«Dimambro ME, Lillywhite RD & Rahn CR Warwick HRI, University of Warwick, Wellesbourne, Warwick, CV35 9EF Corresponding author: ...»
3.2.6 Carbon and nitrogen content and the C:N ratio Nitrogen was lowest in the mixed MSW compost G and highest in compost J, the compost containing both kitchen and catering waste (Table 3.26). Carbon concentration ranged between 14% and 30%. The C:N ratio was higher in compost G than the other composts.
3.2.7 Nutrients Compost F had the highest NO3–N content, and the lowest NH4–N content. In contrast, G had the lowest NO3–N content and the highest NH4–N content (Table 3.27). Mixed waste compost G had the highest concentration of all secondary nutrients (Table 3.28). With the exception of Fe and B, micronutrient concentrations were highest in mixed waste compost G (Tables 3.29 and 3.30).
3.2.8 Potentially toxic elements Compost G had the highest concentration of all PTEs exceeding PAS 100 limits in Cu, Ni, Pb and Zn (Tables 3.31 and 3.32). Concentrations of Pb, Zn, Cr and Hg in compost G are lower in batch 2 than in batch 1 (see Tables 3.10 and 3.11 for batch 1 results).
3.2.9 Pathogens Total coliforms varied from 20,267 (compost J) to 13,133,333 cfu/g (compost G). The coliform levels were higher in all five composts than for batch 1 (see Table 3.12).
Composts F, G and J contained concentrations of E. coli below the PAS 100 limit of ≤1000.
None of the composts contained Salmonella.
In all composts, the higher compost concentrations (75% and 100% compost) inhibited germination, as shown in figure 3.5. For example, after seven days the higher compost concentrations showed a germination rate of less than 20%, compared to 36% in the control.
Marchiol et al. (1999) noted that delayed germination occurred in a number of grass species grown in MSW compost.
Tomato plants grown in different concentrations of compost; 21 days after sowing Growth In general, dry weight was greatest in the 25% compost treatment and lowest in 100% compost (Figure 3.6). At 25%, compost resulted in a yield increase of over 10% in all five composts. Differences between composts were also observed, with increases in tomato plant dry weight at 25% compost, from 11% (compost G), 27% (A), 34% (F), 41% (J) to 53% (B).
A study comparing three green waste composts found 25% compost to produce the greatest yield in barley plants, with an increase at 50% compost in one of the composts only (HDRA Consultants 2000). In this study, the 50% compost treatment reduced yield in composts A, F and G, but increased yield by over 25% in composts B and J. The 75% compost treatment resulted in a reduction in yield of at least 45% in all five composts, being over 80% in composts F and G. A reduction in yield of over 70% was observed in the 100% compost treatment in A, B, F and G, with a 41% reduction in compost J.
The fresh:dry weight ratio of the tomato plants varied from 10 to 14 (Figure 3.7).
The fresh:dry weight ratio of tomato plants grown in peat or different concentrations of composts (25%, 50%, 75% or 100%) When compost is used in agriculture it is generally incorporated into the soil, and so a concentration of 25% compost or less is expected. This study has shown that the 25% compost treatment did not suppress germination of tomatoes, and growth was slightly greater than when no compost was used.
3.3.1 Comparison of composts
In general, results of batch 1 composts were similar to batch 2, but not identical. For example, pH, electrical conductivity and C:N ratio were comparable in both batches (table 3.34). In addition, the C:N ratio in the mixed MSW compost G was considerably higher than the four source segregated composts in both batches, probably due to the higher proportion of paper and card waste in the MSW.
Interestingly, there were differences in the pathogen levels between the two batches, as illustrated in table 3.35. In all five composts, coliforms were lower in batch 1.
These results indicate that, in general, compost properties are similar for each compost, but that there are variations between batches. With the exception of the microbial analyses, variations between batches are generally lower than variations between the different types of compost.
WRAP recommends regular testing of compost to ensure that the nutrient and contaminant levels are known before compost use. Indeed, prior to compost application it is necessary to ascertain the N concentration, so that application rates can be adjusted, in order to comply with the nitrates directive, if applicable.
