首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   142篇
  免费   1篇
林业   3篇
农学   2篇
  12篇
综合类   16篇
农作物   1篇
水产渔业   6篇
畜牧兽医   63篇
园艺   1篇
植物保护   39篇
  2022年   1篇
  2018年   2篇
  2013年   2篇
  2011年   2篇
  2010年   5篇
  2009年   1篇
  2008年   1篇
  2007年   3篇
  2002年   2篇
  1999年   2篇
  1998年   8篇
  1997年   3篇
  1996年   6篇
  1995年   4篇
  1992年   3篇
  1991年   4篇
  1990年   6篇
  1989年   6篇
  1988年   3篇
  1987年   6篇
  1986年   3篇
  1985年   8篇
  1984年   6篇
  1983年   3篇
  1982年   4篇
  1981年   6篇
  1980年   1篇
  1979年   1篇
  1978年   1篇
  1977年   3篇
  1976年   4篇
  1975年   5篇
  1974年   2篇
  1973年   2篇
  1972年   1篇
  1970年   5篇
  1969年   2篇
  1968年   2篇
  1963年   1篇
  1962年   2篇
  1958年   1篇
  1957年   3篇
  1954年   1篇
  1953年   2篇
  1952年   1篇
  1950年   1篇
  1949年   2篇
排序方式: 共有143条查询结果,搜索用时 15 毫秒
71.
The rate of transformation of inorganic phosphorus fractions and forms was investigated in a chronosequence of weakly weathered soils developed on windblown sand. The net decline of acid-extractable Ca-P during 104 years of soil development was approximately 2600 kg/ha m profile. This fraction was progressively removed from surface horizons and had virtually disappeared from the oldest soil. Residual inorganic P, considered to be included apatite which is not extracted by 0·5 and 1N HC1, was relatively constant in profiles of the chronosequence but decreased in the oldest soil. In the younger soils, the NH4F-P fraction was low but, with increasing time, the weight of this fraction increased, initially in surface horizons. The net accumulation of non-occluded secondary inorganic P during 104 years was 360 kg/ha m profile and this was largely due to an increase in the NH4F-P, rather than the 1st NaOH-P fraction. There was no net shift from non-occluded to occluded secondary inorganic P. Although apatite is initially present as inclusions within primary minerals in the wind-blown sand, an extensive loss of P has occurred during 104 years.  相似文献   
72.
The crystallinity of soil kaolinites as a function of clay particle-size and soil age was investigated in soil chronosequences of the Shingle House Creek and Hawkesbury River alluvial terraces in south-eastern Australia. The youngest soils (late Holocene) in each sequence are texturally uniform Entisols containing kaolinite and illite. The oldest soils (Pleistocene to late Tertiary) are Ustalfs with strong textural differentiation and are predominantly kaolinitic. With increasing age, textural B horizons are increasingly enriched in kaolinite and in particles of fine clay (< 0.2 μm) size. In two sub-fractions of the fine clay (0.2-0.06 μm; < 0.03 μm), no corresponding changes were observed in the crystallinity of kaolinites (as measured by the index, C k) with age. However, values of C k were significantly higher in the coarse clay (2-0.2 μm) than for both fine clay fractions in all except the Ultic Paleustalf of the oldest, possibly late Tertiary, terrace of the Hawkesbury River sequence. In this soil, C k values are low in all three clay-size fractions.
In these sequences, the effects of both clay particle-size and soil age were identified in the crystallinity of kaolinites. Disorder as a result of pedogenesis, however, was associated only with the most prolonged weathering and the strongest soil textural differentiation.  相似文献   
73.
Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge.  相似文献   
74.
The embryonic collection techniques in dogs present a vast methodological variation and low recovery rates. The objectives were to compare and describe two techniques as to the recovery of canine embryos, on the 12th day after the first mating or artificial insemination. Embryos were recovered through uterine horn flushing in vivo, before performing the ovariohysterectomy (OHE) (Group 1; n = 9) or ex vivo, immediately after the OHE (Group 2; n = 9). In total, 43 and 47 embryonic structures were recovered in Groups 1 and 2, respectively. There was no significant difference (p > 0.05) between groups on recovery rates (72.8% and 81.0%, respectively). We inferred that both in vivo and ex vivo techniques allow a high rate of embryonic recovery; in the collection technique prior to the OHE, it is essential to carefully handle the reproductive system during the trans‐surgical period and that the 12th day (D12) after the first mating/artificial insemination is an efficient option for the high recovery rate of morulae and blastocysts.  相似文献   
75.
