OM
Soil organic matter is made up of plant and animal residues in different stages of decomposition, cells of soil microorganisms, and many types of decomposed substances.
We can distinguish either "living" and "dead" organic matter:
• "Living": plant roots and soil animals. These provide organic materials to the soil that eventually become part of the soil organic matter cycle.
• "Dead", where we have 3 subgroups:
• Active soil organic matter:
Primarily made up of fresh plant and animal residues that break down in a very short time, from a few weeks to a few years.
It shows lots of biological activity.
• Passive soil organic matter or humus:
This is not biologically active, meaning it provides very little food for soil organisms.
It may take hundreds or even thousands of years to fully decompose.
• Slow soil organic matter is somewhere in between active and passive soil organic matter.
It consists primarily of detritus, partially broken down cells and tissues that are only gradually decomposing.
It may take a few years to a few decades to completely break down.
Illustration of active and passive organic matter
There are four main processes in the soil organic matter cycle, and all of them rely on soil microbes:
• decomposition of organic residues,
• release of nutrients (mineralization),
• release of carbon dioxide (respiration), and
• transfer of carbon from one soil organic matter ‘pool’ to another.
Humus is thus the "end product" of decomposition of organic matter. It gives the soil its dark brown colour. Usually, humus represents the majority of total soil organic matter.
Humus consists of:
• Humic Acids
• Fulvic Acids
• Humin
Compost is not humus! Compost is plant material that is slightly decomposed. Even aged, well-rotted compost is still only slightly decomposed. Once added to your garden compost will continue to decompose for several years.
C:N Ratio
This is the carbon-to-nitrogen ratio of organic matter. There is always more carbon than nitrogen in organic matter. For example, a ratio of 20:1 means that there is 20g of carbon for each 1g of nitrogen in that organic matter.
The lower the C:N ratio, the more rapidly nitrogen will be released into the soil for immediate crop use.
A C:N ratio > 35 results in microbial immobilization and thus longer decompostation.
A ratio of 20–30 results in an equilibrium state between mineralization and immobilization.
Most soil organic matter comes from plant tissue:
• This plant tissue consists largely of water: 60-90%.
• The remaining dry matter consists of carbon (C), oxygen, hydrogen and small amounts of sulphur (S), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). These nutrients are very important for plant growth.
This brings us directly to the benefits of organic matter:
• Provides nutrients. These nutrients become available as the organic matter is decomposed by microorganisms. Because it takes time for this breakdown to occur, organic matter provides a slow release form of nutrients.
• Improves soil structure. As organic matter decays into humus, the humus molecules 'cement' particles of sand, silt, clay and organic matter into aggregates which will not break down in water. This cementing effect, together with the weaving and binding effect of roots and the mycelium of fungi gives the soil 'structure".
• Improves drainage. These larger, stable aggregates have larger spaces between them, allowing air and water to pass through the soil more easily. (*)
• Holds moisture. These aggregates can also retain relatively large amounts water increasing the soil's water retention capacity.
• Improves cation exchange capacity. Humus has an enormous surface area, which is negatively charged. This means it can attract and hold huge quantities of cations (positively charged ions) such as magnesium (Mg2)+, potassium (K+), ammonium (NH4+) and calcium (Ca2+) until the plant needs them. Clays also have this capacity, but humus has a much higher CEC than clays.
(*) Remark: in sports turf midst there are lots of discussions about the role of organic matter. On sand-based fields OM can rapidly accumulate around the base of the grass plants. A "thatch" layer is formed that compromises the drainage capacity. Thatch build-up can be caused by several factors, amongst which the relatively low microbiological activity in such soil profiles and over-application of nitrogen fertilisers. Adequate maintenance is needed to maintain the organic matter within the preferred ranges.
WRC / WHC
Water retention capacity (WRC) or water holding capacity (WHC) is the ability of a soil to physically hold water.
It is commonly expressed as v/v (percent of volume) either w/w (percent of weight).
WRC is primarily controlled by:
• soil texture and
• organic matter.
Soils with smaller particles (silt and clay) have a larger surface area than those with larger particles (sand). A larger surface area allows a soil to hold more water. Based upon the percentage of sand, silt and clay in a soil, we can distinguish 12 soil texture categories. This is visually represented in a soil texture triangle.
These 12 soil types have a different water holding capacity:
pF
The relation between the volumetric water content in your soil and the water potential (i.e. the suction force applied to that water) is expressed in a water retention curve or pF - curve.
The name pF is short for "Potenz" (or "exponentiation") and "Freier energie" (or "available energy").
• On the X-axis the volumetric percentage of water is plotted;
• On the Y-axis the logarithm of the suction force, expressed in centimetres "water head".
As the water retention capacity primarily is controlled by soil texture (and organic matter), the shape of the pF-curve changes with soil texture. A common shape for a clay, silt and loam soil is:
• A saturated soil has a pF close to 0.
• Due to gravity water will drain from the biggest pores, until an equilibrium is reached at +/- pF2. This is called Field Capacity (FC).
• When plants absorb water, more water is removed from the soil: the pF increases. When pF4.2 is reached, the remaining water is no longer available for the plants. This is called Wilting Point (WP).
• The amount of Plant Available Water (PAW) is situated between pF2 and pF4.2.
• An oven-dried soil sample has pF7: no more water is left.
The amount of Plant Available Water (PAW) in a clay soil is much higher than in a sandy soil:
At FC, the volumetric water content in this sandy soil is +/- 8%; at WP merely 2%. This gives 8-2 = 6% PAW in a sandy soil.
In this clay soil, the volumetric water content is +/- 47%; at WP +/- 28%. This gives 47-28 = 19% PAW.
This also is the reason why a clay soil with 20% of water will feel dry and a sandy soil with 10% of sand will feel humid. The WRC of soils can be increased by adding soil amendments suited for that purpose.
To conclude:
• the total Water Holding Capacity WHC is not directly related to the amount of water available for the plants.
• the amount of Plant Available Water PAW is the volume of water that remains in the soil after free drainage minus the volume of water that is too strongly bound to the soil matrix.
The following figure gives a good overview of the PAW in relation to the 12 soil types:
Soil Type
EC
Soil Electrical Conductivity (EC) is a measure of the amount of salts in soil (salinity of soil).
Soil electrical conductivity gives us an indication about the total amount of salts, not the presence of specific salts.
It is an important indicator of soil health. It has an impact on:
• crop yields,
• crop suitability,
• plant nutrient availability and
• soil microbiological activity.
A too high EC will disrupt the soil water balance and hinder plant growth.
Soils with high salt contents occur naturally in arid and semiarid climates. However, salt levels can increase as a result of cropping, irrigation, land use and application of fertiliser and compost.
Some examples:
Cropping or crop rotation is important to maintain a well-balanced soil.
Irrigation: irrigation in amounts too low to leach salts, or with water high in salts, allows salts to accumulate in the root zone, increasing EC.
Land use: management that leads to low organic matter, poor infiltration, poor drainage, saturated soil, or compaction can increase EC.
Fertiliser: nitrogen fertilizer application can increase salinity.
Compost: the amount of salts in a compost should be monitored and partially determines its quality.