SC Master Gardener Course: Soil & Plant Nutirtion

Wendy S. Delmater
By Wendy S. Delmater on Fri, Jan 3, 2014 - 5:39pm

Clemson Master Gardener Training Manual

As many of you know, I've been taking a Master Gardener course online through the Clemson University Cooperative Extension. I'm going to do a series of posts on the various topics studied, to try and share what I've learned.  I will strive to give you useful information for any climate. This post will be about Soil and Plant Nutrition.

Soil has the following components: air, water, and minerals (with a small amount of organic material, maybe five percent). Air and water typically run 25% each, with sand, silt, and clay making up the rest. Soil texture can be anywhere on a continuum of coarse (sand), medium (silt), or fine (clay). The smaller the particulate, the more water it can hold, so plain sand will dry out too soon and fine clay will hold so much water that there is no room for air. Aim for a sandy loam, if possible. 

Smaller particles have dramatically larger surface areas and that's where your plants pick up minerals. Anions and cations are the positive and negatively charged particles that hold or repel mineral nutrients. Tiny soil particles (mineral clay colloids) and humus have a negative charge and hold positively-charged nutrients, while repelling negative ones. Nitrogen is negatively charged, which is why it is not held in the soil and needs to be constantly renewed. If you get your soil analyzed, look at the Cation Exchange Capacity, or CEC, which tells you how well you plants can take up mineral nutrients from the soil. CEC can be increased by adding organic matter.

Soil organic matter can be fresh green plants, partly decomposed or fully decomposed (humus). Adding organic matter increases the amount of water retention in sandy soils and improved drainage in clay soils. It stores nutrients and plant micronutrients, which become available as the organics decompose. One thing I learned was that just growing beans in a plot does not improve the soil, you have to turn the dead plants into the soil to get the benefit of a legume's ability to fix nitrogen from the soil.  Cover crops can be grown to stop erosion, help control weeds, and be turned into the soil to add organic matter. Non-legume cover crops add less nitrogen but more organic matter. Be careful not to use anything that will get "woody" as a cover crop: white clover is fine, for example, but red clover gets woody and is slow to break down. It has a high C:N - Carbon to Nitrogen ratio.

Stable soil has a CH ration of 10:1 or 12:1. If the C:N ratio gets over 30:1 decomposition will "rob" your plants of any available nitrogen until the "woody" material breaks down. It will not affect soil nitrogen to use something with a high C:N ratio as a surface mulch, but it will affect your pants if you mix it in.

Another very Important factor is soil pH. Most plants like a pH of 6 to 7 (slightly acidic) because pH affects a plant's ability to pick up nutrients from the soil. Other than the carbon, hydrogen and oxygen plants get from water and air, there are 14 nutrients. The macro-nutrients are Nitrogen, Phosphorus and Potassium: the N-P-K you see on fertilizer bags.  Plants also need fairly large amounts of calcium, magnesium, sulfur. The micronutrients are like plant vitamins:  iron, copper, manganese, molybdenum, copper, boron, zinc, chlorine, and nickel. All of these nutrients and the pH will be tested in a soil sample. To get your soil analyzed give your agricultural cooperative extension about three cups of soil taken from the root zone of your plants, not just scraped off the surface.

If you have any questions on this topic, please list them in the comments.

,

6 Comments

Hrunner's picture
Hrunner
Status: Gold Member (Offline)
Joined: Dec 28 2010
Posts: 256
Good, concise soil info, Wendy

Keep the updates coming!

Mackay's picture
Mackay
Status: Member (Offline)
Joined: Mar 12 2013
Posts: 17
Great stuff

This is nice, clear stuff Wendy. Thanks!

When things warm up this year, I'm about to start some serious gardening for the first time since I was a teenager in the 1980s. I've been reading so much over the past year or two that I'm kind of suffering paralysis by analysis.

A couple of questions:

1 - Do I judge my soil type based on what's there now (acidic, fairly heavy forest soil) or what's there after I add compost, manure, straw etc. (I'm planning on sheet mulching a part of the garden and double-digging another part)? I mean, if I sheet mulch, won't the composition of the existing soil be a lot less important in my calculations? Or will the initial forest soil remain a major factor year after year despite inputs?

