David had me pick up some soil samples he’d taken. He owns a small organic farm in the next county. I brought the samples home, air-dried them, then screened them using a wide-mouthed plastic jar with quarter-inch holes in the lid. Screening removes impurities like bug parts, roots, worms, leaves, tiny pebbles, etc. Labs just want to test soil nutrients, not lots of other tagalong debris. These impurities can inflate test readings due to biological concentration.

Crop Comments: Soil testing without BSPs is like swimming without flippersAfter shaking these dried samples in the jar, I poured the screenings into sandwich bags – then sent them to the Dairy One Lab in Ithaca. After about a week, the soil test results arrived in my inbox. Since David doesn’t have a phone, I made copies of the results, snail-mailing them to him. Immediately, he wrote me back, asking questions about Base Saturation Percentage (BSP).

BSP is the proportion of a soil particle’s surface occupied by positively charged soft metals. This grouping includes calcium (Ca), potassium (K), magnesium (Mg) and sodium (Na). These elements are in ionized form, meaning they have all lost one or more electrons. Losing electrons makes them positively charged, qualifying them to be called cations. Negatively charged particles (ones acquiring electrons) are anions.

Let’s presume that the soil particle surface is round, comparing it to the surface of a volleyball. That ball’s surface is broken down into 18 slightly curving rectangles. Next, imagine that instead of 18 surface sub-divisions, there are 100; they could be divided up amongst the four types of soft metal cations. (The term “cation” is fairly interchangeable with “base.”)

In addition to the above four soft metals, there’s another cation on this microscopic scene: hydrogen (H). So we’ve got four soft metal cations, as well as H cations, competing for those hundred sites. In an ideal soil micro-environment, soil structure would boast at least 6% organic matter and pH 7.0. Of those 100 micro-panel job sites, 82 would be occupied by Ca cations, 14 by Mg, three by K, one by Na and zero by H. These numbers always total 100, thus the logic of comparing volleyball panels to percentages.

However, when the pH starts dropping below 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 of 6.4 freeing up 14 panels to H cations and a pH of 6.2 liberating 17 panels to H cations. Those Hs are effectively displacing soft metal cations. Remember, pH is defined as the negative logarithm of the H ion concentration.

David asked me why his Mg BSPs had 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. Why was this value increasing? He’d managed these fields for eight years. During that time none of those fields received any cow manure, because they were two miles away from the home farm.

During the preceding three years he had applied (per acre) 1.5 tons layer manure, 20 lbs. of sol-u-bor, 8 lbs. of zinc sulfate and 60 lbs. of sulfur. He said he planned to apply gypsum and bone meal for the next cropping season. He had applied bone meal – a natural phosphorus (P) source – on fields closer to home by adding it to cow manure. His soil test readings pegged sulfur (S) at 15 lbs./acre (even with that application) and showed that Ca BSP had dropped down to 59%.

When I wrote back to David, I told him that most likely what caused higher Mg BSP readings 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 also pretty high. Chicken manure does very little for shoring up K. Its N:P:K 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 cow nutrition.

Cow manure spread on these fields should prove to be a less expensive method for getting K to those fields compared to organic sulfate of potash. His soil tests showed that BSPs for K averaged 1.3% compared to the targeted 3%.

I recommended that he apply gypsum – running about 20% Ca and 16% S – at 300 lbs./acre. This would drop about 50 lbs. S/acre and not raise pH anymore, since it was already averaging 7.1. Gypsum (calcium sulfate) is a buffering material, not a liming material like calcium carbonate. The Ca in gypsum will nudge Ca’s BSP a little higher.

We try to have the Ca:Mg ratio stabilize at 5:1 or 6:1. Then some of that S in the gypsum will chemically link with the excess soil Mg – which caused that high Mg BSP of 22% – to create magnesium sulfate (Epsom salt), enabling it to escape soil and enter the plant. Magnesium is the cornerstone element of chlorophyll, the compound responsible for the green pigment in all plants everywhere.

Speaking of “green,” let me quote Certified Crop Advisor Tom Kilcer’s wisdom on proper minimum grass stubble height: “Raising the cutter bar is critical for survival and a high yield of cool-season grasses. Grasses regrow using the existing leaf tissue, not from root reserves like legumes. Cutting grass close leaves no regrowth and the stand quickly dies out. Multiple studies found a four-inch cutting height is critical to both survival and total yield. If you pull out after harvest and the field is not still green, you screwed up future yield and replaced it with weeds. For mixed grass/legume stands, the more grass, the more critical for the cutting height to be correct.”