Size Matters (Particle, That Is)

U.S. Standard sieves Nos. 10 (2 mm) and 20 (841 µm). Sieves are used to determine particle separate size fractions for material quality and mechanical properties evaluation and classification.

Granted, the last two AJ dispatches were  heavy hitting on the technical side of things. And apologies to all who skipped reading them which I’m sure is most of you. Now, how about sharing something a little more down to earth for a change? What I had in mind was a recent encounter with that humble, often neglected but ever so useful soil conditioner, lime. Comes a time in the course of one’s daily rounds when something happens to recall a basic truth, self-evident, incontestable it would seem, yet worthy of relearning. This moment came to me back in November while applying lime to a field that had, according to a certain soil test, gone quite acid with a measured pH of 5.3. The same test came with a recommendation to apply 1 ton/acre agricultural limestone to move the pH back up to where it should be, a mildly acidic 6.0 to 6.2. The telltale moment struck when I realized that the hopper on my Gandy drop spreader was running out much faster than it should have, which was set to run the last calibration I had marked for this particular brand of pelletized lime. Something wasn’t right, so I stopped to investigate.

I didn’t notice anything amiss with the lime pellets or applicator. No complicated technology here, just your basic pellets dropping from a slot at the bottom of a hopper in a stream in accordance with Newton’s second law at a rate predetermined by manual calibration hand-written in my field logbook. I know it’s considered old school, but I still carry a logbook everywhere I go even though I could easily store the notes in a Google Drive file and access them from my mobile phone, provided there’s cellular service which is, sadly, not guaranteed anywhere around here. I have field books dating back decades, inscribed with everything from parts serial numbers, seeding and ground application equipment calibrations, planting and harvesting dates, to measured voltages from long-ago surplussed batteries and alternators, both at work and at home. So, it would take no small effort to transcribe every note, and it wouldn’t preserve the patina of sweat, dirt and oily grime that embellishes a well-paged field logbook.

Anyway, to get back on point, the applicator checked out OK, but I still suspected something was wrong because the coating of pellets was thicker than normal. I grabbed a sample of lime pellets from the hopper to compare with others back in the cargo container where lime and granular fertilizer are stockpiled. The pellets looked identical, but I noticed something else. In the stockpile there were mixed two brands of lime with similar packaging. We’ll call them brand x and brand y; both are commonly available pelletized lime. At first, I didn’t pay attention to this because lime is lime, right?

Wrong! I put a sample of each brand into separate plastic beakers for comparison. Here’s what they looked like:


The difference was like night and day! Note the particle sizes of brand x pellets on the left are finer than those of brand y on the right. Because brand x was finer, it was dropping out at a faster rate than brand y, the product for which the calibration was gated. That would explain the thicker coating of lime pellets on the ground. How much was the application rate off? After doing a second calibration with brand x at the same gate setting, it appeared the hopper was delivering lime pellets at a rate over two-fold greater than it should have.

In other words, I was applying 2+ ton/acre of pelletized lime.

This was a teaching moment, I thought. As a student in Ag Mech 418, I learned about the importance of machinery calibration. This upper-division engineering course drilled deep into the nitty-gritty of ground application equipment calibration: physics, math, chemistry et al., in a unified manner that has stood the test of time for me. It also instilled the need for calibrating virtually all instruments involved in metering and metrology regardless of application. Therefore, I should have known about mixing lime products, but it caught me off guard.  Since information about particle size and lime quality must, by government fiat, be printed legibly on the product packaging, I compared the two sieve analyses. Here’s how they stacked up:

Note that both labels list seven nominal sieve sizes from 8 mesh to 100 mesh. In material mechanical properties analysis, particle size fractions are determined using U.S. Standard Series or Tyler Equivalent sieves. Standard U.S. sieves are designated by number, or screen size opening as printed on the packaging labels above. The mesh number indicates the diameter of the opening, or rather, the number of openings per unit linear measure: the larger the mesh number, the smaller the diameter opening. Also, note that while the number of screens and nominal diameters are the same, the amount of material passing is not. We can observe the amount of material passing screen sizes 8 and 10 is the same for both brands, 100%. But the amount passing 20, 40, 50, 60, and 100 mesh varies. For brand x, 70% passes 100 mesh, while only 50% passes for brand y. Thus, brand x is shown to have finer distribution of particle separates compared to brand y. The label states that the percentage passing the designated screen sizes was tested before pelletizing. In this sense, it’s no different from the sieve analysis of any aglime material. But the end result, after agglomeration, is that brand x is still finer than brand y.

Every U.S. state has a procedure for evaluating the quality of aglime, whether pelletized or not, including sieve analysis testing fineness of grind. This information, along with calcium carbonate equivalence (CCE) is used to calculate effective neutralizing value (ENV). The ENV provides a basis for lime recommendation and adjusting lime application rate, where appropriate. Here, I won’t go into how ENV factors into calibrating lime spreaders but interested readers are free to review Section 3.5 of Technical Note 3 Soil Acidity and Liming for more information.

So, the source of my calibration problem could be traced back to substituting one pelletized lime product for another that wasn’t exactly the same in terms of particle size separates. The lesson learned (or re-learned in this case) is simple: don’t mix products, and never assume two products are the same just because they’re called the same thing in the trade. Product formulation varies depending on many upstream factors, some of which may be hidden from the end user. If you’re using Acme brand granular fertilizer and switch to Co-Op brand which is a little cheaper this year, you need to re-calibrate your applicator for Co-Op even if it’s the same formulation. The two brands may look alike, but chances are there’s a measurable difference in handling and flowability that will impact application. The same applies to any liquid product where properties like concentration, % active ingredient, viscosity, density, and combining ability affect ground application. Always compare product formulations of different origin even if they’re deemed  “identical” by regulatory entities. And check the calibration of ground application equipment annually or whenever switching out products. Impellers, pumps, gates, and nozzle tips all get worn out at some point and should be replaced on a regular schedule. Even the wire mesh openings in a U.S. Standard sieve get bigger over time from constant abrasion.

In the end my calibration mishap didn’t cost much and probably won’t change the soil pH drastically since it only affected a relatively small area. Disking and field conditioning should further dilute the carbonates in the affected zone. But for broad-acre operations, poorly calibrated equipment can yield a substantial waste of money, damage crops and risk polluting our soil and water. In this view, basic human-machine interactions like calibration still contribute tremendously to agricultural efficiency, productivity, and environmental protection. Can your satellite imagery do all that?



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