全文获取类型
收费全文 | 278696篇 |
免费 | 15543篇 |
国内免费 | 687篇 |
专业分类
林业 | 24046篇 |
农学 | 14086篇 |
基础科学 | 2985篇 |
44918篇 | |
综合类 | 30052篇 |
农作物 | 17538篇 |
水产渔业 | 17457篇 |
畜牧兽医 | 112739篇 |
园艺 | 6396篇 |
植物保护 | 24709篇 |
出版年
2021年 | 2511篇 |
2020年 | 2918篇 |
2019年 | 3732篇 |
2018年 | 4301篇 |
2017年 | 4732篇 |
2016年 | 4938篇 |
2015年 | 4377篇 |
2014年 | 5742篇 |
2013年 | 16304篇 |
2012年 | 7267篇 |
2011年 | 9505篇 |
2010年 | 7835篇 |
2009年 | 8377篇 |
2008年 | 8882篇 |
2007年 | 7954篇 |
2006年 | 8292篇 |
2005年 | 7620篇 |
2004年 | 7391篇 |
2003年 | 7181篇 |
2002年 | 6395篇 |
2001年 | 7022篇 |
2000年 | 6808篇 |
1999年 | 6114篇 |
1998年 | 4222篇 |
1997年 | 4280篇 |
1996年 | 3996篇 |
1995年 | 4655篇 |
1994年 | 4011篇 |
1993年 | 3705篇 |
1992年 | 4816篇 |
1991年 | 4994篇 |
1990年 | 4696篇 |
1989年 | 4682篇 |
1988年 | 4140篇 |
1987年 | 4172篇 |
1986年 | 4175篇 |
1985年 | 4416篇 |
1984年 | 3992篇 |
1983年 | 3706篇 |
1982年 | 3018篇 |
1981年 | 2874篇 |
1980年 | 2871篇 |
1979年 | 3555篇 |
1978年 | 3114篇 |
1977年 | 2849篇 |
1976年 | 2731篇 |
1975年 | 2648篇 |
1974年 | 2835篇 |
1973年 | 2813篇 |
1972年 | 2460篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
Neil G. Bayfield 《Biological conservation》1974,6(4):246-251
The extent of accelerated erosion from ground damaged near ski lifts was found to have reached a peak in about 1969, with a marked decline since then. This decline was attributable to reseeding of damaged ground, provision of drains and grading of dirt roads. Burial experiments showed that where erosion debris had covered vegetation, recovery at best took several years, and with depths above about 7 cm, was almost negligible. 相似文献
994.
995.
996.
997.
The relative-intensity X-ray fluorescence method of analysis of rock and soil samples has been investigated and compared with the net-intensity method. Strong, coherently scattered radiation originating from the X-ray tube is shown to be preferable to background radiation as an internal standard, and scattered radiation measured at one wavelength can usefully be applied in the determination of several elements. When the concentrations of an element in two soil samples of different composition (for example concretions and the soil adjacent to them) are compared, the ratio of the relative intensities may be different from the ratio of net intensities by a factor of about two. The concentrations of manganese in thirteen standard rock samples determined by the relative-intensity method are within or very close to the ranges of values reported previously. 相似文献
998.
Infrared spectroscopy and nitrogen analyses suggest that secondary amide groups in protein-like components of soil clay-organic complexes and extracted organic matter decompose above 100°C to yield ammonia which is retained as NH4+ by acid-washed clay-organic complexes. Above about 400°C, other volatile nitrogenous decomposition products are released. Clay surfaces in the clay-organic complexes may catalyse the decomposition. 相似文献
999.
Hydraulic conductivity between saturation and a tension of 100 cm water was calculated with moisture-retention data for nine soil horizons and compared with results from in situ measurements with the crust test. Agreement was good for sandy, apedal soil horizons with simple packing voids but only if matching factors were used. Results were unreliable in clayey, pedal soil horizons in which a few relatively large planar and tubular pores determine K in the measured tension range, whereas the greatest fraction of total porosity is composed of fine pores inside peds that hardly contribute to flow. Varying the number of pore classes (n) and the water-filled porosity at saturation made no significant difference in the calculations for the apedal soils, but drastically changed the shape of the calculated curves for the pedal soils. Matching factors based on Ksat measurement had to be used for all studied soil horizons, indicating that Marshall's pore-interaction model never predicted Ksat accurately. 相似文献
1000.
Electrofocusing in polyacrylamide gel was carried out on humic substances extracted by 0.1 n NaOH, 0.1m Na4P2O7, and by the method of Burns, El Sayed and McLaren (1972) from the upper and lower organic horizons of an alpine podsol. The electrophoretic patterns were characteristic of both humus origin and extractions procedure. The best resolution of the isoelectric bands for the humus were obtained by the Burns-McLaren procedure. The ionic component does not have much influence on the distribution of humus constituents within the electrophoretic pattern, but the presence of phosphate appears to be essential to obtain good separations. 相似文献