Nitrogen Fixation and Legumes: Say What?

How do legumes fix nitrogen?
If the atmosphere is about 80% Nitrogen (N2), but plants cannot use that form, how do they get nitrogen? In this unit, students plant and care for soybeans under one of five different nitrogen/microbe regimes: Full amount of slow release fertilizer (1X), no soil microbes added; Quarter amount of slow release fertilizer (0.25X), no soil microbes added; No slow release fertilizer (0X), soil microbes added; Quarter amount of slow release fertilizer (0.25X), soil microbes added; No slow release fertilizer (0X), no soil microbes added. As the plants grow, students analyze and maintain records of their plant’s progress. Students will become familiar with the vegetative and reproductive stages of the soybean (more information and pictures of these stages can be found at the OSU crop extensions sites). Students keep records of the dates in which they observe these reproductive transitions.

Teacher background

Nitrogen is the most common limiting factor of the nutrients in ecosystems. Nitrogen is naturally made available to plants through a series of steps, the nitrogen cycle. In farming systems, the nitrogen cycle does not provide enough (quickly enough) of the nitrogen required in a field of crops all planted at the same time. That is why farmers apply fertilizers in various nitrogen forms for the different growth stages of plants. However, there is a family of plants, Fabaceae, legumes, that have an adaptation to live in a symbiotic relationship with microbes in the soil that fix bacteria naturally.

Throughout the life cycle of a plant, there are stages for growth (vegetative) and reproduction. These stages follow a step-wise progression, but are affected by other abiotic factors, such as the amount of rain, temperature and the amount of light (photoperiod). Due to the latitude of the earth, there are varying lengths of daylight across the earth, throughout the year, even during the same season. This lesson investigates the effect of those different lengths of daylight on vegetative and reproductive stages of soybean growth. (See teacher handout for more information.)

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