«Dimambro ME, Lillywhite RD & Rahn CR Warwick HRI, University of Warwick, Wellesbourne, Warwick, CV35 9EF Corresponding author: ...»
The incorporation of composts increased potassium in all treatments in the 0-30 cm layer, except for compost F applied at the 250 kg ha-1 N level (Table 4.10). There were no significant differences in potash levels. All composts applied at the 500 kg N ha-1 level increased the concentration of magnesium in the 0-30 cm soil layer (Table 4.11).
4.2 Plant analysis Plant samples were taken pre-top dressing in May (56 days after drilling) and at harvest in August (142 days after drilling) as detailed in Section 2.2.
Visual assessment of the crop four weeks after sowing revealed a reduction in size and vigour, and a yellowing of the leaves in plants treated with compost G, compared to all other treatments. However, after fertiliser application, plants treated with both compost G and fertiliser showed some recovery, becoming greener and more vigorous.
4.2.1 Plant samples prior to top dressing (10th May 2005)
Plant yield on day 50 varied according to compost type and rate. Composts F and J increased yield at both rates, with compost A increasing yield at the 500 kg N ha-1 rate only. A 15% reduction in yield was observed in compost B. The mixed waste compost G caused a significant reduction in plant yield at both concentrations; 62% (250 kg ha-1) and 75% (500 kg N ha-1).
There are a number of possible explanations for the reduced growth caused by mixed MSW compost G. Firstly the high concentration of NH4–N in compost G (2.2 g kg-1); high concentrations of NH4–N in the region 1.2-2.3 g kg-1 have been found to inhibit grass seed germination (O’Brien and Barker 1996). High concentrations of NH4 have been found to decline 7-10 days after compost application, although initial phytotoxic effects can damage seeds and plants may not recover (O’Brien and Barker 1996). In this study, the barley was sown four days after compost application, and so it is possible that the high levels of NH4 in compost G caused a reduction in barley germination and performance.
Secondly, compost G generally had higher concentrations of PTEs and sodium than the other four composts, which may have caused the reduced yield observed in the barley. Indeed, as shown in the plant bioassay (Section 3.30), compost G delayed germination and stunted the growth of tomato seedlings.
Thirdly, compost G had a high C:N ratio. When the C:N ratio is less than 20:1 (as was the case with composts A, B, F and J), composts do not lock up nitrogen. However a C:N of 20:1 may reduce nitrogen availability to plants.
4.2.2 Plant samples at harvest (4th August 2005) Yield: Fertiliser response curve The commercial yield potential for this barley variety (Optic) under optimum growth conditions is 6.5 - 6.9 t ha-1 and the results from this trial agree, ranging from 5.41 (Control) to 6.68 t ha-1 (FERT 125). However, the response to inorganic fertiliser was minimal with a flat response curve. We attribute this to the high levels of soil mineral nitrogen throughout the growing season. The flat response curve has made interpreting the results from the compost treatments difficult.
Yield: Compost treatments Grain yield was significantly affected by both compost type and rate. In comparison to the control, the incorporation of composts A, F and J increased grain yield and composts B and G reduced it. Grain yield increased on average by 10% where the amount of compost incorporated doubled the nitrogen content from 250 kg N ha-1 to 500 kg N ha-1. Applying an additional 125 kg N ha-1 ammonium nitrate to the compost treatments was sufficient to overcome any soil mineral nitrogen immobilisation caused by compost incorporation except in compost G. In a field study using MSW compost, maize showed reduced growth and vigour where compost only was applied, however, the addition of mineral fertiliser allowed the plants to recover (Eriksen et al 1999).
In comparison to the control, compost A increased grain yield by 2% and 19% at the 250 and 500 kg N ha-1 rates respectively. Compost A had no effect on the 1000 grain weight although the grain nitrogen content was less than the control at both rates of compost application.
In comparison to the control, compost B reduced grain yield by 14% and 7% at the 250 and 500 kg N ha-1 rates respectively. In contrast, both 1000 grain weight and grain nitrogen content were greater than the control at both rates of compost application.
