Nitrogen effects of non-legume catch crops

Nitrate leaching during the winter period can be reduced and often prevented by growing catch crops after the harvest of a main crop. However, catch crops which effectively take up residual nitrogen do not necessarily show good nutrient effects on a succeeding main crop. The objective of this experiment was to investigate how the content of soil mineral nitrogen in spring was affected by the time of incorporation of non-legume catch crops and how the yield and nitrogen uptake of a succeeding main crop was influenced. The yield of spring sown onion and white cabbage was significantly increased by catch crop growing the previous autumn. The nitrogen effect of Italian ryegrass corresponded to 50–100 kg N per ha in the vegetables. However, the yield of spring barley was not significantly affected by the nitrogen released from decomposing catch crops. During decomposition of non-legume catch crops, grown at a high level of nitrogen fertility, nitrogen immobilization did not occur.     

Incorporation time of nitrogen catch crops influences the N effect for the succeeding crop

The significance of incorporation date of a catch crop on the nitrogen supply for the subsequent crop, the N effect (N_eff), was examined. Winter rye was grown as a catch crop for 3 years during the autumn, and incorporated on five dates, two in the autumn and three in the spring. Two of the winters had high precipitation, and the N_eff was small at the early autumn incorporation date, but increased when incorporation was delayed into late autumn and further increased by early spring incorporation. In the third winter, which was very dry, the N_eff was negative at all incorporation dates, with the negative effect gradually increasing in value the later the incorporation date. In all 3 years the N_eff was reduced when incorporation was delayed from early spring until later in the spring. The main processes determining this pattern were found to be (1) the risk of leaching of N mineralized after catch crop incorporation, which can reduce the N_eff at early incorporation under wet conditions, (2) pre-emptive competition which can reduce the N_eff when incorporation is delayed until later in the spring, and in dry conditions is already apparent during the autumn, and (3) catch crop growth leading to carbon gain and increased C/N ratio which decreases mineralization and thus the N_eff after delayed incorporation in the spring. Lack of time for catch crop N uptake prior to early incorporation, or lack of time for N mineralization after late incorporation which might also reduce the N_eff did not appear to be important in our experiment. The results show that catch crops grown in high rainfall areas on sandy soils should be incorporated later than those in low rainfall areas on nitrate retentive soils.     

Measured and simulated availability and leaching of nitrogen associated with frequent use of catch crops

Abstract. Results are presented from three years (1992-1995) of a field leaching experiment on a sandy soil in south-west Sweden. Plots of spring cereals, either with or without an undersown perennial ryegrass catch crop, were compared for nitrogen leaching and nitrogen status in soil. Both treatments were ploughed in spring, and other tillage regimes were also identical. Measurements of nitrogen leaching from drains, nitrogen uptake in crops and mineral nitrogen in the soil were made. Two coupled, simulation models, which describe water flow and nitrogen transformations and transport in soil, were used to interpret data and to calculate the nitrogen budget and nitrogen mineralization in the soil.
Nitrogen leaching was 40 50% less in the catch crop treatment compared with the control during years when the establishment of the catch crop succeeded. In the third year of the experiment nitrogen leaching was actually greater in the catch crop treatment (7 kg N/ha). This increase was caused by a poorly established catch crop coinciding with enhanced mineralization of previous catch crop residues. There was no simulated change in soil organic nitrogen in either of the treatments. Simulations showed increased nitrogen mineralization during April-July after incorporation of plant material in spring, especially in the catch crop treatment. However, the increased nitrogen mineralization probably occurred too late for the released nitrogen to be fully available to the main crop.     

Nitrogen mineralization and availability of mixed leguminous and non-leguminous cover crop residues in soil

