Uffda, that’s a lot of potash!

Question: Can you really change the %K base cation saturation ratio?

This winter, we have gotten more questions from farmers asking about the base cation saturation ratio (BSCR) concept. The farmers had attended a series of meetings where the speakers encouraged farmers increase the %K saturation in their soils and apply high potassium fertilizer rates to soils, even though there was little to no chance to get a crop yield response. In short, the BSCR concept revolves around reaching a certain percentage (%) of each base cation in your soil to obtain the “ideal” soil. If you do not have the right percentage of each cation, then you are instructed to apply large amounts of fertilizer to reach this “ideal” balance of each cation. Potassium is the most common nutrient where people fall into the BSCR trap, often suggesting that extra potassium will “fix” their soil.

Since the 1940s, university and industry researchers from around the world have thoroughly debunked the BCSR concept. But sometimes, you just need to show people how this works in the real world to get their attention. To help farmers see the silliness of the BSCR concept, we conducted a simple field project in 2015 to show just one flaw in the failed BSCR concept, focusing on its primary claim that you can actually change the %K saturation in soil. We identified three soils in Manitoba, Minnesota, and North Dakota with low initial %K saturation. The goal was to increase %K saturation into the 4 to 6% range, which is recommended by BCSR promoters. We applied a staggering rate of 1000 lb/acre K2O (1666 lb/ acre potassium chloride, KCl, 0-0-60). You are probably thinking that 1000 lb/acre K2O is a lot of potash! And so did we. We called the project the “uffda” project because my Norwegian grandfather, who farmed in southern Minnesota 60 years ago, would have looked at the high rate and said, “Uffda, that’s a lot of potash!”

But, we failed to achieve the “ideal” 4 to 6% K saturation (Table 1). As expected, soil test K (part per million, ppm) increased substantially, but the %K saturation did not reach the “ideal” soil range even with the enormous potassium fertilizer rate.

Table 1. Soil test potassium and potassium base saturation after application of 1000 lb/acre K2O (1666 lb/acre potassium chloride, KCl, 0-0-60).
Location Soil test K, ppm K saturation, %
initial final initial final
Northwood, ND 156 430 0.6 1.6
Benson, MN 154 290 1.9 3.7
Roseisle, MB 50 330 0.4 2.8

Do you need more proof? Let’s see what the plants said. The soybean plant tissue K concentrations did not change either because the soil test K (ppm) was sufficient and above the 150 ppm critical level (Table 2). Simply put, the proven university-developed soil fertility guidelines would have told you that more potassium was not needed. I know a few farmers who were convinced to follow the BCSR concept, which only wasted their valuable dollars and time on the failed idea.

Table 2. Soybean leaf potassium concentration did not increase with additional potassium fertilizer applied on soils with soil test potassium above the critical level (150 ppm).
Fertilizer K Rate Soybean leaf K, %
lb/acre K2O Northwood, ND

STK 150 ppm

Benson, MN

STK 190 ppm

0 3.2 1.9
20 2.6 1.8
100 2.7 1.8
200 3.0 1.8
1000 2.6 1.9
Soybean leaf K sufficiency level = 1.7 %K

All things considered, there are still good reasons to apply potassium fertilizer (moderate rates) to achieve profitable yield responses. Either way, the base cation saturation ratio (BCSR) concept is a really bad way to justify potassium fertilizer use, and it usually leads to very high fertilizer rates and costs. Here are some good reasons to include potassium in your soil fertility program:

  • Soil test K below 150 ppm (grid or zone soil test)
  • Soil test K below 200 ppm (composite soil test, high soil variability)
  • History of low plant tissue K when no potassium fertilizer is applied; potential compaction
  • Replicated strip trials showing profitable crop yield response to moderate potassium fertilizer rates
  • Chloride required for small grains; potassium chloride (0-0-60-50Cl) is the most common chloride source

High soil pH and calcium carbonate inflate base cation saturation and cation exchange capacity (CEC)

Soil pH is a soil chemical property that measures soil acidity or alkalinity, and it affects many soil chemical and biological activities. Soils of the northern Great Plains and Canadian Prairies often have high soil pH (>7.3) and contain calcium carbonate (free lime) at or near the soil surface. It is the calcium carbonate in soil that maintains high soil pH and keeps it buffered around pH 8.0. The calcium carbonate originates from soil formation processes since the latest glacial period.

