Sources of Nutrients:

Plants require water, carbon dioxide and the range of trace minerals called as nutrients to grow. They get these nutrients from soil. Nutrients available in the given soil finally depend on the rock from which soil was made. If plants grown from this soil die and decay where they have grown then their nutrients are recycled. Though, if plants have been grown for agriculture then they are removed from area in which they have grown and their nutrients can't be recycled. So in soils which are used for cropping, necessary nutrients constantly have to be replaced. Aside from decomposition of organic substances, atmosphere and soil or sediments are also sources of nutrients in soils.

i) Atmosphere:

Atmosphere acts as a reservoir for several elements. These elements when released may increase availability of minerals for uptake by autotrophs. Inorganic substances that comprise carbon, nitrogen and water among others are part of air and are involved in mineral cycles of the ecosystem.

ii) Soil or Sediments:

Apart from peat that is derived from plant material soil is made up of silicate minerals of different kinds, composed largely of oxygen and silicon, with lesser amounts of iron, aluminum, sodium, potassium, calcium and magnesium, and variable and much smaller amounts of all other elements. As rainwater saturated with carbon dioxide percolates through silicate minerals it dissolves out simple soluble cations, like Na⁺, K⁺ and Ca²⁺, and anions such as Cl^-, in the procedure of weathering. Residue comprises largely of oxides of silicon, aluminum and iron. Without ability of soils to bind both cations and anions necessary plant nutrients would be rapidly washed out of soil, and plant life would possibly not have developed in forms we know.

iii) Decomposition of Organic Substances:

Soil organic matter is fraction of the soil which consists of plant or animal tissue in different stages of breakdown (decomposition). Quickest increases are got with sources which are high in carbon like compost or semi-solid manure.

Sources of organic materials comprise:

• crop residues
• animal manure
• compost
• cover crops (green manure)
• perennial grasses and legumes

With careful management preservation and accumulation of soil organic matter can assist to enhance soil productivity resulting in greater farm profitability.

Nutrient Limitations:

Several factors affect growth of crops in any agricultural system, mainly climatic conditions and soil characteristics. Climatic environmental factors comprise moisture, temperature, and light energy. Soil characteristics comprise soil structure and composition, soil biology, pH, nutrient availability and any procedures affecting availability. All of these factors are interrelated.

Temperature:

Most agricultural plants grow between 150 and 400, each crop requiring the specific range for different growth processes. Soil temperature affects soil air composition and thus soil moisture, which in turn modify soil biological processes affecting nutrient availability. Nutrient absorption and uptake are also affected. Soil pH may experience changes with temperature that is thought to be relative to microorganism activity.

Moisture:

Movement of nutrients to roots and nutrient uptake are restricted by inadequate water, as most soil nutrients are water-soluble. Too much water can result in nutrient loss by leaching. Micro-organism activity is subject to soil moisture levels, very high or very low results in decreased nutrient transformations which process nutrients in plant-available forms.

Soil Structure and Composition:

Soil texture (determined by size of mineral and organic matter particles) and soil structure (how soil particles are aggregated) influence nutrient retention capacity by altering porosity, compaction, and the cation exchange capacity (CEC: capacity of the soil for ion exchange of positively charged ions between soil and soil solution; the majority of plant nutrients are cations).

pH:

Plant nutrients are most available between pH of 6.2 to 6.8. In acidic soils below pH 5.5, and in alkaline soils, most (cationic) nutrients change form and are unavailable to plants.

Biological Activity:

Biological activity liable for nutrient transformations, making them more available for plant uptake, is most active in soil containing organic matter; soil which is warm and moist for some part of year, pH between 6.2 and 6.8 is perfect for several beneficial organisms.

Other Nutrients:

Some nutrients become less bioavailable when another nutrient is in excess.

Cycling of minerals and nutrient pool:

The movement of nutrient elements through the different components of the ecosystem is known as nutrient cycling. Nutrients are never lost from ecosystems; they are recycled time and again indefinitely. Amount of nutrients, like carbon, nitrogen, phosphorus, calcium, etc., present in soil at any given time, is referred to as standing state. It differs in different types of ecosystems and also on the seasonal basis. Another name of nutrient cycling is biogeochemical cycles (bio: living organism, geo: rocks, air, and water).

Characteristics of Biogeochemical Cycles:

In biogeochemical cycles, nutrient elements generally enter the living system through vegetation. This is due to animals are not capable to free and absorb nutrient elements from soil or atmosphere as plants can. Hence, the ecosystem depends on plants not only to supply the essential nutrients to maintain the flow of energy but also to fix solar energy. Solar energy keeps the earth warm adequate so that chemical reactions are possible and living organisms can perform their essential life processes. Any kind of biogeochemical cycle should have the given characteristics:

• Should be able to circulate nutrient element from environment to organisms and back to the environment
• There should be involvement of plants, animals and microbes
• Presence of nutrient reservoir
• Occurrence of chemical changes.

Kinds of Biogeochemical Cycles:

There are two basic kinds of biochemical cycles. These are:

• sedimentary
• gaseous cycles

Sedimentary cycles have soil and sediments as reservoir and safety valve of the system whereas the gaseous cycles, it is the air which serves as reservoir and safety valve. Some typical examples of sedimentary cycle are: phosphorus, sulphur, and gaseous cycle are nitrogen and carbon.

