分蘖期土壤水分对旱稻矿质养分吸收的影响

在池栽条件下研究了不同水分处理对旱稻吸收氮磷钾及分蘖生产效率的影响。结果表明:土壤水分对旱稻吸收N、P、K矿质元素有明显影响。随着土壤水分含量的提高,植株对矿质元素的吸收量显著提高。在分蘖初期,65%水分处理的植株对N、P、K的吸收量分别只有100%的74.1%,43.9%和50.8%,而在分蘖后期,则分别只有100%的41.4%,18.6%和27.5%。旱稻矿质元素的分蘖生产效率以在85%土壤含水量时最高,其N、P、K的平均分蘖生产效率分别比100%高35.7%,140%和81.8%。平均分蘖生产效率表现为P>K>N。     

不同水、氮供应条件下夏玉米养分累积动态研究

在遮雨棚内进行了微区试验,采用不同水、氮素供应研究了玉米N、P、K吸收累积动态。结果表明,植株生物量和N、P、K吸收量,随生育期延长而持续增加;而植株的N、P、K含量,则呈下降趋势。植株生物量和N、P、K吸收量随时间的变化,可用S曲线方程描述。玉米生长期间干物质与养分吸收并非同一速率,前期上升快,至最高峰后缓慢下降。在N、P、K三要素中,N、K吸收速率高,上升快,下降也快;P吸收速率低,上升慢,下降亦慢。养分最大吸收速率出现的时间以K最早,N次之,P最晚。但三者均早于干物质最大累积速率出现的时间。水分和氮素供应增加养分最大吸收速率及养分吸收量,也可增加生育前期的养分含量,但不改变养分累积变化趋势和养分吸收速率的变化趋势。水分和氮素供应促进了营养体养分向子粒的运转,提高了养分在子粒中的分配比例,从而提高了子粒产量。     

平衡栽培体系中的农业栽培措施(五)

3灌溉施肥（续四） 植物主要通过根系吸收矿质元素。阳光中的热辐射给叶片周围的空气加温，迫使植株体内的水分通过叶片上的气孔蒸腾到大气中，这样在植株体内造成了水分亏缺，产生了对根外水分（在大多数情况下是土壤水分）的拉动力。     

水磷耦合对冬小麦幼苗营养元素含量的影响

本文研究了速效水范围内水磷耦合对冬小麦幼苗的N、P、K吸收量的影响。结果表明:不同速效水水平对小麦苗期N、P和K的吸收有明显影响。在不同的P水平处理时最高N、P吸收值都出现在速效水高的处理上,低水时N、P吸收也较低。中P与高水配合有利于N、P的吸收。低速效水时提高P水平可促进P吸收,但不能显著提高N吸收。速效水的水平对K吸收影响不如对P吸收影响明显,品种间差别不大。结果还显示土壤低P处理显著降低植株幼苗对P素的吸收,不同速效水水平时增加土壤P可以提高植株吸P量。土壤缺P对小麦植株K吸收有一定影响,水磷耦合后品种间的K吸收量差异加大。     

基质栽培甜瓜矿质营养吸收规律的研究

以温室基质栽培的网纹甜瓜'甜甜1号'为试材,探讨了植株的干物质积累和矿质元素的吸收规律。结果表明,温室内基质栽培网纹甜瓜的生育期为100天,采收时植株总生物量为136g时,单株矿质元素的需求总量为11 09g。营养生长期和结果中期是植株干物质积累的两个重要时期(营养生长期1.27gplant-1d-1、结果中期2.19gplant-1d-1),应增施肥料,以满足植株快速生长的需要。甜瓜对矿质元素N、P、K、Ca、Mg需求的比例,营养生长期为41∶4∶32∶19∶4,结果中期为28∶5∶37∶25∶6。甜瓜的P需求量在结果前期最大(15 55mgplant-1d-1),N、K、Ca、Mg的需求量在结果中期最大,N为52.58mgplantd、K为82.33mgplant-1d-1、Ca为54.59mgplant-1d-1、Mg为12.42mgplant-1d-1。     

