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Improving corn crops without fertilizer A team from the US is shedding new light on a neglected aspect of agricultural research and is adding one piece to the puzzle of how to grow food sustainably in an arid climate. Corn (also known as maize), the preferred staple crop in many countries, requires large amounts of nitrogen for its growth. Usually expensive synthetic fertilizer is necessary to sustain high yields. It has become the mission of Pennsylvania State University graduate Ylva Besmer and her colleagues to discover ways to improve corn yield for subsistence farmers in Zimbabwe without the use of chemical fertilizer. Ylva and her colleagues have discovered that for subsistence farmers in the semi-arid tropics, the proper selection of legumes coupled with an increase of naturally occurring mycorrhizal fungi effectively sustains or increases corn yields without the use of fertilizers. Regional difficulties The subsistence farmer in the semi-arid tropics is facing many challenges including drought, depredation by wild and domestic animals before harvest, and an unreliable supply of chemical inputs. In Ylva’s work she has attempted to address the third obstacle. Discovering natural fertilizers "The government of Zimbabwe no longer provides a subsidy for fertilizer, resulting in significantly lower corn yields" says Ylva, a doctoral candidate in ecology. "The old-fashion use of legumes (members of the bean family) in crop rotation may prove to be a solution to this problem because of their ability to fix nitrogen and, thus, provide nitrogen for subsequently grown corn. We have shown in Zimbabwe, however, that legume growth and nitrogen fixation can be limited by the availability of phosphorus in the soil. “In order to improve nitrogen fixation in legumes, somehow phosphorus availability has to be increased.” The challenge The objective of this project has been to understand the role of symbiotic microorganisms, such as arbuscular mycorrhizal fungi (AMF) and Rhizobium bacteria, in the subsistence farming system, to promote their activity and to observe the effect on crop yield. The team from Pennsylvania has been able to show that by increasing the abundance of AMF in pots, nodulation in two legumes increases. The challenge it now faces is to determine how fungal activity can be enhanced in the field by using practices that would be acceptable to and affordable by the subsistence farmer. The team from Pennsylvania is currently testing the effect of tillage timing and fallow and is investigating whether practices that optimise fungal activity, such as continuous cropping, might drain the soil of valuable water and create a weed problem. The dilemma is how to resolve these issues: in order for the farmer to change any current practice, significant benefits have to be demonstrated. Building on previous research Much of the previous research on AMF has focused on comparisons between non-mycorrhizal and mycorrhizal plants where large benefits have been detected. In Ylva’s work, the question concerns the effect of an enhanced mycorrhizal inoculum potential, where the control plants are also mycorrhizal. This, in addition to very low levels of phosphorous in the soils where we are working, makes it very challenging to demonstrate large effects. AMF, although not a substitute for phosphorous, allow the plant to utilize more of the chemical in the soil, thereby perhaps buying the farmer time. For continuous farming, however, nutrients that are removed from the field by harvesting need to be replenished at some point in time. There is no way around that. What are Mycorrhizal Fungi (MF)? Mycorrhizal fungi are organic organisms commonly found in nature. They colonize the roots of many plant species including legumes. These fungi live symbiotically with their hosts, absorbing phosphorus from the soil, and transporting it to the root systems. In preliminary tests Ylva and her colleagues have shown that enhanced mycorrhizal colonization of a number of legumes grown in soil in Zimbabwe increases nitrogen content indirectly by increasing phosphorus uptake. "We first used peanut because it is a legume that is commonly grown by subsistence farmers. Peanut growth and nitrogen content was strongly limited by phosphorus availability, and by amending the soil with mycorrhizal fungi, peanut nitrogen content was significantly increased," Ylva reports. Spreading the word "In temperate agro-ecosystems, mycorrhizal fungal abundance can be increased by reducing fallow periods and tillage," explains Ylva. "We want to take these lessons learned from temperate systems and try to apply them appropriately to the semi-arid tropics to increase mycorrhizal fungal activity.” Building on success Ylva asserts, "While peanut was a logical crop to study, it may not be the best legume to use in Zimbabwe to enhance soil fertility for corn production because most of the nitrogen resides in the nut, which is harvested and removed from the soil. “Another legume, commonly called lablab, looks promising because it grows more vigorously and its stems and leaves contain more nitrogen. "The best way to use any legume to increase soil fertility is to plough most of the plant back into the soil or let animals graze on the plants and allow their manure naturally to fertilize the field.” Research in detail Role of phosphorous (P), mycorrhizal fungi and Rhizobium for groundnut growth and nitrogen (N) content Objective: To determine the effect of altered microbial activity on growth, nodulation and nitrogen content in groundnut. The hypothesis is that nitrogen2 fixation is limited by phosphorous and that an enhanced arbuscular mycorrhizal fungi (AMF) abundance can enhance plant phosphorous uptake, which will stimulate nitrogen 2 fixation. Methods: Groundnut was used as a bioassay and planted in soil collected from low- phosphorous alfisol with low nodulation and moderate mycorrhizal colonization. Plants were grown for 6 weeks in a greenhouse in soil amended with either phosphorous [2 g superphosphate pot-1 (19% P2O5) which roughly corresponds to the nationally recommended phosphorous application rate], mycorrhizal inoculum (2000 spores pot-1 of Glomus intraradices), a commercially available Rhizobium inoculum (seeds soaked in a solution consisting of 5 g of commercial inoculum in 200 mL of H20 plus 1 mL of solution added in each hole prior to planting), a fungicide Benomyl (200 mL of 0.1% solution added one day prior to planting), or control. At harvest, shoot nitrogen and phosphorous content, VAM colonization and nodule numbers were determined. Conclusion: Increased mycorrhizal colonization increases the phosphorous content of the plant, which significantly increases shoot N content (see table below). Experiments showed that there was a strong beneficial effect of phosphorous on the number of nodules but a deleterious effect of phosphorous on mycorrhizal colonization. Results also indicate that inoculations with Rhizobium are ineffectual (?) in this soil since the nitrogen2 fixation is phosphorous limited. Effect of phosphorous, VAM and Rhizobium inoculations on groundnut grown in an alfisol soil collected from Tshlotshlo Site 2. Different superscripts indicate a significant (p£0.05) difference between means. Mean (se), n = 5
Source: Pennsylvania State University researcher Ylva Besmer is working with Roger Koide, professor of horticultural ecology, and Robert Myers, soil scientist with the International Crops Research Institute for the Semi-arid Tropics (ICRISAT), to find ways to increase the presence of the beneficial mycorrhizal fungi in Zimbabwean soils. Their work would not have been possible without the help of their collaborators at ICRISAT (International Crops Research Institute for the Semi-Arid Tropics) in Zimbabwe and funding from The National Geographic Society and the Root Biology Program at the Pennsylvania State University. 11/09/02 << country/story review |
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