首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   402篇
  免费   5篇
  国内免费   14篇
林业   31篇
农学   11篇
基础科学   38篇
  83篇
综合类   170篇
农作物   7篇
水产渔业   13篇
畜牧兽医   45篇
园艺   11篇
植物保护   12篇
  2024年   2篇
  2023年   1篇
  2022年   2篇
  2021年   5篇
  2020年   3篇
  2019年   3篇
  2018年   1篇
  2017年   3篇
  2016年   7篇
  2015年   12篇
  2014年   12篇
  2013年   15篇
  2012年   13篇
  2011年   18篇
  2010年   14篇
  2009年   28篇
  2008年   22篇
  2007年   24篇
  2006年   23篇
  2005年   16篇
  2004年   3篇
  2003年   7篇
  2002年   17篇
  2001年   13篇
  2000年   7篇
  1999年   15篇
  1998年   11篇
  1997年   21篇
  1996年   6篇
  1995年   10篇
  1994年   15篇
  1993年   14篇
  1992年   18篇
  1991年   9篇
  1990年   12篇
  1989年   7篇
  1988年   3篇
  1987年   1篇
  1986年   1篇
  1984年   1篇
  1957年   3篇
  1954年   1篇
  1953年   2篇
排序方式: 共有421条查询结果,搜索用时 15 毫秒
41.
土壤粘土矿物中层状硅酸盐与氧化物共存及晶质和非品质共存。土壤中含钾的粘土矿物主要有水云母、蛭石和绿泥石等。这些粘土矿物影响土壤钾素的固定和释放,进而影响土壤钾的生物有效性。有研究表明,土壤中的层状硅酸盐矿物彼此之间存在着一种相互演变的关系。例如在云母风化初期,通常会出现水化云母,在钾比较丰富的条件下,就会出现伊利石;水化云母进一步风化并释放出更多的钾以后,则形成蛭石和蒙脱石。相反,蛭石和蒙脱石在吸收了钾离子并脱水后,也可以向伊利石及云母类矿物转变。  相似文献   
42.
土壤中~(137)Cs的化学性质及其分布规律   总被引:6,自引:1,他引:6  
李仁英  杨浩  唐翔宇 《核农学报》2001,15(6):371-379
本文对13 7Cs的土壤化学行为及剖面运移规律进行了论述 ,并阐明了13 7Cs可作为中长期土壤侵蚀作用的有效示踪剂的理论基础  相似文献   
43.
山东主要果园土壤的粘土矿物组成及其吸附特性   总被引:2,自引:0,他引:2  
研究了山东主要果园土壤中粘土矿物的组成、类型及其对P、K和Cu、Zn、Pb金属元素的吸附特性。结果表明:山东主要果园土壤的粘土矿物类型存在着明显的差异,淋溶较强,酸度较大的棕壤(简育湿润淋溶土)中粘土矿物以高岭石占优势,对P的吸附较强,但对K+吸附固定较弱。含游离C aCO3较高的潮土(淡色潮湿雏形土)和褐土(简育干润淋溶土),对P有较强的吸附和沉淀作用,使土壤磷的有效性降低。而砂姜黑土(钙积潮湿变性土)由于含有较高的蒙脱石和1.4 nm过渡矿物,对K+具有很强的吸附和晶穴固定作用,因此砂姜黑土中磷肥和钾肥的有效性均较低,在施肥上应采取集中施肥和保持较湿润土壤环境等措施,以提高养分有效性。砂姜黑土和潮土对Cu、Zn、Pb金属元素的吸附显著地大于棕壤和褐土,主要的影响因素是不同土壤的粘粒含量和粘土矿物的类型的差异。同一土壤对Pb的吸附量远远大于对Zn和Cu的吸附量,主要取决于金属元素本身的化学性质和胶体的吸附特性。  相似文献   
44.
在我国热带、亚热带地区,水土流失严重的第四纪红色粘土广泛分布,因其不良的理化特性和水热条件,在实施水土保持植物措施时,植物很难正常生长和发育。作者在强度侵蚀的第四纪红粘土上,进行了营养穴种植胡枝子的试验研究。结果表明,营养穴种植的胡枝子长势良好,而浅沟条播的胡枝子主根下扎困难,生长缓慢。  相似文献   
45.
有机酸对高岭石, 针铁矿和水铝英石吸附镉的影响   总被引:12,自引:0,他引:12  
LIAO Min 《土壤圈》2006,16(2):185-191
Effects of organic acids (oxalic, acetic, and citric) on adsorption characteristics of Cadmium (Cd) on soil clay minerals (kaolinite, goethite, and bayerite) were studied under different concentrations and different pH values. Although the types of organic acids and minerals were different, the effects of the organic acids on the adsorption of Cd on the minerals were similar, i.e., the amount of adsorbed Cd with an initial solution pH of 5.0 and initial Cd concentration of 35 mg L^-1 increased with increasing concentration of the organic acid in solution at lower concentrations, and decreased at higher concentrations. The percentage of Cd adsorbed on the minerals in the presence of the organic acids increased considerably with increasing pH of the solution. Meanwhile, different Cd adsorption in the presence of the organic acids, due to different properties on both organic acids and clay minerals, on kaolinite, goethite, or bayerite for different pHs or organic acid concentrations was found.  相似文献   
46.
