Tuesday, May 13, 1997

 

University of California Workshop on Compost Use for Pest Management in Agriculture

 

Assessing Compost Maturity And Suitability For Agricultural Uses

 

T.K. Hartz

Department of Vegetable Crops

University of California

Davis, CA 95616

 

The composting of yard and landscape wastes is widely seen as an integral part of the municipal solid waste recycling programs; numerous compost facilities have begun operation throughout the state. Agriculture is projected as the end user of much of this composted green waste. However, there is considerable variation in composting technique, and duration, among commercial composting operations; there can also be seasonal and/or geographical variability in the original feedstocks used. This project was conducted to characterize the physiochemical and biological properties of CGW as affected by composting technique and duration. Specific objectives were:

 

  1. evaluate a number of analyses as predictors of compost maturity and suitability for particular agricultural uses.

     

  2. monitor the composting process to document the effects of management practices on speed of composting.

     

  3. relate time/temperature exposure during composting to important compost characteristics (weed seed survival, nitrogen mineralization/immobilization, phytotoxicity, etc.).

 

 Materials and Methods

 Four composting facilities in central and southern California were studied; these operations differed substantially in their management practices, but all were using municipal yard and landscape waste as the main feedstock. At each site, two windrows were sampled at approximately 3 week intervals over a 11-15 week composting period. At each site one element of windrow management (turning, watering, temperature control, or feedstock blending) was manipulated differently between the two windrows studied.

 The four composting facilities sampled were:

 

  1. A commercial composter in San Bernadino County composting yard and landscape wastes from surrounding municipalities. This operation uses a modified 'in-vessel' composting technique in which the ground green waste feedstock is packed into large polyethylene bags originally designed for silage fermentation. The bags were 3 m (10 ft.) in diameter and approximately 56 m (185 ft.) long. Turning and watering are eliminated; temperature and aeration are controlled by the configuration of air vents placed on the bag and the quantity of air pumped into the bag through porous pipes on the bottom of the bag. The two bags monitored (from April to July, 1995) differed primarily in temperature management. This operation was designated 'Source 1'.

     

  2. A farming operation in San Benito County that composts green waste from San Jose. This operation utilizes the 'Luebke' method of windrow composting which features intensive management (frequent turning, daily monitoring of temperature, CO2 level and moisture, etc.). One windrow of 100% green waste was compared with a windrow of blended feedstocks (approximately 40% green waste, 40% aged dairy manure, 10% wheat straw and 10% clay soil, by weight) composted from June to September, 1995. This operation was designated 'Source 2'.

 

  1. A farming operation in Stanislaus County composting green waste from San Jose, in conventional windrows. Turning frequency (2 or 4 week intervals) was the variable studied over the composting period of November, 1995, to February, 1996. This operation was designated 'Source 3'.

 

  1. A commercial composter in San Diego County composting municipal yard and landscape wastes. Composting was done in conventional windrows. The variable of interest was water management. The normal water volume applied was compared with twice that amount; watering frequency (approximately 5-10 day intervals) was not varied between treatments over the composting period of October, 1995 to January, 1996. This operation was designated 'Source 4'.

 

Each windrow was instrumented with continuously recording thermisters placed 45-60 cm (18-24 inches) from the outside of the windrow. At the end of the composting period the temperature data were analyzed to determine the cumulative exposure above 40, 50, 60 or 70° C (104, 122, 140 or 158° F). At each sampling date, composite samples of 15-20 liters of compost were collected from each windrow. Extracts (in 2N KCl) of these fresh samples were prepared for determination of ammonium (NH4) and nitrate (NO3) nitrogen concentration. The remainder of the samples were air-dried and stored for later determination of all other parameters.

 After the collection of all compost samples from all sources was completed, the samples were screened through 12 mm (0.5 inch) mesh before a battery of evaluations was performed. Mineral N (NH4+ and NO3-) concentration was determined in 2N KCl extracts of dried compost. Electrical conductivity (EC) and pH were measured in 2:1 (deionized water:dry compost) extracts. Total N was determined by the combustion method of Sweeney (1989), total C by combustion and subsequent measurement by thermal electric conductivity. Total P was determined by a colorometric technique following microwave acid digestion (Kingston and Jassie, 1986), total K by atomic emission spectroscopy following 2% acetic acid extraction.

