Page 495 of Ricklefs talks about Nitrogen Fixation…
Let's talk about Peanuts!
Peanuts are part of the pea family and therefore help to reduce nitrogen to biologically useful forms. Peanuts in the shell (think about the brown bag of unshelled peanuts you buy outside Fenway Park) grow 1-3 inches underground, and are the tip of the roots of the peanut plant!
Now, let's talk about Nitrogen!
Rhizobium is a type of bacteria that has a symbiotic relationship with the roots of certain legumes - like peanuts - and is dependent on extremely low oxygen conditions in order to be efficient. Peanuts are stuffed underground and therefore provide an ideal living condition for this bacteria. Lots of other fancy chemical interactions occur inside root node due to the presence of this bacteria, enabling this bacteria to remain in continuous supply.
Now, let's talk about Nitrogen and Peanuts!!!!!!
In sum, rhizobium and the peanut work together to "fix" nitrogen into the soil. Nitrogen is a key component to healthy, fertile soil. In fact, in agriculture, the idea of crop rotation is directly related to the quantity of nitrogen in the soil. One year a farmer will plant peanuts (or soybeans) in his field, charging the field up will lots of delicious nitrogen. The following year, a smart farmer, instead of replanting a nitrogen-fixer, would plant a maize (Go Blue!) (I mean, a maize like millet, sorghum, or especially corn) on that field. Corn LOVES nitrogen. It EATS IT UP! So, by rotating fields, the corn benefits from the work the peanut did on the soil the previous year. The thing is, corn eats up so much nitrogen that it shouldn't be planted on that same field for another three years. (Ha!!! Go tell that to our American Industrial Corn Growers! Why do you think they love using fertilizer so much?…). But, resting the field for a year and then replanting peanuts or soybeans to help with natural regeneration of the soil is a nice thing to do for that nutrient starved land.
Now, consider this:
The peanut is the main cash crop of Senegal (a country in West Africa). In Senegal, there is a region called the peanut basin, because it is quiet suitable for growing that beloved nut! The thing is, desertification has been encroaching further and further to the south in Senegal (due to it's proximity to the Sahara desert), so annual rainfall has decreased over the past several decades. Because rainfall has decreased, so has foliage. Foliage is also the fodder on which the animals (that the people need to plow their fields) graze…
The leaves of the peanut plant are like a delicious dessert to ravenous animals (think cows and donkeys). So, each year when the Senegalese harvest their peanuts, they rip the entire plant - roots and all - out of the ground, pluck the peanuts off one by one and then feed the rest of the plant to the animal upon which they rely to plow their field. This causes several issues:
1) soil erosion
2) decreased biodiversity on the field - leaving an empty field
3) nothing left to fix nitrogen into the soil
Instead of ripping the plant out of the ground, if they removed only the peanuts, the plant would continue to fix nitrogen into the soil because not all of the roots have mature peanuts attached to them. But without their animals to plow their fields, they wouldn't have enough food to eat…
So overtime, combined with the encroaching Sahara, the peanut basin is becoming weaker and weaker. And when the farmers plant corn, they must purchase expensive fertilizer to juice up the soil with nitrogen. But, just as was the case in 2008, the rains came hard and heavy this year and washed away their fertilizer (and their money and their food).
But I must say, my favorite Senegalese dish is Maffe (pronounce mah-fay) - white rice covered in a spicy peanut sauce :)