全文获取类型
收费全文 | 65401篇 |
免费 | 2813篇 |
国内免费 | 2671篇 |
专业分类
林业 | 5394篇 |
农学 | 4816篇 |
基础科学 | 2163篇 |
10298篇 | |
综合类 | 14176篇 |
农作物 | 3592篇 |
水产渔业 | 2694篇 |
畜牧兽医 | 21373篇 |
园艺 | 2174篇 |
植物保护 | 4205篇 |
出版年
2023年 | 497篇 |
2022年 | 1117篇 |
2021年 | 1483篇 |
2020年 | 1422篇 |
2019年 | 1520篇 |
2018年 | 2440篇 |
2017年 | 2915篇 |
2016年 | 1953篇 |
2015年 | 1586篇 |
2014年 | 1853篇 |
2013年 | 3241篇 |
2012年 | 3126篇 |
2011年 | 4017篇 |
2010年 | 3741篇 |
2009年 | 3380篇 |
2008年 | 3297篇 |
2007年 | 3510篇 |
2006年 | 2148篇 |
2005年 | 1853篇 |
2004年 | 1338篇 |
2003年 | 1319篇 |
2002年 | 1367篇 |
2001年 | 1433篇 |
2000年 | 1349篇 |
1999年 | 1066篇 |
1998年 | 616篇 |
1997年 | 591篇 |
1996年 | 498篇 |
1995年 | 543篇 |
1994年 | 498篇 |
1993年 | 422篇 |
1992年 | 640篇 |
1991年 | 600篇 |
1990年 | 561篇 |
1989年 | 587篇 |
1988年 | 465篇 |
1987年 | 406篇 |
1986年 | 407篇 |
1979年 | 495篇 |
1977年 | 389篇 |
1976年 | 472篇 |
1975年 | 488篇 |
1974年 | 526篇 |
1973年 | 484篇 |
1972年 | 440篇 |
1970年 | 372篇 |
1969年 | 437篇 |
1968年 | 460篇 |
1967年 | 455篇 |
1966年 | 439篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Subsurface drip irrigation and reclaimed water quality effects on phosphorus and salinity distribution and forage production 总被引:3,自引:0,他引:3
M.P. Palacios-Díaz V. Mendoza-Grimn J.R. Fernndez-Vera F. Rodríguez-Rodríguez M.T. Tejedor-Junco J.M. Hernndez-Moreno 《Agricultural Water Management》2009,96(11):1659-1666
In the Canary Islands, water scarcity is one of the constraints for agricultural activity. Non-conventional water resources generally represent more water volume than conventional ones. The distribution of these resources frequently permits the possibility of a conjunctive use of desalinated (DW) water and reclaimed municipal wastewater (RW). Field testing with both water qualities and different irrigation systems is necessary for optimal site-specific management. The objective of this work was to evaluate soil salinity and phosphorus distribution, and alfalfa yield in a 20 month field experiment carried out in the island of Gran Canaria, using municipal RW and freshwater (FW) under subsurface drip irrigation (SDI). Phosphorus speciation was performed both in irrigation waters and in soils (Olsen's inorganic, organic, and microbial). RW had large EC values (2.4 dS m−1) with a remarkable nutrient load contribution and an average total P around 3 mg L−1, predominantly hydrolysable forms, while FW had very low salinity and negligible amounts of P. For the RW treatment a salt gradient was established, causing plant mortality between the irrigation lines. The study of P speciation allows describing P distribution and plant uptake in terms of P forms. Large values of microbial P were produced for the two irrigation waters around the emitters, especially for FW.A faster P-cycling could have contributed to the significantly larger inorganic P contents observed in FW irrigated soils, in spite no external sources were added by the irrigation water. 相似文献
952.
董玉 《农业机械化与电气化》2009,(1):39-40
介绍并对比了基于神经网络和模糊控制的瞬态空燃比控制方法,二者均不要求控制对象为非常精确的模型。特别在应用的非线性理论建模还不十分完善的情况下。神经网络控制器和模糊控制器的这个优点更为突出。 相似文献
953.
