How Sweet is Your Soil?

The term pH stands for the potential Hydrogen and is measured on a scale between 0 and 14.  What’s being measured is the concentration of hydrogen (H) ions — the more hydrogen ions there are, the more acid the thing being measured is. Things having a pH below 7 are considered acid, while those above 7 are basic or alkaline; neutral is right in between at 7.  Battery acid, vinegar and orange juice are very acidic materials while milk of magnesia, household ammonia and lye are very basic.  Blood, milk, and pure water fall into the neutral range. The scale is not a simple arithmetic scale, but a logarithmic one.  Each pH unit represents a tenfold increase or decrease in relative acidity or alkalinity.  For example, if your soil has a pH of 6, it is 10 time more acid than a desired soil of pH 7.  If the pH was measured at a pH of 5, it would be 100 times more acid than the optimum pH of 7. 

The pH of your soil impacts plant growth in four ways:

  • affects the availability of essential nutrients to the plant
  • the level, type, and vigor of biological activity in the soil
  • the root’s ability, at the cellular level, to absorb both water and nutrients
  • the solubility of toxic materials

Plant roots absorb nutrients like nitrogen and phosphorus only when they are dissolved in water. Nutrients that are in either very acid or very alkaline soils dissolve very slowly, if at all. If they are not available to the plant, they are considered “locked up.” Some minerals that are present in small (and safe) quantities in the neutral pH range are much more available in acidic soil (aluminum, manganese, and iron) or alkaline soil (molybdenum) becoming toxic to the plant. The chart illustrates the materials availability at different pH levels – the thicker the bar, the more available the nutrient.  Nitrogen (nitrates), the most abundant and readily used nutrient in the soil, abundant at pH 6 – 7.5 is almost non-existent at a pH below 5. There may be plenty of nitrogen in the soil, but low soil pH means the plant can’t use it. Conversely, iron – an essential component of healthy green leaves, is potentially toxic at the low pH range, but significantly less so even in the neutral range.

Availability of nutrients in soil is dependent on the pH
Chart courtesy of Clemson University

An essential component to growing great roses is soil with a neutral pH – falling right in the middle of the range, as it affects nearly all soil properties that roses need to grow, thrive, and fight off diseases. Ever heard a seasoned gardener or farmer refer to a soil as being sweet?  Farmers used to taste soil to see if it was acid (sour like vinegar) or alkaline (more bitter). If it was on the bitter side, they’d call it “sweet” since it was the opposite of sour. 

How do you measure the pH of your soil?  The easiest way is to purchase a simple test kit available at your local nursery.  Or use a soil pH meter – a hand-held battery-operated device that will give you an instant reading. Be sure to take samples from a variety of areas within the garden; there may be differences due to surrounding trees, soil content, drainage, and other factors. 

By Nanette Londeree, Master Rosarian

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