Abstract. The lower Murray population of bony bream is subject to an annual epidemic of the oomycete Saprolegnia (principally S. parasitica ) and the bacterium Aeromonas hydrophila. The epidemic is species-specific; it mainly affects adults whose susceptibility may be increased by stress due to winter cold. Lesions occur on the mid-flank and are characterized by an external mycelium, epidermal erosion, scale loss, hypodermal and muscular oedema, haemorrhage, myofibril degeneration and by the presence of Saprolegnia hyphae at all stages of infection. Although A. hydrophila is common in advanced lesions there is no significant systemic bacterial infection. This appears to be a primary mycotic dermatitis and is noteworthy because Saprolegnia is best-known as a secondary pathogen.  相似文献   
76.
The quantification of the essential amino acid requirements of a species is a prerequisite to the formulation of biologically optimized diets. In this study, crystalline L -lysine was used in an attempt to determine the quantitative lysine requirement of juvenile Haliotis midae . Two microencapsulation techniques [gelatine/acacia and cellulose acetate phthalate (CAP)] were used to retard leaching of crystalline L -lysine incorporated into semipurified test diets. An assessment of the efficacy of the encapsulation techniques, revealed that despite effective lysine supplementation, H. midae fed semipurified test diets containing encapsulated crystalline L -lysine failed to promote significant improvements in either growth, feed or protein efficiency ( P  > 0.05). The failure of the crystalline L -lysine to illicit growth and nutritional responses is discussed.  相似文献   
77.
78.
The responses of onion (Allium cepa). Veronica persica, Matricaria matricarioides and Stellaria media to post-emergence applications of methazole were measured in field and glasshouse experiments. Stellaria media was the most susceptible species and V. persica the least. Plants of all species became more tolerant the larger they were at the time of treatment, and this was most pronounced in onion. Onion generally retained less spray per unit of dry weight than the other three species and retention was less on old compared with young plants, whereas with the weed species, this did not change appreciably with age. There was a progressive increase in the amount of structured crystalline wax on successive onion leaves which resulted in larger contact angles between droplets and the leaf surfaces and lower spray retention per unit of dry weight. There was less wax development on the leaf surfaces, increased spray retention, and increased susceptibility to methazole in onion treated pre-emergence with ethofumesate thus confirming that these factors are interrelated. While the increased tolerance of onion to methazole with age could be explained in part by decreased retention of herbicide, this was not so for the weed species, and other factors must determine their change in tolerance with age.  相似文献   
79.
Thirty separate soil samples were taken from different locations at the Brimstone farm experimental site, Oxfordshire, UK. Incubations of isoproturon under standard conditions (15 °C; ?33 kPa soil water potential) indicated considerable variation in degradation rate in the soil, with the time to 50% loss (DT50) varying from 6 to 30 days. These differences were confirmed in a second comparative experiment in which degradation rates were assessed in 11 samples of the same soil in two separate laboratories using an identical protocol. There was a significant negative linear relationship (r2= 0.746) between the DT50 values and soil pH in this group of soils. In a third experiment, degradation rates of the related compound chlorotoluron were compared with those of isoproturon in 12 separate soil samples, six of which had been stored for several months, and six of which were freshly collected from the field. Degradation of both herbicides occurred more slowly in the stored samples than in the fresh samples but, in all of them, chlorotoluron degraded more slowly than isoproturon, and there was a highly significant linear relationship (r2=0.916) between the respective DT50 values.  相似文献   
80.
Fifty-five bitches were examined once, at different intervals for pregnancy. Manual Papation, B-mode real-time ultrasoungraphy, and radiography were used to confirm pregnancy and to count the number of fetuses present. Predicted numbers were compared to whelping data obtained from owners. Radiography was confined to the last trimester of pregnancy and had an overall accuracy of 100% in pregnancy detection and 93% in correct litter size determination. A proor qualtiy radiograph contributed to the one incorrect count estimate. Ultrasound and palpation were used in all three trimester phases starting 3 weeks post coitus. For pregnancy detection ultrasound was 94% accurate (no false positives) and palpation was 88% accurate (one false positive). For fetal counting, ultrasound was 36% accurate and palpation was 12% accurate. Recommendations for the use of ultrasound and radiography in pregnancy evaluation are reviewed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号