Nutrient Requirements

The requirements for plant growth include light, water, an adequate soil structure for roots to grow in and nutrients. Elements essential for the growth of plants are called nutrients. The nutrients used in the largest amounts are the non-mineral elements, i.e. carbon, hydrogen and oxygen. These elements are mainly taken up as carbon dioxide (CO₂) from the air, and water (H₂O) by the roots (see also the nutrient cycle).

Increasing the supply of light, carbon dioxide, water and mineral nutrients from the deficiency range increases the growth rate and crop yields.

However, fertilisation increases crop yield only if the plant nutrients supplied is one of the most limiting growth factors. No yield increase is to be expected when fertilising crops that are mainly limited by factors other than nutrient supply, e.g. water, light, pH, salinity, light or temperature, etc. (JOENSSEN et al. 2004).

Nutrients can be divided into the two categories: macro nutrients and micro nutrients. The six elements normally classified as macro nutrients are: nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg). The plant roots mainly take up these nutrients in ionic form.

N is the element that plants require in greatest amounts and frequently the most limiting nutrient for plant growth. The use of N is usually higher than the total use of the other macro nutrients and micro nutrients together. N is essential to the photosynthesis and healthy cell growth and reproduction. It is vital in producing chlorophyll (which gives the leaves good green colour) and amino acids. It also promotes shoot and leaf growth.The plant takes up N as ions of nitrate (NO₃-) and ammonium (NH₄+). The main natural sources in soil of plant available N are degradation of organic matter and N fixation by microorganisms living in symbiosis with the roots of legumes.

P is taken up by the plants in the form of phosphate ions (at pH 5- to 7 mainly as HPO₄2-and H₂PO₄-). P is required for cell growth and plant reproduction, for the conversion of light energy to chemical energy during photosynthesis and is crucial for flower and fruit formation. Too little phosphorus can result in stunting, but too much can cause bitter flavour in crops. Symptoms of phosphorus deficiency are often mistakenly attributed to viral diseases. The supply of plant-available P comes from dissolution of soluble phosphates in the soil and from mineralisation of organic matter. P is not easy to dissolve in water, so plants often miss P if the level of organic matter in the soil is low.

K can be taken up in large amounts by many crops. High water solubility and natural K-content in the soil often results in a good supply of plant-available K, e.g. of K+. However, crops such as vegetables need large amounts of K and therefore additional K fertilisation may improve plant growth. K enhances plant health, reduces pest sensitivity, increases share of vitamins, proteins and oil in the plants. It increases drought tolerance and helps building up fibre in the plants.

S is also highly water-soluble and most crops need it in somewhat smaller amounts than P. S is a component of amino acids and proteins and important for plant growth.Yearly fertilisation with S is often needed. S is taken up as SO₄2+.

The addition of Ca and Mg is often not needed since soils with acceptable pH contain enough Ca and Mg. The most important macro nutrients that need to be added frequently during crop production are therefore N, P, K and S.

Micro nutrients are as essential for plant growth as macro nutrients, but are taken up in fairly small (micro) amounts. The elements normally considered micro nutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn) (FRAUSTO DA SILVA 1997 and MARSCHNER 1997). Most of the micro nutrients are needed for the formation of different enzymes. These micro nutrients are normally available in sufficient quantities through initial soil content and mineralisation of organic material. Only in special circumstances does scarcity of micro nutrients limit plant growth. When human excreta or manure are used as a fertiliser, the risk for such deficiency is minimal as excreta contain all micro nutrients necessary for plant growth.

In addition to the mere nutrients, plants also need a supporting soil structure for their roots to grow in. A high organic matter content in the soil improves the soil structure and supports diverse microbial community in the soil.It provides for airy crumbs and pore space in which particles of sand, clay and silt are held together allowing microorganisms and insects to feed on and to easily decay the nutrients in the soil and making them plant available. It also improves the water and nutrient retention capacity and the capacity to buffer pH and pollutants like heavy metals by holding them in the soil. Organic matter furthermore moderates the soil temperature and prevents the soil from heating up or cooling down to fast. Due to the improved water and nutrient withholding capacity, the vulnerability to environmental shocks such as droughts and floods is considerably reduced (see also soil degradation).

Fertilisation is a way to provide plants with nutrients that are missing in the soil. One can use either organic or inorganic (synthetic) fertilisers. Inorganic fertilisers provide the bulk of plant nutrients needed, and are produced artificially, while organic fertilisers (such as compost, treated faeces or urine) are produced by natural processes, and have the capacity to improve the physical, chemical and biological properties of the soil (e.g. its organic matter content, pH-value, texture, nutrient and water-retention capacity).

In terms of natural resources that are needed, food production requires mainly water, arable land that steadily supplies nutrients or synthetic fertiliser and the organic substrate for plant growth. These vital resources are often distributed unevenly around the world and many soils have been depleted or damaged by inappropriate agricultural practices. At present the earth is losing 25 billion tons of nutrient rich topsoil annually (WWI 2005), which results in considerably reduced productivity, and hence decreased food security. According to a study by the United Nations Environmental Programme (UNEP), some 2 billion hectares of vegetated land have been degraded since 1945 globally, which is equivalent to 17% of the worldwide productively used land (see also soil degradation).

In former centuries, the loss of soil fertility, which is inherent to all agricultural systems, has been solely compensated through the application of animal manure, human excreta, and compost or through long fallow periods.

Today, in extensive agriculture, compensation of soil degradation is often not practised. When inorganic fertilisers were introduced in the mid 19th century, fertilisation with industrially produced macro- (and micro-) nutrients became the prevailing and most common practice to meet key nutrient requirements for food production.

Inorganic fertiliser, i.e. mineral, synthetic or artificial fertilisers, refers most often to the macro nutrients P and K gained through mining of phosphate or potash rocks or the recovering of N form air.

However, these inorganic fertilisers are expensive to produce because their production relies on the availability of fuel (e.g. 1 litre of cryogenic distillation used to recover N uses approximately 1 litre of petrol oil). Furthermore, artificial fertilisers are bound to the world market controlled by large and powerful multi-national companies. Furthermore, mainly P and K reserves in rocks are predicted to be depleted in the next hundred years, if their exploitation follows the same trend as to day. The use of synthetic fertiliser is therefore not a reasonable solution for the majority of small-scale farmers in developing countries. In addition, the often inappropriate transporting infrastructure, particularly to remote areas, makes these commercial fertilisers even more expensive then their actual market value. Apart from that, industrial fertiliser cannot replace the loss of organic matter, microorganisms and many micro nutrients equally at stake in degraded topsoils (because they generally contain only some macro nutrients, i.e. NPK) and despite of the fertiliser use a negative nutrient balance in most soils is observed.

Wastewater and excreta, together with animal manure and composted solid waste have the potential to cover plant nutrient requirement with minimal or no application of synthetic fertilisers or freshwater resources. They can serve as important sources for soil amelioration, as they deliver all relevant nutrients and water needed for plant growth, as well as organic matter. Indeed, the aspect of growing food and achieving food security is historically strongly linked with the idea of reusing liquid and solid waste from households in agriculture. Throughout centuries, human and animal excreta played a crucial role in maintaining soil fertility and providing essential plant nutrients for food production. If agriculture is to be sustainable, these concepts should be revived.

See also (e.g.):

• Use of urine and faeces in agriculture (see this category in reuse and recharge section)
• Reuse of wastewater (see both reuse categories in reuse and recharge section)
• Linking up sustainable sanitation water management and agriculture