Expert Advice

An Overview of the General Soil pH by Region in the U.S.

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Earth Science

We may not realize it, but we use chemistry solutions in our daily lives all the time. Whether it’s taking an antacid to balance the chemical composition of our stomach contents for digestive purposes, or spraying chemicals on surfaces in our home when we clean, we initiate chemical reactions on a daily basis without even thinking about it.

Those chemical reactions also take place outdoors on a daily basis, in the soil of our yards and gardens. Just as our stomach needs the right balance of acid and alkaline in order to digest the food we eat, plants need the right balance of acid and alkaline in their soil in order to absorb and digest the nutrients in that soil, referred to as soil pH. This balance of acid and alkaline is measured on a scale of pH, which stands for the “power of hydrogen.” It refers to the concentration of hydrogen ions in the soil, which affects each plant’s ability to absorb available nutrients.

Understanding Regional Differences in pH

When we talk about plants being “native” to a particular area of the country, that means they have developed over many generations to be perfectly suited to the pH of the soil in which they have grown. When we take a plant out of its native habitat, we usually have to give it more or less water, and a specific type of fertilizer, in order to create a micro-habitat that is favorable to that plant’s health and growth.

If we think for a moment about how different the hot, dry desert of Arizona is when compared with green, rainy Oregon, we can understand that there is a lot of variety in the climates around the U.S., and subsequently a variety in soil pH. As a general rule of thumb, soils that receive a lot of rainfall are generally more acidic than those that are dry. There are, however, many additional factors that determine the pH of a region as well.

Acidic Soils in the U.S.

Generally speaking, soils in the U.S. are moderately acidic in the Eastern and Southeastern portions of the U.S. and the Pacific Northwest, which includes the western portions of Washington, Oregon and Northern California. Rainfall is greater in these areas of the country, and over time, rain leaches away the alkaline elements in soil, creating a more acidic soil.

In addition to receiving more rainfall than other areas of the country, these regions have more tree cover as well. Fallen leaves and pine needles slowly decay and contribute to the acidic level of the soil. The rocks underlying the soil also impact a soil’s pH, as can be seen in these regions of the country as well, where granite rock contributes to the acidic character of the soil.

In addition to rainfall, organic matter, and underlying rocks, a final contributor to the acidic nature of soil is the harvesting of crops. When crops absorb the alkaline elements they need for growth, and those crops are then pulled from the ground, they leave behind a more acidic soil.

Alkaline Soils in the U.S.

Generally speaking, alkaline, or “basic,” soils in the U.S. are found in the Western half of the country, with the exception of the Pacific Northwest areas noted above. Here, there is much less rainfall to leach alkaline elements from the soil, and fewer trees to contribute their acidic leaves and needles to the soil. Furthermore, the underlying rocks in these areas are more likely to be alkaline shale or limestone, rather than acidic granite.

Neutral Soils in the U.S.

Soil in the Midwest and Great Lakes areas tends to be closer to neutral, because of a mix of factors. Although these areas have less rainfall and tree cover than the Eastern and Southeastern areas of the U.S., they have also been subjected to intensive farming over a number of generations, which has helped to increase the acid levels in what might otherwise be alkaline soils.

The Melting Pot Effect

While soil tends to have similar characteristics based on the region it’s in, soil within a region can still vary greatly. Whether transported for use in construction, agriculture, or landscaping endeavors, bags, bins, and barge-loads of different soils (as well as specific additives such as peat moss and compost) have been transported across the country to “amend” soils, creating a variable “melting pot” of soil conditions. Many regions of the U.S. now reveal a complex and ever-changing map of checkerboard micro-climates with differing pH values in close regional proximity.

The transportation of plants out of their “native” habitats can also result in adaptive changes over time, and the eventual development of new plant variants that can thrive more easily in their newly-native soil. In the same way that we humans are adapting to our current diet with the help of antacids, plants are adapting to new soil conditions with the help of amendments. The fundamental pH of a particular region may not change, but a plant’s ability to adapt and thrive can increase with the help of a little understanding of soil chemistry.