Right before Christmas just past, DS asked me to pick up a few soil samples he’d just taken. He runs a small organic dairy farm one county away from me. I picked the samples up, brought them home, air-dried them, then screened them, using a wide-mouthed plastic jar in whose lid I had drilled a couple dozen quarter-inch holes. I go the extra mile in efforts to remove impurities like parts of bugs, worms, roots, leaves, tiny pebbles, etc. Soil labs just want to analyze for soil nutrients, not lots of other tag-along debris. (These impurities, except for the pebbles, tend to inflate most readings because of biological concentration.) After shaking the raw samples in the jar, I bag up the screenings in sandwich bags, then send them to the lab (usually Dairy One in Ithaca). DS’s lab results arrived in my inbox on Jan. 5. I mailed DS copies of his soil test results. On Jan. 21, he wrote me back, asking a question about base saturation percentages (BSP).
BSP, expressed simply, is the proportion of a soil particle’s surface that is occupied by positively charged soft metals. This grouping includes calcium (Ca), potassium (K), magnesium (Mg) and sodium (Na). They are in ionized form, meaning they have all lost one or two electrons. Losing electrons is what makes them positively charged, and we call these positive-charged particles cations. Negatively charged particles, ones acquiring electrons, are called anions. Let’s presume that the soil particle surface is round, then compare it to the surface of a volleyball. A volleyball’s surface is broken down into 18 slightly curving rectangles. Imagine that instead of 18 surface subdivisions, we have 100; they could be divvied up amongst the four types of soft metal cations. The term “cation” is also fairly interchangeable with “base.”
In addition to the above soft metals (Ca, K, Mg, Na), there’s another cation on the scene: hydrogen (H). So, we’ve got four soft metal cations, as well as H cations competing for those hundred sites. In an ideal world, the optimum soil structure would boast at least 6% organic matter and pH 7.0. Of those hundred micro-panel sites, 82 would be occupied by Ca cations, 15 by Mg cations, three by K cations, one by Na cations and zero by H cations. However, when the pH starts to drop from 7.0, H cations start squatting on some of those mini-panels. For example, in two randomly chosen soil test results in my files, I found a pH 6.4 freeing up 14 panels to H cations, and a pH 6.2 liberating 17 panels to H cations. Those Hs are effectively displacing soft metal cations; one could sensibly compare this phenomenon to electoral redistricting.
DS’s main question was “Why have my Mg BSPs been increasing?” In 2016, a certain group of his fields averaged 18% Mg BSP; the same group averaged 20% in 2019, and 22% in 2021. He was curious as to why this value kept going up. He told me that he had been managing these fields, which he owned, for eight years; during that time, none of the fields had received any cow manure, because the fields are two miles away from the home farm. He said during the last three years he has applied (per acre) 1.5 tons of layer manure, 20 pounds of sol-u-bor, eight pounds of zinc sulfate and 60 pounds of sulfur (S). He said he plans to apply gypsum and possibly bovine bone meal this year. He had applied bone meal, a major natural phosphorus source, on fields closer to home, by adding it to cow manure. His soil test readings pegged sulfur at about 15 lbs./acre even with that S application, and that average Ca BSP had dropped down to 59%.
I wrote back to DS, telling him that what most likely caused the higher Mg BSP reading was that a lot more Ca than Mg was pulled out of the soil by the mixed mostly grass crops. No doubt K extraction was pretty high also. Chicken manure does very little for shoring up K; its nitrogen (N) to phosphate to potash ratio tends to run about 4:2:1. The comparable ratio for cow manure is 3:1:2. The difference in these ratios is due largely to the fact that grains, which comprise most of poultry diets, run much lower in K than do roughages, which hopefully provide the bulk of the dairy cow’s nutrition. Somehow getting cow manure to these fields should prove to be a less expensive source of K than organic sulfate of potash. BSPs for K on these fields averaged 1.3%; recall we were aiming for 3%.
I told him it was a good idea to apply gypsum, which runs about 20% Ca and 16% S, at a rate of 300 lbs./acre. This would drop about 50 lbs./acre of S and not raise pH anymore. It already was averaging 7.1. Gypsum, chemically, is primarily calcium sulfate, thus a buffering material, not a liming material like calcium carbonate. The Ca in 300 pounds of gypsum, about 60 pounds of that element, will help nudge Ca BSP a little higher; another goal is to try to have the Ca:Mg BSP ratio weigh in at 5:1 or 6:1. Some of that S in the gypsum will chemically link up with the excess Mg which caused that high BSP (22%), to create magnesium sulfate (Epsom salt), enabling the Mg to escape soil and enter the plant. Lest anybody forgets, Mg is the cornerstone element in chlorophyll, the compound responsible for the green pigment in all plants everywhere.