Effect of phosphorus application in legume cover crop rotation on subsequent maize in the savanna zone of West Africa

The benefit of planted fallow with legume cover crops may be limited on P deficient soil. A trial was conducted at two P deficient sites in northern Nigeria to test the hypothesis that application of P to legume cover crop fallow can substitute for N application to subsequent maize. Mainplots consisted of leguminous fallows followed by unfertilized maize, or native (mostly grass) fallows followed by maize with 0 or 40 kg N ha⁻¹ (Kaduna) and 0, 30 or 60 kg N ha⁻¹ (Bauchi). Three rates of P (0, 9, and 18 kg ha⁻¹) were applied to fallow sub-plots as single superphosphate. In the first year, dry matter accumulation of lablab (Lablab purpureus) responded to P application, while mucuna (Mucuna cochinchinensis) dry matter did not. Lablab mulch dry matter during the dry season was significantly increased by previous season P application while mucuna was not. Previous fallow vegetation was a significant factor for maize growth in the second year but the interaction with P applied to the fallow was not significant at P < 0.05. Substantial and similar yield increases were achieved with application of N fertilizer to maize and from application of 9 kg P ha⁻¹ to previous lablab. Depending on local economic circumstances, a good use of expensive inorganic fertilizer might be to apply P sources to cover crop legumes to profit from additional N benefits as well as residual effects of applied P. This revised version was published online in June 2006 with corrections to the Cover Date.     

Grain legume rotation benefits to maize in the northern Guinea savanna of Nigeria: fixed-nitrogen versus other rotation effects

The yield increases often recorded in maize following grain legumes have been attributed to fixed-N and ‘other rotation’ effects, but these effects have rarely been separated. Field trials were conducted between 2003 and 2005 to measure these effects on maize following grain legumes in the northern Guinea savanna of Nigeria. Maize was grown on plots previously cultivated to two genotypes each of soybean (TGx 1448-2E and SAMSOY-2) and cowpea (IT 96D-724 and SAMPEA-7), maize, and natural fallow. The plots were split into four N fertilizer rates (0, 30, 60 and 90 kg N ha⁻¹) in a split plot design. The total effect was calculated as the yield of maize following a legume minus the yield following maize, both without added N and the rotation effect was calculated as the difference between rotations at the highest N fertilizer rate. The legume genotypes fixed between 14 and 51 kg N ha⁻¹ of their total N and had an estimated net N balance ranging from −29.8 to 9.5 kg N ha⁻¹. Positive N balance was obtained only when the nitrogen harvest index was greater than the proportion of N derived from atmosphere. The results also indicated that the magnitude of the fixed-N and other rotation effects varied widely and were influenced by the contributions of the grain legumes to the soil N-balance. In general, fixed-N effects ranged from 124 to 279 kg ha⁻¹ while rotation effects ranged between 193 and 513 kg ha⁻¹. On average, maize following legumes had higher grain yield of 1.2 and 1.3-fold compared with maize after fallow or maize after maize, respectively.     

锌肥有效施用的土壤条件研究

通过田间试验、相关分析研究了松嫩平原黑土和黑钙土的锌肥有效施用条件。结果表明 :当土壤 p H>7.2、石灰含量 >3.9%、砂粒含量 >40 %、代换量 <15 cm ol/ kg、有机质含量 <2 %、土壤有效锌含量低于缺锌临界值0 .8m g/ kg时 ,施用锌肥具有明显的增产效果。而在高磷情况下会降低土壤锌有效性 ,易引起玉米缺锌加重。吉单10 1和四单 8两个玉米品种对锌缺乏最为敏感。     

Adapting the potentially mineralizable N concept for the prediction of fertilizer N requirements

Quantification of N dynamics in the ecosystem has taken on major significance in today's society, for economic and environmental reasons. A major fraction of the available N in soils is derived from the mineralization of organic matter. For decades, scientists have attempted to quantify the rate at which soils mineralize N, but the complexity of the N cycle has made this a major task. Further, agronomists have long sought soil test methods that are practical, yet will provide accurate means of predicting the amounts and rates of release of N from soils. Such tests would allow us to make more precise fertilization decisions. This paper discusses the potentially mineralizable N concept, first promoted by Stanford and colleagues [61, 62, 64], and suggests how it may be incorporated into deterministic models, such as CERES and LEACHM, so as to provide more accurate estimates of N mineralization under field conditions. We also suggest how the potentially mineralizable N concept may be coupled to quick, routine laboratory methods of determining available soil N, such as the hot 2M KCl extracted NH₄-N method recently developed by Gianello and Bremner [35], and used together with deterministic N models, such as CERES, for predicting probable fertilizer N requirements.     

