Crop rotation is one of the key principles of organic farming, but it is also practiced by most good farmers.  The idea, roughly speaking, is to not plant the same crop in the same field year after year.
There are a number of reasons why crop rotation is a core principle of sustainable agriculture.  One has to do with soil fertility:  different types of plants remove different nutrients from the soil and leave others behind.  Corn, for example, famously takes a lot of nitrogen to grow.  But it also puts back a tremendous amount of biomass into the soil when just the ears are harvested (some farmers harvest the entire plant to feed to livestock).  Beans and peas, on the other hand, pull nitrogen out of the air into their roots, creating their own fertilizer.
Different crops are also susceptible to different pests and diseases.  Tomatoes, potatoes, and peppers are all in the same plant family and are susceptible to many of the same diseases such as Late Blight, which was responsible for the Irish Potato Famine.  Yet the foliage of tomatoes and potatoes (but not peppers) is toxic to many common insect pests.  Broccoli and other cruciferous vegetables are attacked by several types of caterpillars and aphids, but when their residue is tilled in to the soil, it creates gases that are toxic to many common plant diseases.  Alternating crops prevents pests and diseases from building up too much in a field.
Crops also have varying abilities to compete with weeds.  Carrots and onions, for example, grow slowly and must be weeded frequently to avoid being overtaken by weeds.  Crops like tomatoes, squash, beans, corn and broccoli are vigorous, fast growing plants that take less care.  Planting crops from the latter category for a year or two can be a way to “clean up” a weedy field.
So far I have been referring almost entirely to vegetable crops, since that is what we grow at Terra Firma.  But in the larger world of farming, there are also crops like sunflowers (for oil and seeds), wheat, and hay.  These are extremely important rotation crops for larger scale farmers.
For just about any farmer, the term “rotation crop” has another meaning:  a crop which is not as profitable as their main crop or crops.  These are generally referred to as “cash crops”.  Good farmers try to practice a four-year crop rotation, meaning they only plant a particular crop once every four years in a particular field.  Ideally you would have four cash crops to rotate through your fields, but many farmers only have two.  The other two years they are growing “rotation crops”.  With luck, these crops make a small profit but often they are just paying the bills and keeping employees working.
Farmers face numerous limitations on what they can grow due to soil, climate, water, season length or other factors. Some may not have enough crop choices to be able to practice even a four-year rotation.  The market also plays a critical role:  very few farmers will grow a rotation crop if they know the price is too low to make a profit.
Crop rotation is the number one reason why farmers listen with bafflement and frustration when the media or consumers talk about making food purchasing decisions based on how much water a crop uses.  Tomatoes, for example, might take half as much water to grow, per pound, then any other vegetable crop.  But from a farmers’ perspective, it doesn’t matter.  You can’t just grow tomatoes every year in a particular field.
And if a crop uses lots of water, but is the lynchpin in a farmer’s rotation — as strawberries are for many farmers in coastal California — it also doesn’t matter.  The only reason they grow other crops is to rotate with the strawberries.  Take the strawberries away and they would go bankrupt.
One rotation crop that seems to cause great anger in the comments sections of the internet is Alfalfa Hay, especially when it is exported.  And yet alfalfa, which is a legume, plays a critical role in many farmers’ rotations by resting the soil for three years while producing a small income.
At Terra Firma we practice a delicate and complicated balancing act factoring in consumer demand, profitability, human resources, weather and sustainability concerns like water resources in addition to the basic principles of crop rotation.  It’s a never ending process, and just when we think we have it right, something changes.