Crop Rotation – Is There a Benefit for Gardeners?

Crop rotation is one of those techniques that seems to be necessary for anyone growing vegetables. Read any book or online source about growing food and you soon run into the recommendation that crop rotation will make plants grow better, reduce pests and prevent diseases. The Alabama A&M Extension Office says, “it’s an essential part of planning a home garden“. It is a common practice in agriculture and if it works there, why would it not also be a good idea for the back yard garden?

As you learn a bit more about this technique you soon realize it’s not a simple technique to implement.  You have to know your plant families and you need to set up a 3 year schedule, or is it a 4 year schedule? And every author seems to have different rules for selecting the next crop. If there are many so-called “best” systems, which one should you use?

Let’s have a closer look at crop rotation and decide how it should be used by home gardeners.

History of Crop Rotation

Crop rotation was practiced by farmers in ancient Rome, Greece and China. Ancient Middle Eastern Farmers rotated crops as early as 6000 BC. All these people can’t be wrong.

What is Crop Rotation?

Here is a simple definition. Crop rotation in a vegetable garden means not planting the same crop or member of a plant family in the same location every year.

Starting with this basic principal, you can now make it as complex as you want. Some people use a 3 year rotation where the same crop is not grown in the same spot until the 4 th year. Others use a 4 year rotation, or even a 7 year rotation. Some will leave the ground fallow in one of those years and others will plant a cover crop for the full year.

You also need to decide which crop follows your current crop. If you planted tomatoes this year, which crop comes next? Some suggest that since tomatoes are heavy feeders you should follow them with a legume which adds nitrogen to the soil. Others won’t follow a root crop with another root crop, claiming that alternating with a different root system helps improve soil tilth. Some people use plant families where radishes, beets and carrots are in different families. So is it OK to have root crops follow each other if they are in different families?

Whole books have been written about “the right way” to rotate crops and everyone seems to have a different formula.

This is always a big red flag to me. If experts on the subject can’t agree on something as simple as the number of years for the “best” rotation period, how  much science can there really be in all of this?

Crop Rotation Reduces Pests

Although most discussions talk about “pests” in a general way, what they are really talking about are pests that overwinter in soil near the crop, including pests like the Colorado potato beetle,  wireworms and nematodes.

Crop rotation can control pests but it works best when the insect has very low mobility between the end of one crop year and the end of the next and when it has a restricted list of host plants. Not many insects match these criteria. Many insects that overwinter in soil, hatch as flying adults in spring and simply fly to their food so they won’t be controlled very well by crop rotation.

Other pests like corn earworms, cabbage loopers, and potato leafhoppers, travel hundreds of miles to a winter home and then return in spring. Crop rotation does not work for them either.

Insects that can be controlled stay in the soil near this years crop hoping that the same crop will be available to them next year. If you come along and plant something different that they don’t eat, they starve and die out. So when the initial crop returns in 3 or 4 years the pest is gone. Crop rotation does work for a small number of pests.

Crop Rotation Prevents Disease

This also applies only to diseases that overwinter in the soil. In gardening circles I see a lot of comments that say something like, “crop rotation will prevent blight in tomatoes”. First of all there are at least 3 kinds of tomato blight; early, septoria leaf spot and late (septoria is not really a blight but many people call it a blight) . The first two can overwinter on plant debris and in soil so rotation may help. However, late blight does not overwinter in soil, at least not in cold climates, so no amount of crop rotation is going to prevent the disease.

Diseases are controlled by growing a non-susceptible crop until the disease has died out or is at such low levels that it won’t affect the next crop.  To make this work requires good knowledge of the disease life cycle and the susceptibility of other crops. This also points out the misconception that a 3 year or 4 year cycle is the solution. The best cycle duration really depends on the disease being controlled.

Diseases that don’t die out quickly without a host, are not suitable for control by crop rotation.

It is also not a simple matter of following one family type by a different one, since some diseases affect more than one family. For example, Phytophthora capsici causes blight in cucurbits, peppers, and lima beans. Following peppers with lima beans will not prevent the disease.

Crop rotation does work for many diseases including carrot root dieback, clubroot, verticillium wilt, potato scab and lettuce white mold, provided that the right crops are used and that the appropriate durations are followed. For example, clubroot disease caused by Plasmodiophora brassicae is reduced when tomato, cucumber, phaseolus bean and buckwheat are grown, as well as when the plots were fallowed.

Crop Rotation Improves Soil Fertility

Apparently this is a major reason for crop rotation. Some crops are heavy feeders and they suck a lot of nutrients out of the soil, resulting in low levels for the following year. Planting heavy feeders in the same spot year after year depletes the soil nutrients, therefore it is a good idea to follow heavy feeders with either a light feeder like lettuce or with a legume that adds nitrogen back into the soil.

The idea that a legume crop adds nitrogen to soil is mostly a myth, which I have discussed in Do Legumes Add Nitrogen to the Soil? Crops like peas and beans do fix nitrogen but they keep it for themselves, with most of the nitrogen ending up in the harvested seeds. A leguminous cover crop where the seeds are not harvested will add nitrogen to soil.

What about those heavy feeders reducing nutrient levels? This seems to make a lot of sense, but when I looked for evidence that the soil actually gets depleted, I found none. It is true that a crop like corn reduces nitrogen levels and it may fall below optimal levels for another good crop, but the reduction for other nutrients is limited. Even if corn reduces nutrient levels, why would growing lettuce next, increase the levels?

