«Dimambro ME, Lillywhite RD & Rahn CR Warwick HRI, University of Warwick, Wellesbourne, Warwick, CV35 9EF Corresponding author: ...»
Compost has been found to have a substantial buffering capacity, and generally has a pH above neutral. Thus compost application could reduce liming costs in agriculture. Inorganic fertilisers can reduce soil quality in a number of ways if applied for a number of years. For example, continued application of NH4 fertiliser tends to reduce soil pH.
The application of MSW compost to sodic soils has been found to have a number of benefits.
Sodic soils are highly alkaline and have high concentrations of sodium which adversely affect crop production and soil structure. Compost application increases the concentration of other salts which displace the sodium (Kochba et al. 2004). This, combined with the enhancement of soil structure by compost could result in the reclamation of sodic and saline soils.
Organic matter has been used as a soil improver and fertiliser for centuries. Although the application of inorganic fertiliser to agriculture is now common practice, using composts derived from green wastes in agriculture is slowly making a come back. Compost contains variable amounts of N, P and K, and is a valuable source of plant nutrients.
Composts have been used successfully as a fertiliser in variety of field crops ranging from grass to maize, grains and broccoli (Rodrigues 2000; Szmidt 1997). The fertilising effect of compost is due to its capacity to release N and other plant nutrients. The application of compost to agricultural soil increases soil organic matter (SOM). As SOM undergoes mineralization, N is released and becomes available to the crop. In many cases, around 25% of available N in compost is released in the first year, with a release of 10% each year thereafter (Sikora and Szmidt 2001). However, this is not a consistent figure, with N release rates varying according to a number of factors including SOM, temperature, moisture and texture. One study of four vineyard soils in Germany showed a 4-33% N release rate in the first year of compost application (Nendel et al. 2005).
1.4.2 Compost chemical characteristics As mentioned above, BMW consists of a number of organic wastes including food and green waste, paper and cardboard. The type of feedstock will greatly influence the response of crops to the applied compost. The C:N ratio of the compost is one of the principal factors. For
example, C:N ratios of MSW compost used in studies on different crops varies from 16:1
(Hadas and Portnoy 1997) to 27:1 (Brito 2001) and 30:1 (Crecchio et al. 2001) to 40:1 (Eriksen et al. 1999). The C:N ratio controls residue breakdown and mineralization of the N contained in the compost. C:N ratios tend to be lower in vegetable wastes and higher in straw, paper and cardboard.
The Soil Association suggest a C:N ratio in the range of 20:1 to 40:1 in mature composts.
Above this threshold N becomes immobilised i.e. the N is no longer available to the crop.
Compost with a C:N ratio greater than 30:1 applied to soil can actually immobilise available N, causing N deficiency.
Trace elements occur in the environment, soils and plants. Plants acquire the minerals that they need from the soil solution. Six macronutrients (N, K, P, S, Mg, Ca) and eight micronutrients (B, Cl, Cu, Fe, Mn, Mo, Ni, Zn) are essential for plant growth. However, other trace elements, termed potentially toxic elements (PTEs) or heavy metals can also be taken up by plants. The presence of PTEs above a certain concentration can have detrimental effects on plant growth and development. Fertilisers, pesticides and sewage sludge added to soils often contain traces of PTEs. For example, Cd, Cu, Pb, Ni and Zn are all found in inorganic fertilisers (Epstein et al. 1992). PTEs can be toxic to plants, contaminate water and affect human health.
Composts derived from MSW contain PTEs, often due to the presence of solder and lead acid batteries in the waste stream (Mamo et al. 2002). The principal PTEs in MSW compost are Cd, Cr, Cu, Hg, Ni, Pb and Zn. According to the United Nations Environment Programme, the most significant potential environmental problem arising from compost use is its potential to convey PTEs to the soil (UNEP 1996).
The physical appearance of composts can be significantly influenced by the volume of noncompostable elements (NCEs) present. These NCEs are predominantly glass, plastic, stones and metal. Generally composts produced from mixed waste contain a greater proportion of NCEs than source segregated waste composts.
