maximum yield were used, different relationships between yield and tissue test for P values were required fordifferent harvest, both in the same or different years.     

Identification of nutrients limiting cassava yield maintenance on a sedimentary soil in southern Benin,West Africa

Market opportunities will drive intensification of cassava production and fertilizer will play a role in this. A trial was initiated on 15 farmers fields (replications) in one village territory in Benin on a relatively fertile sedimentary soil site to identify nutrients limiting cassava yield using nutrient omission plots over three cropping years. There was no response to fertilizer in the first year when fresh root yields in the unamended control averaged 19.1 t ha^–1. In the second year, the control yield was 16 t ha^–1 and there were significant reductions from withholding P (3.5 t ha^–1) and K (2 t ha^–1) from a complete fertilizer regime. Nutrient balance after 1 and 2 years (cumulative) showed substantial P and K deficits in unamended plots. In the third year, the control yield was 12.9 t ha^–1 and effects of withholding K (5.3 t ha^–1), P (5.0 t ha^–1) and N (3.0 t ha^–1) were statistically significant. Soil K was a significant source of variation in yield in the third year. In the third year of annual nutrient additions soil P and K in the top 0.3 m were increased by 37 and 40%, respectively. Based on the cumulative nutrient balance calculation, the annual application needed to compensate nutrient depletion was 13 kg N, 10 kg P, and 60 kg K ha^–1. Partial budget analysis based on these amounts of fertilizer suggested that investment was clearly justified in the third year of continuous cropping at current low cassava prices.