Agglomeration of gypsum, limestone, or gypsum-limestone mix

A process for making a granular fertilizer product for use as a source of calcium secondary plant nutrient, to produce agglomerated granules of sufficient strength, proper size, and adequate weight to be suitable for mechanical dispensing and application to and into the soil, from fine particles of a calcium source that is characterized by poor solubility in water and by slow conversion in the soil to a form in which it is useful to plant life growing in the soil, said calcium source being selected from a mixture of fine particles of limestone and fine particles of gypsum, said particles of limestone having an individual particle-size distribution of about 15 to 25 percent plus 20 mesh, about 10 to 25 percent minus 20 plus 40 mesh, about 15 to 30 percent minus 40 plus 100 mesh, about 10 to 25 percent minus 100 plus 325 mesh, and about 15 to 25 percent minus 325 mesh, U.S. Standard Testing Sieve, said particles of gypsum having an individual particle-size distribution of about 0.1 to about 2 percent minus 20 plus 40 mesh, about 10 to 20 percent minus 40 plus 100 mesh, about 30 to 50 percent minus 100 plus 325 mesh, and about 30 to 50 percent minus 325 mesh, U.S. Standard Testing Sieve, the relative proportions of said gypsum and said limestone being from about 80 parts to about 95 parts by weight of said gypsum and from about 20 parts to about 5 parts by weight of said limestone, said gypsum selected from the group consisting of sulfogypsum, mineral gypsum, and mixtures thereof, said particles being present in an amount to comprise from about 90 percent to about 96 percent by weight of said granules, dry basis, and comprising:

• applying to said particles a small but effective amount of a binder so as to bind said fine particles together in the form of granular agglomerates, said binder after distribution of the granules in the soil being capable of permitting the utilization of the secondary plant nutrient calcium values of said particles;
• drying and screening the agglomerated granules to produce granular agglomerated product having sizes in the range from between about 60 percent to about 90 percent minus 6 plus 16 mesh, Tyler Standard Screen Scale;
• said binder being selected from the group consisting of a water-soluble metal or ammonium salt of lignosulfonic acid and mixtures thereof, and applied so that the average crushing strength of a sample of said granules, selected to have a size range of minus 7 plus 8 tyler Standard Screen Scale, is at least about three pounds per granule.