Phosphate Fertilizers

by W. H. PIERRE

O.FNLY A LITTLE more than a century ago Sir John Lawes of England first produced soluble phosphate fertilizers by treating ground bones with sulfuric acid. About 25 years later his process was apphed in the United States to phosphate rock, deposits of which had been found in South Carolina. Little was known then about the use of phosphate fertilizers, but information on the phosphate needs of Ameri- can soils gradually accumulated, and the phosphate fertilizer industry soon became well established. Today, nearly 4 million tons of phosphate rock are mined annually for the production of superphosphate fertilizers and for direct appHcation to the soil. American farmers spend about 200* million dollars annually for phosphorus, the important plant food element that is added to soils through the use of phosphate fertihzers. Of the total amount of phosphorus found in soils, only a small per- centage is in a form readily available for use by plants. Most of it is found in compounds from which plants cannot obtain sufficient amounts for rapid growth and maximum yields. Cropping results in a continuous removal of the most available soil phosphorus. Furthermore, as H. T. Rogers in Virginia and O. R. Neal in New Jersey have shown, it is this small but most valuable portion of the total soil phosphorus that is most readily lost by erosion. But the problem of phosphate fertilization is not simply one of adding to soils an amount of phosphorus equal to that removed by crops or lost by erosion. Soils differ greatly in the kinds of phosphorus compounds they contain and in their ability to pass along to the plant the phosphate added in fertihzers. Moreover, crops vary in their ability to use the phosphorus compounds of the soil and in their response to phosphate fertilizers. The kinds of phosphate fertilizers used and the way in which 554 PHOSPHATE FERTILIZERS 555 they are applied are other factors that affect the returns obtained. Only through a better understanding of such factors can farmers bring about the most efficient use of phosphate fertilizers and insure adequate consumption for soil improvement and conservation. The amount of phosphate as well as other fertilizers used on American farms reached an all-time peak in 1946, largely because of the great war-born demand for agricultural products and the relatively high level of farm income. Despite variations through the years, associated with changes in farm income, the trend in the use of phosphate fertilizers has been definitely upward. As with other fertilizers, the use of phosphate fertilizers varies in the different States. Even in the East, where rainfall is not the limiting factor in crop production, large differences exist; in 1943, seven Southeastern States used 224,609 tons of phosphorus in fertilizers, or more than 40 percent of the total used in the United States. The use of phosphate fertilizers in the various States is in sharp contrast to the calculated amounts of phosphorus removed in harvested crops. Five Corn Belt States—Ohio, Indiana, Illinois, Iowa, and Missouri—for example, removed in harvested crops in 1943 more than 30 percent of the 740,000 tons of phosphorus removed in the entire country, according to calculations made by J. H. Stallings. And even though half of the phosphorus contained in crops may be returned to the soil in manures, cropping results in a much heavier drain of phosphorus from the soils of the Corn Belt and other North Central States than from those of the Eastern States. Eastern farmers use relatively large amounts of phosphate fertilizers because their soils are inherently less productive and have been farmed longer than the soils to the west and because of the kinds of crops they grow and their type of farming. Potatoes and truck crops usually give a high net return from fertilizers because of their high acre value, and ordinarily receive at least a ton of fertilizer to the acre. Tobacco and cotton get much more than are usually applied to grain and hay crops. Most of the phosphate fertilizer used in the United States consists of ordinary superphosphate, containing 16 to 22 percent PgOr,, and concentrated superphosphate, containing 40 to 45 percent PsOr». Both materials contain monocalcium phosphate, a form readily available to crops. Finely ground phosphate rock also is used for direct application to the soil. It is less readily available to crops than is superphosphate, but when used with legumes in the cropping system it has proved effective in building up the productive level of acid, phosphorus-deficient soils. Since about 1934, extensive investigations have been in progress to determine the value of several new phosphate materials prepared by the Tennessee Valley Authority for use as fertilizers. Most of the experimental work was done with concentrated or triple superphosphate and with two new products, calcium metaphosphate and fused tricalcium phosphate. 556 YEARBOOK OF AGRICULTURE

