More advanced topics- Soil pH

As I learned from my daughter's grade 8 science class notes pH stands for potential of Hydrogen, and it's determined by measuring the number of hydrogen ions. Water has a neutral pH with a balanced number of hydronium and hydroxide ions. This pH is measured as 7. Anything less is acidic and anything above is basic (or alkaline). Lemon juice is 2.2 pH, so it is highly acidic. The best range for plants is between 5.5 to 7.5 (so acidic is better than alkaline), although some plants have evolved to survive in soils outside of that range (like Arabian coffee and highbush blueberry).




Soils tend to acidify over time in a warm, humid environment. Rain water is slightly acidic due to higher concentrations of carbon in the air. As well, it leaches ions from the soil. Excessive fertilizer containing ammonium nitrate (used to add nitrogen into the soil), and some plants acidify the soil around their roots to help absorb certain nutrients.

Salts, calcium, manganese and potassium can make a soil more alkaline.

If a soil is too acidic it can inhibit plants from taking up the last three nutrients, and also an acidic soil can poison the plant with aluminum, which is a natural component of most soils. Slightly acidic soils do retain moisture very well, however too acidic soils retain too much moisture and won't drain, causing roots to suffer moisture stress.

A common and practical method for measuring soil pH is using litmus test strips. Litmus is a water-soluble mixture made from lichen dye extracts, and was first used in 1300 CE by a Spanish alchemist by the name of Arnaldus de Villa Nova. The word litmus comes from the Old Norse word litr, which means colour. Litr is also the name of both a giant and a dwarf who are killed by Thor. The name appears a few times as a dwarven name, and once for a giant. Litmus paper turns blue for acid (below pH 4.5) and red for alkali (above pH 8.3). Although, what we tend to use today is called a Universal Indicator strip, which uses a variety of chemicals laid down on a strip of paper and gives a more nuanced indication of pH. If it only turns red or blue it's a litmus strip. If it could become a rainbow of colours then it's a Universal Indicator.

If you want to increase the acidity of soil you can add peat moss or pine needles. The most common way to decrease acidity is to add agricultural lime. Compost is naturally slightly acidic, but less so than peat. I've also read that some stables add lime to their manure piles to decrease the odour, so if you're picking up some from a local farmer make sure to ask about that. Manure tends to naturally be higher in calcium and magnesium (therefore less acidic), which makes phosphorus more available to plants and reduces aluminum toxicity.

Sandy soils tend to be more acidic and clay soils tend to be more neutral.

 Honestly, unless I'm growing blueberries, or something with highly specific needs I just throw compost and (composted) manure into the soil and have at 'er. My husband drinks a lot of coffee, so I put the grounds as mulch around my blueberries. It works great, and the blueberries love it. Using pine needles as mulch is also a very good idea for them.


So, now you know.




More reading-

Soil pH - from Rodale's Organic Life

http://www.rodalesorganiclife.com/garden/beginners-guide-organic-gardening
https://en.wikipedia.org/wiki/Soil_pH
https://en.wikipedia.org/wiki/PH
https://en.wikipedia.org/wiki/Litmus
https://en.wikipedia.org/wiki/Litr
http://www.rodalesorganiclife.com/garden/how-get-accurate-soil-test
http://www.rodalesorganiclife.com/garden/soil-chemistry
http://animalwaste.okstate.edu/welcome-folder/pt98-7manureraisessoilph.pdf