全文获取类型
收费全文 | 2191篇 |
免费 | 774篇 |
国内免费 | 9篇 |
专业分类
林业 | 157篇 |
农学 | 100篇 |
基础科学 | 224篇 |
1506篇 | |
综合类 | 653篇 |
农作物 | 32篇 |
水产渔业 | 25篇 |
畜牧兽医 | 108篇 |
园艺 | 5篇 |
植物保护 | 164篇 |
出版年
2025年 | 41篇 |
2024年 | 89篇 |
2023年 | 97篇 |
2022年 | 111篇 |
2021年 | 100篇 |
2020年 | 117篇 |
2019年 | 119篇 |
2018年 | 104篇 |
2017年 | 171篇 |
2016年 | 193篇 |
2015年 | 115篇 |
2014年 | 100篇 |
2013年 | 134篇 |
2012年 | 176篇 |
2011年 | 179篇 |
2010年 | 157篇 |
2009年 | 115篇 |
2008年 | 136篇 |
2007年 | 130篇 |
2006年 | 104篇 |
2005年 | 81篇 |
2004年 | 50篇 |
2003年 | 46篇 |
2002年 | 36篇 |
2001年 | 37篇 |
2000年 | 35篇 |
1999年 | 24篇 |
1998年 | 27篇 |
1997年 | 21篇 |
1996年 | 21篇 |
1995年 | 19篇 |
1994年 | 16篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1991年 | 21篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有2974条查询结果,搜索用时 15 毫秒
1.
通过对山西省永济市气象局72年安装的长春DYYZ2型雨量传感器和山西省技术装备中心研制的采集器为例,分析乡镇自动雨量站的常见故障,提出了排除办法及维护时的注意事项。 相似文献
2.
《Communications in Soil Science and Plant Analysis》2012,43(17-18):2648-2662
This study was designed to evaluate phosphorus (P) losses from rotationally grazed pastures. Sod of tall fescue was transplanted into boxes 100×18.4 cm2. Treatments (13) imposed in triplicate were a no P control, inorganic P at rotational requirements, inorganic P at seasonal requirements, and 10 plots representing a paddock in a 20‐day, alternate‐day grazing rotation. Treatments were isonitrogenous; three rotations were conducted. Inorganic fertilizers were applied on day 1 of the rotation. Dairy slurry from grazing cows was applied daily to the respective treatments. Rainfall was simulated after each rotation, and continuous runoff was collected for 30 min. Total and dissolved reactive P in runoff were lower from control than from P‐amended treatments. No differences were observed among P‐amended plots. The small size of the boxes and naturally high variability in fescue sod did not allow for an accurate assessment of the risk of P losses from rotationally grazed pasture. 相似文献
3.
4.
In this study we determined soil moisture storage, evapotranspiration (ET) and light interception in an agroforestry trial consisting of pine trees grown over (1) control (bare ground), (2) ryegrass/clovers (Lolium perene/Trifolium spp.), (3) lucerne (Medicago sativa), and (4) ryegrass only during the third growing season between 1992 and 1993. The results show that:
相似文献
1. | In the period when rainfall was frequent and exceeded the evaporative demand (Epot), ET and depletion of soil moisture were not affected by the ground cover treatments. During summer when rainfall was less frequent, but with moisture readily available in the soil profile, ET was associated with plant canopy, and was significantly higher for the pasture ground covers than for the control. Therefore, the more rapid growth by lucerne caused higher ET in this ground cover than in the ryegrass/clovers ground cover in which the pasture was slow growing. At the end of the study period, total ET was in the following order: lucerne (757 mm) > ryegrass/clovers (729 mm) > Control (618 mm). |
2. | ET was dominated by pasture transpiration (Ep) during most of the growing season, but by tree transpiration (Et) in winter when large parts of the pasture canopy was shaded. Ep was always at least 16% higher for lucerne than for ryegrass/clovers species as a result of a greater radiation intercepted by the former. |
3. | Fraction of incoming radiation intercepted by the tree crowns was in the following order: control > ryegrass > ryegrass/clovers > lucerne. At the end of the one-year period, fraction of intercepted radiation was 140% greater for control than for lucerne ground cover. |
4. | The control produced the largest tree crowns, which were almost twice the tree crowns in the lucerne ground cover which produced the smallest trees. Accordingly, the trees in the control intercepted more radiation and rainfall, with the former being lost to evaporation, than the trees in the pasture. |
5. | The fractions of radiation intercepted and ET accounted for by the trees and pastures were associated with the proportion of the plot area they occupied. |
5.