Calcium metaphosphate is produced when the elementary phosphorus issuing from a phosphate-reduction furnace is burned to P20r, and is allowed to react with phosphate rock heated to 1,200° C. Because it contains about 65 percent P2O0, it is a concentrated phosphate^ as compared with ordinary superphosphate. A summary prepared by the Ten- nessee Valley Authority of 758 experiments conducted in the seven Tennessee Valley States with cotton, corn, small grains, and hay shows that tiie yields with calcium metaphosphate averaged 99 percent as high as with ordinary superphosphate. On calcareous soils and on certain acid soils where it is not well incorporated into the soil, calcium metaphosphate has been found inferior to superphosphate. Fused tricalcium phosphate is produced by heating phosphate rock to 1,500°-1,600° C. in the presence of water vapor. The process causes a disruption of the apatite structure of the phosphate rock and the loss of fluorine, so that a tricalcium phosphate that contains about 30 percent P2O5 is formed. The degree to which fluorine is removed and the fineness of grinding influence the cost and the availability to crops. Experiments conducted since 1941, and summarized by the Tennessee Valley Authority, have resulted in the establishment of tentative maximum limits of 0.4 percent fluorine and 40-mesh size as standards that insure relatively high availability to crops along with economy of production. In general, the availability of fused tricalcium phosphate on acid soils has been found to be slightly less than that of superphosphate. Like calcium metaphosphate it has been found to be inferior to superphosphate on calcareous soils and when appUed as a top dressing. The main advantage of these newer forms of phosphate, as weU as of concentrated superphosphate that contains about 45 percent P2O5, is the economy in cost of transportation and handling. If further work shows that they can be produced as economically as superphosphate they should find greatly increased use. Because concentrated phosphates do not contain sulfur, that element would need to be added separately to sulfur-deficient soils if concentrated phosphates come into general use.

Soil Requirements and Efficient Use

Some of the most significant field and laboratory investigations relat- ing to the use of phosphate fertilizers have been concerned with the determination of the phosphorus requirement of different soil types under various farming systems, and with more efficient methods of use. The results have given farmers a sounder basis for determining the need for phosphorus anci estimating the amount required. That the efficiency in use of phosphate fertilizers depends also on other soil conditions and on crop requirements is illustrated by some recent investigations in Mississippi and Iowa. In the South, oats and corn often PHOSPHATE FERTILIZERS 557 have not shown profitable responses from phosphate fertihzers despite the low amounts of soluble phosphates in the soils and low average yields. The recent work of Russell Coleman in Mississippi shows, however, that this lack of response to phosphate may be due largely to inadequate amounts of nitrogren. Where no nitrogen was applied* in fertilizers, an application of 200 pounds of superphosphate (16-percent P2O5) an acre did not increase the yields ; but where 48 pounds of nitrogen was used, the phosphate increased the yield of oats by 17.3 bushels and that of cotton by 215 pounds of seed cotton an acre. Similar results were obtained by L. B. Nelson, Kirk Lawton, and C. A. Black in Iowa. In 22 experiments with oats conducted in different parts of the State in 1945 they found that the use of superphosphate (20-pcrcent P2O5) at the rate of 200 pounds an acre increased the yields by only 3.1 bushels an acre where no nitrogen had been applied, but by 7.8 bushels an acre where 40 pounds of nitrogen was used. Studies have also been continued on better methods of applying phosphate fertilizers. In general, band placements have been found to be most satisfactory, but the best methods vary considerably with the kind of soil, the crop, and the kind and amount of fertilizer used. On soils that combine strongly with the phosphate to make it relatively unavail- able to crops, the application of soluble phosphates in bands rather than broadcast has been found to be particularly advantageous. With relatively insoluble forms of phosphate, however, mixing of the fertihzer with the soil appears desirable. Investigations on the deep placement of fertilizers have been stimulated by the desire to increase the rate of fertilization in order to obtain maximum production. This is particularly true of nitrogen fertilization. In experiments of W. H. Metzger and Floyd Davison in Kansas, the placement of phosphate fertilizer in the row at a depth of 6 inches resulted in considerably greater yields of sorghum'than did more shallow placement. Experiments with corn in several Midwestern States, however, have shown no such advantage. It remains to be estabhshed, therefore, whether deep placement of phosphate is desirable, especially where only small amounts are applied and where the plants are likely to suffer from phosphorus deficiency before the root system contacts the deep placement zone. The greatest advantage of deep placement would seem to be in the case of nitrogen fertilizers and in areas of limited summer rainfall.

