Fertilizers can be placed into the categories of organic and inorganic fertilizers (composed of simple chemicals and minerals). Organic fertilizers are 'naturally' occurring compounds manufactured through a natural processes. Inorganic fertilizers are manufactured through chemical processes using naturally occurring deposits, while chemically altering them e.g. concentrated triple superphosphate.
Organic fertilizers contain essential nutrients to improve the health and productivity of soil and encourage plant growth. Organic nutrients increase the abundance of soil organisms by providing organic matter and micronutrients for organisms such as fungal mycorrhiza, which aid plants in absorbing nutrients. Chemical fertilizers may have long-term adverse impact on the organisms living in soil and a detrimental long term effect on soil productivity of the soil.
Definitions of 'organic'
There can be confusion as to the veracity of the term 'organic' when applied to agricultural systems and fertilizer. The problem is related to the marketing and colloquial uses of the term. Minerals such as mined rock phosphate, sulfate of potash and limestone are considered organic fertilizers, although they contain no organic (carbon) molecules. This is but one of many ambiguities in the usage of the term organic.
Synthetic fertilizers, such as urea and urea formaldehyde, are considered organic in the sense of the organic chemistry, and can be supplied organically (agriculturally), but when manufactured as a pure chemical is not organic under organic certification standards.
Naturally mined powdered limestone, mined rock phosphate and sodium nitrate, are inorganic (in a chemical sense) in that they contain no carbon molecules, and are difficult to harvest, but are approved for organic agriculture in minimal amounts.
The common thread that can be seen through these examples is that organic agriculture attempts to define itself through minimal processing (e.g. via chemical energy), as well as being naturally-occurring (as is, or via natural biological processing) This is a contradictory stance however, because high-concentrate plant nutrients (in the form of salts) obtained from dry lake beds by farmers for centuries in a very minimal fashion are excluded from consideration by most organic enthusiasts and many governmental definitions of organic agriculture. One of the main tenants of organic lifestyle marketing is that organic fertilizers are completely different than chemical fertilizers. No such dichotomy exists. There is substantial overlap between the two.
Health and sustainability issues
Many inorganic fertilizers do not replace trace mineral elements in the soil which become gradually depleted by crops. This depletion has been linked to studies which have shown a marked fall (up to 75%) in the quantities of such minerals present in fruit and vegetables. Deficiencies in zinc, copper, manganese, iron and molybdenum can limit the growth of broad acre crops and pastures.
There are concerns regarding arsenic, cadmium and uranium accumulating in fields treated with synthetic fertilizers. The phosphate minerals contain trace amounts of these elements and if no cleaning step is applied after mining, the continuous use of phosphate fertilizers leads towards an accumulation of these elements in the soil. High levels of lead and cadium can also be found in many manures or sewage sludge.
Phosphate fertilizers replace inorganic arsenic naturally found in the soil, displacing the heavy metal and causing accumulation in runoff. Eventually these heavy metals can build up to unacceptable levels.
Another problem with inorganic fertilizers is that they are now produced in ways which cannot be continued indefinitely. Potassium and phosphorus come from mines and such resources are limited. Nitrogen sources are effectively unlimited (forming over 70% of atmospheric gases), however, nitrogen fertilizers are presently made using fossil fuels such as natural gas and coal, which are limited.
For Feed & Pasture
Nitrogen fertilizer is often synthesized using the Haber-Bosch process, which produces ammonia. This ammonia is then used to produce other compounds (notably anhydrous ammonium nitrate and urea) which can be applied to fields. These concentrated products may be used as fertilizer or diluted with water to form a concentrated liquid fertilizer, UAN. Ammonia can also be used in combination with rock phosphate and potassium fertilizer to produce compound fertilizers.
The production of ammonia currently consumes about 5% of global natural gas consumption, which is somewhat under 2% of world energy production.
Natural gas is overwhelmingly used for the production of ammonia, but other energy sources, together with a hydrogen source, can be used for the production of nitrogen compounds suitable for fertilizers. The cost of natural gas makes up about 90% of the cost of producing ammonia. The price increases in natural gas in the past decade, along with other factors such as increasing demand, have contributed to an increase in fertilizer price.
Nitrogen-based fertilizers are most commonly used to treat fields used for growing corn, barley, wheat, canola, soybean, etc. One study has shown that the application of nitrogen fertilizer on off season cover crops can increase the biomass of these crops, while having a beneficial effect on soil nitrogen levels for the cash crop planted during the summer season.
