Rock phosphate/sulfur fertilizer composition

A fertilizer composition which provides high soil mobility of phosphorus comprising an intimate mixture of 1 to 100 micron size ground rock phosphate and sufficient sulfur of 1 to 5 micron size in a weight ratio of P.sub.2 O.sub.5 :S in the range of 1:0.5 to 1:2.0 to convert said rock phosphate to a soluble form of phosphorus when said sulfur is oxidized by soil bacteria to sulfuric acid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to fertilizer compositions which provide high 
phosphorus nutrient value to soil, in particular the invention relates to 
a composition of rock phosphate and sulfur. 
Related Art 
It has been long known that a mixture of rock phosphate and sulfur can 
serve as a source of phosphorus to soil in a form usable by plants. 
Furthermore, it is well known in agriculture to use ground rock phosphate 
(40-100 microns) on acid soil as a source of P.sub.2 O.sub.5, with normal 
applications being 1500 to 2000 pounds per acre every three years. 
U.S. Pat. No. 1,777,908 to Bordero discloses that mixtures of phosphate and 
sulfur in the form of finely divided or impalpable powder were used as 
fertilizers, however, to improve the state of division of the sulfur 
relative to the phosphate, the sulfur was sublimed onto the rock 
phosphate. 
The mixture of phosphate and sulfur for soil enrichment was only marginally 
useful and with the development of the superphosphates it has not been a 
viable fertilizer composition or method of soil enrichment for years. 
The problem which the farmer faces is obtaining high phosphate levels 
during a growing season (about 150 days). This has been met by a number of 
materials which are considered to be equal as sources of phosphate by the 
American Society of Agronomy, namely, normal super phosphate (0-18-0), 
triple super phosphate (0-46-0), phosphoric acid (0-54-0), diammonium 
phosphate (18-46-0), and monoammonium phosphate (11-52-0). 
These materials are all manufactured products, whereas the rock phosphate 
is a ground mineral, hence, a substantially cheaper source of phosphate if 
it can be utilized to provide a high soil phosphate content. In a 
sufficiently acid soil the rock phosphate is useable, however, many soils 
are not acid and some treatments such as limestone application raise the 
pH to the point where rock phosphate is not practical. 
In the present invention the utilization of sulfur of a very small and 
particular particle size is the central feature which allows the phosphate 
content of the rock phosphate to be utilized as a phosphate source during 
growing season in place of other phosphate sources. 
The great advantage of the present improved mixture is the elimination of 
sulfuric acid plants, reduced air pollution, reduced cost, and reduced 
hazardous waste material at phosphate mines when the present compositions 
are substituted for superphosphate. An incidental benefit is an increase 
in calcium mobility in the soils with the present compositions. The 
present compositions have also been found to perform at substantially the 
same level for phosphorus availability as the super-phosphates. 
SUMMARY OF THE INVENTION 
One aspect of the present invention is a fertilizer composition comprising 
an intimate mixture of ground rock phosphate containing phosphorus in a 
substantially water insoluble form and having a particle size in the range 
of 1 to 100 microns and sulfur having a particle size in the range of 1 to 
5 microns, said rock phosphate, calculated on the basis of P.sub.2 
O.sub.5, and sulfur being present in the weight ratio of P.sub.2 O.sub.5 
:S in the range of 1:0.5 to 1:2.0. The sulfur is generally present in an 
amount of at least 60% of the stoichiometric amount calculated as H.sub.2 
SO.sub.4 to convert the phosphate to a soluble phosphorus compound. An 
excess of sulfur may be present. Preferably sulfur content is about 70 to 
200% of the stoichiometric amount calculated as H.sub.2 SO.sub.4 based on 
the phosphorus present in the rock phosphate. 
Another aspect of the present invention is a method of fertilization to 
provide improved soil phosphorus mobility comprising applying from about 
100 to 800 pounds per acre of a composition as described above to soil. 
The present compositions may be a dry mix or a water suspension thereof. 
