Crop Rotation A Lecture By

Mr. Allah Dad Khan

Crop rotation

Crop rotation refers to the practice of growing different types of crops (or

none at all) in the same area over a sequence of seasons. Historians believe

farmers in the Middle East practicedcrop rotation as early as 6,000 B.C.,

although they didn’t fully understand the science behind it

What is Crop Rotation?

The simplest rule of crop rotation is not to grow the same thing in the same

place two years running. In fact, the larger the gap between a crop occupying

the same piece of ground the better. Some pests may be present at high

levels initially but a gap of three or four years will see their numbers fall to

acceptable levels without a host to sustain them.

There are many different systems for rotating crops, some fairly crude and

some quite complex, designed to ensure that following crops utilise nutrients

left by previous crops. The simplest is a three year crop rotation but a four

year crop rotation plan or. as I use, a five year crop rotation may suit you

better. I have seen eight and ten year plans but these are probably too

complex for gardeners

Who Invented Crop Rotation and When?

The concept of crop rotation goes into prehistory so answering the

question of who invented crop rotation and when is not easy. Early crop

rotation methods were mentioned in Roman literature, and from the

Middle Ages to the 18th Century a three year rotation was practiced by

farmers including a fallow year

Principles of Crop Rotation

The first principle of any crop rotation is to have the largest possible gap

between potatoes occupying the same piece of ground. The same applies

for brassicas, the cabbage family.

Keep lime away from potatoes because it increases the chances of them

getting scab. Conversely, brassicas like a limey soil. So potatoes should be

planted as far away from the application of lime as possible and brassicas can

go in to soil that has been limed.

Root crops such as carrots and parsnips do not want soil that has been

manured the previous autumn. It will cause them to fork and split.

Where possible, keep plants of the same family together as their

requirements will be similar

Advantages of crop rotation

Prevents soil depletion

Maintains soil fertility

Reduces soil erosion

Controls insect/mite pests. Crop rotation as a means to control to insect pests is most effective when the pests are present before the crop is planted have no wide range of host crops; attack only annual/biennial crops; and do not have the ability to fly from one field to another.

Reduces reliance on synthetic chemicals

Reduces the pests' build-up

Prevents diseases

Helps control weeds

Why Rotate Crops?

Effective crop rotations are a foundation of organic cropping systems. Organic farmers recognize

that crop rotation is necessary to maintain field productivity. Expert farmers design their

rotations to (1) earn income and (2) increase soil quality or build “soil capital” (sidebar 2.1).

Crop rotation and a crop rotation plan and records are required for organic certification of a

field or farm

Why follow Crop Rotation?

If you grow the same crop in the same place year after year you will get a

build up of pests and diseases specific to that crop. Different crops take

different levels of nutrients from the soil and inevitably these become

unbalanced, exhausting one nutrient but leaving a lot of another. This is

often referred to as a 'sick soil'. Even the addition of fertilizers is unlikely

to help since it is likely the trace elements and micro-nutrients are

depleted in the same way.

Why Rotate?

• Soil quality Crop rotation practices such as manuring, composting, cover

cropping, green manuring and short pasturing cycles improve soil quality by

maintaining or increasing soil organic matter content. Organic matter serves

as the primary food source for soil microorganisms. These organisms provide

many benefits, including holding the soil particles together, releasing minerals

for plant uptake, enhancing the downward movement of water and air, and

providing pathways for root growth. Rotations including crops with a variety

of rooting depths make use of water and nutrients throughout the soil, aid in

loosening compacted soil and increase topsoil over time.

Fertility

Fertility Well-planned rotation schedules take into account the preceding

year’s crops, ensuring that nutrients are available for crops grown the

following season.

It is important to consider the nutrient needs of each crop to ensure they will

be met. The addition of leguminous crops in a rotation can provide nitrogen

for following crops. Including crops with a variety of rooting depths allows

crops to retrieve water and nutrients not accessed by those grown in previous

rotations. Some plants are also effective at making nutrients more available

by using less soluble forms, making them accessible for later crops.

Pest Management

Pest management Pests are most easily kept in balance when different crops

are grown over a number of years. Rotate susceptible crops at intervals to

inhibit the buildup of their specific pest organisms. Rotation length should be

based on the amount of time soil-borne pathogens remain viable in the field.

A four-year rotation using crops not susceptible to the same pathogens will

generally minimize problems from soil-borne pathogens, with some

exceptions (Table 1). Two years is considered enough time to reduce the

incidence of foliar diseases.

Legumes

Legumes Legumes are an important addition to a crop rotation plan because

they fix atmospheric nitrogen, which can be used as a replacement or

supplement for inorganic nitrogen fertilizer. The total N contribution varies

among species, but 50-200 lbs N/acre can be expected from a good legume

cover or cash crop stand. Unlike highly soluble nitrogen fertilizers with a

significant potential to leach, N supplied by legume

Cover Crops

Cover crops and green manures Cover crops are also an important component

of a crop rotation plan and should be utilized when fields are not being used

for production. Cover crops and green manures are those crops grown

specifically for the benefits they provide. They may be incorporated into the

soil or left as a residue on the soil surface. Their benefits include increased

organic matter, improved soil structure, enhanced drought tolerance,

increased nutrient availability for plants, protection against soil erosion,

weed suppression, penetration of compacted subsoils and nutrient cycling.

