Soil fertility

What does "soil fertility" mean?

Fertility of a soil is defined by its ability to provide all essential nutrients in adequate quantities and in the proper balance for he growth of plants – independent of direct application of nutrients – when other growth factors like light, temperature and water are favorable. This ability does not depend on the nutrient content of the soil only, but on its effi ciency in transforming nutrients within the farm’s nutrient circle.
In transformation of nutrients soil organisms play a key role. They breakdown biomass from crop residues, green manures and mulch and contribute to build up of soil organic matter, including humus, the soil’s most important nutrient reservoir. They also play an essential role in transferring nutrients from the soil organic matter to the mineral stage, which is available to plants. Soil organisms also protect plants from disease and make the soil crumbly.
A fertile soil is easy to work, absorbes rain water well, and is robust against siltation and erosion. It fi lters rain water and supplies us with clean drinking water. It neutralizes (buffers) acids, which pass through contaminated air to the soil surface, and decomposes pollutants such as pesticides rapidly. And last but not least a fertile soil is an effi cient storage for nutrients and CO 2. In this way a fertile soil prevents the eutrophication of rivers, lakes and oceans and contributes to the reduction of global warming.
In the context of biological agriculture soil fertility is thus primarily the result of biological processes, not of chemical nutrients. A fertile soil is in active exchange with the plants, restructures itself and is capable of regeneration. The biological properties can be observed in the soil’s conversion activity, in the presence and the visible traces of the organisms in it. The communities of microorganisms are robust and active at the right moment. In the self-regulating ecological equilibrium animals, plants and microorganisms all work for each other It is the responsibility of farmers to understand soil ecology to the point that they can create or restore the conditions for a robust balance in the soil. If a soil does not regularly bring good yields, farmers should investigate the reasons for it.

Properties of a fertile soil

A fertile soil:

is rich in nutrients necessary for basic plant nutrition (including nitrogen, phosphorus, potassium, calcium, magnesium and sulphur);
contains suffi cient micronutrients for plant nutrition (including boron, copper, iron, zinc, manganese, chlorine and molybdenum);
contains an appropriate amount of soil organic matter;
has a pH in a suitable range for crop production (between 6.0 and 6.8);
has a crumbly structure;
is biologically active;
has good water retention and supply qualities.

Appropriate amount of plant nutrients

There are 16 essential nutrients that plants need in order to grow properly. Out of the 16 essential elements hydrogen, carbon and oxygen are obtained mainly from the air and from water. The other essential elements come from the soil and are generally managed by the farmers. Some of these nutrients are required in large amount in the plant tissues and are called macro (major) nutrients. Others are required in small amount and are called micro (minor) nutrients. Macronutrients include nitrogen (N), phosphorus (P) potassium (K), calcium (Ca), magnesium (Mg), and sulphur (S). Of these N, P and K are usually depleted from the soil first because plants need them in large amounts for their growth and survival, so they are known as primary nutrients. Ca, Mg and S are rarely limiting and are known as secondary nutrients. Where soils are acidic lime is often added, which contains large amounts of calcium and magnesium. Sulphur is usually found in sufficient amounts from the slowly decomposing soil organic matter. The micro nutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo), and zinc (Zn). Recycling organic matter such as crop residues and tree leaves is an excellent way of providing micronutrients to growing plants.
Plant roots require certain conditions to obtain nutrients from the soil:

First, the soil must be sufficiently moist to allow the roots to take up and transport the nutrients. Sometimes supplying water to plants will eliminate nutrient deficiency symptoms.

Second, the pH of the soil must be within a certain range for nutrients to be releasable from the soil particles.

Third, the temperature of the soil must fall within a certain range for nutrient uptake to occur.

Fourth, the nutrients must be within the root zone in order for the roots to access them.

The optimum range of temperature, pH and moisture is different for different species of plants. Thus, nutrients may be physically present in the soil, but not available to plants. A knowledge of soil pH, texture, and history can be very useful for predicting what nutrients may become deficient.
On the other side, too much of any nutrient can be toxic to plants. This is most frequently evidenced by salt burn symptoms. These symptoms include marginal browning of leaves, separated from green leaf tissue by a slender yellow halo. The browning pattern, also called necrosis, begins at the tip and proceeds to the base of the leaf along the edge of the leaf.

