Farmers often practice crop rotation to help enrich agricultural fields. Why are beans and other legumes often used in the rotations?

Question

A.  Beans fix nitrogen.
B.  Bacteria in legume root nodules are nitrifiers.
C.  Legumes enrich the soil by forming heterocysts.
D.  Legumes remove toxins from the soil.
E.  Less carbon dioxide.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
 
Gather Content
· What do you already know about nutrient cycling? How does it relate to the question?
· What other information is related to the question? Which information is most useful?
 
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

  Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

B.  Bacteria in legume root nodules are nitrifiers.

Clarify Question
· What is the key concept addressed by the question?
        o For generations, farmers have used legumes in crop rotations because they improve the soil, making later crops more productive. How do legumes improve the soil?
· What type of thinking is required?
        o This is an “evaluate” question, so use your knowledge about nutrient cycling to evaluate the relative merit of each hypothesis. Remember, a hypothesis is an educated guess, so you should be able to back up your choice with facts.
 
Gather Content
· What do you already know about nutrient cycling? How does it relate to the question?
        o There are three nutrients crucial for plant growth: nitrogen, phosphorous, and potassium. Plants also need a lot of organic material, such as decomposing leaves, to make the soil fluffy and allow air and water to pass through it. Growing legumes improves which of these four essential soil characteristics?
· What other information is related to the question? Which information is most useful?
        o Most plants and animals cannot use nitrogen gas. Nitrogen gas consists of two nitrogen atoms bound together with three covalent bonds. This makes the molecule very difficult to break apart. Only organisms with nif genes are able to break apart nitrogen gas and turn it into nitrate or ammonia, compounds plants and animals can use. These organisms are called nitrifiers.
        o In oceans and lakes, most nitrogen fixation is done by cyanobacteria. These bacterial colonies form specialized cells called heterocysts, where nitrogen fixation takes place.
 
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o After reviewing the background information, cover up the options and try to answer the question as if it were a short answer question. Now, compare your answer to the options. Are you able to cross some out that are clearly incorrect? How many do you have left?
        o After crossing out the ones you know are incorrect, compare the options you have left. What do you know about legumes such as beans and peas? How can that help you answer the question?

  Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked you to explain how legumes improve the soil. You had to compare the five hypotheses and pick the best one.
        o If you got the correct answer, well done!         o If you got an incorrect answer, where did the process break down?
            · Were you able to cross off “Less carbon dioxide”? Carbon dioxide isn’t important for soil nutrients. More important aspects are nitrogen, phosphorous, potassium, and organic matter.
            · Did you get stuck between “Beans fix nitrogen” and “Bacteria in legume root nodules are nitrifiers”? Plants do not have the nif genes. Only bacteria and archaea are able to fix nitrogen. Legumes seem to fix nitrogen because they form a symbiosis with nitrogen-fixing bacteria in their roots.