Agrogeology is broadly defined as a study of geological processes that influence the distribution and formation of soils, and the application of geological materials in farming and forestry systems as means of maintaining and enhancing soil productivity for increased social, economic and environmental benefits. It is mostly coined ‘geology in the service of agriculture’ and agrogeologists are concerned with issues such as the replenishment of soil fertility in areas where agrominerals have been depleted by unsustainable farming methods It combines the knowledge of pedologists (soil scientists) and farmers with that of geologists and process engineers.
- Soil scientists define the soil limitations and needs
- geologists find, delineate and characterize the geological raw materials that address those needs
- process engineers contribute by concentrating the agro minerals and transforming them into more plant available forms.
1. Minerals from rocks as agricultural nutrients.
Most rocks which form the host for minerals are formed or deposited through different processes. These minerals may be formed through geological processes of crystallization, recrystallization, deposition/weathering etc depending on the environments in which they occur. However, some minerals are a source of elements that are very useful and needed in agriculture and if collectively used in agriculture are called agrominerals.
Soil minerals serve as both sources and sinks of essential plant nutrients. As primary minerals that originally formed at high temperatures and pressures in igneous and metamorphic rocks are weathered in soils, they release plant nutrients into the soil solution.
With the exception of nitrogen, all plant nutrient resources for farming systems are of geological origin. In natural systems, nitrogen is ‘harvested’ from the air by legumes or through Nitrogen-fixing organisms and recycled into the soil. There is also a large amount of soil nitrogen that derives from the weathering of nitrogen-bearing parent materials such as mica-schists and nitrogen-rich siliciclastic rocks Other nutrients critical for plant growth, like potassium, calcium, magnesium, phosphorus, sulphur and some micronutrients are supplied by geological resources referred to as rocks. Weathering of these rocks, as well as organic inputs, atmospheric deposition, and re-sedimentation of soil materials through erosion, supplies most of the nutrients essential for plant growth. It has been widely recognized that the removal of nutrients from soils through repeated harvesting, leaching, soil runoff and erosion is too high to retain enough soil nutrients in the soil for sustained crop production. The result is that soils are now ‘mined.’
2. Essential plant nutrients
(a) Macro nutrients
- Primary and secondary (Nitrogen, Phosphorus Potassium & Calcium, Magnesium, Sulphur)
(b) Micronutrients
- iron, copper, Zinc, Molybdenum cobalt, nickel, chlorine, boron, manganese
(c) Structural Plant nutrients
- from water and the atmosphere ( carbon,hydrogen & oxygen)
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The most important minerals that may be useful include, phosphorus from phosphate rocks/apatite and rock phosphate is a type of phosphorus found in sedimentary rocks. It is often mined and used as an agricultural fertilizer sodium from feldspars and calcium and potassium from nepheline synites/limestones, nitrates/nitrogen and phosphorus from guanos, local agricultural lime from travertine and carbonatites, sulphur from sulphide rocks (pyrites), sulphates from gypsum and fossil fuels (coal), copper and zinc from their ores, magnesium from dolomites, iron from their iron ore, etc. Rocks containing potash silicate minerals such as feldspar and nepheline occur widely as potential sources of potassium for use in soils where these minerals weather rapidly. In other instances, perlitic rocks enhance aeration of the soil, pumice control evaporation while vermiculites and zeolites store moisture. Amphiboles and pyroxenes are vital reservoirs of magnesium, iron, calcium, silicon and most of the micronutrients.
3. Minerals of concern in agrogeology
Some minerals found in soils are toxic to humans, marine and soil ecosystem. Unfortunately most of them occur in natural environments and their background concentrations are naturally above recommended concentrations. In that regard, most regulators have maximum permissible limits in fertilisers (using absolute values)
The following are the minerals of concern.
(a) Cadmium
- The concern is that toxicity in plants may not be visible yet very toxic to mammals. Symptoms of cadmium poisoning in plants include stunting, leaf roll and chlorosis.
(b) Arsenic
- Non-essential metalloid to plants and animals
- It is classified as carcinogenic and inhibits plant growth resulting in decreased crop yield
(c) Mercury
- Toxic to fish, soil and terrestrial ecosystems
- It can induce seed injury and limits crop yields
(d) Chromium
- Other than occupational exposure people get exposed to chromium through food chain in vegetables and Fruits
(e) Nickel, Cobalt, Copper, Iron, Selenium, Molybdenum
When these enter the food chain through crop production they can become toxic to humans and animals.
- Selenium can be very toxic to other plants and humans although it is proven to be very effective in certain types of grasses as a nutrient.
- Poorly managed Nickel can result in contaminated soils and may not be suitable to other crops though it may help in some deficient pea nuts.
- Copper, Iron and Molybdenum are essential micronutrients but can end up in the food chain due to excessive usage as a fertilizer or pesticides.
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