Chemical Reactions in Fertilizer Mixtures DECOMPOSITION OF DOLOMITE' IMESTONE and dolomite KENNETH c- BEESON A N D WILLIAM ROSS causing loss of available PzO5 If the temperature of the mixhave long been used as Fertilizer Research Division, ture is maintained above norBureau of Chemistry and Soils, conditioning agents infermal, however, tricalcium phostilizer mixtures and more reWashington, D. C. phate is formed as a result of cently as correctives of their s e c o n d a r y reactions involving potential acidity (16). The difdiammonium phosphate, calcium sulfate, and dicalcium phosference in the relative effects of added limestone and dolomite phate. The loss of available PZOS that occurs under these on the availability of the PzO6 in superphosphate and superconditions varies with the time and temperature of storage, the phosphate mixtures has been studied by MacIntire and his codegree of ammoniation, and the concentration of PZOs. workers (19-16). They showed that, although limestone deWith a view to finding a suitable laboratory method far creased the citrate solubility of the superphosphate in storage, measuring the availability of magnesia to plants, Smith (20, no appreciable loss of available PZOsoccurred a t ordinary tem21) determined the solubility of the magnesia in dolomite and peratures when the limestone was replaced by high-grade doloother minerals in various solvents, including 4 per cent citric , mite. Both materials react with superphosphate in the presacid adjusted to a pH of 4.0 with ammonia. The determinaence of moisture to evolve carbon dioxide, but the products of tions with this acid ammonium citrate solution were made the reaction are different. Thus it was found that, although both on the dolomite alone and in association with other mathe reactions with dolomite proceed only to the formation of terials in a commercial mixture. The proportion of the total the available diphosphates of calcium and magnesium, more magnesia recovered was less for the dolomite in the mixture basic compounds may be formed in mixtures with limestone. than when the same dolomite was treated separately. The reactions of dolomite with different components of ferFor many years it has been customary to use materials in tilizer mixtures have also been reported in several of the earfertilizer mixtures that contain magnesium as well as other lier papers of this series as well as in the publications of other minor plant food elements as incidental constituents, and the investigators. Beeson and Ross (4, 19) found that monoamneed for including magnesium in fertilizers to provide for demonium phosphate reacts with dolomite in the presence of ficiencies of this element in the soil was therefore not generally moisture to evolve carbon dioxide and form phosphates of calrecognized until use was made of concentrated fertilizers precium, magnesium, and ammonium, but no loss of ammonia or pared from relatively pure materials (6, s). It has now bedecrease in the availability of the phosphoric acid occurred at come a practice of many fertilieer mixers to include a quantity ordinary temperatures, regardless of the proportions of the reof a water-soluble magnesium compound, such as calcined acting materials. Reversion of phosphoric acid and even loss kieserite, in the formulation of their mixtures. Dolomite difof ammonia may occur, however, when monoammonium phosfers from kieserite and other sources of water-soluble magnesia phate is stored with an excess of dolomite a t temperatures in that it is water insoluble. If it can be shown, however, that above normal. dolomite through reaction with other constituents of the fertiDolomite also reacts with diammonium phosphate, but loss lizer mixture can supply a considerable portion of the required of ammonia, as well as of carbon dioxide, occurs a t ordinary soluble magnesia as well as a residual supply, it should logitemperatures in mixtures of this kind (3). cally replace the higher priced sources of magnesia in part a t Keenen and Morgan (11) showed that two reactions occur least. during storage of ammoniatedmixed fertilizers containingdoloThe present investigation was undertaken, therefore, to demite, The dolomite first reacts with the monoammonium termine the rate and extent to which the reactions that dolophosphate, converting a portion of it to diammonium phosmite undergoes in fertilizer mixtures affect the solubility of the phate and substantially reducing water-soluble Pz06 but not magnesia. The work differs from that of Keenen and Morgan 1 For the earlier papers in this series, see literature citations 8, 3, 4,18.19.
L
TABLEI. FORMULAS OF FERTILIZER MIXTURES,IN POWDS -Nonammoniated 7 3 - 9 - 5 7 886 886 886
.. .. .. .. .. ..43.. .. .. 43 43
Superphosphate (20.33% Pior) Double superphosphate (46.66% PzOd Ammonia* Urea (46 6'3' N) Ammoni;mosulfate (20.85 N, Sodium nitrate (16.35% Potassium chloride (56.5% Kz0) Dolomiteb Filler (sand)
144 61 177 95 594
8
144 61 177 380 309
---
Total b
144 61 177 190 499
2000 2000 2000
Mixtures-6-18-10-
c 7 -
til,
.86. . .+7l, . . . 7% .86. 86
288 122 354 95 286
288 122 354 190 190
288 122 314 380
886
-
Ammoniated Mixtures 3-9-5 r--6-18-10886 886 $$O.. $+O..
.24.3 . . . . . 24.3 . . . . . .24.3 ... .144. . . . .144 . . . . . 144 ....
61 177 96 612.7
61
$: :517.7
61 E%
48.5
.....
288 122 354 95 322.5
48.5
+70..
48.5
288''
288..
