Fertilizer production is also an environmental concern. For every ton of phosphoric acid produced, five tons of phosphogypsum are generated. Phosphogypsum is a solid material that results from the reaction of phosphate rock with sulfuric acid. Although it is nearly identical to natural gypsum, it may contain small amounts of sand, phosphate, fluorine, radium, and other elements present in phosphate ore. Federal regulations restrict both use and research involving phosphogypsum because of its radium content and require phosphogypsum to be stacked on the ground. A limited amount of phosphogypsum, with a minimal radium content, is used as an agricultural soil amendment. During the past 50 years, more than 700 MT have accumulated in Florida alone. These enormous stacks, some covering an area of more than 300 hectares and up to 60 m high, have settling ponds on top that contain highly acidic water that can overflow into waterways. New regulations have been enacted to guard against potential spills (Johnson and Traub 1996).
Mycorrhiza offers an alternative to many problems in an ecofriendly, sustainable, and economical way besides creating employment and facilitating poverty reduction. In situations where the native mycorrhizal inoculum potential is low or ineffective, providing appropriate fungi for the plant production system is worth considering. With the current state of technology, inoculation is best for transplanted crops and in areas where soil disturbance has reduced the native inoculum potential. The first step in any inoculation program will be to obtain an isolate that is both infective, and able to penetrate and spread in the root, and effective, or able to enhance the growth and stress tolerance of the host. Individual isolates of mycorrhizal fungi vary widely in these properties, so screening trials are important to select isolates that will perform successfully. Screening under actual cropping conditions is best because indigenous mycorrhizal fungi, pathogens, and soil chemical and physical properties will influence the result.
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