Fertilizer inputs, nutrient balance, and soil nutrient-supplying power in intensive, irrigated rice systems. I. Potassium uptake and K balance

Research in many countries indicates a negative K balance in intensive, irrigated rice systems but comparative studies across different environments are few. Using a uniform sampling methodology, we measured K uptake, K use efficiency, and K balance in six different fertilizer treatments of long-term fertility experiments with rice at 11 sites in five Asian countries. Depending on the absolute yield level, K uptake requirements of rice ranged from 17 to 30 kg K per ton of grain. For yields greater than 8 t ha^-1, total K uptake exceeded 200 kg ha^-1. The K balance at most experimental sites was negative, with an average net removal of 34–63 kg K season^-1. There was significant depletion of soil K reserves at many sites. Based on these data, we estimated that the amount of K cycled annually from the soil into rice plants is 7–10 million t in irrigated rice systems of Asia. About 1 million t of this total amount is removed with the harvested grain. Present recommendations for K addition in most intensive irrigated rice domains are insufficient to replace K removal. However, response to K can only be expected on soils with deficient supply capacity and where other nutrients, particularly N and P, are not limiting. Efficient K management for rice must therefore be based on the K input/output balance, the achievable yield target, and the effective K-supplying power of the soil.     

Phosphorus Mass Balances for Successive Crops of Fertilised Rainfed Rice on a Sandy Lowland Soil

Raising and sustaining rice yields in the rainfed lowlands requires an understanding of nutrient inputs and outputs. On sandy lowland rice soils, managing phosphorus (P) supply is a key factor in achieving increased yields and sustainable production. Phosphorus inputs, rice yields, and crop P uptake were used to quantify P requirements of rice: together with results on soil P fractions, P balance sheets were constructed over five consecutive cropping seasons on a sandy Plinthustalf near Phnom Penh, Cambodia. Grain yields ranged from 665 to 1557 kg ha⁻¹ with no added P. Average yields increased significantly with P fertiliser application over five consecutive crops by 117, 139 and 140% when the phosphate fertiliser was applied at 8.25, 16.5 and 33 kg P ha⁻¹, respectively. Without added P fertiliser, a net loss of 1.2 kg P ha⁻¹ per crop was estimated with straw return and 2.0 kg P ha⁻¹ per crop with straw removed from the field, whereas, with added P fertiliser, there was a net P gain in the soil of 5.6 or 9.5 kg ha⁻¹ per crop when straw was removed and returned to the soil, respectively. After one crop, the addition of P fertiliser significantly (P < 0.01) increased recovery in all soil P fractions. Across five successive crops, repeated application of 16.5 and 33 kg P ha⁻¹ rates resulted in progressive P accumulation in the soil, especially a labile NaOH–Po pool, but had no effect on yields and P uptake of rice. By contrast, 8.25 kg P ha⁻¹ per rice crop was generally adequate for grain yields of 2.5–3.0 t ha⁻¹ and to maintain soil P pools.     

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 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.     

