Soil testing

[MontyJ]

I don't have a lot of time. Work has been crazy lately and working on the other house is starting to take up more and more of my free time. But...

I want to talk about the cations and base saturations a bit. For this to make a little more sense, you need to understand that there are both acidic and basic cations. Hydrogen (H) and Aluminum (Al) are both acidic. Calcium (Ca), Potassium (K) and Magnesium (Mg) are all basic. Basic, in this sense, doesn't mean basic as in common, everyday variety, but alkaline, if you will. The opposite of acidic in other words. OK, onward.

Let's look at another soil sample:

pH : 6.0
Buffer pH : 6.7
Phosphorus (P) : 152ppm (very high)
Potassium (K) : 193ppm (optimum)
Calcium (Ca) : 2122ppm (low optimum)
Magnesium (Mg) : 200ppm (low-medium)
Sulfur (S) : 16ppm (low-medium)
Boron (B) : 0.6ppm (low medium)
Copper (Cu) : 2.2ppm (optimum
Iron (Fe) : 168ppm (very high)
Manganese (Mn) : 92 (very high)
Zinc (Zn) : 4.7ppm (optimum)

Organic Matter (OM) : 4.6

CEC : 15.1

Saturations:

K : 3.3
Ca : 70.3
Mg : 11.0
H : 15.4

If you actually do the math, you will find that the CEC differs slightly from what the lab reported. Remember that the formulas are only for a close estimate.

What stands out in this test report are two things: Good CEC, low pH. The total base saturation is 84.6%, leaving 15.1% for H. H is acidic. Also Fe and Mn are very high, both are a factor of a low pH. As pH increases, both Fe and Mn become less available to plants.

So, what's the cure? Adding lime will work, but there are many different types of garden lime. In this case, Dolomite would be the correct choice. Why? Dolomite has a higher Mg content than many other types of lime. Mg is low in this test having only 11% of the CEC sites. So adding 25-30 pounds per 1000 square feet would increase the Mg CEC saturation (as well as adding some Ca) causing the pH to rise which would, in turn, cause less Fe and Mn to be available. That would result in the Ca and Mg rising to optimum levels and the Fe and Mn to decrease to optimum levels. Since the pH is balancing out and additional Ca and Mg are available on the CEC sites, that must mean the H% is now lower (it has to be if the pH went up).
Using this example, you can see how pH is affected by the saturations.

A few other things of note:
The S and B are low. I would wait until after a season of letting the dolomite work before adjusting either one. It may sound strange to want to add sulfur to an acidic soil, but the amount needed to bring it up to a more desirable level is very small and would have almost no affect on pH. Boron, on the other hand, is a very tricky thing to adjust. It can be plant toxic at levels as low as 4ppm. Once the pH levels out, I would re-test and see where the B is. If still low, I would add one can of 20 Mule Team borax per 1000 square feet.

Finally, let's look at the that very high P level. Personally, I wouldn't call that very high. High maybe. Remember that P does not occupy CEC sites and is not mobile in the soil. The soil has to reserve it in place. Most of the P in the soil is in the form of organo-phosphates and inorgano-phosphates. It's not always available to plants. When people speak of their concern about fertilizer run-off it's really P they are talking about (even if they don't know it). Since it's not held in the soil via the CEC, some of it is free to run off. Much of it gets locked into the soil through chemical bonding, but if a field is over fertilized with phosphate ferts, run-off will occur. Adding a balanced fertilizer to this garden would be a mistake.

If this seems broken and dis-jointed, it's because I can only type a little at a time in the early morning or late evening and save it as a document to post all at once. My apologies.

 

[897tgigvib]

I would not at all say disjointed, but it shows me some terms I still don't understand. I can very much understand that! If I were to try to explain 4th semester music theory to a person who only had 1st semester music theory same thing would happen, but with some placements of where the knowledge ends it can be worked out.

I still couldn't quite grasp buffer ph compared to ph, but I'm sure it's back there in the posts. Maybe a bit of refresher on it.

Still don't understand saturation and the numbers...I think also how saturation and cec work together, and when it is spoken as cec saturation.

You have a lot of projects going on! I hope everyone in your family is helping how they can. I'm sure they are.

 

[MontyJ]

Buffer pH refers to a number used by the lab. Basically, soil lab uses a liming chemical on the test sample. The resulting pH is the buffer pH. The amount of buffering chemical used never changes. For example: The lab has two samples. Sample A has a pH of 6.0, sample B is also at 6.0. Each is given the same 10mg of buffering chemical. Each sample is tested for pH again. Since sample A is a clay based soil, the pH (buffered) is now 6.7. Since sample B was sandy loam, it's buffered pH is 7.2. Clay soils typically have a higher CEC than sandy soils. Therefore there are more exchange sites to be occupied, so it takes more material ie, lime to raise the pH in a clay based soil. That's how the lab determines how much lime to recommend if a pH adjustment is needed. In this example, sample A would need more lime than sample B. Sample B has a lower CEC than sample A so there are fewer exchange sites to be occupied.

Saturations are a percentage of the CEC sites occupied by the individual cations of Ca, K, Mg, H, Al and Na. Remember that these are the positively charged ions. In a well balanced soil, the pH will be high enough to completely lock out Al form the sites. Soil tests provide the saturation levels as a percentage. In the example test above K holds 3.3% of the CEC exchange sites, Ca 70.3%, Mg 11%. Those are the desirable cations. They are also basic cations. Again basic as in opposite of acidic. So those are the "Basic Cation Saturations" or how much the cation exchange sites are occupied by basic ions. H and Al are acidic. Neither is a plant nutrient, so the more CEC sites they hold, the worse it is becasue that limits nutrients available to plants. But becasue both H and Al are cations, they will occupy CEC sites if possible. In the example below, H is occupying the remaining 15.4%. So the total "Base Saturation" is 84.6%. You can get it higher by increasing the base cations in the soil (or liming). Since plants need Ca and Mg much more than K, those are the two you would add. I never want to see K higher than Mg. Since Mg is low, increasing it (along with Ca to a lesser extent) would increase the CEC sites occupied by those to cations, thereby reducing the percentage of sites occupied by H. The result is a higher pH.

To understand how that works, we would have to get into chemistry at the atomic level and consider that Ca is 40 times heavier than H and Mg is 24 times heavier than H and a lot more stuff I don't fully understand. What I believe is that any cation heavier than H (which is all of them) is able to drive H from the CEC site. That of course depends on whether the cation is there and available in the first place.

 

[Ethan]

I came across almost all the post in that thread, now if it's not healthy, by all means start with a soil test. Or if I'm planting a very temperamental plant, start with soil test. But if I have a healthy garden, I would not change anything regardless what a soil test said, because its too much costly for me.

 

[897tgigvib]

I see your point Ethan. If it aint broke, don't fix it.

 

[catjac1975]

I have had my soil tested and plan to do it again this spring. The problem was the recommended amendments were all for chemical fertilizers and prescribed per acre. I'm sure there is a conversion available-I just winged it.

 

[seedcorn]

I have had my soil tested and plan to do it again this spring. The problem was the recommended amendments were all for chemical fertilizers and prescribed per acre. I'm sure there is a conversion available-I just winged it.

There are conversion tables. Problem is , they are just averages. Can take samples of what you are putting on.

 

[MontyJ]

An acre is about 43500 square feet. Take the recommended amounts per acre and divide by 43.5. That will give you the amount per 1000 square feet. Example:

625 pounds per acre

625/43.5 = 14.4 pounds per 1000 square feet.

14.4/10 = 1.44 pounds per 100 square feet.