2 - Also, I just read Gaia's Garden by Toby Hemenway. He said nitrogen fixers deposit a significant amount of nitrogen throughout their life cycle due to root die-back, which is brought about almost daily by weather changes and other conditions. His argument was quite convincing to my novice brain. However, I had just read Steve Solomon's Gardening When it Counts, and he, like you, contradicted Hemenway. Any hunch as to whether Hemenway may be right? I'm actually thinking of planting nitrogen-fixing cover crops on part of my garden right away because I won't be able to take care of all the space that I want to clear but I want to improve the soil for the 2015 season.

3 - And would the nitrogen be added effectively with a no-dig method? Or would it dissipate too quickly because it's all on the surface?

Thanks much!

Mackay

Wendy S. Delmater's picture
Wendy S. Delmater
Status: Diamond Member (Offline)
Joined: Dec 13 2009
Posts: 1988
answers, based on what you told me

Q1 - Do I judge my soil type based on what's there now (acidic, fairly heavy forest soil) or what's there after I add compost, manure, straw etc. (I'm planning on sheet mulching a part of the garden and double-digging another part)? I mean, if I sheet mulch, won't the composition of the existing soil be a lot less important in my calculations? Or will the initial forest soil remain a major factor year after year despite inputs?

Answer 1. Do testing every year. Do a baseline now, and then heck the area you've sheet mulched next year. Track the changes. Use your local cooperative extension for the soil analysis.

2 - Also, I just read Gaia's Garden by Toby Hemenway. He said nitrogen fixers deposit a significant amount of nitrogen throughout their life cycle due to root die-back, which is brought about almost daily by weather changes and other conditions. His argument was quite convincing to my novice brain. However, I had just read Steve Solomon's Gardening When it Counts, and he, like you, contradicted Hemenway. Any hunch as to whether Hemenway may be right? I'm actually thinking of planting nitrogen-fixing cover crops on part of my garden right away because I won't be able to take care of all the space that I want to clear but I want to improve the soil for the 2015 season.

Answer 2. I've not read Gaia's garden, yet, but it's on my list. According to the Intro to Permaculture course at NC State, as well as the Clemson Master Gardener course, if you pull nitrogen-fixing plants, the nitrogen does not stay in the soil. They taught me to put the string ".edu" in any searches that I do for information. This way  you get research and not hearsay, a real problem on the web. I'm not saying that amateurs have nothing to teach us--I am a big fan of the guys who wrote Paradise Lot, for example--but verify your sources. 

Q3 - And would the nitrogen be added effectively with a no-dig method? Or would it dissipate too quickly because it's all on the surface?

Answer 3 - Nitrogen leaches out of soil when it rains. As I mentioned in my post, nitrogen particles the size and composition that plants can use are negatively charged just like the clay colloid particles that the other, positively charged minerals "stick" to (the process is called adsorbtion). Think of two same-poles of magnets pushing each other away: that's how nitrogen reacts to your soil. No-dig gardening will supply a lot of nitrogen, but you may have to keep adding more, depending on the weather and other factors. By the away soil testing does NOT check for nitrogen. It can't; the stuff washes away too quickly for a test to be accurate.

Mackay's picture
Mackay
Status: Member (Offline)
Joined: Mar 12 2013
Posts: 17
Thanks Wendy! The .edu hint

Thanks Wendy!

The .edu hint should be particularly useful. It should cut out a lot of noise.

You also answered my unasked question about the frequency of soil testing.

Cheers,

Mackay

pinecarr's picture
pinecarr
Status: Diamond Member (Offline)
Joined: Apr 13 2008
Posts: 2244
Thanks Wendy!

Wendy, thanks so much for this series of articles you've started.  I am struggling (with limited time for research, and what appears to be a "brown thumb" as a result of a need to know more/better) to learn more about soil composition and nutrition.  So your tutorials from your Master Gardening classes and gardening experience are really appreciated!

Oliveoilguy's picture
Oliveoilguy
Status: Platinum Member (Offline)
Joined: Jun 29 2012
Posts: 578
Great Job Wendy

I've finally come to understand that you feed the soil and it will nourish the plants. Aquaponics is a good analogy. We feed the fish and they fertilize the plants indirectly. I'll be looking forward to your articles. There is so much to learn here.

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.
Login or Register to post comments