In comparison to the control, compost F increased grain yield by 21% and 31% at the 250 and 500 kg N ha-1 rates respectively. The 1000 grain weight was lower than the control and the grain nitrogen greater than the control at both rates of compost application. We suggest that these large increases in yield can be attributed to the high levels of nitrate in the compost.
Nitrates are easily absorbed by the crop, and so composts with high nitrate levels could promote early growth and development, even before the top dressing is applied.
In comparison to the control, compost G reduced grain yield by 33% and 28% at the 250 and 500 kg N ha-1 rates respectively. In contrast, both 1000 grain weight and grain nitrogen content were greater than the control. Although compost G restricted seedling germination and establishment, the plants that did survive were not adversely affected at the later growth stages. Compost from MSW reduced yield in lettuce and garden cress, with composts produced from source segregated BMW having no detrimental effects on germination or growth (Gajdos 1997). This reduction in yield in the mixed MSW compost could be due to the high levels of salts and PTEs (see section 3.2) or high C:N ratio.
In comparison to the control, compost J increased grain yield by 5% and 14% at the 250 and 500 kg N ha-1 rates respectively. The 1000 grain weight was lower than the control and the grain nitrogen greater than the control at both rates of compost application.
Composts A, B, F and J applied at 250 + 125 kg N ha-1 produced a higher barley yield than the control. Moreover, with the exception of compost F, yield was greater in the compost + inorganic fertiliser treatment than compost alone; this was also observed for MSW compost in potato and maize (Mkhabeka & Warman 2005). A Swedish study found that applying compost (50 kg N ha-1) in combination with fertiliser (50 kg N ha-1) resulted in a higher grain yield of oats and barley than using compost alone (100 kg N ha-1) (Svensson et al. 2004), as was seen in compost B. They recommended that compost should not be used as the sole fertiliser in intensive grain cropping, rather it should be regarded as a soil conditioner which must be complemented with mineral N.
Grain analysis The barley grain was analysed for P, K and C (Table 4.15), Ca, Mg and Cu (Table 4.16) and Fe, Mn, Na and Zn (Table 4.17).
The application of all the composts resulted in significant, but numerically slight, increases in the phosphorus, potassium, calcium, magnesium, copper and sodium concentrations in the barley grain. In all cases compost G resulted in the highest levels. However, these results are not significant in the terms of this trial since all the fertiliser treatments also increased concentrations of the same elements. Concentrations of manganese and carbon contents were unaffected by any of the compost treatments.
Straw carbon content was between 41.8% and 44.3% and was not significantly affected by the application of composts. Nitrogen content was between 0.30% and 0.51% and was significantly affected by the application of composts. However there was no clear relationship between application rate and nitrogen content except where compost G was applied, where the nitrogen content was 37% and 53% greater than the control with the 250 and 500 rates, respectively.
Straw phosphorus concentrations were between 0.05 and 0.14 mg kg-1 dry weight and were significantly affected by compost application. There was little variation between composts A, B, F and J, however, compost G resulted in high values that were nearly double the control.
The application rate was not a significant factor in phosphorus concentration.
Straw potassium concentrations were between 1.05 and 2.19 mg kg-1 dry weight. The application of compost G resulted in significantly higher values. The application rate was also a significant factor in potassium concentration however no obvious relationship could be found between rate and concentration.
Calcium concentrations in straw ranged from 0.36 to 0.51 mg kg-1 dry weight. Statistically, the application of composts was significant but the application rate was not. Compost G gave the highest readings but no clear pattern was obvious within the other composts. The higher application rate actually resulted in lower concentrations.
Straw magnesium concentrations were between 0.8 and 0.12 mg kg-1 dry weight. Both compost type and application rate were statistically significantly different although the spread of the data is small with only compost G appearing higher that the rest.
Straw copper concentrations were between 2.0 and 2.99 mg kg-1 dry weight. The application of composts was significant. Compost A concentrations were less than the control while composts B, G, G and J were greater than the control. Concentrations within the compost treatments were comparable for both application rates.