Whereas non-leguminous cover crops such as cereal rye (Secale cereale) or annual ryegrass (Lolium multiflorium) are capable of reducing nitrogen (N) leaching during wet seasons, leguminous cover crops such as hairy vetch (Vicia villosa) improve soil N fertility for succeeding crops. With mixtures of grasses and legumes as cover crop, the goal of reducing N leaching while increasing soil N availability for crop production could be attainable. This study examined net N mineralization of soil treated with hairy vetch residues mixed with either cereal rye or annual ryegrass and the effect of these mixtures on growth and N uptake by cereal rye. Both cereal rye and annual ryegrass contained low total N, but high water-soluble carbon and carbohydrate, compared with hairy vetch. Decreasing the proportion of hairy vetch in the mixed residues decreased net N mineralization, rye plant growth and N uptake, but increased the crossover time (the time when the amount of net N mineralized in the residue-amended soil equalled that of the non-amended control) required for net N mineralization to occur. When the hairy vetch content was decreased to 40% or lower, net N immobilization in the first week of incubation increased markedly. Residue N was significantly correlated with rye biomass (r=0.81, P<0.01) and N uptake (r=0.83, P<0.001), although the correlation was much higher between residue N and the potential initial N mineralization rate for rye biomass (r=0.93, P<0.001) and N uptake (r=0.99, P<0.001). Judging from the effects of the mixed residues on rye N Concentration and N uptake, the proportion of rye or annual ryegrass when mixed with residues of hairy vetch should not exceed 60% if the residues are to increase N availability. Further study is needed to examine the influence of various mixtures of hairy vetch and rye or annual ryegrass on N leaching in soil. Received: 10 March 1997     

残落物混合分解对杨树-农作物复合系统土壤碳氮矿化的影响

采用室内培养的方法研究杨-麦、杨-花生等不同复合经营模式下,杨树叶与农作物秸秆混合后对土壤碳、氮矿化及土壤微生物量的影响。结果表明:(1)单一模式中,花生叶处理的有机碳矿化累积量最大,花生茎秆、杨树叶处理次之,小麦秸秆处理最低。混合处理有机碳矿化累积量依次为杨树叶-花生叶>杨树叶-花生茎秆>杨树叶-小麦秸秆,且培养结束时,混合物表现出明显的促进作用;(2)土壤微生物量碳、氮与各残落物氮含量、C/N比存在显著的相关性;(3)杨树叶、小麦秸秆及其混合物处理的土壤矿质态氮含量均低于对照,而添加花生叶、花生茎秆以及它们与杨树叶的混合物使矿质态氮含量高于对照。试验说明杨-麦、杨-花生复合模式均能有效提高土壤微生物的生物量,调节碳的动态及氮的供应,而选择种植含氮量高的农作物更有利于促进残落物分解和养分归还,这对深入研究林-农复合系统的模式筛选、结构优化及可持续经营具有一定的现实意义。     

Plant availability of catch crop sulfur following spring incorporation

Catch crops might reduce sulfate leaching and thereby increase the overall sulfur (S)‐use efficiency in crop rotations. At two experimental sites in Denmark (a sandy loam and a coarse sand), S uptake of catch‐crop species was measured. Furthermore, net release of S following incorporation of this material (S contents 0.13%–1.03%, C:S ratios of 40–329, and lignin contents of 1%–10.8%) was investigated in a pot experiment with spring barley in sandy soil. The catch crops showed huge differences in their ability to sequester S. The best catch crops (legumes on sandy loam), sequestered 10–12 kg S ha^–1, and the poorest catch crops (ryegrass and sorrel on coarse sand) sequestered less than 3 kg S ha^–1. The S‐mineralization rates were highest for crucifers (57%–85% of total S added) and lowest for legumes (up to 46% of total S added). Differences can partly be explained by the C:S ratio, whereas no significant relationship was found with the lignin content of the incorporated catch crops. Catch crops may help to avoid S deficiency and increase synchrony between plant demand and available soil S in a crop rotation. However, the release of S will not fulfil the need of S‐demanding crops and even for cereals, the mineralization will most often only make a contribution. In the case of legume catch crops, it is advisable to use a supplemental S source.     

Nitrogen and carbon mineralization of surface-applied and incorporated winter wheat and peanut residues

Laboratory incubation experiments were conducted to study the C and N mineralization dynamics of crop residues (fine roots and straw) of the two main crops (winter wheat and peanut) in the Chinese Loess Plateau under different ways of incorporation. The C mineralization patterns of the soil amended with winter wheat residues differed greatly, and the highest C mineralization was observed in the treatment with winter wheat straw incorporated (39% of the total added C mineralized). The way of straw placement had only a minor effect on the pattern of C mineralization for peanut. Generally, winter wheat residues showed a stronger immobilization than peanut residues during the incubation period, without any net N release. Winter wheat straw incorporated showed the strongest N immobilization with 35 mg kg⁻¹ (equivalent to 27% of added N) immobilized at the eighth week. This study indicated that retaining crop residues at the soil surface in the dry land soils of the Chinese Loess Plateau is beneficial for C sequestration. It also showed that N immobilization occurs only during a limited period of time, sufficient to prevent part of the mineral N pool from leaching, and that net N mineralization can be expected during the subsequent cropping season, thus enhancing synchronization of N supply and demand.     

