Physical characterization of bulking agents before and after composting Caractérisation physique d adjuvants organiques avant et après compostage

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1 Scientific registration n o : 1236 Symposium n o :19 Presentation: poster Physical characterization of bulking agents before and after composting Caractérisation physique d adjuvants organiques avant et après compostage KRITZ Göran, ROBERTSSON Martin, KHALED Tarek, JÖNGARD Hans- William Swedish University of Agricultural Sciences, Department of Horticulture, P.O. Box 55, S Alnarp, Sweden Introduction Most research on compost quality have focused on plant nutrients, heavy metals and other toxic elements. However, this paper deals with compost quality from a physical point of view. Until recently, physical investigations usually have been used for soils and substrates. But increased quality requirements on compost products have resulted in an increasing interest for this kind of research. Physical characteristics are of great importance to the root environment when the compost is used as a substrate, but also to the microbial population during the previous composting process. Compost raw material can be classified into two groups regarding physical properties: materials with poor structure and structure creating materials. Materials with poor structure are usually wet and rich in nitrogen. Most household wastes belong to this group, but also wastes from the food industry. With a high nitrogen content (C/N- ratio below 30) there is a large risk for ammonia formation during the composting process. A compost which is based only on material with poor structure will have severe problems with the oxygen supply. This will result in anaerobic conditions and formation of phytotoxic substances. Structure creating materials is usually found in garden wastes (except grass cuttings). Other structure creating materials are sawdust and disintegrated straw. These components have a high carbon content, thus creating a higher C/N-ratio when mixed with nitrogen rich materials helping to lower the ammonia formation by providing microbial available energy. The structure creating materials are usually dry and can absorb a surplus of water when mixed with material having a poor structure. The main function of the structure creating materials or the bulking agents, is however to increase the proportion of free airspace in the compost. 1

2 A literature study in this field has shown that the number of physical investigations concerning composted materials for use in the horticultural sphere is small (examples are: Jespersen and Willumsen, 1993; Lamanna et al., 1991; Nappi and Berberis, 1993). The aim of this study was to evaluate changes in physical properties that occurs in the bulking agent during the composting process. The fundamental was to determine the change in the pore size distribution as a result of the composting. Basically the water content at different water potentials was determined. By the following relation the pore size was determined for corresponding capillary height of rise: h = / d where h = capillary height of rise (meter) and d = equivalent pore size diameter (meter). The capillary height of rise was also expressed as pressure (1 m water column = 10 kpa; Figure 1). This provided a connection between pore size and water content at different water potentials. Material and methods Bulking agents. Sawdust from pine and barley straw, before and after composting were investigated. Particle sizes less than 1.5 mm were used. Composting, equipment and procedure. Composting took place in small scale bio reactors constructed of 3 l static vacuum flasks (Dewar vessels). The process was totally self heating, 100 ml air per min was supplied from the bottom of the flask. Ammonium nitrate was added to achieve C/N=30. Composting ended after 17 days when temperature was close to ambient. Physical investigation, equipment and procedure. A reference method established within the International Society for Horticultural Science, ISHS, was used to determine water content between the potentials -1 and -10 kpa and some other parameters (Gabriëls and Verdonck,1991). The equipment used was comprised in a plastic box with a drainage hose to create a sandbed of 12 cm thickness. Double rings for sampling (volume cm 3 each) were placed on the sandbed. By the means of an adjustable water table, connected via a hose to the sandbox, it was possible to regulate the water potentials at different levels. Bulk density (g/cm 3 ), total porosity (per cent), air-filled porosity (per cent) and water content (per cent) were determined at the water potential -1 kpa. The water content at water potentials of -5 and -10 kpa were also determined. The total porosity was calculated from the values of bulk density and particle density (g/cm 3 ) as follows: TP = 100 (1 - BD / PD) where TP = total porosity (per cent), BD = bulk density (g/cm 3 ) and PD = particle density (g/cm 3 ). To determine the particle density for each substrate, the following equation was used: PD = 100/((100 - A) A / 2.65) 2

3 where A = ash content (per cent), the constants 1.50 and 2.65 were used as particle density values (g/cm 3 ) for organic and inorganic material respectively. To determine the ash content, two randomized samples were ignited at 600 o C for eight hours. For determination of water content at the water potential kpa (physical wilting percentage) a ceramic plate extractor was used (method description, see Cassel and Nielsen, 1986). The water content remaining at this water potential is unavailable for plants. In calculations, the same bulk density was used as above. Each result in this investigation is normaly given as a mean value of four replications. The exeptions are the sandbed investigations for samples after composting. Because of small quantities of material produced in the bio reactors only two replications were used. The precision of the mean values are determined by the standard error. In this study we used some definitions from De Boodt and Verdonck (1972): air space or air filled porosity, AFP, as air percentage at the water potentials larger than -1 kpa; easily available water, EAW, as percentage water between -1 and -5 kpa; water buffering capacity, WBC, as percentage water between -5 and -10 kpa. Unavailable water, UAW, was used as percentage water at potentials smaller than kpa (Hillel, 1980) and the percentage interval between -10 and kpa was called as difficultly available water, DAW UAW DAW WBC 0 BS1 BS2 PS1 PS2 LP MP Substrates Figure 1. Solid material (SM), air-filled porosity (AFP) at - 1 kpa, and water content at different water potentials for barley straw, before (BS1) and after (BS2) composting, pine sawdust before (PS1) and after (PS2) composting as well as earlier investigated Sphagnum peats, low (LP) and moderate (MP) humified. For the four water potentials corresponding pore size diameters in mm, PSD, are given. EAW = easily available water (-1 to -5 kpa), WBC = water buffering capasity (-5 to -10 kpa), DAW = difficultly available water (-10 to kpa) and UAW = unavailable water (< kpa). EAW AFP SM P, kpa PSD mm