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

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

Nitrogen fertilizer in leucaena alley cropping. I. Maize response to nitrogen fertilizer and fate of fertilizer-15N

Field microplot experiments were conducted in the semi-arid tropics of northern Australia to evaluate the response of maize (Zea mays L.) growth to addition of N fertilizer and plant residues and to examine the fate of fertilizer¹⁵N in a leucaena (Leucaena leucocephala) alley cropping system, in which supplemental irrigation was used. Leucaena prunings, maize residues and N fertilizer were applied to alley-cropped maize grown in microplots which were installed in the alleys formed by leucaena hedgerows spaced 4.5 metres apart. The¹⁵N-labelled fertilizer was used to examine the fate of fertilizer N applied in the presence of mulched leucaena prunings and maize residues.Application of leucaena prunings increased maize yield while addition of N fertilizer in the presence of the prunings produced a further increase in maize production. There was a significant positive interaction between N fertilizer and leucaena prunings in increasing maize production. The addition of maize residues in the presence of N fertilizer and leucaena prunings decreased maize yield and N uptake and increased fertilizer¹⁵N loss from 38% to 47%. Maize recovered 24–79% of fertilizer¹⁵N in one cropping season, depending on application rate of N fertilizer and field management of plant residues. About 20–34% of fertilizer¹⁵N remained in the soil. More than 37% of fertilizer¹⁵N was apparently lost from the soil and plant system largely through denitrification when N fertilizer was applied at 40 kg N ha^–1 or more in the presence or absence of plant residues. Application of N fertilizer improved maize yield and increased the contribution of mulched leucaena prunings to crop production in the alley cropping system.     

Manure and fertilizer contributions to soil mineral nitrogen and the yield of forage maize

Fifteen field trials were conducted to evaluate soil mineral N measurement as a means for quantifying the total N supply to forage maize and so to form the basis for fertilizer recommendations on a crop-specific basis. In every trial, 4 rates of cattle manure N (nominally 0, 80, 160, 240 kg N per ha) and 4 rates of ammonium nitrate (0, 50, 100, 150 kg N per ha) were factorially combined. Soil mineral N measurements were made before manure application, at the time of maize drilling, 7-10 weeks after drilling and after harvest. Measurements on control treatments which received no manure or ammonium nitrate showed extensive net mineralisation of soil N (mean 140 kg N per ha) in the 7-10 weeks after drilling followed by a decrease due to crop uptake, and probably net immobilisation, of approximately the same amount by harvest. This net mineralisation was probably the reason why only one trial showed a significant dry-matter yield response to ammonium nitrate. Results indicated that , to be useful for N recommendations, soil mineral N measurements should be taken 7-10 weeks after drilling. Only if the amount of mineral N at this time is less than expected crop N offtake should fertilizer N be applied. A mean of around 64% of the N applied in ammonium nitrate could be accounted for in soil mineral N after harvest of the maize, although this was reduced to 24% in the single trial where a dry-matter response to ammonium nitrate was recorded.     

Detectability of15N-depleted fertilizer N in soil and plant tissue during a corn-rye crop rotation

Use of¹⁵N-depleted fertilizer materials have been primarily limited to fertilizer recovery studies of short duration. The objective of this study was to determine if¹⁵N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn (Zea mays L.) -winter rye (Secale cereale L.) crop rotation. Nitrogen as¹⁵N-depleted (NH₄)₂SO₄ was applied at five rates (0, 84, 168, 252, and 336 kg N ha^–1) to corn. Immediately following corn harvest a winter rye cover crop treatment was initiated. Residual fertilizer N was easily detected in the soil NO ₃ ^- -N fraction following corn harvest (140-d after application). Low levels of exchangeable NH ₄ ⁺ -N (<2.5 mg kg^–1) did not permit accurate isotope-ratio analysis. Fertilizer-derived N recovered in the soil total N fraction following corn harvest was detectable in the 0 to 30-cm depth at each N rate and in the 30 to 60 and 60 to 90-cm depths at the 336 kg ha^–1 N rate. Atom %¹⁵N concentrations in the nonexchangeable NH ₄ ⁺ -N fraction did not differ from the control at each N rate. Nitrogen recovery by the winter rye cover crop reduced residual soil NO ₃ ^- -N levels below the 10 kg ha^–1 level needed for accurate isotope-ratio analysis. Atom %¹⁵N concentrations in the soil total N fraction (approximately one yr after application) were indistinguishable from the control plots below the 168, 252, and 336 kg ha^–1 N rate at the 0 to 30, 30 to 60, and 60 to 90-cm depths, respectively. Recovery of residual fertilizer N by the winter rye cover crop was verified by measuring significant decreases in atom %¹⁵N concentrations in rye tissue with increasing N rates. The greatest limitation to the use of¹⁵N-depleted fertilizer N as a tracer of residual fertilizer N in a corn-rye crop rotation appears to be its detectibility from native soil N in the total N pool.Research partially supported by grants from the National Fertilizer and Environmental Research Center/TVA and the Virginia Division of Soil and Water Conservation.     