Nitrogen Contribution of Peanut Residue to Cotton in a Conservation Tillage System

ABSTRACT

Leguminous crops, particularly winter annuals, have been utilized in conservation systems to partially meet nitrogen (N) requirements of succeeding summer cash crops. Previous research also highlights the benefits of utilizing summer annual legumes in rotation with non-leguminous crops. This study assessed the N contribution of peanut (Arachis hypogaea L.) residues to a subsequent cotton (Gossypium hirsitum L.) crop in a conservation system on a Dothan sandy loam (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) at Headland, AL during the 2003–2005 growing seasons. Treatments were arranged in a split plot design, with main plots of peanut residue retained or removed from the soil surface, and subplots as N application rates (0, 34, 67, and 101 kg ha^{? 1}) applied in fall and spring. Peanut residue did not influence seed cotton yields, leaf N concentrations, or plant N uptake for either growth stage or year of the experiment. There was a trend for peanut residue to increase whole plant biomass measured at the first square in two of three years. Seed cotton yields and plant parameters measured at the first square and mid-bloom responded favorably to spring N applications, but the recommended 101 kg N ha^{? 1} did not maximize yields. The results from this study indicate that peanut residue does not contribute significant amounts of N to a succeeding cotton crop, however, retaining residue on the soil surface provides other benefits to soils in the southeastern U.S.     

Green Manuring with Clover and Ryegrass Catch Crops Undersown in Small Grains: Effects on Soil Mineral Nitrogen in Field and Laboratory Experiments

Abstract

In three field trials in southern Norway, Italian ryegrass (Lolium multiflorum Lam.), white clover (Trifolium repens L.) or subterranean clover (T. subterraneuni L.) was undersown in spring grain at three N fertilizer rates and ploughed under in late October as a green manure for a succeeding spring grain crop. The content of topsoil (0-20 cm) mineral nitrogen was determined during the growth of the grain crop, after grain harvest and after ploughing. In addition, mineralization of nitrogen and carbon was measured in green-manured soil incubated at 15°C and controlled moisture conditions. During grain crop growth, ryegrass tended to reduce soil mineral N compared with the other treatments. After grain harvest, in a small-plot experiment where extra nitrate was added, ryegrass reduced soil nitrate N (0-18 cm) from 4.2 to 0.4 g m^?2 within 13 days, while the clovers had negligible effect compared with bare soil. Up to 9.4 g N m^?2 was present in above-plus below-ground ryegrass biomass at ploughing. In incubated ryegrass soil, there was a temporary net N immobilization of up to 0.9 g N m^?2 as compared with unamended soil. In clover-amended soil, mineral N exceeded that in unamended soil by up to 5 g N m^?2.     

Effect of catch crops on the content of soil mineral nitrogen before and after winter leaching

Since large amounts of nitrogen may be lost by leaching during the winter period, investigations have been carried out to find suitable crops for catching nitrogen during the autumn. The plant species phacelia, common sunflower and italian ryegrass were sown at three times during the late summer. The soil was analysed for mineral nitrogen at the end of November just before incorporation of the catch crops and in the middle of April the following year. The dry matter production and the uptake of nitrogen decreased as the establishment was postponed and the growth period thereby decreased. When sown in the middle of July or at the beginning of August phacelia and italian ryegrass accumulated 150 kg N per ha in the above ground plant parts. The content of soil mineral nitrogen in November was reduced by growing catch crops during the autumn. The content of soil mineral nitrogen increased as the growth period of the catch crops decreased. The content of mineral nitrogen in the upper 50 cm soil layer in April was, irrespective of plant species, increased by catch crops. When no crop was grown the difference between the content of soil mineral nitrogen measured before and after the winter period indicated a net loss of 144 kg of N per ha in 0–100 cm soil depth. When italian ryegrass was sown in the middle of July the previous year the content of soil mineral nitrogen found in April was 59 kg per ha higher compared to the content found in November.     