 The presence of phytotoxic compounds was examined through a tomato seed bioassay. Air-dried compost (20g) and 100 ml deionized water were mixed and shaken for 2 hr. The solution was filtered, then diluted 2:1 (deionized water:compost extract). Three ml of diluted extract were used to wet filter papers in a petri dish; 10 seed of '3155' tomato were placed on the wetted paper. Three replicate dishes were prepared for each CGW sample. The dishes were incubated at 22° C (72° F) for 3 days, then rated for percent germination and length of radicle. These parameters were used to calculate a germination index, which compared the performance of seeds incubated in CGW extracts to those incubated in deionized water. The calculation was:

(% germ. in compost/% germ. in water)*(mean length in compost/mean length in water).

Weed seed survival in CGW was determined by a 6 week assay. CGW (2 liter volume/sample) was placed 2.5 cm (1 inch) deep in trays in a greenhouse at 25± 5° C (77± 9° F). The samples were kept moist throughout the period; germinating weeds were counted and identified by species.

 Nitrogen mineralization/immobilization behavior was measured by controlled-environment aerobic incubation. Blends of 10% CGW/90% soil mix (50% silt loam/50% coarse sand) were moisture equilibrated under 25 centibar pressure, then incubated at constant moisture for 14 days at 30°C (86°F). The change in mineral N concentration (as measured in 2N KCl extracts) over the incubation period represented net mineralization/immobilization. There were 3 replicate samples per CGW sample. An N mineralization index was calculated, relating the change in mineral N concentration to that in unamended soil mix.

 The suitability of CGW as a constituent of a potting medium was evaluated in a 6 week greenhouse experiment. Plugs of vinca (Vinca minor cv. 'Pink cooler') were planted in 5 cm (2 inch) diameter pots filled with a 1:1 blend (by volume) of CGW and perlite. There were 12 pots per CGW sample. The plants were fertilized once per week with a 100 PPM N solution (as 20-20-20 plus minor elements). After 6 weeks of growth in a greenhouse at 23± 3° C (73± 5° F) the plants were harvested and total top fresh and dry weights were determined. To help determine whether differences in plant growth were related to physical properties of the blend, air-filled porosity, bulk density and water-holding capacity were measured on triplicate samples by the method of Bragg and Chalmers (1988).

 Results

 Temperature profiles and windrow management:

 A wide range of temperature profiles were encountered in the various windrows studied. Source 2 had by far the lowest temperatures (Table 1), by design; the Luebke method of composting emphasizes temperature control, achieved by manipulating the windrows through watering, turning, the addition of less active material (eg. straw), etc. At Source 2, windrow A (100% yardwaste) was dramatically hotter than windrow B, which contained only 40% yardwaste and 60% less easily degraded materials (aged manure, straw and soil).

 Windrows from all other sources developed substantially higher temperatures, for more prolonged periods. Neither windrow turning frequency (Source 3) nor water management (Source 4) substantially affected windrow temperature. Manipulation of aeration within the compost bags (Source 1) did result in differential temperature profiles between bags; in bag 1 temperature >60°C (140°F) was limited to the first 20 days of composting while bag 2 continued to experience >60°C until day 70.

 

Physiochemical characteristics:

 Initial physiochemical characteristics, and their change over time, were similar in compost from all sources (Table 2). Total N concentration ranged from 1.1 to 1.5% in the original feedstocks, and remained quite constant over the composting period. Rapid oxidation of carbon in the initial 3-9 weeks of composting resulted in rapidly decreasing C/N ratios over that period, the ratio stabilizing thereafter. Final C/N ratios varied from 12 to 16. Initial P and K concentration varied from 0.16 to 0.27 and 0.5% to 1.2%, respectively. K concentration was quite stable over the compost period, while P increased marginally.