Iron concentrations in straw ranged from 23.1 to 44.0 mg kg-1 dry weight. The spread of the data is narrow although the application of composts was significant. Composts A, B and F were comparable to the control but composts G and J gave higher values especially at the higher application rate. The higher application rate gave higher results.
Manganese concentrations in straw ranged from 8.53 to 13.55 mg kg-1 dry weight. The application of composts was significant. While composts A, B, F and J were comparable to the control, compost G again gave the highest values. There was no effect from different application rates.
Straw sodium concentrations were between 423 and 989 mg kg-1 dry weight. Both compost application and rate significantly affected sodium concentrations. All composts increased sodium in comparison to the control and the higher application rate resulted in higher concentrations. Compost G had the highest result at both application rates.
Zinc concentrations in straw ranged from 3.67 to 12.59 mg kg-1 dry weight. Compost application was significant in comparison to the control with all the composts treatments having higher results than the control. The application rate was not significant although the higher rate gave higher concentrations.
This sector of the report has highlighted many examples where the application of compost G has resulted in significantly higher levels of elements in both the grain and straw. We suggest that this is not a response to the compost but that it is luxury uptake due to the reduced biomass accumulation within this treatment. The toxic elements contained within compost G initially reduced seed germination and plant establishment however once the plants were growing they had greater resources to exploit in comparison to other treatments.
Nitrogen uptake in compost treatments With the exception of compost F, the application of composts at the 250 kg N ha-1 rate reduced nitrogen uptake in the grain in comparison to the control. This suggests that existing soil mineral nitrogen was immobilised by the application of the composts and that the increase where compost F was applied was the result of higher levels of nitrate. Where composts were applied at the 500 kg N ha-1 rate, composts A, F and J increased nitrogen uptake in grain in comparison to the control whereas composts B and G reduced it. It is likely that the higher nitrogen content at the 500 kg N ha-1 rate was sufficient to overcome the immobilising effect of the compost itself. The overall nitrogen uptake at the 500 kg N ha-1 rate was 13% higher than at 250 kg N ha-1.
The immobilising effect of the composts was highlighted again where an additional 125 kg N ha-1 was applied to the composts. With the exception of compost A, even with this additional mineral fertiliser the nitrogen uptake in grain for the compost treatments was reduced in comparison to the control (FERT 125 kg N ha-1). However this result is not unexpected since many workers have found that composts release little or no nitrogen in their first year. The differences in nitrogen uptake in grain and straw due to both compost type and rate were significant and the pattern was the same for nitrogen uptake in straw.
Total nitrogen uptake increased with the increasing rate of mineral nitrogen in the FERT treatments. The application of composts had mixed effects on total nitrogen uptake. At the 250 kg N ha-1 rate composts B and G reduced total nitrogen uptake in comparison to the control, while composts A and J gave comparable and compost F higher uptakes. At the 500 kg N ha-1 rate composts B and G reduced total nitrogen uptake in comparison to the control and A, F and J increased it. The application of compost F at the 500 kg N ha-1 rate gave comparable total nitrogen uptake to the 42 kg N ha-1 FERT rate.
Nitrogen uptake in the compost + FERT treatments was considerably higher than compost alone. However, only the application of compost A resulted in a higher total nitrogen uptake in comparison to the control (FERT 125 kg N ha-1) demonstrating again that the application of composts is likely to lead to immobilisation of soil mineral nitrogen in the first year of application. Compost G significantly reduced nitrogen uptake suggesting that it reduced establishment rather than nitrogen uptake; this view is supported by the grain nitrogen content which was higher than the control.
Although the composts contained either 250 or 500 kg ha-1 nitrogen, the proportion of the nitrogen that was recovered by the growing crop was minimal, only composts F and J showed positive recovery rates. However, in subsequent years as the compost breaks down in the soil, more nitrogen will be released. Under normal field conditions, not more than 10 to 15% of the total nitrogen in the compost is available in the year of application. Nitrogen recovery rates using ammonium-nitrate ranged from 75% at 42 kg ha-1 to 38% at the highest rate of 209 kg ha-1.