生姜对氮、磷、钾吸收分配规律研究

通过田间试验研究了高产生姜生长特性及对氮、磷、钾的吸收分配规律。结果表明,生姜幼苗期生长缓慢,虽持续时间占全生育期的52.6%,但干物质积累量较少,对N、P2O5、K2O的吸收速率较低,吸收量仅占全生育期的24%左右; 发棵期植株生长迅速,N、P2O5、K2O的吸收速率分别达3.5、1.3 和4.9 kg/(hm2·d),吸收量约占全生育期的30%左右; 根茎膨大期对N的吸收速率略有降低,而P2O5、K2O则仍分别高达1.3和5.1 kg/(hm2·d),N、P2O5、K2O吸收量分别占全生育期的44.0%、45.5%和47.1%。随生长的进行,N、P2O5、K2O在茎、叶中的分配率呈降低的趋势,而在根茎中的分配率则逐渐增加; 在根茎膨大期,根茎中的N、P2O5、K2O分配率分别达50.0%、61.0%和46.5%。生姜全生育期对N、P2O5、K2O的吸收比例约为2.5∶1.0∶3.8。本试验条件下,每生产1000 kg生姜根茎产品,分别吸收N、P2O5、K2O约4.67、1.90和7.25 kg。     

不同生育期缺水和补充灌水对冬小麦氮磷钾吸收及分配影响

采用盆栽试验研究了不同生育期缺水和补充灌水对冬小麦N、P、K吸收及在体内分配的影响。结果表明 ,水分缺乏不仅抑制作物生长 ,还降低养分吸收。分蘖、拔节和灌浆期是作物需水关键期 ,这些时期缺水 ,生长量降低36.1%～72.3% ;N吸收降低 40.6%～72.0% ,P降低 40.5%～72.4% ,K降低 25.9%～69.5% ;而越冬期缺水 ,生长和养分吸收降低较少。不同生育期补充灌水也不一定能使小麦生长和养分吸收增加。拔节期补水生长量增加幅度最高 ,达 18.2% ;N、P、K吸收量增幅也最大 ,分别为 14.5% ,15.6%和 38.2%。其次是越冬和灌浆期补水。分蘖期补水 ,生长量和养分吸收量却显著降低。越冬、拔节、灌浆期补充灌水 ,虽能促进小麦整株生长 ,增强养分吸收 ,但养分向子粒的转移和分配并不相应增强。越冬和灌浆期补水 ,子粒吸N量分别提高 5.5%和 20.9% ,吸P量提高 9.1%和 6.2% ;拔节期补水 ,N、P吸收量反而分别降低 19.6%和 13.5%。 3个时期补水 ,子粒吸K量均降低3.0%～26.7%。     

微曝气生态浮床水芹吸收NP的特性及其对系统去除NP贡献的研究

针对污染河水黑臭缺氧、NH4^＋—N含量高等问题,研发了一种“漂浮载体悬挂弹性生物膜填料＋水生植物并辅以人工微曝气系统”的微曝气生态浮床系统,以漂浮植物水芹为例研究了系统中水芹对N、P的吸收特性和去除作用。结果表明,随着水芹的生长其生物量干重显著增加,生长80d左右时总生物干重在2497．2—3144．4g·m^-2之之间,上、下部生物量比平均为13．4。不同部位水芹N、P的含量不同,总的趋势为含N量叶〉根〉茎,含P量茎〉根〉叶。不同生长时间水芹N、P含量及其吸收速率不同：随着水芹的生长,组织内N、P含量逐渐降低,N的吸收速率总趋势为60-80d〉35-60d〉1-35d,P的吸收速率总趋势为35-60d〉1-35d〉60-80d。而随着水芹的生长吸收N、P的总量却在逐渐增加,吸收N的总量从17．69g·m^-2增加到61．66g·m^-2,吸收P的总量从4．99g·m^-2增加到13．55g·m^-2,这主要取决于自身的生物量和N、P的含量。水芹对N、P的积累主要集中在上部,分别占N、P吸收总量的92．2％-93．4％、92．5％～93．1％。水芹生长35、60、80d时,吸收N量占系统TN去除量的比率分别为4．50％、6．06％和6．87％,水芹对P的吸收量分别占系统去除P总量的18．53％、26．82％、22．00％。水芹对N、P的吸收仅是微曝气生态浮床净化系统去除N、P的一个途径,但水芹根际微生物的作用不可忽视。     

