全文获取类型
收费全文 | 88447篇 |
免费 | 27153篇 |
国内免费 | 1256篇 |
专业分类
林业 | 11365篇 |
农学 | 6956篇 |
基础科学 | 5574篇 |
20656篇 | |
综合类 | 48354篇 |
农作物 | 4483篇 |
水产渔业 | 2999篇 |
畜牧兽医 | 9209篇 |
园艺 | 2442篇 |
植物保护 | 4818篇 |
出版年
2025年 | 1303篇 |
2024年 | 2644篇 |
2023年 | 2671篇 |
2022年 | 2951篇 |
2021年 | 3008篇 |
2020年 | 3175篇 |
2019年 | 3710篇 |
2018年 | 2385篇 |
2017年 | 3705篇 |
2016年 | 4470篇 |
2015年 | 3810篇 |
2014年 | 4949篇 |
2013年 | 5581篇 |
2012年 | 7438篇 |
2011年 | 7393篇 |
2010年 | 5913篇 |
2009年 | 5780篇 |
2008年 | 5263篇 |
2007年 | 6204篇 |
2006年 | 5455篇 |
2005年 | 4518篇 |
2004年 | 3455篇 |
2003年 | 2959篇 |
2002年 | 2271篇 |
2001年 | 2050篇 |
2000年 | 1828篇 |
1999年 | 1485篇 |
1998年 | 1293篇 |
1997年 | 1149篇 |
1996年 | 1115篇 |
1995年 | 1118篇 |
1994年 | 1019篇 |
1993年 | 907篇 |
1992年 | 826篇 |
1991年 | 681篇 |
1990年 | 626篇 |
1989年 | 564篇 |
1988年 | 443篇 |
1987年 | 311篇 |
1986年 | 158篇 |
1985年 | 59篇 |
1984年 | 36篇 |
1983年 | 27篇 |
1982年 | 20篇 |
1981年 | 22篇 |
1980年 | 24篇 |
1979年 | 19篇 |
1978年 | 17篇 |
1976年 | 10篇 |
1962年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 20 毫秒
1.
Field experiments were conducted to determine the direct and residual contributions of legumes to the yield and nitrogen (N) uptake of maize during the wet seasons of 1994 and 1995 at the University Farm, Abubakar Tafawa Balewa University, Bauchi, Nigeria, located in the Northern Guinea savannah of Nigeria. Nodulating soybean, lablab, green gram and black gram contributed to the yield and N uptake of maize either intercropped with the legumes or grown after legumes as a sole crop. Direct transfer of N from the nodulating soybean, lablab, green gram and black gram to the intercropped maize was 24.9–28.1, 23.8–29.2, 19.7–22.1 and 18.4–18.6 kg N ha–1 , respectively. However, the transfer of residual N from these legumes to the succeeding maize crop was 18.4–20.0, 19.5–29.9, 12.0–13.7 and 9.3–10.3 kg N ha–1 , respectively. Four years of continuous lablab cropping resulted in yields and N uptake of the succeeding maize crop grown without fertilizer N that were comparable to the yields and N uptake of the succeeding maize crop supplied with 40–45 kg N ha–1 and grown after 4 years of continuous sorghum cropping. It may therefore be concluded that nodulating soybean, lablab, green gram and black gram may be either intercropped or grown in rotation with cereals in order to economize the use of fertilizer N for maize production in the Nigerian savannah. 相似文献
2.
3.
Little is known about the effect of fertilization on the N uptake of sunflowers. A 42 factorial trial with 0, 60, 120 and 180 kg N ha−1 and 0, 15, 30 and 45 kg P ha−1 was conducted over three years. The N content and concentration of leaves, stems and capitula were determined at three growth stages. High N levels increased the N content and concentration of all plant parts at all growth stages sharply. High P levels increased the N content of all plant components through better growth. P has an inconsistent effect on N concentration but tended to decrease it. After flowering the crop assimilated 20 to 25 % of the total N. This implies that N applied can still be applied and utilized by the crop at a late stage. This should be substantiated by further research. 相似文献
4.
Root development of sugar beet plants on a sandy loess site with regard to nitrogen nutrition.
Root development of sugar beet plants in a sandy loess soil (Haplic Phaeozem) was observed from the early seedling stage up to harvest by measuring at first the greatest vertical and lateral extension of the root systems of single plants and later the rooting density of the whole plant stands (auger method, profile wall method).
During the seedling stage not only the subsoil, but also large parts of the topsoil between the plants remained unoccupied by the root systems. In this phase the greatest lateral extension of single roots reaches nearly the length of the greatest leaf of the plant. With the closure of the canopy the rooting density in the topsoil accounts to 1–2 cm cm−3 .
In summer roots penetrate to a depth of 100–150 cm with rooting densities of 0.1 to 1 cm - cm−3 . Thus, the plants gain not only access to water reserves, but sometimes meet remarkable amounts of nitrate which under the relatively dry conditions of the region tends to accumulate in 60–120 cm depth and – when taken up by the beet plants in the late stage of growth – affects crop quality negatively. 相似文献
Root development of sugar beet plants in a sandy loess soil (Haplic Phaeozem) was observed from the early seedling stage up to harvest by measuring at first the greatest vertical and lateral extension of the root systems of single plants and later the rooting density of the whole plant stands (auger method, profile wall method).