 The initially rapid oxidation of carbon was reflected in a substantial increase in % ash over the first 3-9 weeks of composting. Final ash content varied from 55-67% for compost containing 100% green waste; the higher ash content (76%) of Source 2, windrow B reflected the inclusion of soil in the initial feedstock. EC varied widely among sources, reflecting differences in windrow management, evaporative potential at each site, and salt content of the original feedstock (particularly evident with the inclusion of aged manure to windrow B at site 2). A consistent trend in pH was evident. All windrows began the composting period at or below pH 7.0, and rose steadily until pH neared or exceeded 8.0.

 Mineral N concentration was highly variable, but an overall pattern was evident (Table 3). Extracts of fresh compost showed high NH4-N and very low NO3-N concentration during the early stages of composting. As composting proceeded, NH4-N levels generally declined, with NO3-N increasing substantially in a few late samples. Air-drying samples before extraction yielded quite different values, with NH4-N decreasing dramatically and NO3-N generally increasing.

 N mineralization index also showed a clear trend. A high level of N immobilization was observed in samples from all sources collected early in the composting period. The degree of immobilization generally decreased over time, but nearly all samples continued to show some level of immobilization, even after more than 100 days of composting.

 Few viable weed seeds survived the initial weeks of composting. In the greenhouse assay less than one germinating weed per 2 liter sample was observed in compost from Sources 1, 3 and 4. Approximately 3 weeds per sample were observed in compost from Source 2. A total of fourteen weed species were observed, but only California burclover (Medicago hispida Gaertn.), which accounted for more than 50% of all germinating weeds, occurred with any regularity.

 Clear trends were apparent in the degree of phytotoxicity of compost extracts, as measured by the tomato seed germination index (Table 4). Although a statistically significant reduction in germination index was found only in composts from Sources 2 and 4, compost from all sources showed a trend toward increasing seedling vigor (decreasing phytotoxicity) with increased duration of composting; a germination index of 100 represented seedling vigor equal to those germinated in deionized water. Despite the 15:1 dilution employed, EC may have been a confounding factor in this assay, so an adjusted germination index was also calculated, based on the influence of osmotic potential on tomato seed germination reported by Dahal and Bradford (1990). The relative rank of compost samples using this adjusted index did not differ substantially from the original data.

 The growth of vinca plants in compost:perlite potting medium showed a similar trend, with plant growth over the 6 week period increasing with increased duration of composting (Table 4). The physical properties of the media (air-filled porosity, water holding capacity, bulk density) were not strongly correlated with vinca growth. Thorough leaching to remove salts precluded EC as a factor. The influence of CGW on nitrogen nutrition was apparently not a significant factor in plant growth, given the similarity in tissue N concentration across CGW samples.

 A correlation matrix of physiochemical characteristics of CGW samples and the corresponding biological parameters is presented in Table 5. In general, days of composting was more closely related to biological characteristics than cumulative hours above any threshold temperature. NH4-N concentration, whether extracted fresh or dry, was negatively correlated with phytotoxicity and N mineralization index, whereas NO3-N concentration was positively correlated with mineralization index and vinca growth. C/N ratio, although statistically correlated with N mineralization index, was a less effective predictor than mineral N variables or length of composting.

 

Discussion

Despite the very different techniques employed by the cooperating composters, compost from all windrows studied showed similar trends over time in both physiochemical and biological characteristics. Relatively rapid changes were evident through the first 6-9 weeks of composting, with subtle yet significant changes continuing throughout the period studied. At the end of the sampling period none of the windrows of 100% yard and landscape waste were 'mature' (biologically stable), as evidenced by their continued high temperature and immobilization of nitrogen. In current practice few commercial operations compost longer than the 3-4 month period monitored in this study; it is likely that the sale and utilization of biologically immature compost is common. There are no easily performed laboratory procedures that can document CGW maturity, but these are several good indicators of immaturity:

 

  1. less than 2-3 months of active composting
  2. temperature above 50°
  3. C/N ratio > 16-18
  4. high NH4-N concentration (>100 PPM) in fresh compost; NH4-N in dried CGW is less diagnostic
  5. low NO3-N concentration (< 20 PPM) in either fresh or dry compost, in the absence of leaching rain.

 

The significance of the degree of compost maturity varies with the intended end use:

 

  1. as a surface mulch in orchard, vineyard or landscape uses.