不同氮肥形式对玉米氮、磷、钾吸收及氮素平衡的影响

通过黑土区田间试验,研究不同氮肥对玉米N、P、K吸收量、肥料利用率、玉米产量和氮素平衡的影响。结果表明:玉米植株N、P、K积累大小顺序为NKP;玉米对N的吸收主要集中在籽粒和叶片,分别占总吸N量的51.77%~55.32%和34.42%~36.49%,P在籽粒中的吸收量高达57.08%~64.75%,K的吸收主要集中在叶片,占总吸K量的37.15%~54.16%;等量磷钾肥条件下,施用等量不同形式氮肥可以显著促进作物地上部养分的吸收、提高作物产量,N、P、K含量和产量分别最高增加43.94%,33.35%,69.05%,16.77%,且收获期土壤铵态氮大于硝态氮;半控比掺混肥显著提高玉米植株、各器官氮磷钾吸收量和产量,得到最高的氮、磷、钾肥利用率,分别为58.57%,45.49%,89.07%,且收获期土壤硝态氮与铵态氮含量最低,氮素盈余最少,降低对环境污染。     

长期施肥对南方黄泥田水稻养分吸收利用的影响

黄泥田为南方广泛分布的一类中低产田，为探明长期不同施肥模式对水稻养分吸收利用的影响，基于南方黄泥田连续 35年长期定位试验平台，设置 1)不施肥(CK)，2)单施化肥(NPK)，3)化肥 +牛粪(NPKM)，4)化肥 +稻秆还田(NPKS)4个处理，于试验的第 35年分析水稻各生育期的植株养分与累积吸收利用。结果表明，长期施肥均提高了水稻籽粒和秸秆产量，其中籽粒产量增幅 61.1%～ 97.7%，以 NPKM与NPKS处理最高，其籽粒产量分别较 NPK处理提高 22.7%和 20.4%。与 CK相比，各施肥处理成熟期籽粒 N、P、K吸收量分别增幅 137.6%～206.2%、86.0%～ 172.4%和 71.6%～142.5%，茎叶N、P、K吸收量分别增幅98.4%～240.8%、145.7%～419.8%、94.4%～142.8%，且均以 NPKM处理最高。施肥的土壤有机质、碱解N、速效 K含量分别较 CK增幅 21.0%～42.1%、34.0%～ 86.0%和 40.9%～127.3%，差异均达显著水平，有机质、碱解 N及有效 P含量以 NPKM处理提升最为显著，速效 K含量以 NPKS处理提升最为明显。不论是籽粒、茎秆还是地上部植株，成熟期 N、P、K养分吸收量均与土壤有机质含量呈极显著正相关，且 N、P、K养分吸收量分别与相应的土壤碱解 N、有效 P及速效 K含量呈显著正相关。综合考虑施肥对水稻产量、养分利用与稻田土壤肥力的影响，NPKM与 NPKS处理优于 NPK，而 NPKM与 NPKS二者产量基本相当。土壤有机质是影响黄泥田水稻植株N、P、K养分吸收累积的关键因子。     

Soil-profile distribution of primary and secondary plant-available nutrients under conventional and no tillage

Nutrient distributions under no tillage (NT) compared with conventional disk-and-bed tillage (CT) management in the warm, humid region of the southeastern USA need to be assessed so that future placement, quantity, and type of fertilizers can be altered, if necessary, to efficiently match crop demands. We determined soil-profile distributions of pH, N, P, S, K, Ca, Mg, Na, Zn, Fe, Mn, and Cu to a depth of 0.9 m at the end of 8.5 years of continuous CT and NT management on a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentral Texas. Most dramatic changes occurred within the 0–0.05 m depth, where soil under NT had lower pH, Fe, and Cu than under CT, but greater P, K, Zn, and Mn. Greater P and K under NT than under CT also occurred below the till-zone (0.15–0.3 m). At a depth of 0–0.3 m, soil under NT contained greater amounts of extractable P, K, Zn, Fe, Mn, and Cu than under CT. Nitrogen fertilization had little effect on nutrient distributions, except resulting in greater extractable K at 0–0.05 m and greater nitrate at 0–0.15 m. Few changes in soil-profile distributions were observed for extractable S, Ca, Mg, and Na. Long-term continuous use of NT on this fine-textured, high-fertility (except for N) soil had no apparent adverse effects on nutrient distributions relative to CT, but enhanced conservation and availability of P, K, Zn, Fe, Mn, and Cu near the soil surface where crop roots proliferate.     