Residual Value of Phosphate Fertilizers

Phosphate fertilizers have long been used along with barnyard manure, because of the fact that manure is low in phosphorus as compared with nitrogen and potassium. Investigations by Alvin R. Midgley and David E. Dunklee in Vermont have re-emphasized the value of the practice. 558 YEARBOOK OF AGRICULTURE

They found that greater increases in crop yields result from applying the manure and phosphate together than when applied separately. When soluble phosphate fertilizers are added to soils they form new compounds that are largely insoluble in water and only partly available to plants. For that reason usually not more than 10 to 20 percent of the phosphorus in fertilizers is used by the crop to which it is applied. The remainder accumulates in the soil although much may be lost by erosion. A major problem in the efficient use of phosphate fertilizer in areas of intensive use, therefore, is to know the amount of phosphorus that accumulates, how it may be kept in forms most available to plants, and to what extent fertilizers may be needed by succeeding crops. There is some evidence to show that on soils used largely for such crops as citrus, potatoes, tobacco, and vegetables, a large accumulation of phosphorus has taken place. Early in 1944 soil scientists at Beltsville initiated a cooperative study with a number of State agricultural experiment stations for a compre- hensive study of the problem. Samples of soil collected from 425 fields in the important potato-producing areas of Alabama, Maine, Maryland, North Carolina, New Jersey, New York, and Virginia were analyzed for total and readily soluble phosphorus. Field experiments are also being conducted on selected farms for determining to what extent present changes in phosphate fertilization practices might be warranted. The results obtained thus far show that the plowed layer of soil in many of the older potato fields has accumulated as much as 800 pounds of phosphorus. There has also been a marked increase in readily soluble phosphorus, the amount depending on the chemical characteristics of the soil and on the farm management practices followed. The longer the soil has been farmed under intensive fertilization the greater has been the accumulation of phosphorus. Other evidence that the needs for phosphate fertilizers may be ma- terially affected by past fertilization is shown by the results of a 15-year experiment conducted at the Alabama Agricultural Experiment Station under the direction of Garth W. Volk and L\ E. Ensminger. Where the superphosphate (20-pcrcent P20n) application of 300 pounds an acre annually was reduced one-half after 5 years, the yield of cotton remained fairly stable at about 95 percent of the yield obtained where the 300- pound annual application was continued. Where the applications of phosphate were omitted after the first 5 years, the yields dropped to about 85 percent within 3 years and to 65 percent within 8 to 10 years. Even after 8 years, however, the yields were still about twice as high as where no phosphate had been applied during the entire 15-year period. The amounts of superphosphate used in this experiment were, of course, much higher than those usually applied in a legume-livestock or grain system of farming. Not only is the amount of residual phosphorus PHOSPHATE FERTILIZERS 559 less where the rates of fertiHzation are lower, but the availability of the residual phosphorus is also lower. Moreover, the very fine sandy loam soil on which the experiment was conducted was relatively low in clay. Soils high in clay, especially those high in reactive iron and aluminum compounds, usually show low residual effects from applied phosphate.

Needs and Potential Use

Although our farmers have used phosphate fertilizers for more than 75 years, the amounts now used are still inadequate from the standpoint of both efficient production and soil conser\^ation. That conclusion was reached in 1944 by the State Production Adjustment Committees of the State agricultural experiment stations working in cooperation with De- partment statisticians. Their estimates of the amount of fertilizers that could be used profitably under generally prosperous economic conditions are that for the whole country nearly four times more phosphate fertilizer is needed than was used before the war ( 1935-39) and more than twice as much as was used in 1944. The suggested increase over 1944 ranges from 25 percent in the Northeastern States to approximately 250 percent in the Corn Belt and Lake States. Although 8 Corn Belt and Lake States account for 47 percent of the total suggested increase (compared to only 34 percent for the 20 States of the Southeast, Appalachian, and Northeast regions), the increased needs in the latter States should not be minimized. Those States have large acreages of rolling and eroded soils that should be in permanent pasture or are now producing only scant and poor vegetation. Many field experiments have shown that phosphate and lime are the primary essen- tials in the establishment of a good vegetative cover of high quality legumes and grasses. Although in many of the States several times more phosphorus is added in fertilizers than is removed in crops, much of the consumption is concentrated in areas growing cash crops of high value per acre. In areas of concentrated use, phosphorus accumulation will no doubt lead to reduction in acre applications, but on large acreages of poor crop and pasture land increases in use are essential to good land use and conservation. Millions of acres of pastures in the Central States also need improvement through the use of phosphates. According to Donald B. Ibach of the Department, less than 6 percent of the permanent pasture in the humid region was fertilized in 1943. The need for improving the phosphate status of many soils is also emphasized by the advances made in the development of better adapted and higher yielding crop varieties. It is evident that improved crop varieties make a greater demand on the soil for phosphorus and other elements than do lower yielding varieties, and that a higher phosphate level is 560 YEARBOOK OF AGRICULTURE

