Organic and inorganic sources of nutrients

The majority of nutrient inputs to agriculture come from commercial mineral fertilizers. Organic manures play a significant but lesser role in providing nutrients, leaving aside their beneficial effects on physical, chemical and biological soil properties. However, there are no reliable global data on the production and consumption of organic sources. Appreciable amounts of nutrients can also be brought in with atmospheric deposition and with irrigation water.

Organic forms of nutrients are synthesized by or originate from organisms. These organic sources of nutrients include crop residues, farmyard manure, human wastes and biologically fixed nitrogen.

Plants cannot absorb the organic forms of nutrients, which must be mineralized before they can be used by crops and other plants. Fertilizers generally provide nutrients in their inorganic, or mineral, forms that are readily available for use by plants. However, the most common manufactured nitrogen fertilizer is urea, which is scientifically speaking an organic form of nitrogen.

Although inorganic sources of nutrients are often called “manufactured” or “chemical” fertilizer, all of these nutrient forms exist in nature and are a vital part of the natural cycles that provide plant nutrition.

Organic sources provide fewer nutrients than most people think, but they are usually excellent for improving other aspects of soil fertility. Inorganic sources contain only plant-available nutrients and therefore have no direct influence on soil structure or the presence of microorganisms. This is why agronomists generally consider it optimal to use both organic and inorganic sources together, a technique called Integrated Plant Nutrition Management (IPNM).

Good nutrient management begins with recycling of available organic sources of nutrients, such as farmyard manure. However, there are limits to the potential for recycling these sources. These include:

• There are not enough organic sources of nutrients to meet the global needs of today’s population.
• There is sometimes a mismatch between spatial concentrations of manures and the amount of locally available land on which to spread them.
• Recycling sewage sludge from urban to agricultural areas may raise issues about food safety and hygiene.
• Transport of both animal manures and sewage sludge over long distances has environmental impacts through the consumption of fossil fuels.
• The relatively low nutrient concentration of these sources limits the economic feasibility of transporting them.