Assessing the effectiveness of conservation measures: Resolving the “wicked” problem of the Steller sea lion

“Wicked” problems are those that are complex and that change when solutions are applied. Many conflicts in conservation fall into this category. The study approached the problem of how to constrain the apparent wickedness of a problem in the conservation management of a species by using simple empirical indicators to carry out iterative assessment of the risk to a population and to document how this risk evolves in relation to the addition of new data and the implementation of management actions. Effects of high levels of uncertainty within data and also concerning population structure were examined through stochastic simulation and by exploration of scenarios. Historical trends in the example used, the Steller sea lion, showed rapid declines in abundance in some regions during the 1980s. The current total population is 130,000-150,000 Steller sea lions through Alaska and British Columbia and this number has been stable since about 1990 in spite of regional differences in population dynamics. Regional differences in the sequence of changes in the number of pups and non-pups, suggested that an internal re-distribution of juveniles could have happened between 1980 and 1990. Current productivity also appears close to the long-term mean. Stochastic population projection using various scenarios showed that, based upon this history, the risk of extinction for the population has declined and is below reasonable thresholds for considering the population to be endangered. The trends in risk suggest that management actions taken since 1990 have probably been effective. Consequently, the conservation management objectives for the Steller sea lion are probably being met. The approach provides a mechanism, based upon experience and scenario analysis, for exploring future policy options and may help to constrain the debate amongst stakeholders about the cost-benefit trade-offs associated with different options.     

Enhanced establishment and growth of giant cardon cactus in an eroded field in the Sonoran Desert using native legume trees as nurse plants aided by plant growth-promoting microorganisms and compost

To evaluate the feasibility of long-term desert reforestation technology of mixed vegetation, cardon cactus (Pachycereus pringlei) seedlings from indoor and outdoor nurseries were planted in the field adjacent to one seedling of potential legume nurse trees: mesquite amargo (Prosopis articulata), yellow palo verde (Parkinsonia microphylla), and blue palo verde (Parkinsonia florida). Some of the planting holes were also supplemented with common dairy compost. Additionally, the combinations of legume tree–cactus were inoculated with either a consortium of desert arbuscular mycorrhizal (AM) fungi, plant growth promoting bacteria (PGPB; the diazotroph Azospirillum brasilense Cd, and the phosphate solubilizer Paenibacillus sp.), or a mixture of all. The field experiments were evaluated periodically during 30 months for survival and growth. Cardons reared in an outdoor screen house survived better in the field than those reared in a controlled growth chamber and hardened later outdoors. Association with any legume nurse tree increased survival and enhanced growth of untreated cardons. For cardons growing alone, application of either compost, AM fungi, and all the treatments combined increased survival. For these plants, no treatment affected plant growth during the first 3 months after transplanting. Later, all treatments, except for AM fungi, enhanced plant growth. However, only 2 years after transplanting the enhanced growth effect of AM fungi was also significant. In the presence of the legume nurse trees, transient positive effects on cardon growth were recorded. General evaluation after 30 months of cultivation showed that the treatments positively affected cardon growth when growing alone or in combination only with mesquite amargo but not with the other two legume trees. This study proposes that young legume trees have the capacity to enhance survival and growth of cardon cactus, depending on the legume cactus combination. Additional treatments such as compost or PGPB can either amplify the effect or else attenuate it.     

Karyotypic and genome size variation in Cajanus cajan (L.) Millsp. (pigeonpea) and some wild relatives

Detailed karyotypes and 4C DNA amounts have been studied in five cultivars of Cajanus cajan and 20 species belonging to Cajanus, Rhynchosia, Dunbaria, Flemingia and Paracalyx. C. cajan shows intraspecific variability and its karyotype is most similar to that of C. cajanifolius (sect. Cajanus) in the morphology and number of satellite chromosomes and the lack of any correlation between chromosome size within the complement and asymmetry. Karyotypes of C. lineatus and C. sericeus belonging to sect. Atylia are similar with respect to maximum r-index and the ratio of longest and shortest chromosomes in their respective complements. C. acutifolius (sect. Frutocosa) is distinct in having a very low ratio (1.44) between the longest and the shortest chromosomes, while in C. albicans, C. goensis, C. scarabaeoides (sect. Cantharospermum) there is no chromosome pair with r-index >2.0. C. mollis and C. volubilis (sect. Volubilis) show similarity with regard to ratio between the longest and the shortest chromosomes and a distinct chromosome pair with an arm ratio of 2.7 and C. platycarpus is distinct (sect. Rhynchosoides) in having the smallest ratio (1.36) between the longest and the shortest chromosomes. 4C DNA amounts in Cajanus and Rhynchosia vary between 3.28 pg to 11.69 pg and 4.92 pg to 11.13 pg respectively, while in Dunbaria, Flemingia and Paracalyx these vary between 4.70 and 7:06 pg. In the genus Cajanus karyotypic features and 4C DNA amounts agree with the sectional classification and bring out a close relationship between C. cajan and C. cajanifolius. This is supported by studies based on seed protein patterns, isozyme analysis, trypsin and chymotrypsin inhibitor patterns, RFLP and RAPD data and crossability relationships. A clear relation between DNA amount and abnormalities effecting the extent of pairing and recombination in interspecific hybrids shows the importance of this study in developing future breeding programme involving C. cajan and its wild relatives that are of potential value.     

