ARCHIVE: Fertiliser Manual (RB209)

Compost

Composts made from source-separated biodegradable inputs are valuable sources of plant available nutrients and organic matter. They are generally made from landscaping and garden ‘wastes’ (green compost), and additionally can contain kitchen/catering ‘wastes’ (green/food compost).

Compost that meets the requirements of the Quality Compost Protocol and the associated product standard (BSI PAS 100) is called ‘Quality Compost’, and may be spread on agricultural land without an Environmental Permitting Regulations exemption or a Standard Permit (which other green or green/food composts require). BSI PAS 100 is a specification that requires controls on inputs, the composting process and product quality, and requires the compost producer to establish and implement a quality management system. The Quality Compost Protocol defines the point at which compost can be treated as a product and ceases to be waste. Before spreading on the field, users are advised to consult their produce purchasers concerning any possible use restrictions imposed by the food supply chain. Where Quality Compost is used in agriculture and field horticulture the following requirements should be met to protect human health and the environment:

  • Records of input materials to the composting process.
  • Analysis of the Quality Compost giving characteristics and nutrients that will be supplied at a recommended application rate to meet crop needs and legislative requirements.
  • Soil analysis results, including heavy metals, prior to field spreading.

Nutrient content of compost

Composts are a valuable source of stable organic matter and crop available nutrients, which can be used by growers to meet crop nutrient requirements and to maintain soil fertility (see the example below). Green compost is the most commonly applied product to land, although increasing amounts of green/food compost will be produced in the next few years.

Based on the analysis of a large number of green compost samples, typical nutrient content data are summarised in the table below. Typical analysis data are also summarised for green/food composts, although they are based on more limited sample numbers. The nutrient content of compost products will vary depending on the source materials and treatment process. Most composts are supplied with specific nutrient content data and other relevant information.

Typical total nutrient contents (fresh weight basis)

Compost type

DM(%)

              
   

Total (kg N/t)

Readily available
(kg N/t)

Phosphate
(kg P2O5/t)

Potash
(kg K2O/t)


(kg SO3/t)

Mg
(Kg MgO/t)

Green

60

7.5

<0.2

3.0

5.5

2.6

3.4

Green/food

60

11

0.6

3.8

8.0

3.4

3.4


Nutrient supply from compost

The available field experimental data indicate that green compost supplies only very small amounts of crop available nitrogen and that inorganic fertiliser nitrogen application rates should not be changed for the next crop grown. In the case of green/food compost, the available experimental data indicate that around 5% of the total nitrogen applied is available to the next crop grown (irrespective of application timing). Following the repeated use of green and green/food composts long-term soil nitrogen supply will be increased.

As little work has been done on the availability of compost phosphate to crops, it is appropriate to extrapolate from work on livestock manures and sewage sludge which suggests that around 50% of the phosphate will be available to the next crop grown, with the remainder released slowly over the crop rotation. Around 80% of compost potash is in a soluble form and is readily available for crop uptake. Soil analysis will indicate if the phosphate and potash applied in compost is maintaining the target Index for both nutrients. Composts also supply useful quantities of sulphur and magnesium, although there are no data on availability to the next crop grown. Composts also have a small liming value that can balance the acidifying effects of inorganic fertiliser nitrogen additions to soils.

Example.

30 t/ha of green compost is applied in autumn to a sandy soil before drilling winter barley (8 t/ha grain yield, straw baled). The soil is at P Index 2 and K Index 2-. Allowing for the green compost nutrient supply saves up to £153/ha. This potential saving will be less where soil P or K Indices are above maintenance levels.

 

 Nitrogen (N)

 Phosphate
(P2O5)

 Potash
(K2O)

 Financial
Saving (£/ha)

1. Estimated total nutrients in green compost (kg/t)

 

 

 

 
Analysis provided by green compost supplier or typical
values from table above.

7.5

3.0

5.5

 
2. Estimate available nutrients in green compost (kg/t)        
Nitrogen (see above)

 NILa

     
Phosphate and potash (see above)

 

1.5

 4.4

 
3. Nutrients supplied by green compost that
are equivalent to inorganic fertiliser (kg/ha)
       

30t/ha supplies 225 kg/ha total nitrogen

 NIL

 90b

 165b

 
Potential saving from green compost use      

 £153/had

4. Nutrient requirements for barley (kg/ha)        
Section 4 of this book

160

 70c

 95c

 
5. Inorganic fertiliser needed for the barley crop (kg/ha)        
Stage 4 minus Stage 3

160

 NIL

NIL

 
Actual saving for next crop due  to green compost use      

 £99/ha

6. Surplus compost phosphate for subsequent crops that are equivalent to inorganic fertiliser (kg/ha)

 

     
Stage 3 minus Stage 4

 NIL

20

70

 
Saving for subsequent crops from green compost use      

£54/ha

a. Nitrogen availability is negligible or, for practical purposes, nil (see text above)
b. Total phosphate and potash content used in calculations to maintain soil P and K Indices.
c. Nutrients required for maintenance of soil reserves (soil P Index 2 and K Index 2-)
d. Saving for next crop plus value of surplus compost phosphate and potash which will contribute to the nutrient requirement of future crops.
Assumed fertiliser costs: Nitrogen 60p/kg; phosphate 60p/kg; potash 60p/kg.

ADLib logo Content provided by the Agricultural Document Library
© University of Hertfordshire, 2011