全文获取类型
| 收费全文 | 42033篇 |
| 免费 | 13996篇 |
| 国内免费 | 842篇 |
专业分类
| 林业 | 3363篇 |
| 农学 | 2312篇 |
| 基础科学 | 2597篇 |
| 24421篇 | |
| 综合类 | 16746篇 |
| 农作物 | 1268篇 |
| 水产渔业 | 78篇 |
| 畜牧兽医 | 2156篇 |
| 园艺 | 579篇 |
| 植物保护 | 3351篇 |
出版年
| 2025年 | 680篇 |
| 2024年 | 1813篇 |
| 2023年 | 2055篇 |
| 2022年 | 1905篇 |
| 2021年 | 1917篇 |
| 2020年 | 2017篇 |
| 2019年 | 2265篇 |
| 2018年 | 1838篇 |
| 2017年 | 2533篇 |
| 2016年 | 2938篇 |
| 2015年 | 2029篇 |
| 2014年 | 2323篇 |
| 2013年 | 3250篇 |
| 2012年 | 4065篇 |
| 2011年 | 2995篇 |
| 2010年 | 2319篇 |
| 2009年 | 2400篇 |
| 2008年 | 2176篇 |
| 2007年 | 2333篇 |
| 2006年 | 2016篇 |
| 2005年 | 1720篇 |
| 2004年 | 1327篇 |
| 2003年 | 1139篇 |
| 2002年 | 907篇 |
| 2001年 | 868篇 |
| 2000年 | 743篇 |
| 1999年 | 553篇 |
| 1998年 | 485篇 |
| 1997年 | 500篇 |
| 1996年 | 430篇 |
| 1995年 | 456篇 |
| 1994年 | 422篇 |
| 1993年 | 329篇 |
| 1992年 | 268篇 |
| 1991年 | 256篇 |
| 1990年 | 181篇 |
| 1989年 | 160篇 |
| 1988年 | 101篇 |
| 1987年 | 78篇 |
| 1986年 | 40篇 |
| 1985年 | 13篇 |
| 1984年 | 6篇 |
| 1983年 | 2篇 |
| 1982年 | 2篇 |
| 1981年 | 3篇 |
| 1980年 | 4篇 |
| 1976年 | 1篇 |
| 1963年 | 1篇 |
| 1962年 | 5篇 |
| 1955年 | 1篇 |
排序方式： 共有10000条查询结果，搜索用时 0 毫秒
241.
研究了铜在红壤和砂姜黑土中的吸附和解吸过程以及有机污染物邻苯二胺对铜在两种土壤中吸附和解吸行为的影响。结果表明 ,砂姜黑土较红壤对铜具有更高的吸附量 ,同时 ,被吸附的铜离子从土壤中的脱附百分数也是砂姜黑土大于红壤 ,吸附在砂姜黑土中的铜较红壤更易被 1molL- 1 MgCl2 所置换。pH 3 .5～ 7.5范围内 ,土壤对铜的吸附量均随溶液pH的升高而升高 ,呈现S形。邻苯二胺增加了红壤对铜的吸附量 ,同时也增加了铜的脱附百分数。而邻苯二胺虽未改变砂姜黑土对铜的吸附量 ,但却显著改变了铜的脱附百分数。文中对邻苯二胺与铜的交互作用过程及其机理进行了推测 相似文献
242.
J. Huang E. Scudiero W. Clary D. L. Corwin J. Triantafilis 《Soil Use and Management》2017,33(2):191-204
The volumetric soil water content (θ) is fundamental to agriculture because its spatiotemporal variation in soil affects the growth of plants. Unfortunately, the universally accepted thermogravimetric method for estimating volumetric soil water content is very labour intensive and time‐consuming for use in field‐scale monitoring. Electromagnetic (EM) induction instruments have proven to be useful in mapping the spatiotemporal variation of θ. However, depth‐specific variation in θ, which is important for irrigation management, has been little explored. The objective of this study was to develop a relationship between θ and estimates of true electrical conductivity (σ) and to use this relationship to develop time‐lapse images of soil θ beneath a centre‐pivot irrigated alfalfa (Medicago sativa L.) crop in San Jacinto, California, USA. We first measured the bulk apparent electrical conductivity (ECa – mS/m) using a DUALEM‐421 over a period of 12 days after an irrigation event (i.e. days 1, 2, 3, 4, 6, 8 and 12). We used EM4Soil to generate EM conductivity images (EMCIs). We used a physical model to estimate θ from σ, accounting for soil tortuosity and pore water salinity, with a cross‐validation RMSE of 0.04 cm3/cm3. Testing the scenario where no soil information is available, we used a three‐parameter exponential model to relate θ to σ and then to map θ along the transect on different days. The results allowed us to monitor the spatiotemporal variations of θ across the surveyed area, over the 12‐day period. In this regard, we were able to map the soil close to field capacity (0.27 cm3/cm3) and approaching permanent wilting point (0.03 cm3/cm3). The time‐lapse θ monitoring approach, developed using EMCI, has implications for soil and water use and management and will potentially allow farmers and consultants to identify inefficiencies in water application rates and use. It can also be used as a research tool to potentially assist precision irrigation practices and to test the efficacy of different methods of irrigation in terms of water delivery and efficiency in water use in near real time. 相似文献
243.