Fertility status of soils of the derived savanna and northern guinea savanna and response to major plant nutrients, as influenced by soil type and land use management

Although the fertility status of soils in the West African moist savanna is generally believed to be low, crop yields on farmers' fields vary widely from virtually nil to values near the potential production. The soil fertility status was evaluated for a number of farmers' fields selected at random in 2 villages (Zouzouvou and Eglimé) representative for the derived savanna (DS) benchmark area and in 2 villages (Danayamaka and Kayawa) representative for the Northern Guinea savanna (NGS) benchmark area. The relation between soil fertility status and soil type characteristics and fertilizer use was explored. In an accompanying missing nutrient greenhouse trial, the most limiting nutrients for maize growth were determined. While soils in the DS villages were formed on different geological units, soils in the NGS villages could be differentiated according to their position on the landscape. Generally, soils in the DS contained a smaller amount of silt (104 vs. 288 g kg^–1), a larger amount of sand (785 vs. 584 g kg^–1), C (9.3 vs. 6.3 g kg^–1), N (0.7 vs. 0.5 g kg^–1), Olsen-P (10.7 vs. 5.4 mg kg^–1), and had a higher CEC (7.0 vs. 4.8 cmolc kg^–1) than soils in the NGS villages. The large silt content of the soils in the NGS is a reflection of the aeolian origin of the parent material. Within the benchmark areas, general soil fertility characteristics were similar in the villages in the NGS, except for a larger amount of particulate organic matter in Kayawa than in Danayamaka. This may also have led to a significantly larger amount of ammonium-N content in the 0–20 and 20–40 cm soil layers in Kayawa compared to Danayamaka (42 vs 24 kg N ha^–1 in the 0–20 cm soil layer). Differences in topsoil soil characteristics between the DS villages were a reflection of differences in clay quality (kaolinitic vs. 2:1 clay minerals) of the parent material and past fertilizer use. The Olsen-P and exchangeable K contents were observed to increase with increased fertilizer application rate in both benchmarks, while fertilizer application rate had no significant effect on the organic C or total N content of the soil nor on its ECEC. The response of maize shoot biomass production to applied N was similar for both benchmarks (biomass accumulation in the treatment without N was, on average, 55% of the biomass production in the treatment which received all nutrients), while soils in the NGS responded more strongly to applied P than soils in the DS (37% vs 66% of biomass production in the treatment which received all nutrients). The more favourable P status of soils in Eglimé (DS) was attributed to the more intense use of P fertilizers, as a result of government-supported cotton production schemes. Response to cations, S or micronutrients were neglegible. A significant linear relationship was found between the soil Olsen-P content and the response to applied P up to levels of 12 mg kg^–1 in the topsoil. Above this level, a plateau was reached.     

7. The efficacy and loss of fertilizer N in lowland rice

Nitrogen fertilization is a key input in increasing rice production in East, South, and Southeast Asia. The introduction of high-yielding varieties has greatly increased the prospect of increasing yields, but this goal will not be reached without great increases in the use and efficiency of N on rice. Nitrogen enters a unique environment in flooded soils, in which losses of fertilizer N and mechanisms of losses vary greatly from those in upland situations. Whereas upland crops frequently use 40–60% of the applied N, flooded rice crops typically use only 20–40%. There is a great potential for increasing the efficiency of N uptake on this very responsive crop to help alleviate food deficits in the developing world.This article reviews current use of N fertilizers (particularly urea) on rice, the problems associated with rice fertilization, and recent research results that aid understanding of problems associated with N fertilization of rice and possible avenues to increase the efficiency of N use by rice.     