Neutral soil pH

Soil pH, its acidity or alkalinity, is highly relevant to how readily available nutrients become in soil, known as solubility of nutrients. In Africa, about one-third of the soils are acidic or prone to acidity and another one-third is either saline or alkaline and both are difficult to manage. Plants differ in their sensitivity to a low or high pH level. Some plants can withstand or even prefer a somewhat low pH level, others a higher one.
Soils with pH less than 6.5 and which respond to liming may be considered as acid soils. When potassium, calcium and magnesium leach from the soil, it becomes acidic. This may happen if there is a lot of rain (or irrigation water) that washes nutrients away, or if too much mineral nitrogen fertilizers are applied.
In acid soils plant roots do not grow normally due to toxic hydrogen ions. Phosphorous gets immobilized and its availability is reduced. Most of the activities of beneficial organisms like Azatobacter and nodule forming bacteria of legumes are adversely affected as acidity increases under acidic conditions, the bacteria fi x less nitrogen and decompose less organic matter, which results in fewer available nutrients.
Addition of lime or compost with high pH (8) will help to neutralize acidity and to increase the pH, so that the availability of nutrients will be increased.
Alkaline soils are formed due to concentration of exchangeable sodium and high pH. Irrigated soil with poor drainage may lead to alkali soils. In coastal areas, if the soil contains carbonates, the ingression of sea water leads to formation of alkaline soil due to sodium carbonate deposition.
pH of alkaline soil can be corrected through application of gypsum: For every 1 milli-equivalent of exchangeable sodium per 100 gram of soil about 1.7 tonnes of gypsum should be added to an acre of land. If the requirement is 3 tonnes per acre, application should be done in one dose. If the requirement is 5 or more tonnes per acre, application should be done in 3 split doses. Application of molasses or growing of green manure crops and their incorporation in the field can also help to correct an alkaline soil.

Crumbly structure

Plant roots prefer soil with a crumbly structure, like well-made bread. Such soil is well-aerated and the plant roots are able to penetrate easily. This allows them to grow both wide and deep allowing them to access more nutrients to support good growth.
Soil aggregation is also an important indicator of the workability of the soil. Soils that are well aggregated are said to have “good tilth”. A good soil structure also contributes to reduction of erosion of topsoil, as water infi ltrates more easily into the soil and the aggregates resist to the raindrops.

High biological activity

Even if we cannot see most soil organisms doing their work, the majority of soil organisms are very important to the quality and fertility of soils. They contribute to the transformation of crop residues and organic fertilizers to soil organic matter, to the improvement of plant health by controlling pest and disease organisms and to helping release nutrients from mineral particles. High biological activity is an indicator of fertile soil.
Most soil organisms prefer the same conditions as plant roots: humid conditions, moderate temperatures, air and organic material are best for them. Most are very sensitive to changes in soil moisture and temperature. Their activity is generally low when soils are dry, very wet or too hot. If the soil is compacted, dried out, baked by the sun, or is poor in organic matter, it becomes like a piece of concrete and soil organisms cannot do a good job. Even the bacteria, as tiny as they are, cannot work in a dead soil. Good air circulation within the soil is crucial for their development. Activity is highest in warm and moist soils when “food” is available.