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

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

Africa — how much fertilizer needed: Case study of Sierra Leone

The quantities of nitrogen, phosphorus and potassium supplied by an average African soil cleared from bush fallow, assuming no losses, were approximated. Values ranged from 23 to 120 Kg N ha^–1, 1.8 to 12 Kg P ha^–1, 47 to 187 Kg K ha^–1, depending on type of fallow, length of fallow, drainage and extent of depletion of native supplies. Additional amounts of 4 to 5 Kg N ha^–1, 4 to 6 Kg P ha^–1 and 14 to 20 Kg K ha^–1 are obtained from the ash.Using crop nutrient removal data and approximate efficiencies of native and fertilizer N, P and K, fertilizer requirements at the reconnaissance level were estimated for selected target yields. For newly cleared uplands at cropping/fallow ratio of 2:7, N fertilizer requirements for cassava (30 t ha^–1), maize (4 t ha^–1), and sweet potato (16 t ha^–1), were 138, 98, 42 kg ha^–1 respectively. Wetland rice (4 t ha^–1) required 55 kg N ha^–1. Corresponding P fertilizer requirements for cassava, maize, sweet potato, upland rice (1.5 t ha^–1) and ground-nut (1 t ha^–1) were 190, 80, 30, 30 and 16 kg P ha^–1 respectively. Wetland rice required 83 kg P ha^–1. Substantial residual values of applied P are to be expected. Cassava required 60 kg ha^–1 of K on newly cleared land. In soils of lowered nutrient status higher N, P, and K fertilizer requirements were indicated for all crops.Land use data from Sierra Leone were used to illustrate how the total quantities of N, P and K fertilizers in a country in the forest zone of Africa can be approximated. Fertilizer needs in Sierra Leone were in decreasing order P > N K. N, P and K requirements were estimated to be 10,000 t, 20,000 t and 4,000 t respectively. The nutrient balance sheet method described in this paper is a useful tool to estimate the order of magnitude of fertilizer requirement at selected target yields for countries in Africa.     

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.     

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.     

Phosphorus management in continuous wheat and wheat-legume rotations

The sustainability of cropping systems is closely related to the judicious use of fertilizers. Little research has been conducted on the management of P in rotations in Morocco. The purpose of this study was to determine the effects of direct, cumulative, and residual P on wheat (Triticum aestivum) and chickpea (Cicer arietinum L.) yields under field conditions in two cropping systems: continuous wheat (W-W) and chickpea-wheat (CP-W). Experiments were conducted in 1994–96 at two locations in the arid and semiarid regions of Morocco. Phosphorus was applied the first year at rates of 0, 9, 18, and 27 kg P ha^–1 on both wheat and chickpea. The second year, plots were split into treatments with P and without P fertilizer. The changes in NaHCO₃-P in soil showed that after two years of cropping, P rates of 9 and 18 kg P ha^–1 were needed to increase and maintain soil test P level in the range where a third successive crop could be grown without fertilization at locations 1 and 2, respectively. Also, soils with the same initial NaHCO₃-P soil test levels required different amounts of fertilizer P to produce maximum yields. Inclusion of chickpea in the rotation resulted in a greater response to residual P by wheat at location 2. Differences in wheat grain yield between rotations were not significant. The maximum increase in yields above the nil-P treatment due to the highest amount applied in the the previous year was 1.3 t ha^–1, obtained for continuous wheat at location 2. Though the residual P effect was evident in this study, it did not produce maximum yields. Yields (GY, DM) could be predicted by the inclusion of both P applied in previous year (PR) and P applied in the current year (CP) by the following model: GY or DM = a + b*Ln(RP+1) + c*Ln(CP+1). Based on the range of P rates used in this study, a single P application for a 2-year rotation is not a suitable practice in these soils. The application of 18 kg P ha^–1 each year is recommended for continuous wheat, and 9 kg P ha^–1 the first year plus 18 kg P ha^–1 the second year is recommended for chickpea-wheat rotations. We suggest that either using single large applications of P or performing repeated applications should take into account the range of targeted application rates.     

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.     

Impact of organic residue management on nitrogen use efficiency in an annual rice cropping sequence of lowland Central Thailand

Field experiments were conducted in Central Thailand under a rice–fallow–rice cropping sequence during consecutive dry and wet seasons of 1998 to determine the impact of residue management on fertilizer nitrogen (N) use. Treatments consisted of a combination of broadcast urea (70 kg N ha^–1) with rice straw (C/N 67) and rice hull ash (C/N 76), which were incorporated into the puddled soil 1 week before transplanting at a rate of 5 Mg ha^–1. Nitrogen-15 balance data showed that the dry season rice recovered 10 to 20% of fertilizer N at maturity. Of the applied N, 27 to 36% remained in the soil. Loss of N (unaccounted for) from the soil–plant system ranged from 47 to 54% of applied N. The availability of the residue fertilizer N to a subsequent rice crop was only less than 3% of the initial applied N. During both season fallows NO₃-N remained the dominant form of mineral-N (NO₃+NH₄) in the aerobic soil. In the dry season grain yield response to N application was significant (P=0.05). Organic material sources did not significantly change grain yield and N accumulation in rice. In terms of grain yields and N uptake at maturity, there was no significant residual effect of fertilizer N on the subsequent rice crop. The combined use of organic residues with urea did not improve N use efficiency, reduced N losses nor produced higher yields compared to urea alone. These results suggested that mechanisms such as N loss through gaseous N emissions may account for the low fertilizer N use efficiency from this rice cropping system. Splitting fertilizer N application should be considered on the fertilizer N use from the organic residue amendment.     

