Biofertilizers - the natural way?
The nitrogen fixing capacity of legumes is one of nature's gifts to agricultural sustainability but that gift can be enhanced with a little help from man. Biofertilizers, or rhizobia bacteria, which are applied as an inoculant to legume seed before sowing, are gaining in popularity with farmers and agricultural scientists alike. They are claimed to be cheap, easily used, and effective.
Communal farmers in Zimbabwe found that inoculated soybean seeds yielded, on average, more than double those that had not been inoculated. In fact they performed better than fields treated with commercial ammonium nitrate fertilizer and at a fraction of the price. Farmers buy small, sealed polythene packets containing 100g of a suitable strain of rhizobia adsorbed on to an organic substrate. This quantity is sufficient for inoculationg 50kg of seed or enough for one hectare. The seed is simply wetted and the correct amount of biofertilizer added and mixed by hand, making sure that the seeds are evenly coated. The seeds are then ready for planting.
Some strains of rhizobia are more efficient than others at providing the most nitrogen to the growing plant and, therefore, the highest yields. Farmers need the best rhizobia strain for the specific legume crop, or even the specific cultivar, that they plan to grow. With thousands of rhizobia strains known, how can the perfect match be found when, out in the field, many other farming variables make accurate measurement of efficient nitrogen take-up impossible? The answer is to use nuclear technology, specifically the nitrogen-15 isotope, to label and trace the amount of nitrogen taken up by the plant and, therefore, the relative efficiency of a particular inoculant. The International Atomic Energy Agency has provided assistance to the governments of several countries to promote the use of biofertilizers, among them Bangladesh where it has been found that the biological nitrogen fixation of chickpeas and groundnuts can be doubled if inoculated with an appropriate strain of rhizobia. Even more significant is that this more efficient nitrogen fixation improves grain yields by up to 50% and still leaves some nitrogen in the soil for the following crop, reducing the need and cost of applying commercial, inorganic nitrogen.