Compost applied as a non-incorporated surface mulch is usually intended primarily to suppress weeds and conserve moisture. As such, the most significant requirement is that the material has gone through a thermophyllic phase sufficient for control of weed seeds or pathogenic fungi. In this study, acceptable weed control was accomplished in 3-6 weeks, with several hundred hours of exposure above 50° C (120° F). Based on exhaustive studies of soil solarization (passive solar heating), control of common plant pathogens would also be expected with that degree of heating (Katan and DeVay, 1991). It has been suggested that surface application of compost may, over time, suppress the activity of common soil-borne root pathogens such as Phytophthora spp. There is solid evidence of pathogen suppression in nursery media by 'mature' compost (Hoitink et al, 1993), but compelling evidence is lacking for a similar level of suppressive activity in typical field situations, particularly when the compost is not incorporated. It is also questionable whether very young, immature compost (which supports a radically different microbial community than stable, mature compost) would have significant disease suppressive potential.

 

  1. as a soil amendment for vegetable or field crop production.

Compost amendment of field soil is done for one or more of the following reasons: nutrient value, improved soil physical properties, and enhanced microbial activity. This study, and previous work (Hartz and Schrader, 1996), document that short-term release of N is unpredictable, with immature compost typically immobilizing N rather than releasing it. At normal application rates (<10 tons/acre) the degree of N immobilization would be modest, but at higher rates short-term effects on N availability could be substantial. When using CGW of uncertain maturity a substantial period (a month or more) between compost application and planting would be prudent. This would also allow any phytotoxic effects to be minimized, although at normal application rates phytotoxicity is unlikely to be a significant issue; incorporating 10 tons/acre in the top six inches represents an addition of only 1% by weight. In the long term, a program of repeated CGW applications will enhance soil fertility, but quantifying the economic value of this approach vis-à-vis traditional fertilization is problematic.

 Improved soil structure and reduced surface crusting with application of organic amendments has been well documented, and could be expected with a program of CGW amendment, regardless of compost maturity stage. Overall microbial activity would be enhanced, even with immature compost (Hartz, unpublished data). The economic value of these changes in the soil environment would vary greatly, depending on original soil conditions and the crops to be produced. Substantially modifying soil moisture holding capacity or cation exchange capacity would require massive amendment and would be unlikely to be of economic benefit.

 

  1. as a constituent of potting mix.

This is by far the highest value use of CGW, with the most exacting requirements. This study and others (Hartz and Burger, unpublished data) show significant differences in the performance of CGW samples of varying levels of maturity. Variable plant response appeared to be affected less by physical than biochemical properties. More mature compost has less NH4-N and phytotoxic organic compounds, and is less likely to immobilize significant amounts of N. Also, material that is still actively composting (>50° C, 120° F) is unlikely to have substantial disease suppressive ability (Hoitink et al, 1993). Production of woody plant material appears to be less sensitive to variability in CGW samples than bedding plant production. Following these steps will minimize the risks inherent in utilizing CGW in potting mix:

 

  1. use material that has had at least 3 months of active composting
  2. allow CGW to 'cure' on-site before use, ideally for a month or more. Do not use material as long as the temperature in the pile is >50° C (120° F)
  3. limit the amount of CGW in the mix to 1/3 or less by volume
  4. leach thoroughly before use.

 

Acknowledgments: This project was supported by the Carlsbad Agricultural Grant Program of the Resource Conservation District of Greater San Diego County, and Davis Street Station for Material Recycling and Transfer.

 Literature Cited

 Bragg, N.C. and B.J. Chalmers. 1988. Interpretation and advisory applications of compost air-filled porosity (AFP) measurements. Acta Horticulturae 221:35-45.

Dahal, P. and K.J. Bradford. 1990. Effects of priming and endosperm integrity on seed germination rates of tomato genotypes. J. Experimental Botany 41:1441-1453.

Hartz, T.K. and W.L. Schrader. 1996. Suitability of municipal green waste compost for horticultural uses. HortScience 31 (in press).

