Chapter 37 - Soil and Plant Nutrition

Soil is home to a live, diverse ecosystem.

The top layers of soil include a diverse range of living creatures that interact with each other and with the physical environment, from which plants take virtually all of the water and nutrients they require. This intricate ecology may take centuries to create, but human mismanagement can destroy it in a few years.

To understand why soil must be maintained and why some plants thrive where they do, the basic physical features of soil must first be considered: its texture and composition.

The size of the particles in soil determines its texture. Soil particles can range in size from coarse sand (0.02–2 mm) to silt (0.002–0.02 mm) to tiny clay particles (less than 0.002 mm). These different-sized particles are formed as a result of rock weathering. Mechanical fracture is caused by water freezing in rock fissures, and chemical fracturing is caused by weak acids in the soil.

When organisms enter the rock, they speed up the disintegration process by chemical and mechanical mechanisms.

For example, roots release acids that dissolve the rock, and their development in cracks causes mechanical fracturing.

Weathering causes mineral particles to combine with living organisms and humus.

The attached image depicts the soil horizons.

The most fertile topsoils—those that sustain the most plentiful growth—are loams, which are made up of about equal parts sand, silt, and clay.

Loamy soils have enough tiny silt and clay particles to offer an adequate surface area for mineral and water adhesion and retention.

Plants are fed by the soil solution, which is made up of water and dissolved minerals in the pores between soil particles. Water flows from bigger gaps in the soil after a heavy rain, while smaller spaces hold water because water molecules are attracted to the negatively charged surfaces of clay and other particles. In sandy soils, the wide gaps between soil particles are typical.

The capacity of soil particles to bind numerous nutrients is determined by their surface charges. Because the majority of soil particles in productive soils are negatively charged, they do not bind negatively charged ions (anions), such as the plant nutrients nitrate (NO3 - ), phosphate (H2PO4 - ), and sulfate (SO4 2- ).

As a result, these nutrients are quickly lost by leaching and water percolation through the soil. Positively charged ions (cations) as potassium (K+ ), calcium (Ca2+ ), and magnesium (Mg2+ ) attach to negatively charged soil particles and are thus less easily lost by leaching.

Roots, on the other hand, take mineral cations from the soil solution rather than directly from the soil particles.

A live, complex ecosystem exists in the soil.

Soil contains soil particles of varying sizes produced from the disintegration of rock. The size of soil particles influences the availability of water, oxygen, and nutrients in the soil.

The makeup of soil includes both inorganic and organic components. Topsoil is a diverse environment teeming with bacteria, fungus, protists, animals, and plant roots.

Some agricultural methods can deplete soil minerals, deplete water supplies, and encourage erosion. Soil conservation aims to mitigate this harm.

Plant roots take nutrients from the earth. Macronutrients, or elements that must be consumed in significant quantities, include carbon, oxygen, hydrogen, nitrogen, and other key components of organic molecules. As cofactors of enzymes, micronutrients, or substances required in minute quantities, often have catalytic activities.

A mobile nutrient deficiency generally affects older organs more than younger ones; the opposite is true for nutrients that are less mobile within a plant. Macronutrient deficits are the most common, with nitrogen, phosphorus, and potassium deficiencies being the most common.

Instead of customizing the soil to the plant, genetic engineers are tailoring the plant to the soil.

Plant feeding frequently includes interactions with other species.

Rhizobacteria get their energy from the rhizosphere, a microorganism-rich ecosystem that is tightly linked to roots. Plant secretions help the rhizosphere's energy demands. Some rhizobacteria generate antibiotics, while others increase the availability of nutrients to plants. The majority of them are free-living, although others dwell inside plants. Plants get the majority of their nitrogen via bacterial breakdown of humus and gaseous nitrogen-fixing.

Nitrogen-fixing bacteria convert atmospheric N2 to nitrogenous minerals, which plants can use as a nitrogen source in organic synthesis. The most effective mutualism between plants and nitrogen-fixing bacteria occurs in the nodules generated by Rhizobium bacteria growing in legume roots. These bacteria consume sugar from the plant and provide it with fixed nitrogen. In agriculture, legume crops are cycled with other crops to replenish soil nitrogen.

Mycorrhizae are mutualistic fungi-root connections.

Mycorrhizae's fungal hyphae absorb water and minerals, which they then pass on to their plant hosts.