The very fine particle size allows for the preparation and storage of 
solutions which are relatively stable, however, normal viscosifying 
materials such as hydroxyethyl cellulose and clays may be added to improve 
suspension stability. 
In addition to the phosphate and sulfur other components such as urea, 
ammonium nitrate, potash, trace metals and the like which may be desirable 
nutrients for a crop may be added to the suspensions or dry mix as 
appropriate. 
Rock phosphate is a mineral, which is generally given the chemical 
structure Ca.sub.3 (PO.sub.4).sub.2, whitlockite, which is essentially 
insoluble in water, although the phosphorus present is reported as P.sub.2 
O.sub.5, as is conventional in the art. The sulfur employed is elemental 
sulfur and may be ordinary rhombic sulfur, although any form in the 
appropriate particle size may be use.

DETAILED DESCRIPTION OF THE INVENTION 
In order for elemental sulfur to be reactive in soils, it must first be 
oxidized to sulfuric acid. This is accomplished largely by sulfur 
oxidizing bacteria of the genus Thiobacillus. In the presence of oxygen 
and water, the bacteria oxidize the sulfur to sulfuric acid which reacts 
with minerals and other insoluble materials, leading to nutrient 
mobilization. Thus, the oxidation increases the quantity of soluble 
phosphate, potassium, calcium, manganese, aluminum, magnesium, iron and 
zinc. Although other groups of microorganisms are responsible for sulfur 
oxidation, by far the most important are the chemautrotrophic bacteria, 
namely: Thiobacillus thiooxidans and Thiobacillus thioparus. Thiobacilli 
are unique soil bacteria which can obtain their energy requirements for 
growth from the oxidation of inorganic sulfur compounds. Differences 
between the members of Thiobacilli include the degree of acid tolerance 
and the range of inorganic sulfur compounds which they are capable of 
oxidizing. T. thiooxidans and T.thioparus differ in one physicological 
characteristic and requirement, that being optimum pH range. The former 
thrives under extremely acid conditions with optimum pH ranging from 
2-3.5, while the latter prefers a pH of around 7.0. Other than this, both 
species are aerobic, motile, autotrophic, gram negative, short rods. 
In the chemoautotrophic bacteria, the oxidative process is the means by 
which energy is provided. Sulfates are formed as by-products of their 
metabolism. These sulfates are released into the soil as sulfuric acid. 
The result is a lowering of the pH. 
In biological oxidation of sulfur, the general reactions that take place in 
the soil are as follows: 
EQU 2S+30.sub.2 .fwdarw.2SO.sub.3 
EQU SO.sub.3 +H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 
The sulfuric acid produced may react with tri calcium orthophosphate (rock 
phosphate) also called tri calcium phosphate, in the following way: 
EQU CA.sub.3 (PO.sub.4).sub.2 +3H.sub.2 SO.sub.4 .fwdarw.3CaSO.sub.4 +2H.sub.3 
PO.sub.4 
The sulfuric acid formed reacts with insoluble rock phosphate leading to 
nutrient mobilization. The soluble form of phosphate may be phosphoric 
acid. This is a proposed mechanism and is not intended to limit the 
invention as described. The soluble phosphoric acid is a ready source of 
phosphorus for plants. 
It has been found that the solubility of the phosphate in the rock 
phosphate (tri calcium orthophosphte) is directly proportional to the 
oxidation rate of the sulfur; and the oxidation rate of the sulfur is 
directly proportional to particle size. Sulfur of the size in the range of 
1 to 5 microns as employed in the present composition produced lower soil 
pH than a 40.+-.20 micron sulfur, e.g., pH 2.4 after 189 days at 5000 
pounds per acre for the claimed sulfur and 3.8 for the 40.+-.20 micron 
sulfur sandy acid soil. Thus indicating a more rapid reaction with soil 
bacteria for the present sulfur component than for a similar fine sulfur. 
Another observation on these two micro sulfurs was that in a high lime clay 
soil, the present sulfur had a greater oxidation to SO.sub.4 after 75 days 
at a rate of 5000 pounds per acre than the 40.+-.20 micron sulfur at 
10,000 pounds per acre and 50% more oxidation than the 40.+-.20 micron 
sulfur at the same rate of application. 
A study was carried out comparing the present composition with super 
phosphate. A P deficient Cecil sandy soil (Typic paleudult) was used for 
both studies. 
The composition according to the present invention was a blend of rock 
phosphate, particle size 20-40 microns (passes through 350 mesh), 1 part 
by weight and elemental sulfur, particle size 1-2 microns, 1.06 parts by 
weight to give a 0-18-0-14 (N-P.sub.2 O.sub.5 -K.sub.2 O-S) fertilizer. 
The normal superphosphate was 0-18-0-12 analysis (sulfur was added as 
sulfuric acid during manufacture in the conventional manner.) 
Plots were treated with fertilizer specified in the following tables and 
one month later planted with NK 1003 wheat at the rate of 1 bu/Ac. All 
plots were top dressed with 60 pounds of N as NH.sub.4 NO.sub.3, three and 
one-half months thereafter. The test results are reported in TABLES I, II 
and III. In TABLE I the yield (bushels per acre), test weight (pounds per 
bushel) and test area, (grams per 1.5 square feet) are reported. In TABLES 
II and III, whole plant and tissue samples were dried at 70.degree. C., 
ground to pass 20 mesh sieve for analysis by standard procedures. 
The fertilizers compositions were applied as water suspensions. As can be 
seen from the TABLES, the present compositions give slightly better yields 
than conventional super phosphate-sulfur compositions. 
The composition of the present invention produced the greatest increase of 
P in plant tissue during the early growth period, when phosphorus content 
is the greatest problem. 
TABLE I 
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TREATMENT YIELD TEST WT. TEST A 
EXAMPLE BU/A lb./Bu g/1.5' 
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1. Control 37.0 52.9 48.2 
2. 200 lbs/A Invention 
59.6 53.7 60.3 
(36 lbs P205-38 lbs/A S) 
3. 400 lbs/A Invention 
57.3 53.2 63.2 
(72 lbs P205-76 lbs/A S) 
4. 600 lbs/A Invention 
59.2 53.8 62/7 
(108 lbs P205-114 lbs/A S) 
5. 400 lbs/A Super Phosphate 
56.0 52.8 57.3 
(80 lbs P205-48 lbs/A S) 
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TABLE II 
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Phosphorus Content (%) 
1st 4th 5th 5th 
MONTH AFTER PLANTING 
Whole Whole Flag Whole 
Example Plant Plant Leaf Plant 
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1. Control .19 .22 .21 .16 
2. 200 lbs/A Invention 
.24 .28 .23 .17 
(36 lbs P205-38 lbs/A S) 
3. 400 lbs/A Invention 
.30 .29 .23 .17 
(72 lbs P205-76 lbs/A S) 
4. 600 lbs/A Invention 
.28 .29 .23 17 
(108 lbs P205-114 lbs/A S) 
5. 400 lbs/A Super 
.20 .27 .23 .15 
(80 lbs P205-48 lbs/A S) 
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TABLE III 
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Sulfur Content (%) 
4th 5th 5th 
MONTH AFTER PLANTING 
Whole Flag Whole 
Example Plant Leaf Plant 
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1. Control 0.20 0.24 0.08 
2. 200 lbs/A Invention 
.25 .27 .13 
(36 lbs P205-38 lbs/A S) 
3. 400 lbs/A Invention 
.25 .32 .13 
(72 lbs P205-76 lbs/A S) 
4. 600 lbs/A Invention 
.29 .34 .14 
(108 lbs P205-114 lbs/A S) 
5. 400 lbs/A Super .25 .26 .12 
(80 lbs P205-48 lbs/A S) 
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