Potassium Release Characteristics in Relation to Plant Uptake in Soils of Major Cropping Systems in Kurnool District

Plants feed not only from exchangeable K but also from non-exchangeable K, which mainly consists of K trapped in the interlayer of non-expanding clay minerals. A major contribution of non-exchangeable K by crop removal has been reported, particularly in soils under continuous cropping without K application (Srinivasa Rao et al., 2007).

In the absence of external K supply, most of the crop K needs are met from soil K reserves. In boiling 1N HNO₃ extracts, some of the non-exchangeable K is released through the breakdown of primary and secondary minerals (Patiram & Prasad, 1983).

Crop uptake and response to potassium show a good correlation with boiling 1N HNO₃ extractable K in soils (Sharma & Sekhon, 1992; Sivaprasad, 2014).

Hence, there is a need to study the potassium release characteristics of soil in relation to plant uptake. The present investigation has been undertaken to study the potassium releasing characteristics of thirty soils in major cropping systems in relation to plant uptake in soils of Kurnool district, Andhra Pradesh.

Based on the predominance of cropping systems, different locations were identified in major cropping systems of Kurnool district, Andhra Pradesh. Surface soils were collected from 60 locations, and out of these, 30 samples were screened for the present investigation based on potassium (K) status.

The texture of the soils varied from sandy loam to clay. The pH of the soils ranged from 6.9 (neutral) to 8.4 (slightly alkaline), and electrical conductivity (EC) ranged from 0.10 to 0.69 dS m⁻¹, indicating that the soils were non-saline. The organic carbon content ranged from low (0.21%) to medium (0.59%).

The soils were low to medium in available nitrogen (N), ranging from 159 to 307 kg ha⁻¹. In contrast, available phosphorus (P₂O₅) was high in all villages (68 to 169 kg ha⁻¹), while available potassium (K₂O) ranged from medium to high (154 to 2088 kg ha⁻¹).

The cation exchange capacity (CEC) of the soils varied between 13.03 cmol (p⁺) kg⁻¹ to 29.91 cmol (p⁺) kg⁻¹. The base saturation of the soils ranged from 62.7% to 88.1%, indicating that most of the soils were medium fertile in nature.

Potassium Release Parameters

Potassium release parameters, namely Step-K, Constant Rate-K, and Cumulative-K, were derived using the Haylock (1956) method, as modified by McLean (1961), using 1N HNO₃ as an extractant.

This method involved the removal of exchangeable K by:

• Soaking 5g of soil in 50 ml of 0.01N HNO₃ overnight.
• Leaching the soil with 10 ml of 0.01N HNO₃ (4-5 times).
• Boiling the soil with 1N HNO₃ (1:10 soil: 1N HNO₃) exactly for 10 minutes, followed by cooling and filtering.
• Continuing extractions with the same reagent until K release from the soil reached a more or less constant rate.

The Step-K value was computed by subtracting the amount of Constant Rate-K from the amount of K released in each step of successive extraction. The total amount of K released in all the extractions was considered Cumulative-K.

An important precaution was that the soil sample should reach boiling in 3 minutes, after which it was allowed to boil for 7.5 minutes.

Pot Culture Experiment

A pot culture experiment was conducted using 5 kg of 2.0 mm sieved soil collected from different cropping systems in earthen pots. The P-3396 maize hybrid was used as the test crop.

A common recommended dose of nitrogen (N) and phosphorus (P₂O₅) was applied to all treatments as per the recommended dose (250:60 kg ha⁻¹ of N and P₂O₅, respectively). However, potassium (K) was not applied to the crop.

The maize seedlings (three per pot) were sown, and at 10 days after sowing (DAS), two plants were removed and incorporated into the same pot. Only one plant was maintained per pot.

The crop was harvested at 60 DAS, and plant samples were collected, processed, and analyzed for K content using the tri-acid digest method (HNO₃ : HClO₄ : H₂SO₄ - 9:4:1), followed by determination with a flame photometer (Piper, 1966).

The dry matter and K content were measured, and potassium uptake was calculated using the formula:

\text{Nutrient Uptake (g pot⁻¹)} = \frac{\text{Nutrient Content (%) × Dry Matter Production (g pot⁻¹)}}{100}

The formula given by Panse and Sukhatme (1978) was used to calculate the coefficients of correlation.

Potassium Release Characteristics

The highest mean constant rate-K was observed in the rice-rice cropping system, while the lowest was found in the groundnut-groundnut cropping system (Table 1). All investigated soils recorded a constant rate-K lower than the critical value of 0.2 cmol (p⁺) kg⁻¹ or 78 mg kg⁻¹, as suggested by Metson (1969). This indicated that these soils had low K-supplying power to plants. Additionally, the non-exchangeable potassium pool could only slowly replenish the water-soluble and exchangeable K fractions, significantly affecting crop growth.

Step-K is an estimation of mineral K that is potentially available over time under intensive cropping systems (Haylock, 1969). The highest mean Step-K was observed in the maize-maize cropping system, while the lowest was found in the groundnut-groundnut cropping system (Table 1). The higher Step-K in the maize-maize cropping system resulted from a change in potassium equilibrium in a forward direction (Krishna Kumari et al., 1984). The reserve potassium involved in the replenishment process of available K during the growing period is mainly released from clay and micas, with fine-textured soils showing a better K-supplying power. These findings align with those of Datta and Sastry (1993) and Pal and Durge (1993).

Some soils in the present investigation recorded Step-K values lower than the critical value of 1.5 cmol (p⁺) kg⁻¹ or 585 mg kg⁻¹, as suggested by Hunsigi and Srivastava (1981).

The highest mean cumulative-K was observed in the maize-maize cropping system, whereas the lowest was in the groundnut-groundnut cropping system. Lower amounts of cumulative K (less than 1500 mg kg⁻¹ with 1N HNO₃, as suggested by Srinivasa Rao et al. (2007)) were observed in all soils except those of Srinagaram and Rythunagaram in the present investigation. Lower cumulative K combined with continuous cropping could lead to the depletion of soil K reserves, ultimately resulting in potassium deficiency (Table 1).

Table 1. K release parameters in different cropping system

Table 2. Dry matter, K content and K uptake of maize crop in experimental soils

Table 3. Correlation co efficient between potassiumrelease parameters and plant parameters

Correlation Coefficient (r) Between K Release Parameters and Plant Parameters

Among the K release parameters, cumulative-K and step-K showed maximum positive and significant correlation with dry matter (r = 0.907, 0.891), K content (r = 0.897, 0.884), and K uptake (r = 0.891, 0.879), while constant-K showed the minimum correlation (Table 3).

Step-K and cumulative-K exhibited maximum positive and significant correlation, indicating that soils are rich in available potassium and can supply potassium to crops on a short-term basis. However, external application is needed, especially for high K-requirement crops. The minimum correlation with constant-K suggests that K availability is reduced, leading to a low K-supplying power on a long-term basis.

The potassium release characters were the highest in the maize-maize cropping system and lowest in the groundnut-groundnut cropping system.

All investigated soils recorded less constant rate-K than the critical value, indicating that these soils had low K-supplying power to plants. Additionally, the non-exchangeable potassium pool could slowly replenish the water-soluble and exchangeable K fractions, significantly affecting crop growth.

Lower amounts of cumulative-K were observed in all soils except in the soils of Srinagaram and Rythunagaram under the present investigation. Lower cumulative-K and continuous cropping would lead to the depletion of soil K reserves, resulting in K deficiency.

Constant-K showed a minimum correlation, indicating a low K-supplying power on a long-term basis. Hence, judicious and frequent application of potassic fertilizers is required for better crop production.