Soil pH

What is Your Soil’s pH and What to Do About It

The Basic Idea

Acidity and basicity are measured on a pH scale of 1 to 14. A pH of 7 is considered neutral, lower numbers are acidic and higher numbers are basic. Depending on a soil’s pH some nutrients may or may not be able to be absorbed by plants.

The ideal soil pH for nutrient uptake is slightly acidic at about 6.8 on the pH scale. For optimum uptake of nutrients, soils that are not around 6.8 should be amended, or at least improved some.

Soils that are too acidic should be limed in order to raise soil pH; soils that are too basic should be acidulated in order to lower soil pH. Liming soils is effective in raising soil pH for long periods of time; acidulation, on the other hand, is unfortunately much shorter-lived.

Compost tends to be acidic, and can help basic soils with a bit of work.  Pine straw (pine needles) are also quite acidic and will help acidify soil as they leach compounds into your soil.

Property owners of basic soils who don’t want to invest much time in their soil may have an easier time gardening by planting base-tolerant plants than trying to lower their soil pH.

The Details

Like us, plants must have certain nutrients in order to function properly and survive. Plants can get their nutrients from the air, water, or the soil in which they grow. Of these three sources, the soil is probably the greatest source for plant nutrients, but whether or not these nutrients are actually available for plant uptake depends largely on the acidity or basicity of the soil. If soils are too acidic then some nutrients can’t be absorbed; if soils are too basic then other nutrients can’t be absorbed. Ideally, a soil should have a good balance such that it is neither very acidic nor very basic.

blueberriesHow acidic or basic (or alkaline as some prefer to call it) a soil is, is measured on a pH scale of 1 to 14 where a pH of 7 is neutral and pH numbers that become continually less than 7 become increasingly acidic and pH numbers that become continually more than 7 become increasingly more basic.

Interestingly there is a trend between average rainfall per year of a given region and that region’s soil pH. Areas that experience heavy rainfall generally have acidic soils. Areas that experience little rainfall generally have basic soils. To use two extremes as an example: rain forests have very acidic soils while deserts have very basic soils.

Most of the nutrients that are required by plants for their maintenance have an ideal pH range in which they are best absorbed by plant roots. Scientists have found that the ideal overall pH for optimum absorption of most nutrients is 6.8, slightly acidic. With 6.8 being the ideal, a good gardener should seek to amend his soil to achieve a soil pH of 6.8 or at least come close to it. Any soil that has a pH range within 6.3 to 7.3 is considered good.

I saw a graph like the one below during my schooling, but I was not able to find a link to it online, so with the information I had I drew one up myself.

It gives you a general idea of the absorption ability of different nutrients depending on pH.  You can see, for example, that the absorption of iron is decreased as the soil becomes more alkaline.  Iron deficiency is very common in arid parts of the world.

If you live in rainy regions of the world, your soil is likely to be acidic. Acidic soils are fairly easy to correct by adding basic soil amendments to the soil called limes. Soil limes are usually compounds of calcium or magnesium, such as calcium carbonate or magnesium carbonate.

These compounds are chemically basic and thus act to neutralize the acid in acidic soils once they are applied. Coincidentally, calcium carbonate and magnesium carbonate happen to be the major ingredients in over-the-counter antacids, such as Tums or Rolaids.

One thing to note is that liming a soil takes time; generally, a limed soil takes up to a year to reach the desired pH increase. So don’t be alarmed if your soil is still acidic shortly after a lime application. The great thing about liming acid soils is that the effects of the lime can last for several years.

If you live in a desert, or more arid region of the world, your soil is likely to be basic. Basic soils, unfortunately, are quite a bit more difficult to change than acidic soils. Unlike acidic soils, basic soils by their nature are very strongly buffered; meaning that basic soils strongly resist changes in pH, especially changes in pH that are acidic.

High pH or basic soils can be amended by a process called acidulation. Acidulation is best done by adding elemental or pure sulfur to a soil. Once in a soil, sulfur is converted into sulfuric acid by bacteria. Though this may seem like an effective amending process (it does help a little), the buffering capacity of basic soils is so great that the effects of acidulation are short-lived.

One good thing is that compost, organic matter, and manure tend to be acidic, so a gardener in a dryer area can improve the nutrient- and water-holding capacity of his/her soil by adding compost or manure, and at the same time bringing the pH down a little.

There are a handful of plants, such as blueberries or rhododendrons, which must have an acidic soil in order to grow.  Wherever you plant these plants, be sure the soil is acidic or you won’t get a very healthy plant, even if it manages to grow a little.


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