Effect of some fungicide seed treatments on the survival of Bradyrhizobium japonicum and on the nodulation and yield of soybean [Glycine max. (L) Merr.]

Several commercial fungicide seed treatments were evaluated for their possible effect on the survival of Bradyrhizobium japonicum on seeds and on the nodulation and yield of soybeans in a greenhouse and a field experiment. quinolate Pro (carbendazim and oxine copper), Vitavax 200FF (carboxin and thiram), and Monceren (pencycuron) had a small effect or no effect on the survival of B. japonicum and on the nodulation and yield of soybeans. They can thus be considered compatible with soybean seed inoculation. Germipro UFB (carbendazim and iprodione), Apron 35J (metalaxyl), and Tachigaren (hymexazol) decreased B. japonicum survival and the nodulation and yield of soybeans and thus cannot be considered compatible with soybean seed inoculation.     

Einfluss der düngungsmethoden auf die entwicklung von zuckerrübenkrankheiten

Während neunjähriger (1994-2002) Untersuchungen in der Versuchsanstalt Ba?cyny (Fahlerde, Fall-Gleyboden, ausgebildet aus leichtem Lehmboden) wurde der Befall mit Krankheiten an der Zuckerrübe in Abhängigkeit von der Düngungsmethode bewertet. Folgende Methoden wurden angewendet: A - Mineraldünger, B - Stalldung und Mineraldünger, C - Stroh-, Gründung und Mineraldünger, D - Kompostdünger und Biopräparate (ökologische Düngung). Auf den Objekten A, B, C wurden bei konstanter Phosphor-Kalium-Düngung die Stickstoffgaben von 0 bis 180 kg.ha^?1 differenziert. Auf dem ökologischen Objekt D ohne Mineraldünger wurden statt Stickstoff die Biopräparate P₅₀₀ und P₅₀₁ verabreicht. Der geringste Befall von Krankheiten wurde an der Zuckerrübe auf jenem Objekt festgestellt, das mit Kompost und den Biopräparaten gedüngt worden war. Im Rahmen der konventionellen Düngungsmethoden wurde allgemein die höchste Intensität der Krankheiten an der mit Stroh, Gründung und Mineraldünger gedüngten Zuckerrübe beobachtet. Ausnahmen von dieser Regel bildete das Vorkommen von echtem Mehltau und Keimlingsfäule. Auf den Objekten A, B, C nahm die Intensität der Krankheiten mit der Steigerung der Stickstoffgabe zu. Auch die auf Objekt D verabreichten Biopräparate stimulierten die Entwicklung der Krankheiten. The effect of various fertilization methods on the diseases of sugar beet was estimated on the basis of nine-year studies (1994-2002) conducted in the Experimental Production Farm in Ba?cyny on loess-based, sedimentary-gley, light-clay soil. The following fertilization methods were applied: A - mineral fertilization, B - mineral and manure fertilization, C - straw, green manure and mineral fertilization, D - ecological compost and biodynamic preparations fertilization. In the objects A, B and C fertilized with phosphorus and potassium, various doses of nitrogen ranging from 0 to 180 kg/ha^?1 were used. In the ecological object D, where mineral fertilization was not used, biodynamic preparations P₅₀₀ and P₅₀₁ were applied instead of nitrogen. The lowest ratio of the diseases of sugar beet was observed in the object fertilized with the compost and biodynamic preparations. In view of the conventional fertilization methods, generally the highest incidence of diseases was noted in the sugar beet fertilized with straw, green manure and mineral fertilizers. The exception was the occurrence of powdery mildew and the black leg. In objects A, B and C the incidence of the diseases increased with the increase in nitrogen doses. The applied biodynamical preparations in the object D stimulated the development of diseases too.     

