Compost Seed and Pathogen Viability Tests

 Milt McGiffen ( and Jer Sawma

Department of Botany and Plant Sciences

University of California, Riverside, CA 92521-0124


 Pathogens and weeds introduced in compost can cause serious harm to farmers. The process to reduce pathogens (PRP), in which the compost pile is repeatedly brought to high temperatures for several weeks and then turned, is intended to kill these pathogens. However no studies show that the pathogens are killed in practice. This study examines the effect of composting on the seeds of six weeds, one species of nematode, and two species of fungus.


 The experiment was conducted at California Biomass, a greenwaste composting facility in Thermal California. California Biomass uses a two part composting process. Raw material is first placed in large pile 5 meters tall by 46 meters long by 46 meters wide. This pile is kept moist for six months so that it degrades significantly and begins to hold water better. The material is then removed, ground and placed in a finishing pile 46 meters long 2.6 meters high and 5 meters wide. It is then turned and watered every two weeks for up to 9 weeks.


Six species of weeds, cheeseweed (Malva neglecta), yellow nutsedge (Cyperus esculentus), annual bluegrass (Poa Annua), crabgrass (Digitaria sanguinalis), wild mustard (Brassica kaber), and yellow sweetclover (Mehlotus officianahs), two species of fungus (Macrophomina phaseolina and Sclerotium rolfsii) and one nematode Heterodera schachtii were selected for the experiment. These species of weed and fungus have a wide range of heat tolerances and therefore some were expected to survive composting better than others were. All seeds, fungus and nematodes were enclosed in small packets made of no-see--um netting. Four packets of each species were placed in each of 8 wire envelopes. The envelopes were then buried in the finishing pile at 1 meter and 0.5 meter depths in four locations on the pile. A thermocouple was buried with each of the wire envelopes to record the temperatures.


The cages were first buried on 11/5/99. On 11/23/99 the cages were dug up, one set of packets was removed from each, the pile was turned, and the cages were then reburied. On 12/8/99 packets were removed and the wire envelopes were again reburied. Temperature data from 11/5/99 to 11/23/99 was lost, but data from 11/23/99 to 12/17/99 is available.


 Results and Discussion

 No seeds or fungus survived the first period of burial. All uncomposted seeds except Malva had significant germination. Malva has dormancy dependent on a hard seed coat, so when scored the germination proceeded quickly. Germination percentages of Malva are currently being redone. Too few nematodes were placed in the compost pile to make an accurate assessment of viability. Temperatures averaged near 63 C in the pile and varied a few degrees according to the day-night cycle and temperatures overall maximum temperatures ranged between 58 C and 68 C. Temperatures from the first week in the pile were probably comparable to those in the following weeks because measurements taken by thermometer less than 24 hrs after the compost pile was built showed temperatures between 57 C and 60 C.


The compost pile achieved higher temperatures after the third time it was turned than after the second. This is contrary to what was expected because the amount of carbon that fuels microbial respiration decreases with composting. Possibly moisture variation influences the compost piles temperature more than age.


Both piles tended to have hotter temperatures at 0.5 meters than at 1 meters. This is probably because the pile was better aerated at 0.5 meters.

Table 1. Viability of plant pathogens after composting 

Species Date Recovered Depth of Burial (M) % Viability
M. phaseolina 11/23/99 0.5 0
M. phaseolina 11/23/99 1.0 0
M. phaseolina n/a Control 100
M. phaseolina 12/8/99 0.5 0
M. phaseolina 12/8/99 1.0 0
M. phaseolina n/a Control 100
S. Rolfsii 11/23/99 0.5 0
S. Rolfsii 11/23/99 1.0 0
S. Rolfsii n/a Control 72
S. Rolfsii 12/8/99 0.5 0
S. Rolfsii 12/8/99 1.0 0
S. Rolfsii n/a Control 72

 Table 2. Viability of weed seed after composting. 

Species Date Recovered Depth of Burial (M) % Germination
B. kaber 11/23/99 0.5 0
B. kaber 11/23/99 1.0 0
B. kaber n/a Control 33
B. kaber 12/8/99 0.5 0
B. kaber 12/8/99 1.0 0
B. kaber n/a Control 33
C. esculentus 11/23/99 0.5 0
C. esculentus 11/23/99 1.0 0
C. esculentus n/a Control 46
C. esculentus 12/8/99 0.5 0
C. esculentus 12/8/99 1.0 0
C. esculentus n/a Control 46
D. sanguinalis 11/23/99 0.5 0
D. sanguinalis. 11/23/99 1.0 0
D. sanguinalis n/a Control
D. sanguinalis. 12/8/99 0.5 0
D. sanguinalis 12/8/99 1.0 0
D. sanguinalis n/a Control -
M. neglecta 11/23/99 0.5 0
M. neglecta 11/23/99 1.0 0
M. neglecta n/a Control -
M. neglecta 12/8/99 0.5 0
M. neglecta 12/8/99 1.0 0
M. neglecta n/a Control -
M. officianalis 11/23/99 0.5 0
M. officianalis 11/23/99 1.0 0
M. officianalis n/a Control 42
M. officianalis 12/8/99 0.5 0
M. officianalis 12/8/99 1.0 0
M. off cianalis n/a Control 42
P. annua 11/23/99 0.5 0
P. annua 11/23/99 1.0 0
P. annua n/a Control 95
P. annua 12/8/99 0.5 0
P. annua 12/8/99 1.0 0
P. annua n/a Control 95