Potassium Cycling and Balance in Agriculture

Potassium Content in Soil and Availability

Potassium is present in soil mainly in the form of salts and in an absorbed state—available (exchangeable) and unavailable (nonexchangeable). It is well retained by soil, except for light soils with low absorption capacity.

Total potassium content often exceeds nitrogen and phosphorus content and is determined by soil texture. Clayey and loamy soils are especially rich in potassium, where the content reaches 2–3%. Sandy, sandy loam, and peat soils are poor in potassium—up to 0.1%.

However, the total potassium content, due to the characteristics of exchange reactions, does not mean sufficient supply to plants, since only about 1% of its total content is available to plants. Therefore, the characteristic of potassium supply is its amount in mobile forms.

Based on availability to plants, all potassium compounds in soil are divided into five groups:

1. Potassium contained in soil minerals—aluminosilicates. A sparingly available form of potassium. However, some minerals (muscovite, biotite, and nepheline) are capable of transforming some potassium into available forms under the action of carbon dioxide and certain organic acids released by plant roots. The rate of potassium transition from nonexchangeable to exchangeable forms depends on soil type. For sod-podzolic soils it is 15–30 kg/ha per year; for leached chernozems, about 60 kg/ha.

2. Absorbed, or adsorbed-bound by soil colloids, potassium is the main source of plant nutrition. Its content in soil can range from 50 to 300 mg per 1 kg of soil. During the growing season, plants use only part of the exchangeable potassium, which is determined by soil properties, biological characteristics of plants, weather conditions, etc.

3. Water-soluble forms of potassium—the most available form. They constitute 10–20% (about 1% according to E. Rübenzam and K. Raue) of exchangeable potassium. According to MAA, in unfertilized sod-podzolic soil during the spring-summer period, the content of water-soluble potassium forms was 1.5–5 mg/kg of soil. It is formed as a result of chemical and biological action on minerals. It partially passes into water-soluble form from the exchangeable state as a result of displacement from the soil absorption complex, as well as from fertilizers.

4. Biogenically bound potassium, i.e., that contained in the biomass of soil bacteria, plant residues, and biota. Its share can reach, for example, in sod-podzolic soils 40 kg K₂O per 1 ha. It passes into available form only after the death and mineralization of residues.

5. Soil-fixed potassium. Potassium can be fixed in the mineral part of soil in a nonexchangeable state. The process proceeds most actively under conditions of alternating wetting and drying of soil and predominates in soils of heavy particle-size composition containing clay minerals montmorillonite and hydromicas, which are characterized by intracrystalline adsorption of cations, unlike kaolinite.

Potassium fixation into nonexchangeable form is intensified in an alkaline environment and predominates in solonetz soils. Chernozems fix potassium to a greater extent than sod-podzolic soils.

Soils have a certain limit of potassium fixation from fertilizers: for sod-podzolic soils it rarely exceeds 200 kg/ha; for chernozems it can reach 300–700 kg K₂O per 1 ha. The use of potassium fertilizers makes it possible to achieve complete saturation of the potassium fixation capacity.

The optimal content of exchangeable potassium in soil at which maximum crop yield is observed is 170–225 kg/ha for sod-podzolic and gray forest soils.

In the main subtypes of chernozems, the optimal content of mobile potassium, depending on soil, crop, and determination method, is according to Chirikov from 130 to 200 mg/kg; according to Machigin—up to 400 mg/kg.

Potassium Cycling in Agriculture

Potassium cycling in biocenoses proceeds intensively. Potassium content in the biomass of biocenoses varies from 20 kg/ha for deserts to 2000 kg/ha for oak forests.

The closed cycle of nutrient turnover in natural biocenoses, due to the accumulating activity of plants, leads to potassium accumulation within the root-inhabited layer and gradual enrichment of the upper horizons with this element.

In agrocenoses, potassium cycling and balance are influenced mainly by economic activity: availability of fertilizers, specialization of farms.

Total potassium reserves in soils exceed nitrogen and phosphorus reserves by 5–50 times. D. N. Pryanishnikov, when assessing the potassium balance for the country as a whole, assumed its deficit of 20–22 kg/ha per year.

The main expenditure item of the potassium balance in agriculture is farm removal with products.

Annually, from 40 to 310 kg/ha of potassium can be removed with the plant harvest. These values are calculated, in particular, for cereals at average yields; with increasing productivity they will naturally increase.

For an objective assessment of the potassium balance, it is necessary to take into account the distribution of removal between marketable and non-marketable parts of the products. Thus, in wheat grain, 15% of potassium from total farm removal is contained; in straw—85%. The less potassium is contained in the marketable part of the harvest removed from the farm and the more in the non-marketable part remaining in the field or farm, including in feed, the less potassium is removed from on-farm cycling. Thus, farm specialization determines the on-farm potassium balance.

Part of potassium can be lost from the root-inhabited soil layer as a result of infiltration: on light soils about 5%, on heavy soils—2% of the amount applied with fertilizers. The intensity of leaching is influenced by soil texture, water regime, fertilizer rates, and biological characteristics of crops.

Part of potassium can be lost from water and wind erosion. According to averaged data, this value is 4–8 kg/ha. It is generally considered that the expenditure items of erosion losses are compensated by inputs with seeds (about 2 kg/ha) and precipitation (2–6 kg/ha).

Some part of exchangeable potassium can pass in soil into a fixed (nonexchangeably absorbed) state, becoming unavailable to plants. The subarable soil layers also participate in supplying plants with potassium, thereby reducing potassium consumption from the arable layer. Thus, in experiments on sod-podzolic soils, sunflower and lupine on average consumed about 32% of potassium from total removal from the subarable horizons.

Potassium Balance

Potassium balance (Б_K) is determined by the formula:

where

Б_K

— potassium balance, kg/ha K₂O.

Input items of the balance:

K_у

— input with fertilizers K_у = (K_м + K_op), where K_м — input with mineral fertilizers;

K_op

— input with organic fertilizers;

K_с

— input with seeds (sowing material);

K_ро

— input with plant residues;

K_о

— input with precipitation.

Expenditure items of the balance:

K_ву

— removal with the harvest of main and by-product;

K_ро

— removal with plant residues;

K_п

— losses from soil K_п = (K_пв + K_пэ), where K_пв — leaching losses;

K_пэ

— losses from erosion processes.

Potassium input with fertilizers and seeds is determined by chemical composition and seeding rates. The amount of potassium input with atmospheric precipitation varies within 2–6 kg/ha. Potassium removal with the harvest (K_в) is determined by chemical composition and the amount of main and by-product.

Potassium losses from erosion (K_пэ), according to averaged data, are 3–5 kg/ha. Potassium losses from leaching depend on soil texture, precipitation amount, fertilizer rates, and crops.

Potassium losses from fertilizer leaching (K_пв) are: for light soils—5%, for heavy soils—2% of the applied amount.

References

1. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. by B. A. Yagodin. Moscow: Kolos.

2. Evtefeev Yu. V., Kazantsev G. M. (2013). Fundamentals of Agronomy: Textbook. Moscow: FORUM.

3. Mineev V. G., Sychev V. G., Gamzikov G. P., et al. (2017). Agrochemistry. Textbook. Ed. by Mineev V. G. Moscow: Publishing House of the D. N. Pryanishnikov All-Russian Research Institute of Agrochemistry.