Caliban07
Aquarium Advice Addict
Soil. It is at the bottom of the food chain but is the corner stone of life on earth. It is the mixture of minerals, organic matter, gases, liquids, and countless organisms that together support life on Earth. It is a medium for plant growth, it is a means of water storage, supply and purification, it is a modifier of Earth's atmosphere, it is a habitat for organisms, all of which, in turn, modify the soil. There are more micro-organisms in a handful of soil than there are people on earth. A single gram of soil may contain 2.5 billion bacteria, 400,000 fungi, 50,000 algae and 30,000 protozoa. The top sediment layer may contain 1000 times more bacteria then the overlying water. Among all of these micro-organisms are bacteria capable of nitrification and de-nitrification. Topsoil is the most active horizon of soil.
The four most common elements of the earths crust- oxygen, silicon, aluminium and iron form the mineral backbone of soil- sand, silt and clay.
Soil consists of 45% minerals, 25% water, 25% air & 5% organic matter and stores approximately 10% of the world’s carbon dioxide emissions. It provides all the nutrients required for successful plant growth. If soil can support the growth of terrestrial plants, then it can support the growth of aquatic plants.
10kg of a typical soil in a 50 gallon aquarium would provide plants with a 330,000 months supply of iron. It would provide boron for 7,700 months, calcium for 2,700, copper for 18,800, potassium for 875 magnesium for 2500, manganese for 125,000, molybdenum for 4000, nitrogen for 63, phosphorus for 290, sulphur for 440 and zinc for 5,600 months. It is an extremely concentrated source of micro nutrients.
What makes soil so significant is its exceptional ability to store plant nutrients whilst resisting pH change. In fact, long time submerged soils are very stable in terms of pH. Even when waters pH is lowered to 5.0 for 60 days the sediment can maintain its ambient alkaline pH. A pH of 6.6 is considered ideal for plants. Anything lower can cause toxicity to plants and anything higher nutrients become less available to plants. Due to both biological and chemical forces the pH gravitates towards neutral after 2-4 weeks of submergence. The same could be said for the soils redox potential.
Redox potential is the solutions ability to accept electrons. Because oxygen (the optimal electron acceptor) content in solution is high so is the redox potential (a voltage difference between a platinum electrode and a reference hydrogen electrode in solution expressed in millivolts mV) The deeper in to the soil bed the more oxygen has been used up by aerobic bacteria meaning there are reduced electron acceptors. Bacteria must turn to other electron acceptors like nitrate. This would occur +250mV. As we go deeper in to the soil (-200mV), efficient electron acceptors have become extremely depleted. Bacteria must resort to a process known as fermentation to accept electrons. This is difficult for plants as this is a very inefficient process for obtaining energy. A redox of around +70 to +120mV is considered optimal for plants. Provided the soil substrate is not too deep long time submerged soils are more stable in terms of redox which helps facilitate and support plant growth.
Cation exchange capacity should be thought of as the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution. Nearly all plant nutrients are taken up from the soil water solution in the form of ions, either cations or as anions. In an effort to gain nutrients, plants will release ions to the soil. Bicarbonate (HCO3-) and hydroxyl (OH-) anions released from plant roots enhance the absorption of nutrient anions; similarly, hydrogen cations are released in exchange for cations as nutrients. As a result, nutrient ions are pushed into the soil water solution from their sequestration on colloids (soil particle) to become available to plants. A high cation exchange capacity means that soils are more fertile as they have the ability to attract and hold cations ready to be taken up by the plants roots. Soil supports plants root growth by providing three major mechanisms for nutrient uptake. Mass flow of water, diffusion within water and root interception by their growth
There are an enormous amount of chemical processes and reactions taking place in the soil and at the plant root that are made much more significant and efficient by a higher CEC capacity. Hydrogen Ions associated with low soil pH can displace cations from the soil colloids and reduce CEC.
Different soils have differing compositions of clay, sand, silt and organic matter (humus) depending on there geological origins. Organic matter is the remains of algae, bacteria, plants, dead leaves and fish following decomposition. Although organic matter may represent only a small fraction of soils weight (2%), it may cover 90% of the surface area of soil particles. Organic matter eventually decomposes in to humic substances known as ‘humus‘, the penultimate state of decomposition of organic matter. It is composed of the very stable lignins (30%) and complex sugars (polyuronides, 30%), proteins (30%), waxes, and fats that are resistant to breakdown by microbes.
