Extension: Relationship between soil, plants and fertilizers


In ecosystems, relationships exist between all the actors on the ground. Soil supports plants, plants support insects and herbivores, and insects and herbivores support creatures higher up the food chain. Because the condition and fertility of soil influences which plants can thrive, soil also influences the character of ecosystems.

Soil basics

The inorganic components of soil ā€“ sand, silt and clay ā€“ from when rocks and minerals break down into smaller particles. Soil is usually described in terms of the relative proportions of sand, silt, and clay in the mixture.

Sandy soils contain 85-100% sand particles. Clay soils contain 40-100% clay particles. The composition of the soil considered ideal, the loam, is 40% sand, 40% silt and 20% clay.

Water, air and organic matter are the other components of soil. A good soil composition is 25% water and 25% air, held in the pores between the soil particles. Water flows quickly through sand particles because they are large and loose.

Clay is the smallest soil particle and has better water-holding capacity.

Organic matter is present in very small amounts, typically 5% or less, but is important for soil structure, infiltration and water-holding capacity, and for supporting biological soil organisms that contribute to soil fertility.

How Soil Affects Fertility and Plants

The soil of North Georgia is generally clay-rich, low in organic matter, and acidic.

Organic matter comes from living tissue, such as dead leaves, grass clippings and dead insects. As soil organisms like bacteria, beetles, earthworms consume these tissues, they release nutrients which plants use for their growth.

In our environmental conditions of high heat and precipitation, organic matter quickly escapes from the soil.

Although nutrients are present in the soil, their availability to plants is determined by soil pH, a measure of acidity or alkalinity. Important plant nutrients such as phosphorus, potassium, calcium, magnesium and several trace minerals are more available to plants when the soil pH is between 6.0 and the neutral point of 7.0.

It seems like a small difference, but a pH of 6.0 is ten times more acidic than 7.0. Outside this range, critical nutrients become less available to plants, while others are released from the soil in toxic amounts.

The optimum pH range varies by plant. Blueberries, azaleas, and rhododendrons work best in a pH range of 4.5 to 5.2. Bermuda grass and tall fescue do best between 5.5 and 6.5, and vegetables need a pH of 6.0 to 6.5.

Our activities can damage the ground. Walking, driving and parking on the floor apply enough pressure to break the particles together and close the pores.

This compacted soil cannot retain water or air, which plant roots need to survive and grow.


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