首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   10930篇
  免费   1401篇
  国内免费   5873篇
  2024年   22篇
  2023年   398篇
  2022年   450篇
  2021年   544篇
  2020年   668篇
  2019年   785篇
  2018年   733篇
  2017年   735篇
  2016年   692篇
  2015年   667篇
  2014年   670篇
  2013年   850篇
  2012年   681篇
  2011年   646篇
  2010年   557篇
  2009年   727篇
  2008年   674篇
  2007年   749篇
  2006年   672篇
  2005年   631篇
  2004年   554篇
  2003年   570篇
  2002年   444篇
  2001年   424篇
  2000年   372篇
  1999年   367篇
  1998年   284篇
  1997年   275篇
  1996年   286篇
  1995年   243篇
  1994年   237篇
  1993年   219篇
  1992年   176篇
  1991年   146篇
  1990年   178篇
  1989年   149篇
  1988年   136篇
  1987年   91篇
  1986年   84篇
  1985年   63篇
  1984年   61篇
  1983年   24篇
  1982年   95篇
  1981年   39篇
  1980年   34篇
  1979年   34篇
  1978年   13篇
  1977年   10篇
  1976年   13篇
  1974年   10篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
71.
The effect of soil salinity and soil moisture on the growth and yield of maxipak wheat (Triticum aestivum L.) was studied in a lath-house experiment in whih, chloride-sulphate salt mixtures were used to artificially salinize a sandy loam soil from Al-Jadyriah Baghdad. Five soil salinity levels of ECe's equal to 1.7 (Control) 4.2, 5.8, 8.1, 9.4 and 11.0dSm–1 were prepared and used at 3 levels of available soil moisture depletion, namely, 25, 50, and 75% as determined by weight. Both growth (vegetative) and yield components were studied throughout the growing season.Results showed that increasing the soil salinity from 1.7 to 11.0 dSm–1, and decreasing the available soil water from 75 to 25% resulted in independent and significant decreases in Mazipak wheat growth and yield components at different stages of plant development. Root growth showed more sensitivity to both available soil water and soil salinity level than other components. It has been concluded that at soil salinity levels of more than 8.0 dSm–1, available soil water became a limiting factor on wheat growth and the maintenance of 75% of available soil water during the growth period is recommended to obtain satisfactory grain yield.  相似文献   
72.
The level of vesicular-arbuscular mycorrhizal (VAM) infection in the roots of Leucaena grown in a sand-soil mixture in the greenhouse increased rapidly with time and reached a peak value of 84% at 30 days from planting. The pattern of immobile nutrient uptake and accumulation closely paralleled that of the development of infection, particularly during the first 10–30 days after planting. Significant changes in dry matter yield were also observed only after a significant portion of the root length was colonized byGlomus aggregatum. The development of VAM infection was not accompanied by growth depression at any of the sampling periods. However, VAM roots had very high levels of Cu which was not translocated to shoots. It is hypothesized that such a diversion of Cu by the endophyte from the host could cause growth depression under conditions where the soil volume is supplied with sub-optimal levels of Cu. Contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Series No. 3186.  相似文献   
73.
Victor Chude 《Plant and Soil》1988,112(2):293-295
The profile distribution of total and extractable B was determined in 16 Nigerian cacao-growing soil profiles formed from different parent materials. Total B for all soils ranged from 8 to 54μgg−1 with a mean of 24μgg−1. The soils formed from sandstones in the rainforest zone contained higher amounts of total B than soils derived from basement complex. Boron extractable in hot water, in 0.1% CaCl2, and in 1N NH4OAc varied from 0.13 to 1.38, 0.44 to 1.20 0.03 to 0.56μgg−1 respectively. The corresponding means were 0.66, 0.75 and 0.27μgg−1 B. Soils on metamorphic rocks gave the highest values. All extractable B values were related to organic matter while only CaCl2-extractable B correlated with total B. Generally total and extractable B values were higher in the top soils than in the subsoils.  相似文献   
74.
75.