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

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

Comparative response of bread and durum wheat cultivars to nitrogen fertilizer in a rainfed Mediterranean environment: soil nitrate and N uptake and efficiency

A 3-year multi-site study was carried out on rainfed Vertisols under Mediterranean conditions in southern Europe to determine the influence of the N fertilizer rate on soil nitrates, N uptake and N use efficiency in bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L. var. Durum Desf.) in rotation with sunflower (Heliathus annuus L.). Nitrogen fertilizer rates were 0, 100, 150 and 200 kg N ha⁻¹ applied in equal proportions at sowing, tillering and stem elongation. The experiment was designed as a randomized complete block with a split plot arrangement and four replications. Nitrogen harvest index (NHI), N uptake/grain yield (NUp/GY), N use efficiency (NUE), N utilization efficiency (NUtE), N uptake efficiency (NUpE) and N apparent recovery fraction (NRF) were calculated. Differences were observed in N use efficiency between the two modern bread wheat and durum wheat cultivars studied. In comparison to durum, bread wheat displayed greater N accumulation capacity and a more efficient use of N for grain production. While under N-limiting conditions, the behavior was similar for both wheat types. No difference was noted between wheat types with regard to changes in soil residual levels over the study period at the various sites. The 100-kg ha⁻¹ N fertilizer rate kept soil nitrates stable at a moderate level in plots where both wheat types were sown.     

Response of rainfed bread and durum wheat to source,level and timing of nitrogen fertilizer on two Ethiopian Vertisols: II. N uptake,recovery and efficiency

In trials conducted at 2 highland Vertisol sites in Ethiopia in 1990 and 1991, 2 locally popular wheat cultivars, 1 spring bread wheat (Triticum aestivum L.) and 1 durum wheat (T. durum Desf.), were supplied with nitrogen (N) fertilizer at 0, 60 and 120 kg N ha^–1 in the form of large granular urea (LGU), standard urea prills or ammonium sulfate. N was applied all at sowing, all at mid-tillering or split-applied between these two stages (1/3:2/3). While durum wheat exhibited the highest N concentration in grain and straw, bread wheat, because of its higher productivity, resulted in a greater grain and total N uptake. In general, split application of N and use of LGU as N source enhanced grain and total N uptake, apparent N recovery and agronomic efficiency of N, particularly under severe water-logging stress. Where significant, the interactions among the experimental factors substantiated the superior responsiveness of the bread wheat cultivar to fertilizer N, and the beneficial effects of split N application and LGU as an N source. Split application of N tended to nullify the positive effects of LGU, presumably by approximating the delayed release of N achieved with LGU. Considering the potential benefits to Ethiopian peasant farmers and consumers, split application of N should be advocated, particularly on water-logged Vertisols; LGU could be an advantageous N source assuming a cost comparable to the conventional N source urea.     

Fertilizer requirements for specified yield targets. I. Theoretical derivation of mathematical models for the computation of soil and fertilizer nutrient efficiencies

The conventional deduction procedure of computation of soil () and fertilizer () nutrient efficiencies for the amount of fertilizer required for specified yield targets does not make provision of the amount of soil nutrient derived by crops from the available pool of soil nutrients not accounted for in the amount extracted by a soil test procedure. The derivation of two mathematical models, viz., Tamil Nadu Agricultural University Model I [TNAU Model I] and Model II [TNAU Model II] is reported in this paper which aim at computing the soil () and fertilizer () nutrient efficiencies not accounted for by the conventional method.In the case of TNAU Model I, the relationship between the nutrient uptake (U) and the soil (S) and the fertilizer (F) nutrients was established by assuming a functional relationship of the type U =S +F such that 0 1 and 0 1. In TNAU Model II the same relationship was established as U =S +F + such that 0 1, 0 1 and > 0. The term in the latter model is a measure of the amount of soil nutrient the crop absorbs from a slowly available pool of nutrients not accounted for in the amounts extracted by the soil test procedure employed or applied through fertilizer.The field verification of these models is reported elsewhere.     