122 354 190 227.5
122 a54 380 37.5
327.7 ... - - ----2000.0 2000.0 2000.0 2000.0 2000.0 2000.0
2000 2000 2000
2.74% on basis of superphosphate and 6.3% on basis of double superphosphate. Ground to pass a 30-mesh sleve; eqmvalent to 95.5% CaCOa; contained 17.1% MgO, 4
1176
OCTOBER, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
(11)in that the latter investigation was limited to a study of the rate and extent to which dolomite reactions in ammoniated mixtures affect the availability of the P206 present. A study was also made of the comparative rate of dolomite decomposition in ammoniated and nonammoniated mixtures of a single- and double-strength grade.
Materials and Composition of Mixtures
1177
cent. None of the other materials apart from the dolomite contained any appreciable quantity of magnesia. The dolomite in the mixtures was therefore the chief source of all water-soluble and citrate-soluble magnesia found in the tests. The determinations of the water-soluble and citrate-soluble magnesia were not made as measures of the availability of the magnesia but as measures of the change in the solubility of the magnesia in the dolomite. That the magnesia in the dolomite used in these tests is somewhat soluble in neutral ammonium citrate solution is shown in Figure 1. It is, however, sufficiently insoluble for any increase in the solubility of its magnesium content through
The formulas of the mixtures used in this investigation are given in Table I. The materials used in the mixtures were of commercial grade. The superphosphate and double superphosphate were made from Florida land-pebble rock. The dolomite was from the same source as that used in previous work (4) but was ground to pass a 30mesh (0.500-mm.) sieve. It had a magnesia content ol 17.1 per cent and a calcium carbonate !+ equivalent of 95.2. The sample had a relatively 30 low solubility in acid ammonium citrate solution as shown by a comparison of its solubility in this zs reagent with that of samples 12A and l5B used , by Smith (31) in his study of the relative solu'b 20 bil ities of various magnesic limestones. The mixtures comprised two general groups-a n o n a m m o n i a t e d and an ammoniated group. 3 ,J Urea was used in the first group as a source of nitrogen in place of the ammonia added to the second group. Each group was divided into two sub10 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LO groups consisting of an ordinary mixture (3-9-5) Dolomite per 100 cc. of neutral Rmmoniunt frate Jolufion - grams and a high-analysis mixture (6-18-10). SuperOF SAMPLE SIZEON SOLUBILITY OF MAGNESIA PRESENT FIGURE 1. EFFECT phosphate was used as the source of phosphorus in IN DOLOMITE IN NEUTRAL AMMONIUM CITRATE SOLUTION the low-analysis mixtures, and double superphosphate was used in the high-analysis mixtures. chemical reactions in the fertilizer mixture to be readily deterThe physiological acidity (17) of the fertilizer materials in mined. It has been shown (9, 20) that such compounds as the 3-9-5 mixture was equivalent to 190 pounds of the dolotrimagnesium phosphate and magnesium ammonium phosmite ;eer ton, and that of the fertilizer materials in the highphate are entirely soluble in neutral ammonium citrate soluanalysis mixture to double this amount. The amount of dolotion when treated with this solution according to the procedure mite actually used in this study varied from 95 to 380 prescribed for determining citrate-insoluble PzOb( I ) . pounds per ton, Separate portions of each mixture were stored a t temperatures of 30 O , 45 ",and 60' C. @eo,113O , and Reaction of Dolomite with Superphosphate and these portions were sampled a t regular intervals. 140" F,), A rapid loss of moisture from the bottles containing the mixstudy was made Of the factors affecting the reactions Of tures was prevented by the insertion of long capillary tubes in in dolomite-superphosphate mixtures, preliminary to tha?rubber stoppers. It was thus possible to maintain a moisture of about7per cent throughout the time of storage. the work with mixtures* According to MacIntire and Shaw (14)dolomite reacts with superphosphate a t ordinary temperatures to form the diphosphates of calcium Analysis and magnesium represented in Equation ': Each sample withdrawn from the mixtures in storage was air-dried, ground to pass a 30-mesh sieve, and analyzed for 2Ca(HzP04)z + CaaOs.MgCOs = 3CaHP04 + carbon dioxide, for nitrogen, and for total water-soluble, and MgHP04 2Hz0 2CO2 (1) citrate-insoluble Pz06( I ) . In addition, the magnesia in the Dimagnesium phosphate is only slightly soluble in water, water extract obtained in the determination of water-soluble P2O5 and that in the ammonium citrate extract obtained in the but in contact with the acid salt, monocalcium phosphate, its determination of citrate-insoluble PzOswere determined and solubility should be increased. It would be expected also that as the reaction between the monocalcium phosphate and the classified, respectively, as water-soluble and citrate-soluble dolomite proceeded, the water-soluble magnesia would demagnesia. Total P2O5 was determined in each sample as a basis for expressing the values found for water-soluble and crease while the citrate-insoluble magnesia would increase, This was confirmed by experiment. Thus, in mixtures of 1 citrate-soluble magnesia in terms of the original mixture. A series of tests showed that, although the water-soluble part of
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WATER-SOLUBLE PzOsIN FERTILIZER 190 POUNDSOF