Simulated acid rain (SAR) at three pH levels (pH 4, 3, 2) was applied to only the top or both the top and roots of 4-month-old
Japanese black pine seedlings repeatedly for two months. Then the seedlings were inoculated with a virulent isolate (S10)
of pinewood nematode,Bursaphelenchus xylophilus. The exposure to SAR at any level did not kill the seedlings. When inoculated with nematodes, however, the nematode population
build-up was significantly higher in the seedlings pretreated with SAR at any level than in the control at the 7th and 17th
day after inoculation, and subsequent disease symptom development was also significantly accelerated by the exposure to SAR
at pH 2 and 3. This result indicates that even acid rain at pH 4 has the potential for promoting population growth of pinewood
nematodes in 4-month-old Japanese black pine seedlings. 相似文献
6.
Experiments were undertaken to identify an efficient method of selection for yield in early segregating generations of narrow-leaf lupin crosses. A first series of experiments involved simulation of F2 populations with mixtures of ten cultivars and advanced breeding lines. The performance of genotypes in mixtures sown at different densities was compared with their monoculture [commercial density) yields m the same and the following year. These comparisons provided a basis for assessing the value of different planting designs and different selection criteria in early generation selection for yield. Correlated monoculture yield responses to simulated selection for single plant yield were similar for the honeycomb design and standard selection procedures at both low and commercial densities. These responses were much higher than that for grid selection in a low density population. Correlated monoculture yield responses to selection for other plant characters were substantial only for flowering time and number of pod bearing branches under certain conditions. These responses were greatest for selection in a low density mixture and in the year following selection. A procedure involving selection of the earliest flowering plants and selection within this group of plants with the highest number of pod bearing branches should be an effective approach to early generation yield selection in the narrow-leaf lupin. 相似文献
7.
Summary Alfalfa (Medicago sativa L.) forage yield may be reduced by moderate levels of salinity. Increasing forage yield in saline environments through plant breeding could partially ameliorate this problem. However, prior indirect selection either in cell culture, at germination, or during seedling growth has not resulted in agronomically relevant levels of salt tolerance. This study was conducted to determine whether mean forage yield of alfalfa grown in saline environments could be increased. To reduce the number of possible selection schemes, forage yield data for five harvests from Sonora and its parental cultivar African grown in a greenhouse and irrigated with 0, 30, 60, or 80 mM NaCl were used to simulate one cycle of selection at each NaCl environment. Greatest gains in forage yield were expected under non-saline conditions with decreased gains under saline conditions. Selection based on yield in regrowth harvests was also predicted to be more efficient than that based on seedling harvest. Experimental populations were developed from Sonora and African at each NaCl level using phenotypic selection for increased mean forage yield for harvests 3 to 5. After each of two cycles of selection, forage yield from selected, control (plants selected randomly under non-saline conditions), and parental populations was evaluated at 0, 30, 60, and 80 mM NaCl. Gains in forage yield were observed in African when the selection and evaluation environments were the same, but in Sonora this was observed only at 0 NaCl. Selection at 0 NaCl did not increase forage yield at 60 or 80 mM NaCl in either germplasm source. This suggests that selection for increased forage yield may be successful at low to moderate NaCl levels in germplasm with ample variation for yield when grown in saline environments, while selection for increased forage yield in non-saline conditions may be ineffective. 相似文献
8.
旱农区不同种植模式作物最佳补灌时期和适宜补灌量研究 总被引:17,自引:3,他引:17
The effect of supplementary
irrigation with catchment rainfall on crop yield increment and its water use efficiency
was studied on upland field with different cropping patterns to make full use of the
natural rainfall. The results showed that the crop yield and water use efficiency was
increased significantly after 45 mm~90 mm supplementary irrigation, as compared with no
irrigation. The optimum total amounts of supplementary irrigation for all three cropping
patterns were 90 mm. The optimum amounts and periods of irrigation for mono-cultured wheat
were 8mm before sowing and 82 mm at its boot stage, those for film-covered mono-cultured
were 90mm at its big horn stage, and those for multipled cropping wheat and film-covered
corn were 4mm before wheat sowing and 19 mm both at jointing and boot stages, and 24 mm
both at jointing and big horn stages of corn. The results were suitable for northern
upland fields. 相似文献
9.
乐昌峡水利枢纽右岸高边坡风化层深厚,岩石强度弱,大坝右岸坝肩开挖后,将形成约215m的开挖边坡,边坡处于降雨强度大的地区,在施工期边坡开挖过程中的变形稳定,直接影响到碾压混凝土大坝的施工工期,十分关键.为了验算边坡开挖中和开挖后的稳定性,首先采用有限元法,对边坡开挖过程进行模拟,给出边坡的变形、屈服区等应力应变情况,利用强度折减法得到边坡的抗滑稳定安全系数,其次,用刚体极限平衡法计算安全系数,并与有限元的计算结果进行对比分析.在缺少渗流基本资料情况下,对深厚风化层的稳定状态,在极限平衡分析中,进行受降雨影响下的敏感性分析.最后对该工程提出加强观测和排水等措施,对边坡的稳定具有指导意义. 相似文献
10.