4 Table 1. Mean values for bulk density (BD), total porosity (TP) and air-filled porosity (AFP) at the potental - 1 kpa for barley straw and pine sawdust before and after comosting as well as earlier investigated Spagnum peats, low and moderate humified. Standard error values are given in parenthesis. Substrate BD TP AFP g/cm 3 % % Straw, before composting, BS (0.0009) Straw, after composting, BS (0.0021) Sawdust, before composting, PS (0.0003) Sawdust, after composting, PS (0.0004) Low humified peat, LP (0.0008) Moderate humified peat, MP (0.0015) Results and discussion The pore size distributions changed for the two bulking agents as a result of the composting process (Figure 1). The straw material (BS1 and BS2) showed a higher decrease of pores larger than 0.3 mm than the sawdust material (PS1 and PS2). The result can be explained by the high activity in the bio reactor measured as temperature, carbondioxide and oxygen levels. The air percentage for the corresponding water potential interval larger than -1 kpa is called the air filled porosity, AFP (Figure 1 and Table1). For straw before composting (BS1), the AFP was 64 per cent and after (BS2) 42 per cent to be compared with sawdust (PS1 and PS2) 57 and 50 respectively. The big decrease of pores with diameters larger than 0.3 mm for the straw material was counter balanced by a big increase of pores smaller than 0.3 mm. The pore size interval mm corresponds to the percentage water between the potentials -1 to -5 kpa called EAW, which increased from 2 to 17 (BS1 compared with BS2; Figure 1 and Table 2). Also in the next two intervals the volume increased due to the composting. The interval mm and mm corresponds to the percentage water between -5 and -10 kpa (WBC) and between -10 kpa and kpa (DAW) respectively (Figure 1 and Table 2). For the last pore size interval, smaller than mm, corresponding to the percentage water at potentials lower than kpa and called unavailable water, UAW, the composting process was expected to result in an increasing value. But it was not the case. We can not give any explanation to this fact. 4

5 Table 2. Mean values of water percentage at different water potential intervals for barley straw and pine sawdust before and after comosting as well as for earlier investigated Spagnum peats, low and moderate humified. Standard error values are given in parenthesis. EAW = easily available water (-1 to -5 kpa), WBC = water buffering capasity (-5 to -10 kpa), DAW = difficultly available water (-10 to kpa) and UAW = unavailable water (<-1500 kpa). Substrate EAW, % WBC, % DAW, % UAW, % Straw, before composting, BS1 Straw, after composting, BS2 Sawdust, before composting, PS1 Sawdust, after composting, PS2 Low humified peat, LP Moderate humified peat,mp 02.1 (0.29) 0.9 (0.35) 17.2 (0.35) 10.4 (0.14) 16.8 (0.53) 3.3 (0.92) 23.5 (0.94) 08.6 (0.18) 20.9 (0.25) 0.4 (0.36) 7.9 (0.31) 04.9 (0.03) 16.8 (0.86) 6.9 (0.17) 11.5 (0.18) 05.5 (0.09) 36.6 (1.47) 7.4 (0.06) 24.8 (0.10) 07.2 (0.08) 32.6 (0.96) 3.6 (1.01) 29.5 (0.91) 13.3 (0.07) In contrary to the barley straw material, the percentage for the pore size interval mm (EAW) decreased for the pine sawdust material after composting (Figure 1 and Table 2). The EAW values fo the two agents after composting are the same,17 per cent. The presented Sphagnum peat materials were used as comparison objects as well known substrates (Khaled and Kritz, unpublished material). The percentage pores larger than 0.3 mm (corresponding to the AFP) is 19 for low humified peat (LP) and 12 for modrate humified (MP; Figure 1 and Table 1). Compared with the two agents these values are very low. In contrary the EAW values are much higher than the values for agents. The comparison between these peats showed that increasing humification gave an increasing percentage pores smaller than 0.03 mm. Conclusion o This study has shown the possibility to determine the change of the two bulking agents in pore size distribution caused by the composting process. o The barley straw with a high percentage of pores larger than 0.3 mm has shown a substantial decrease of these pores after composting. 5

6 o The pine sawdust with a percentage of pores larger than 0.3 mm not so far from that of barley straw has not shown so big decrease of these pores after composting. o This fact means that if we want to use a bulking agent, active during the composting process, barley straw could be a good choice, but if we want a more stable agent pine sawdust would be suitable. References Cassel, D. K. & Nielsen, D. R Field capacity and available water. Methods of soil analysis, Part1. Rev. Physical and Mineralogical Methods. Amer. Soc. of Agron. Monogr.9 De Boodt, M. & Verdonck, O The physical properties of the substrates in horticulture. Acta Horticulture 26, p Gabriëls, R. & Verdonck, O Physical and chemical characterization of plant substrates: towards a European standardization. Acta Horticulturae 294, p Hillel, D Fundamentals of soil physics. Academic Press, New York. Lamanna, D., Castelnuovo, M. & D'Angelo, G Compost-based media as alternative to peat on ten pot ornamentals. Acta Horticulturae 294, p Melby Jespersen, L. & Willumsen, J Production of compost in heat composting plant and test of compost mixtures as growing media for greenhouse cultures. Acta Horticulturae 342, p Nappi, P. & Barberis, R Compost as growing medium: chemical, physical and biological aspects. Acta Horticulturae 342, s Keywords: physical properties, composting, bulking agent Mots-clés : propriété physique, compostage, adjuvant organique 6