In the oldest commercial wine district of Australia, the Hunter Valley, there is the threat of soil salinization because marine sediments underlie the area. To understand the risk requires information about the spatial distribution of soil properties. Electromagnetic (EM) induction instruments have been used to identify and map the spatial variation of average soil salinity to a certain depth. However, soils vary with depth dependent on soil forming factors. We collected data from a single‐frequency and multiple‐coil DUALEM‐421 along a toposequence. We inverted this data using EM4Soil software and evaluated the resultant 2‐dimensional model of true electrical conductivity (σ – mS/m) with depth against electrical conductivity of saturated soil pastes (ECp – dS/m). Using a fitted linear regression (LR) model calibration approach and by varying the forward model (cumulative function‐CF and full solution‐FS), inversion algorithm (S1 and S2), damping factor (λ) and number of arrays, we determined a suitable electromagnetic conductivity image (EMCI), which was optimal (R2 = 0.82) when using the full solution, S2, λ = 3.6 and all six coil arrays. We conducted an uncertainty analysis of the LR model used to estimate the electrical conductivity of the saturated soil‐paste extract (ECe – dS/m). Our interpretation based on estimates of ECe suggests the approach can identify differences in salinity, how these vary with parent material and how topography influences salt distribution. The results provide information leading to insights into how soil forming factors and agricultural practices influence salinity down a toposequence and how this can guide soil management practices. 相似文献
244.
～50 cm与0～100 cm土层土壤湿度的转换关系研究 总被引：8，自引：0，他引：8
对郑州、南阳2个测点多年来0～100cm土壤湿度、降水(灌溉)及作物发育期资料进行了分析。结果表明,按土壤湿度随时间的变化规律,可将其划分为秋季缓慢下降期、冬季相对稳定期、春季迅速下降期和夏季增墒期4个时段。对0～50cm与0～100cm土层土壤湿度进行回归分析,当前期降水量在10mm以下时,用直线或二次曲线拟合效果良好;当前期降水(灌溉)量大于10mm时,0～50cm与0～100cm土壤湿度之比与前期降水(灌溉)量呈双曲线或二次曲线关系,拟合曲线考虑降水(灌溉)时回归效果有所提高。同一土壤类型、不同时段,或同一时段、不同的土壤类型,拟合的方程不同,反映出不同层次土壤湿度关系的复杂性。 相似文献
245.
腐植酸专用肥分层异位同播技术对玉米生长的影响 总被引：2，自引：1，他引：2
为简化农艺措施, 降低生产成本, 本文通过田间试验法, 以腐殖酸玉米专用肥和无机复混肥的实用技术作对比, 研究了玉米腐殖酸专用肥分层异位同播施肥技术对玉米全生育期内干物质动态积累､养分动态吸收､产量及经济效应的影响｡结果表明:全生育期内, 随着生育期的延长, 各处理玉米干物质积累和养分吸收均逐渐增加, 直到成熟｡在成熟期, 与无机复混肥常规施肥技术相比, 腐植酸专用肥分层异位同播技术的玉米根条数增加 8.1 条/ 株, 叶面积指数增加了20.1 %, 地上部干物质积累增加2 354.25 kg/ hm2 ;植株体内积累氮､磷､钾量分别提高50.43､18.87､24.70 kg/ hm2 ;获得12 147.35 kg/ hm2 的籽粒产量, 纯收入增加4 934.6 元/ hm2 ｡ 相似文献
246.
华北黄泛平原潮土土壤养分与土壤粒级的关系研究 总被引：1，自引：0，他引：1
根据第二次全国土壤普查资料,利用数理统计的方法研究了华北黄泛平原潮土土壤粒级与土壤养分含量之间的关系。研究结果表明,土壤耕层养分含量与土壤砂粒成负相关,与小于0.01 mm的各粒级成正相关。其中土壤有机质含量主要受砂粒(1~0.05 mm)和细粘粒(<0.001 mm)含量的影响,土壤全氮含量主要受细粘粒含量的影响,土壤全磷含量主要受砂粒和粗粉粒(0.05~0.01 mm)含量的影响,土壤速效磷含量主要受细粘粒含量的影响,土壤速效钾含量主要受细粉粒(0.01~0.005 mm)含量的影响。 相似文献
247.
