Keywords: Fluid fertilizers, Polyphosphoric acid, Suspension fertilizers.

The use of fluid fertilizers in significant quantities is less than 20 years old. Nevertheless, growth has been so rapid that in the United States the consumption of mixed liquid fertilizer is now over 45 million tons per year (about 12% of the total fertilizer tonnage), and all fluid fertilizers (including nitrogen solutions and anhydrous ammonia) supply about 30% of the total tonnage.

Liquids have not been accepted as rapidly in other parts of the world, for several reasons. The main usage is in France and the United Kingdom.

The main problems in mixed liquid fertilizer production and use are raw material cost, nutrient concentration, and product quality. All three areas have been improved by the introduction of polyphosphates, which reduce cost by allowing use of wet process phosphoric acid, increase concentration because of the high solubility of ammonium polyphosphates, and improve quality by sequestering impurities and holding them in solution. The new form of phosphate has been adopted widely, to the extent that about 70% of the liquid fertilizer produced in the United States now contains part or all of the phosphate in the poly form.

The basic production method for polyphosphate-based liquid fertilizer is a two-step process in which polyphosphoric acid ('super acid') is first made by dehydrating wet process orthophosphoric acid and the super acid is then ammoniated to give a 10-34-0 base solution. There is a growing trend, however, to a direct process in which the ortho acid is dehydrated and ammoniated in a single operation.

The suspension method, in which fluid fertilizer is made by adjusting composition to give a slurry of crystals in saturated solution and the suspension is then stabilised by addition of clay. is another way of reducing raw material cost and increasing nutrient concentration. Wet process acid can be used because the impurities remain suspended with the salt crystals, and the grade can be as high as most of those available as solid mixed fertilizers.

One of the major advantages of fluid fertilizers is the uniformity of composition and absence of segregation when small amounts of micronutrient sources are mixed with the major materials. This is also true for pesticide-fertilizer mixtures, with the added advantage that the complete and uniform ground coverage afforded by liquid fertilizers is quite desirable in pesticide application. Such advantages are pushing consumption of fluid fertilizers to higher levels.

Developments in application are keeping pace with advances in manufacturing procedures. The intrinsic labour-saving advantage in application of liquid fertilizers is being exploited by use of high-flotation application equipment, large applicator tanks, and applicator booms covering a swath up to 60 feet wide. Application rates of up to 60 acres per hour are becoming common.

The basic advantages that have pushed liquid fertilizers to the present level of consumption seem likely to continue their effect. At the same time, research developments are reducing the drawbacks of high raw material cost, low nutrient concentration, and low product quality - problems that have plagued liquid fertilizer producers in the past. As a result it is expected that fertilizers in the fluid form will occupy a position of continually increasing importance in the future.

A V Slack and F P Achorn, Office of Agricultural and Chemical Development, Tennessee Vallay Authority, Muscle Shoals, Alabama, USA.

64 pages, 9 figures, 3 tables, 40 references.