Compost

The economic well-being of New Zealand and most other nations depends greatly on the soil and how well its productivity is maintained. Soil is the essence of human life and health, as it is the source of most of our food.

The soil, however, does far more for society than just produce food. A healthy or good quality soil can act as an environmental filter by cleansing air and water. When managed properly soil can favourably affect the carbon-dioxide balance, and therefore help us in combating global climate change. Soil is the ultimate receptor and incubation chamber for the decomposition and detoxification of organic wastes, and for recycling nutrients from these materials back to plants. If the ecological balance in the soil is damaged permanently (mismanagement), the soil can work against us; it can pollute the air and water, and cease producing abundant and nutritious food. Many people believe that, within a community, there is a strong link between the health and quality of the soil, food quantity and quality, and health, well-being and prosperity of the people who are part of the community (Doran et al., 1992).

Organic farming is based upon maintaining the quality and health of the soil. The quantity and quality of the humus in the soil determined to a large extend the Soil Quality. Maintaining or increasing the humus buffer of the soil is therefore a vital part of OFS. Soil Organic matter and humus have a significant effect on soil and crops. (see box 1). This makes Composting a vital part of farming practices and some understanding of the processes involved can be off great benefit to the farmer.

Processes

Composting can be seen as a method that transforms organic wastes, residues and manure into more or less stable new organic compounds (humus) under controllable conditions.

Composting can have three different effects.

First there is the effect of the transformation of nitrogen compounds. Transforming the nitrogen from fresh organic matter is of major importance in biological or organic farming.

Nitrification is the transformation of ammonia to nitrate from organic material.

NH4+ --> NO32-

This process has an optimum temperature of around 30 to 40degrees Celsius. Net results highest under spring conditions.

Humification is the transformation of mineral nitrogen into organic nitrogen (proteins, humus compounds). The accumulation of humus compounds in a compost heap has an optimum temperaturerange from 12 to 30 degrees Celsius. Net results highest under autumn conditions.

Next to the nitrogen transformation there is the sanitary effect of composting. In this case relatively high temperatures and the time of composting are important. Most weed seeds, fungi's spore and bacteria are killed at a temperature of 55 to 60 degrees. Long term composting leads to the destruction of weed seed’s by antibiotic’s and other physiological/biotic substances. This can be of major importance with horticultural wastes and weeding wastes.

A third important effect of composting is loss of volume and moisture. Moisture losses occur due to water evaporation.

Dry matter losses can vary among 30 to 50% after 1 year of composting. Moisture evaporation can be as high as 7 liters per kg dry matter. Energy freed by thermophilic bacteria (bacteria that have an optimum growth rate at relative high temperatures) when they digest carbohydrates causes a lot of the evaporation.

Quality

The compost quality depends on the purpose of the end product:

  • compost with a high nitrogen mineral fraction (Nm), quality comparable to inorganic fertilizers and liquid manure’s.
  • compost with high relatively easy degradable nitrogen compounds which within a year of applying to the field will free nitrogen through mineralisation (Ne), quality is comparable with fresh and dynamic organic matter.
  • compost with a high residual nitrogen in stable macromolecules (Nr), this has a quality similar to humus.

The compost with a high Nr fraction is more suitable as a "soil conditioner" and improves or maintains the soil structure. The Ne and especially Nm fractions contribute more towards nutrient availability for crops.

The time, temperature of composting and the C/N ratio of the original material are the main factors that influence the outcome.

Carbon/Nitrogen (C/N) Ratio

Starting of with organic matter that has an average C/N ratio of 33/1 makes it theoretically possible to capture all nitrogen in biomass1(Steiner puts big emphasis on preventing N losses from the system). In this case sufficient C is available to supply micro-organisms for their maintenance, growth and reproduction.

C/N ratio lower than 33/1 creates nitrogen losses to the environment. Not all sources of C are easy digestible. Straw and hemicellulose are relatively easy digestible. Woody material, lignin, is very hard to digest for micro-organisms. Increasing the C/N ratio, when it is too low, of the compost with woody materials like bark, saw dust, pruning material, etc. do not give enough digestible carbohydrates and thus nitrogen is lost from the heap.

If the C/N ratio is too high in the end product then it can create immobilisation after application. This means that crops manured with this compost can suffer from nitrogen deficiencies.

