全文获取类型
收费全文 | 274279篇 |
免费 | 15868篇 |
国内免费 | 666篇 |
专业分类
林业 | 24100篇 |
农学 | 14083篇 |
基础科学 | 2964篇 |
44123篇 | |
综合类 | 29267篇 |
农作物 | 17359篇 |
水产渔业 | 17295篇 |
畜牧兽医 | 110712篇 |
园艺 | 6388篇 |
植物保护 | 24522篇 |
出版年
2020年 | 2899篇 |
2019年 | 3689篇 |
2018年 | 4176篇 |
2017年 | 4534篇 |
2016年 | 4931篇 |
2015年 | 4386篇 |
2014年 | 5748篇 |
2013年 | 16418篇 |
2012年 | 6731篇 |
2011年 | 8760篇 |
2010年 | 7571篇 |
2009年 | 8116篇 |
2008年 | 8128篇 |
2007年 | 7197篇 |
2006年 | 7692篇 |
2005年 | 6962篇 |
2004年 | 6780篇 |
2003年 | 6691篇 |
2002年 | 5961篇 |
2001年 | 6653篇 |
2000年 | 6252篇 |
1999年 | 5811篇 |
1998年 | 4227篇 |
1997年 | 4293篇 |
1996年 | 3998篇 |
1995年 | 4617篇 |
1994年 | 4008篇 |
1993年 | 3712篇 |
1992年 | 4769篇 |
1991年 | 5058篇 |
1990年 | 4721篇 |
1989年 | 4770篇 |
1988年 | 4242篇 |
1987年 | 4231篇 |
1986年 | 4295篇 |
1985年 | 4517篇 |
1984年 | 4113篇 |
1983年 | 3810篇 |
1982年 | 3076篇 |
1981年 | 2894篇 |
1980年 | 2891篇 |
1979年 | 3675篇 |
1978年 | 3184篇 |
1977年 | 2987篇 |
1976年 | 2811篇 |
1975年 | 2698篇 |
1974年 | 2984篇 |
1973年 | 2984篇 |
1972年 | 2702篇 |
1971年 | 2606篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
B Stavric R Klassen W Miles 《Journal of the Association of Official Analytical Chemists》1979,62(5):1020-1026
A method is described for the simultaneous quantitation of trace amounts of alpha- (alpha-NA) and beta-naphthylamines (beta-NA) with detectability in the 0.1 ppb range and sensitivity of 50 picomoles in certified food grade amaranth (FD&C Red No. 2; C.I. Food Red 9; CI 16185). The amaranth sample is extracted with benzene, and the evoporated residue is derivatized with perfluorooctanoic anhydride. The resulting derivatives are separated by gas-liquid chromatography and identified and quantitated by mass spectrometric monitoring of the m/e at 539.04. The method was used for quantitation of alpha-NA and beta-NA in randomly chosen samples of amaranth. Of 11 samples from different manufacturers, 5 were free of the beta-isomer; the remaining samples contained up to 1.2 ppb beta-NA. The concentration of alpha-NA ranged from no detectable amount to 970 ppb; the majority of the samples contained less than 7 ppb. 相似文献
962.
963.
Little information exists concerning the effects of long-term fungicide programs on the qualitative and quantitative characteristics of fusaria in turfgrasses and in turfgrass soils. These effects were studied on Poa pratensis L, turfgrasses at three locations, using 14 fungicides, one nematicide, and five mixed fungicide programs. Some fungicides increased the numbers of fusaria in soil and thatch, some had no effect, and others greatly reduced the numbers. Changes in Fusarium species' compositions occurred independently from the changes in propagule numbers. The proportion of Fusarium-colonized turfgrass crowns was generally higher in fungicide-treated than in nontreated turfgrasses. 相似文献
964.
Phytotoxicity was demonstrated in the aqueous extract of wheat (Triticum aestivum, L.) straw suspensions. When extracts were incubated under anaerobic conditions, the development of phytotoxicity was greater at 20°C than at 10°C. The toxic products formed during incubation depended upon the incubation medium. Acetic and butyric acids were the major toxins produced in liquid straw fermentations during the first 2 weeks. However, after that time the acids did not account for the total toxicity, suggesting formation of unidentified phytotoxins. Acetic, propionic and butyric acids were the toxins formed in sand culture. The development and accumulation of phytotoxins were favored where the sand-straw mixtures were water-saturated. The implications of these findings to wheat cultural practice is discussed. 相似文献
965.
