Characterization of Pyrolysis Char for Use as an Agricultural Soil Amendment
Keith Harris1, Julia Gaskin1, Leticia Sonon2, and K.C. Das1
1Dept. of Biol. & Ag. Eng., 2AESL, College of Ag & Env. Sci University of Georgiai, Athens, GA

Introduction:
The Southeastern Coastal Plain in the United States is a major agricultural production area; however, soils are typically low in cation exchange capacity (CECi), nutrient content, and organic carbon content. For example, Tifton
loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) has organic carbon, CEC, and base saturation
of less than 1%, 5 cmol kg-1, and 40%, respectively in the Aphorizon.

Studies of charcoal in forest and agricultural systems indicate it affects soil fertility parameters and may affect
overall productivity (Tryon, 1948; Glaser et al., 2002; Briggs, 2005). This study reports preliminary screening of pyrolysis chars from several feedstocks for nutrient concentrations and CEC.

Material and Methods:
Char: Peanut hull pellets (PN), pine chips (PC), pine bark (PB), sawdust (SD), and hardwood chips (HW) produced at
380, 400, and 420° C under ambient pressure with steam as the carrier gas. Char was ground and sieved to < 420 μm.

Nutrient analysis: Total C and N - LECO CNS-2000. Nutrients were extracted with Mehlich I solution and
measured on a Thermo Jarrell-Ash model 61E ICP.

Leachate analysis: 40 mls of DI water was added to10 g of selected chars pyrolyzed at 426° C and shaken @ 200 rpms
for 5 minutes on an orbital shaker, filtered through a 0.2 μm filter, and frozen until analysis. The samples were washed 5 times and then the char was placed in an oven at 60° C overnight to dry. This process was repeated two more times for a total of 15 washings per feedstock. Washings 1-9 and 11 were analyzed for nutrients on a Thermo Jarrell-Ash model 61E ICP.

pH: Char was mixed 1 to 1 vol/vol with DI water and measured with Fisher Scientific AR15 digital pH meter.

CEC: Prior to analysis samples were leached with deionized water to remove soluble salts. Na-acetate/ethanol/ NH4-
acetate replacement method was used to determine CEC. Na was analyzed by atomic absorption on a Perkin Elmer PE 4100ZL.

Results and Discussion:
The available nutrient content of char varied with feedstock and pyrolysis temperature (Table 1). PN char pyrolyzed at 426°C (PN 426) contained the highest amounts of Mehlich I P, K, and Ca. PN pyrolyzed at 371°C (PN 371) contained the highest total N (2.03 %), but only a very small fraction was plant available based on a 24-day N mineralization study (data not shown).

Nutrient concentrations in the char leachate decreased by approximately 50% during the first three washings and
leveled off in washings 4 and 5. After overnight drying, nutrient concentrations increased to near the initial leachate concentrations for K, Ca, Mg, and S (Figure 1). The sum of the nutrient concentrations in the leachate
exceeds the Mehlich I concentrations for several nutrients (Table 2). Wetting and drying char may open pores and allow access to nutrients that are initially not available.

The pH of the char was related to feedstock and temperature. The pH of PN, SD, PC, HW chars increased slightly with temperature. The pH of PB had an inverse relationship with temperature. PN chars were alkaline (range 10.09 - 10.88 S.U.) and had higher concentrations of basic cations such as Ca, Mg, and K. The other feedstocks were slightly acidic to near neutral (range 5.84 – 7.16 S.U.).

Char CEC varied with feedstock and pyrolysis temperature(Figure 3). CEC ranged from 44.0 cmol kg -1 char for PN 402
to 14.1 cmol kg -1 char for the HW pyrolyzed at 426° C. CEC was significantly lower for feedstocks pyrolyzed at 420° C. PN chars had a significantly higher CEC than other feedstocks.

Conclusions:
This preliminary screening indicates chars form several different feedstocks have potential to improve the both CEC
and serve as a nutrient source. The properties of char varied by feedstock and pyrolysis temperature. Ongoing studies are evaluating variability within feedstock, temperature, and carrier gas, as well as char effects on soil moisture holding capacity, plant growth, and yields.