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   1331592篇
  免费   116772篇
  国内免费   1384篇
  2021年   17901篇
  2020年   12671篇
  2019年   16178篇
  2018年   17625篇
  2017年   16386篇
  2016年   28061篇
  2015年   42488篇
  2014年   50556篇
  2013年   76881篇
  2012年   36078篇
  2011年   24192篇
  2010年   43306篇
  2009年   44689篇
  2008年   22964篇
  2007年   21141篇
  2006年   26368篇
  2005年   27408篇
  2004年   26457篇
  2003年   24135篇
  2002年   22499篇
  2001年   35268篇
  2000年   32619篇
  1999年   32099篇
  1998年   25314篇
  1997年   25222篇
  1996年   24802篇
  1995年   22908篇
  1994年   22757篇
  1993年   21770篇
  1992年   27725篇
  1991年   26028篇
  1990年   24465篇
  1989年   25241篇
  1988年   23181篇
  1987年   21740篇
  1986年   20521篇
  1985年   22397篇
  1984年   22236篇
  1983年   19744篇
  1982年   20218篇
  1981年   19427篇
  1980年   18098篇
  1979年   18225篇
  1978年   17286篇
  1977年   16525篇
  1976年   15715篇
  1975年   15259篇
  1974年   15466篇
  1973年   15818篇
  1972年   13486篇
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
161.
162.
163.
Nitrogen fertigation of greenhouse-grown cucumber   总被引:2,自引:0,他引:2  
Summary This greenhouse study investigated the response of trickle-irrigated cucumber (Cucumis sativa cv. ‘Petita’) to three N levels applied with every irrigation via the irrigation stream. The plants were grown in pots filled with 12 kg of soil. Water containing 5.8, 11.8, or 17.8 mmol N/l, and uniformly supplied with 2.0 and 3.9 mmol/l of P and K, respectively, was applied two to three times daily. In all treatments of 0.3 leaching fraction was allowed. The resulting total N applications were 15.7, 31., and 47.2 g N/plant. The total amount of water applied was 1851/plant. Total N and NO3-N, in lajinae and petioles, increased with increasing N level whereas P and K in generated decreased. Although different NO3/NH4 ratios in the treatments may have influeced the response to N, it could be concluded that the highest yield was obtained with 11.8 mmol N/1 due to increased number of fruit. In the root volume of this treatment the NO3-N concentration in the soil solution was aroun 7 mmol/1 for most of the growing season. The dry matter concentration of fruits was not affected by the N levels. It was concluded that 11.8 mmol N/1 applied with every irrigation via the irrigation stream is adequate to cover the needs of greenhous-grown cucumber for higher yield (9.42 kg/plant over a harvesting period of 93 days).  相似文献   
164.
OBJECTIVE--To test the hypothesis that farmers are at high risk of hip osteoarthritis and to investigate possible causes for such a hazard. DESIGN--Cross sectional survey. SETTING--Five rural general practices. SUBJECTS--167 male farmers aged 60-76 and 83 controls from mainly sedentary jobs. All those without previous hip replacement underwent radiography of the hip. MAIN OUTCOME MEASURES--Hip replacement for osteoarthritis or radiological evidence of hip osteoarthritis. RESULTS--Prevalence of hip osteoarthritis was higher in farmers than controls and especially in those who had farmed for over 10 years (odds ratio 9.3, 95% confidence interval 1.9 to 44.5). The excess could not be attributed to any one type of farming, and heavy lifting seems the likely explanation. CONCLUSIONS--Manual handling in agriculture should be limited where possible. Consideration should be given to making hip osteoarthritis a prescribed industrial disease in farmers. There may be wider implications for the prevention of hip osteoarthritis in the general population.  相似文献   
165.
166.
Synthesis of prenylquinones in chloroplasts   总被引:3,自引:0,他引:3  
  相似文献   
167.
168.
169.
170.
An earlier report (1) of an adverse effect of high doses of oxytocin on human memory included results of studies on women receiving oxytocin as part of the treatment to induce 2nd trimester therapeutic abortion. These women served as their own controls. We have now been able to study a group of women who have been treated in all ways like the original group, with the exception that they did not receive oxytocin. The results from this external control corroborate the finding that oxytocin affected memory.  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