Nigel J.H. Smith 《Biological conservation》1979,16(3):165-176
Turtles have long served man as an important food resource in Amazonia. During the colonial period, Portuguese traders disturbed nesting beaches on a large scale in search of eggs which were converted into oil for cooking and lighting. Exploitation was increasingly intense by the end of the 19th century and turtle populations had declined drastically. Although legislation has been passed to protect them, enforcement is lax and turtles face increasing pressure from man not only for food but as a result of habitat disturbance. 相似文献
966.
This paper reviews the breeding distribution and numerical status of selected species of seabirds at 23 oceanic island groups in the Southern Ocean (between latitudes 35 and 70°S), based on census data and population estimates which have become available through increased scientific endeavour in the region during the last 15 years or so. The paper focuses on the avifauna of the Prince Edward islands, in the southern Indian Ocean, and points to the importance of these islands as a breeding station and sanctuary for seabirds. Only one oceanic island group, the Crozets, contains more species of breeding seabirds than the Prince Edward islands. Substantial segements (about 10% and greater) of the world breeding populations of the king and macaroni penguins, and the wandering, grey-headed, yellow-nosed and sooty albatrosses are based at the Prince Edward group. We recommend that Prince Edward Island (sensu stricto) should be a wilderness area, and that development at Marion Island should be kept to an absolute minimum. 相似文献
967.
This paper describes the main features of the Sangay wilderness area in the Ecuadorian Andes. It is a region of great diversity with life zones ranging from tundra to subtropical rain forest. The conservation value of the area is assessed and the feasibility of protection determined. Boundaries based mainly on the limits of human interference are put forward with management proposals for its establishment as a National Park. 相似文献
968.
A triangular plot of heather moor at Kindrogan was used for teaching botanical analysis for eight years, using the same set of random coordinates to locate sample points. Because of the method of point location, there was a gradient of trampling pressure across the plot, and there were also parts of the plot that were never analysed. At the end of the eight-year period heather cover and mean height were found to have declined with increasing levels of trampling. Mean numbers of species/m2 did not however decline, and were actually minimal on untrampled ground. With moderate levels of trampling, quadrats that had been analysed for eight years contained more heather litter than those that had not been analysed before. No decline in species frequencies was found in previously analysed quadrats. The value of the plot for teaching purposes does not seem to have declined, and may have actually increased over the period of use. 相似文献
969.
E J de Vries J Zeeman R J Esser B Borsje F J Mulder 《Journal of the Association of Official Analytical Chemists》1979,62(6):1285-1291
Vitamin D is determined in preparations containing other fat-soluble vitamins by high performance liquid chromatography (HPLC). The unsaponifiable residue is extracted and separated from interferences by reverse phase chromatography; the fraction corresponding to vitamin D3 is collected and quantitated using normal phase chromatography (amylalcohol-n-hexane as mobile phase) by measuring the vitamin D3 and pre-vitamin D3 peaks at 254 nm. Previtamin D3 content is calculated as vitamin D3 with a conversion factor (determined on the equipment used). Application of the method to vitamin AD3 mixtures in oils gives 98-102% recovery. The reproducibility, using an external standard, is 2-3%, calculated as the coefficient of variation; with an internal standard, the coefficient of variation is 1-1.5%. The method measures potential vitamin D3 content in preparations containing greater than or equal to 200 IU/g in the presence of all known vitamin D3 isomers, vitamin A, and vitamin E. 相似文献
970.
Karel V. Sykora 《Biological conservation》1979,16(2):145-162
The effect of the extreme drough of 1976 has been studied to obtain information on the reaction of the vegetation of moorland pools following a fall in the ground water level. The degree of change appeared to be dependent on the nutrient status of water and soil and on the relief. The vegetation of metatrophic and guanotrophic moorland pools showed the most drastic change. 相似文献