Crop Comments: Why Soil Base Saturation Percentages Are Very Important

According to Steve Culman, Ph.D., soil scientist at Ohio State, cation exchange capacity (CEC) is a fundamental soil property used to predict plant nutrient availability and retention in the soil. It is the potential of available nutrient supply, not a direct measurement of available nutrients.

Soil CEC typically increases as clay content and organic matter (OM) increase because cation exchange occurs on surfaces of clay minerals, OM and roots. A gram of clays has a much higher total amount of reactive surface area than a gram of sand (with silt particles falling somewhere in the middle).

I’ll illustrate this relationship using Rubik’s cubes. A 2x2x2 Rubik’s cube boasts four plastic squares on each of its six sides, totaling 24 squares. A 4x4x4 cube has 16 squares on each of its six sides, for a total of 96 squares. The bigger cube displaces the same volume as eight of those smaller cubes. Those eight smaller cubes each boast an area of 24 squares. So the eight smaller cubes enjoy double the surface area (2x2x6x8 = 192) compared to the single larger cube. So smaller soil particles (clays) boast surprisingly higher surface areas than sands and even silts. Greater surface areas correspond to higher CECs.

David had me gather some soil samples. He owns a small organic farm in the next county. I air-dried the samples, then screened them using a wide-mouthed plastic jar with quarter-inch holes in the lid. (Labs just want to test soil nutrients, not lots of other tag-along debris. These impurities can inflate test readings due to biological concentration.) After shaking these dried samples in the jar, I poured the screenings into bags and sent them to the Dairy One Lab in Ithaca. After a week, the test results arrived. Since David doesn’t have a phone, I made copies of the results and mailed them to him. He wrote me back, asking questions about base saturation percentages (BSP).

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

Let’s presume that the soil particle surface is round, like the surface of a volleyball. The ball’s surface is broken down into 18 slightly curving rectangles. Next imagine that instead of 18 surface subdivisions, there are 100. They could be divided up against four types of soft metal cations. (Cation, chemically, means base).

In addition to the above soft metals, there’s another cation: hydrogen (H). So we have four soft metal cations, as well as H cations, competing for those 100 sites. In an ideal soil micro-environment, soil structure would boast at least 6% OM with a pH of 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 from 7.0, H cations start squatting on some of those panels.

For example, in two randomly chosen soil test results in my files, I found pH 6.4 freeing up 14 panels to H cations, and pH 6.2 liberating 17 panels to H cations. Those Hs are displacing soft metal cations. (pH is 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 had averaged 18% Mg BSP.

The same group averaged 20% in 2019 and 22% in 2021. Why was this value increasing? He’s managed these rental fields for eight years. During that time, none of these fields received any cow manure because they were two miles from the home farm. During the preceding three years he had applied (per acre) 1.5 tons of layer manure, 20 lbs. of sol-ubor, 8 lbs. of zinc sulfate and 60 lbs. of sulfur (S).

He said he planned to apply gypsum and bone meal for the next cropping season. He’d applied bone meal (a natural phosphorus (P) source on fields closer to home) by adding it to cow manure. His soil test readings pegged S at 15#/acre (even with that application) and showed that Ca BSP had dropped down to 59%.

I told David what most likely caused higher Mg BSP readings was that a lot more Ca than Mg was being pulled out of the soil by the mixed mostly grass crops. No doubt K extraction was also pretty high, since chicken manure does very little for shoring up K. Its N:P:K ratio runs 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 roughages do, which 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 than organic sulfate of potash. His soil tests showed that BSP for K averaged 1.3% compared to the targeted 3%.

Then I recommended that he apply gypsum (20% Ca and 16% S) at 300#/ acre. That would drop about 50# S/acre and not raise pH any more (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 Mg (which caused that high Mg BSP of 22%) to create magnesium sulfate (Epsom salt), enabling Mg to escape soil and enter the plant.

Remember: Mg is the cornerstone element of chlorophyll, the essential compound in all plants.

by Paris Reidhead