3.4 Compost analysis summary
Analysis was performed on 12 composts supplied from UK commercial composting plants.
Ten composts were produced from 100% source segregated BMW which included paper, cardboard and green, fruit, vegetable, meat and kitchen wastes. One compost was 72% mixed MSW plus 18% source segregated BMW and the other compost was 100% mixed MSW.
Composts were analysed for a number of parameters including pH, electrical conductivity, carbohydrates, nutrients and contaminants.
Chemical compost characteristics In general, compost pH was greater than 7. Nitrogen content varied between 1.0 to 2.2% with the higher levels found in composts containing source segregated kitchen or meat wastes.
Phosphorus concentration ranged from 23 to 247 mg kg-1 and potassium between 1851 and 6615 mg kg-1. Total salts were higher in mixed waste composts (15-23 g kg-1), predominantly due to high concentrations of Ca, S and Na. Levels of PTEs in the 10 source segregated BMW composts were much lower than the limits for composts in the UK (PAS 100). However, the 100% mixed MSW compost exceeded the PAS 100 levels in four of the seven heavy metals tested (Cu, Ni, Pb and Zn).
Physical compost characteristics The MSW composts contained higher levels of physical contaminants (glass, plastic and metal) than the source segregated BMW composts.
Compost as a growing medium The composts tested can be used as a growing medium for plants such as tomatoes, without deleterious affects on plant growth at a rate of 25% compost to 75% peat. Concentrations of compost above 50% were found to reduce the growth of tomato plants.
4 RESULTS AND DISCUSSION OF THE FIELD TRIAL
4.1 Soil analysis Soil samples were taken on three occasions: prior to compost application, pre-fertiliser treatment and post-harvest. Samples were taken to two depths: 0-30 cm and 30-60 cm. Further details are available in section 2.3.2.
4.1.1 Prior to compost application (9th March 2005) Soil samples were taken from the three blocks prior to compost application. Soil pH ranged from 6.67 to 7.05 which was lower than the pH of the composts (7.67-8.68). Soil conductivity ranged from 1.72 to 2.18 mS cm-1. Total soil carbon was 0.8%, and total soil nitrogen 0.05% (Table 4.1).
Soils contained higher levels of NO3–N compared to NH4–N (Table 4.2). Mineral N ranged from 20 to 30 kg ha-1 in each layer, equating to around 50 kg ha-1 in the 0-60 cm soil layer.
Phosphorus (P) and potassium (K) were highest in the top soil layer (0-30 cm).
4.1.2 Prior to top dressing (5th May 2005) The incorporation of composts increased the amounts of total carbon and nitrogen in the 0-30 cm layer (Table 4.3). In the period between compost incorporation (9th March) and pre top dressing (5th May) the percentage of carbon and nitrogen increased in all compost treatments compared to the control although the increases were not significant. The incorporation of composts made little or no differences to the percentage of carbon and nitrogen in the 30-60 cm soil layer.
The 0-30 cm soil layer contained slightly more moisture than the 30-60 cm soil layer (Table 4.4). Moisture in the 0-30 cm layer ranged from 12.8 to 14.9% and 11.1 to 12.9% in the 30-60 cm layer. There were significant differences in both layers. In the 0-30 cm layer the incorporation of composts resulted in a significant increase in moisture content whereas in the 30-60 cm layer the same treatment resulted in a significant decrease. However the absolute values are small so we would not expect to see any effects on growing conditions.
Soil pH ranged from 6.7 to 7.3 in the 0-30 cm layer and from 6.6 to 6.8 in the 30-60 cm layer (Table 4.4). The range of values spans a maximum of 0.53 and there were no significant differences so we would not expect soil pH to have an effect on growing conditions.
Mineral nitrogen levels in the 0–30 cm layer (Table 4.5) showed some variation suggesting that mineralisation did occur from some of the composts. In treatments where 250 kg N ha-1 was applied in the composts, levels of mineral N did not differ significantly from the control.
