Soil Nutrients

What is Soil?

Soil is home to a living, complex ecosystem. The upper layers of soil contain a diverse range of life forms and it is in these layers where plants find the water and nutrients that they need to live.

Soil has two basic physical properties: texture and composition.

Soil texture

Soil texture depends on the size of the soil particles which can range from coarse sand to microscopic clay particles. The varying size of soil particles is due to the weathering of rock.

When rock is weathered, mineral particles are released. These minerals, which are inorganic materials, mix with living organisms and humus, the organic material produced by the decay of animal and plant matter. The combination of these substances forms the topsoil. The topsoil and the succeeding layers are called soil horizons, each of which can be millimeters to meters deep.

The amount of surface area accessible to a soil particle determines its capacity to react with water and nutrients (Fig. 1). The type of topsoil that leads to the most vigorous plant growth is loam soil.

Loam soil is composed of sand, silt, and clay in equal proportions. Loam has small soil particles in the form of silt and clay, creating enough surface area for mineral and water adhesion and retention.

The mixture of water and dissolved minerals in the spaces found in the gaps between soil particles is called soil solution and it is what provides plants the nutrients they need. Sandy soils and clayey soils are not as conducive to growth as loam soil because the spaces between soil particles are not efficient in retaining water.

• Sandy soils have large spaces between soil particles. As such, it usually does not retain enough water to enable plants to grow vigorously.

• Clayey soils have tiny spaces between soil particles. As such, it usually retains too much water causing the roots to suffocate from the lack of oxygen.

Soil composition

Soil has both inorganic and organic components. Let's discuss each of these briefly.

Inorganic components

As mentioned earlier, soil is a product of the weathering of rocks. This means that the minerals present in soil can be traced back to the mineral composition of rocks.

The capacity of soil particles to bind various nutrients is determined by their surface charges. Because the majority of soil particles in productive soils are negatively charged, they do not attract negatively charged ions including plant nutrients nitrate (NO_3-), sulfate (SO₄^2-), and phosphate (H₂PO₄ -). Such nutrients are easily depleted by leaching due to water percolation.

On the other hand, positively charged ions stick to the negatively charged soil particles so they are less susceptible to leaching. Positively charged ions include potassium (K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺).

Organic components of soil include life forms and humus

The main organic component of topsoil is humus, which is made up of organic material created by bacteria and fungi, decaying animals and plants, feces, and other organic waste.

• Humus keeps clay particles from sticking together, resulting in crumbly soil that retains water while being porous enough to supply roots with oxygen.

• Humus helps the soil exchange positively-charged ions and store mineral nutrients that eventually return to the soil while microorganisms decompose organic materials.

A wide variety of life forms inhabit the topsoil, including bacteria, fungi, algae, insects, earthworms, and plant roots. The interaction of these organisms with each other and their environment affect the physical and chemical properties of soil. For example, roots produce acids, lowering soil pH.

Soil nutrients for plants

Plants require 17 particular elements known as essential nutrients to grow and develop.

Types of soil nutrients

While plants need these nutrients, they must be supplied in appropriate amounts. Excessive or deficient nutrients can be detrimental to the growth and development of a plant.

The 17 essential nutrients are identified and classified as follows:

Essential nutrients must be broken down into negatively charged or positively charged ions before plants can use them. Likewise, organic materials must be broken down into elemental or ionic forms for plants to use them.

Plants take in nutrients–with the exception of carbon–through their roots. Carbon is absorbed through the plants’ leaf pores called stomata.

Effect of soil pH on nutrient availability

A plant’s optimal pH depends on organic matter content and plant type. The pH of the soil influences how securely nutrients are bound to soil particles. Many nutrients become unavailable to the plant when the soil pH is either too high (basic) or too low (acidic) since they can no longer dissolve in soil water.

As such, nutrient availability is closely related to the pH of the soil solution. A lower soil pH increases the solubility of plant nutrients copper (Cu), manganese (Mn), zinc (Zn), and iron (Fe). This means that acidic soils increase nutrient availability. However, toxic levels of soil nutrients (and even non-nutrient elements like Aluminum) can be released when soil pH is less than 5.5.

Different plants also have different pH requirements. Azaleas and blueberries, for example, thrive on soils with a pH of around 5.0, independent of organic matter content. Asparagus, on the other hand, can withstand basic soil with a pH of up to 8.0.

In most cases, the optimal soil pH of plants falls somewhere between 6.5 to 7.0.

Importance of soil nutrients

Without nutrients a plant may be unable to complete its life cycle—its seed may not germinate; the plant may not form healthy roots, stems, leaves, or flowers; it may be unable to produce seeds; or the plant itself may die.

Because symptoms of nutrient problems vary depending on the nutrient and the plant type, it is difficult to tell if the soil has nutrient-related problems just by looking at them.

It is thus important to have the soil tested by an agricultural soil testing facility to see whether nutrients need to be added to the soil.

3 ways nutrients are removed from soil

When nutrients are removed from the soil, soil fertility declines. There are many ways by which nutrients are removed from the soil. We will discuss three examples: growing crops, soil erosion, and leaching.

Growing crops

On a specific plot of land, one might notice that crop yield tends to decline with time. Moving to an uncultivated area may temporarily restore crop yield, but the same pattern will be observed over time. This is because growing crops can remove nutrients from soil.

For this reason, fertilizers–mineral nutrients added to soil–are used to restore soil fertility, enabling the seasonal cultivation of crops at fixed locations.

Soil erosion

Soil erosion causes nutrient loss in soil; nutrients are carried away by various agents including wind and water (Fig. 2).

Farmers can reduce erosion by planting windbreak rows, terrace hillside crops, and cultivating crops in a contour pattern. They can also use a plowing method known as no-till agriculture in which narrow furrows for seeds and fertilizer are dug using a special plow, causing minimal disturbance to the soil.

Leaching

When water in the soil is drained, the soil solution may move from the root zone to the soil below. This process is called leaching. Leaching can have both environmental and economic consequences. Leached nutrients may contaminate groundwater reservoirs, and fertilizers are depleted from agricultural fields.