A crop can only develop properly and produce a sufficiently high yield if the soil pH is regulated according to its requirements. The right soil pH allows the plant to develop a large root system, which enables it to take up nutrients from deeper in the soil.
A high yield of a crop depends on many conditions, not on fertilization alone. Farmers make a crucial mistake if they think that just by applying a lot of fertilizer, they will be able to compensate for the negative effects of a defective soil or for deficiencies in cultivation or plant care. Soil is not a static, immutable commodity; its properties are constantly changing. Many soils become increasingly acidic and their nutrient reserves are depleted.
Soil pH is a measure of the relative acidity or alkalinity of a soil and is defined as the ratio of hydrogen ions (H+) to hydroxyl ions (OH–). A distinction is made between acidic (predominantly hydrogen ions), neutral (a balance of hydrogen and hydroxide ions) and alkaline (predominantly hydroxide ions) soil reactivity. Since it was difficult in practice to handle the difference in concentrations, a special scale (pH) was developed based on the logarithm of the concentrations. The acid-base scale (pH) ranges from 0 to 14, where 7.0 indicates neutral soil, a value below 7.0 indicates that the soil is acidic (predominance of H+ ions) and a value above 7.0 is alkaline (predominance of OH– ions).
The soil buffer depends on the agronomic class and humus content of the soil. Therefore, to change the soil pH by one unit on a heavier, more humus-rich soil with a lower initial pH, a greater load of active Ca2+ must be supplied than on a lighter, lower humus soil with a higher initial pH. It is also important to note that the pH is the active acidity. Apart from this, we distinguish between latent acidity, exchangeable acidity and hydrolytic acidity. In diagnostic practice, pH is measured in the presence of potassium salt solution. This method also allows the exchangeable cations to be partially included in the measurement. Therefore the pH measured in KCl is usually lower than the pH measured in water, and the level of difference depends on the soil properties.
The soil pH determines the chemical, biological and physical properties of the soil. This ultimately translates into the quantity and quality of yields obtained on the farm, so the farmer’s knowledge of the role of pH and crop requirements related to pH is extremely important for good and efficient crop production management. Knowing the requirements and tolerance of soil pH for different crop species allows the farmer to set up a proper crop rotation according to the current soil condition and to plan a program of treatments to improve soil pH.
Soil pH largely affects the management and availability of macronutrients (N, P, K, S, Ca, Mg), micronutrients (Mn, Mo, B, Fe, Zn, Cu), as well as heavy metals. It influences the toxicity of some elements, e.g. aluminium, which at low pH (acid soil) may pose a threat to the development of plants by limiting the development of their root system and thus is unable to effectively supply the plant with mineral salts. As a result, the very mobile nitrogen is washed out of the root system and ends up in the groundwater, which makes the economic effect of fertilizing with this component very low. It also affects the processes of mineral weathering and nutrient storage in the soil (sorption), which determines the level of abundance in available forms of nutrients. Phosphorus in very acidic soils forms water-insoluble bonds with aluminium and iron.
Thus, despite a potentially high phosphorus abundance in the soil, phosphorus is not available to plants. An increase in soil pH caused by liming changes the chemical form of phosphorus to a form that is more soluble in water and thus more available to plants. Low pH also significantly alters the uptake of potassium by plants. On acidic soils, even slight over-fertilisation, which exceeds the plant’s potassium requirement, causes potassium to be taken up in excess of the plant’s needs. The mineral composition of plants is then adversely affected. They contain too little calcium and magnesium in relation to potassium. This is particularly dangerous in fodder crops. Animals fed on these plants suffer from the metabolic disease grazing tetany.
Magnesium in acidic soils is usually found (as well as phosphorus) in a form that is not available to plants. In addition to magnesium, a regulated soil pH increases the bioavailability of molybdenum. These two elements are exceptions, however. The plant availability of the others decreases with increasing soil pH. They are converted to chemical forms that are unavailable to plants.