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Extension: Relationships between soil, plants and fertilizer
-courtesy Forsyth County Extension

In ecosystems, relationships exist between all players in the field. Soil supports plants, plants support insects and herbivores, and insects and herbivores support critters further up the food chain. Because the condition and fertility of soil influence which plants can thrive, soil also influences the character of ecosystems. 

Soil basics

The inorganic components of soil – sand, silt, and clay – from when rocks and minerals break down into smaller particles. Soil is usually described based on the relative proportions of sand, silt, and clay in the mix. 

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

Water, air, and organic matter are the other components of soil. Good soil composition includes 25 percent water and 25 percent air, held in pore spaces between soil particles. Water runs quickly through sand particles because they are large and loose. 

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

Organic matter is present in the least amount, usually 5 percent or less, but it is important for soil structure, water infiltration and holding capacity, and supporting biological soil organisms that aid soil fertility. 

How soil affects fertility and plants

North Georgia soil is typically high in clay content, low in organic matter, and acidic. 

Organic matter comes from living tissues, like fallen leaves, grass clippings, and dead bugs. As soil organisms like bacteria, beetles, earthworms consume these tissues, they release nutrients that plants use for growth. 

Under our environmental conditions of high heat and rainfall, organic matter quickly leaches out of the soil.

While nutrients are present in soil, how well they are available to plants is determined by soil pH, a measure of acidity or alkalinity. Important plant nutrients like phosphorus, potassium, calcium, magnesium, and several trace minerals, are most available to plants when soil pH is between 6.0 and the neutral point of 7.0. 

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

The optimal pH range varies by plant. Blueberries, azaleas, and rhododendrons perform best in a 4.5 to 5.2 pH range. Bermudagrass and tall fescue do best in the 5.5-6.5 range, and vegetables need a 6.0-6.5 pH.

Our activities can damage soil. Walking, driving, and parking on soil applies enough pressure to smash particles together and close pore spaces. 

This compacted soil can’t hold water or air, which plant roots need to survive and grow.

How fertilizer affects soil and plants

Nitrogen is essential for plant growth, but plants can’t use its organic form. Soil microbes convert organic nitrogen into a form usable by plants, but the process may not be timely or sufficient for plant needs. 

Chemical fertilizers provide nitrogen in forms readily available to plants, but uptake by plants is still dependent on correct soil pH.

While chemical fertilizer provides necessary energy for actively growing plants, it does not correct problems with soil structure. Applying too much fertilizer can injure plant leaves and roots and kill soil organisms.

Fertilizing at the wrong time, such as when plants are entering the dormant season, can lead to winter injury and spring diseases. Fertilizer applied to dormant plants or in excess of plant needs is washed into streams and lakes where it can fuel algae blooms.

Improving the soil-plant ecosystem

Adding organic matter, such as composted plant materials and animal manures, to soil is one of the best ways to improve soil structure, health, and fertility. 

Organic matter reintroduces soil organisms to damaged soil, relieves compaction, improves structure, and increases infiltration of rainwater. For new plantings, work several inches of organic matter into the soil. 

On established lawns, leave grass clippings and periodically apply a ¼ inch topdressing of compost or composted manure during the growing and dormant seasons. 

UGA Extension strives to translate the science of life for use in everyday living. Forsyth County Extension is supported by the University of Georgia, Forsyth County Board of Commissioners, Forsyth County Board of Education, and United Way of Forsyth County.