The clay minerals of more than 200 soil samples collected from various sites of Fujian Province were studied by the X-ray diffraction method and transmission electron microscopy to study their distribution and evolution.Montmorillonite was found in coastal solonchak,paddy soils derived from marine deposit,lacustrine deposit and river deposit,and some lateritic red soil,red soil and yellow soil with a low weathering degree.Chlorite existed mainly in coastal solonchak and paddy soil developed from marine deposit.1.4nm intergradient mineral appeared frequently in yellow soil,red soil and lateritic red soil.The content of 1.4nm intergradient mineral increased with the decrease of weathering degree from lateritic red soil to red soil to yellow soil.Hydrous micas were more in coastal solonchak,paddy soils derived from marine deposit,lacustrine deposit and river deposit.and puple soil from purple shale than in other soils.Kaolinte was the most important clay mineral in the soils iun this province.The higher the soil weathering degree,the more the kaolinite existed.From yellow soil to red soil to lateritic red soil,kaolinite increased gradually,Kaolinite was the predominant clay mineral accompanied by few other minerals in typical lateritic red soil. Tubular halloysite was a widespread clay mineral in soils of Fujian Province with varying quantities.The soil derived from the paent rocks rich in feldspar contained more tubular halloysite.Spheroidal halloysite was found in a red soil and a paddy soil developed from olivine basalt gibbsite in the soils in this district was largely“primary gibbsite” which formed in the early weathering stage.Gibbsite decreased with the increase of weathering degree from yellow soil to red soil to lateritic red soil.Goethite also decreased in the same sequence while hematite increased.  相似文献   
47.
Clay addition to light-textured soils is used to ameliorate water repellency and to increase nutrient retention. However, clay addition may also increase the potential to bind organic matter and thus C sequestration. Divalent calcium ions (Ca2+) play an important role in binding of organic matter to clay because they provide the bridge between the clay particles and organic matter which are both negatively charged. In the first experiment, quartz sand was mixed with clay isolated from a Vertosol at rates of 0, 50 and 300 g kg-1, finely ground mature wheat residues (20 g kg-1) and powdered CaSO4 at 0, 5 and 10 g kg-1. Soil respiration was measured over 28 d. Compared to the sand alone, addition of isolated clay at 300 g kg-1 increased cumulative respiration with a stronger increase than that at 50 g kg-1. Addition of CaSO4 increased electrical conductivity, decreased sodium adsorption ratio and reduced cumulative respiration. The latter can be explained by enhanced sorption of organic matter to clay via Ca2+ bridges. In a second experiment, isolated clay or subsoil of the Vertosol without or with powdered CaSO4 at 10 g kg-1 were used for a batch sorption with water-extractable organic C (WEOC) from wheat straw followed by desorption with water. Addition of 10 g kg-1 CaSO4 increased sorption and decreased desorption of WEOC in both subsoil and isolated clay. In the third experiment, subsoil of the Vertosol was used for a batch sorption in which WEOC was added repeatedly. Repeated addition of WEOC increased the concentration of sorbed C but decreased the sorbed proportion of the added WEOC. This indicates that sorption of WEOC may be underestimated if it is added only once in batch sorption experaments.  相似文献   
48.
海南岛北部玄武岩发育而成的土壤粘土矿物研究   总被引:2,自引:0,他引:2  
对海南岛北部玄武岩上发育的成土年代系列土壤的研究表明,土壤中高岭石含量随风化成土年代的增加而逐渐增高,至富铁土时含量已达最高,之后含量下降。三水铝石含量逐渐增加。赤铁矿含量和结晶程度也随风化成土年代增加而增高。磁赤铁矿也有增加的趋势。  相似文献   
49.
多年来,人们一直习惯于选择砂质土壤培育桑苗,而粘性土往往因土质结构不良影响育苗效果,形成了育苗空间的局限性,一定程度上制约了蚕桑生产的发展.为探索粘土地区繁育桑苗的新路子,笔者于2001年进行了粘性土质育苗试验.现将试验结果报告如下:  相似文献   
50.
农六师一○五团地处天山北麓,准噶尔盆地南缘,三屯河下游洪积扇淤积平原下游,土壤质地以重粘土为主,占全团土地面积的90%以上,重粘土保水保肥性极强,易耕性差,棉花出苗困难,为解决这一难题,在多年的生产实践中逐步摸索总结出以下主要技术.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号