What has an influence on Soil Fertility?

Farmers know that the fertility of the soil depends on many factors. For the plants to grow they
need to get from the soil suitable conditions for root growth, appropriate supply of water and
nutrients available for uptake by roots. If certain soil conditions are not suitable, plant growth can
be inhibited. For example water logging, acidity, compaction or shortage of nutrients can tremendously
decrease the yields of some crops.

Why organic matter is so important?

Soil organic matter helps to build up a loose and soft soil structure with a lot of cavities
(pores). This leads to better aeration, better infiltration of rain or irrigation water and an easier
penetration of roots.
The visible parts of organic matter act like tiny sponges which can hold water up to five times
their own weight. Therefore in dry periods more water is available for the plants for a longer
time. This is especially important in sandy soils.
The non-visible parts of organic matter act like a glue, sticking soil particles together thus forming
stable crumbs. Such aggregates improve the soil structure, especially in clay and sandy
soils.
Beneficial micro-organisms and other soil organisms such as earthworms also feed on organic
material thus decomposing it. As these organisms require sufficient humidity and aeration,
soil organic matter provides a suitable environment for them.
Organic matter has a great capacity to retain nutrients and release them continuously. It thereby
increases the capacity of the soil to supply the plants with nutrients and reduces nutrient
losses by leaching. This is especially important in sandy soils as they naturally retain very few
nutrients.
Organic matter also prevents soils from becoming too acidic.

Soil Testing

Most people have strong trust in any scientific work. Therefore, when it comes to soil fertility,

farmers might also think of getting their soil analysed in a laboratory. Though chemical soil

testing may yield valuable information to specific questions, farmers should not expect too

much of it.

For example there are some inherent problems related to analysing nutrient contents: For

the plant, the total content of a certain nutrient in a sample is not always relevant, as the

nutrient may be absorbed to minerals so strong that it is not available to the plant roots (e.g.

Phosphorus, see chapter 4.1.3). Therefore, some tests treat the sample with solvents in

order to simulate the fraction of the nutrient available to plants. This might be a realistic

simulation for conventional farming. In organically managed soils, however, the higher activity

of soil organisms can result in a better availability of the nutrient, thus the result of the test

is not fully appropriate. The content of other nutrients such as nitrogen is extremely fluctuating

within few days, so that it highly depends on the point of time when the sample is taken.

Still, chemical soil analysis can be useful in some cases, e.g. to analyse the level of acidity

of the soil (pH) or to detect deficiency of nutrients such as Potassium (K) or Zinc (Zn).

Organic farmers might be especially interested in knowing and monitoring the content of soil

organic matter.

Chemical soil analysis on pesticide residues is highly complicated as one must know which

pesticide to look for, and they are very costly. Physical testing, e.g. related to water retention

capacity or soil structure can yield interesting information, but samples must be taken very

carefully. Biological analysis, e.g. of the activity of soil organisms, must be done in specially

equipped laboratories and is rather costly. Altogether, the use of soil analysis on the farm

level is limited due to the scientific methods, the availability of suitable laboratories and the

costs involved. If soil tests are used, make sure that the relevant aspects are investigated

Mycorrhiza – a beneficial fungus

A major part of the soil microbial biomass is composed of fungi. Important representatives of
the soil fungi are the "mycorrhizae" that live in association (symbiosis) with plant roots. Both
the plant and the fungus profit from the association: the plant gets nutrients collected by the
fungus and the fungus receives assimilates ("food") from the plant in exchange. Mycorrhizae
are present in all types of soils, but not all crops can get into a symbiosis with the fungus.
Mycorrhizae have several functions, which are of high interest for the farmer:
They enlarge the rooting zone of plants and can enter into small soil pores
They dissolve nutrients such as phosphorus from mineral particles and carry them to the
plant
They make soil aggregates more stable thus improving the soil structure
They preserve moisture and improve the water supply to the plants
Mycorrhiza formation depends on the soil conditions, the crops that are grown and the
management practices:
Soil tillage and burning of biomass drastically harm the mycorrhizae
High nutrient levels (especially phosphorus) and chemical pesticides suppress the symbiosis
Mixed cropping, crop rotation and the cultivation of perennial plants encourage mycorrhiza
Practice mulching to stabilize soil temperature and moisture
Among the naturally occurring species of mycorrhizae, not all show the same efficiency to
take up phosphorus from the soil. That is why artificial inoculation of specific mycorrhiza
varieties can improve their use. Inoculation, however does not reduce the importance of
offering appropriate living conditions for these organisms.

Soil Organic Matter

Besides mineral particles, soil contains smaller or larger quantities of organic matter or humus,
resulting from the decomposition of biomass. Though in most agricultural soils of the tropics it
makes only a few percent or even less than one percent of the total solid material, it is of tremendous
importance for the soil fertility. Its functions are described in detail in chapter 3.2.2.
Organic matter is mainly present in the top layer of the soil, which is subject to continuous
transformation processes. The active part of soil organic matter can be further decomposed by
soil organisms. The resulting structures can recombine themselves to form very stable humus
structures, which can remain in the soil for many years. This long term soil organic matter or
humus contributes a lot to improve the soil structure.

Mineral Particles in Soil

Soil consists of mineral particles, organic matter and pores. Mineral particles originate from
subsoil and rock, which gets crushed to smaller and smaller pieces through physical and chemical
weathering processes.
The mineral soil particles are divided into four groups according to their size:
Gravel and stones: particles larger than 2 millimetres
Sand: particles from 0.05 to 2 millimetres; they can be felt between the fingers
Silt: particles from 0.002 to 0.05 millimetres
Clay: particles smaller than 0.002 millimetres


The difference between sand, silt and clay is not visible to the naked eye. Still it is important to
distinguish between them, as the properties of the soil is very much dependent on the composition
of the different particle sizes. Soils having equal amounts of clay, silt and sand are ideal
for agricultural use. Such a soil is called loam.
Mineral particles contain nutrients which are slowly released in the process of weathering.
Plant roots and some micro-organisms can actively dissolve nutrients from mineral particles
and use them for their growth. The plants need minerals to build up organic matter and for
physiological processes.

Activities that increase the level of soil organic matter:

 leaving crop residues on the field, instead of burning or wasting them, as they are the major
source of biomass
applying compost: this is very effective, as part of the organic matter in compost is already
stabilised and will remain in the soil for a longer time than fresh plant material
applying organic manures: as they contain organic material, they help to increase the content
of organic matter; at the same time, they can speed up decomposition as they are rich in nitrogen
and thus stimulate soil organisms
mulching with plant materials or agro-wastes: especially applying hardy material (rich in fibres
or wood) will increase the organic matter content, as it will remain in the soil for a long time;
in addition, it helps to reduce erosion
using green manures or cover crops: green manures grown on the same field will contribute
biomass both from the leaves and roots; material grown on another site contributes only the
leaves; the younger the plant material is, the faster will it decompose, thus releasing the
nutrients faster but adding less to the built up of soil organic matter
Suitable crop rotation: including crops in the rotation which build up soil organic matter (e.g.
pastures); especially perennials and crops with a dense root system (e.g. pastures) are very
beneficial
reducing soil tillage: each tillage will speed up the decomposition of organic material, as it
aerates the soil and stimulates soil organisms
avoiding soil erosion: all methods listed before will be in vain unless soils are prevented from
erosion; it carries away those parts of the soil which contain most humus and are most fertile