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.     

Response of upland rice to phosphorus in an Ultisol in the humid forest zone of West Africa

Phosphorus (P) deficiency is one of the major limiting factor for crop production in highly weathered soils in the humid tropics. Field experiments were conducted for two years (1992 and 1993) to determine P response and efficiencies of upland rice cultivars in an Ultisol, low in available P, in the forest zone of Cote d'Ivoire. The rice cultivars tested were selected from a large number of entries tested earlier for acidity tolerance.Grain yields of the cultivars were significantly increased by P application. There was little further response in grain yield at higher rates than 60 kg P ha^–1. The rice cultivars differed in agronomic and physiological P efficiencies and the efficiencies were higher at lower rates of P. The rooting depths of the cultivars were increased by application of P at the lowest application rate (30 kg P ha^–1).The results suggest that P fertilization of soil acidity-tolerant upland rice cultivars can significantly improve the productivity of the Ultisols.     

Modelling heavy metal and phosphorus balances for farming systems

Accounting for agricultural activities such as P fertilization in regional models of heavy metal accumulation provides suitable sustainable management strategies to reduce nutrient surpluses and metal inputs in agricultural soils. Using the balance model PROTERRA-S, we assessed the phosphorus (P), cadmium (Cd) and zinc (Zn) flux balances in agricultural soils of a rural region in Switzerland for different farm types and crop types. The P requirements of crops on arable farms were mainly supplied by commercial fertilizers and sewage sludge, while on animal husbandry farms P fertilizer demands were met by animal manure alone. Metal accumulation in soil was very different between the balance units. Estimated net Cd fluxes ranged between 1.0 and 2.3 g ha^–1 yr^–1 for arable farm types, 0.6 and 2.0 g ha^–1 yr^–1 for dairy and mixed farm types, and 9.1 and 17.8 g ha^–1 yr^–1 for animal husbandry farm types. Largest net Zn fluxes of 17.9–39.8 kg ha^–1 yr^–1 were estimated for animal husbandry farms, whereas for arable farm types net Zn fluxes of 101–260 g ha^–1 yr^–1 and for dairy and mixed farm types of 349–3360 g ha^–1 yr^–1 were found. The results indicate that P management is a primary factor determining the variation of these net Cd and net Zn fluxes. The latter were highly sensitive to the Zn/P concentration ratio in animal manure, atmospheric deposition and crop concentrations. Variation of net Cd fluxes resulted mainly from uncertainty in crop concentrations, atmospheric deposition, leaching parameters and uncertainty in Cd/P concentration ratio of commercial fertilizers. In addition, element balances were sensitive to empirical assumptions on fertilization strategy of farmers, such as the partitioning of manure between balance units.     

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.     

Effect of green manuring with Sesbania aculeata and nitrogen fertilization on the performance of direct-seeded flood-prone lowland rice