The fate of sheep urine-N applied to an upland grass sward at four dates representing widely differing environmental conditions, was followed in soil (0–20 cm) and in herbage. Urine was poured onto 1-m2 plots to simulate a single urination in August 1984 (warm and dry), May (cool), July and August 1985 (cool and wet) at rates equivalent to 40–52 g N m−2. The transformation of urine-N (61–69% urea-N) in soil over a 6–7 week period followed the same general pattern when applied at different times during the season; rapid hydrolysis of urea, the appearance of large amounts of urine-N as ammonium in soil extracts, and the appearance of nitrate about 14 days after application. The magnitude of “apparent” nitrification however differed markedly with environmental conditions, being greatest in May 1985 when a maximum of 76% of the inorganic soil N was in the form of nitrate. At all other application dates nitrate levels were relatively low. With the August 1984 application soil inorganic N returned to control levels (given water only) after 31 days but considerable amounts remained in soil for 60–90 days with the other applications. Weekly cuts to 3-cm indicated that increases in herbage dry matter and N yields in response to urine application were greatest in absolute terms after the May 1985 application and continued for at least 70 days with all applications. Relative to control plots the May application resulted in a 3-fold increase in herbage DM compared with corresponding values of 6-, 5-, and 7-fold increases with the August 1984, July and August 1985 applications. Recovery of urine-N in herbage was poor averaging only 17% of that applied at different dates, while recovery in soil extracts was incomplete. The exact routes of loss (volatilisation, leaching, denitrification or immobilisation) were not quantified but it is evident that substantial amounts of urine-N can be lost from the soil-plant system under upland conditions.  相似文献   
76.
应用电子探针对植物根际和根内营养元素微区分布的探讨   总被引:3,自引:0,他引:3  
用电子探针可检测出玉米、大豆根际和根内含有Na,Mg,Al,Si,P,S,Cl,K,Ca,Ti,Fe,Cu和Zn 13种元素。这些元素在根际土壤、粘液层和根组织内的含量分布有一定的规律性。除Si,Al,Ca,Fe在根际土壤中峰值较高外,Ti仅在土壤中达到可检测量;S,Fe和Zn富集在粘液层,Mg,P,Cl只在根组织内才有较明显的峰。这些规律可作为区分根—土界面的参考指标。K含量在根内明显高于根际土壤,并由表皮层到中柱径向增加;Ca则与K不同,且受植物种类的影响。  相似文献   
77.
Gametophyte populations inAthyrium brevifrons were analysed with respect to population size and surviving area (%) of individual thalli in a transplant garden at Sapporo during 5–26 April 1983, to study the safe-microsite for gametophyte establishment in nature. Spores dispersed in August 1982 germinated and grew into thalli of various widths (<10 mm); 10.3% of the thalli matured by early October 1982. Maturation was attained by gametophytes of width 4–7 mm. The number of gametophytes gradually decreased with increasing width. By April 1983, 20.5% of total gametophytes were mature with a mode of 5–6 mm in width. The relative number of gametophytes with surviving area of 2–20% increased and that of 85–100% decreased in accordance with collection days delayed until after snow-melt. Surviving area (%) on gametophyte of all widths decreased with decreasing soil moisture contents. In particular, immature gametophytes of 2–4 mm width showed a significant correlation (P<0.01) between soil moisture content and relative number of gametophytes with 0–20% surviving area and mean surviving area (%) of every width of thalli. The spring desiccation might be a factor that reduces or limits gametophyte populations in nature.  相似文献   
78.
79.
80.
Dicotyledonous plants subjected to Fe-deficiency stress can decrease pH in the rhizosphere by proton excretion and reduce ferric iron by an activated reduction system in the plasma membranes of the root or by reductants released from the roots. The efficiency by which these plants take up Fe may strongly depend on their cation-anion balance. This study presents results of two experiments conducted to evaluate the effect of K, growth stage and cultivar on ionic balance and Fe acquisition of peanut (Arachis hypogaea L.) plants.Potassium applications to the high calcareous soil (30.3% CaCO3) favoured proton release, but did not ameliorate plant Fe acquisition. At the earliest stages of plant growth, anion uptake exceeded cation uptake due to intensive N uptake. With time, a shift in the ionic balance was observed as a result of predominant cation uptake. It appears that the relationship between H/OH-ion release and Fe nutrition of peanut plants is actually a complex phenomenon under soil conditions and depends on some soil parameters, such as CaCO3 content. Even by enhanced H-ion release Fe nutrition of plants can be impaired if soil CaCO3 is too high.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号