1.5 Project rationale
There are significant drivers to enhance the collection and management of organic waste materials in the UK. These drivers include the EU Landfill Directive and Waste Strategy 2000 (DETR 2000). In order for these objectives to be met increasing amounts of BMW will need to be composted. Indeed, by 2010 the UK may need to compost and find alternative methods of disposal for up to 15.5Mt BMW by 2020. In 2003/04 2.0Mt BMW was composted to produce 1.2Mt of compost (The Composting Association 2005). Thus an eight fold increase in composting is necessary to meet the 2010 target.
To date, research in the UK regarding the use of compost has focused on green waste composts as a peat substitute or soil conditioner (for example: Peatering Out Ltd 2005). This is due to the majority of operations composting green waste only. Results from the few UK field trials investigating green waste composts (Parkinson et al. 1999, HDRA Consultants 2000, Ward et al. 2005) have revealed a number of benefits to both the crop and soil. These findings may not be directly transferable to mixed BMW composts. The feedstock materials and the final product in BMW composts may differ in terms of both physical and chemical properties to those of green waste composts.
Agricultural land covers over 70% of the land area of England and Wales (www.defra.gov.uk). Thus, the potential area for spreading composted BMW is substantial. If long term and repeated applications of composted BMW to land is to become the solution to the UK's current waste problem, then both short and long term field trials will be required to assess its impact. It should be highlighted that the Animal By-Products Order, the Water Directives and the Nitrates Directive must be complied with when compost is applied to land, as for any agrochemical, manure or sewage application. Moreover, the compost should be of agricultural benefit to both the soil and the crop. Thus the compost can be regarded as a positive input into the agricultural system.
Assuming an average compost N content of 1-1.5%, and a maximum permissible application rate of 250 kg N ha-1 year-1 in Nitrate Vulnerable Zones (NVZs), an area of 375,000ha would be required to spread 15Mt compost. Cereal production is the largest agricultural sector in the UK, producing 22.3Mt cereals in 2003, including 14.8Mt wheat and
6.6Mt barley. The land area used for cereal production in the UK is approximately 3,100,000ha (www.defra.gov.uk). This large area could easily absorb the increasing volumes of compost which will be produced, and even benefit from the applications of compost.
In order to assure farmers that BMW composts are safe to use, and do indeed have an agricultural benefit, field trials demonstrating this are imperative. Field trials should be supported by articles in the popular farmer’s press and with agreement of the major retailers, whose support is necessary to make it work.
1.6 Project aims
The overall aim of the work described in this report is to improve our understanding of the effects of applying BMW composts to agricultural land. This project will investigate the application of composted BMW to cereals, using BMW compost sourced from a variety of UK companies using a range of organic feedstocks. Such work is necessary to demonstrate to farmers whether compost produced from BMW can be safely used in agriculture without causing detrimental damage to the crop, soil or the environment.
The objectives of the study were to assess:
1. the physical and chemical characteristics of composts produced from BMW
2. the effects of applying BMW composts to agricultural land on both the crop and soil
2.1 Compost acquisition Two batches of compost were acquired. Batch 1 was an initial screening exercise and was performed on 12 composts from 11 companies. Following screening, five composts were chosen for the field experiment. These are referred to as batch 2.
The majority of the compost feedstocks were 100% source segregated, with the exception of composts G (100% MSW) and L (72% MSW from a Resource Recovery Facility). A variety of composting methods were used including open windrow and in-vessel systems. Composts A, B and J were first composted for up to two weeks in-vessel, followed by maturation in open windrows.
Batch 1 composts were stored at 4oC prior to analysis. The composts were analysed as detailed in section 2.2.
Five composts were selected for use in the field trial: A, B, F, G, J. They represented a range of feedstocks, from mixed MSW to source segregated green waste mixed with vegetable waste, and contained a range of N values, C:N ratios and proximate characteristics. For details please refer to section 3.
Batch 2 composts were obtained in bulk at the end of February 2005 for the field trial. Subsamples were taken for analysis in accordance with BS EN 12579, and were stored at 4oC.
The composts were analysed as detailed in section 2.2. The remainder of the compost for the field trial was stored undercover until required (section 2.3).
2.2 Compost analysis
The composts were analysed according to the British Standards (BS) for soil improvers and growing media, which are used by the Publicly Available Specification (PAS) 100 compost accreditation scheme (WRAP 2005), as detailed in table 2.2. For comparison, the composts were also compared with the APEX compost standards (APEX 2004).
