全文获取类型
收费全文 | 60584篇 |
免费 | 3976篇 |
国内免费 | 5014篇 |
专业分类
林业 | 4745篇 |
农学 | 3747篇 |
基础科学 | 3251篇 |
23742篇 | |
综合类 | 22919篇 |
农作物 | 2398篇 |
水产渔业 | 577篇 |
畜牧兽医 | 3136篇 |
园艺 | 1099篇 |
植物保护 | 3960篇 |
出版年
2024年 | 364篇 |
2023年 | 1575篇 |
2022年 | 2352篇 |
2021年 | 2399篇 |
2020年 | 2391篇 |
2019年 | 2612篇 |
2018年 | 2075篇 |
2017年 | 3220篇 |
2016年 | 3835篇 |
2015年 | 2799篇 |
2014年 | 3294篇 |
2013年 | 4077篇 |
2012年 | 5399篇 |
2011年 | 4289篇 |
2010年 | 3362篇 |
2009年 | 3381篇 |
2008年 | 2902篇 |
2007年 | 3151篇 |
2006年 | 2625篇 |
2005年 | 2181篇 |
2004年 | 1629篇 |
2003年 | 1436篇 |
2002年 | 1090篇 |
2001年 | 1046篇 |
2000年 | 907篇 |
1999年 | 689篇 |
1998年 | 582篇 |
1997年 | 616篇 |
1996年 | 522篇 |
1995年 | 541篇 |
1994年 | 506篇 |
1993年 | 395篇 |
1992年 | 307篇 |
1991年 | 313篇 |
1990年 | 210篇 |
1989年 | 187篇 |
1988年 | 116篇 |
1987年 | 90篇 |
1986年 | 49篇 |
1985年 | 15篇 |
1984年 | 7篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1963年 | 2篇 |
1962年 | 8篇 |
1956年 | 3篇 |
1955年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
971.
《Communications in Soil Science and Plant Analysis》2012,43(2):123-132
In this study, soil physical properties were evaluated in the top 40 cm of cleared forestland that had been subjected to continuous cultivation for 7 years to ascertain selected crop or crop combinations that influenced the soil physical properties the most. There was no significant effect of crop treatment on particle‐size distributions over 6 years of cultivation. In year 7, clay values were significantly (p = 0.05) greater in plots grown with solely cassava (SC) and solely maize (SM) than in the plots grown with solely pigeon pea (SP). The soil depth effects over the 7 years were significant on the clay content. The mean values of bulk density, pore‐size distribution, and hydraulic conductivity obtained from each plot fluctuated over the years. The bulk density values in 1998 ranged from 1.29 to 1.43 g cm3, but from 1999 to 2004, the range was from 1.12 to 1.40 g cm3. Thus, bulk density generally decreased when compared with their respective values in 1998. The greatest decrease of ≈ 22% was in 2000. More than 70% of the macroporosity values were significantly less than their respective values in 1998. The greatest decease of 72% was obtained from SM plots in 2001. All the microporosity were significantly more than the 1998 values. All the increases were >100% of the original values. These increases were reflected in the variations of total and saturated hydraulic conductivity (Ks) values. However, in 2004, Ks values decreased in the plots grown to C + P, SP, and SM. Generally, the C + M + P mixture appeared to be the most consistent in improving micro‐ and total porosities and Ks among the crop treatments. 相似文献
972.
Manganese (Mn) deficiency may be induced by adding large quantities of iron (Fe), provided that soil manganese is marginally deficient. Results of a greenhouse study showed that iron soil application did not influence shoot dry matter yield of dry bean due to the fact that the iron:manganese ratio in aerial parts of dry bean was higher than 0.4. A foliar spray of 2% iron sulfate significantly reduced it probably due to the high level of shoot iron and iron:manganese ratio greater than 4. Iron application decreased concentration/uptake of shoot manganese due to the iron-manganese antagonistic relationships. Mangenese soil application is not an effective method in correction of manganese deficiency induced by iron fertilizers. Iron did not affect root manganese uptake, indicating that manganese absorption was not affected by iron application. Both manganese/iron soil tests are recommended in calcareous soils with manganese soil test in marginal range. 相似文献
973.
人工低产林改造对生态恢复区土壤理化性质的影响 总被引:1,自引:0,他引:1
为了解人工低产林改造对土壤理化性质的影响,于2010年9月-2011年8月对甘肃定西华家岭生态恢复区4个人工低产林改造后的混交林和1个杨树纯林、1个荒草坡对照点的土壤主要理化性质进行了研究.结果表明,杨树纯林的土壤有机质、总N和总P含量均处于较低水平,分别低于荒草坡10.98、0.78、0.10 g/kg;低产林改造后,土壤含水量、有机质和总N含量均有所升高,且随着改造时间的延长呈现出明显的增加趋势.改造8、18、25、31 a的混交林土壤含水量分别高出杨树纯林2.01、7.79、11.38、12.07个百分点,有机质含量分别高出2.96、15.23、20.80、24.96g/kg,总N含量分别高出0.41、1.02、1.48、1.96 g/kg;总P含量在改造初期有所增加,后期维持稳定,改造8、18、25 a的混交林总P含量分别高出杨树纯林0.02、0.06、0.10 g/kg,改造31a和25 a的混交林总P含量相等;低产林改造后土壤pH值略有降低.结果显示,人工低产林改造大大改善了该区土壤环境质量,有利于该区生态环境的恢复重建. 相似文献
974.
