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Key departmental publications, e.g. annual reports, budget papers and program guidelines are available in our online archive.

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On-farm Composting of Municipal and Commercial Organics as an Environmentally and Socially Sustainable Resource Recovery Scheme for Rural Communities

June 2003,
Environment Australia

8 Compost Use

Compost should be used in accordance with good agricultural practice. This means that the applied compost should be fit for purpose, should not have excessive contamination levels and the rate and timing of applications should be in accordance with plant nutrient requirements and with consideration to environmental conditions. Nutrients provided by means of compost use need to be accounted for in nutrient budgets for individual crops as well as for the entire cropping sequence.

High application rates of nutrient rich compost and inadequate timing of compost application can result in detrimental environmental effects through leaching and ground water contamination. Oversupply of nutrients and subsequent leaching is particularly a problem if compost is used in addition to synthetic fertilisers and if nutrients contained in the organic fertiliser are not accounted for. As a result of this situation and in the face of massive ground water pollution problems (nitrate, phosphorus), Germany legislated to restrict the use of compost to 30 dry tonnes per hectare over a three-year period. The NSW Biosolids Guidelines established a 'nitrogen limited biosolids application rate' (NLBAR) and require that nitrogen from biosolids is utilised and its release to the groundwater is prevented or minimised. However, maximum allowable biosolids application rates do not apply to Grade A products and the maximum agricultural application rate is set at 1,200kg total nitrogen per hectare. On the other hand, BFA in their organic production standard recommend an application rate of 20 tonnes per hectare.

However, usually the proper use of compost products does not result in an oversupply of nutrients and leaching. In fact, the use of different recycled organic products can be used wisely to manage soil nutrients and to prevent environmental problems. In Biala (2000) it was reported that the use of 100 tonnes (fresh matter) of pasteurised, i.e. immature compost resulted in soil nitrate levels that were identical to the control (50-100kg nitrate [NO3] / ha, 0-90 cm depth). However, the same quantity of mature compost resulted in soil nitrate levels (0-90 cm) that reached between 200 and 250kg per hectare. If this happens in a period of high rainfall and reduced plant growth, considerable amounts of nitrate would be leached out of the root zone.

The proposed organics recycling model suggests that all generated compost be used on-farm. This requires the farm to have sufficient land and to show adequate production levels (cropping) to ensure that applied nutrients are absorbed and utilised. The hypothetical small, medium and large on-farm composting operations presented later in the report (Section 9) are assumed to generate 531 tonnes, 2,367 tonnes and 4,734 tonnes of compost per year, respectively. If the BFA recommended application rate of 20 tonnes per hectare were used, approximately 27 ha, 118 ha and 237 ha of suitable land, respectively, would be required to apply the available compost.

It was surprising to learn that the farmer did not use nutrient budgets as a tool to manage his organic farm and ensure that soil nutrient levels are maintained and not depleted. While organic farmers can replenish and supply nitrogen relatively easily through the use of legumes, evidence is emerging that phosphorus is being depleted on many organic (broad acre) farms and that this is posing a major problem (Evans 2002, McDonald 2002, Penfold 2002). The same concerns have been raised for sulphur in organic farms in Europe (Hansen, 2003, Phillips, 2003). These problems can be largely avoided by using compost that is derived from off-farm sources. Generally, compost is able to provide all essential plant nutrients at satisfactory levels, except for nitrogen.

The supply of plant nutrients through the use of compost was determined for an application rate of 20 tonnes per hectare, as recommended by BFA. Calculation of applied nutrients per hectare was based on analytical results obtained for Compost I (Table 6). The amount of nutrients applied and their availability over a three-year period are shown in Table 7. It can be seen that not all nutrients are fully plant available during the first year after compost use. Usually only about 40-50% of total nitrogen contained in the compost becomes available for plant uptake (compare Biala and Wynen, 1998). It has been assumed that micronutrients contained in compost, such as manganese, copper or molybdenum become fully plant available during a three year period. However, this has still to be confirmed by further research into the effects of compost use.

Table 7 Nutrient application through the use of 20 t/ha of compost (Compost I)
Component Unit Quantity
Availability 1)
Year 1 Year 2 - 3 Total
Dry matter kg 16,200.0      
Water kg 3,800.0      
Organic matter kg 4,860.0      
Organic carbon ∧ kg 2,825.6      
Total nitrogen kg 153.9 13.4 46.2 59.6
Ammonium - N kg* 1.2      
Nitrate - N kg* 18.8      
Ammonium + Nitrate - N kg* 20.0 20.0    
Total phosphorus as P kg 59.9 30.6 30.6 61.2
P Soluble kg* 1.4      
Total potassium ** kg 97.2 77.8 19.4 97.2
Total magnesium ** kg 71.3 21.4 49.9 71.3
Total manganese g 8,262.0     8,262.0
Total boron g 194.4     194.4
Total copper g 648.0     648.0
Total molybdenum g 599.4     599.4
Total zinc g 1,944.0     1,944.0
∧ derived from organic matter value by dividing it by 1.72
* calculated on the basis of assumed bulk density of 1L = 0.6 kg
** based on average K and Mg concentrations found in compost Ia and II
1) Availability of various nutrients based on figures presented by Biala and Wynen (1998)

Table 7 shows that the application of 20 tonnes (fm) of compost per hectare supplies approximately 33kg of nitrogen, 30kg of phosphorus, 77kg of potassium and 21kg of magnesium per hectare in the first year after compost use. In addition, various amounts of trace elements are supplied, ranging from 194 g of boron to 8.26kg of manganese per hectare.

A comparison between the amount of nutrients supplied by 20 tonnes (fm) of compost and nutrients removed by wheat or barley (yield 3.3 t/ha, Table 8) demonstrates that the applied compost is able to supply more than sufficient phosphorus and potassium and contribute to the nitrogen supply. However, additional nitrogen has to be supplied by other means to meet nutrient demand of agricultural crops and facilitate high yields.

Table 8 Nutrient needs on broad acre farms (kg/ha)
 
Wheat
Barley
Nutrients removed (Cooke 1975)    
N
66.0
57.8
P
9.9
9.9
K
33.0
24.8
Nutrients removed (Dalal and Probert 1997)    
N
44.3
39.5
P
6.6
5.4
K
17.4
18.3
Source: adapted from Cooke (1975) and Dalal and Probert (1997).
Note: Nutrient removal for a yield of 3.3 t/ha.

Biala and Wynen (1998) showed that nutrients (N, P, K, Mg, Ca) contained in green organics compost could be valued at more than $30 per tonne (fm). But not only nutrients are important and valuable for plant growth and agriculture, so is soil structure and organic matter. This is increasingly recognised in Australia and is now promoted as 'Carbon Based Agriculture' (Paulin, 2003). Vogtmann et al. (1989) attempted to value organic matter (carbon) that is supplied through compost use by assessing the costs of alternative supplies, i.e. green manuring. Production cost per tonne of organic matter (dm) from green manure was estimated to be around $59 in Germany in 1989. On this premise, organic matter contained in composted garden organics with 30-40 per cent organic matter levels was valued between $17.70 and $23.60 per tonne of dry matter.

Based on levels of macro nutrients (N, P, K, Mg, Ca) and organic matter found in the generated compost, it could be valued at about $40 per tonne (dm). If micronutrients were taken into account also, its value may reach approximately $45 per tonne (dm).