Effect of climate on the recovery in crop and soil of15N-labelled fertilizer applied to wheat

Data was assembled from experiments on the fate of¹⁵N-labelled fertilizer applied to wheat (Triticum spp.) grown in different parts of the world. These data were then ranked according to the annual precipitation-evaporation quotient for each experimental location calculated from the average long-term values of precipitation and potential evaporation. Percentage recovery of¹⁵N fertilizer in crop and soil varied with location in accordance with the precipitation-evaporation quotient. In humid environments more¹⁵N fertilizer was recovered in the crop than in the soil, while in dry environments more¹⁵N fertilizer was recovered in the soil than in the crop. Irrespective of climatic differences between locations 20% (on average) of the¹⁵N fertilizer applied to wheat crops was unaccounted for at harvest. Most of the¹⁵N fertilizer remaining in the soil was found in the 0–30 cm layer. The most likely explanation of these differences is that wheat grown in dry environments has a greater root:shoot ratio than wheat grown in humid environments and, further, that the residue of dryland crops have higher C/N ratios. Both factors could contribute to the greater recovery of¹⁵N fertilizer in the soil in dry environments than in humid ones.     

Timing of N fertilization on N2 fixation, N recovery and soil profile nitrate dynamics on sorghum/pigeonpea intercrops on Alfisols on the semi-arid tropics

Cropping systems and fertilizer management strategies that effectively use applied nitrogen (N) are important in reducing costs of N inputs. We examined the effect of time of N application on dry matter (DM) and grain yield (GY), N accumulation, the N budget in crop from soil, fertilizer and atmosphere, and the fertilizer N use efficiency (estimated by the conventional difference method, and the direct ¹⁵N recovery by the crops), in a sorghum/pigeonpea intercropping system on an Alfisol (Ferric Luvisols (FAO); or Udic Rhodustalf (USDA) in India. Fertilizer N was applied at planting (basal) and at 40 days after sowing (delayed). Nitrogen was applied only to the sorghum rows in the intercropping treatment. Nitrogen derived from air (N_dfa) was estimated by the¹⁵ N natural abundance method, and N derived from fertilizer (N_dff) was estimated by the ¹⁵N isotope dilution method. Delaying N fertilization till 40 days after sowing (DAS), rather than applying at sowing increased DM and GY of the sorghum, but not of pigeonpea. Delaying N fertilization to sorghum for 40 days significantly (p<0.001) increased ¹⁵N recovery in shoot from 15 to 32% in sole crop, and from 10 to 32% in intercrop. Similarly, there was a significant (p<0.001) increase in N recovery (by the difference method) from 43 to 59% in sole crop and from 28 to 71 % in intercrop sorghum. Fertilizer N recovery by sole crop pigeonpea (14%) was higher than intercrop pigeonpea (2–4%). Pigeonpea fixed between 120–170 kg ha-¹ of atmospheric N throughout the cropping season. Although there was a marked difference in nitrate-N (N0₃-N) concentrations between basal and delayed treatments at planting, no difference was observed in N0₃-N concentrations in soil solution between the treatments at 40 DAS. Our data on N accumulation by plants showed that the rate of N depletion or disappearance from the soil solution was 2–3 times faster than N accumulation by plants, suggesting that an appreciable amount of N0₃-N would disappear from soil solution in the top soil without being utilized by crops during the initial growth stage. This revised version was published online in August 2006 with corrections to the Cover Date.     

Yield response of maize (Zea mays L.) to crop residue management on two major soil types Of Alemaya, Eastern Ethiopia: I. Effects of varying rates of applied and residual N and P fertilizers

Field trials were conducted on two soil types for seven years (1988–1994) to investigate grain yield response of maize to crop residue application as influenced by varying rates of applied and residual N and P fertilizers. Yearly application of N and P fertilizers at both one-half and full recommended rates resulted in grain yield increases of more than 500 and 1100 kg ha-1, respectively over application of only crop residue. Moreover, grain yield responses due to residual N and P fertilizers applied only during the first year were found to be comparable to the yearly applications of these fertilizers. Rainfall and soil type have exerted considerable influences on the grain yield response obtained in this study. Grain yield exhibited a corresponding decrease with decreasing rainfall. Grain yield increases on Typic Pellustert were relatively higher than on Typic Ustorthent.     

