In tropical agroecosystems, limited N availability remains a major impediment to increasing yield. A 15N-recovery experiment was conducted in 13 diverse tropical agroecosystems. The objectives were to determine the total recovery of one single 15N application of inorganic or organic N during three to six growing seasons and to establish whether the losses of N are governed by universal principles. Between 7 and 58% (average of 21%) of crop N uptake duringthe first growing season was derived from fertilizer. On average, 79% of crop N was derived from the soil. When 15N-labeled residues were applied, in the first growing season 4% of crop N was derived from the residues. Average recoveries of 15N-labeled fertilizer and residue in crops after the first growing season were 33 and 7%, respectively. Corresponding recoveries in the soil were 38 and 71 %. An additional 6% of the fertilizer and 9.1 % of the residue was recovered by crops during subsequent growing seasons. There were no significant differences in total 15N recovery (average 54%) between N from fertilizer and N from residue. After five growing seasons, more residue N (40%) than fertilizer N (18%) was recovered in the soil, better sustaining the soil organic matter N content. Long-term total recoveries of 15N-labeled fertilizer or residue in the crop and soil were similar. Soil N remained the primary source of N for crops. As higher rainfall and temperature tend to cause higher N losses, management practices to improve N use efficiency and reduce losses in wet tropical regions will remain a challenge.
Fertilization is an important management strategy of yams (Dioscoreaspp.) especially when grown in degraded soils. A field study evaluated the leaf numbers, leaf area indices, crop growth, yields, and nitrogen (N) and potassium (K) use efficiencies ofD. alataandD. rotundatain Côte d'Ivoire when grown in two contrasting soils with and without fertilizer.D. alatahad a lower number of leaves per vine, although leaf area indices were higher, and the leaves were retained for a longer period than inD. rotundata. In all situations, the yields ofD. alatawere significantly higher, and fertilizers promoted growth of shoots, roots, tubers, and, thus, final yields especially in the low fertile savanna soil. The beneficial impact of fertilizer on yields was significantly lower in the fertile forest soils. The nutrient use agronomic efficiencies indicated the impact of both N and K in promoting yields especially under nonfertilized conditions.
This survey was carried out to obtain information on seed establishment practices of dairy and sheep farmers of the central districts of New Zealand, from South Auckland to Canterbury. Sheep and dairy farmers in different regions did not differ significantly in their pasture establishment practices. Many sowed pasture seed every year, and old pastures were renovated rather than new land developed. Most either broadcast or drilled their seeds; a small proportion of farmers used both methods. An increase in drilling is anticipated. Farmers used, or intend using, simple pasture mixtures containing 2-4 grass species. Single pasture species have been, and will be used either for renovating old pastures, establishing special purpose leys, or for seed production. Ryegrass was the most popular grass species and 'Grasslands Nui' the most widely used cultivar. Farmers generally sowed their pastures in autumn, irrespective of region. They relied on their experience to select pasture species, and the seeds were purchased from seed merchants who made up the seed mixture. Seed rates used by farmers were 30--35 kg/ha.
Weeds are a perennial problem in coconut plantations and cause significant losses in the nut yield.The occurrence of a wide range of weeds also causes difficulties in their eradication.The influence of five different weed management practises on the distribution and composition of the soil weed seed bank in coconut plantations in the low-country dry zone of Sri Lanka was evaluated. The treatments imposed included the application of glyphosate (N-[phosphonomethyl]-glycine), cover cropping with Pueraria phaseoloides, tractor harrowing, tractor slashing, and tractor plowing.All the treatments were applied twice per year, except for the cover cropping treatment. In terms of a reduction in the weed biomass, the application of glyphosate and cover cropping (Pueraria) were more efficient in reducing the ground weed population.These methods were very effective in reducing the weed seed density in the top soil layers. Plowing and harrowing significantly reduced the seed bank in the top soil layers and shifted significant numbers of weed seeds to deeper soil profiles. However, the total germinated weed seed count increased by 123.5, 691.4, 1133.1, and 1216.5% in the 10-15, 15-20, 20-25, and 25-30 cm soil depths, respectively, compared with the initial germinated weed seed count in the plowing treatment. Considering all the soil layers, the decline in the germinating weed seed count was very high in the treatment plots with cover cropping and the application of glyphosate; thus, these are considered to be the best practises to reduce the germinating weed seed count in the soil of coconut plantations.
Potassium is major nutritional factor influencing plants' ability to overcome stress. Thus the effect of this nutrient on plant growth and symbiotic nitrogen fixation could be critical. A study was conducted to evaluate the influence of three levels of potassium fertiliser (0·1, 0·8 and 3·0 mM potassium) and two day/night temperature regimes (30°C/23°C—supra‐optimal and 23°C/18°C—optimal) on the vegetative growth and symbiotic nitrogen fixation of two legumes. The crops selected were faba bean, an amide producing temperate species, and French beans, a ureide‐producing tropical legume. Potassium increased relative growth rates of faba bean to a greater extent than in French beans with 3·0 mM potassium under the high temperatures. The influence of potassium was less evident at the lower temperature. Specific leaf weights of both species were reduced by potassium indicating the influence of this nutrient in the process of translocation of photosynthates. Total nitrogen concentrations of both species were lower and potassium concentrations were greater under supra‐optimal temperatures. Nodulation was not observed in faba beans at this temperature. Potassium increased nodulation and nitrogen fixation of faba bean under optimal temperatures and at both temperatures in French beans. In contrast, specific activity of nodules and percentage nitrogen derived from fixation were not affected by potassium at both temperatures. Potassium reduced the impact of high temperatures especially in temperate faba beans when compared with tropical French beans by enhancing vegetative growth, nodulation and total tropical nitrogen fixation.
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