Chemical soil properties

Exchanges

One of the most important properties of the soil is its ability to retain (= sorb) ions (e.g. nutrients ) and release them when required.

The accumulation of nutrients takes place on the surfaces of the mineral and organic soil particles ( sorption complex ), mainly on the colloids ( clay minerals , humic substances ).

Ion exchange

The nutrients are released through an exchange of ions , e.g. B. 1 calcium ion (Ca ²⁺ ) against 2 hydrogen ions (H ⁺ ). Withdrawal of nutrients and leaching does not reduce the amount of ions bound in the soil , but their ratio to one another. The total amount of ions that a soil contains in an exchangeable bond is called its exchange capacity (AK). It means its exchange capacity for cations (KAK), which makes up the vast majority of exchangeable ions. The most important exchangeable cations are calcium (Ca ²⁺ ), magnesium (Mg ²⁺ ), potassium (K ⁺ ) and sodium (Na ⁺ ). With increasing soil acidification, hydrogen and aluminum (Al ³⁺ ) take their place . For the nitrogen balance of the soil it is important that the ammonium ion (NH ₄⁺ ) is also sorbed and thus protected from leaching. The exchange capacity for anions (AAC) is much lower than that for cations . Exchangeable anions which are important for plant nutrition are phosphate (PO ₄³⁻ ), sulfate (SO ₄²⁻ ), nitrate (NO ₃^- ) and chloride ions (Cl ^- ). The latter two are only very weakly bound and therefore easily washed out. This also applies to a lesser extent to sulphate ion.

Base saturation

The cations calcium, magnesium, potassium and sodium are called basic cations ( bases ). Their percentage share of the exchange capacity is called base saturation . Soils with high base saturation are used when the proportion of basic cations is more than 70% and less when it is less than 30%. If ions are added to the soil from outside (e.g. by acids or fertilizer salts ), some of the sorbed ions are exchanged.

pH value, acidification, buffering

PH value

The pH value is a measure of the degree of acidity ( acidity ) of a soil or the soil reaction . Soils in the humid climatic area have pH values in the range from 3.0 (very strongly acidic) to around 7.5 (weakly alkaline). The pH numbers are the simplified notation for the amount of free hydrogen ions in one liter of water . The concentration of hydrogen ions decreases by a factor of 10 with each pH level. At pH 3 there are 10 ⁻³ (0.001) mol H ⁺ ions in 1 l soil solution .

Acidification

Decreasing pH numbers indicate an increase in the hydrogen ion concentration, which is known as soil acidification . The causes of acidification are:

The production of H ⁺ ions as a result of respiration (CO ₂ excretion) by roots and microorganisms . Through soil respiration , around 10 kg of H ⁺ ions / ha and year are produced on agricultural land . 500 kg calcium carbonate / ha are required for their neutralization .

The formation of organic acids during humification (formation of humic acids ), but also in the case of impaired decomposition of organic matter due to a lack of air (formation of lactic acid , butyric acid, etc.). Poorly plowed beet leaves can, for. B. up to 10 kg H ⁺ ions / ha can be released.

The implementation of physiologically acidic fertilizers , e.g. B. ammonium sulfate . 100 kg nitrogen , fertilized as ammonium nitrogen, can release up to 7 kg H ⁺ ions / ha. The equivalent amount of lime is 350 kg calcium carbonate .

Acid input from the atmosphere (acid rain). The proportion of acid rain in the total acid input is often overestimated. In soils used for agriculture, it usually accounts for less than 10%, but up to 60% in soils used for forestry.

The acidity of a soil has multiple meanings for soil development, soil properties and plant growth. When the pH values of the originally base-rich soils drop, clay migration and structural disintegration begin. The biological activity decreases. Phosphoric acid is set. At higher degrees of acidity (pH below 5.0) free Al ³⁺ ions and heavy metals appear in mineral soils , which have a more or less toxic effect on the growth of the cultivated plants. The range 7.4 to 6.5 is called the neutral range in practice. The pH value of the soil can be adjusted to the desired level by adding lime. The pH range to be aimed for is set lower for sandy and very humus soils than for clay-rich soils, since they require less lime and a lack of trace elements and increased humus degradation occur due to heavy amounts of lime .

buffering

The buffering of a soil is its resistance to changes in the pH value when it is supplied with H ⁺ or OH ^- ions. This resistance is important because plants and soil organisms react very sensitively to sudden changes in acidity. The buffering capacity of a soil is primarily determined by the cation exchange capacity and the base saturation of a soil. A sudden supply of H ⁺ ions in the soil solution is neutralized by exchanging them for basic cations of the sorption complex . Conversely, a sudden supply of OH ^- ions is neutralized by releasing H ⁺ ions into the soil solution in exchange for basic cations. The pH value only changes when the buffer capacity is exhausted. Because of the much higher amount of sorbed H ⁺ ions, a soil with a high exchange capacity ( clay soil ) needs much larger amounts of lime to raise the pH value than a sandy soil with poor sorption . Calcareous soils have a very large buffering capacity against acids.