Drilling Powdered Agricultural Limestone
RECENT ADDITIONS TO our knowledge of sour soils and of the requirements for successfully growing legumes, indicate that the practice of applying two to three tons of coarsely ground limestone per acre in order to grow certain legume crops bids fair to undergo modification. The well-recognized necessity of liming the soil long in advance of seeding the legume crop is evidence of the sluggishness with which the common agricultural limestone becomes effective. Though relatively insoluble, limestone becomes much more soluble when extremely finely divided. The wider recognition of this fact and the fuller understanding of the functions of limestone in growing legumes are contributed to the impending changes in our liming practice. It has been recently emphasized that the one of the beneficial effects of liming is due to the fact that it supplies the element calcium. Sour soils present a problem not only because of their sourness, but also because of their deficiency in this plant nutrient. This suggests that the lime of the Romans and other ancient agricultural nations may well be called “forgotten fertilizer.” Laboratory studies and field trials suggest the fitness of this newer cognomen, when they demonstrate that 300 pounds of finely ground limestone, drilled with the legume at seeding time, is enough to establish red clover, sweet clover, and other lime-loving plants on soils giving tests of two to three ton lime requirement. Farm trials in cooperation with the Missouri Agricultural Experiment Station, dating from 1926 and in gradually increasing number, suggest the wisdom of giving consideration to finely ground limestone as a fertilizer as well as a soil-sweetening agency.
Trials of limestone of different fineness have always indicated that the extremely fine material is the most important part of the limestone. Early experimental data by the Pennsylvania Agricultural Experiment Station and others following have pointed out that only when limestone is ground finely enough to pass a hundred-mesh sieve, does its effectiveness approach that of the soluble hydrated lime or quick lime. During the early development of the agricultural limestone market, the product offered was of widely varying fineness, for the problems of crushing the stone prohibited much else. It was a by-product. Now science points to the need for a specially prepared material whose effectiveness demands that it be very finely ground. At present with newer types of stone-crushing machinery which can produce at low cost a limestone ground finely enough to pass a hundred-mesh sieve, and with an entire rearrangement of our economic household in progress, we may well contemplate changes in our liming practice. We may well consider this powdered limestone ground finely enough for most of it to pass a hundred-mesh sieve as the standard product for general use. Its economy is an especially weighty argument for its adoption, since its rates of application are low enough to cut the cost of material to one-half, and the amount handled to one-tenth. Savings in cost of transportation, the convenience of handling a bagged product, and the reduction of the labor of application to a one-man basis instead of a community affair, will direct attention to this newer practice of using the powdered or hundred-mesh limestone.
The powdered limestone must be drilled into the soil. The old methods of spreading limestone broadcast will not suffice. By drilling it, certain areas within the soil are given a concentrated application from which the plant may take its supply of calcium, while adjacent soil areas are not subjected to the radical changes in chemical and biological conditions that would be created by complete neutralization of acidity. The fertilizer drill presents itself as the logical machine for drilling this fine limestone, and wherever it is available its use is recommended. The scarcity of fertilizer drills in some sections of the country raises the question, “Why not drill it with an ordinary grain drill?” The problem of distributing limestone powdered finely enough to pass a hundred-mesh sieve has recently been given some attention in the hope of learning something about its drillability, and the possibility of drilling it with a regular grain drill, as well as the fertilizer drill. The tests reported in this paper indicate that the ordinary grain drill may help solve the distribution problem created by the change from coarsely to finely ground limestone in agricultural practice.
A test was run on the fertilizer distributing part of a superior combined grain and fertilizer drill equipped with the finger or star-wheel type of fertilizer feed. The stone, furnished by the Columbia Quarry Company and ground by a crusher using air draft to separate the fine material had the sieve analysis as given in table 1. The method of testing consisted in filling the fertilizer hopper with the powdered stone, hauling the drill with a tractor for a distance equivalent to one-tenth acre, collecting the stone delivered by each delivery opening in a separate container attached just below the drill box, weighing this, and making calculations on the acre basis. The distributing fingers were set at the high speed (gear 6), and the test run with different gate openings. The results are shown in table 2, which compares the amount of stone delivered to the amounts of fertilizer that would be delivered according to the sowing table furnished with the drill. The weight of stone delivered in the test varied from 70 to 81% or an average of 75% of that given in the fertilizer sowing table for the different gate settings. It should be borne in mind, however, that the sowing tables for fertilizer furnished with a drill, can be only approximately at best as different fertilizers vary considerably in weight and drillability. In drilling finely ground limestone through a drill, the rate of application can be determined approximately by noting the acreage covered, as indicated by the drill surveyor, while a known amount of material is drilled.
Attempts were made to drill fine limestone through ordinary grain drills, both of the internal double-run type and of the fluted-roll type of force feed. It was found that this fine, fluffy material would not feed through. Tests were next run on a McCormick-Deering drill equipped with agitators. The drill was of the double-rod type, with three-finger spider stirrers on the lower rod and with two-arm paddle throats beneath the fluted rolls open, and with them closed. The results are shown in table 2. The tests indicate that this drill can be easily made to deliver 200 to 350 pounds of this powdery material by setting it the same as for 6 to 12 pecks of oats per acre. Since it would be difficult to get agitators for some of the older grain drills, a homemade agitator was made and tested in a Superior grain drill, this particular drill being equipped with the internal double-run type of force feed. The agitator was made essentially as follows: A one-inch gas pipe was run lengthwise on top of the drill hopper and mounted in improvised wood bearings. The outer end of this pipe was equipped with a crank so that the pipe could be oscillated back and forth in its bearings. Vertical arms were attached to this pipe and allowed to project downward beside each of the drill runs in the bottom of the hopper. The first design of this agitator failed to work because these vertical arms did not work in close enough proximity to the wheels which carry the seed through the runs.
Table 1
Fineness of Powdered Limestone Used
| Mesh of test screen | Per cent caught on test screen | Per cent passing through test screen |
| 40 | 2.0 | 98.0 |
| 60 | 0.3 | 97.3 |
| 80 | 1.1 | 96.6 |
| 100 | 1.6 | 95.0 |
| 200 | 16.8 | 78.2 |
| 300 | 46.2 | 32.0 |
| Through | 32.1 |
During the test, the agitator was worked back and forth by hand twice for each revolution of the ground wheel of the drill. Very little effort is required to operate the agitator, and doubtless wood blocks bolted to wheel spokes could be made to operate it. The results of the test are given in table 2, and indicate that the internal double-run type of force feed will drill fine limestone when a suitable agitator is used in the drill hopper.
With coarse limestone for our past experience, the opinion naturally arises that the drilling of limestone through the regular seed runs of grain drills will cause excessive wear and consequent inaccuracies in subsequent drilling of grain and seed. The trials reported here did not include tests on this point, though such are contemplated. The finely ground limestone, however, is widely different from the commonly used, coarsely ground agricultural limestone. This finer material is impalpably fine, and resembles flour more than it does the common agricultural limestone. Its fluffy nature and ease of agitation through failure to pack as does the coarser material suggests that any wear on the drill by such material would be relatively small. Further, it is of neutral reaction—neither acid nor alkaline—with no corroding effect on metal. While there may be some wear, the seriousness of this remains to be established by experiment or experience. In all of the trials there was a surprising uniformity in the rate of delivery per spout, and calculations based on the lowest spout rate and the highest spout rate were never widely different. The variations were certainly within the limits of accuracy commonly demanded in such farm operations.
The results of these trials as given in table 2 indicate that present drilling machinery will distribute the finely powdered limestone effectively and in rates that easily accommodate the amounts suggested by soil experiments with this fine limestone.