Wednesday, July 28, 2010

Vertisols, veritably difficult.

Once upon a time, I took a class on tropical soil management (SOS 5132) by Dr. Hugh Popenoe, at the University of Florida.  Each day, we would get a slide show of pictures from farms all over the world that highlighted low-input agriculture.   It wasn't my typical technical science class; we learned from stories.  This post is on one of the many cool things I learned. 
I got this photo from here
According to the USDA-NRCS, vertisols are high shrink/swell content clays that have deep wide cracks in the dry season. They shrink (to the point of cracking) when drying and swell when absorbing moisture.  Also according to the USDA-NRCS, I live on a classic vertisol, the Houston Black Clay (associated sob story and fun fact).  The state soil of Texas.

According to my class notes, vertisols typically have a wavy, bumpy surface due to all of their shrinking and swelling.  They are usually dark brown, and are located on flat or low slope soils.  Although they have a high exchange capacity (ability to hold nutrients), their massive* structure makes it easy to erode.  

The shrinking and swelling can damage roots, and the soil is hard to plow unless it has just the right soil  moisture (what that magic number is, I can't tell you, it's magic).  If you plow it when it is too wet, you will be creating clods that harden as they dry, almost to the strength of  rocks.  

So how can you manage it? The notes say two things: 1) "Well, annuals, pasture and rice do best on the soil."  This makes sense to me since my soil developed with prairie vegetation from a parent material of calcareous clays and marls  2) "You must manage soil moisture, and cultivate at right moisture content."  

I have a third tip.  If you build a house on it, enjoy the options of either watching grass grow or the cracks on your wall grow**!!  They are about the same :-l

By the way, remember my post on the five soil forming factors?  Well, vertisols and alfisols (a better drained soil order) are both found in same climactic zone, with the same type parent material, but different topography. They both developed from and found on basic parent materials like andecite, limestone, and basalt.

Links for further study:
*massive in this case means structureless
** from your house foundation shifting

Sunday, July 18, 2010

Plants care about soil pH

Fact: Soil pH* affects the availability of nutrients for plants.

Here is a figure that shows the relationship of soil pH with nutrient availability to plants.  The thicker the color, the greater the availability of that nutrient to the plant.  For the most part, plants can get the most nutrition at pH values between 6-7.
Question: Why?
Answer: Well, one could write a book on this, a looooooong book, but this is a blog post, so let us settle with the main and demand!

For the most part, soil has a negative charge.  Negative charges attract positive charges (or cations*).  Hydrogen has a positive charge...

  • ...and when there is  a lot of positively charged hydrogen (i.e., less basic, lower pH), there is more of it available to bind to the negatively charged soil.  When more hydrogen is bonded to the soil, there is less of a chance for other positively charged nutrients to bind to the soil.  Without binding to the soil, they have a chance to nourish the plant, but they are also more easily leached away (like when it rains).
  • ...and when there is less hydrogen (i.e., more basic, high pH), there is less of it available to bind to the soil.  When less hydrogen is bonded to the soil, there is more of a chance for other positively charged nutrients to bind to the soil instead of being allowed to nourish the plant. (Link to reference)
More learning resources:
In case you ever found yourself in front of a classroom of middle schoolers, and all you had were test tubes, plugs, scoops, pipettes, graduated cylinders, universal indicator solution, beakers, three different soil types, a stop watch, a color chart pH scale, and nothing to talk about, I propose this lesson plan for you: Measuring pH in soil.  I found this at the University of Texas Environmental Science Institute.  It also has other learning resources for teachers and students.

Also,  I found a webpage with great links to nutrient management teaching modules.  Although they might be easier to read if you have some science background, I am still impressed with their clarity.


Soil pH: a measure of the soil’s acidity, or hydrogen (H+) concentration.  
pH = -log[H+], 
where [H+] = the hydrogen ion concentration. 
Because of the negative sign in the definition for pH, low pH soils have more hydrogen than high pH soils.  
Acidic soil: a soil with pH values <7 (high hydrogen concentration)
Alkaline soil: a soil with pH >7 (low hydrogen concentration)
Cations: We defined cations here.  They are ions that carry a positive charge of electricity.
Cation Exchange Capacity: the total negative charge on soil, which is a good measure of the ability of a soil to retain and supply nutrients to a crop.

Friday, July 9, 2010

As the soil turns.

Hello dear readers! I went on a quick trip to Swallow Falls State Park in Maryland.  Swimming in the waterfall was so much fun!  I took a few pictures as I thought about the difference between rocks and soil.

There are 5 factors that work together to form soil:
  1. Parent material - It can be mineral and/or organic (i.e. decomposing plants).
  2. Climate -  Different environmental factors (i.e. temperature and precipitation) affect chemical and physical weathering
  3. Living organisms - Among other things, they help organic matter biodegrade, and can mix soil layers/horizons into each other.  Their chemical reactions can cause chemical weathering.
  4. Topography - For example, valleys accumulate soil and water, hilltops shed them.  This affects the type of soil created. 
  5. Time - Neat factoid: sometimes a very chronologically old soil won't be very developed, and vice versa.
For more, there are quick explanations at NASA and the NRCS.

The Soil Science Society of America gives us a two part definition of soil. This is the first part: 
 "The unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants."
Below, I'm sharing with you my pictorial exploration of a plant's contribution to soil development.

Plant pioneers.  This guy might trap tiny rock fragments or organic debris which will grow his soil pile.  To be honest though, erosion is high here.  It was on a slope.  This soil might not develop further.

These guys have a small part in a very big process.  I imagine their roots will help make this crack a little bigger, making it able to hold even more soil, allowing even bigger plants to grow here in the future.

Growing sideways out of a crack between two rock layers.  Not much opportunity for soil development here, but don't tell her!

This shrub (tree?) has chutzpah.  It looks like it is growing out of the rock.

It looks like these guys are the furthest along in helping to develop soil, but they still have a looooong way to go!  When these plants die, they will contribute organic matter to the soil for new plants.  

Even if these little bits of soil are chronological Methuselahs, environmental factors prevent them from developing horizons.  Mayhaps erosion rates are faster than the rate of soil development.  If so, the soil would be characterized as an Entisol.  Entisols are the youngest (meaning least developed) of the 12 soil orders. 

And let's end on a lithologic music note...