The current and residual value of superphosphate for lupins grown in rotation with oats and wheat on a deep sandy soil

In a field experiment on a deep pale-yellow sand in a 600 mm per annum rainfall Mediterranean environment of south-western Australia, six levels of phosphorus (P) as superphosphate (O up to 546 kg P ha^–1) were applied once only, to the soil surface, before sowing lupins (Lupinus angustifolius). The lupins were grown in a continuous arable cropping rotation with, in successive years, oats (Avena sativa), wheat (Triticum aestivum), lupins. Five such rotations were started in the experiment from 1985 to 1989. The experiment continued until the end of 1990.The relationship between lupin seed (grain) yields and the level of P applied was measured in the year of P application for five successive years (1985 to 1989). The relationship had the same general form but it varied between years, largely due to different maximum yields (yield plateaux) in each year.The residual value of superphosphate applied three years previously was measured for lupins on two occasions (1988 and 1989) relative to superphosphate applied in the current year. The residual values was different in the two years. The superphosphate applied three years previously was about 30% as effective as freshly applied superphosphate in 1988, and 12% as effective in 1989.At each harvest, the relationship between grain yield and the P concentration in the grain differed for different species. However, for each species at each harvest, the relationship was similar regardless of when the P was applied in the previous years. Thus each species had the same internal efficiency of P use curve, and yields varied only with P concentration in tissue.Bicarbonate-extractable soil P was determined on soil samples taken in mid-July of 1989 and 1990. These soil test values were related to grain yields at harvest. The relationship between yield and soil test values had the same general form but varied for different species within years and for each species between years. It also varied for each species within years depending on the year the P was applied.     

Fertilizer requirements for specified yield targets. II. Field verification of mathematical models for the estimation of soil and fertilizer nutrient efficiencies

Sugarcane response data from field experiments conducted between May 1979 and August 1981 on a sandy clay loam soil (Udic Haplustalf) of Coimbatore district, Tamil Nadu State, India were used in the present investigation. Soil () and fertilizer () nutrient efficiencies for the amount of fertilizer required for specified cane yield targets were computed from this data by three procedures, viz., conventional deduction procedure, Tamil Nadu Agricultural University Model I [TNAU Model I] and Model II [TNAU Model II].In the case of nitrogen, both TNAU Model I and TNAU Model II gave more realistic estimates of and than those determined by the conventional deduction procedure. The differences in the predicted amounts of fertilizer nitrogen required between these two models were well within the permissible limits of variation indicating that both these approaches can be followed for the amount of nitrogen required for specified yield targets.The Olsen's procedure for available phosphorus estimation was inadequate to explain the relationship between soil available phosphorus and sugarcane response as indicated by results obtained using the TNAU Model II. The incorporation of the term in this model caters for the actual situation in the field in respect of the relationship between soil and fertilizer phosphorus availabilities and phosphorus uptake by sugarcane proving usefulness of this model for assessing the amount of phosphorus required for specified cane yield targets.The results indicated that a considerable amount of potassium from the soil reserve was released into the soil available pool due to a priming effect. This fraction was preferentially absorbed by sugarcane compared to the fractions extracted by 0.1 N HNO₃ as indicated by results obtained using the TNAU Model II. In this case too, the actual situation regarding the relationship between soil and fertilizer potassium availabilities and potassium uptake by sugarcane is catered for by this model proving its superiority over the other two procedures for assessing the amount of potassium required for specified yield targets.     

Recovery of 15N-labelled fertilizer applied to winter wheat and perennial ryegrass crops and residual 15N recovery by succeeding wheat crops under different crop residue management practices

The recovery of ¹⁵N-labelled fertilizer applied to a winter wheat (120 kg N ha^–1) and also a perennial ryegrass (60 kg N ha^–1) crop grown for seed for 1 year in the Canterbury region of New Zealand in the 1993/94 season was studied in the field. After harvests, ryegrass and wheat residues were subjected to four different residue management practices (i.e. ploughed, rotary hoed, mulched and burned) and three subsequent wheat crops were grown, the first succeeding wheat crop sown in 1994/95 to examine the effects of different crop residue management practices on the residual ¹⁵N recovery by succeeding wheat crops. Total ¹⁵N recoveries by the winter wheat and ryegrass (seed, roots and tops) were 52% and 41%, respectively. Corresponding losses of ¹⁵N from the crop-soil systems represented by un-recovered ¹⁵N in crop and soil were 12% and 35%, respectively. These losses were attributed to leaching and denitrification. The proportions of ¹⁵N retained in the soil (0-400 mm depth) at the time of harvest of winter wheat and ryegrass were 36% and 24%, respectively. Although the soil functioned as a substantial sink for fertilizer N, the recovery of this residual fertilizer by subsequent three winter wheat crops was low (1-5%) and this was not affected by different crop residue management practices.