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   42061篇
  免费   13952篇
  国内免费   858篇
林业   3363篇
农学   2312篇
基础科学   2597篇
  24421篇
综合类   16746篇
农作物   1268篇
水产渔业   78篇
畜牧兽医   2156篇
园艺   579篇
植物保护   3351篇
  2025年   680篇
  2024年   1813篇
  2023年   2055篇
  2022年   1905篇
  2021年   1917篇
  2020年   2017篇
  2019年   2265篇
  2018年   1838篇
  2017年   2533篇
  2016年   2938篇
  2015年   2029篇
  2014年   2323篇
  2013年   3250篇
  2012年   4065篇
  2011年   2995篇
  2010年   2319篇
  2009年   2400篇
  2008年   2176篇
  2007年   2333篇
  2006年   2016篇
  2005年   1720篇
  2004年   1327篇
  2003年   1139篇
  2002年   907篇
  2001年   868篇
  2000年   743篇
  1999年   553篇
  1998年   485篇
  1997年   500篇
  1996年   430篇
  1995年   456篇
  1994年   422篇
  1993年   329篇
  1992年   268篇
  1991年   256篇
  1990年   181篇
  1989年   160篇
  1988年   101篇
  1987年   78篇
  1986年   40篇
  1985年   13篇
  1984年   6篇
  1983年   2篇
  1982年   2篇
  1981年   3篇
  1980年   4篇
  1976年   1篇
  1963年   1篇
  1962年   5篇
  1955年   1篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
 通过对水泥工程的类型、组成、占地以及施工和生产工艺、土石方量及弃渣的分析,总结出水泥工程在建设与生产过程中的水土流失特点,水泥工程在建设期间的水土流失重点区域是厂区,而在生产运行期则是石灰石和黏土矿,此外在水泥生产厂区还会产生大量的粉尘。水泥工程除了应遵循"生产建设项目水土保持准入条件通用条款"中的条件外,对照涉及水泥产业的法律、法规、规范、政策、标准、发展纲要和行业发展规划,提出了水泥工程项目的否决、限批和缓批的13条水土保持准入条件。根据水土保持新国标和有关法规的规定,结合典型案例分析,提出须修正主体设计的5项准入条件和须修改水土保持方案的4项准入条件。  相似文献   
162.
马驰 《干旱区研究》2020,(3):591-597
基于Sentinel-2A遥感数据,结合白城市表层土壤采样的全盐含量化验值,利用统计与拟合分析的方法,建立土壤盐渍化遥感监测模型,对研究区表层土壤含盐量进行反演分析.结果 表明:①研究区土壤的反射率与含盐量呈正相关,相关系数在Sentinel-2A第5波段(中心波长为0.705 μm)达到最大值,为r=0.902,利用...  相似文献   
163.
刮板式水稻育苗平盘播土装置的设计与试验   总被引:2,自引:0,他引:2  
分析了常用的一些水稻毯状秧苗工厂化育秧播土装置的特点,根据Mohr-Coulomb的土壤抗剪强度定律,设计出刮板式播土装置。并对生产上常用的砖红壤、水田土等床土进行了播土试验。试验表明,该播土装置能够适用于不同特性床土在不同播土量要求下的播土作业,播土均匀性较高,变异系数均小于10%,盘土厚度相差不大,符合农艺生产要求。  相似文献   
164.
土壤墒情自动化监测及应用   总被引:1,自引:1,他引:1  
介绍了"德州市气象局土壤墒情信息综合处理系统"的系统内容,从数据收集与处理、图表生成、土壤墒情信息发布等方面介绍了其软件功能的实现,以期促进德州市干旱监测工作的开展。  相似文献   
165.
    
