Science Biweekly

Every farmer can attest to the benefits of crop rotation. Interestingly, the idea of crop rotation quickly followed the birth of agrarian societies. The Indus Valley Civilization was one of the original cultures to practice crop rotation nearly 4000 years ago and benefited from their ingenuity (Worthington, 2016).

The Indus flourished along Pakistan and northwest India and assisted civilization in the development of modern agriculture. Although the Indus civilization has disappeared, the technique of crop rotation is highly relevant. For those that plant gardens, crop rotation can help maximize yield and minimize the amount of exogenous fertilizer applied to the soil each year.

The four field crop rotation is a standard and proven method to enhance garden health. Although there are multiple varieties, the idea behind crop rotation is simple; allow the soil to regenerate lost nutrients from a previous crop while a new crop utilizes untapped resources in the soil to produce a desired product.

A garden can be divided into four quadrants to apply the technique of a four field crop rotation (Figure 1). The first quadrant is the legumes (eg. peas, beans), which is the regenerative crop for nitrogen.

Nitrogen is necessary for biological organisms to create amino acids for proteins and nucleotides for DNA. Planet Earth is rich with nitrogen gas (N2), as it accounts for 78% of Earth's atmosphere; however, with the exception of a few select bacteria, atmospheric nitrogen is unusable for most organisms (Flynn, 2015).

The bacterial families Rhizobiaceae and α-Proteobacteria live mutualistically within the root nodules of legumes. In this partnership, the legumes provide nutrients for the bacteria while the bacteria convert nitrogen gas to ammonia (NH3), which can then be utilized for the production of amino acids and nucleotides.

Excess NH3 is released into the soil and will provide a future reserve of nitrogen for plants that are unable to host nitrogen-fixing bacteria. Ultimately, the rich soil produced by legumes and their bacterial comrades provide an excellent source of nitrogen for the remaining crops.

The second quadrant should be planted with Alliaceae (onion, garlic) and other root like vegetables (carrots, potatoes, etc). These root plants do not require excessive amounts of nitrogen so they will not deplete this resource for the plants that follow their rotation. Typically, root plants rely on potassium which can be added exogenously if desired.

The third quadrant should be planted with fruits such as peppers, melons, and tomatoes. These plants require nitrogen and phosphorous, but the nitrogen balance is crucial as an excess of nitrogen will lead to a decrease in crop output as the plants create more foliage than fruit. By following the root vegetables nitrogen levels will be suitable for fruit barring plants to yield a generous bounty.

The fourth quadrant should be planted with leafy vegetables (eg. spinach, lettuce, herbs). Again, these plants require nitrogen to be healthy and productive. The nitrogen in the soil will be drained after a crop of leafy plants; therefore, the year following a leafy crop, legumes should be planted in this quadrant to replenish the soil.

Crop rotation has been practiced for thousands of years and is a sustainable method for an organic garden. Crop rotations do not account for other minerals such as phosphorous or potassium, which will need to be replenished in the soil by fertilization. If you haven't planted your garden yet, take a few minutes to plan a garden-rotation and benefit from the years of plentiful harvests.

References

1. Worthington, D. (2016). Over 4,000 Years Ago, the Indus Grew Rice and Used Crop

Rotation. Retrieved from NewHistorian website: http://www.newhistorian.com/

4000-years-ago-indus-grew-rice-used-crop-rotation/7594/

2. Flynn, R., & Idowu, J. (2015). Nitrogen Fixation by Legumes. Retrieved from New Mexico State University website: http://aces.nmsu.edu/pubs/_a/A129/