Hoitink, H.A.J., M.J. Boehn and Y. Hadar. 1993. Mechanisms of suppression of soilborne plant pathogens in compost-amended substrates. p. 601-621. In: H.J. Hoitink and H.M. Keener (eds.). Science and engineering of compost: Design, environmental, microbiological and utilization aspects. The Ohio State University, Columbus, Ohio.

Katan, J. and J.E. DeVay. 1991. Soil solarization. CRC Press, Boca Raton, Florida.

 Kingston, H.M. and L.B. Jassie. 1986. Microwave energy for acid decomposition at elevated temperatures and pressures using biological and botanical samples. Anal. Chem. 58:2534-2541.

Sweeney, R.A. 1989. Generic combustion method for determination of crude protein in feeds: collaborative study. J. Assoc. Off. Anal. Chem. 72:770-774. 

 

Table 1. Cumulative hours of exposure above various threshold temperature during composting.

 

 

 

Cumulative hours above (°C)

 

Source

 

Windrow

Days of

composting

 

40°

 

50°

 

60°

 

70°

1

A

(moderate temperature)

 

20

 

482

 

406

 

274

 

0

 

 

49

1174

773

274

0

 

 

84

1909

926

274

0

 

 

111

2550

1115

274

0

1

B

(high temperature)

 

26

 

526

 

430

 

214

 

17

 

 

43

895

804

418

17

 

 

64

1399

1192

647

17

 

 

96

2168

1696

661

17

2

A

(100% green waste)

 

10

 

216

 

180

 

130

 

0

 

 

33

590

452

197

0

 

 

54

773

526

197

0

 

 

74

976

526

197

0

 

 

82

1051

598

197

0

2

B

(green waste/soil/

manure/straw blend)

 

10

 

216

 

180

 

30

 

0

 

 

33

707

383

130

0

 

 

54

707

383

130

0

 

 

74

707

383

130

0

 

 

82

707

383

130

0

3

A

(2 week turning interval)

 

22

 

508

 

461

 

313

 

0

 

 

48

1129

1080

436

0

 

 

69

1595

1283

449

0

 

 

89

1828

1283

449

0

 

 

109

2070

1283

449

0

3

B

(4 week turning interval)

 

22

 

509

 

504

 

360

 

8

 

 

48

1132

1055

617

15

 

 

69

1550

1153

617

15

 

 

89

1776

1153

617

15

 

 

109

2012

1153

617

15

4

A

(normal watering)

 

21

 

493

 

452

 

353

 

88

 

 

42

973

898

536

101

 

 

63

1453

1282

685

104

 

 

84

1837

1446

691

104

 

 

105

2309

1699

722

104

4

B

(extra watering)

 

21

 

455

 

442

 

370

 

54

 

 

42

959

934

696

96

 

 

63

1410

1318

834

96

 

 

84

1742

1318

834

96

 

 

105

2174

1520

834

96

Table 2. Physiochemical characteristics of compost throughout the composting period.

 

 

 

%

 

Compost

source

 

Windrow

Days of

composting

 

N

 

P

 

K

 

Ash

 

C/N ratio

 

EC

 

pH

1

A

(moderate temperature)

 

0

 

1.1

 

0.16

 

0.8

 

37

 

28

 

6.9

 

6.5

 

 

20

1.6

0.2

0.9

51

16

7.2

7.3

 

 

49

1.5

0.22

0.8

56

14

6.3

7.4

 

 

84

1.6

0.24

0.8

51

15

5.5

7.6

 

 

111

1.6

0.25

0.7

56

14

4.5

7.5

 

 

118

1.6

0.19

0.9

67

14

10.7

7.8

1

B

(high temperature)

 

0

 

1.3

 

0.16

 

0.9

 

33

 

26

 

7

 

6.6

 

 

26

1.5

0.22

1.0

49

18

7.4

7.1

 

 

43

1.7

0.24

1.0

46

15

6.6

7.4

 

 

64

1.6

0.22

0.9

54

14

6.3

7.4

 

 

96

1.9

0.25

0.9

55

13

6.5

7.3

 

 

103

1.7

0.24

0.7

59

15

4.1

7.6

2

A

(100% green waste)

 

0

 

1.6

 

0.21

 

