全文获取类型
收费全文 | 1556篇 |
免费 | 131篇 |
国内免费 | 242篇 |
专业分类
林业 | 137篇 |
农学 | 144篇 |
基础科学 | 78篇 |
586篇 | |
综合类 | 600篇 |
农作物 | 123篇 |
水产渔业 | 8篇 |
畜牧兽医 | 104篇 |
园艺 | 30篇 |
植物保护 | 119篇 |
出版年
2024年 | 12篇 |
2023年 | 36篇 |
2022年 | 89篇 |
2021年 | 78篇 |
2020年 | 67篇 |
2019年 | 94篇 |
2018年 | 71篇 |
2017年 | 87篇 |
2016年 | 108篇 |
2015年 | 93篇 |
2014年 | 84篇 |
2013年 | 123篇 |
2012年 | 120篇 |
2011年 | 97篇 |
2010年 | 89篇 |
2009年 | 61篇 |
2008年 | 69篇 |
2007年 | 62篇 |
2006年 | 60篇 |
2005年 | 50篇 |
2004年 | 41篇 |
2003年 | 40篇 |
2002年 | 35篇 |
2001年 | 17篇 |
2000年 | 29篇 |
1999年 | 31篇 |
1998年 | 20篇 |
1997年 | 26篇 |
1996年 | 26篇 |
1995年 | 20篇 |
1994年 | 19篇 |
1993年 | 13篇 |
1992年 | 12篇 |
1991年 | 11篇 |
1990年 | 11篇 |
1989年 | 15篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1982年 | 1篇 |
1962年 | 2篇 |
排序方式: 共有1929条查询结果,搜索用时 15 毫秒
991.
992.
993.
Effects of repeated manure and fertilizer phosphorus additions on soil phosphorus dynamics under a soybean-wheat rotation 总被引:4,自引:0,他引:4
Soil P availability and efficiency of applied P may be improved through an understanding of soil P dynamics in relation to
management practices in a cropping system. Our objectives in this study were to evaluate changes in plant-available (Olsen)
P and in different inorganic P (Pi) and organic P (P0) fractions in soil as related to repeated additions of manure and fertilizer P under a soybean-wheat rotation. A field experiment
on a Typic Haplustert was conducted from 1992 to 1995 wherein the annual treatments included four rates of fertilizer P (0,
11, 22 and 44 kg ha–1 applied to both soybean and wheat) in the absence and presence of 16 t ha–1 of manure (applied to soybean only). With regular application of fertilizer P to each crop the level of Olsen P increased
significantly and linearly through the years in both manured and unmanured plots. The mean P balance required to raise Olsen
P by 1 mg kg–1 was 17.9 kg ha–1 of fertilizer P in unmanured plots and 5.6 kg ha–1 of manure plus fertilizer P in manured plots. The relative sizes of labile [NaHCO3-extractable Pi (NaHCO3-Pi) and NaHCO3-extractable P0 (NaHCO3-P0)], moderately labile [NaOH-extractable Pi (NaOH-Pi) and NaOH-extractable P0 (NaOH-P0)] and stable [HCl-extractable P (HCl-P) and H2SO4/H2O2-extractable P (resisual-P)] P pools were in a 1 : 2.9 : 7.6 ratio. Application of fertilizer P and manure significantly increased
NaHCO3-Pi and -P0 and NaOH-Pi, and -P0 fractions and also total P. However, HCl-P and residual-P were not affected. The changes in NaHCO3-Pi, NaOH-Pi and NaOH-P0 fractions were significantly correlated with the apparent P balance and were thought to represent biologically dynamic soil
P and act as major sources and sinks of plant-available P.
Received: 23 October 1997 相似文献
994.
The hardsetting surface of many Alfisols in Australia is a limiting factor in crop production. Better soil management systems are required to improve productivity. This study reports the effects of several untested management systems, involving the combined effects of tillage (deep ripping, deep mouldboard ploughing and disc ploughing), irrigation layout (permanent beds, border ditch) and crop rotation treatments (single cropping, double cropping and pasture), on soil structure and the production of irrigated soybeans [Glycine max (L) Merr.] on a hardsetting Alfisol. It was conducted under a system of controlled traffic near Trangie, NSW, Australia. The double cropping and pasture treatments increased total water entry, soil organic carbon and soybean leaf water potential. Consequently, soybean yield was increased by as much as 58%. The retention of cereal and pasture stubble increased total water entry by slowing the rate of irrigation water advance, reducing the potential for slaking and increasing the number of continuous vertical macropores. The increases in soil water were associated with a reduction in soil strength; a key limiting factor in crop growth on hardsetting Alfisols. Deep tillage or the use of permanent beds did not affect soybean yields under the conditions reported in this paper, although the combination of disc ploughing and furrow irrigation was successful. 相似文献
995.
于2008年采用静态暗箱-气相色谱法对人工手插和机插2种水稻种植方式下CH4和N2O排放进行田间观测,研究稻麦轮作条件下机插水稻CH4和N2O的排放特征及其温室效应。结果表明,水稻生长季CH4排放通量人工手插水稻和机插水稻均呈先升高后降低的变化趋势,N2O仅在水稻搁田期间有明显排放,机插和人工手插水稻CH4平均排放通量分别为4.68、4.39 mg.m-2.h-1,N2O平均排放通量为92.80、111.33μg.m-.2h-1。与人工手插水稻相比,机插水稻增加CH4排放总量14%,减少N2O排放总量11%,使稻季排放CH4和N2O所产生的全球增温潜势(GWP)和"单位产量的GWP"分别提高8%和10%。在稻麦轮作条件下采用机插水稻种植方式,水稻生长期间排放的CH4和N2O所形成的温室效应有提高的趋势。 相似文献
996.
