Soil testing

[MontyJ]

Just got home from work. I'm waiting for my daughter to call so I can go to her new house and help her get the frozen pipes thawed. I promise to get to CEC this evening. Meanwhile:

Oldguy-The DIY kits are useless IMHO. They use reactant chemical analysis which is far from accurate.

Bobm-When I say area size makes no difference, that is what I mean. What you are describing are different soils on the same property. Each soil type must be tested individually. So, if you have 3 acres of one type of soil, that's one test, 5 acres of another soil type is another test. The test results are not affected by area, as long as the soil is the same type. Soil tests do not take into account the area size, only the soil and what is in it. If a 1/4 acre garden tests at 1545 ppm Ca and a 5 acre garden tests at 1545 ppm Ca, it's the same thing. If I have 20 acres of the same soil type and send in one sample from 19 acres and another sample from the 1 remaining acre, the test results will come back the same for both plots (all things being equal and one area not having any ammendments that the other doesn't). So, whether I have a 100 square foot garden or a 1000 square foot garden, if the nutrient levels are the same, the test results will be the same, regardless of size.

A lot of this has to do with CEC which I will try to explain this evening.

 

[seedcorn]

MontyJ... when you say area size makes NO BEARING ... take my 20 acre property in Cal. for example. It is at 503' +/- elevation of rolling high desert with clay soils deposited by errosion from the high Sierra Nevada Mts. ( mountains rise 500'-1000' within 1/2 mile to 3000 ' within 3 miles )over eons of time. There is meandering black, white, red, and yellow soils changing at about every 10+/- feet deposited by creeklets and flood waters. Some areas of my pasture even has white salt deposits on the soil surface where no grass grows. So how can I get an accurate soil test ? :tools

A soil sample is only good for that 1/2 pound of soil. Bob, yours represents the majority of real life situations. To do it for better results, you have to take a lot of individual sample in each soil type by elevation. Then to diagnose the field because then you have to adjust for border ground, how fertilizer was previously applied, how soil was worked. To take 1/2 pound of soil and then apply those numbers to a small amount of dirt is one thing. To apply those numbers over a large area is just plain wrong. Then to try to fine tune an area based on these numbers is worthless. Now if you are using these numbers to grow 1 large pumpkin or a potted plant, great numbers. Or if you are trying to fix a problem, it gives you direction to start. Just to mess with some of you, take samples, send them in. Next day, take samples again, send them in. Guaranteed, different results. Now which one do you use to fine tune with? Soil test re good, but realize what they are and are not.

In real life, CEC's will be raised with organic matter but it is a very slow process in a decent sized area.

An acre of soil 6" deep weighs 2,000,000 pounds. You are going to pull .5 pound for soil sample and fine tune off of it? Key is "fine tune" vs. identify problem.

 

[MontyJ]

Yep, soil tests are still worthless. Yep, that horse is still dead. For those that still want to learn how the soil works, read on:

This is pretty long,

CEC or Cation Exchange Capacity is not as complicated as people make it out to be. It's simple chemistry and math. Simply put, the CEC is the soils ability to retain nutrients. That doesn't mean all nutrients, but those with a positive electrical charge, or cations. Nutrients with a negative electrical charge are called anions.
The positive nutrients (cations) include calcium (Ca), potassium (K), magnesium (Mg), Sodium (Na) and even copper (Cu). There may be others that I don't remember right now, but those are the biggies. Other positive elements found in the CEC are hydrogen (H+) and aluminum (Al). However, H+ and Al are not plant nutrients. Plants do not need or use these molecules, but they must be accounted for.
Anions include Phosphate (PO4/3) and Sulfate (SO4). Anions are not included in the CEC (which makes sense because CEC is the Cation exchange capacity), but they do play a role.

I have to explain what an ion is, either Cation or Anion. An ion is simply a molecule that has either a positive or negative charge. Cations are positive, Anions are negative. The CEC is a way to scientifically measure how many exchangeable Cations the soil in question can hold.

CEC is measured in Meq/100g or milligram equivalents per 100 grams of soil. Don't freak out, it's not that hard. The comparison factor used is Hydrogen or H+. One milligram of H+ occupying all of the CEC sites in 100 grams of soil would equal a CEC of 1. That is the base for comparison.

Now, I really don't want to go into how a lab determines how much H+ it takes to occupy all of the CEC sites in a particular sample, but I will if I have to. Basically, all nutrients are driven out of the soil and then the amount of H+ it took to fill all of the sites is measured. That is how CEC is determined in a lab.

************Heavy math time! If you hate math, skip down some!!*****

You can also estimate CEC fairly closely (as I did in the earlier post). It's nothing more than a mathematical equation:

If your soil test results are in ppm you will have to convert them to lbs/acre. You simply multiply ppm * 2 to get pounds per acre.

