Soil Science and Plant Nutrition

Plants, like men need food in order to survive and to maintain their existence here on earth. These foods needed by the plants are merely composed of thirteen essential nutrients which are drawn out by the plants from the soil. These mineral nutrients are organized into two groups: the macronutrients and the micronutrients. The first group of nutrients is still divided into subgroups the primary and the secondary nutrients. The primary nutrients which are composed of nitrogen, phosphorus and potassium are the chief nutrients requirements of the pants for because large amount of these are utilized by plants for development and continued existence. The secondary nutrients on the other hand are magnesium, sulfur and calcium. This is the reason why lime is applied in most of the soil for fertilization because calcium and magnesium is added to the soil when lime is applied. The micronutrients on the other hand, are the nutrients needed by the plants in little amounts. The micronutrients are as follows: copper, iron, chloride, molybdenum, manganese, zinc and boron. The presence of these nutrients in plants is the main reason why tree leaves and cut grass are left in the soil for decomposition. This manner, the organic matter provides the soil with micronutrients and macronutrients. These micronutrients along with the macronutrients are acquired by the plants from the soil by using their sole mechanism: the roots.

The Roots

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Plants acquire its food mainly from the soil through its roots. The root which is the ones that holds the plants firmly and stabilizes the whole body is also the main mechanism of plants in order to obtain food from the soil. Not later during the germination of the seeds, the seed begins to grow the embryonic root or the radicle into the soil. The radicle grows into the soil and not out of the soil as a reaction to the force of gravity pulling the growing root into the soil. As the seed continues to grow, its roots lengthen as fresh new cells are being added to the interconnected root system. And as it grows longer, it generates root hairs and lateral roots which make the root of the plant gain more of where it was based. Deep within the root, there are internal cells which are particular in the absorption and the generation of solutes which is needed by the plant in order to continue developing.

Nutrient Extraction

The food needed by the plants to develop and maintain like the food required by men to survive is also made up of nutrients. These nutrients are the ones utilized by the plants and extracted from the soil via the roots and distributed to the rest of the parts of the plant via the stems and the branches. The roots of the plants attain these nutrients from the soil by making use of three different individual schemes: the mass flow, the interception and the diffusion. These methods used by the plant roots are vital in the absorption and the accomplishment of needed nutrients from the soil. Mass flow is referred to as the movement of minerals salts and other nutrients from the soil towards the roots. After the stream of nutrients towards the plant roots, the cells in the roots aids in capturing and getting hold on to certain nutrients that they can find during the mass flow. This is known as the interception and the second method used by the plant roots in getting nutrients from the soil. These nutrients and minerals captured by the plant cells in the roots are too big and they can not be taken in by the cells undisturbed. Therefore, before the nutrients and minerals pass through the roots and go across the primary linings of the roots, they needed to be dissolved first. In this method, the plant roots emit certain chemicals in the soil that assists in the faster dissolution of the minerals and nutrients needed by the plants after that, the minerals and the nutrients in the soil are absorbed by the roots and distributed to the different parts of the plant (Pittaway, 2002). Figure 1 relates the processes undergone by the nutrients before they are absorbed by the roots of the plants.

 

Fig. 1. Nutrient absorption by the roots in the soil. (Pittaway, 2002)

Arrow 1 specifies the mass flow of the nutrients, minerals salts and water in the soil towards the roots of the plant.

Arrow 2 signifies the interception or the catching of the minerals and nutrients needed by the plants via the cells in the root tips of the plants.

Arrow 3 indicates the release of acids by the roots to the soil in order to prepare the nutrients and minerals for absorption

Shortage in Nutrients

Due to the increasing competition in the absorption and the consumption of nutrients and minerals in the soil, definitely, there is a time that a shortage in the supplies of nutrients and minerals in the soil will be undergone by the plants. This decline in the supply of nutrients in the soil brings about certain re-adaptations of the plants in order for them to amplify the absorption rate and still acquire the usual rate and quantity of nutrients that are absorbed despite the shortage that is taking place. During times of scarcity of nutrients in the soil, some of the plants locate help from bacteria in order to maximize their absorption of nutrients.

Root Nodules

Nodules occur on the roots of certain plants in association with some symbiotic bacteria. The production of these nodules in the roots of plants occurs in nitrogen limited circumstances. Plants such as from the pea family perform relationships with certain types of bacteria known as rhizobia. These nodules that can be seen in the roots of the plants such as plants originating from the pea family accommodates the bacteria which are the ones responsible for the fixation of nitrogen or the transformation of useless forms of nitrogen into forms which can be efficiently utilized by the plants. Moreover, hemoglobin content of the iron harbored in the nodules of the roots assists in the nitrogen ammonia conversion. Nitrogen in the atmosphere is converted to ammonia by these nodules because this is the form of nitrogen that could be easily used by the plants.

Mycorrhiza

The party formed between the bacteria and the plant in the nitrogen fixation by the nodules is not the only example wherein plants make use other life form in assisting them in their nutrition. Another is the Mycorrhiza. This is the symbiotic relationship between the fungus and the plant roots. The fungus and the plant exhibits mutualistic relationship with each other since the plant gives the fungus access to the fixed carbon sugars of the plant which is the source of food of the fungus. Subsequently, the presence of fungus in the roots of the plants raises the rate of the absorption of minerals and water of the plant in the soil.  Mycorrhizal mycelia or the fungus at the roots of the plants can absorb more minerals due to their physical features (Taiz and Zeiger, 2002). One is that these mycorrhizal mycelia are much smaller in size than smallest hairs in the roots thus; these mycelia can explore areas than what the root hairs can do. Another is that because of their difference in size the mycelia have a greater surface area that is exposed in the soil for greater absorption.

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