How much do crops remove from the soil? The above table shows values for some crops. Note that these numbers are pounds per 1,000 pounds of fresh produce. The amounts removed are quite small and on average they are about the same for each crop. Corn and grains do remove much higher levels of nitrogen.

A 53 year crop rotation study, used potatoes followed by various crops including legumes (clover, alfalfa) and it found higher yields when non-legume plants were used. “The superior yields of potatoes after the non-legume hay is thought to result, in part, from a more favorable supply and balance of potassium, calcium and magnesium left by the grass crop.”

Crop rotation can be important in agriculture for balancing out nutrients in soil but this is usually done by growing a cover crop or by growing a perennial grass. Neither of these techniques are common in home gardens. Growing a food crop following another food crop is not going to add nutrients back into the soil.

Crop Rotation Reduces Weeds

Field studies using three different crop rotation systems, rice-wheat, maize-wheat and soybean-wheat showed that crop rotation significantly affects the density and species composition of the weed seedbank. Some weeds grow much better in a monoculture and have difficulty growing in a rotation due to changes in competition and maybe allopathic effects.

A limited number of studies have looked at the effects of crop rotation without the use of herbicides, however they have generally found greater diversity and lower density with rotation.

Agriculture vs a Garden

The above discussions deal with agriculture and there is certainly scientific evidence that certain crop rotations work in agriculture. Most of these involve crops that gardeners don’t grow, such as corn followed by perennial alfalfa.

It is important to understand that not all crop combinations work in agriculture. Garden writers assume that every combination has merit, but most have not been tested by science, so we don’t really know.

The most common way to schedule crop rotation in gardens is by crop families. When you look at agricultural practices and the scientific studies you realize that it is mostly based on specific crops, not plant families. Gardeners probably use plant families to simplify the whole thing, but that does not mean it works.

Here is just one example of how complex things can be. “Most root-knot nematodes have a very wide host range. Thus, growers who have a root-knot nematode problem may find it difficult to control it through crop rotation. Cotton growers who have an infestation of Meloidogyne incognita can often plant peanuts in subsequent years to reduce nematode populations. Unfortunately, peanut is an excellent host for race1 M. arenaria, which can be found in fields that also contain M. incognita. Growers who have a problem with M. javanica can employ sweet pepper as a rotational crop, but not if they also have M. incognita. This example demonstrate the importance of understanding which Meloidogyne species is present.”

Gardeners treat all nematodes as one pest, and peppers, peanuts, and carrots (a common host) are all in different “plant families”.

Crop Rotation in Agriculture?

Many farms will grow one crop and then grow a different crop in the following year. They don’t try to grow 10 crops and rotating them. The research studies also looked at growing one crop at a time. In such cases it is much more likely that pests and diseases die out while they wait for the right crop to be grown again.

Smaller farms, or market gardens operate similar to home gardens who tend to grow multiple crops in the same year. This is a completely different environment which makes control of pests and diseases much more difficult.

Location, Location, Location

Let’s compare agriculture to gardening and look at the location of crops. In agriculture the carrots are miles away from the potatoes. When a farmer rotates those crops, this years crop will be miles away from where it grew last year if it is even grown at all. That makes it hard for soil based pests and diseases to find the new crop.

Compare that to a home garden. They tend to grow the same crops every year, and if they use crop rotation, the carrots will be 20 feet from where they grew last year. That gap may be big enough to stop a microscopic nematode but it’s not much of a barrier for most pests and diseases that can travel many miles.

Much of what works in agriculture, is much less likely to work in a home garden because of size. You might move the tomatoes 30 feet to the left, but the pests are laughing at your efforts to fool them.

Dirty Shovels Cause Problems

You are a home gardener that has diligently rotated your crops, but have you washed your shovel?

Why is that important? The main reason to rotate crops in a garden is to prevent soil borne pests and diseases from finding the crop’s new location. There is no point in crop rotation if you are going to use a dirty shovel or trowel and help pests move from one spot to the other.

Gardens and Nutrient Levels

Nutrient levels are critical in agriculture and crop rotation might help reduce the amount of fertilizer they need. They also test soil and only apply what is needed. Gardens operate differently. They tend to over fertilize and then crop rotation is not needed to solve a deficiency.

Even if nutrient levels are low, it is much easier for gardeners to add fertilizer or compost, than to follow a complex 4 year rotation plan.

Gardens and Weeds

Gardeners hate weeds, but pulling them is much easier than doing crop rotation. Besides, you should be mulching, in which case you will have few weeds.

The Bottom Line on Crop Rotation

Crop rotation in agriculture does work in some specific cases but it is not the panacea that gardening experts make it out to be. Agriculture also combines the technique with fallow fields, season long cover crops and perennial grasses.

Agriculture and gardening are very different, and crop rotation is unlikely to provide much of a benefit in most home gardens. It is also complex and depends very much on pest and disease identification, which is not common in gardens.

It might make sense in special cases where you have identified a specific disease or pest and where a known rotation has been shown to work. The solution probably involves not growing the problem crop for a number of years because simple crop rotation without excluding the infected crop rarely works in a small area.

Crop rotation might make a bit more sense in large gardens, such as market gardens, but even there the benefits are limited because most of these gardens grow all crops, every year.

Most of the hype around this gardening technique is just that, hype. Use it if you know it will solve your problem, otherwise, don’t worry about it.