Soil pH levels. What every gardener should know

Soil pH is a measure of soil acidity or alkalinity, with values on a scale of 0.0 (most acidic) to 14.0 (most alkaline). Acidic soils have a low pH value (less than 5.5), while alkaline soils have a high pH value (more than 8.0). Soils with a pH between these values are considered to be more or less neutral.

pH stands for Potential Hydrogen. pH is the measure of hydrogen ion concentration, the element that spurs the formation of acids in the soil. The pH factor of soil reflects its acidity level, which is important to consider because all plants require different levels for proper growth. The soil’s acidity level also affects the dispersal of other important nutrients in the soil, and an imbalance can block a plant’s ability to absorb them. Testing pH levels is important, particularly when planting a garden for the first time in new soil whose acidity is unknown. This can be done using a home kit or by sending soil samples to your local country extension.

pH Requirements

When planning a new garden, it’s important to know if your soil is suited to the types of plants you will grow. The soil’s pH is rated on a scale of 3.5 to 9.0, and most plants do best in soil that tests within the neutral range of 6.0 to 7.0. Growth may still occur if the soil tests higher or lower than this, but plants may exhibit the effects of an improper balance through poor development and fruiting.

Effects of pH Imbalances

Major effects of extremes in pH levels include gaps in nutrient availability and the presence of high concentrations of minerals that are harmful to plants. In very alkaline soil, certain micronutrients such as zinc and copper become chemically unavailable to plants. In very acidic soil, macronutrients such as calcium, magnesium and phosphorous are not absorbed while others reach toxic levels.

Nutrient Availability

Important nutrients are absorbed by plants at varying levels of effectiveness based upon the soil’s acidity level. Nitrogen, potassium, sulphur, magnesium and iron are available along a broader range of acidity, while the availability of phosphorus, manganese, copper, boron and zinc lessens as alkalinity increases. Molybdenum, a trace nutrient, increases in availability proportionate to the soil’s alkaline level.

Additional Effects

In addition to affecting how nutrients are dispensed to growing plants, pH levels also influence microorganic activity that contributes to the decomposition of organic materials. A neutral pH is ideal for microbial action that produces chemical changes in soil, making nitrogen, sulphur and phosphorus more available. A pH that is either too high or too low may also interfere with the effectiveness of pesticides by changing their basic composition or weakening their ability to kill unwanted insects. Correcting very acidic soil usually involves working lime into the soil a few weeks before planting, while correcting alkaline soil normally calls for the addition of gypsum, which also reduces the high sodium content often found in such soil.

Soil pH is a measure of soil acidity or alkalinity, with values on a scale of 0.0 (most acidic) to 14.0 (most alkaline). Acidic soils have a low pH value (less than 5.5), while alkaline soils have a high pH value (more than 8.0). Soils with a pH between these values are considered to be more or less neutral.

Optimum levels

Slightly Acidic – pH 6.1 to 7.0. This is usually the best level for most plants. Nutrients are readily available, bacteria and earthworm activity is optimum, resulting in good renewal of soil with new matter being quickly broken down

How do you adjust the pH?

Before attempting to change your soil’s pH, you must know its current level. This will determine how much you need to raise or lower it, if at all. A simple soil test can be done at home or by a soil-testing laboratory. You must also know your soil’s texture, be it clay, sand, or something in between. More material is needed to change the pH level of a clay soil than for a sandy soil because the charged surfaces of clays make them more resistant to pH changes than the uncharged surfaces of sand particles.

Generally, limestone is used to raise a pH level, and sulphur is used to lower it. Limestone is relatively pure calcium carbonate, but dolomitic limestone is a mix of calcium carbonate and magnesium. Kilogram for kilogram, dolomitic limestone neutralizes more acidity than pure limestone and adds magnesium to the soil, perfect for gardens where this nutrient is naturally low.

Limestone and sulphur are available in powdered or pelletized form, with the latter being easier to spread uniformly and causing less of a health hazard from dust. Neither limestone nor sulphur is soluble in water, so mix these materials thoroughly into the top 6 inches of soil when quick action is needed. Or else, just lay the material on top of the ground, and let it gradually work its way down

Plants need light, air and nutrients to thrive. They derive their nutrients from soil or, in hydroponics, nutrient-rich water. In soil gardens, you need to water the garden frequently so the good bacteria, fungi and microbes can add nutrients to the soil for the plants to use. Plants also need water for cellular function. The type of water used, both in soil gardening and in hydroponics, can affect plant growth.

There’s no need to fear soil science. Make friends with it, grab a testing kit, and get to know your soil. Your garden will thank you!