However, where 500 kg N ha-1 N was applied in the composts, levels of mineral nitrogen were significantly higher than the control. The increase ranged from 3.3 kg N ha-1 (compost F) to 9.6 kg N ha-1 (compost J). Levels of mineral nitrogen in the 30-60 cm layer showed no significant effect resulting from the incorporation of composts.
Although not significant, where 250 kg N ha-1 was applied in the composts, there is some evidence of soil nitrogen immobilisation when compared to the control plots. Any immobilisation where 500 kg N ha-1 N was applied is masked by the extra nitrogen mineralised from the composts.
It is also possible that the effects of incorporating the composts could have been more pronounced, since high levels of spring mineralisation may have masked the effects. Between drilling and top-dressing, a period of only 57 days, the mineral nitrogen levels in the control plots increased from 50 kg/ha to 150 kg/ha. This amount of nitrogen mineralisation is unusual on the sandy loam soils at Wellesbourne.
4.1.3 Post-harvest soil samples (8th August 2005)
Incorporation of composts increased the carbon content in the 0-30 cm soil layer in comparison to the control. Levels of carbon were highest using composts G and J and lowest in the control (Table 4.6). There were no significant differences in the 30-60 cm soil layer.
Levels of total nitrogen in the 0-30 cm soil layer were highest where compost J was incorporated with 0.07% (250 and 500 treatments), compared to 0.05 or 0.06% for the other treatments. There were no significant differences in the 30-60 cm soil layer.
Where composts were incorporated, soil moisture levels at harvest in the 0-30 cm layer were generally higher than the control (Table 4.7) but not significantly so. There were no differences in the 30-60 cm soil layer.
In this study, soil pH was slightly higher in composts A, B and G compared to the control although this was not statistically significant. The incorporation of composts did lead to significant differences in the pH of the 0-30 cm layer although the narrow spread of the data suggests that pH will not influence the results. There was no effect on the 30-60 cm soil layer.
Where compost is incorporated annually, an increase in soil pH would be anticipated, as observed in a number of studies (Mkhabeka & Warman, 2005). A UK field study comparing three composts found a slight increase in soil pH with two years of compost application (HDRA Consultants, 2000). The ability of compost to raise the soil pH is one of the many advantages it has over inorganic fertilisers (Mkhabeka & Warman 2005), although these beneficial effects take time to become apparent.
The incorporation of composts did not result in any significant differences in electrical conductivity (Table 4.8). Even though the buffering effect of the soil is large, over time an increase could be expected with repeated compost applications.
The incorporation of composts should over time lead to an increase in soil organic matter as determined by loss on ignition although it does require multiple applications. However even within the limited duration of this trial we did find some increase in soil organic matter. The increases were not significantly different between the two sampling dates, 9th March (Table 4.1) and 8th August (Table 4.8)) but were quantifiable. Although the differences are small, the incorporation of composts did increase the organic matter when compared to the control and the double application rate resulted in greater organic matter in comparison to the single rate.
The mineral N level in the control plots at harvest was 57 kg ha-1 (Table 4.9). Comparing the soils that received the two rates (250 & 500 kg N ha-1) of compost application showed minimal differences in mineral N at harvest. Plots where compost, without the additional ammoniumnitrate, had been applied were lower in mineral N than the control. This suggests that incorporation of composts had an immobilising effect on existing soil mineral N. Where an additional 125 kg ha-1 of ammonium-nitrate had been applied mineral N levels were higher.
Where composts A and F were incorporated this additional fertiliser N may have overcome any immobilising effect and allowed some mineralisation of contained compost nitrogen.
Composts are known to mineralise their nutrients slowly and we would expect that more nitrogen would become available in the next 4-6 years. Moreover, regular applications of compost will ensure that the reported benefits of composts on soil structure, moisture retention and disease suppression will occur. In this study, the maximum permitted quantity of compost was applied (determined according to maximum N application in nitrate vulnerable zones in the UK; 250 kg N ha-1). This amount was equivalent to less than 45 t ha-1 compost, a layer of compost less than 1 cm thick before incorporation.