ESTIMATING SHORT- AND MEDIUM-TERM AVAILABILITY TO CEREALS OF NITROGEN FROM ORGANIC RESIDUES

Overused soil resources and the build-up of organic residues from industrial processes have resulted in increased risk of environmental contamination. Recycling of organic residues from industry by incorporation into agricultural soil, can provide valuable organic amendment as well as supply nutrients to crops. The effect of applying organic amendments to an agricultural sandy soil on the nitrogen nutrition of wheat (Triticum aestivum L.) and residual effects on the growth of a following maize crop (Zea mays, L.), were assessed under semi-controlled environmental conditions and were compared to nitrogen mineralization prediction obtained from an aerobic incubation. Six different organic residues (composted municipal solid waste, secondary pulp-mill sludge, hornmeal, poultry manure, the solid phase from pig slurry and composted pig manure) were added to a Cambic arenosol, incubated or used in pot experiments, to evaluate and try to predict the availability to crop plants of nitrogen released from these materials. Poultry manure was the most effective amendment in making nitrogen available and enhancing nitrogen uptake by wheat plants resulting in greater dry matter yield. The dried solid phase from pig slurry and hornmeal were also beneficial to wheat growth. There was a greater recovery of nitrogen (N), from organic materials studied, by a maize crop. Poultry manure was the residue that provided a greater residual effect on N supply to maize.     

Nitrogen release from differently aged Raphanus sativus L. nitrate catch crops during mineralization at autumn temperatures

In temperate climates with surplus precipitation and low temperatures during autumn and winter, nitrate catch crops have become crucial in reducing nitrate leaching losses. Preferably, the N retained by the catch crop should remain in the soil and become available to the next main crop. Fodder radish (Raphanus sativus, L.) has emerged as a promising nitrate catch crop in cereal cropping, although the course of remineralization of residue N following termination of this frost‐sensitive crucifer remains obscured. We incubated radish residues of different age (different planting and harvest dates) with a loamy sand soil; mineralization of residue N was determined after 1, 2, 4 and 7 months of incubation at 2 °C and 10 °C. Incubations with soil only and with residues of white mustard (Sinapis alba, L) and perennial ryegrass (Lolium perenne, L.) were included as references. Using linear regression, net N release was fitted to plant chemical characteristics (initial concentrations of N, fibre fractions, lignin and C/N ratio). Residue C/N ratio (ranging from 10 to 25) and N concentration (ranging from 17 to 40 mg N/g dry matter) showed superior fits to net N release at both temperatures (r², 0.64–0.94) while fibre analyses provided inferior fits (r², 0.12–0.64). This was true across planting date and plant age. Net N release after 7 months of incubation at 2 °C and 10 °C accounted for up to 40% and 50% of residue N, respectively. During most of the incubation period, nitrate dominated the mineral N pool at both temperatures. The N mineralization and nitrification potential at these low soil temperatures suggest that a considerable fraction of the N captured by nitrate catch crops may be remineralized, nitrified and thus available for plant uptake but also for loss by leaching and denitrification.     

Effect of N supply on apparent recovery of fertilizer N as crop N and Nmin in soil during and after cultivation of winter cereals

Field trials were conducted over two years to investigate the effect of increasing N supply on apparent fertilizer N recovery by winter cereal crops (4 × wheat and 2 × barley) and on non‐recovered N. Apparent fertilizer N recovery was calculated by comparing N in fertilized and unfertilized crops. Non‐recovered N is defined as N which was neither found in crops nor soil mineral N (N_min = NH₄‐N + NO₃‐N). At N supply levels according to common farming practice (N_cfp = 190 to 220 kg N/ha), 60— 93% of the fertilizer N was recovered in crops at harvest, while at high N supply levels of 265 to 273 kg N/ha 58—76% of fertilizer N was recovered. There were small differences in soil N_min in 0—200 cm between N_cfp and unfertilized plots, but substantial increases in N_min occurred at the highest N supply. Amounts of non‐recovered N differed substantially between sites (maximum value of 84 kg N/ha). Non‐recovered N increased with increasing N rate on only 3 out of the 6 sites, indicating that N immobilization was not necessarily dependent on N rate. The fate of non‐recovered N was studied for a further year by growing catch crops on the sites after cereal harvest. N re‐mineralization deduced from changes in catch crop N and in N_min indicated that non‐recovered N had been immobilized in the soil. At three sites, crop N uptake was found between milk‐ripe stage and harvest (19 to 60 kg N/ha) suggesting substantial uptake of N mineralized from soil. However, grain yields were lower with N rates below N_cfp, indicating that late net soil N mineralization could not compensate for reductions in N fertilizer rate in these trials.     