1.0

 

26

 

23

 

10.2

 

5.8

 

 

10

1.4

0.23

1.1

37

21

6.8

7.3

 

 

33

1.6

0.27

1.2

51

15

9.1

8.3

 

 

54

1.5

0.25

1.1

54

16

9.8

8.4

 

 

74

1.5

0.3

1.2

58

14

11.2

8.3

 

 

82

1.5

0.24

1.1

55

16

11.1

8.3

2

B

(green waste/soil/

manure/straw blend)

 

 

0

 

 

1.1

 

 

0.17

 

 

0.9

 

 

45

 

 

21

 

 

40

 

 

6.5

 

 

10

0.9

0.28

1.3

66

17

18.4

8

 

 

33

1.0

0.26

1.2

71

13

24.2

8.2

 

 

54

1.1

0.3

1.3

74

11

25.1

8.3

 

 

74

1.1

0.26

1.2

76

12

23.2

8.3

 

 

82

1.0

0.25

1.4

76

12

25

8.2

3

A

(2 week turning interval)

 

0

 

1.1

 

0.18

 

0.5

 

53

 

24

 

5.4

 

6.6

 

 

22

1.3

0.19

0.6

52

19

5

6.9

 

 

48

1.2

0.18

0.6

53

20

5

7.1

 

 

69

1.3

0.22

0.5

57

17

4

7.5

 

 

89

1.2

0.19

0.5

61

17

3.6

7.7

 

 

109

1.4

0.2

0.5

56

16

3.6

7.8

3

B

(4 week turning interval)

 

0

 

1.1

 

0.18

 

0.5

 

53

 

24

 

5.4

 

6.6

 

 

22

1.1

0.18

0.5

49

21

4.4

7

 

 

48

1.3

0.17

0.6

47

22

4.3

7

 

 

69

1.1

0.21

0.6

59

18

4.2

7.4

 

 

89

1.3

0.2

0.6

57

15

4.9

7.7

 

 

109

1.3

0.2

0.6

61

16

4.9

7.8

4

A

(normal watering)

 

0

 

1.5

 

0.27

 

1.2

 

41

 

20

 

11

 

6.9

 

 

21

1.4

0.24

1.1

47

18

12.9

7

 

 

42

1.5

0.28

1.2

53

16

14.7

7.3

 

 

63

1.4

0.26

0.9

66

13

14.5

7.7

 

 

84

1.6

0.29

1.0

62

12

14.7

8

 

 

105

1.6

0.28

1.1

62

13

16

7.9

4

B

(extra watering)

 

0

 

1.5

 

0.24

 

1.2

 

37

 

22

 

11

 

7

 

 

21

1.7

0.29

1.3

39

18

13.1

7.3

 

 

42

1.5

0.26

1.0

48

18

12.3

6.8

 

 

63

1.5

0.28

0.9

63

12

14

8

 

 

84

1.4

0.29

1.0

61

13

15.4

8

 

 

105

1.3

0.28

1.1

64

12

16.9

8

Table 3. Effects of source, management, and duration of composting on compost mineral N concentration (PPM)

and N mineralization index.

 

 

 

NH4-N

 

NO3-N

 

 

Source

 

Windrow

Days of

composting

 

Fresh

 

Dry

 

 

Fresh

 

Dry

N mineralization indexz

1

A

(moderate temperature)

 

20

 

110

 

18

 

 

5

 

12

 

-20

 

 

49

13

6

 

4

2

-23

 

 

84

107

14

 

16

35

-23

 

 

111

67

20

 

42

77

-5

 

 

118

46

8

 

152

161

-4

1

B

(high temperature)

 

26

 

328

 

48

 

 

4

 

12

 

-25

 

 

43

181

377

 

3

40

-24

 

 

64

72

13

 

164

38

-10

 

 

96

74

14

 

162

209

5

 

 

103

18

11

 

21

12

-5

2

A

(100% green waste)

 

10

 

 

 

27

 

 

 

 

2

 

-18

 

 

33

 

8

 

 

5

-20

 

 

54

 

8

 

 

12

-20

 

 

74

 

7

 

 

15

-10

 

 

