14.5 Alternative sources of fertilisers

Organic fertilisers (ESCSK)

Organic fertilisers are not manufactured by man, but come from natural sources. Examples are manure, blood and bones, guano, compost and kelp products. Organic fertilisers contain lower levels of nutrients and might take longer than inorganic fertilisers to be absorbed as they generally have to degrade first. The advantage of organic fertilisers is that they increase the organic component of the soil. This improves the physical structure of the soil, which in turn increases the soil’s water-holding capacity. The nutrients also tend to be released slower than those of inorganic fertilisers, decreasing their contribution to water pollution.

Manure (ESCSM)

Manure is a solid waste product from animals that is widely used as an organic fertiliser in agriculture. It contains high levels of nitrogen, phosphorus, potassium and other nutrients. Manure decomposes over time through bacterial and microbial action and in the process releases these nutrients into the soil. This slow release mechanism is a great benefit to farmers as it limits the leaching of nutrients into the ground water, making it available to plants over a longer period of time. Manure also adds organic matter to the soil, increasing the quality of the soil itself.

Plants can only absorb nutrients that are dissolved in water. When manure decomposes, the nitrogen compounds are converted to a form that is soluble in water, for example nitrates (\(\text{NO}_{3}^{-}\)). The nitrates are dissolved in the moisture in the soil and plants are then able to absorb the nitrogen compounds through their root system.

Guano (ESCSN)

Guano is the excretion of seabirds, bats and seals. Guano consists of ammonia, uric acid, phosphoric acid, oxalic acid and carbonic acid and also has a high concentration of nitrates. The particularly high levels of phosphorus make this an effective phosphorus fertiliser. Guano was mined off the West Coast of South Africa as early as 1666. Since the 1840’s, large scale mining of guano caused the reserves to be depleted and mining was stopped by the turn of the century.

Crop rotation (ESCSP)

Crop rotation is a farming method that is used to manage the nutrients in soil naturally. When the same crop is grown repeatedly in the same place, it eventually depletes the soil of specific nutrients. With crop rotation, one type of crop that depletes the soil of a particular kind of nutrient, is rotated with another type of crop which replaces the depleted nutrient. For example, legumes, like beans or peas, have nodules on their roots which contain nitrogen-fixing bacteria. These bacteria help ‘fix’ or change nitrogen into a soluble form. Legumes are therefore often alternated with plants requiring nitrogen, and soy beans can therefore be followed by maize.

Lime (ESCSQ)

Agricultural lime, or crushed limestone, can be used as an alternative fertiliser. Lime increases the pH of soil, making the soil less acidic, and more soluble for nitrogen, potassium and phosphorus compounds. These nutrients will therefore be more readily available for absorption by plants.

Potash (ESCSR)

Potash is the common name for various mined and manufactured salts that contain potassium in the water-soluble form. The name is derived from pot ash, which was the main method of manufacturing potassium salts before the industrial era. Ashes from a fire were soaked in a pot of water after which the water was filtered out. The water, containing the leached potassium salts, was then evaporated to obtain a white powder known as potash.

Potash is a rich source of potassium and could contain potassium carbonate (\(\text{K}_{2}\text{CO}_{3}\)), potassium chloride (\(\text{KCl}\)), potassium sulfate (\(\text{K}_{2}\text{SO}_{4}\)) or potassium nitrate (\(\text{KNO}_{3}\)). Up to the 19th century potash was manufactured in asheries using wood ashes, but these declined in the late 19th century when large-scale production of potash from mineral salts was established in Germany.