Understanding pH and Why it is Important

I've attached some photos of the raised bed gardens that I've installed this year, showing the progression of the plantings. They look fantastic so far and we're all looking forward to some wonderful fresh vegetables very soon.

This week we'll spend a little time working on how to make great garden soil out of the good soil you have. Soil is the medium that garden plants grow in and it doesn't just happen to be great by accident, it requires a concerted effort and lots of patience. Here's the first thing you need to know:  

• “A plant grows in direct proportion to the least available nutrient.”

In order for the nutrients to be used, the first step to good soil is to measure and adjust the pH.

PH is a measure of “potential hydrogen ions” (defined as the negative logarithm of the hydrogen ion concentration) in a compound (soil), and registers on a scale from 0-14.

Compounds are considered “acidic” if the pH is between 0 and 6.5 (lemon juice, vinegar and battery acid are in this range); “neutral” if the pH is between 6.5 and 7 (pure water, saliva and blood are neutral); and “alkaline” or “basic” if the pH is between 7 and 14 (seawater, ammonia and bleach fall in this range).

Most soils fall between 4 and 8. Without even looking at your soil I am fairly confident that if you live in Sudbury your soil is too acidic. At the opposite extreme, if you live in the desert your soil is probably too alkaline.

When you apply fertilizer, you’re making sure your plants get a lot of each of the nutrients they need in order to have good, healthy growth. However, if the pH of your soil is not right, it can prevent the fertilizer from being effective.

In other words, if the pH isn’t close to what the plants require some of the nutrients can’t be dissolved in water. For example potassium doesn’t dissolve readily in acidic conditions. If the nutrients aren’t dissolved the plant can’t absorb them. Some of the symptoms are: the plant won’t grow; it won't produce to its full potential,; it won't be able to fight off diseases.

Here are preferred pH levels for some common plants:

• Rose 5.5-7
• Lily 6-7
• Azalea 4.5-5
• Mums 6-7.5
• Holly 5-6
• Lilac 6-7.5
• Rhododendron 4.5-6
• Tomato 5.5-7.5
• Onion 5.8-7
• Cucumber 5.5-7
• Spinach 6-7.5
• Beans 6-7.5
• Corn 5.5-7.5
• Rhubarb 5.5-7
• Maple Trees 6-7.5
• Oaks 5-6.5
• Spruce 4-5
• Pines 5-6
• Apple trees 5-6.5
• Blueberries 4-5
• Peach 6-7.5
• Raspberries 5.5-7
• Strawberries 5-6.5
• Kentucky bluegrass 5.5-7.5

Soil pH is very easily determined. In the old days people used to taste their soil and if it had a sweet taste it was alkaline or if it had a sour taste it was acidic. Not being above eating a little dirt I’ve tried it and I can’t honestly tell any difference.

I find it much more accurate to use a soil pH tester from the local hardware store or garden store. It has two prongs sticking out the bottom and a meter with a moving arrow on the front and gives instant results that are good enough for my needs. There are also home kits with dyes and color charts, and you might like them better. The means doesn't matter, only the end result.

If you’re starting a new garden or you’ve moved to a new area, you might want to get a soil test kit from the Cooperative Extension Service at UMass. Amherst and they’ll send you a fantastic analysis with recommend additives and quantities to make your soil pH correct. That way you’ll have a starting benchmark to compare all your future tests against.

To test your own soil, you need to start with distilled water, because it contains no chemicals or minerals in it that might change the test readings. I get it at the drug store in town (it’s most popular use is in steam irons) but have been known to use rainwater when I don’t have any. Mix this with a little of the soil in a glass container (plastic is OK but never use a metal container) and stick in the probes.

Once you have your results, you’ve got to add soil amendments to change the pH in the direction needed. If your soil is too acidic you add ground limestone. It is also sold as hydrated lime, dolomite (with extra magnesium) and calcitic lime. Lime not only replaces hydrogen ions and raises soil pH; it also supplies two nutrients – calcium and magnesium to the soil.

You can also add fireplace ash, crushed marble, or crushed oyster shells. Bone meal used to be recommended, but it is not recommended anymore because of the remote possibility of spreading mad cow disease.

If your soil is too alkaline you will need to add ground rock sulphur (also called flowers of sulphur) if it’s available. You could also add sawdust, composted leaves, wood chips, cottonseed meal, leaf mold, or peat moss. As to which is the better additive, the plants don’t really care so go with what makes you happy or what’s on sale.

The best way to adjust pH is gradually, over several seasons, and never try to change the pH more than 1 point per season. There's an urban legend about the guy who added so much lime to his garden that it turned to cement. I'm sure there's probably a video on YouTube about it somewhere.

Contrary to what most literature will make you believe, it’s not an exact science and won’t happen overnight – be prepared to wait as it may take a year or more for the pH adjustment to make nutrients available to your plants. Also, allow plenty of time, a minimum of one month, between adding amendments and planting, as most of them are very caustic to new plants.

The good news is that no matter what you do, it's really difficult to "break" your soil. So take it slowly, make adjustments regularly, and keep records of what you did so you can do it again, or avoid doing it again depending on your results.

Some garden trivia for the week:

When you give someone roses, the color can have a meaning:

• Red = Love and respect
• Deep pink = Gratitude, appreciation
• Light pink = Admiration, sympathy
• White = Reverence, humility
• Yellow = Joy, gladness
• Orange = Enthusiasm, desire
• Red & yellow blend = Gaiety, joviality
• Pale blended tones = Sociability, friendship