By,Pillai Aswathy viswanathPG 1 BotanySt. Thomas college kozhencherry

Soil characters influencing nutrient availability

Plants are unable to grow in pure water, the reason is that in addition to water plants also require a number of mineral elements.These are take up as inorganic ions from the soil solution.

The soil support the plant and it is the main reservoir of water and nutrients for plant.

The soil is a complex physical, chemical and biological substrate which is made up of mineral matter , air, water and organic materials

It is a heterogenous material containing solid, liquid and gaseous phase

All of these phase interact with mineral elements

The solid particles may be inorganic, organic or a mixture of both.

Inorganic particles, often called mineral particles

Organic particles was formed by the decay of organism

The inorganic particles of solid phases provided a reservoir of pottasium, calcium, magnesium, and iron

Also solid phase are organic compounds containing nitrogen, phosphorus and sulphur

The liquid phase of the soil constitutes the soil solution, which contains dissolved mineral ions

Plants obtain most of their oxygen and carbon from the air by photosynthesis and hydrogen is obtained directly or indirectly from the water in the soil

These three element together make up the 90%of fresh plant tissue

How ever plants cannot survive with out the much smaller quantity of essential nutrients

So the nutrients like nitrogen, phosphorous, pottassium,calcium magnesium and sulphur obtained from the soil

So the characters of the soil play a big part in the plants ability to extract water and nutrients

Soil characters like size , charge of soil particles and soil pH will influence the nutrient availability for plants

Soil particles are classed on their size : Anything over 2mm in diameter is

referred to as gravel, pebbles or stones Particles from 2mm to 0.5mm are called

sand particles 0.5mm to 0.002mm are called silt

particles Any particle under 0.002 mm is referred

to as clay

Sand and silt are similar in composition and are formed by physical and chemical break down of rocks

The texture of a soil depends on the reletive mixture of sand ,silt and clay particles

Soil texture has an important role in nutrient management because it influences nutrient retention.

For instance, finer textured soils tend to have greater ability to store soil nutrients

More nutrients can be adsorbed by gram of clay particles than by a gram of sand or silt particles

Because the clay particles provide a much greater surface area for adsorption

Also soil texture influence in water holding capacity and water infiltrating rates

In the soil ,the larger portion of plants nutrients is bound up in complex compounds that are unavailable to plants

The complex compounds are gradually changed in to the simpler compounds by chemical weathering

Thus the fertility of a soil depends in parts on how easily the complex compounds be change to simpler forms this is referred to as the availability of a nutrients

Plant nutrients are composed of single elements eg;phosphorus

compound of elements eg; ammonium nitrate

Most of the soil nutrients that a plant take up must be in a soluble form (mixed with water)

When an atom is in water it is usually becomes electrically charged and is called an ion

An ion with positive charge is called a cation

An ion with a negative charge is called an anion

Cation include sodium ,potassium, calcium magnesium and aluminium

Anions include chloride ,nitrate, sulphate, carbonate and borate

Both organic and inorganic soil particles have predominantly negative charge on their surfaces

As a result mineral cations become adsorbed on the negative surface of soil particles

And also found dissolved in the soil solution and it will later absorbed by the roots

This cation absorption is an important factor in the soil fertility

Mineral cation adsorbed on the surface of soil particles are not easily lost when the soil is leached by water and they provide a nutrients reserve available to plants roots

Mineral nutrients adsorbed in this way can be replaced by other cations in a process known as cation exchange

The degree to which a soil can adsorb and exchange ions is termed its cation exchange capacity and is highly dependent on the soil type

The soil with higher cation exchange capacity generally has a larger reserve of mineral nutrients

Mineral anions such as nitrate and chloride tend to be repelled by the negative charge on the surface of soil particles and remain dissolved in the soil solution

Thus the anion exchange capacity of most agricultural soil is small compared to the cation exchange capacity

Among anions, sulphate in the presence of calcium forms gypsum

Gypsum is only slightly soluble, but it releases sufficient sulphate to support plant growth

Phosphate ions may bind to soil particles containing aluminum or iron because the positively charged iron and aluminum ions have hydroxyl group that exchange with phosphate

As a result, phosphate can tightly bound, and its mobility and availability in soil limit plant growth

Once the nutrients are in the soil solution ,they can be adsorbed by the plant’s roots

The soil solution is the medium by which most soil nutrients are supplied to growing plants

The soil solution can be neutral ,acid, or alkaline this is called soil pH

When the soil solution contain more H ions it is acidic

When there are fewer H ions the soil solution is alkaline

The level of the acidity and the alkalinity in a soil affects the availability of soil nutrients

Hydrogen ion concentration (pH)is an important property of soil because it affects the growth of plant roots and soil microorganism

Root growth is generally favored in slightly acidic soil,at pH values between 5.5 and 6.5

Soil pH determines the availability of soil nutrients

CO2 is produced from decaying organic matter

CO2 is also released from the respiring roots

It dissolves in water and forms carbonic acid which lowers the pH of the soil

Ammonia and hydrogen sulphide released from decaying organic matter ultimately get oxidized to form nitric acid and sulphuric acid

When the soil becomes acidic it will promotes the weathering of rocks that releases ca, Mg, K and Mn and increases the solubility of carbonates, phosphates and sulphate

There for , the solubility of nutrients facilitate and are more easily available to roots

Major factors that lower the soil pH are decomposition of organic matter and the amount of rain fall

Carbon dioxide is produced as a result of the decomposition of organic materials and equilibrates with soil water in the following reaction:

CO2 +H2O H+HCO3

This reaction releases hydrogen ions, lowering the pH of the soil

In arid regions, the weathering of rocks releases ca, Mg, K and Mn to soil, but because of the low rain fall, these ions do not leach from the upper soil layers, and the soil remains alkaline

Iron is absorbed best in acidic soil whereas molybdenum is absorbed best in alkaline soils

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