Humus’s multiple negative charges means that it can powerfully attract and bind cations such as copper, iron, potassium, calcium ammonium and magnesium. Humus makes up 60-80% of the organic matter in terrestrial soils. the CEC of humus is many times greater than that of clay. Small amounts of humus may remarkably increase the soil's capacity to promote plant growth however, most of the soil's CEC occurs on clay and humus colloids or soil particles. Clay has 10,000 times more surface area than sand which gives clay a much greater capacity to bind plant nutrients and support bacterial colonisation. It has a much higher CEC than sand.
Another vital component of this electrochemical phenomena is the aid in formation of soil aggregates via flocculation. Flocculation occurs when small particles in a solution lose their repelling forces and begin to attract one another. The small particles then bond together to form “flocs”. Under most circumstances, a flocculant is necessary to begin the flocculation process. The most common flocculants are iron, aluminum, magnesium, and calcium. When soil colloids form flocs it gives the soils its structural composition in the form of aggregates. These inconsistent structural aggregates form pores, channels, chambers and cracks which provides a favourable environment for microbial activities and facilitates the root growth of plants. In addition Animals, soil mesofauna and micro-organisms mix soils as they form burrows, allowing any gases to move about. In the same way, plant roots open channels in soils. This movement in soil helps prevent low redox conditions and hydrogen sulphide build up as is commonly seen in sand and gravel substrates. As gases are moved bubbles are released which enables oxygen to enter. It is this force that also helps to reduce turbidity in solution as the ‘flocs’ of soil colloids are quickly ‘pulled’ back towards the soil. Therefore, soils submerged for longer periods will resist soil entering solution and will see that the soil settles again quickly.
These conditions made possible by soil allow the colonised micro-organisms to carry out essential bacterial processes such as nitrification, denitrification, aerobic and anaerobic decomposition of organic matter in to humic substances, fermentation, hydrogen sulphide and methane oxidation. The formation of polysacharride substances in bio films also acts as an adhesive stopping soil particles from entering the water and causing turbidity.
All in all a submerged healthy topsoil consisting of decent mineral consistency (clay, sand, organic matter) will stabilise in terms of pH and redox. This facilitates the growth of plants which in turn purify the water. There are things to be aware of before setting up a soil substrate tank. The initial submergence of a terrestrial soil sets of a large number of chemical and biological reactions which can be detrimental to plants and fish. However, if the soil stays submerged these reactions slow and the soil begins to stabilise. It is important to choose a soil that does not have added fertilisers. Fertilised soils often contain large amounts of sulfates which are beneficial to terrestrial plants but in an environment with potentially anaerobic substrate conditions these sulfates may be converted to hydrogen sulphide which is extremely toxic to fish. After a couple of days of submergence, oxygen in the soil will be depleted. A displacement of cations begins to take place that can take weeks to stabilise. This is what causes algae in new set ups. This can be avoided by mineralising the soil beforehand. Mineralising involves wetting and drying the soil a number of times to encourage the growth of bacteria that further break down the soil resulting in less cation displacement and less algae. Try to go no more than 2.5 inches in depth and include a cap if necessary. Gravel is preferable as its porosity will prevent severely anaerobic conditions. Poking the substrate initially will release trapped air bubbles and stop anaerobic pockets from forming. Generally this is not a problem as plant roots can oxygenate their own root surroundings. If plants appear to be struggling and float to the top because of damaged roots and fish lose their appetite, suspect hydrogen sulphide poisoning. The rotten egg odour accompanied by H2S is also unmistakable. Once soils have stabilised though (16 weeks) expect increased efficiency, minimal algae and a healthy tank that can support the growth of many different plant species.
I wrote this article to hopefully encourage the use of soils as a substrate and to reinforce some of the points and work in Diana Walstads book ‘Ecology of the Planted Aquarium‘. I believe that using a soil substrate will provide your plants with all the benefits of nature which in turn can only benefit your fish. Soil substrates can be used with all planted tank setups and will bring many physiological advantages over other substrates. Using soil as a substrate will provide an extra step towards the ecological ideology of a aquatic ecosystem and who knows your plants just might thank you for it.
If you have any questions please feel free.