954.
955.
956.
立式多级筒袋泵吸入装置的优化设计 总被引:1,自引:0,他引:1
通过试验和数值计算,研究了立式多级筒袋泵的空化性能及泵内空化流场.为改善立式多级筒袋泵的吸入性能,分析了几种叶轮几何参数对泵空化的影响。研究结果表明:采用叶片进口断面面积较大的双吸式首级叶轮,且在首级叶轮的两侧进口加设诱导轮,大幅提高了立式多级筒袋泵的空化性能,使泵的空化比转速达1479;基于均衡混合流假设的空化模型,可合理预测泵的平均空化性能,模拟的空化流场有助于了解水力设计诸因素对泵内空化发展的影响;在设计诱导轮及首级叶轮时,选取较大的叶片进口安放角有利于改善泵装置的吸入性能,同时有利于发挥诱导轮的功效. 相似文献
957.
Suitability of mapped sequence tagged microsatellite site markers for establishing distinctness, uniformity and stability in aromatic rice 总被引:6,自引:0,他引:6
R.K. Singh R.K. Sharma A.K. Singh V.P. Singh N.K. Singh S.P. Tiwari T. Mohapatra 《Euphytica》2004,135(2):135-143
At present, testing for distinctness, uniformity and stability (DUS) of crop varieties relies on a set of morphological characters.
These characters suffer fromthe limitations of number, interaction with the environment in which the variety grows and subjectivity
in decision-making. The potential of DNA-based markers such as sequence tagged microsatellite site (STMS), for establishing
DUS merits investigation. In the present study, a set of 55 mapped STMS markers, selected from 12 linkage groups of rice genome,
was used to examine distinctness of 23 aromatic rice genotypes including the commercially important Basmati varieties. Forty-one
of these markers (74.5%) showed polymorphism between the varieties. The number of alleles per locus ranged from 2–4 with an
average of 2.3. The polymorphism information content (PIC) of the markers varied from 0.083 to 0.665 with an average of 0.338.
All the varieties could be differentiated from each other at a low probability (0.07×10-13) of identical match by chance. The marker-based clustering of the varieties corresponded with the known phenotypic classification,
thereby providing confidence in the distinctness established by the mapped STMS markers. The utility of these markers to study
uniformity and stability was analysed using a commercially important crossbred Basmati rice variety Pusa Basmati 1(IET-10364)
that contributes about 40–50% of Basmati rice export from India. Genotyping of twenty individual plants, grown from the nucleus,
breeder, foundation, certified and farmer's saved seed samples using all the 55 markers revealed no variation among the plants.
These observations suggested that the set of mapped markers employed in this study could be further used for establishing
distinctness of aromatic rice varieties and for studying DUS of the important commercial variety Pusa Basmati 1.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
958.
959.
Nitrogen Accumulation and Translocation for Winter Wheat under Different Irrigation Regimes 总被引:12,自引:0,他引:12
The translocation of pre‐anthesis nitrogen to the grain is an important source for winter wheat. The relation between the nitrogen translocation and irrigation regime was studied in the field under a rain‐proof trough shelter. Nitrogen (N) translocation amount, N translocation efficiency decreased with a decline in irrigation amount or by excessive irrigation. Compared with different organs, the leaf and stem had higher N translocation amounts, and contributions to grain for both cultivars – Jinan 17 and Lumai 21, indicating that stem also is a major N source for grain development. The contribution of pre‐anthesis total above ground N to grain N ranged from 57 to 76 %, indicating the importance of pre‐anthesis storage of N for achieving high grain N concentrations. Grain nitrogen and yield (kg ha?1) were positively and significantly correlated with the N translocation amounts and contributions, respectively, suggesting that the sink strength may be involved in the translocation of N from a vegetative organ to the grain. N harvest index (NHI) was significantly correlated with N translocation efficiency, suggesting that the latter is a prerequisite for increasing grain N and improving grain quality. The experiment showed that N translocation status is enhanced by better irrigation practices, but limited by severely deficient or excessive irrigation. 相似文献
960.