necessary if full advantage is to be realized from crop improvement programs aimed at higher acre yields and greater efficiency in production. Moreover, it is a well-established fact that poor crop quality and lower feeding value are more often associated with phosphorus deficiencies in soils than with deficiencies of any other mineral element. This is particularly true of pastures where phosphates increase not only the phosphorus content of the forage, but also the proportion of the more desirable and nutritious plant species. Like other practices aimed at soil improvement and conservation and at greater efficiency in production, thé practice of phosphate fertilization must be considered a part of a unified program of good soil management. Its place in this program varies with the soil, with the crop grown, and with the system of farming. It is a practice, therefore, that must be adapted to the individual farm.

THE AUTHOR W. H. Pierre is professor of soils and head of the Department of Agronomy, Iowa State College, Arnes, Iowa.

ACKNOWLEDGMENT Dr. Charles A. Black, associate professor of soils, Iowa State College, assisted in the preparation of this article.

FOR FURTHER READING

Alway, F. J., and Nesom, G. H.: Effectiveness of Calcium Metaphosphate and Fused Rock Phosphate on Alfalfa^ Journal, American Society of Agronomy, volume 36, pages 73-88, 1944.

DeTurk, E. E.: The Problem of Phosphate Fertilizers^ Illinois Agricultural Experi- ment Station Bulletin 484, 1942. Maclntire, W. H., Winterberg, S. H., Hatcher, B. W., and Palmer, George: Fused Tricalcium Phosphate: Relation of Dei^ree of Defluorination to Fertilizer Value of Quenched Fusions of Rock Phosphate, Soil Science, volume 57, pages 425-442, 1944. Mehring, A. L.: Fertilizer Expenditures in Relation to Farm Income, Better Crops with Plant Food, volume 28, No. 8, pages 10-16, 47-48, 1944.

Midglcy, Alvin R., and Dunklce, David E. : The Availability to Plants of Phosphates Applied with Cattle Manure, Vermont Agricultural Experiment Station Bulletin 525, 1945. Neal, O. R. : Removal of Nutrients from the Soil by Crops and Erosion, Journal, American Society of Agronomy, volume 36, pages 601-607, 1944.

Rogers, H. T.: Plant Nutrient Losses by Erosion from a Corn, Wheat, Clover Rotation on Dunmore Silt Loam, Soil Science Society of America, Proceedings, (1941), volume 6, pages 263-271, 1942.

Smalley, H. R. : The Phosphate Problem is Complicated by Many Factors, Fertilizer Review, volume 19, No. 3, pages 8-12, 14, 1944. FARMERS in the 17 Western States look hope- their next year's supply of irrigation water. fully to snow-capped mountains like these for How deep this snow is, so shall the harvest be.

WATER FROM MOUNTAIN SNOW

Snow was first measured for its water content snow readily converted into water inches. in this country by Charles A. Mixer in Maine Now, a thousand or more snow surveyors face about 1900. He proved that snow—a frozen sudden storms, snowslides, and exposure to mass of air and water—varies too widely in measure each year the snowfall in the Western density for its depth alone to show how much States where irrigation farming is the back- water it will make. In 1908 Dr. J. E. Church bone of cropland agriculture. The surveys of the University of Nevada developed the usually start in December; measurements are Mt. Rose sampler, a forerunner of the Federal taken on or about the first of each month until sampler currently used. With it, a snow the spring thaws set in. The Soil Conservation sample can be taken and weighed and the Service is responsible for compiling the data. Water that flows from this white, frozen mass hydro-electric power companies, flood control is all-important, not only to farmers, but to agencies, and many others in the valleys below.

Natural lakes and specially constructed reser- and equitable distribution can be made of it voirs hold the melted snow so that controlled according to the amount nature has provided. 'í'^Hl :/f f

PROPERLY USED, TH_ SPELLS PROFIT OR LOSS

Nearly all of the water for irrigating some 24 the West comes from snow laid down on million acres of flat and fertile cropland of mountain watersheds during the winter months.

To find out what the snow water supply will be, This is one type of motorized equipment now hardy snow surveyors, traveling in pairs for used in a few areas of the country to make at safety, trek high into the mountain watersheds least a part of the long trip to the carefully se- to measure the water content of the snow pack. lected courses where snow samples are taken. Shelter cabins that have been built and main- Out on the course a snow sample is tal

The sample is immediately weighed on special of water in the snow. Measurements are noted hand scales with markings that show the inches and carefully recorded for future compilation. ■' ÍF'.W

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At the end of a day's work these weather-worthy the summer, are litesa\crs to the snow sur- cabins, supplied with food by pack train during veyors who often travel 25 miles or more a day.