Assessing the fertility of soil

Soil analysis
Farmers may find getting their soil analysed in a laboratory helpful to know more about the fertility of their soils. Soil analysis, however, often has limited relevance as nutrient uptake depends on many soil factors, such as biological activity. While soil analysis may provide good results for soils fertilized with mineral fertilizers, the higher activity of soil organisms in organically managed soils can result in better nutrient availability, making the results of a test not fully appropriate or accurate. In addition, the content of nitrogen in soil fluctuates extremely within just a few days, so that the amount in the sample is highly dependent on the point of time when the sample is taken.
Chemical soil analysis can be useful to analyse the level of acidity in soil (pH) or for detecting deficiencies or toxicities of nutrients such as Phosphorus (P), Potassium (K) or Zinc (Zn). Organic farmers may especially be interested in knowing and monitoring the content of soil organic matter (Corg). For soil that has presented problems such as low yields during several consecutive years, doing the traditional analyses of P, pH and Corg can certainly give an indication as to what should be done to improve soil fertility.
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 related to water retention capacity or soil structure can yield interesting information, but samples must be taken very carefully. Biological analysis of the activity of soil organisms must be done in specially equipped laboratories and is rather costly.
If soil tests are used, farmers should make sure that the relevant aspects are investigated and that the results of the test are critically discussed with an extension officer. For most farmers in Africa, it may be more appropriate to use a spade diagnosis and dig a soil profile to better understand their soils, and invest in soil fertility in general. Extension officers should encourage farmers to watch the activity of soil organisms that are decomposing plant material and also watch the fate of the plant material upon degradation. This may be part of a spade diagnosis, but could also be a first step to recognizing the soil as a living and active ecosystem.

General fertility status of African soils and its management

Africa has a wide range of soils and climatic conditions. Its soils are inherently low in fertility because they developed from poor parent material, are old and lack volcanic rejuvenation. In addition to low inherent fertility, about 65 % of the arable land in Africa is degraded due to water and wind erosion, loss of nutrients, physical deterioration and salinization. Soil fertility is highly heterogeneous with large on-farm variation from field to field and nearly as much variation on a local level as across all of Africa.
Several attempts have been made in the management and development of soil fertility in Africa. In the 1960s and 1970s soil fertility in agricultural systems was maintained through long term bush fallows of 10 or more years, but this is no longer being practiced because of increased shortage of land. The appropriate use of external inputs such as mineral fertilizers, lime, irrigation water and improved cereal germplasm was championed because it was believed to be able to alleviate constraints that are associated with crop production. However, unlike Asia and Latin America where the application of these technologies has boosted agriculture production leading to the `Green Revolution`, this did not work in the case of Africa due to the diversity of the agro-ecologies and cropping systems, variability in soil fertility, weak institutional arrangements and policy
failure. The Low External Input Sustainable Agriculture (LEISA) paradigm was also championed in the research and development agenda in the 1980s, which advocated a shift from external input only, to limited external inputs and optimal use
of locally available resources in efficient manner. But nevertheless, lack of sufficient organic resources and labour intensiveness of the LEISA technologies had caused a shift in soil fertility management. Significant progress has also been
made in the combined use of organic and mineral fertilizers in the mid-1980s and 990s. However, a lot of challenges still remain to be overcome in soil fertility management.
Today most African soils are defi cient of organic matter due to repeated ploughing, erosion of the topsoil, mono-cropping and lack or insuffi cient supply of organic materials. These soils have low capacity to retain and supply nutrients to plants, high nitrogen leaching and phosphate fi xation potential, low to medium water holding capacity, weak soil structure and are deficient in minor nutrients.

Challenges associated with mineral fertilizers

The nutrients in mineral fertilizers are highly soluble, easily taken up by the plant, but also easily leached out of the soil (especially nitrogen). They have to be applied cautiously so as to not to end up polluting streams or groundwater, which causes health problems in humans. Nitrates found in well water, for instance, are known to cause methaemoglobinaemia, also known as ‘blue baby syndrome’, where the blood is short of oxygen.
When plants receive nutrients in the form of mineral fertilizers through the soil water, they are forced to grow quickly, making them vulnerable to diseases and attractive to pests. On the other hand, when nutrients are supplied through biological activity from the decomposition process or humus, for instance, then the flux of nutrients (although water soluble) is slower and in more continuous supply compared to mineral fertilizers where nutrients are only available for a short period of time.
Mineral fertilizers are salts that may help to neutralize alkalinity such as ammoniated fertilizer. In African acid, infertile, red soils in arid and semi-arid climates, however, ammoniated fertilizers contribute to acidity, increasing problems with plant nutrition
Mineral fertilizers are very expensive for most farmers in Africa. Farmers who take out a loan to buy farm inputs depend on a good harvest to pay back the credit. Repayment becomes a problem when crops fail due to other reasons or when crop returns are low.
Reliance on mineral fertilizers cannot halt the continued degradation of African soils, because these fertilizers only address the mineral fraction of the soil and ignore, if used solely, the role and potential of soil organic matter and the need to implement other soil conservation measures to maintain soil fertility.

