Phosphate Fertilizer

Were it not for their accessibility to sources of fertilizers, certain eastern and southern agricultural lands would be called marginal more often than is now the case. These lands have long been served by the phosphate deposits of Florida and Tennessee; by the potash mines of Europe, and by the nitrate deposits of Chile, with products deliverable at many close-by ports by water transportation, and by oke ovens, widely distributed, which supply byproduct ammonia at low production and distribution costs. Hence soil fertility in these areas has long since become not a matter of nature but of soil management. It is not a coincidence that this area of relatively heavy fertilizer application is accessible to relatively low-cost supplies.

The term "heavy application” is used comparatively. The comparison is with the vaster areas of the West and Middle West, where at no time have fertilizer supplies been accessible except when imported from long distances at freight charges representing a disproportionate part of their cost. This cost is not necessarily prohibitive, for cost must be measured in terms of profits from use; but relatively fertilizers are high in the West and unquestionably their costs have been an effective deterrent to their more general use in that section.

Federal surveys have determined the location and extent of the fertilizer resources of the West. Considerable research has been conducted in the Fertilizer Technology Division of the Bureau of Chemstry and Soils to develop feasible methods for their commercial utilization—methods capable of employing locally available raw materials, and yielding high-grade products susceptible of low-cost distribution. Abundant supplies of raw materials have been found for the production of potash, phosphates, and nitrates, the essential ingredients of commercial fertilizers; and substantial progress has been made in the development of an appropriate technology.

Potash industries are now established in southern California and New Mexico. They produce with highly developed technology an excellent grade of potassium chloride. Despite their distance from the East, and the resulting high freight charges, they supplied in 1933 almost 40 percent of the potash used in the East. Abundant raw materials in addition are represented by the polyhalite deposits of Texas and New Mexico, the alunites of Utah, the leucitic larvas of Wyoming, and the natural brines of Nebraska and Utah.

Of phosphate rock there is a superabundance. The phosphate deposits of Idaho, Wyoming, Montana, and Utah represent the world’s greatest known phosphate reserves.

The great coal deposits of Wyoming, and the supplies of natural gas of that and other States, represent inexhaustible sources of basic raw materials for nitrate production from the air by the modern synthetic methods. Ammonia synthesis has freed the farmers of this country from exclusive dependence on foreign nitrate deposits, and brought close to the farm an inexhaustible supply at costs far below those formerly paid. But the great nitrate plants of the East, while at the door of the eastern farmer, are still far removed from the farms of the West.

Here are raw materials of such abundance, diversification, and distribution as to offer the potentialities for fertilizer manufacture adequate to the all-time needs of western agriculture.

In their utilization there should be applied a technology representing the most recent developments in chemical engineering. These developments involve a radical departure from established processes. The three basic plant-food elements must be combined into high-analysis compounds to eliminate freight charges on useless ingredients, so as to make wide distribution possible. The operation must be profitable if private capital is to be employed. These are problems with which the Department is now engaged.

As the American fertilizer trade is organized, the mixture sold is designed to supply the plant-food elements in which the average soil is apt to be deficient, and to which the growing plant makes most ready response. Without discounting the relative importance of any one plant food, emphasis has been placed in the past on phosphates. Many years of experience on a diversity of soils and crops has shown that a mixture is so much better than the separate ingredients used singly, that for the sale of one, supplies of the other two are essential. For a satisfactory fertilizer industry for the West, therefore, the production of all three elements is required. At present, potash produced in the West must seek its market in the East where supplies of phosphates and nitrates are abundant.

With the blast-furnace process now under large-scale demonstration in comparison with the electric-furnace method by the Tennessee Valley Authority at Muscle Shoals, Ala., the question of profitable operation will be answered. It appears to be the most feasible method of processing the western phosphates, and is designed specifically for that use.

As a nucleus around which to build a well-balanced fertilizer industry, the phosphate blast furnace affords the basis of new activities that bid fair to become an essential part of the industrial and agricultural development of the Northwest which now seems certain as the result of current water-power and irrigation enterprises. Such an industry would assure to that vast area the many, enduring benefits represented by abundant supplies of low-cost plant food.

PHOSPHATE Fertilizer Prepared by Treating Phosphate Rock With Steam at High Temperatures

Domestic phosphate rock consists principally of fluorapatite, an insoluble compound which contains calcium phosphate and fluorine. Recent laboratory studies have shown that when phosphate rock, containing about 5 to 10 percent of silica, is heated in the presence of water vapor at about 1,400° C., the fluorapatite is decomposed, upwards of 95 percent of the fluorine is volatilized and 80 percent or more of the phosphoric oxide (P₂O₅) is converted into the citrate-soluble (available) condition.

