• You may not notice the plants underfoot in your yard, or those that sprout up as weeds in your garden. Different plants, however, play different roles in ecosystems and affect your soil health in various ways. It is likely that your lawn and garden contain a mixture of grasses and legumes, plants that seem alike at first glance, but differ in several ways, with important implications for your landscape.
  
  

Root Structure

• If you uproot a legume and a grass, the root structures differ. Root nodules provide one of the chief defining characteristics of a legume. These nodules appear as small, round swellings on the roots of legume plants. Grasses, on the other hand, have fibrous roots without nodes, in addition to underground stems called rhizomes present in some species, according to the North Carolina State University TurfFiles website.
  
  

Nitrogen Sources

• The structural differences between the roots of grasses and legumes have important implications for how plants acquire nitrogen, the most important nutrient required for plant growth. Although nitrogen forms the most abundant element in the Earth's atmosphere, it occurs in a form that plants cannot use, requiring them to extract it from the soil instead. Legumes, however, form a mutually beneficial relationship with rhizobia bacteria that live in their root nodules. These bacteria can "fix" atmospheric nitrogen, breaking it down into a form that plants can use, according to retired Harvard biology professor John W. Kimball. Because of this relationship, legumes do not need to use soil nitrogen and have a reduced impact on soil fertility. Planting legumes as a cover crop and tilling them under boosts nitrogen levels in the soil for future crops and plantings. Grasses, on the other hand, use their roots to extract nitrogen from the soil like any other non-legume plant, a process that, over time, takes it toll on your soil fertility.
  
  

Leaf Structure

• Legumes belong to the family Fabaceae, and grass belongs to the family Poaceae, families with only a distant relationship within the plant kingdom. This means that you can tell one from the other quite easily by looking at physical characteristics. Grasses have long, narrow leaf blades and leaf veins that run parallel to each other, explains TurfFiles. Their stems are rounded or flat. Legumes are broadleaved plants. Leaves are wide and nourished by branching veins or veins that emerge from the midrib that extends down the center of the leaf.
  
  

Flowers and Seeds

• Both grasses and legumes belong to the flowering plant division. Their flowers, however, differ greatly from each other. Legumes produce noticeable and often showy flowers in a variety of colors and shapes. These flowers develop into seeds contained within pods: Peas and beans are two familiar examples. Grasses also develop flowers, but these are large clusters called inflorescences, according to naturalist Jim Conrad of Backyard Nature. Grass flowers are usually colorless and inconspicuous, lacking familiar flower traits such as petals and upright stamens.