Factors influencing soil fertility improvement in Sub-Saharan Africa

Creating long-term soil improvement can be challenging due to the following circumstances:

Cultural beliefs and remains of early extension. The way land or soil is managed in many areas in Africa is deeply embedded in the cultural beliefs. Some common practices that are generally not good for soil fertility include cutting of trees, burning of bush and crop residues, and deep ploughing of the entire field. Some practices were brought through early extension messages and are not cultural beliefs per se. For example, long back extension used to recommend a clean field without any trees yet the farmers had been practicing some form of agroforestry where they used to leave some trees uncut. Or, cutting and burning of certain crop residues like cotton and tobacco is regulatory in some African countries as a preventive measure for pests and disease.
Migratory communities. With ever moving communities such as pastoralists and shifting cultivators, soil protection becomes very challenging. Communities move to new areas, cut all trees and burn bushes to grow crops or graze animals for 2 to 4 seasons. When the soil becomes less productive, the community moves to a new area. Since these communities do not stay in a given area for a long time, there is little incentive to undertake soil conservation measures.
Land tenure systems. Most farmers do not own the land on which they are farming; it is either customarily owned or rented land. Such tenure systems, which do not provide security to the farmer, are major obstacles to soil conservation. Farmers in such situations find no incentive to invest in soil conservation measures, especially if the lease is short term. In some cases, farmers are also not allowed to plant long-term crops including trees.
Scarcity of organic materials. During land preparation, potentially good mulching materials from slashed bushes, crop residues and weeds are instead burnt to clear the way for digging or ploughing. Farmers have numerous other uses for crop residues such as fodder for animals, roofi ng, fencing or fuel for cooking or using the ashes for soap production. Sometimes, even cow dung is dried and used as fuel for cooking meals. Such competition for organic materials for the various household needs limits availability of these materials for soil conservation needs. Scarcity of organic material is more pronounced in dry climates.
Fuel needs. Most households in Africa use and produce fi rewood or charcoal for their fuel and income needs. As a result, many forests and individual trees have been cut, rendering the land susceptible to degradation.
High population densities. Growing population is causing land use intensity, which is increasingly putting pressure on marginal land such as forests, wetlands and steep slopes as well as challenges related to land fragmentation. Such circumstances render soil improvement very diffi cult.
Climate change. High temperatures and water scarcity alternating with high inter-annual variability and erratic rain distribution in space and time, cause severe drought and fl ooding in some areas. This will lead to a reduction in the area suited for intensive agriculture. Lack of water also limits crop growth and unpredictable rains make timely sowing and successful establishment of crops difficult. Increase in soil temperature aside having negative effect on crop growth accelerate soil degradation processes and patterns. Together, these harsh climatic factors, coupled with poor soil management, have reduced soil fertility by contributing to erosion and to general soil and water degradation.
Inadequate use of synthetic fertilizers. Farmers have neither access to nor can they afford the fertilizers due to high prices as a result of the removal of subsidies, transaction costs, poor infrastructure, poor market development and inadequate access to credit facilities. The use of inorganic fertilizer in Africa is also limited by inherent low conversion efficiency due to lack or excess supply of water, soil compaction, inappropriate application, and low soil organic matter content of the soils. Phosphorus fertilizers are often poorly available, as they are easily fixed in some soils. Sole reliance on inorganic fertilizers cannot sustain the productivity of soil since it has very little effect on soil structure, resistance to erosion, moisture retention and biological activity.
Low availability of organic soil amendments and fertilizers. Application of organic sources of nutrients such as manure, compost and others not only provides some nutrients to the present and the following crops, but also improves the physical, chemical and biological quality of the soils. On most farms availability of animal manures or organic materials for compost production are low and access to organic soil amendments and fertilizers from surrounding farms or nearby industrial production is very limited. Better availability and integrated use of farm-own and affordable foreign organic and mineral fertilizers may significantly contribute to higher productivity and resilience of farms.