Soils with high pH and calcium carbonate create analytical challenges in determining cation exchange capacity (CEC) and subsequent base cation saturation ratio (BCSR) calculations. Cation exchange capacity is the amount of positive-charged cations (e.g. ammonium, calcium, hydrogen, magnesium, potassium, sodium) held on negative-charged soil particles, like clay and organic matter. Fine-textured soils (high clay content) and organic soils (high organic matter content) have high CEC, while coarse-textured soils (low clay content) have low CEC. The BSCR is the relative proportion of base cations in soil.

The routine laboratory method to determine CEC is the summation method, where all extractable cations on soil particles are added together. The assumption is that all positive-charged cations extracted are held on negative-charged exchange sites on soil particles. The assumption falls apart in soils with pH > 7.3 because the soil test method also extracts calcium from the naturally occurring soil mineral calcium carbonate, which is not held on cation exchange sites. The resulting amount of extractable calcium is inflated. The summation procedure still sums together the inflated calcium result, producing an inaccurate and inflated CEC result. Any subsequent base cation saturation calculations are similarly flawed.

To illustrate the inflated CEC problem, AGVISE Laboratories conducted a laboratory experiment. The experiment showed that more calcium carbonate in soil increased the amount of extractable calcium (Table 1). Furthermore, the inflated amount of extracted calcium also inflated the CEC because all the extractable cations are summed together. The correct CEC is 24 cmolc/kg, yet the inflated CEC could be 150 to 200% higher with increasing calcium carbonate content. Adding calcium carbonate to soil did not increase the inherent CEC sources, i.e. clay and organic matter, yet the laboratory CEC result increased. In reality, the ability to hold more cations did not change. This highlights the analytical challenge in determining CEC via summation method on soils with high pH and calcium carbonate.

Table 1. Effect of calcium carbonate addition on extractable cations and cation exchange capacity.
Calcium carbonate equivalent (CCE) pH EC Ca Mg K Na CEC
% dS/m —————– ppm —————– cmolc/kg
0 7.5 0.43 3350 730 220 45 24
1 7.6 0.51 6150 730 220 50 37
5 7.7 0.52 7480 720 215 45 44
10 7.8 0.50 7240 650 180 40 42
Abbrev.: pH (1:1); EC, electrical conductivity (1:1); Ca, calcium; Mg, magnesium; K, potassium; Na, sodium; CEC, cation exchange capacity via summation of cations.

The subsequent base cation saturation calculations were also affected, showing much lower percent potassium saturation as calcium carbonate content increased (Table 2). This presents a major challenge in using the base cation saturation ratio (BCSR) concept to guide soil fertility and plant nutrition on soils with high pH and calcium carbonate. Without an accurate analytical method, the BCSR concept loses its grounding and any practical application.

Table 2. Effect of calcium carbonate addition on base cation saturation ratio and cation exchange capacity.
Calcium carbonate equivalent (CCE) Ca sat. Mg sat. K sat. Na sat. CEC
% ————————– % ————————– cmolc/kg
0 71 26 2.4 0.8 24
1 82 16 1.5 0.6 37
5 85 14 1.2 0.4 44
10 86 13 1.1 0.4 42
Abbrev.: Ca, calcium; Mg, magnesium; K, potassium; Na, sodium; CEC, cation exchange capacity via summation of cations.

In general, the routine CEC method via summation of cations is quite accurate on soils with pH less than 7.3 and no calcium carbonate. However, the routine method has clear challenges on soils with pH greater than 7.3. The resulting CEC and subsequent base cation saturation calculations are inflated and/or flawed. This is why the BCSR concepts is extremely misleading on soils with high pH.

5 Things You Should Know About Calcium

1. Calcium (Ca) is abundant in soils of the upper Midwest, northern Great Plains, and Canadian Prairies; calcium deficiency in agronomic crops is rare

Calcium makes up about 3.6% of the Earth’s crust, and it is relatively abundant in agricultural soils across the region. In soils with pH greater than 6.0, Ca is the dominant cation (positively charged ion) on the cation exchange capacity (CEC). Since most soils in the region have a pH of 6.0 or above, calcium is very abundant and soils with low soil test Ca (less than 500 ppm) are rare (Figure 1).

Soil samples with soil test calcium below 500 ppm in 2020

Figure 1. AGVISE regional soil test summary. AGVISE has created regional summaries for the past 40 years. You can find more soil test summary data, including Montana and Canada, here.