The aim of the study was to elucidate the spatial distribution of soil microorganisms and enyzme activities in a long‐term wastewater treated soil. Soil was sampled from a plough layer of the Ah horizon of a sandy Haplic Luvisol which was either (1) irrigated with municipal wastewater for almost 100 years, or (2) no more irrigated since 20 years, or (3) never received wastewater. The samples were fractionated by wet sieving to obtain seven size fractions of organic and mineral soil particles, and a separate silt+clay fraction. The individual soil samples contained between 1.2% (never irrigated) and 4.1% (long‐term irrigated) organic particles by weight, but these particles harboured up to 47.8% of the total soil carbon and 41.7% of nitrogen, and thus represented an important storage of energy and nutrient for microorganisms. In total, however, the highest C and N amounts were accumulated in the silt+clay fraction, whereas coarser mineral particles which dominanted by weight in the Haplic Luvisol were low in C and N. The highest numbers of bacteria, actinomycetes and fungi per gram of the individual soil fractions were found in organic particles of the long‐term irrigated soil. Less nutrient‐dependent oligotrophic bacteria were for the most part associated with the silt+clay fraction, irrespective of the soil treatment with wastewater. Similar to microbial counts, also the ATP content, as a measure of active microbial biomass, and the activities of β‐glucosidase, β‐acetylglucosaminidase, and proteinase were higher in the long‐term irrigated soil than in that which was never irrigated. In most cases slightly enhanced values of microbiological and biochemical parameters were still detectable 20 years after the wastewater irrigation was terminated. The values of the individual parameters decreased in all soil samples under testing in general gradually with decrease in size of the organic soil particles. In conclusion, the coarse soil organic particles > 5 mm and the silt+clay fraction < 0.05 mm represent the sites with the highest microbial inhabitance, ATP contents and enzyme activities in the Ah horizon of an Haplic Luvisol. Long‐term wastewater irrigation resulted in an increase of microbial counts, total biomass and soil enzyme activities.  相似文献   
166.
张连生 《油气储运》2000,19(5):14-15
在地温较低的情况下,成品油管道顺序输送5号轻柴油时,容易出现管壁结蜡现象,使混油量增大,在分析了5号轻柴油质量数据和长输管道各段地温6变化的基础上,提出了“临界点”地温,给出了在“临界点”地温的条件下输送5号轻柴油时应采取的措施,为管道安全经济运行提供了保障。  相似文献   
167.
168.
华北平原属干旱与半干旱缺水地区,城市植被建设在定位、水资源分配、循环与再利用等方面存在诸多问题,表现为:人工植被多为“耗水型绿色拼盘”、植源性污染日趋加重、城市枯枝落叶层的清除、城市湿地生态系统建设的忽略、大量硬化地面导致大气降水利用率低等。建议:城市植被建设应重视城市森林、湿地生态系统的恢复与建设,结合城市环境工程建设,提高大气降水利用率和节水治污等措施,促进城市建设的可持续发展。  相似文献   
169.
土壤组分对广东省酸性水稻土磷吸附参数的影响   总被引:7,自引:2,他引:7  
Soil components affecting phosphate sorption parameters were studied using acid paddy soils derived from basalt, granite, sand-shale and the Pearl River Delta sediments, respectively, in Guangdong Province.For each soil, seven 2.50 g subsamples were equilibrated with 50 mL 0.02 mol L-1 (pH=7.0) of KCl containing 0, 5, 10, 15, 25, 50 and 100 ng P kg-1, respectively, in order to derive P sorption parameters (P sorption maximum, P sorption intensity factor and maximum buffer capacity) by Langmuir isotherm equation. It was shown that the main soil components influencing phosphate sorption maximum (Xm) included soil clay, pH,amorphous iron oxide (Feo) and amorphous aluminum oxide (Alo), with their effects in the order of Alo >Feo > pH > clay. Among these components, pH had a negative effect, and the others had a positive effect.Organic matter (OM) was the only soil component influencing P sorption intensity factor (K). The main components influencing maximum phosphate buffer capacity (MBC) consisted of soil clay, OM, pH, Feo and Alo, with their effects in the order of Alo > OM > pH > Feo > clay. Path analysis indicated that among the components with positive effects on maximum phosphate buffer capacity (MBC), the effect was in the order of Alo > Feo > Clay, while among the components with negative effects, OM > pH. OM played an important role in mobilizing phosphate in acid paddy soils mainly through decreasing the sorption intensity of phosphate by soil particles.  相似文献   
170.
    
Abstract

Temporal changes in the scores of selected soil fertility indices were studied over six years in three different cases of organic crop rotation located in southern, eastern and central Norway. The cropping history and the initial scores of fertility indices prior to conversion to organic cropping differed between the sites. Crop yields, regarded as an overall, integrating fertility indicator, were in all rotations highly variable with few consistent temporal trends following the first year after conversion. On the site in eastern Norway, where conversion followed several years of all-arable crop rotations, earthworm number and biomass and soil physical properties improved, whereas the system was apparently degrading with regard to P and K trade balances and contents in soil. On the other two sites, the picture was less clear. On the southern site, which had a relatively fertile soil before conversion, the contents of soil organic matter and K decreased during the six-year period, but the scores of other fertility indices showed no trends. On the site in central Norway, there were positive trends for earthworm-related indices such as worm biomass and tubular biopores, and negative trends for soil porosity. The results, especially those from the eastern site, illustrate the general difficulty in drawing conclusions about overall fertility or sustainability when partial indicators show divergent trends. Consequently, the study gave no unambiguous support to the initial working hypothesis that organic farming increases inherent overall soil fertility, but rather showed that the effect varied among indicators and depended on status of the cases at conversion. It is concluded that indicators are probably better used as tools to learn about and improve system components than as absolute measures of sustainability.  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