Prior to analysis, the composts were prepared according to British Standards (BS 2000). Five replicates per compost type were used for % carbon, % nitrogen and carbohydrate analysis.
For all other methods three replicates were used.
2.2.1 Water holding capacity The water holding capacity was only measured on batch 2 composts. Fresh compost samples were sieved to 11mm.
Method to measure field capacity Galvanised metal cylinders with a volume of 195cm3 were used. The base of each cylinder was covered with fine nylon fabric (3 threads/mm) and secured with a rubber band. The cylinders were filled with compost and placed in a cold water bath lined with capillary matting for 24h, by which time the samples were saturated. The cylinders were then placed on the sand surface in a sand tower for 24h. Then, still on the sand surface, a pressure of 50cm suction (equivalent to field capacity) was applied for 72h. Samples were weighed, dried at 103oC for 48h and then re-weighed.
Method to measure wilting point The method and apparatus are as described by Heining (1963). A known volume of fresh compost, 23cm3, was weighed, soaked in distilled water overnight, then placed in the apparatus and 220lb/in2 pressure was applied for 14 days. The compost was weighed, dried at 103oC for 48h and re-weighed.
Calculation for available water content Available water content; the difference between field capacity and wilting point (as % water on a dry weight basis) was calculated from the above measurements.
2.2.2 Carbohydrate analysis Analysis of water soluble carbohydrate (WSC), cellulose and lignin concentrations were performed on oven dried (80oC) compost. Samples were milled using a rotary mill to pass through a 2mm sieve.
WSC 20ml H2O was added to 0.2g compost and incubated for 2h in a boiling water bath. The water-soluble fraction was separated by centrifugation at 5,000rpm for 20min then made up to a known volume. WSC was determined using the phenol-H2SO4 assay (Dubois et al. 1956).
Cellulose and lignin The solid residue remaining after centrifugation was dried for 16h at 100oC. Lignin was determined using a method based on Ritter et al. (1932). The compost residue was treated with 4ml 72% H2SO4 in an iced water bath for 2h. The acid was diluted to 0.6M strength and reluxed for 2h on a hot plate. Following cooling the solution was filtered through a Whatman GF/A filter. Cellulose was determined by measuring sugars in the filtrate using the phenolH2SO4 assay (Dubois et al. 1956). Material remaining on the filter was dried for 24h at 100oC to determine the acid-insoluble material. The filter was placed in a furnace at 400oC for 16h to determine ash content. Lignin was determined by subtracting the weight of ash from the weight of acid-insoluble material.
2.2.3 Bioassay A bioassay was performed on the batch 2 composts, using a modified version of the PAS 100 standard protocol the “Method to assess contamination by weed propagules and phytotoxins in composted materials” (WRAP 2005).
The composts were combined with a peat mix consisting of 10 litres sphagnum peat, medium
grade, 60g limestone and 40g fertilizer (17:17:17), to give five different treatments:
Treatment 1 0 % compost: 100% peat 2 25% compost: 75% peat 3 50% compost: 50% peat 4 75% compost: 25% peat 5 100% compost: 0% peat Growth conditions The experiment was carried out in July 2005 at Warwick HRI. Trays (210x150x50mm) were filled with the compost mixtures and wetted thoroughly. Ten tomato seeds (variety Moneymaker) per tray were sown, and lightly covered with compost. The trays were situated in a temperature controlled glasshouse (20-25oC) on moist capillary matting, and were also watered from above when required. A fully randomised and blocked experiment was established. Each treatment was replicated five times.
Once a week for four weeks, the number of tomato plants in each tray was recorded. After four weeks the above ground part of the tomato plants were harvested. Fresh weights were recorded, and after oven drying for 48h dry weights were recorded.
2.3 Field trials 2.3.1. Location and design The field trial was located at Warwick HRI, Wellesbourne (Lat: 52:12:11 N, Lon: 1:36:07 W). The soil was a sandy loam of the Wick series (Whitfield 1974). In 2003 winter wheat (Hareward) was grown, followed by winter barley (Pearl) in 2004. In autumn 2004 a base dressing of 250 kg ha-1 P and 240 kg ha-1 K was applied.