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   6705篇
  免费   283篇
  国内免费   8篇
林业   231篇
农学   344篇
基础科学   83篇
  834篇
综合类   362篇
农作物   246篇
水产渔业   202篇
畜牧兽医   3697篇
园艺   105篇
植物保护   892篇
  2023年   32篇
  2022年   36篇
  2021年   56篇
  2020年   88篇
  2019年   90篇
  2018年   102篇
  2017年   103篇
  2016年   114篇
  2015年   93篇
  2014年   131篇
  2013年   493篇
  2012年   296篇
  2011年   350篇
  2010年   177篇
  2009年   175篇
  2008年   331篇
  2007年   321篇
  2006年   272篇
  2005年   277篇
  2004年   286篇
  2003年   257篇
  2002年   248篇
  2001年   234篇
  2000年   217篇
  1999年   197篇
  1998年   54篇
  1997年   54篇
  1996年   57篇
  1995年   65篇
  1994年   39篇
  1993年   45篇
  1992年   105篇
  1991年   116篇
  1990年   135篇
  1989年   110篇
  1988年   100篇
  1987年   84篇
  1986年   67篇
  1985年   72篇
  1984年   66篇
  1983年   44篇
  1982年   30篇
  1981年   41篇
  1980年   44篇
  1979年   45篇
  1975年   25篇
  1974年   27篇
  1973年   26篇
  1972年   35篇
  1971年   32篇
排序方式: 共有6996条查询结果,搜索用时 31 毫秒
81.
A study of the water balances of The Fayoum irrigated lands and Lake Qarun was made to investigate the management of the irrigation system and the efficiency of irrigation water use. The two water balances are strongly interrelated. The drainage flow to Lake Qarun and the water level of the Lake are in delicate balance. A rise in Lake level causes the inundation of adjacent land. Management of The Fayoum water balance assumes control over irrigation water flows, but this control has technical and organizational limitations. Also discussed is the influence of irrigation practices in The Fayoum on the water balance (e.g., the autumn flushing of fields and farmers' preference for not irrigating at night in winter). Notwithstanding a high overall efficiency, irrigation efficiency during the winter is low. The reasons for this are given, together with the constraints against improving system management. Improved uniformity of the division and application of irrigation water will enable a better technical control of flows and will result in better water management in The Fayoum. Abbreviations: FID — Fayoum Irrigation Department, 1 feddan (fe) — 0.4 ha, 1 mcm — 1 million cubic metres: an average annual flow of 3.17 m3/s gives 100 mcm, m3/fe.year — supplied volume (m3) per surface area (fe) per year: 1000 m3/fe.year equals 240 mm/year, MSL — Mean Sea Level  相似文献   
82.
Summary As hardly any data can be found on the water requirement and yield response to water of Aloe vera, an irrigation experiment on transplanted aloe poles was conducted on the Caribbean island of Aruba. Compared to non-irrigated treatments irrigation shows a positive influence on growth, number of developed leaves, raw gel figure and gel production per plant. The amount of irrigation water to apply can be best calculated based on crop coefficients (Kc) between 0.20 and 0.30. Treatment based on a Kc value higher than 0.30 did not further increase production. A lysimeter study conducted over a relatively short period revealed that the crop coefficient for Aloe vera varied between 0.25 and 0.40 which is characteristic of the so-called agave/pineapple group.  相似文献   
83.
This work describes the analysis of the uncertainty linked to the annual direct and indirect losses of different nitrogenous compounds at the scale of a group of farms. The nitrogen (N) forms taken into account are: ammonia (NH3), nitric oxide (NO), nitrous oxide (N2O), dinitrogen (N2) and nitrate (NO3). The gaseous N emissions for the different components of the farms are estimated with a selection of adapted emission factors. The NO3 losses at the farm scale are calculated as the difference between the surplus of the farm-gate N balance and the gaseous N emissions.  相似文献   
84.
Whole-farm design models quantitatively analyze the effects of a variety of potential changes at the farm system level. Science-driven technical information is confronted with value-driven objectives of farmers or other social groupings under explicit assumptions with respect to exogenous variables that are important drivers of agricultural systems (e.g., market conditions). Hence, farm design is an outcome of objective specification and the potential of a system. In recent publications, whole-farm design modelling has been proposed to enhance (farm) innovation processes. A number of operational modelling tools now offers the opportunity to assess the true potential of whole-farm design modelling to enhance innovation. In this paper, we demonstrate that it is not trivial to find niches for the application of goal-based farm models. Model outcomes appeared not to match questions of farm managers monitoring and learning from their own and other farmers’ practices. However, our research indicates that whole-farm design modelling possesses the capabilities to make a valuable contribution to reframing. Reframing is the phenomenon that people feel an urge to discuss and reconsider current objectives and perspectives on a problem. Reframing might take place in a situation (i) of mutually felt dependency between stakeholders, (ii) in which there is sufficient pressure and urgency for stakeholders to explore new problem definitions and make progress. Furthermore, our research suggests that the way the researcher enters a likely niche to introduce a model and/or his or her position in this niche may have significant implications for the potential of models to enhance an innovation process. Therefore, we hypothesize that the chances of capitalizing on modelling expertise are likely to be higher when researchers with such expertise are a logical and more or less permanent component of ongoing trajectories than when these researchers come from outside to purposefully search for a niche.  相似文献   
85.
Nitrous oxide (N2O) emissions make up a significant part of agricultural greenhouse gas emissions. There is an urgent need to identify new approaches to the mitigation of these emissions with emerging technology. In this short review four approaches to precision managements of agricultural systems are described based on examples of work being undertaken in the UK and New Zealand. They offer the opportunity for N2O mitigation without any reduction in productivity. These approaches depend upon new sensor technology, modeling and spatial information with which to make management decisions and interventions that can both improve agricultural productivity and environmental protection.  相似文献   
86.
87.
88.
89.
90.
Landscape Ecology - Tritrophic interactions may be affected by local factors and the broader landscape context. At small spatial scales, carnivorous enemies of herbivorous insects use...  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