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.

Link to image source.

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 idea...supply 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.

*Definiciones:

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.

Link to image source

The Skinny on Pine Needles

Pine needles were recently thrust into my consciousness by two friends. So, if you don't mind, let's discuss the skinny on pine needles, shall we?

But before we delve, side note! I did learn one consideration in its sustainability of use during my internet perusing.  Harvesting pine needles off the ground, like harvesting any other crop, ultimately reduces the nutrients in the soil, so conscientious pine needle collectors may want to give something back for the taking.

Moving on, did you know how awesome they are? They are a pretty good mulch! Look at all these reasons! Particularly on hillsides, since they interconnect with each other and stay put. But many people believe that pine needles, as mulch, add acidity. Perhaps they read this link (look under wheat straw). But what if you didn't think it was acidic at all, because you read this link? Or what if you were like, who cares, I read this pine needle apologist! So I asked the expert who wrote this, where he originally writes that pine needles should be put on acid-loving plants. He is Dr. Don Rakow, Executive director of Cornell Plantations at Cornell. He explained his text succinctly:

Pine needles have been shown to be slightly acidic, usually in the range of pH 6.0 – 6.5. This is not very different from average rainfall pH. The acidity is related to the presence of hydrogen ions in the volatile terpenes in the needles. For a review of a somewhat rigorous series of experiments that examined this subject, check out: Pine Straw (Pine Needle) Mulch Acidity: Separating Fact From Fiction Through Analytical Testing by Scott Jacobs.

So there you have it, it is acidic, but only temporarily and not enough to worry about. So all the supposedly conflicting information from reliable sources out there, were all right in their own way. So I guess this is another YES WE CAN, agree moment, heee.

By the way, you may be curious about terpenes.  They are "compounds produced by plants from isoprene, a hydrocarbon that has its five carbon atoms arranged in a branched chain." They smell really good, and in the leaves of a conifer tree suffering from air pollution, they change in chemical composition and amount (more evocative terpene-talk here, from where I got this info). Wow.

Pine needles by any other name would still smell like terpenes.

(P.S. One day I'll read the instruction manual on my camera, and maybe get a better pic for you)
(P.P.S. Speaking of soil myths and mulch, here are some interesting tidbits on mulch selection from an Australian soil blogger)

What are cations, why do I care.

Soil is so interesting, it makes me want to stare into space.

What are cations? 
They are ions that carry a positive charge of electricity (Reference 4). The common soil cations are calcium (Ca+2), potassium (K+), magnesium (Mg+2), sodium (Na+), and hydrogen (H+) (What is an ion, you ask.  An atom or molecule that has lost or gained a charge -by losing or gaining an electron).
Why do we care?
Many of these cations are nutrients for plants.  Different soils have a different capability to hold these nutrients (and other non-nutrients e.g. aluminum Al+3) and store them. This is quantified by cation exchange capacity.  The higher the exchange capacity, the less likely your favored cations will leach away from your beloved plant roots (and in tangential news, the lower your "base saturation", the less likely the mineral will give that cation up and let it go into the soil water solution for roots.  What is base saturation? I'll get back to you with an eloquent reminder, just... remind me).