82

 

7

 

 

15

 

2

B

(green waste/soil/

manure/straw blend)

 

 

10

 

 

 

 

 

15

 

 

 

 

 

 

2

 

 

-17

 

 

33

 

7

 

 

30

-15

 

 

54

 

5

 

 

10

-13

 

 

74

 

6

 

 

20

-8

 

 

82

 

15

 

 

28

 

3

A

(2 week turning interval)

 

 

 

441

 

121

 

 

8

 

1

 

-26

 

 

22

126

12

 

2

42

-19

 

 

48

34

13

 

2

61

-20

 

 

69

158

10

 

9

18

-16

 

 

89

96

11

 

4

13

-17

 

 

109

 

 

 

 

 

 

3

B

(4 week turning interval)

 

22

 

226

 

18

 

 

2

 

1

 

-17

 

 

48

100

11

 

4

93

-18

 

 

69

47

14

 

4

18

-20

 

 

89

153

9

 

7

35

-14

 

 

109

108

8

 

4

55

-17

4

A

(normal watering)

 

21

 

245

 

49

 

 

5

 

20

 

-29

 

 

42

265

147

 

4

1

-49

 

 

63

102

32

 

2

7

-23

 

 

84

117

10

 

14

19

-13

 

 

105

602

15

 

7

89

-12

4

B

(extra watering)

 

21

 

170

 

48

 

 

7

 

28

 

-27

 

 

42

511

377

 

15

1

-49

 

 

63

76

13

 

6

21

-21

 

 

84

268

14

 

9

25

-18

 

 

105

182

11

 

55

121

-14

z change in mineral N concentration (PPM) during a 14-day incubation of a 10% compost/90% soil blend relative to

unamended soil; negative numbers indicate net N immobilization.

Table 4. Effect of source, management and duration of composting on compost phytotoxicity and suitability as a nursery medium.

 

 

 

 

Phytotoxicityz

 

Vinca growth

(g/plant)

 

 

Media physical characteristics

Compost source

 

Windrow

Days of

composting

 

Raw

 

Adjusted

 

 

Fresh wt.

 

Dry wt.

Vinca tissue

% N

Porosity

(% volume)

Water holding

(% volume)

Bulk density

(g/ml)

1

A

(moderate temperature)

 

0

 

82

 

89

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

20

75

82

 

2.0

0.27

4.6

0.15

0.48

0.19

 

 

49

93

99

 

2.3

0.32

4.4

0.15

0.51

0.21

 

 

84

109

115

 

1.8

0.30

4.4

0.14

0.46

0.21

 

 

111

106

111

 

2.1

0.29

4.5

0.13

0.46

0.23

 

 

118

93

104

 

2.6

0.38

4.3

0.13

0.46

0.26

1

B

(high temperature)

 

0

 

67

 

74

 

 

 

 

 

 

 

 

 

 

 

 

 

 

26

75

82

 

1.9

0.27

4.5

0.15

0.49

0.19

 

 

43

80

87

 

2.0

0.26

4.6

0.14

0.49

0.21

 

 

64

87

93

 

2.4

0.33

4.4

0.11

0.52

0.22

 

 

96

107

114

 

2.8

0.37

4.6

0.10

0.52

0.23

 

 

103

108

112

 

2.4

0.38

4.5

0.13

0.46

0.22

2

A

(100% green waste)

 

0

 

13

 

23

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10

57

64

 

0.9

0.15

4.0

0.24

0.35

0.2

 

 

33

51

60

 

1.4

0.20

4.2

0.18

0.45

0.23

 

 

54

49

59

 

1.3

0.16

4.3

0.15

0.47

0.28

 

 

74

82

93

 

1.6

0.21

4.2

0.10

0.5

0.33

 

 

82

89

100

 

1.7

0.22

4.2

0.09

0.53

0.34

2

B

(green waste/soil/

manure/straw blend)

 

 

0

 

 

36

 

 

76

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10

55

73

 

1.5

0.19

4.4

0.13

0.47

0.28

 

 

33

81

105

 

1.2

0.16

4.3

0.09

0.45

0.33

 

 

54

25

50

 