References:
Ecology of the Planted Aquarium third addition by Diana L. Walstad
https://www.quickcrop.ie/blog/2014/01/top-10-interesting-facts-about-soil
http://www.soilquality.org.au/factsheets/cation-exchange-capacity
https://en.wikipedia.org/wiki/Soil
http://www.tech-faq.com/flocculation.html
Soil oxidation-reduction in wetlands and its impact on plant functioning S.R. Pezeshki and R. D Delaune 2012
The chemistry of submerged soils F.N Ponnamperum 1972
Plant Adaptations Ci.coastal.edu
The four most common elements of the earths crust- oxygen, silicon, aluminium and iron form the mineral backbone of soil- sand, silt and clay.
Soil consists of 45% minerals, 25% water, 25% air & 5% organic matter and stores approximately 10% of the world’s carbon dioxide emissions. It provides all the nutrients required for successful plant growth. If soil can support the growth of terrestrial plants, then it can support the growth of aquatic plants.
10kg of a typical soil in a 50 gallon aquarium would provide plants with a 330,000 months supply of iron. It would provide boron for 7,700 months, calcium for 2,700, copper for 18,800, potassium for 875 magnesium for 2500, manganese for 125,000, molybdenum for 4000, nitrogen for 63, phosphorus for 290, sulphur for 440 and zinc for 5,600 months. It is an extremely concentrated source of micro nutrients.
What makes soil so significant is its exceptional ability to store plant nutrients whilst resisting pH change. In fact, long time submerged soils are very stable in terms of pH. Even when waters pH is lowered to 5.0 for 60 days the sediment can maintain its ambient alkaline pH. A pH of 6.6 is considered ideal for plants. Anything lower can cause toxicity to plants and anything higher nutrients become less available to plants. Due to both biological and chemical forces the pH gravitates towards neutral after 2-4 weeks of submergence. The same could be said for the soils redox potential.
Redox potential is the solutions ability to accept electrons. Because oxygen (the optimal electron acceptor) content in solution is high so is the redox potential (a voltage difference between a platinum electrode and a reference hydrogen electrode in solution expressed in millivolts mV) The deeper in to the soil bed the more oxygen has been used up by aerobic bacteria meaning there are reduced electron acceptors. Bacteria must turn to other electron acceptors like nitrate. This would occur +250mV. As we go deeper in to the soil (-200mV), efficient electron acceptors have become extremely depleted. Bacteria must resort to a process known as fermentation to accept electrons. This is difficult for plants as this is a very inefficient process for obtaining energy. A redox of around +70 to +120mV is considered optimal for plants. Provided the soil substrate is not too deep long time submerged soils are more stable in terms of redox which helps facilitate and support plant growth.
Cation exchange capacity should be thought of as the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution. Nearly all plant nutrients are taken up from the soil water solution in the form of ions, either cations or as anions. In an effort to gain nutrients, plants will release ions to the soil. Bicarbonate (HCO3-) and hydroxyl (OH-) anions released from plant roots enhance the absorption of nutrient anions; similarly, hydrogen cations are released in exchange for cations as nutrients. As a result, nutrient ions are pushed into the soil water solution from their sequestration on colloids (soil particle) to become available to plants. A high cation exchange capacity means that soils are more fertile as they have the ability to attract and hold cations ready to be taken up by the plants roots. Soil supports plants root growth by providing three major mechanisms for nutrient uptake. Mass flow of water, diffusion within water and root interception by their growth
There are an enormous amount of chemical processes and reactions taking place in the soil and at the plant root that are made much more significant and efficient by a higher CEC capacity. Hydrogen Ions associated with low soil pH can displace cations from the soil colloids and reduce CEC.
Different soils have differing compositions of clay, sand, silt and organic matter (humus) depending on there geological origins. Organic matter is the remains of algae, bacteria, plants, dead leaves and fish following decomposition. Although organic matter may represent only a small fraction of soils weight (2%), it may cover 90% of the surface area of soil particles. Organic matter eventually decomposes in to humic substances known as ‘humus‘, the penultimate state of decomposition of organic matter. It is composed of the very stable lignins (30%) and complex sugars (polyuronides, 30%), proteins (30%), waxes, and fats that are resistant to breakdown by microbes.
Humus’s multiple negative charges means that it can powerfully attract and bind cations such as copper, iron, potassium, calcium ammonium and magnesium. Humus makes up 60-80% of the organic matter in terrestrial soils. the CEC of humus is many times greater than that of clay. Small amounts of humus may remarkably increase the soil's capacity to promote plant growth however, most of the soil's CEC occurs on clay and humus colloids or soil particles. Clay has 10,000 times more surface area than sand which gives clay a much greater capacity to bind plant nutrients and support bacterial colonisation. It has a much higher CEC than sand.
