Plant pH Preference
By: Milo Shammas, Founder & Formulator
Proper pH gets results
Basic requirements for a successful garden are highly visible, such as sunlight, water, and organic amendments. Unfortunately, you cannot see pH. It is a hidden abstraction buried deep in the soil, but an understanding of pH is invaluable. For example, proper pH makes it easier for roots to absorb essential nutrients, so it supports plant growth. Neglect of this important consideration of plant fertility will result in poor growth. Many times, gardeners will diagnose a plant as being chlorotic (nitrogen, iron, or magnesium deficient) and will add more nutrients to the soil without checking soil pH first. Desired results are not achieved, because soil pH will always be a limiting factor. Learn your soil’s pH; it’s worth getting it right.
Soil pH basics
The fundamental understanding of soil pH is simple. It is simply a way to measure a condition, not so different from using a ruler. Acidity and alkalinity are measured as pH (parts Hydrogen), expressed in a logarithmic scale from zero to 14. Specifically, acidity is associated with an increase in hydrogen ions, and alkalinity is with an increase of hydroxyl ions. The differences affect how molecules (or potential plant nutrients) will interact in the soil.
A pH of 7.0, like that of pure water, is neutral, because the concentrations of hydrogen ions and hydroxyl ions are equal. As the pH number goes lower than 7, the hydrogen ions increase and the soil becomes more acidic; as the pH number goes higher than 7, the concentration of hydroxyl ions increases and the soil becomes more alkaline.
Most garden plants grow best in a slightly acidic soil, with a pH between 6.0 and 7.0. Going from one pH number to the next makes a big difference, because the numbers change by factors of 10. A pH of 6 is 10 times more acidic than a pH of 7 and pH of 5 is 100 times more acidic than a pH of 7.
Acid soils are common on the East Coast, where rain leaches out calcium, and the underlying rock is largely magnesium. They typically range from pH 4 to 6, while alkaline soils in the dry Southwest arise from calcium-rich rock and range from pH 7 to 9.
pH affects nutrient uptake
You can lay out a nutrient feast for your plants, but roots may not be able to absorb it well if the soil pH is not appropriate. Phosphorous provides a great example. In importance to plants, phosphorous is a close second to nitrogen. The plant needs phosphorous for DNA, membrane development, and energy production. Phosphorous also participates in photosynthesis and sugar formation. But most of the phosphorous in soil particles is tied up in insoluble forms, unavailable to roots. It must be in solution in the form of orthophosphate for the plant to absorb it. At very low pH levels, the phosphorous is tightly bound to iron and aluminum in the soil. At high pH levels the phosphorous is locked in insoluble forms of calcium and magnesium.
However, there is a pH window between 6 and 7 where the mineral grip on phosphorous loosens and more orthophosphate dissolves. Also, the pH of orthophosphate in solution determines its electrical charge, which affects uptake. Around pH 6.0, orthophosphate has one negative charge, the form most favored for root absorption. Above pH 7.5, most of the orthophosphate has two negative charges and is absorbed more slowly.
Because of pH and the major effect it has, adding phosphorous to a garden can be tricky. Just because you put on three pounds does not mean three pounds will be available. Adding a fertilizer such as super phosphate temporarily makes more orthophosphate available, but most of the added phosphorous gradually turns to insoluble forms.
The good news is that the role of pH is less dramatic with other plant nutrients. Nitrogen, usually taken up by roots as nitrate nitrogen (NO³), is very soluble in the soil solution. Its uptake by roots is not influenced by soil pH. The same is true with potassium, which clings tightly to soil particles in layers of crystalline sandwiches and does not leach rapidly like nitrate.
Plants require minute amounts of other minerals, including calcium, magnesium, sulfur, iron and zinc, etc. Although the ideal pH for these elements varies, a pH of 6.5 is usually adequate to make sufficient amounts available. In alkaline soils, poor growth and yellow leaves are often encountered because plants cannot absorb enough iron and other nutrients since they become insoluble.
Measurement and the organic advantage
The first step toward proper pH is measuring your gardens pH values now. Kits that are available at your local nursery will give you a good approximation of your soil pH value. More exact results come from agricultural experiment stations or private labs.
If it turns out the soil is too alkaline, the acid level can be increased by mixing in materials such as peat moss or soil sulfur. Clay soils demand more of these amendments. The low pH of acid soils can be counteracted by mixing in crushed limestone or wood ash. The amount needed depends on the pH change and the soil texture.
Know what pH you have in your garden. A clearer understanding of your plants’ growing requirements can be achieved more easily when you know your soil’s pH. Knowledge equals success! The great thing about being an organic gardener is that you will typically not have to concern yourself with pH as being a serious concern.
Organic materials tend to bring soils to a slightly acid range after they have gone through decomposition and have contributed to the humus reserve in the soil. Alkali soils can be lowered and acidic soils can be elevated to a more neutral level. Organic components and microbes help to stabilize the soil and create a pH environment that is more productive for plants and microbes equally.