1.4

0.17

4.2

0.14

0.49

0.39

 

 

74

76

100

 

1.5

0.20

4.2

0.13

0.53

0.46

 

 

82

90

115

 

1.4

0.17

4.4

0.14

0.54

0.47

3

A

(2 week turning interval)

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

22

78

83

 

1.9

0.26

4.5

0.13

0.49

0.19

 

 

48

87

92

 

2.0

0.29

4.6

0.09

0.53

0.23

 

 

69

90

94

 

2.6

0.37

4.5

0.14

0.46

0.26

 

 

89

123

126

 

2.8

0.38

4.5

0.13

0.43

0.27

 

 

109

116

120

 

2.7

0.39

4.3

0.14

0.49

0.28

3

B

(4 week turning interval)

 

0

 

 

 

 

 

 

 

 

 

 

 

 

22

105

110

 

1.9

0.25

4.7

0.13

0.49

0.21

 

 

48

90

94

 

1.9

0.26

4.6

0.17

0.41

0.22

 

 

69

83

87

 

2.7

0.35

4.5

0.12

0.49

0.25

 

 

89

96

100

 

2.3

0.30

4.5

0.14

0.47

0.27

 

 

109

106

110

 

2.8

0.39

4.5

0.10

0.52

0.26

 

Table 4. Continued

 

 

 

 

Phytotoxicityz

 

Vinca growth

(g/plant)

 

 

Media physical characteristics

Compost

source

 

Windrow

Days of

composting

 

Raw

 

Adjusted

 

 

Fresh wt.

 

Dry wt.

Vinca tissue

% N

Porosity

(% volume)

Water holding

(% volume)

Bulk density

(g/ml)

4

A

(normal watering)

 

0

 

51

 

62

 

 

 

 

 

 

 

 

 

21

22

35

 

1.4

0.20

4.4

0.14

0.5

0.19

 

 

42

28

43

 

1.6

0.23

4.3

0.17

0.46

0.19

 

 

63

59

74

 

2.4

0.32

4.5

0.11

0.52

0.23

 

 

84

52

67

 

2.5

0.33

4.6

0.08

0.53

0.25

 

 

105

53

69

 

2.6

0.38

4.4

0.07

0.59

0.26

4

B

(extra watering)

 

0

 

48

 

59

 

 

 

 

 

 

 

 

 

21

36

49

 

1.6

0.22

4.4

0.17

0.41

0.22

 

 

42

30

42

 

1.6

0.24

4.3

0.17

0.45

0.18

 

 

63

33

47

 

2.1

0.29

4.4

0.11

0.53

0.24

 

 

84

56

71

 

2.2

0.32

4.6

0.09

0.55

0.23

 

 

105

45

62

 

2.4

0.30

4.5

0.10

0.55

0.22

z phytotoxicity expressed as germination index of tomato seed in compost extract; higher numbers represent less phytotoxicity. 'Adjusted' values represent correction

for EC of compost extract.

Table 5. Correlation matrix of biological and physiochemical compost characteristics.

 

 

 

Phytotoxicityz

 

Adjusted

Phytotoxicityz

N mineralization

index

 

Vinca

dry wt.

Length of composting (days)

.45*

.48*

.49*

.62*

Hrs > 40°C

.17

.15

.15

.52*

Hrs > 50°C

.26

.22

.18

.85*

Hrs > 60°C

-.09

-.15

-.12

.67*

NH4-N (fresh)

-.47*

-.47*

-.41*

-.27

NH4-N (dry)

-.37*

-.41*

-.72*

-.12

NO3-N (fresh)

.20

.23

.51*

.25

NO3-N (dry)

.25

.26

.51*

.41*

Total N (%)

-.02

-.11

-.06*

.27

C/N ratio

.12

-.01

-.37*

-.09

Media porosity

 

 

 

-.29

Media water holding capacity

 

 

 

.36*

Media bulk density

 

 

 

-.31

*correlation significant at the 5% level of probability (p < .05, n=40)

z phytotoxicity expressed as germination index of tomato seed in compost extract; higher numbers represent

less phytotoxicity. 'Adjusted' values represent correction for EC of compost extract.