Another vital component of this electrochemical phenomena is the aid in formation of soil aggregates via flocculation. Flocculation occurs when small particles in a solution lose their repelling forces and begin to attract one another. The small particles then bond together to form “flocs”. Under most circumstances, a flocculant is necessary to begin the flocculation process. The most common flocculants are iron, aluminum, magnesium, and calcium. When soil colloids form flocs it gives the soils its structural composition in the form of aggregates. These inconsistent structural aggregates form pores, channels, chambers and cracks which provides a favourable environment for microbial activities and facilitates the root growth of plants. In addition Animals, soil mesofauna and micro-organisms mix soils as they form burrows, allowing any gases to move about. In the same way, plant roots open channels in soils. This movement in soil helps prevent low redox conditions and hydrogen sulphide build up as is commonly seen in sand and gravel substrates. As gases are moved bubbles are released which enables oxygen to enter. It is this force that also helps to reduce turbidity in solution as the ‘flocs’ of soil colloids are quickly ‘pulled’ back towards the soil. Therefore, soils submerged for longer periods will resist soil entering solution and will see that the soil settles again quickly.
These conditions made possible by soil allow the colonised micro-organisms to carry out essential bacterial processes such as nitrification, denitrification, aerobic and anaerobic decomposition of organic matter in to humic substances, fermentation, hydrogen sulphide and methane oxidation. The formation of polysacharride substances in bio films also acts as an adhesive stopping soil particles from entering the water and causing turbidity.
All in all a submerged healthy topsoil consisting of decent mineral consistency (clay, sand, organic matter) will stabilise in terms of pH and redox. This facilitates the growth of plants which in turn purify the water. There are things to be aware of before setting up a soil substrate tank. The initial submergence of a terrestrial soil sets of a large number of chemical and biological reactions which can be detrimental to plants and fish. However, if the soil stays submerged these reactions slow and the soil begins to stabilise. It is important to choose a soil that does not have added fertilisers. Fertilised soils often contain large amounts of sulfates which are beneficial to terrestrial plants but in an environment with potentially anaerobic substrate conditions these sulfates may be converted to hydrogen sulphide which is extremely toxic to fish. After a couple of days of submergence, oxygen in the soil will be depleted. A displacement of cations begins to take place that can take weeks to stabilise. This is what causes algae in new set ups. This can be avoided by mineralising the soil beforehand. Mineralising involves wetting and drying the soil a number of times to encourage the growth of bacteria that further break down the soil resulting in less cation displacement and less algae. Try to go no more than 2.5 inches in depth and include a cap if necessary. Gravel is preferable as its porosity will prevent severely anaerobic conditions. Poking the substrate initially will release trapped air bubbles and stop anaerobic pockets from forming. Generally this is not a problem as plant roots can oxygenate their own root surroundings. If plants appear to be struggling and float to the top because of damaged roots and fish lose their appetite, suspect hydrogen sulphide poisoning. The rotten egg odour accompanied by H2S is also unmistakable. Once soils have stabilised though (16 weeks) expect increased efficiency, minimal algae and a healthy tank that can support the growth of many different plant species.
I wrote this article to hopefully encourage the use of soils as a substrate and to reinforce some of the points and work in Diana Walstads book ‘Ecology of the Planted Aquarium‘. I believe that using a soil substrate will provide your plants with all the benefits of nature which in turn can only benefit your fish. Soil substrates can be used with all planted tank setups and will bring many physiological advantages over other substrates. Using soil as a substrate will provide an extra step towards the ecological ideology of a aquatic ecosystem and who knows your plants just might thank you for it.
If you have any questions please feel free.
References:
Ecology of the Planted Aquarium third addition by Diana L. Walstad
https://www.quickcrop.ie/blog/2014/01/top-10-interesting-facts-about-soil
http://www.soilquality.org.au/factsheets/cation-exchange-capacity
https://en.wikipedia.org/wiki/Soil
http://www.tech-faq.com/flocculation.html
Soil oxidation-reduction in wetlands and its impact on plant functioning S.R. Pezeshki and R. D Delaune 2012
The chemistry of submerged soils F.N Ponnamperum 1972
Plant Adaptations Ci.coastal.edu