Potential calcium deficiencies are most common on sandy soils with strongly acidic pH (pH less than 5.0). Luckily, the fix for low soil pH also fixes potential Ca deficiencies. To correct soil pH, agricultural limestone is applied to raise soil pH to 6.0 or 6.5, if growing sensitive crops like alfalfa or clover. When limestone (calcium carbonate) is applied in tons per acre, more than enough calcium is also applied and sufficiently increases soil test Ca, providing ample calcium for optimal crop growth and development. Throughout the region, soils with low soil pH are more common in the higher rainfall areas to the east and south (Figure 2), and liming is a standard practice to correct soil pH and provide calcium.

Soil samples with soil pH below 6.0 in 2020, for 5 things you should know article

Figure 2. Soil samples with pH below 6.0 in 2020, where lime application may be required. The number of fields with low pH has increased over time and will continue to do so because soil acidification is a natural process. Keep watch for low soil pH, especially in western North Dakota and South Dakota. You can find more soil test summary data, including Montana and Canada, here.

2. Multiple calcium fertilizer sources exist; some increase pH, others do not

Agricultural limestone is the most common lime source and is available in two flavors: calcitic (calcium carbonate, <5% magnesium) or dolomitic (calcium-magnesium carbonate, >5% magnesium). Limestone quarries exist in southern Minnesota and Iowa, but the northern Great Plains is virtually devoid of mineable limestone. Industrial waste lime (spent lime) is another good lime source and available from sugar beet processing plants and water treatment plants throughout the region. Any of these liming materials will supply enough calcium to increase soil test Ca if soil pH is increased above pH 6.0.

Gypsum (calcium sulfate) is another calcium source, but it does not change soil pH. Gypsum is sometimes used to increase soil test Ca if the producer does not want to increase soil pH with lime application. This situation is common in irrigated potato production where increased soil pH may increase soil-borne diseases like common scab of potato. Gypsum is not a lime source, so it will not increase soil pH.

3. There is no “ideal” base cation saturation range or ratio for calcium

Suggestions that Ca and other base cations (magnesium, potassium) are required in a certain percentage or ratio in soil are not supported by modern science. Recent research done at several universities shows a wide range of base cation ratios in soil will support normal crop growth (see links below). What is important is that a sufficient soil test amount of each base cation (Ca Mg, K) is present in soil to support plant growth and development.

4. Soils with pH greater than 7.3 will have falsely inflated soil test Ca and cation exchange capacity (CEC) results

Soils with pH greater than 7.3 will contain some amount of naturally occurring calcium carbonate (CaCO3), shown on the soil test report as carbonate (CCE). The calcium soil test method will extract Ca on cation exchange sites and some Ca from calcium carbonate minerals, resulting in an inflated soil test Ca result. Starting with inflated soil test Ca, the routine cation exchange capacity (CEC) calculation is also inflated. For example, a soil with pH 7.8 and 3.0% CCE may report CEC at 60 meq/100 g, but the correct CEC is only 27 meq/100 g. To obtain accurate CEC results on soil with pH greater than 7.3, a special displacement CEC laboratory method is required. For soils with pH less than 7.3, the routine CEC calculation method will provide accurate soil test Ca and CEC results. High soil salinity (soluble salts, EC) can also inflate CEC results.

5. Calcium is not an environmental risk to surface or ground water

Calcium is one of the major dissolved substances found in surface and ground waters, especially in the northern Great Plains and Canadian Prairies. In fact, water hardness is determined from the amount of dissolved calcium and magnesium in water. There is already so much calcium found in natural waters in the region that calcium fertilizer additions to soil are negligible. Water hardness does affect the effectiveness of some herbicides and may cause tank-mixing issues, but is not an environmental concern.

Bonus: Just because your tomatoes have had blossom-end rot does NOT mean your soil is Ca deficient!

If you are a backyard tomato grower, you may have experienced blossom-end rot before, where the blossom-end of developing fruits turn brown and mushy while still on the plant. Yes, the problem is caused by low calcium in the tomato plant, but not necessarily because the soil has low soil test Ca. Blossom-end rot is primarily caused by inconsistent soil moisture. Adequate soil moisture is required to maintain a consistent supply of calcium moving to the plant root, which might run short if watering is inconsistent. To keep blossom-end rot away from your garden, just try to be more consistent with watering, especially during dry periods.

Resources on Base Cation Saturation Ratios

Cation Exchange: A Review, IPNI

Soil Cation Ratios for Crop Production, UMN

A Review of the Use of the Basic Cation Saturation Ratio and the “Ideal” Soil, SSSA Journal