全文获取类型
收费全文 | 80345篇 |
免费 | 18228篇 |
国内免费 | 29040篇 |
专业分类
林业 | 8887篇 |
农学 | 8604篇 |
基础科学 | 9296篇 |
35483篇 | |
综合类 | 33216篇 |
农作物 | 6729篇 |
水产渔业 | 6355篇 |
畜牧兽医 | 9347篇 |
园艺 | 2803篇 |
植物保护 | 6893篇 |
出版年
2024年 | 1042篇 |
2023年 | 1232篇 |
2022年 | 1784篇 |
2021年 | 1799篇 |
2020年 | 3292篇 |
2019年 | 6659篇 |
2018年 | 6245篇 |
2017年 | 7212篇 |
2016年 | 7883篇 |
2015年 | 8092篇 |
2014年 | 7104篇 |
2013年 | 7535篇 |
2012年 | 7527篇 |
2011年 | 6503篇 |
2010年 | 5474篇 |
2009年 | 4876篇 |
2008年 | 3765篇 |
2007年 | 3965篇 |
2006年 | 3560篇 |
2005年 | 3202篇 |
2004年 | 2833篇 |
2003年 | 2458篇 |
2002年 | 2174篇 |
2001年 | 1815篇 |
2000年 | 1585篇 |
1999年 | 1690篇 |
1998年 | 1504篇 |
1997年 | 1495篇 |
1996年 | 1313篇 |
1995年 | 1377篇 |
1994年 | 1306篇 |
1993年 | 1091篇 |
1992年 | 1075篇 |
1991年 | 897篇 |
1990年 | 715篇 |
1989年 | 672篇 |
1988年 | 595篇 |
1987年 | 433篇 |
1986年 | 427篇 |
1985年 | 213篇 |
1984年 | 202篇 |
1983年 | 195篇 |
1982年 | 198篇 |
1981年 | 232篇 |
1980年 | 193篇 |
1979年 | 150篇 |
1978年 | 130篇 |
1975年 | 109篇 |
1965年 | 137篇 |
1964年 | 140篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
A disease causing the decline of papaya (Carica papaya) plants was noticed in the Jordan Valley in 1982. The disease caused severe yellowing of the upper leaves and dieback of the apex. The disease was not transmissible mechanically and continued efforts to associate viruses and viroids have so far been unsuccessful. Epidemiological observations suggested that the disease is airborne and probably caused by a mollicute. In order to test this hypothesis, plots were covered by an insect-proof net and with white nets providing 15, 30 and 50% shade. The 30%, 50% and insect-proof nettings provided complete protection, and the 15% netting reduced disease incidence to <2%, compared with>37% in the uncovered control plot. Spraying plants at weekly intervals with a commercial whitewash solution was also found to be effective in reducing disease incidence. The advantages of using a range of netting field treatments as a simple means for obtaining information on the epidemiological nature of a new disease are described. 相似文献
2.
3.
SUMMARY: Aoshio is hypoxic milky blue-green seawater observed in some eutrophic bays. Previous studies have shown that colloidal sulfur causes the coloration and that the source of aoshio water is attributed to coastal anoxic bottom water. Occurrences of aoshio have been reported in limited areas of coastal seawater, although hypoxic transparent water seems rather universal. Promotion in auto-oxidation of sulfide by metal ions in seawater was investigated to explain the occurrences of aoshio. Artificial seawater containing 10 μM metal ion was assayed for the sulfide oxidation rate. The velocity constant which represents the oxidation rate within the initial 30 min and the amount of reacted sulfide in 2 h were determined by oxygen monitoring and sulfide quantification, respectively. Fe2+ and Cu2+ enhanced the initial 30 min reaction. Fe2+ , Fe3+ and Ni2+ increased the amount of reacted sulfide in 2 h, forming whitish turbid water. Seawater from a suspected source of aoshio water was also assayed for the auto-oxidation rate of sulfide. The oxidation rate in water from 12 m depth was 13–19 times higher than the artificial seawater without an addition of heavy metal ions. More than 15% of the oxidation rate in 12 m deep seawater was explained by dissolved iron in the seawater. 相似文献
4.
Daisuke Shiode Yasuzumi Fujimori Fuxiang Hu Susumu Shimizu Teisuke Miura 《Fisheries Science》2001,67(1):110-117
SUMMARY: Flume-tank experiments were performed to examine the depth response of a new type of depth-controlled gillnet. Variations of net depth were investigated as the warp was paid out and wound up for different changes of warp length, main sinker weights, and winch speeds. In most experiments, when the warp finished paying out, the net continued to descend and then ascended slightly to an equilibrium depth (overshoot phenomenon). The overshoot distance was nearly constant when the warp was wound up and increased linearly with increasing winch speed when the warp was paid out. An increase in winch speed reduced net settling time, which converged on a constant value for both paying out and winding up. 相似文献
5.
6.
7.
工科院校在强调专业教育的同时,没能同样重视审美教育。加强对工科院校大学生的审美教育,有助于培养正确的审美观,提高审美能力;有助于完善知识结构,提升人文精神;
有助于形成独立完善的人格,提升人格境界;有助于培养个性和谐、德智体美全而发展的高素质人才。 相似文献
8.
Soil pH declined from 5.9 to 5.0 in 8 years beneath plantations of Eucalyptus saligna (Sm.) in Hawaii. In stands of Albizia falcataria, (L.) Fosberg, the soil pH change was more dramatic, declining from 5.9 to 4.6. We measured several components of soil acidity beneath four mixtures of the two tree species to gain insight on the processes responsible for the decline in soil pH. These components were studied using an empirical method of comparing acid quantity, degree of neutralization (depletion of base cations), and acid strength. The decline in soil pH differed between species as a result of differences in the degree of neutralization of the soil exchange complex; the larger decrease in soil pH under Albizia was produced by greater acidification of the exchange complex. Empirical titration curves suggested that differences in acid strength moderated the divergence in soil pH beneath the species. Had the acids accumulating in the soil under Albizia been as strong as those in the Eucalyptus soil, the difference in soil pH would have been greater. Though the two species had contrasting effects on soil pH, the differences in degree of neutralization, responsible for the pH decline, were small compared with differences in the amount of cations stored in tree biomass. Continued supply of nutrient cations (from weathering or fertilization) will ultimately control both the extent of soil pH decline and the level of productivity sustained by the forest. 相似文献
9.
10.
澳洲坚果引种试种研究初报 总被引:12,自引:0,他引:12
澳洲坚果无性系从1979年开始引入中国,并在南亚热带作物研究所(南亚所)试种,现已在南亚热带地区七省扩大试种达200hm^2,各地生长均良好,已有少量结果。南亚所植区澳洲坚果13龄树平均单株年产已达6kg(壳果),抗寒性良好,在-4℃时仍未受害,病虫害不严重(曾发生过花序枯萎病、速衰病、茎干溃疡病),但风害严重。澳洲坚果的适生温度在10℃-30℃,以最冷月(1)平均气温≤10℃,年降雨量在1000 相似文献