全文获取类型
收费全文 | 78000篇 |
免费 | 5126篇 |
国内免费 | 7908篇 |
专业分类
林业 | 5360篇 |
农学 | 4956篇 |
基础科学 | 7082篇 |
31028篇 | |
综合类 | 27919篇 |
农作物 | 2760篇 |
水产渔业 | 2238篇 |
畜牧兽医 | 2999篇 |
园艺 | 1318篇 |
植物保护 | 5374篇 |
出版年
2024年 | 504篇 |
2023年 | 1837篇 |
2022年 | 2674篇 |
2021年 | 2739篇 |
2020年 | 2940篇 |
2019年 | 3233篇 |
2018年 | 2712篇 |
2017年 | 4014篇 |
2016年 | 4826篇 |
2015年 | 3481篇 |
2014年 | 4091篇 |
2013年 | 5482篇 |
2012年 | 6824篇 |
2011年 | 5728篇 |
2010年 | 4523篇 |
2009年 | 4401篇 |
2008年 | 3915篇 |
2007年 | 4358篇 |
2006年 | 3712篇 |
2005年 | 3061篇 |
2004年 | 2394篇 |
2003年 | 2046篇 |
2002年 | 1608篇 |
2001年 | 1517篇 |
2000年 | 1257篇 |
1999年 | 1041篇 |
1998年 | 885篇 |
1997年 | 808篇 |
1996年 | 725篇 |
1995年 | 721篇 |
1994年 | 648篇 |
1993年 | 501篇 |
1992年 | 425篇 |
1991年 | 405篇 |
1990年 | 301篇 |
1989年 | 250篇 |
1988年 | 154篇 |
1987年 | 133篇 |
1986年 | 64篇 |
1985年 | 29篇 |
1984年 | 13篇 |
1983年 | 11篇 |
1982年 | 3篇 |
1981年 | 7篇 |
1980年 | 9篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1963年 | 3篇 |
1962年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
121.
122.
对梅州市五华县几个石灰岩矿区周边土壤中铅含量进行了测定,结果表明,污染土壤的铅含量远远超出了国家土壤环境质量三级标准(对土壤生态环境产生影响的最低浓度值),对生态产生影响。根据内梅罗污染指数评定得出,矿区土壤铅污染达到重污染的程度。提出了减轻铅污染对生态环境危害的控制污染源措施和改良对策。 相似文献
123.
This paper reports the effects of irrigation amount and partial rootzone drying (PRD) on water relations, growth, yield and wine quality of Vitis vinifera cv. ‘Tempranillo’ during two consecutive years in a commercial vineyard with a deep, light-clay soil located in Requena, Valencia, Spain. Partial rootzone drying applied at two amounts (100% and 50% of the estimated crop evapotranspiration), was compared to conventional drip irrigation, and also to rainfed vines. Results showed that the effects of irrigation amount on yield and wine quality were different between years. In 2003 with low yield values (around 6.3 t ha−1) irrigation did neither affect grape production nor wine quality. However, in the following year, with much higher general yield (17 t ha−1), the high irrigation dose increased yield by 30% compared to rainfed vines and it also increased must total soluble solids and wine alcohol content. In both seasons, PRD did not significantly affect physiological parameters, nor growth, yield or fruit and wine quality, when compared to the same amount of water applied by conventional drip irrigation. Overall these results suggest that, under our experimental conditions, it was the irrigation amount rather than the system of application what affected vine performance, indicating the difficulties of successfully employing the PRD type of irrigation with a drip system in heavy and deep soils. 相似文献
124.
125.
126.
本文研究了木捆排在无限水域中的运行水阻力问题,提出了排型参数(L/B)和(B/T)对木捆排运行水阻影响程度的试验数据和图表,探讨了降低木捆排运行水阻力的途径及措施。 相似文献
127.
根据在北京郊区的试验,认为土壤水冻融过程可分为三阶段,其中第二阶段有聚墒现象,平均聚墒8.6mm,所聚的墒在融化过程中又逐渐消失。气温回升至-2℃左右是冻层下部由冻到融的转折温度。冬灌水保存到来年春季的数量有限,本试验中1m 土层内平均不足20mm。 相似文献
128.
国产硝酸磷肥含氮27%,磷13.5%。所含速效氮中硝态氮均占一半。研究麦田不同水分对硝酸磷肥中硝态氮在土壤中扩散结果显示,硝态氮在土壤中下移深度与土壤湿润峰的深度基本一致,在小麦越冬期有表聚现象。当灌水量为田间持水量时,硝态氮移动深度为40~50cm,是在主要根系活动层内,这对小麦吸收有利,不会产生氮的流失。等氮磷条件下,硝酸磷肥与磷铵加尿素在北方地区有同样肥效。 相似文献
129.
本文估计了褐壳蛋鸡洛岛红(AA)和浅花苏赛克斯(BB)在正反反复选择(RRS)下的近交系数,分析了近交对选择标准饲料利用率及其相关性状和繁殖性状的影响。试验群体由两纯种及其正反交组合和相应的对照组组成。经过7个世代的RRS和近交试验,试验组AA和BB(括弧里为对照组1AA和1BB)用系谱材料计算出的平均累积近交系数分别为2.58%和3.23%(9.33和8.17%)或每代为0.43%(1.35%),用有效群体含量估计出的AA与BB的相应数值为2.91%与2.83%(5.84%与5.92%)或每代为0.42%(0.86%)。1%近交系数导致的孵化率和饲料利用率的近交衰退较严重(0.5%和0.4%),其次是产蛋性状(约0.3%),而体重和蛋重只有0.1%左右。在本研究中,通过连续7代的RRS与合适的留种率等措施,成功地将闭锁群体的近交系数控制在3%以下的低水平,消除或减轻了近交对饲料利用率等重要经济性状的不利影响。 相似文献
130.
Abstract. In Australia, stubble burning and tillage are two of the major processes responsible for the decline of soil organic carbon concentration in cropped soils, and the resulting soil degradation. However, the relative importance of these two practices in influencing the soil organic carbon concentration and the long-term impact on soil quality and productivity are not clear. The effects of stubble burning as practised by farmers in southeastern Australia were evaluated in two field trials, one of 19 years duration, the other of 5 years. Conventional tillage (three tillage passes) led to greater loss of soil organic carbon than stubble burning. Loss of total soil organic carbon attributed to stubble burning in the 0–10 cm layer was estimated to be 1.75 t C ha−1 over the period of the 19-year trial, equivalent to 29% of that lost due to tillage. In the 5-year trial, no change in soil organic carbon due to stubble burning was detectable. Changes in soil quality associated with stubble burning detected in the longer trial included a reduction in macro-aggregate stability, and increases in pH and exchangeable K+ . Only the latter two were detected in the shorter trial. A higher mean wheat yield (average 0.15 t ha−1 ) following stubble burning was observed in the 19-year trial but not in the 5-year trial. Research to monitor the longer term effects of stubble burning is needed, and to identify conditions where loss of soil organic carbon is minimized. 相似文献