Soil pH or reaction is one of the most important elements for accurately understanding soil characteristics. We have already discussed various types of agricultural soil (loam, clayey, sandy, etc.) by analyzing their physical structure. However, when it comes to soil pH, our focus shifts to the chemical characteristics of the soil. The pH level directly influences the ability of plants to absorb necessary nutrients. Naturally, this significantly affects the ability of plants to inhabit a specific soil. Measuring the reaction with pH allows us to choose the most suitable species for the environment and make any necessary corrections or adjustments.
Let’s better understand what soil pH means and what practical implications it has for our organic cultivations.
Liquid and Solid Phases of Soil
Before delving into the definition of soil pH, let’s start by discussing the liquid and solid phases of the soil. The liquid phase, called circulating solution, consists of water and dissolved nutrients. It is crucial because plants absorb most nutrients along with water from the circulating solution. The composition and characteristics of the liquid phase of the soil vary significantly. To make a correct assessment, it must be related to the solid phase of the soil. Among the factors that determine the composition of the circulating solution are rainfall, irrigation, temperature, fertilization, and soil microbiological activity. Therefore, it seems to be a highly fluctuating value. However, in reality, these fluctuations are mitigated by an important property of the soil in its solid phase: ion exchange. This is a continuous chemical phenomenon that occurs between the solid and liquid phases of the soil.
Soil pH measures the concentration of hydrogen ions (H+) in the circulating solution, i.e., the liquid phase of the soil. The importance of this value is linked to its influence on all natural reactions, including the continuous ion exchange between the liquid and solid phases. Therefore, the soil reaction (or pH) decisively influences the functions of habitability and nutrition of plants. In theory, pH can range from 0 to 14, but these extreme values are not found in agricultural soils. Based on the pH value, soils are classified as follows:
- Very acidic, less than 4.6
- Acidic, between 4.6 and 5.2
- Moderately acidic, between 5.3 and 5.9
- Subacid, between 6.0 and 6.6
- Neutral, between 6.7 and 7.3
- Subalkaline, between 7.4 and 8.0
- Moderately alkaline, between 8.1 and 8.7
- Alkaline, between 8.8 and 9.4
- Very alkaline, greater than 9.4
Most agricultural species prefer a pH close to neutrality, but there are important distinctions and clarifications to be made.
Implications on Agricultural Crops
The most important nutrients for the healthy and thriving growth of crops are nitrogen (N), phosphorus (P), potassium (K). These elements are more available to plants on soil with a pH close to neutrality. If the pH value is below 5.5, nutrient absorption is compromised. Roughly the same can be said for other crucial elements such as calcium (Ca), magnesium (Mg), and molybdenum (Mo), which, however, have greater assimilability with a subalkaline reaction (pH between 7.4-8.0). Soils with acidic or subacidic pH conditions favor the absorption of iron (Fe), manganese (Mn), boron (B), copper (Cu), and zinc (Zn). As for microbial activity, an acidic pH generally favors the development of fungi at the expense of bacteria. The latter promote nitrification and nitrogen fixation processes and thrive under favorable conditions in soils with neutral or moderately alkaline pH. Thus, both too low and too high soil pH values are considered unfavorable. Soils with such characteristics are referred to as having “anomalous reaction”. However, it is possible to correct these types of soils, and we will see how in the next chapter.
The Preferred Soil pH for Plants
Before understanding how to correct soil pH, let’s see what type of soil plants prefer. In nature, plants have a specific predisposition to live and develop in different types of soil. Some naturally thrive in alkaline soils (alkali-loving plants), while others prefer acidic soils (acid-loving plants). Observing spontaneous flora can provide useful indications about the chemical reaction of the soil. For example, the presence of plants like scotch broom, tamarisks, and licorice indicates alkaline soils. Conversely, ferns, lupines, rhododendrons, heather, and blueberries indicate acidic reaction soils.
Below is a list of plants with their associated preferred soil pH range.
Ideal Soil pH for Fruit Trees
- Apple: between 5 and 6.5
- Apricot: between 6 and 7
- Cherry: between 6 and 7.5
- Grapevine: between 6 and 7
- Lemon: between 6 and 7
- Orange: between 5 and 7
- Peach: between 6 and 7.5
- Pear: between 6 and 7.5
- Plum: between 6 and 7.5
- Pomegranate: between 5.5 and 6.5
- Walnut: between 6 and 8
Ideal Soil pH for Vegetables
- Potato: between 4.5 and 6
- Watermelon, Strawberry, and Peas: between 5.5 and 6.5
- Turnip, Cucumber, Eggplant, Pepper, Parsley, Zucchini: between 5.5 and 7
- Carrot, Chicory, Onion, Garlic, Bean, Green Beans, Fava Beans, Lettuce, Tomato, Radish, Spinach: between 6 and 7
- Cauliflower, Cabbage, Savoy Cabbage, Melon, Celery, Pumpkin, Turnip Tops: between 6 and 7.5
- Asparagus, Basil, Swiss Chard from the garden: between 6.5 and 7.5
Ideal Soil pH for Ornamental Plants
- Heather, Gardenia: between 4.5-5.5
- Hydrangea: between 4.5 and 6.5
- Azalea, Rhododendron: between 5 and 5.5
- Bromeliads, Rubber Plant, Fuchsia: pH between 5 and 6
Anthurium, Caladium, Dieffenbachia, Orchids, Philodendron, Pothos: pH between 5 and 6.5
- Ferns: between 5.5 and 6
- Cacti, Cyclamen, Gerbera, Poinsettia, Asparagus Plumosus: between 5.5 and 6.5
- Geranium, Saintpaulia: between 6 and 6.5
- Bougainvillea, Calla Lily, Freesia: between 6 and 7
- Cineraria, Chrysanthemum, Strelitzia: between 6 and 7.5
- Asparagus Sprengeri, Carnation, Hyacinth, Rose: between 6.5 and 7.5
Acidic Soils and Correction
The acidity of soil can have various origins and is closely related to factors such as climate, agricultural use, and the type of rock that formed it.
As seen, a slight degree of acidity is not considered a severe inconvenience since many crops thrive well with soil pH from 6 to 7. However, soils with lower pH can pose a significant problem.
Cultivating acidic soils either requires the use of suitable species or adequate correction.
In organic farming, it’s possible to distribute substances on the soil to raise pH. These include wood ash, which you can easily produce if you have a fireplace or wood stove; quicklime (which you can find here), slaked lime (which you can find here); finely ground limestone (more or less pure calcium carbonate) and dolomite (calcium and magnesium carbonate).
Alkaline Soils and Correction
The excessive alkalinity of soil is almost always due to an excess of calcium carbonate, also known as limestone. It should be noted that a minimum amount of limestone in the soil is necessary because, in addition to the calcium’s nutritional function for plants, it contributes to some important phenomena that occur in the soil.
The alkalinity caused by the presence of limestone is called constitutional alkalinity and usually never reaches values higher than 8-8.4.
However, higher pH levels with an excessive amount of limestone can be harmful to plants. Firstly, this excess causes the formation of a crust on the surface, making it difficult to work the soil. Often, the soil becomes muddy in the presence of continuous rainfall. Another possible problem is the insolubilization of phosphorus and iron (resulting in the typical iron chlorosis of citrus fruits).
Limestone soils are generally clayey. Correcting this type of soil is more difficult, and many farmers limit themselves to choosing the most suitable species. The recommended intervention in organic farming is organic fertilization. To carry it out, mature manure or a large quantity of domestic compost is needed.
Another trick (applicable on small surfaces) is the addition of acidic peat.