1、2代马尾松人工林林分生长特性比较 总被引:2,自引:0,他引:2
采用配对样地法,以不同栽植代数(1、2代)、不同发育阶段(8、15、18、20 a)的马尾松人工林为对象,对1、2代马尾松人工林林分树高、胸径、单株材积及蓄积量生长进行比较分析。结果表明:马尾松连栽后,林分平均胸径、平均树高、单株材积和林分蓄积等指标总体表现为2代好于1代,2代较1代总体分别上升0.61%、3.17%、3.85%和23.82%,但差异均不显著。马尾松连栽并未导致林分平均胸径、平均树高、单株材积及蓄积量生长出现下降趋势,林分生产力水平不但没下降,反而还有一定程度的提高。 相似文献
997.
塔里木绿洲种植制度对棉田土壤养分性状的影响 总被引:5,自引:0,他引:5
为了研究塔里木绿洲种植制度对棉田土壤养分特性的影响,以棉田轮作方式(水稻→棉花、水稻→冬麦→棉花、水稻→冬麦-绿肥→棉花)和连作年限(3、5、8、10、15 a)为试验因素,对0~100 cm土层土壤进行了调查与试验。结果表明:短期轮作对棉田土壤养分含量无影响,棉花连作年限是不同层次土壤养分含量变化的主要影响因素,连作3、8、10、15 a土壤0~20 cm速效氮平均为40.60、48.75、51.96、44.35 mg/kg;速效磷平均为10.16、26.72、27.00、23.37 mg/kg;速效钾平均为184.70、142.60、130.20、105.56 mg/kg;有机质含量分别为10.43 g/kg、12.10g/kg、12.93 g/kg、13.56 mg/kg;无论轮作类型、连作年限长短,从垂直分布上,连作15 a、3 a土壤速效氮F值分别为79.01、299.45;速效磷F值分别为88.99、17.54;有机质F值分别为77.27和171.6。表明长期连作降低了上下层土壤速效氮和有机质含量差异而增加了上下层土壤速效磷含量差异;有机质含量随着土层深度的增加而降低;速效钾含量有随着土层深度的增加而增加的趋势。棉花长期连作有利于增加各层次土壤速效氮、速效磷和有机质含量,同时降低深层土壤速效钾的含量;连作8~10 a耕作层土壤速效氮、有效磷含量达到最大值分别为52.35 mg/kg和27.45 mg/kg。 相似文献
998.
Static experiment was carried out to study rotation behavior of timber beam-column connection. The influence of length of Gongmu and position of dowel on the initial rotational stiffness of the joint was analyzed. Rules of stiffness degradation under cyclic loading were studied as well. The results show that: the moment-rotation curve of typical timber beam-column connection can be simplified as bilinear model; the initial rotational stiffness of the joint is enhanced with the increase of the length of Gongmu, and it is decreased with the inward movement of dowel; under the same conditions, the influence of the inward movement of Gongmu-beam dowel on the initial rotational stiffness of the joint is smaller than that of Dianmu-Gongmu dowel; and stiffness degrades under cyclic loading and there exists obvious transition phenomenon. 相似文献
999.
[目的]研究植物激素对新疆白皮蒜鳞茎盘不定芽生长的影响,为进一步完善大蒜的快繁体系提供依据.[方法]采用二次回归正交旋转组合实验,设计不同的IBA、NAA、6 - BA水平及其组合.[结果]影响新疆白皮蒜鳞茎盘不定芽生长的三因素主次顺序是NAA>6- BA> IBA.其中NAA与IBA交互作用影响显著,但其高浓度有负效应,在最高值预测时去掉了IBA,以降低NAA与IBA交互作用对新疆白皮蒜鳞茎盘不定芽生长的影响,6 - BA单因素影响较其他因素明显,为正效应.[结论]新疆白皮蒜鳞茎盘不定芽生长最佳激素配比为:NAA浓度为2 mg/L,6 - BA浓度为7.5 mg/L,在此条件下每个新疆白皮蒜鳞茎盘不定芽生长最大值为39.22株. 相似文献
1000.
为研究沙地农田不同土壤耕作、覆盖方式和轮作对作物产量、土壤硝态氮含量及积累的影响,在黑河中游边缘绿洲典型沙地进行农田定位试验。试验结果表明,在沙地农田生态系统,相对传统的三耕两耱的耕作方式,减少秋耕对玉米和大豆产量没有显著影响;相对于地膜覆盖处理,麦秆覆盖处理使玉米减产11.3%~18.8%,使大豆产量增加6%;不同处理0~40cm土壤硝态氮积累量存在显著差异,作物收获后传统耕作处理土壤硝态氮的平均积累量比少耕处理低8.5%~22.7%,覆膜处理比覆麦秆处理低7.0%~8.8%,玉米连作处理比玉米-大豆轮作处理高54.9%。分析结果表明,减少秋耕、麦秆代替地膜覆盖均有利于土壤硝态氮在作物根系集中层的积累,玉米大豆轮作对土壤硝态氮积累和淋失的影响仍需进一步研究;该试验研究的继续深入将对维持绿洲农田环境的健康和提高沙地农田生态系统生产力提供重要的理论依据。 相似文献