The results of experiments with Florida land-pebble phosphate rock show (fig. 51) that no increase in the citrate solubility of the phosphoric oxide occurs until about 63 percent of the total fluorine is volatilized. From that point, however, the citrate solubility of the phosphoric oxide increases with increase in the percentage of the total fluorine volatilized. Removal of only 30 to 60 percent of the fluorine causes the citrate solubility of the phosphoric oxide to decrease below that of the phosphoric oxide in the untreated rock.

The process seems to have possibilities for the production of cheap phosphate fertilizer. It can be carried out in direct-fired rotary kilns and is applicable to all of the regular commercial grades and types of phosphate rock produced in this country at present.

Some of the properties of the product, which for convenience may be called calcined phosphate, are summarized briefly, as follows:
The product is obtained in the form of a sintered or semifused clinker which, unlike superphosphate, requires no aging and needs only to be ground to the desired fineness for fertilizer purposes. It is practically insoluble in pure water, is weakly alkaline in reaction, has no deleterious effect on fertilizer bags and machinery, and should prevent, to a considerable extent, the increase in soil acidity caused by the use of ammonium salts as fertilizers. Although the alkalinity of the material is sufficient to cause some loss of ammonia from ammonium salts in fertilizer mixtures, it is believed that it will be possible to overcome this disadvantage.

The properly prepared material should contain about 30 percent or more of citrate-soluble (available) phosphoric oxide, as compared with about 19 to 21 percent in the best grades of ordinary superphosphate. The chemical nature of the available phosphate in calcined phosphate is not definitely known but it is believed to be similar to that of the phosphate in basic slag, the phosphatic byproduct of the smelting of high-phosphorus iron ores, which is widely used as a fertilizer in Europe. Calcined phosphate not only is superior to superphosphate in physical properties but it markedly improves the physical properties of fertilizer mixtures in which it is present.

FIGURE 51.—Relation between volatilization of the fluorine from phosphate rock and the citrate solubility and nutrient value of the phosphoric oxide (P₂O₅). Until more than 60 to 65 percent of the fluorine is volatilized from phosphate rock by the calcination process the solubility of the phosphoric oxide in neutral ammonium citrate and the growth of millet are depressed below the results obtained with the raw phosphate rock. With greater removal of the fluorine both citrate solubility and plant growth are increased. (See fig. 52.)

Because of its low fluorine content, calcined phosphate has possibilities as a substitute for bone meal in the preparation of mineral feeds for livestock. Also, the fluorine volatilized during the manufacturing process is a possible source of fluorine compounds for industrial and technical purposes and for use as insecticides.

Finally, the high citrate solubility of calcined phosphate indicates that it should be an efficient fertilizer material.

In order to determine the plant-food value of calcined phosphate, greenhouse pot. experiments were carried out with millet as a test crop, using a phosphorus-deficient Norfolk loamy fine sand soil. In the preparation of the calcined phosphates used in these tests about 50 to 97 percent of the fluorine content of the phosphate rock was volatilized and the citrate solubility of the phosphoric oxide in the products ranged from about 7 to 86 percent. Tests were also made with ordinary superphosphate and dicalcium phosphate as standard sources of phosphoric oxide. The phosphates were applied in 4―12—6 fertilizer mixtures at the rate of 240 pounds of total phosphoric oxide per acre, equivalent to 1 ton of the complete mixture per acre. The growth of millet resulting from the different treatments is shown in Figure 52.

FIGURE 52.—Growth of millet with calcined phosphate. Phosphate treatment: 1, superphosphate, 2, dicalcium phosphate; 3, raw Florida pebble phosphate rock, 3.85 percent fluorine; 4, calcined phosphate, 2.08 percent fluorine; 5, calcined phosphate, 1.49 percent fluorine; 6, calcined phosphate, 0.85 percent fluorine; 7, calcined phosphate, 0.4 percent fluorine; 8, calcined phosphate, 0.27 percent fluorine; 9, calcined phosphate, 0.1 percent fluorine. The percentages of fluorine removed from the phosphate rock in the preparation of the calcined phosphates were as follows: No. 4, 50; no. 5, 64; no. 6, 79; no. 7, 90; no. 8, 93; and no. 9, 97.

Calcined phosphates from which only 50 to 64 percent of the fluorine had been removed (groups 4 and 5) gave smaller crop growths than did the untreated phosphate rock (group 3). With the removal of greater percentages of fluorine larger increases in growth were obtained (groups 6 to 9), and the calcined materials from which 90 to 97 percent of the fluorine had been removed (groups 7 to 9) gave better results than did ordinary superphosphate and dicalcium phosphate (groups 1 and 2).

As shown in figure 51, there is a more or less close correlation between the citrate solubility and the plant-food value of the phosphoric oxide in calcined phosphate, and both of these properties are correlated with the proportion of the fluorine removed during the manufacturing process.

Other greenhouse tests with millet and other crops substantiate the results presented here, in showing that the fertilizer value of calcined phosphate, with 90 percent or more of the fluorine removed, compares favorably with that of superphosphate and dicalcium phosphate.