During the seedling stage not only the subsoil, but also large parts of the topsoil between the plants remained unoccupied by the root systems. In this phase the greatest lateral extension of single roots reaches nearly the length of the greatest leaf of the plant. With the closure of the canopy the rooting density in the topsoil accounts to 1–2 cm cm
In summer roots penetrate to a depth of 100–150 cm with rooting densities of 0.1 to 1 cm - cm
5.
A field experiment was conducted with different nitrogen regimes to assess the growth and yield performance of wheat genotypes which differ in nitrate assimilation potential. Genotypic differences in biomass accumulation were observed at different growth stages. The nitrogen treatment had little effect on biomass accumulation at early stages of growth, while at later stages of growth there was enhanced biomass accumulation when N was applied in more than two splits. On an average, genotypes with high nitrate reductase activity (the 'HNR' genotypes) accumulated 14.2 % more biomass than the genotypes with low nitrate reductase activity ('LNR' genotypes) when an extra dose (40 kg N ha−1 ) of nitrogen was given at the time of anthesis. The application of nitrogen in more than two splits increased grain yield of both 'HNR' and 'LNR' genotypes mainly by increasing grain weight per ear. The application of an extra dose of nitrogen (40 kg N ha−1 ) at the time of anthesis increased grain yield of 'HNR' genotypes by 38.5 % as compared to 'LNR' genotypes. 相似文献
6.
江立 《仲恺农业技术学院学报》1996,9(2):1-4,19
本文述了“广东山区开发理论与实践研讨会”所达成共识的观点:广东山区开发具有重要的战略意义,广东山区开发必须走可持续肆展,农业产业化经营、工业化道路,同时必须搞好基础设施建设,组织好商品流通工作,优化资源配置。 相似文献
7.
J. Müller 《Journal of Agronomy and Crop Science》1993,171(4):217-235
Dry Matter Production, CO2 Exchange, Carbohydrate and Nitrogen Content of Winter Wheat at Elevated CO2 Concentration and Drought Stress
Methods of mathematical modelling and simulation are being used to an increasing degree in estimating the effects of rising atmospheric CO2 concentration and changing climatic conditions on agricultural ecosystems. In this context, detailed knowledge is required about the possible effects on crop growth and physiological processes. To this aim, the influence of an elevated CO2 concentration and of drought stress on dry matter production, CO2 exchange, and on carbohydrate and nitrogen content was studied in two winter wheat varieties from shooting to milk ripeness. Elevated CO2 concentration leads to a compensation of drought stress and at optimal water supply to an increase of vegetative dry matter and of yield to the fourfold value. This effects were caused by enhanced growth of secondary tillers which were reduced in plants cultivated at atmospheric CO2 concentration. Analogous effects in the development of ear organs were influenced additionally by competitive interactions between the developing organs. The content and the mass of ethanol soluble carbohydrates in leaves and stems were increased after the CO2 treatment and exhausted more completely during the grain filling period after drought stress. Plants cultivated from shooting to milk ripeness at elevated CO2 concentration showed a reduced response of net photosynthesis rate to increasing CO2 concentration by comparison with untreated plants. The rate of dark respiration was increased in this plants. 相似文献
Methods of mathematical modelling and simulation are being used to an increasing degree in estimating the effects of rising atmospheric CO
8.
V. Slipevi I. Vedrina-Dragojevi L. Balint J. Momirovi-uljat 《Journal of Agronomy and Crop Science》1992,168(2):73-84
Dynamics of the cumulation of phosphorus, potassium, calcium and magnesium in developing seed of four soybean genotypes under the defined climatic conditions in the course of three years was observed. Samples were being taken at weekly intervals from the point at which the grain reached a size suitable to be analyzed up to the full maturity of the seed. By analyzing the results it was found out that the intense cumulation of the minerals tested took place during the stages III and IV of the seed development, that is, in the course of intense cumulation of dry matter and the most expressive metabolic activity in the seed. Investigation of the impact of climatic conditions provides evidence that cumulation of the tested macroelements has been favourably influenced by increased precipitation along with moderate air temperatures within the seed developing period. The highest content of the minerals found with the cultivar Maksimirka independently of the year investigated reveals that mineral cumulation in soybean seed is affected by the cultivar and genetic particularity. 相似文献
9.
10.
Germinating seeds and young plants of winter rape var. Górczañski were vernalized for 56–63 days under conditions of 9-hour day, at the temperature 2 and 5 °C and in continuous darkness at the temperature 2 °C. After vernalization the plants grew under conditions enabling to complete vernalization: in a glass-house at the temperature day/night 15/10 °C and in semi natural conditions of open vegetation hall in the period from June till August. After sub-optimal vernalization further growth of the plants at lowered temperature increased its effectiveness (completion of vernalization). Depending on the degree of the vernalization of the plants the completion of their vernalization was both obligatory, i.e. conditioning the acquisition of the ability of generative development, and facultative i.e. accelerating this development. It has been demonstrated that the population of plants of the examined variety is strongly differentiated not only with respect of vernalization requirements in the particular plants, but also what regards the effectiveness of vernalization completion. New observations have been made indicating that the mechanisms controlling the successive phases of generative development, i.e. phase of forming flower buds and the flowering phase are not identical which may be interpreted as indicating that the "flowering factor" is polymorphous. 相似文献