Crop Families

Crop families Crops within the same family are generally susceptible to the

same insect pests and diseases. A four-year rotation using crops not

susceptible to the same pathogens will generally minimize problems from soil-

borne pathogens, with some exceptions (Table 1). Two years is considered

enough time to reduce the incidence of foliar diseases and insect pests. When

planning a rotation, it is often helpful to map out where the crop families

listed below will be located and how much of each will be planted: Poaceae:

Corn Alliaceae: Onion, garlic, shallot, leeks Chenopodiaceae: Beet, chard,

spinach Cucurbitaceae: Winter and summer squash, cucumber, melon,

pumpkin Brassicaceae: Rutabaga, kale, broccoli, cauliflower, cabbage,

Brussels sprouts, radish, mustard, turnip Fabaceae: Pea, bean Apiaceae:

Carrot, parsley, celery, parsnip Solanaceae: Potato, tomato, pepper, eggplant

Asteraceae: Lettuce Convolvulaceae: Sweet potato Malvaceae: Okra*

Compatibility

It is important to consider crop compatibility when planning a rotation. Some

crops may have beneficial interactions and enhance yield, while others may

have detrimental effects to subsequent crops. For example, many crops

following the cabbage family may have lower yields. Sweet corn is a good

selection to follow the cabbage family because it shows no yield decline.

Potatoes are a good crop to follow sweet corn because research has shown

sweet corn to be one of the preceding crops that most benefit the yield of

potatoes.

Increased Profitability

Some forage legumes have the additional benefit of providing multiple

marketing options. Alfalfa, for example, can be fed to livestock as haylage,

sold as dry hay, or harvested for seed production. Also, the increased organic

matter and availability of nutrients that cover crops can provide create the

potential for healthier and more productive main crops under a wider range

of conditions.

Decreased Weed Pressure

Field tillage, cultivation, and residue management practices all impact the

types and density density of weeds that are present present in a field. When

these practices are performed at the same time each year, certain weed

species are favored, become established, and are increasingly difficult to

control. Consider an example of a six year rotation for corn-belt organic

grains. Corn – beans – corn – small grain – hay – hay. Perennial weed pressure

can be reduced during the first three years of the rotation when annual cash

crops are being aggressively cultivated. Annual weed pressure is reduced by

competition from the hay crop and from the repeated cutting of the hay.

Adding a cereal rye cover crop following the second year of hay to this

rotation might further improve weed management through the allelopathic

qualities of the rye plowdown

Other Benefits

Biodiversity of crops and wildlife

Reduced impact of other pests such as insects and pathogens

Distributed economic risk

Distributed workload Improved on-farm resource utilization

Decreased negative environmental impact

How long should the rotation be?

The longer the rotation the better, but the usual length is 4 years. This means

that crops return to their original site after 4 years. If the soil is already

infected with persistent problems such as eelworm or clubroot, try to extend

the rotation of susceptible crops even further.

There are a few simple rules for crop

rotation:

Don’t follow tomato, peppers or eggplant with potatoes, or each other.

Allow 3 years before replanting the same group in any given bed.

Onions may be planted throughout all groups.

Beets, carrots and radishes may be planted among any group, and replanted

as early crops are removed.

Don’t forget to interplant with companion plants to minimize pesticide use.

See the Companion Plants handout for some ideas on this practice.

Keep good notes so you can duplicate successes.

Beneficial Plants to Include in Rotations

Some plants suppress pathogens in addition to being unsuitable hosts. These

include some cover and green manure crops, as well as cash crops. Including

diseasesuppressive species in a rotation sometimes reduces the time needed

before a particular cash crop can again be produced successfully. Examples

include some legumes and crucifers. These plants suppress pathogens by

stimulating beneficial organisms in the soil and by producing toxic chemicals.

The specific mechanisms involved appear to vary with the crop and the

pathogen. Depending on the mechanism, the beneficial effect can disappear

shortly after incorporation or last for years

Some Diseases That Can Be Managed

with Crop Rotation

Bacterial spot of pepper and tomato. The bacterium causing spot

(Xanthomonas campestris pv. vesicatoria) can be effectively controlled with

rotation because this pathogen cannot survive in the soil once diseased plant

debris decomposes. A minimum of two years without a host crop is

recommended.

Bacterial speck of tomato. This disease is more difficult to control with

rotation than bacterial spot because the pathogen (Pseudomonas syringae pv.

tomato) can survive on roots and leaves of taxonomically diverse weeds.

Therefore, success requires good control of weeds and volunteer tomatoes

during the rotation period. A study on survival of this bacterium showed that

it lived up to 30 weeks on crop debris but less than 30 days just in soil.