Prospects for control of Pythium damping-off of lettuce with Trichoderma,Gliocladium and Enterobacter spp.

Summary Damping-off of lettuce (Lactuca sativa) caused by Pythium ultimum was studied in pots containing a non-sterile potting mix in the glasshouse. Fifty P. ultimum sporangia g^-1 compost reduced the plant stand to 15% and shoot dry weight to 18%, but this reduction was totally prevented by applying Trichoderma harzianum at 2×10⁵ viable propagules g^-1 potting mix. Gliocladium virens and Enterobacter cloacae also alleviated the damping-off. E. cloacae did not adversely affect the action of the fungal antagonists. The co-existence of the bacterial and fungal antagonists was revealed on the root surface and inner surface of the testa by scanning electron microscopy, indicating the compatibility of the biocontrol agents.     

细菌、真菌及植物氮营养信号研究进展

氮素吸收代谢是所有生物生命活动的核心部分,因此,各种生物对氮的吸收与同化过程都有精细的调节。这种调节依赖于生物对体内氮素状况信号及体外（生长介质中）氮素信号的感受。过去几十年中,在细菌、真菌中已经对氮素营养信号有了较好的研究,而植物中则相对缓慢,但也有了一定的认识。本文重点比较细菌、真菌及植物中氮信号系统的组成,以期为进一步认识植物中的氮信号接受与转导系统提供启示。     

Aircraft operations near concentrations of birds in Antarctica: The development of practical guidelines

Aircraft operations have the potential to disturb and to impact negatively on bird life. A gradient of increasing behavioural response is evident in birds when exposed to increasing aircraft stimulus. The most major disturbance is likely to lead to impacts on the health, breeding performance and survival of individual birds, and perhaps bird colonies. A process of revision to policies on aircraft operations contained in management plans for a number of specially protected areas in Antarctica by the United Kingdom, accompanied by consultations made within the scientific community through the Scientific Committee on Antarctic Research (SCAR) and with operational interests through the Council of Managers of National Antarctic Programmes (COMNAP) resulted in new guidelines being adopted by the Antarctic Treaty Consultative Parties in June 2004. The principal recommendations of the guidelines are that bird colonies should not be overflown below 2000 ft (∼610 m) above ground level and landings within 1/2 nautical mile (∼930 m) of bird colonies should be avoided wherever possible. These guidelines are less stringent and less specific than those that were recommended by the SCAR specialist group on birds, and represent a compromise to accommodate operational needs. While the adoption of clear and consistent guidelines for the operation of aircraft in Antarctica is welcome in that this provides practical advice that is likely to reduce incidences of close aircraft/bird encounters, there remains insufficient knowledge of the interactions between aircraft and birds in Antarctica, and the consequent impacts on individual birds and on bird populations. It is important, therefore, that the guidelines adopted are considered interim, and should be kept under scrutiny with revisions made as new and improved research results appear.     

The abundance of heterocystous blue-green algae in rice soils and inocula used for application in rice fields

Summary Algal populations were quantified (as colony-forming units [CFU] per square centimetre) in 102 samples of rice soils from the Philippines, India, Malaysia and Portugal, and in 22 samples of soil-based inocula from four countries. Heterocystous blue-green algae (BGA) were present in all samples. Nostoc was the dominant genus in most samples, followed by Anabaena and Calothrix. In soils, heterocystous BGA occurred at densities ranging from 1.0 × 10² to 8.0 × 10⁶ CFU/cm² (median 6.4 × 10⁴) and comprised, on average, 9% of the total CFU of algae. Their abundance was positively correlated with the pH and the available P content of the soils. In soil-based inocula, heterocystous BGA occurred at densities ranging from 4.6 × 10⁴ to 2.8 × 10⁷ CFU/g dw (dry weight), comprising only a moderate fraction (average 13%) of the total algae. In most soils, the density of indigenous N₂-fixing BGA was usually higher than that attained by applying recommended rates of soil-based inoculum. Whereas research on the practical utilization of BGA has been mostly directed towards inoculation with foreign strains, our results suggest that attention should also be given to agricultural practices that enhance the growth of indigenous strains already adapted to local environmental conditions.Maître de Recherches ORSTOM (France), Visiting Scientist at IRRI