全文获取类型
收费全文 | 41331篇 |
免费 | 2216篇 |
国内免费 | 3836篇 |
专业分类
林业 | 4802篇 |
农学 | 3479篇 |
基础科学 | 3770篇 |
6243篇 | |
综合类 | 16002篇 |
农作物 | 2634篇 |
水产渔业 | 1148篇 |
畜牧兽医 | 3645篇 |
园艺 | 1167篇 |
植物保护 | 4493篇 |
出版年
2024年 | 287篇 |
2023年 | 722篇 |
2022年 | 1210篇 |
2021年 | 1450篇 |
2020年 | 1481篇 |
2019年 | 1710篇 |
2018年 | 1025篇 |
2017年 | 1650篇 |
2016年 | 2102篇 |
2015年 | 1593篇 |
2014年 | 2363篇 |
2013年 | 2435篇 |
2012年 | 3230篇 |
2011年 | 3188篇 |
2010年 | 2356篇 |
2009年 | 2534篇 |
2008年 | 2096篇 |
2007年 | 2413篇 |
2006年 | 2094篇 |
2005年 | 1656篇 |
2004年 | 1367篇 |
2003年 | 1163篇 |
2002年 | 933篇 |
2001年 | 895篇 |
2000年 | 725篇 |
1999年 | 655篇 |
1998年 | 545篇 |
1997年 | 532篇 |
1996年 | 441篇 |
1995年 | 467篇 |
1994年 | 369篇 |
1993年 | 324篇 |
1992年 | 307篇 |
1991年 | 251篇 |
1990年 | 230篇 |
1989年 | 189篇 |
1988年 | 135篇 |
1987年 | 97篇 |
1986年 | 51篇 |
1985年 | 24篇 |
1984年 | 15篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1962年 | 4篇 |
1955年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
J. Müller 《Journal of Agronomy and Crop Science》1993,171(4):217-235
Dry Matter Production, CO2 Exchange, Carbohydrate and Nitrogen Content of Winter Wheat at Elevated CO2 Concentration and Drought Stress
Methods of mathematical modelling and simulation are being used to an increasing degree in estimating the effects of rising atmospheric CO2 concentration and changing climatic conditions on agricultural ecosystems. In this context, detailed knowledge is required about the possible effects on crop growth and physiological processes. To this aim, the influence of an elevated CO2 concentration and of drought stress on dry matter production, CO2 exchange, and on carbohydrate and nitrogen content was studied in two winter wheat varieties from shooting to milk ripeness. Elevated CO2 concentration leads to a compensation of drought stress and at optimal water supply to an increase of vegetative dry matter and of yield to the fourfold value. This effects were caused by enhanced growth of secondary tillers which were reduced in plants cultivated at atmospheric CO2 concentration. Analogous effects in the development of ear organs were influenced additionally by competitive interactions between the developing organs. The content and the mass of ethanol soluble carbohydrates in leaves and stems were increased after the CO2 treatment and exhausted more completely during the grain filling period after drought stress. Plants cultivated from shooting to milk ripeness at elevated CO2 concentration showed a reduced response of net photosynthesis rate to increasing CO2 concentration by comparison with untreated plants. The rate of dark respiration was increased in this plants. 相似文献
Methods of mathematical modelling and simulation are being used to an increasing degree in estimating the effects of rising atmospheric CO
52.
山高坡陡,切割强烈;降水丰富,降水年内分配不均;土层深厚,土体疏松等是盈江县沙化土地形成的主要自然因素。森林植被遭到破坏,坡耕地面积逐年增加等是盈江县沙化土地形成的主要人为因素。盈江县现有沙化土地面积2520.5hm^2,有流动沙地、兰固定沙地、固定沙地、沙改田4个沙化土地类型。通过长期的治理,沙化土地各类型面积及利用状况发生明显变化,总体发展趋势呈现逆转,沙化土地得到有效利用。今后需以治理水土流失为中心,采取生物措施、工程措施、农业措施相结合,开发利用并举的综合治理措施。 相似文献
53.
In 5 herds in which bovine virus diarrhoea virus (BVDV) had been isolated, all animals were bled for virological and serological examination. After the herd blood test, follow up blood tests were made on calves born up to 6 months later in 1 herd, 9 months later in 1 herd and up to 12 months later in 3 herds. Persistently infected animals (PI animals) were removed and after a time period a small herd sample of 10 animals that were born after removal of the PI animals were examined for BVDV antibodies.At the herd blood test a total of 21 PI animals were detected. During the follow up period another 25 PI animals were born.Among animals in the small herd samples collected after removal of the PI animals, antibody positive animals were found in the 2 herds with the shortest follow up period. In the 3 herds with a 1 year follow up period there were no antibody carriers in the herd sample.It seems possible to prevent further spread of infection with BVDV if all animals in the herds as well as animals born during the following year are examined and PI animals removed. 相似文献
54.
55.
本文设计了一种用于日光温室的光周期控制器 ,解决了日光温室自动化光照的问题。该设计采用数字电路 ,光电传感器 ,电光源 ,实现了对白天光照接续的时间和晚间增补光照的时间的自动、准确的控制 相似文献
56.
本文对工程造价管理角度进行了论述,从不同的角度分析了控制工程造价的方法,阐明如 何科学地控制工程造价。 相似文献
57.
畜牧业生产中的环境污染及治理对策 总被引:22,自引:3,他引:19
本文阐述了畜牧业生产中产生的畜禽粪尿、有机废水等废弃物对环境造成的污染,探讨了治理畜牧业污染的措施,提出了发展生态畜牧业的思路。 相似文献
58.
介绍了山地橄榄2种病害:叶斑病、炭疽病;5种虫害:橄榄星室木虱、黑刺粉虱、恶性叶甲、橄榄枯叶蛾、橄榄野螟的主要特征和危害症状,并提出了防治措施。 相似文献
59.
改善E1级胶合板用UF胶粘剂预压性能的研究 总被引:1,自引:0,他引:1
利用氧化淀粉在树脂合成后期、API在调胶时加入分别对脲醛树脂进行改性,探讨改性UF对胶合板预压强度的影响。采用正交实验法对比改性后胶合板的预压性能和改性后胶合板的甲醛释放量的变化。结果表明:随着API量的加大,胶合板的预压性能得以提高;而氧化淀粉改性后则随着量的加大,一定范围内胶合板的预压性能得以提高,加入量继续加大后反而下降。 相似文献
60.
Abstract. Soil samples from a 32-year grassland field experiment were taken from 0–5, 5–10, and 10–15 cm soil depths in February 2002. Plots received annual treatments of unamended control, mineral fertilizer, three rates of pig slurry and three rates of cow slurry, each with six replicates. Samples were analysed for cation exchange capacity (CEC), exchangeable cations (Na+ , K+ , Ca2+ , Mg2+ ), pH and Olsen P. Exchangeable sodium percentage (ESP) was calculated as a sodicity indicator. Mean ESP was generally greater for slurry treatments than the control, with a trend of increasing ESP with application rate. This was particularly marked for cow slurry. At 0–5 cm depth ESP increased from 1.18 in the control to 1.75 at the highest rate of pig slurry and 5.60 at the highest rate of cow slurry. Similar trends were shown for CEC, exchangeable Na+ , K+ and Mg2+ , Ca2+ and Olsen P. The build-up of soil P due to slurry applications, together with this combination of physical and chemical factors, may increase the risk of P loss to surface waters, particularly from soils receiving high rates of cow slurry. 相似文献