Effect of Sugar Water on Plants

Overview

Sugar is food for plants. Ordinarily, plants devote much of their resources to synthesize and transport sugars throughout the plant. Gardeners who want to increase the health of their plants may be tempted to help them out by adding sugar to the soil and water, but ultimately the disadvantages far outweigh the benefits of this approach.

Photosynthesis Creates Plant Sugar

Ordinarily, plants don't take in sugars from their surroundings--they make it themselves. Chlorophyll molecules, primarily in leaves, use the energy from photons of light in order to synthesize glucose out of carbon dioxide and water. Glucose is a sugar, which the plant uses to store and transport energy wherever it's needed in the organism. Specifically, for every six carbon dioxide and six water molecules, a plant is able to create a molecule of glucose and six leftover molecules of oxygen. This chemical reaction is called photosynthesis.

Sugar and Osmosis

A high sugar concentration in the root zone of the plant changes the osmotic potential in the soil. Sugar absorbs water very readily, which will pull it away from the roots. Thus, plants could wither and die from lack of water, even as the soil around them would be full of moisture.

How Sugar Affects pH

Sugar is a base, which means that its pH is higher than the neutral level of seven. Soil pH affects the chemistry of the soil and how readily certain vital nutrients will bind to other elements or become available for absorption by the plant. Most plants thrive in soils that are slightly acid, that is, the pH is lower than seven. If sugar raises pH sufficiently, plants will lose access to certain nutrients.

Sugar Attracts Pests

Plants aren't the only organisms in soil that feed on sugar. A higher sugar concentration can attract and encourage the growth of the local pest population. This can cause further harm to plants.

Glucose vs. Sucrose

Table sugar would further burden plant growth. Glucose is the sugar that most plants make. Its molecular structure consists of six carbon, 12 hydrogen and six oxygen atoms. White, "table" sugar used at home is sucrose. This molecule is nearly twice the size of glucose, comprising 12 carbon, 22 hydrogen and 11 oxygen atoms. Root cells are permeable, but only up to a point; sucrose is much more difficult for root and other plant cells to absorb than glucose, and therefore is not used nearly as efficiently as what the plant can make naturally from the sun.