How is bat guano formed?

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As can be seen, bat guano is made up of a wide range of mineral and organic substances, which give the product unique properties that are not enjoyed by any other naturally-occurring fertilizer.

The formation of bat guano is a complicated process, affected by diverse components, factors and processes. Some say that bat guano is a non-renewable resource, because it takes a long time to form. However, others think that guano can be produced in a short time, without affecting either bats or cave. We believe that both statements have strong foundations and arguments.

All agree that the formation of bat guano depends basically on organic compounds, mineral compounds and humidity. Over the course of time, these compounds produce different types of guano, which we will describe later.

The easiest place to collect guano deposits is from the cave floor, one of the places where the process of guano formation usually begins.

Bats sleep during the night, hanging in clusters from the cave ceiling, dropping excrement, the remains of insects they swallow, mineral substances they scratch from the cave ceiling and their carcasses; this is joined by the water that runs or drips in many caves, carrying a large number of solid and/or dissolved mineral substances.

This organo-mineral mass is subjected to multiple biochemical reactions and biological processes, and is rapidly attacked by the millions of micro-organisms (bacteria, fungi and actinomycetes) and thousands of macro-organisms (insects, myriapods, arthropods, etc.).The conditions are ideal for these organisms which decompose the organic matter, transforming it into different organic acids, chelates, enzymes, antibiotics and other organic compounds, making the product predominantly organic at this first stage. During the process of decomposition, the temperature rises and the process accelerates, as long as humidity levels are suitable.

As this is a continuous process, while the bats remain there, new, fine layers of residue are formed on a daily basis, covering the earlier layers. The agents of decomposition multiply (living organisms, biochemical and biological processes), constantly processing fresh materials in the upper layers of the guano deposit. After a short time, a layer as thick as 1m may be produced, a dark-coloured guano with high levels of organic matter and nitrogen, which is called murcielaguina or fresh guano.

Over a period of time, the process continues and new layers of guano are incorporated; then, the main biological activity of the decomposers moves to the upper layer of the guano deposit, the ammoniac forms of organic compounds are retained, while the soluble forms (nitrates and nitrites) are dissolved in water and filter through to the lower layers or are eliminated from the guano deposit. The original layer is at the bottom and subject to various processes, such as the compounds being dissolved by filtering water, compaction and chemical processes of mineral origin. At this level, the activity of biological and biochemical agents tends to diminish.

This leads to the formation of a second layer of guano, as thick as 3 to 8 m, with less organic matter and more mineral content. This product is generally referred to as bat guano or intermediate guano, because it has a more balanced composition than other guanos. At this stage of formation, biochemical and chemical reactions and physical processes predominate and result in more mineral than organic compounds (sulphates, phosphates, carbonates, etc.), creating a product that is lighter-coloured and more uniform in texture and structure than murcielaguina.

As the process of accumulation, decomposition and formation of guano continues, the third and last layer forms at the bottom, with a thickness of 9 to 20 m. This guano is called fossil or old guano, because it has undergone serious mineralization over time, due to greater activity by physical agents (erosion, percolation, filtration, compaction, etc.) and chemical agents (oxidation, reduction, hydrolysis, the creation of sulphates, phosphates and carbonates, etc.), high concentrations of minerals, greater compaction and lower levels of organic matter and nitrogen than in the upper layers, due to the leaching of organic substances. At this last stage of guano formation, organic activity is minimal, many mineral compounds are leached and only the most stable remain in the fossil guano, such as insoluble phosphates and sulphates, which continue to be deposited at this layer. These are mostly calcium, iron and aluminium phosphates and calcium sulphate.

At this last stage, decomposition is highly advanced, and proportions of the various components of guano vary; organic matter, nitrogen and potassium levels fall, while calcium phosphates, sand and clay levels rise in proportion. This product eventually shows colours ranging from cream to red (red from ferrous compounds), greater compaction, finer particles and a brittle or loose structure (similar to well-dug soil). It is clear that the presence of different organic acids in the first phase of the process and the leaching of alkaline substances during the whole process, mean that bat guano is acidic at all levels, especially the deepest.