土壤团聚体形成的研究进展 总被引：32，自引：0，他引：32
土壤团聚体是土壤结构的基本单位,它的形成与稳定是自然过程与人类活动的综合结果,即受生物与非生物因素如土地利用方式改变、耕作和种植制度等多种因素的影响。由于团聚体的形成原因及土壤环境条件不同,不同粒级的团聚体在土壤肥力上表现出不同的性质和特征。 相似文献
248.
To investigate soil changes from forest conversion and regeneration, soil net N mineralization, potential nitrification, microbial biomass N, L‐asparaginase, L‐glutaminase, and other chemical and biological properties were examined in three adjacent stands: mature pure and dense Norway spruce (Picea abies (L.) Karst) (110 yr) (stand I), mature Norway spruce mixed with young beech (Fagus sylvatica) (5 yr) (stand II), and young Norway spruce (16 yr) (stand III). The latter two stands were converted or regenerated from the mature Norway spruce stand as former. The studied soils were characterized as having a very low pH value (2.9 – 3.5 in 0.01 M CaCl2), a high total N content (1.06 – 1.94 %), a high metabolic quotient (qCO2) (6.7 – 16.9 g CO2 kg–1 h–1), a low microbial biomass N (1.1 – 3.3 % of total N, except LOf1 at stand III), and a relatively high net N mineralization (175 – 1213 mg N kg–1 in LOf1 and Of2, 4 weeks incubation). In the converted forest (stand II), C : N ratio and qCO2 values in the LOf1 layer decreased significantly, and base saturation and exchangeable Ca showed a somewhat increment in mineral soil. In the regenerated forest (stand III), the total N storage in the surface layers decreased by 30 %. The surface organic layers (LOf1, Of2) possessed a very high net N mineralization (1.5 – 3 times higher than those in other two stands), high microbial biomass (C, N), and high basal respiration and qCO2 values. Meanwhile, in the Oh layer, the base saturation and the exchangeable Ca decreased. All studied substrates showed little net nitrification after the first period of incubation (2 weeks). In the later period of incubation (7 – 11 weeks), a considerable amount of NO3‐N accumulated (20 – 100 % of total cumulative mineral N) in the soils from the two pure spruce stands (I, III). In contrast, there was almost no net NO3‐N accumulation in the soils from the converted mixed stand (II) indicating that there was a difference in microorganisms in the two types of forest ecosystems. Soil microbial biomass N, mineral N, net N mineralization, L‐asparaginase, and L‐glutaminase were correlated and associated with forest management. 相似文献
249.
旱作区农业节水形式探讨 总被引：1，自引：0，他引：1
山西省水资源贫乏,旱作区发展农业要抓好以利用天然降水为重点的节水措施.节水形式有:旱地果园、庭院蔬菜、农地经济作物区集流节水;大面积农业区水土保持措施节水;农业耕作措施节水;覆盖保墒、减少蒸腾技术节水.其中集流节水应作为发展的重点,是我国旱地农业中高投入、高产出的一种较为可行的形式. 相似文献
250.
The FAO basal crop coefficient Kcb and the soil evaporation coefficient Ke for a sparse sorghum field in one growing season were estimated using the compartment model (Shuttleworth–Wallace model; S–W model), which separately estimates transpiration from crops and evaporation from the soil. The Kcb had a low value of 0.10 (Kcb, ini) during the initial period, increased rapidly to a maximum of 0.98 (Kcb, mid) at midseason, and decreased rapidly to 0.53 (Kcb, end) by the end of the growing season. The Kcb increased with increasing leaf area index (LAI) rapidly and slowly, respectively, below and above an LAI of 1.0. The Ke showed a positive relationship to increasing soil water content (SWC) at a depth of −15.0 cm, but the soil water content indicated a negative relationship to increasing LAI. Using the dual crop coefficients derived by their relationships to the LAI and soil water content, the actual evapotranspiration (ETa) was estimated and compared to the ETa derived by the Bowen ratio energy budget (BREB) method. The root mean square error (RMSE) of the ETa obtained from the simulated crop coefficient to the ETa obtained by the BREB method was 0.84 mm day−1. Thus, the adjusted relationship of Kcb and Ke allowed a good estimation of the evapotranspiration in an irrigated field with sparse vegetation cover. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献