Losses in Nitrogen Uptake by Tomato and Pepper Due to Infestation by the Weed Barnyardgrass

Abstract

Glasshouse experiments showed that the weed “barnyardgrass” (Echinochloa crus‐galli) competes for nitrogen (N) with tomato and pepper crops. Competition was more severe with pepper than with tomato, and greater in both crops the earlier the weed emerged or the longer it grew with the crops. This competition affected growth attributes, fruit yield and its components, and N uptake in both crops. Shoot N content was also affected in pepper. Significant damage to both crops occurred even when weed emergence was as late in the crop growth season as the beginning of flowering.     

Soil nitrogen supply of wheat in relation to method of cultivation

The supply of soil nitrogen to successive wheat crops was compared for direct-drilled and conventional tillage systems on a red earth in south-eastern Australia. There was no significant effect of tillage on the total plant uptake of nitrogen, nor on the decline of soil nitrogen during a 5-year cropping cycle. During the growth of the last two crops in the cycle, the rates of net mineralization, nitrate leaching from the topsoil and crop-nitrogen uptake were measured, for both tillage systems, by an in situ sequential sampling method. For the first of these crops the rates of net mineralization were unaffected by tillage, but for the second crop the rates were greater on the direct-drilled soil. Variation in the rates of mineralization within both seasons could largely be explained by changes in temperature and soil water content. Leaching of nitrate from the topsoil was consistently greater for the direct-drilled soil, where seedlings initially accumulated less nitrogen than those growing on conventionally-cultivated soil. However, the differences in uptake did not persist, and at maturity similar amounts of nitrogen, representing less than 50% of the seasonal net mineralization, were accumulated in the above-ground parts of crops grown under the two systems.     

北方设施菜地夏季不同填闲作物的吸氮效果比较研究

为筛选出吸氮效果明显的北方设施菜地夏季填闲作物,在北京郊区设施菜地,以甜玉米、高丹草、红叶苋菜、空心菜和小麦等5种不同作物为处理设置试验小区,开展田间监测、土壤和植株样品采集及检测,进行试验数据和资料的统计分析。结果表明,5种作物中,甜玉米生物量大、吸氮量大且速率快,阻控硝酸盐向深层土壤淋溶能力强。本试验条件下,甜玉米生物量和吸氮量分别达到92335kg·hm^-2和330kg·hm^-2;种植甜玉米后,0-120cm土层的硝酸含量减少近140kg·hm^-2,均显著大于同等种植条件下的其他4种作物（P〈0．05）。就减少土壤硝态氮淋失的效果而言,甜玉米是北方设施菜地夏季填闲作物的较好选择。     

Impact of harvest residue management on soil nitrogen dynamics in Eucalyptus globulus plantations in south western Australia

More than 200,000 ha of short rotation Eucalyptus globulus plantations have been established in south-western Australia to supply wood for the pulp and paper industries. Sustaining the productivity of these tree crops over successive rotations will depend in part on maintenance of soil fertility, especially soil nitrogen (N) supply. We investigated the impact of four alternative strategies for management of harvest residues on soil N dynamics in recently logged first rotation plantations. The experiments were conducted over 5 years following harvesting at two sites with contrasting soils—a coarse textured grey sand over laterite (Podzol) with low natural fertility and a relatively fertile red earth soil (Ferralsol). At the grey sand site, 31 t ha⁻¹ of residues containing 219 kg N ha⁻¹ were deposited following harvest while at the red earth site the equivalent figures were 51 t ha⁻¹ of residues and 347 kg N ha⁻¹. Experimental treatments applied included residues burned, removed, retained and retained with double the amount of residues. The impact of treatments on soil nitrogen supply was investigated by incubating intact soil cores in the field to determine rates of net N mineralization. Additionally, the effect of treatments on soil moisture and temperature, the resident pool of soil mineral N and the amount of N potentially available for mineralization was assessed. The mulching effect of retained residues resulted in higher soil moisture where residues had been retained and a trend for soil on these treatments to dry out more slowly with the onset of the dry summer season, especially in the first year following harvest. Diurnal variations in soil temperature were moderated and average soil temperatures were reduced during summer where residues were retained. Concentrations of mineral N in soil were high in the 2 years following harvest at both sites and declined as newly established seedlings developed. At the more fertile site, where mineral N occurred predominantly as nitrate, retention of residues resulted in lower pools of soil mineral N following harvest. The effect of residue treatments on soil mineral N pools was less marked at the grey sand site. Concentrations of potentially mineralizable soil N and the amounts of N mineralized annually were greater where residues were retained at both sites. The results indicate that retention of harvest residues will favour the conservation of N following logging. However, accumulation of soil mineral N following harvesting due to reduced plant uptake will result in leaching of N early in the rotation that is largely independent of residue management. Retaining harvest residues will contribute to enhanced N supply for the next tree crop through mineralization in the long term. However, on some sites, additions of nitrogenous fertilizers will still be required to maximise the rate of tree growth.     