I'll use the test results posted earlier as an example:

K = 172.07ppm
Ca = 1853.16ppm
Mg = 363.43ppm

These are the cations used for determining CEC. Na could also be included if you have the exact buffer pH available. I was not given the buffer pH so I used a lab factor aligned with the pH of the soil, which was given.

So:

K = 172.07ppm * 2 = 344.14 lbs/ac
Ca = 1853.16ppm * 2 = 3706.32 lbs/ac
Mg + 363.43ppm * 2 = 726.86 lbs/ac

Next, we have to obtain the CEC factor of each cation. This deals with atomic weight and whether the cation has one, two or more + charges. For example, Ca actually has 2 positive charges so is actually written Ca++. I know, it's getting scary, but stay with me. The atomic weight of Ca is 40, the atomic weight of H+ is 1. Ca has a valence of 2 (which only means that is has 2 positive charges) so it's milli equivalent is 20. That means it would take 20 milligrams of Ca to occupy the same CEC sites that a single milligram of H+ could occupy. Think about that for a minute. It doesn't matter how much the soil weighs, or what it's volume is. If that particular sample can hold exactly 1 milligram of H+ it can hold 20 milligrams of Ca++. Why 20 and not 40? Because Ca++ has 2 positive charges and each occupies a site. Since Ca equals 20 milli equivalents math tells us that it's equivalent must be 200ppm times 2 gives us 400 lbs/acre.

Whew, that was scary. Are you still with me?

OK, so let's do some more math, yay!

Now that we understand how to derive the cation factors, let's do it:

We have the milli equivalent of Ca and we can determine the rest easy enough if you know the atomic weights of each. Anyone have a table of periodic elements handy? Never mind, here are the factors:

Ca = 400 lbs/ac
K = 780 lbs/ac
Mg = 240 lbs/ac

The formula for calculating CEC is:

(lbs K/780) + (lbs Mg/240) + (lbs Ca/400) * lab factor (it's more accurate if you have the buffer pH)

If you have the buffer pH the formula is:

(lbs K/780) + (lbs Mg/240) + (lbs Ca/400) + [12 * (7 - buffer pH)]
(of course if the buffer pH is higher than 7.0 use 0 ) Also, if you have the buffer pH, you can and should include Na in your calculations.

We don't have the buffer pH so we will use the lab factor (which is a factor used by many labs)

CEC = (K344.14/780) + (Ca3706.32/400) + (Mg726.86/240) * 1.05

.44 + 9.27 + 3.03 * 1.05 = 13.37 = CEC

Obviously, my early morning math skills leave something to be desired. I believe I estimated the CEC to be 11.6. What can I say, I was scribbling on an envelope and drinking coffee at 5:00am.

From the numbers we have already calculated, we can determine base saturations and even milli equivalents for each cation. Using that info we can determine Ca:Mg ratios, K:Mg ratios and the like.

Using that information, we can determine where the soil is trending over time and head off problems before they start.

Most reputable labs will provide CEC and base saturations in the soil test. So you don't really have to worry about all the math we just did I find it interesting though because you can see how soil pH is tied to CEC and nutrient availability.


*******Math class over. You can start reading again.*********

Ok, on with CEC.

We now know that CEC describes exchangeable cations in the soil. What that means is those nutrients Ca, K, Mg, Na occupy sites in the soil. Since most clay and humus is negatively charged, and opposites attract, the positive cations are drawn to the negatively charged soil particles and attach to them.

Plants release CO2 into the soil through their roots which creates carbonic acid when it mixes with water. Carbonic acid contains H+. As H+ is released into the soil it is exchanged on a CEC site with a cation nutrient, one for one. The freed nutrient is then taken up by the plant.

Ok, I'm officially tired now. I'll continue this tomorrow.

Disclaimer time:

I am not an expert on the soil. I have never been to school for soil biology or any other ology. What I know comes from years of reading and studying. If you find something wrong, or disagree, I would love to hear about it.

 

[897tgigvib]

Thank you Monty!!!

I actually understood most of that as I read along. Not that I'll memorize the atomic weights or the formulae, but I sure get the idea.

It took about a minute to get what you meant by * 2. Times two, and call the straight numbers pounds per acre. Those wind up being a lot of pounds!

(made me wonder if they do that with things like gold ore).

It's actually regular 6th grade math. I saw no sin or cosin, nor any epsilons or f(x) or g of f(x). Not even any rootsigns or to the 13th root, whew!

