Potassium Analysis in Soil Samples

Introduction

• Soil containing Feldspar minerals can have up to 20,000 ppm of potassium, but usually much less is available to plants.
• Potassium fertilizer application may be needed for most soils.
• Potassium is extracted from air-dried soil by shaking with a 0.1M CaCl solution for 30 minutes.
• This process effectively displaces the potentially available potassium ions.

Objective

• Determine plant-available potassium in soils.

Method

• Sample preparation:
  • Weigh 2g of air-dried, sieved soil into a 50 ml polyethylene tube.
  • Add 20 ml of 0.1M CaCl solution.
  • Shake for 30 minutes and allow to stand for a few minutes.
  • Filter the supernatant liquid through a dry Whatman No. 2 filter paper.
• Flame Photometer:
  • Set the Flame Photometer to "100" using a 100 ppm potassium solution.
  • Read the soil extract (and CaCl blank) by aspirating the solution, diluting as necessary in CaCl acid solution.
  • Calculate the potassium concentration while considering dilution factors.
• Potassium Standard Dilutions:
  • Prepare standard dilutions of potassium for calibration purposes.
• Calculation:
  • Use the following formula to convert results to mg/kg of soil: Concentration of P (mg/L) x Volume of Extractant (L) / Weight of Soil (kg)

Potassium Status

• Potassium status categorization is based on soil concentration (mg/kg):
  • Very Low: 0-60 mg/kg
  • Low: 61-120 mg/kg
  • Moderate: 121-241 mg/kg
  • High: 241-400 mg/kg
  • Very High: > 401 mg/kg

Measuring Potassium in Soils

• Soils can contain a significant amount of potassium (K), but only a small portion is readily available to plants.
• Potassium is extracted from soil samples using a 0.1M calcium chloride (CaCl) solution, which effectively displaces the available potassium ions.
• The extraction process involves shaking a weighed soil sample with CaCl solution for 30 minutes.
• The supernatant liquid is then filtered, and the concentration of potassium in the extract is measured using a flame photometer.
• The concentration of potassium in the extract is then used to calculate the total potassium content in the soil using a specific formula.
• Soil potassium levels are classified into categories: very low, low, moderate, high, and very high, based on their concentration (mg/kg).
• These levels are used to assess the potassium status of the soil and indicate the amount of potassium fertilizer that may be needed for optimal plant growth.

Potassium in Soil

• Soils with Feldspar minerals can contain up to 20,000 ppm of potassium, but this is not always available to plants.
• Potassium fertilizer is often needed to support plant growth.
• A 0.1M CaCl solution is used to extract potassium from air-dried soil samples.
• The extraction process involves shaking the soil with the CaCl solution for 30 minutes, which displaces potentially available potassium ions.
• The supernatant liquid is filtered through a Whatman No. 2 filter paper.
• A Flame Photometer is used to measure potassium concentration, calibrated using a 100 ppm potassium solution.
• Dilution factors are considered when calculating potassium concentration.
• To convert results to mg/kg soil, use the following formula: [Concentration of K in extract (mg/L) * Volume of extractant (L)] / (weight of soil (kg)).
• Potassium status is classified as follows:
  • Very low: 0-60 mg/kg
  • Low: 61-120 mg/kg
  • Moderate: 121-241 mg/kg
  • High: 241-400 mg/kg
  • Very high: > 401 mg/kg