《Journal of plant nutrition》2013,36(10):2173-2190
ABSTRACT Poor water management and high nitrogen (N) losses are the key problems faced by rice farmers under rainfed inland valley systems. There is a need to evaluate different N fertilizers so as to identify one that could withstand these problems. The performance of polyolefin-coated urea (POCU) was therefore compared with conventional urea in a pot experiment with indica rice (Oryza sativa L. cv. IR36), using two water management systems: 1) Submerged condition referred to as good water management (GWM), and 2) excessive irrigation (over 4000 mm in 120 days) referred to as poor water management (PWM). The study was carried out during 1997 and 1998 cropping seasons under glasshouse conditions. For PWM in 1997, the pots were subjected to leaching only whereas in 1998, they were subjected to both surface runoff and leaching. For both cropping seasons, POCU-treated plants under PWM had a significantly higher grain yield (377.5 and 343.0 g m?2) than urea-treated plants (316.5 and 260.5 g m?2). In addition, POCU-treated plants had a significantly higher number of grains per panicle than urea-treated plants. In 1998, both the partial factor productivity of applied N and the agronomic nitrogen-use efficiency of POCU-treated plants under GWM and PWM were significantly higher than those of urea-treated plants. It can be inferred that (using sandy soils and under PWM), POCU could perform significantly better than conventional urea. This finding is important, considering the usually high nitrogen losses in rice-growing inland valley swamps. 相似文献
975.
I. Khabirov 《Archives of Agronomy and Soil Science》2013,59(5):387-393
An attempt has been made to analyze the possibility to use nonequilibrium thermodynamics for the soil dynamic open systems’ treatment. Entropy change of such a system is expressed by the entropy sum produced within the system and the entropy coming from or going into the outer sphere. In the steady state, dynamic soil‐formation processes occur within an organized structure and are characterized by stable parameters close to equilibrium. Accordingly, when examining soil, one can proceed from the conventional thermodynamic equilibrium. However, the matter of Onzager‐Prigozhin general phenomenological theory applicability to soil processes is more complicated. To study soil stability it is necessary to go beyond the limits of linear thermodynamics. 相似文献
976.
Abstract. The mean extractable sulphur (S) concentration in 315 upland topsoil samples collected in 1988/89 from beneath pasture in NE Scotland was 13 μg S g−1 (range 2–77 μg S g−1 ). More than two thirds of the samples had S concentrations less than that acceptable for productive soils. Continued decreases in atmospheric S inputs may have increased this proportion subsequently. The analysis of herbage S also indicated that two-thirds of the samples were below 0.2% S. A 'respirometric index', namely CO2 produced during cellulose decomposition without added S as a percentage of that produced with added S, was significantly less than 100% in a quarter of the soils. Results of three different extraction procedures suggested that sulphate in the soils was present mainly as free plus adsorbed rather than precipitated forms. Soil extraction identified a significant non-sulphate S fraction, presumably organic S. The variability in extractable S stemmed from a combination of geographical, depositional and local site and soil factors. Extractable S was significantly correlated with soil organic matter content and inversely with soil pH and together these factors explained 37% of the variability. While significant differences in mean concentrations between geographical area, soil association and drainage status were evident, no trends could be observed between the major soil subgroups or with altitude. 相似文献
977.
978.
Fine-scale vegetation patches decline in size and cover with increasing rainfall in Australian savannas 总被引:5,自引:0,他引:5
Ludwig John A. Tongway David J. Eager Robert W. Williams Richard J. Cook Garry D. 《Landscape Ecology》1999,14(6):557-566
Fine-scale vegetation patches (<5 m in width) are critically important in many landscapes because they function to obstruct surface flows of water and wind. These obstructions increase the infiltration of runoff and the capture of nutrients in runoff sediments and in wind-blown soil and litter. The importance of redistribution of runoff into runon patches from spaces between patches (fetches) is likely to be greater in drier than in wetter environments. In this paper we examine the hypothesis that the ratio of fetch to patch decreases as rainfall increases, and that this trend will be most evident on intermediate-textured soils because these soils are more prone to runoff. We measured fine-scale patches on 38 sites with sand, loam or clay soils. Sites were located along a 1000-mm rainfall gradient in the savannas of northern Australia. The width and intercept length of patches and the fetch between patches was measuring along line transects of 100–120 m oriented down slope. We found that the ratio of fetch to patch area did not decrease with decreasing rainfall, but increased on both sand and loam soils. This result was because with increasing rainfall mean spacing between patches disproportionally increased while mean patch size and cover declined. The cover of patches was negatively correlated with tree canopy cover, which significantly increased with rainfall. This negative correlation suggests that in higher rainfall savannas the size and spacing of ground-layer patches is controlled by the tree layer, and that as rainfall decreases this control decreases and runoff-runon processes increasingly structure the landscape. For savannas on clay soils these trends were not significant except that on the highest rainfall sites the cover of ground-layer patches was nearly 100% while trees were absent. 相似文献
979.
980.
Improving the soil environment for seedling emergence and plant growth is possible using a good tillage system. From March 1993 until June 1994, the effect of tillage system and plant residue incorporation practices on the bulk density and the soil strength of the surface layer of a vertisol (very fine, smectitic, thermic, chromic Haploxerert), on a less than 1 % slope, was investigated. The two tillage methods were moldboard (T1) and chisel (T2). The two plant residue incorporation dates were in August (R1) and in October (R2). Results indicated that R1 gave significantly higher bulk density than R2, while the different tillage treatments had no significant effect. Soil strength was significantly lower under T1 and R2 than under the other treatments. 相似文献