1. Talk Dirt-y to Me
Determination of Potassium in Compost
Chloe Collin
Introduction
Composting is the act of decomposing organic materials to help fertilize
crops and gardens. Different types of materials used in compost include
nitrogen-rich (green,) and carbon-rich (brown) materials. The ratio of these
materials is typically 1:1, which makes for a fertile soil. Varying the ratio of
these materials
causes a difference in the levels of other nutrients in the compost, such as
phosphorus and potassium.
Table 1: Examples of brown materials and green materials in composting
Potassium is involved in reactions that activate enzymes, as well as
being involved in the proton gradient of cell membranes. Finding a ratio of
green-to-brown material that will allow for the highest amount of potassium
available in the compost is a reasonable goal for farmers and gardeners.
Ion selective electrodes (ISE) are useful in measuring ion concentration
in an aqueous solution. The relationship between potential (mV) and
concentration (ppm) is logarithmic.
ISE was chosen for this experiment because it affordable, easy to
interpret, and accessible.
Methods
Digestion
• 4 mL of sulfuric acid was added to 1 gram of sample and reacted for 30
minutes
• 3.5 mL of hydrogen peroxide was added, and the solution was heated at
250°C for 30 minutes
• The solution was then cooled to room temperature, and 1.0 mL of
hydrogen peroxide was added
• Solutions were then filtered through a disposable syringe and diluted to
100 mL with deionized water.
Discussion & Future Work
Compost composition (Nitrogen:Carbon) of 75:25, 50:50, 25:75 resulted
in the potassium concentrations 0.179, 0.126, and 0.093 mg K+ / g compost
respectively.
According to these results, a more nitrogen-rich compost mixture will
lead to a higher concentration of potassium. It may be wise to not stray
greater than 75:25, because carbon is still important in plant nutrition.
This analysis is from a total digestion of the compost. Given more time,
potassium analysis using Bray & Kurtz digestion may better represent the
amount of potassium that plants specifically in Wisconsin can intake.
Phosphorus could also be analyzed, because it is also important for
plant growth.
Acknowledgements
Thanks to Dr. Dave Snyder and Mrs. Juli Bowling for their support and help in
the experimental process.
Thanks to Dustin Ziegelman for naming this poster.
Figure 2: Carbon-rich, 50/50, and nitrogen-rich samples, respectively, after
digestion and dilution
Figure 3: Potassium standard calibration curve using a potassium ion
selective electrode
Results
Figure 4: Concentrations of potassium in compost samples in mg K+ / g
compost
Sample Preparation
• Three different samples of compost were created with ratios of
nitrogen:carbon as follows:
25:75 50:50 75:25
• These composts were prepared with coffee grounds, chicken manure, and
food scraps in the green material, and woodchips, eggshells, and twigs in
the brown material.
• The composts were then left alone for approximately three months, and
agitated once per week.
y = -43.927x + 59.894
R² = 0.9923
50
55
60
65
70
75
80
85
90
95
100
-0.8 -0.6 -0.4 -0.2 0
Potential(mV)
-log[K+]
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
N-rich 50/50 C-rich
PotassiumConcentration
(mg/g)
Compost Samples
Analysis
• Potassium standards were created using a 1000 ppm solution of KCl
diluted to 1-5 ppm
• 2 mL of NaCl (ionic strength adjuster) were added to 40 mL of each
solution
• Standards and compost samples were analyzed using a potassium ion
selective electrode and a standard calibration curve (Fig. 3) was
determined
Figure 1: Compost samples pre-digestion