Green manuring of rice with dhaincha (Sesbania aculeata) is widely practised under irrigated puddle-transplanted conditions. In flood-prone lowlands, the rice is established through direct seeding early in the season and flooding occurs after 1–2 months of crop growth following regular rains. The low yields are due to poor crop stands and difficulty in nitrogen management under higher depths of water. The effect of green manuring with dhaincha intercropped with direct-seeded rice vis-à-vis the conventional practice of incorporating pure dhaincha before transplanting was investigated under flood-prone lowland conditions (up to 50–80 cm water depth) at Cuttack, India. Treatment variables studied in different years (1992, 1994 and 1995) were: rice varieties of different plant heights, crop establishment through direct seeding and transplanting, varying length of periods before dhaincha incorporation, and urea N fertilizer levels. Dhaincha accumulated 80–86 kg N ha^-1 in pure stand and 58–79 kg N ha^-1 when intercropped with direct-seeded rice in alternate rows at 50 days of growth. The growth of rice improved after dhaincha was uprooted manually and buried in situ between the rice rows when water depth was 10–20 cm in the field. The panicle number was lower but the panicle weight was higher with dhaincha green manuring than with recommended level of 40 kg N ha^-1 applied as urea. The grain yield was significantly higher with direct seeding than with transplanting due to high water levels (>60 cm) immediately after transplanting. Dhaincha manuring was at par with 40 kg N ha^-1 as urea in increasing the yield of direct-seeded and transplanted crops. The highest yield of direct-seeded crop was obtained when 20 kg N ha^-1 was applied at sowing and dhaincha was incorporated at 50 days of growth. The results indicate that green manuring of direct-seeded rice with intercropped dhaincha is beneficial for substituting urea fertilizer up to 40 kg N ha^-1 and augmenting crop productivity under flood-prone lowland conditions.     

Applications of poultry litter and triple superphosphate fertilizer to a sandy soil: effects on soil phosphorus status and profile distribution

To determine P loadings, added through poultry litter, sufficient to cause downward movement of P from the cultivated layer of a sandy soil, six rates of poultry litter were applied annually for four years to a site in central England. (total loading 0 – 1119 kg P ha^-1). A single extra plot also received an extra 1000 kg ha^-1 as triple superphosphate (TSP; total loading 2119 kg P ha^-1) and three other treatments received 200 – 800 kg ha^-1 P as TSP only. Annual soil sampling in 30-cm increments to 1.5-m depth provided information on P build-up in the topsoil and P movement to depth. There were strong linear trends between P balance (P applied – P removed in crops) and total P, Olsen bicarbonate extractable P and water-soluble P in the topsoil. Phosphorus from TSP and poultry litter fell on the same regression lines, suggesting that both would be equally effective as fertilizer sources. We calculated that 100 kg ha^-1 surplus total P would increase the Olsen extractable P content by c. 6 mg kg^-1 and the water-soluble P by c. 5 mg kg^-1. Thus, relatively large amounts of P would need to be applied to raise soil P status. We found some evidence of P movement into the soil layers immediately below cultivation depth. However, neither soil sampling nor soil solution extracted through Teflon water samplers showed evidence of movement into the deep subsoil (1 m) despite large P loadings.     

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

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

Phosphorus sorption capacity of low activity clay soils of South Western Nigeria and its usefulness in evaluating P requirement of rice

Field and laboratory experiments were conducted on 15 low activity clay soils in Ogun State of Nigeria to evaluate the relationships between P sorption capacity and some soil properties and the use of sorption indices in evaluating the P requirement of rice. Langmuir adsorption capacity (b) varied from 30.9 to 414.3 µg g^–1. Although adsorption capacity was related significantly to a number of soil properties, citrate dithinonite bicarbonate (CDB) extractable Fe was the most important variable accounting for 99% of the variation in adsorption capacity. The solution P concentration (SPC) required to achieve 95% maximum grain yield of rice varied from 0.03 in a sandy clay soil to 0.19 µg ml^–1 in a sandy soil, while the quantity of fertilizer P required to attain the solution P concentration (Standard Phosphate requirement, SPR) varied from 14.1 to 88.7 kg ha^–1. Highly significant power function relationships were obtained between SPC and b (r=0.93) and between SPR and b (r=0.93). The P buffering capacity (PBC) of the soils indicated that the soils are moderately buffered. However, SPR accounted best for the variation in grain yield of rice on the field (R²=0.90). The use of P sorption indices in estimating P needs of rice appears superior to the use of chemical extractants.