So I would not call it heavy math, but some formulae to memorize, some chemical attributes such as atomic weight and typical charges to memorize, but even those, if well presented in a curriculum, should not be beyond a 10th grade agriculture class. This is not even beyond 8th grade science, given a simple and slow progressive presentation.

I do see though how my older mind works!

If I can see how cec and humus compost work, and next how this saturation thing works, I can see then how basic problems in soil can be alleviated with additions of compost or other additives.

Then, only after understanding that, can I get a grip on how microbes added can possibly alleviate certain other problems.

All these as a HOLISTIC approach.

+++++

Monty, please don't be dissuaded from this soils presentation by a few possible naysayers. What you have here is the real scoop on garden soils. I know you don't claim to be expert, but you'll probably find that by doing the explanations, and seeing some good questions, I really think some excellent answers will arrive.

 

[MontyJ]

You're right Marshall. It's very simple math and chemestry. It's all proven science. Once you get past that, understanding the soil becomes so much easier. Understanding why a soil is acidic or alkaline helps to understand why things go wrong. The CEC, not CEC's as some would say, gives you so much information. Soil balance is the key. Did you know that balanced soil will always be at a pH of 6.7? Always. The math doesn't lie. It doesn't matter what type of soil it is. If the cations are balanced correctly, the pH will be 6.7. It's not as easy as it sounds though. Soils with a low CEC will be very difficult to balance. Adding humus to raise the CEC sounds easy, but it's not. As seedcorn said, an acre of soil 6" deep weighs 2,000,000 pounds (that's not really true but I'll let him explain that). Increasing the CEC over an acre of ground would require lots of humus in the form of compost or humates, or the addition of clay. On smaller gardens, this is entirely possible. On very large gardens, maybe not so much. Of course, that all depends on what you have access to.

I'm not going to be dissuaded by the argumentative. It's what they live for. The real shame is that they have to constantly inject their negativity on a subject that others are interested in. It's February, the groundhog lied and I have nothing better to do right now. I'll cover as much of the soil as I can before spring as long as there is interest.

Oh, and seedcorn...I've never grown a giant pumpkin in 1/2 pound of soil...can you tell me how you do it?

 

[majorcatfish]

monty you even think about contacting the learning channel? you would be good on there.

 

[897tgigvib]

I am seeing that there are a lot of subtopics that can be discussed and explained about soils! Once we get the main overview, why don't we start on the subtopics.

 

[baymule]

Monty, you are so sexy when you talk "dirt"

 

[Durgan]

From where I sit, soil testing in a backyard garden, a few square feet at the most, is a total waste of time. Now if one is growing commercially in a relatively large area soil testing is probably warranted.

My patch is around 2000 square feet and I condition my soil by trial and error with great success. I have heavy clay and add compost and wood chips every year. I certainly don't waste time on academic speculation as to the soil condition.

There is a lot more to successful growing than the soil, sunshine, moisture and temperature play a large part.

http://www.durgan.org/URL/?MEUFO 22 September 2008 New Garden Bed Prepared

A new garden bed was prepared. (About 8 by 25 feet)The underlying soil is clay. The pictures depict the procedure.
First the sod was removed with manual kick sod cutter.
The underlying packed clay was hand spaded.
The spaded chunks were broken into small pieces using a rototiller.
The sod was put through a shipper shredder. This absolutely destroys the grass roots.
The broken sod was worked into the underlying soil.
A layer of good compost was applied and worked in.
A layer of wood chips was applied and worked in. This was to apply some fiber.
Since this bed wont be used until next year a cover crop of annual clover was planted.
This process only takes about four hours of applied effort.

http://www.durgan.org/URL/?COUUD 26 September 2008 Completing New Garden Bed

 

[vfem]

You know I feel like such a dope. I spent the last few weeks doing soil sample research. I found out I could send my soil samples in for free to NC State so I'm getting those ready to go.

Just because I jumped the gun, I did a quick soil sampling with the burpee kits (HA!) and the only thing I took to heart was the PH of my soil was high... well over 8.0. So added an acidifier to the soil (I had some for the hydrengeas) at the rate given on the bag. I went through that bag pretty quickly! I added just enough to bring it down to about 7, just enough for 1 unit.

My big thing is I'm shocked! Everyone I talked to at the nursery and my own neighbor insist we have very acidic soil and they are forever going on about 'LIME"! I chose to ignore their advice and do my own research. When I brought it to their attention how alkaline our soil really was... my neighbor waved me off and added more lime to his garden as he insists on doing every year.

So his garden seems to do fine (with some exceptions I go over there and evaluate to help him out occasionally). What kinds of issues actually occur from having soil too alkaline or too acidic? Are these noticeable issues, or problems that are minor and may go unnoticed?!