Nitrogen conserving potential of successive ryegrass catch crops in continuous spring barley

Abstract. The nitrogen (N) conserving effects of Italian ryegrass ( Lolium multiflorum L.) undersown as a nitrate catch crop in spring barley ( Hordeum vulgare L.) were evaluated over a ten-year period in outdoor lysimeters (1.5 m deep, diam. 1 m) with sandy loam soil. Spring barley grown every year received 11.0 or 16.5 g N m⁻² before planting or was kept unfertilized. The N was given either as calcium ammonium nitrate or as ammoniacal N in pig slurry. From 1985 to 1989, ryegrass was undersown in the barley in half of the lysimeters while barley was grown alone in the remaining lysimeters. The grass sward was left uncut after barley harvest and incorporated in late winter/early spring. From 1990 to 1994 all lysimeters were in barley only.
Barley dry matter yields and crop N offtakes were not affected by the presence of undersown ryegrass, although grain yields appeared to be slightly reduced. After termination of ryegrass growing, N offtake in barley (grain+straw) was higher in lysimeters in which catch crops had been grown previously.
The loss of nitrate by leaching increased with N addition rate. Regardless of N dressing, ryegrass catch crops halved the total nitrate loss during 1985–1989, corresponding to a mean annual reduction in nitrate leaching of 2.0–3.5 g N m⁻². From 1990 to 1994, lysimeters previously undersown with ryegrass lost more nitrate than lysimeters with no history of ryegrass. The extra loss of nitrate accounted for 30% of the N retained by ryegrass catch crops during 1985–1989.
It is concluded that a substantial proportion of the N saved from leaching by ryegrass catch crops is readily mineralized and available for crop offtake as well as leaching as nitrate. To maximize benefits from ryegrass catch crops, the cropping system must be adjusted to exploit the extra N mineralization derived from the turnover of N incorporated in ryegrass biomass.     

Soil Nitrogen Depletion by Vegetable Crops with Variable Root Growth

The ability of carrot, leek and white cabbage to deplete the soil inorganic nitrogen (N) pool was studied. All three crops are late-harvested crops with a long growing season, but they have been found to have very different root growth. At their optimal N supply, carrot left 27 kg nitrate-N ha^-1 in the top 100 cm of the soil, leek left 87 kg N ha^-1 and white cabbage left only 11 kg N ha^-1, in accordance with previously published differences in rooting depth among the three crops. Compared at a supply of 160 kg N ha^-1, 52, 65 and 4 kg nitrate-N ha^-1 was left in the soil by carrot, leek and white cabbage respectively. Apart from an extensive root system, white cabbage also had a much higher N-uptake capacity than the two other crops. The significance of differences in root growth, N-uptake capacity and other factors in determining the ability of the three crops to deplete the soil inorganic N pool is discussed.     

Residual value of 15N-labelled fertilizer applied to a maize-cowpea intercropping system

Summary We studied the residual effect of ¹⁵N-labelled fertilizer N, applied to a maize-cowpea intercropping system, on the succeeding crops of maize/wheat and its balance in the crop sequence, in greenhouse and field experiments. The N uptake by succeeding crops was always higher following sole or intercropped cowpea. Under field conditions with fertilizer N applied to first-crop maize, the residual N uptake by the succeeding crop of wheat was 5.8% and after maize-cowpea intercropping it was 7.8%.