Benefits of organic fertilizer
However, by their nature, organic fertilizers provide increased physical and biological storage mechanisms to soils, mitigating risks of over-fertilization. Organic fertilizer nutrient content, solubility, and nutrient release rates are typically much lower than mineral (inorganic) fertilizers. One study found that over a 140-day period, after 7 leachings:
- Organic fertilizers had released between 25% and 60% of their nitrogen content
- Controlled release fertilizers(CRFs) had a relatively constant rate of release
- Soluble fertilizer released most of its nitrogen content at the first leaching
Disadvantages of organic fertilizer
It is difficult to chemically distinguish between urea of biological origin and those produced synthetically. Like inorganic fertilizers, it is possible to over apply organic fertilizers and, as a result, a water soluble soil test should be conducted to determine optimum application rates. Release of the nutrients may happen quite suddenly depending on the type of organic fertilizer used. Organic fertilizers from treated sewage, composts and sources can be quiet variable from one batch to the next. Unless each batch is tested the amounts of nutrient applied are not precisely known.
Environmental risks of fertilizer use
High application rates of nitrogen fertilizers in order to maximize crop yields, combined with the high solubilities of these fertilizers can lead to increased leaching of nitrates into groundwater. The use of ammonium nitrate in inorganic fertilizers is particularly damaging, as plants absorb ammonium ions preferentially over nitrate ions, while excess nitrate ions which are not absorbed dissolve (by rain or irrigation) into groundwater. Nitrate levels above 10 mg/L (10 ppm) in groundwater can cause 'blue baby syndrome' (acquired methemoglobinemia), leading to hypoxia (which can lead to coma and death if not treated).
Nitrogen containing inorganic fertilizers in the form of nitrate and ammonium also cause soil acidification.
Eventually, nitrate-enriched groundwater makes its way into lakes, bays and oceans where it accelerates the growth of algae, disrupts the normal functioning of water ecosystems, and kills fish in a process called eutrophication (which may cause water to become cloudy and/or discolored—green, yellow, brown, or red). About half of all the lakes in the United States are now eutrophic, while the number of oceanic dead zones near inhabited coastlines are increasing.
As of 2006, the application of nitrogen fertilizer is being increasingly controlled in Britain and the United States. If eutrophication can be reversed, it may take decades before the accumulated nitrates in groundwater can be broken down by natural processes.
Storage and application of some nitrogen fertilizers in some weather or soil conditions can cause emissions of the greenhouse gas nitrous oxide (N2O). Ammonia gas (NH3) may be emitted following application of manure/slurry. Besides supplying nitrogen, ammonia can also increase soil acidity (lower pH, or "souring"). Excessive nitrogen fertilizer applications can also lead to pest problems by increasing the birth rate, longevity and overall fitness of certain pests.
For these reasons, it is recommended that knowledge of the nutrient content of the soil and nutrient requirements of the crop are carefully balanced with application of nutrients in inorganic fertilizer. This process is called nutrient budgeting. By careful monitoring of soil conditions, farmers can avoid wasting expensive fertilizers, and also avoid the potential costs of cleaning up any pollution created as a byproduct of their farming.
Hazard of over fertilization
Over-fertilization of a vital nutrient can be as detrimental as underfertilization. "Fertilizer burn" can occur when too much fertilizer is applied, resulting in a drying out of the roots and damage or even death of the plant.
All organic fertilizers, and some specially-formulated inorganic fertilizers are classified as 'slow-release' fertilizers, and therefore cannot cause nitrogen burn. If excess nitrogen is present, some plants can exude the excess through their leaves in a process called guttation.
Environmental toxicity of fertilizer
Toxic fertilizers are recycled industrial waste that introduce several classes of toxic materials into farm land, garden soils, and water streams. The consumption levels of toxic fertilizer are increasing and this is leading to major environmental problems. The most common toxic elements in this type of fertilizer are mercury, lead, and arsenic.
Between 1990-1995, 600 companies from 44 different states sent 270 million pounds of toxic waste to farms and fertilizer companies across the country.
According to the United States
Food and Drug Administration,
a) "Current information indicates that only a relatively small percentage of fertilizers is manufactured using industrial wastes as ingredients, and that hazardous wastes are used as ingredients in only a small portion of waste-derived fertilizers."
b) "The EPA has continually encouraged the beneficial reuse and recycling of industrial wastes."
Steel industry wastes, recycled into fertilizers for their high levels of zinc can include the following toxic metals: