In the early seventeenth century, when our immigrant ancestors first settled what would become the USA, they tasted their soil to determine its potential as farmland. This practice persisted well into the twentieth century. A sweet taste told them the soil was neither acidic nor basic but around neutral; a sour taste indicated acidity and a bitter taste alkalinity.
Acidic and basic describe attributes of a chemical property, but soil itself is neither. It is the soil solution, the free water in contact with soil, that makes soil acidic or basic and gives it a characteristic taste. Those old farmers knew that most of their crops would thrive in a sweet-tasting soil. No doubt many were proficient at accessing their soils this way, but today we test a soil’s “pH”.
pH quantifies the degree of acidity of a solution. The concept was introduced by Danish chemist S.P.L. Sørensen in 1909. “H” stands for hydrogen, but no one knows for certain what “p” actually refers to. Effectively, p is a constant and stands for “negative logarithm”, which is how Sørensen defined pH (the negative logarithm of the concentration of hydrogen ions in a solution). Speculation is that p stands for “power” or “Potenz”, the German word meaning power. Others believe it refers to “potential”. In any event, any chemical calculation involving acids or bases will include p.
Sørensen set the pH scale from 0 to 14. A pH value of 7 is neutral (neither acidic nor basic). Values less than 7 are acidic; those greater than 7 are basic. Because the pH scale is logarithmic, a value of 6 is 10 times more acidic than a value of 7; it is 100 times more acidic than a value of 8 (alternatively, a value of 8 is 100 times more basic than a value of 6).
pH is based on the composition of water: hydrogen (H+) and hydroxide ions (OH-); the chemical formula for water is H2O. Pure water, freshly distilled, is neutral and by definition has equal amounts of H+ and OH-. These ions are attracted to each other by electrostatic charge, and they unite and separate constantly, coming together as water itself and separating as free ions. Stated another way, the different forms of water are in equilibrium with each other, as represented below:
H2O ↔ OH- + H+.
If acid is added to pure water, the concentration of H+ dominates, and the solution is acidic; if base (OH-) is added, the solution becomes basic. As a reference, battery acid (H2SO4) is about pH 1; vinegar is pH 3; human blood is pH 7.2; seawater is pH 8; and drain cleaner (lye, NaOH) is pH 13.3.
Soil is more than a simple repository of minerals, and its characteristics are dramatically influenced by pH. It is composed of variously sized particles, both organic and mineral (inorganic) in origin. The organic portion is derived from products of living organisms or the decomposition of those organisms. Inorganic particles contain no carbon and result from weathering of bedrock. Soil also abounds with life.
Between soil particles is pore space; air and water fill this void. In very fine soils, the pore spaces are very small; in coarser soils, the pore spaces are larger. This is important because pore space determines the degree of air and water conductivity within a soil and affects the ability of soil to retain water and nutrients.
Organic particles come in all different sizes; examples can be found in the thatch layer of a lawn or the debris of a forest floor. A mineral layer lies below this debris; this layer is where most life in the soil occurs and includes most of the organic matter accumulation. It is a layer rich in humus, the smallest of the organic particles.
A boulder could be considered a soil particle, but it is sand, silt, and clay that are the mineral particles vital for plant growth. Sand is the largest (0.1–1 mm in diameter; 25.4 mm per inch) and clay the smallest (≤0.001 mm). Alone, none of them make up an adequate soil, but the three together constitute a “loam”. The ideal garden (or agricultural) soil is a loam composed of roughly equal amounts of sand, silt, and clay. Loams contain more nutrients and humus than sandy soils, have better infiltration and drainage properties than silty soils, and are easier to cultivate than clay soils. They are chemically active and retain water and minerals.
Most plants grow best in soil that is neutral to slightly acidic. At this pH, plant-required minerals (nutrients) are available and microbes convert inorganic nitrogen into plant-usable forms. The essential minerals, by convention, are usually divided into two groups: those needed in relatively large amounts (macronutrients) and those needed in small amounts (micronutrients).
Of the macronutrients, carbon, hydrogen, and oxygen are readily available in air and water. Most soils have enough calcium, iron, and magnesium. But nitrogen, phosphorus, and potassium (not surprisingly, the ingredients in common fertilizer mixtures—called “NPK”) are insufficient in many soils. Also, in more and more soils, there is not enough sulfur. This is largely because of environmental regulations enacted several decades ago that limit sulfur’s release into the atmosphere. The micronutrients (cobalt, manganese, copper, zinc, silicon, molybdenum, boron, aluminum, and chlorine) are needed in tiny amounts only, but if lacking in a soil, acute plant problems result.
These nutrients are available to plants in the form of dissolved mineral salts. Soil particles interact with them, attracting and trapping them. They are metal ions that have positive electrostatic charges, just as hydrogen ions (H+) do. Clay and humus, the smallest soil particles, are negatively charged. Because opposite charges attract, in the soil solution under more or less neutral conditions, salts are attracted to and held by clay and humus in a weak chemical bond.
On a molecular scale, these metal ions (for example, Ca2+, Mg2+, Fe3+) are large relative to H+. Because of this size difference, H+ can displace a metal ion by slipping beneath it and adopting the charge that held it. In a soil that becomes acidic, excess H+ displaces the metal ions (the plant nutrients), which are then leached from the soil and lost to the plant.
Under normal growing conditions, plants use H+ in just this way to obtain nutrients. Root hairs growing among soil particles secrete H+ that causes bound nutrients to be released and to become available for plant absorption. But this process is disrupted at pH extremes.
Basic soils create plant-growth problems too. In many parts of the eastern USA, pin oak (Quercus palustris) is commonly sold in nurseries. It grows well in rich, moist, well-drained, slightly acidic soils but is a poor choice in regions with clay-rich, poorly drained soils. In these soils, pH 7.5 or more is common, and at this pH, iron, which is needed for photosynthesis, is converted to an insoluble form (rust), and the pin oak develops chlorotic (yellow) leaves and eventually dies.
Soils in areas with relatively high rainfall, such as the Northeast, tend to be acidic. Those early immigrants in New England amended their soil, as we do today, with crushed limestone (Ca2CO3) to raise its pH; later in Virginia, George Washington and other tobacco farmers used crushed oyster shells from the Chesapeake Bay for the same purpose. This is effective and necessary in many areas.
Soils of the western USA tend to be basic (alkaline). These soils developed over millennia in relatively low rainfall environments from alkaline parent materials (rock). Lowering pH with acid drenches or amending the soil with iron chelates is possible but effective only temporarily. Mulches and working peat, which is quite acidic, into soil will improve soil structure and lower pH somewhat. A good approach for soil of any pH is to use plants adapted to that pH. In the case of the pin oak, American (eastern) sycamore (Platanus occidentalis) or basswood (Tilia americana) do well in basic soils and would substitute well.
Knowing the pH of garden soil is important information, and it should be a prerequisite to fertilization. Fertilizing an acid soil is futile; nutrients are not retained and will merely contaminate neighboring water sources. And this knowledge may well explain apparent nutrient deficiencies or a lack of vitality in certain plants.
Please see our pH and soil test kit here.