Phosphorus Cycle:

All organisms require abiotic phosphorus to make biotic phosphates for DNA and other cell molecules. Short time cycle of phosphorus occurs when organisms die, are decomposed and phosphorus is reabsorbed in the organism. Most phosphorus is stored in abiotic rocks and erosion can make this phosphorus available for organisms to absorb in phosphorus long cycle time. Though very little phosphorus is required by plants, it is very significant and required for all plant growth.

The Phosphorus which plants use is contained generally in the earth's crust rather than in atmosphere, this phosphorus is released with weathering of rock caused by wind, water and geologic uplift. Recycling of phosphorus tends to be much localized, with plants up taking phosphorus from the soil. Animal's uptake the phosphorus in plants they eat, and return it to environment in their waste. Phosphorus excreted by animals is in the organic form that is converted to the inorganic form by microorganisms.

Sulphur Cycle:

Sulphur is the component of proteins, enzymes and other compounds. It is hardly a limiting nutrient and is generally absorbed as sulfate. Sulphur is significant for functioning of proteins and enzymes in plants, and in animals which depend on plants for sulphur. Plants absorb sulphur when it is dissolved in water. Animals consume the plants, so that they take up adequate sulphur to maintain the health. Most of the earth's sulphur is tied up in rocks and salts or buried deep in ocean in oceanic sediments. Sulphur can also be found in atmosphere. It goes in the atmosphere via both natural and human sources. Natural resources can be bacterial processes, volcanic eruptions, evaporation from water, or decaying organisms. When sulphur enters the atmosphere via human activity, this is essentially a consequence of industrial procedures where sulphur dioxide (SO₂) and hydrogen sulphide (H₂S) gases are emitted on the wide scale. When sulphur dioxide enters the atmosphere it will react with oxygen to produce sulphur trioxide gas (SO₃), or with other chemicals in atmosphere, to make sulphur salts. Sulphur dioxide may also react with water to generate sulphuric acid (H₂SO₄).

Carbon Cycle:

In photosynthesis, organisms convert carbon dioxide in atmosphere in carbon containing compounds like sugars and cellulose. These compounds are then utilized by primary consumers like cattle, which by the procedure of respiration; convert the carbon containing compounds in carbon dioxide and water. These procedures viewed on the global scale, is known as Carbon Cycle. In carbon cycle the main photosynthesizers are the plants, marine algae, phytoplankton, and cyanobacteria. These organisms use carbon dioxide and water to generate carbohydrates and oxygen which photosynthesizers use themselves. Plants do release carbon dioxide from their leaves and roots, and phytoplankton and marine algae and cyanobacteria, release carbon dioxide into water where it maintains in equilibrium with the carbon dioxide of air. Not only is carbon dioxide released by plants, it is also released by animals that eat the plants and animals that eat those animals during process of respiration. Carbon dioxide is also released by combustion of organic carbon sources like wood, coal and oil. The enormous amount of organic carbon resides in bodies of dead plants and animals, along with wastes of living animals. Decomposers, like fungi and other small invertebrates, consume this carbon; these decomposers also released carbon dioxide.

Nitrogen Cycle:

Cycling of earth's limited amount of nitrogen is called as nitrogen cycle and has three principal phases; these are ammonification, nitrification and assimilation.

Ammonification also called as nitrogen mineralization takes place when soil dwelling saprophytic bacteria decompose dead organic matter, that are made up of complex nitrogen containing compounds like proteins and amino acids, and nucleic acids these bacteria utilize nitrogen they get to create their own amino acids and proteins and release excess nitrogen as ammonium that can then be utilized by plants. Nitrification takes place when bacteria oxidize ammonia or ammonium ions, this chemical reaction produces energy that bacteria utilize to convert carbon dioxide in nitrite, hydrogen, and water. As nitrite produced by bacteria is toxic to plants it should be converted to nitrate by another species of bacteria, once converted to nitrate, nitrogen is available for absorption by plants. Incorporation of inorganic nitrogen in the form of ammonium or nitrate in organic compounds like proteins, amino acids and nucleic acids is one of the most significant procedures on earth and is almost equal to photosynthesis and respiration. Greatest source of nitrogen to crop plants is in form of nitrate that is broken-down and reduced to ammonia that can be rapidly included in organic compounds like amino acids. Though most plants receive their nitrogen in the inorganic form like nitrate some plants in artic, where nitrogen availability is restricted, are able to utilize organic nitrogen from dead organisms, these specialized plants are able to do so without going through the procedure of ammonification. Though natural nitrogen fixation is one of the most efficient ways in which nitrogen can be made available to plants it is extremely slow and needs the complete growing season to achieve.

Nutrient Pool or Reservoir:

The primary pools or reservoirs of nutrients in the soil are:

• soil solution
• exchangeable cations and anions
• sorbed cations and anions
• organic matter
• primary and secondary minerals.

With exception of carbon, hydrogen and oxygen, necessary plant nutrients originate mainly from soil pools. Significance of each of these pools differs significantly between elements. For instance, primary pool of nitrogen is organic matter whereas plant-available calcium originates predominately from exchangeable cation pool. In addition, the plant availability of nutrients differs greatly between the different nutrient pools. Nutrients contained in soil solution are readily available for plant uptake while nutrients contained in organic matter and primary minerals should first undergo mineralization and chemical weathering, respectively.

Due to great spatial variability of soil properties on the given landscape, the large number of replicate samples is needed to approximate nutrient pools on the landscape scale. Therefore, to approximate soil nutrient pools, it practically needs the tremendous effort because of large number of replicate samples and analytical measurements needed to generate reasonably rigorous estimates.

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