 

Common Carbon : Nitrogen Ratios
Bark 100 - 130
Sawdust - Beech 100
Sawdust - Spruce 230
Paper 200 - 500
Straw - Oats & Rye 60
Straw - Barley & Wheat 100
Straw - Beans & Peas 40 - 50
Mushroom Compost 40
Mature Stable Compost 10 - 12
Organic Kitchen Waste 12 - 20
Organic Garden Waste 20 - 60
Tea Leaves 30 - 60
Grass Clippings 12 - 25
Chicken Slurry 10
Pig Slurry 5 - 7
Stable Cow Manure 25 - 30

A fast temperature rise will occur in the initial stage of composting. The temperature rise, to more or less 60-70 degrees Celsius, depends on the availability of oxygen in the heap and the amount readily digestible carbon. The next 8-10 weeks the temperature will drop. If the temperature in the compost heap rises above 40 degrees Celsius only thermophilic organisms survive. These are mainly bacteria and actinomycetes. Worms, spring tails and other bigger organisms appear at temperatures below 30 degrees Celsius. The most efficient digestors are the mesophilic micro-organisms (reduced N losses), their optimum temperature range is 25-35 degrees. Between 30-40 degrees (blood temperature!) most biotic/ physiological substances2 are formed and active: enzymes, hormones, vitamins, etc. These substances, if varied enough, contribute to a healthy balanced product and control the populations of pathogens in the compost and after application in the soil.

Using Compost or Organic Material In a Commercial Agricultural System

 

Effect of compost on soil and plant.

The break down of organic matter never gives a uniform product. The final product, called humus (under natural conditions) or compost (managed by men), is always dependant on the matter from which it is broken-down, the conditions under which it is processed and the microbes involved in the decomposition. This complex system forms a huge chemical, biological and physical composition that is a valuable product for soil and plant.

Organic matter that has no direct use as a fertiliser can be composted and contribute to chemical and biological soil fertility and quality. The regular use of compost and other organic matter can provide a sustainable soil fertility that will lead to an optimum production system especially on marginal soils. This is due to the following effects (direct and indirect) of compost on soil and plant:

Direct Effects

  • Chemical Soil Fertility

    1. increase in N and S and in lesser extent P.

    2. increase in C.E.C

    3. amorf oxides form complexes with humus which leads to less fixation

    4. soluble nutrients transform into less soluble and temporary insoluble nutrients

    5. micro elements

    6. biotic/physiological substances

Indirect effects

  • increase in soil structure

  • increase in moisture retention

  • decrease in erodibility

  • increased aeration

  • controlling of pests and harmful organisms

  • neutralising toxins

  • controlling pH

  • stabilising (soil) temperature

  • stimulating soil life (increase in e.g. N-fixing bacteria, trichoderma sp., mycorhyza)

Soil organic matter plays a role in soil weathering and breakdown of soil minerals. This means that soil nutrients are being introduced into the farming system from the soil parent material.

The above mentioned includes that humus has a direct influence in plant uptake. The plant has access to all vital nutrients.

Further more the living environment of the plant, improves by use of compost, stabilises the plant uptake. Through regular compost use, made from a wide variety of organic matter and produced by a divers microbial population, the soil becomes stable and suitable for sustainable use.

Visual Compost Judgement (adapted from E. Pfeiffer)

 

Colour   Points Description
Black Brown Bb 2 Ideal
Dark Brown Db 2 Very good
Black and Wet Bw 1 Anaerobic conditions prevalent
Green Black Gb 1 Same as "Black and Wet", undesirable
Green G 0 Too wet and sticky, anaerobic
Yellow Green Yg 0 Too wet, anaerobic and acidic
Yellow Y 1 Intermediate phase, sometimes aerobic
Grey Gr - If on surface only, okay
White W 0 mould and fungus, needs to be dried
       
Odour   Points Description
Earthy E 2 Very Good
Musty M 1 Intermediate phase
Cellar Odour C 1 Intermediate phase
Sour S 0 Intermediate phase, but too wet and anaerobic
Stinks like Sewage St 0 Too wet, tight and anaerobic
       
Structure   Points Description
Loose L 2 Best result, unless too dry
Crumbly C 2 Best result, unless too dry
Tight T 1 Not ideal
Caked Ca 1 Not ideal
Pulpy P 1 Not ideal
Dusty D 1 Not ideal
Crusty Cr 1 Not ideal
       
Moisture   Points Description
 Dry  D 1 Not ideal
Moist M 2 Material is moist, but no water can be squeezed out. Very good
Wet W 1 If water can be squeezed out it is too wet
Juicy J - If juicy in the center or at the bottom, either turn the pile or make air holes

1 biomass: all organic matter including living organisms like the bacteria and fungi

2 Plants seem to have a hormonal system after all, it is just as their digestive system in the soil and is very much controlled by the quality of the soil organic matter.