Back in their shelter cabin, Surveyors R. A. checking and compiling of their data before Work and Max Wilson do some preliminary starting down the mountains to headquarters. Snow survey data thus obtained are sent to or short-wave radio and are immediately com- central offices by air mail, telegraph, telephone, piled and relayed to waiting water users.

At the end of winter, water experts represent- other data, forecast the stream flow and total ing local, State, and Federal agencies get to- amount of reservoir storage water that will be gether and, with the use of snow survey and available for the forthcoming irrigation season. ►H'^1 sâ-^

HERE ARE RAINDROPS driving into unpro- To determine the destructive effect of rain tected soil at 14 miles an hour. On the re- hitting unprotected soil, this relatively simple bound the splashes may carry as much as 40 machine was built (below). It can imitate percent soil, making a muddy mixture that rain of high or low intensity, large or more rain can and usually does wash away. small drops, and rain at controlled velocity. The soil sample at left, with coins to simulate inches away, is already flecked with mud. The protected surface, was photographed after 45 other sample was snapped 1 Vi hours later and seconds of rainfall. The white splash board, 6 shows what rain splash will do to open soil.

Here, in an open field, is proof that beating Only 1 inch of driving rain on erodible soil (be- raindrops, as well as surface flow, will grad- low) can splash and move as much as 170 tons ually carve away the good earth. Each pedes- an acre, mostly downhill. Soon—too soon— tal of soil is shaped to the rock or crust on top. the light-colored, unproducti\e soil shows up.

lati But not all this splashed soil moves downhill. The farmer who owned this land (below) was The finest and richest part can even be floated surprised to find a rail fence under about out of contour furrows, leaving only the coarser, 4 feet of soil at the base of a long field less fertile soil behind. Good cover crops slope. Good cover crops would have pre- will help prevent destructive splash erosion. vented this; contouring also would have helped. *

Other damage frequently caused by raindrop so that water goes off—not into—the soil, splash is that it seals the surface of the land This puddled soil is practically waterproof.

W. D. Ellison of the Soil Conservation Service lected from a bare field, a field with some shows samples, left to right, of the splash col- cover, and one that had good protective cover. A field of young corn (left) is a wide-open From an eye-high level the young oats crop target to splash erosion. Even when corn has (below) would look like a tight cover—but grown to more than knee height (right) many looking straight down at it we can readily raindrops can get through to bombard the soil. see how the driving rains might raise havoc. Beating rain has little chance of getting through water soaks into it. Of course, all land can- good vegetative cover like this combination of not be kept covered all the time. But by using clover and timothy. The splash process is what we know about protecting the land the prevented, the soil remains stable, and the rate of soil loss can definitely be retarded. THE CONTROL OF W A 1 IK l^ \u.il in the contour furrows prevent downgrade washing care of the land. On one Iowa farm (above, and promote good conservation farming. Be- left) heavy rains washed tons of soil across a low, left, erosion is ruining a pasture; right, cornfield. On another Iowa farm (above, right) vegetation-lined waterways help protect fields. Sedimentation between eroding farm fields and (right). Silting of reservoirs, like this one in the oceans is costing us dearly. An example South Carolina (below, left), is an outstanding is deep gully erosion in Wisconsin (above, left) example of sedimentation. It can be stopped from which a vast amount of sterile sand was in various ways: By gully-control structures washed over nearby bottomland cornfields (right), dams, and revetments to halt erosion. Tilled land on which dead residues from pre- after rain. Subsurface tillers (right), treaders vious crops are left resists erosion better than (below, left), and a two-row seeder behind a bare soil. A photographer removed residues treader are implements for stubble mulching. from a small area (above, left) in a field to An article on Ways to Till the Soil, by F. L. show how surface structure remained intact Duley and O. R. Mathews, begins on page 518. Soil maps help farmers put their land to its and production possibilities of each farm. Be- best permanent use. A soil surveyor (above, low, a Vermont farmer and county agent study left) maps slopes, a factor in good land use. their county maps; later, a farmer plans what to Right, men in the Department's Soil Survey plant and how to improve his land. Nearly Division put data on maps that show soil types a third of our farm land is now mapped.