Crop Rotation and Cover Crops

Crop Rotation in Middle Ages
Source: tenthmedeival.wordpress.com

For millennia farmers have recognized that soil fertility declines as the same fields are used for the same crops year after year.   Over generations and in different cultures, crop rotations based on two, three and four year cycles evolved as a way to keep soils  productive. [If you remember anything from middle school World History, it's probably  that during the Middle Ages the serfs let  one field lie fallow every third season.] Serfs then,  and farmers  now, also   spread animal   manures on fields as  fertilizers. 

Today, backyard  vegetable gardeners with either  just a small, single raised bed or a large fenced garden  try to avoid planting the same vegetables in the same spot year after year.  They know that it is a way to minimize the presence of some insect pests and plant pathogens. Most of these gardeners also  add compost and other fertilizers to sustain soil fertility, but   they may or may not follow  systematic crop rotation and may not use cover crops as part of their garden planning.  So let's try to make a case for  both of these activities...

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Our favorite backyard vegetables, like tomatoes and peppers, and some grains, like corn, have  relatively large appetites for nutrients. Because the root systems of many popular vegetables are relatively shallow, garden soil can become depleted of some  nutrients near the top of its upper  layer (i.e. the soil's A Horizon).  Adding compost  does return some of these nutrients to  garden soil, and the new organic matter  also promotes soil aeration and  helps to retain moisture.  Typically,  dressing a vegetable garden with about 2" of compost annually will do a lot to sustain its fertility.  Mulching also contributes organic matter and should be considered to  be part of the "compost picture." Few of us, speaking from experience, probably add either enough compost or mulch each year to our vegetable beds,  but every little bit   helps...

Source: growers learning.com

 If you don't  have a lot of your own compost, you can buy compost, but probably many gardeners simply yield to the cheaper temptation and supplement soil nutrients for another year by opening  a standard 5-10-5 bag of  synthetic fertilizer. Eventually, use of synthetic fertilizers will lead to a decline of "soil organic content" (SOC). Without new organic matter regularly being added, the diversity of garden  soil's microbial ecology simply declines (fewer types of bacteria and other  lower order critters, i.e. invertebrates  exist there).  It's these creatures that account for the breakdown of  organic matter that releases a fuller range of plant nutrients  into the soil.

Although adding compost is very important for soil fertility, unfortunately, compost alone is not quite enough.  Backyard gardeners also should systematically rotate crops and  use  cover crops in order to sustain and improve  soil fertility and texture.  [It will also get you a long way towards following really recommended  organic practices in your garden.] Together with adding compost, these two other actions will enable you to break  synthetic fertilizer addiction.

Crop Rotation

The above illustration shows a modern four year sequence of vegetable rotation in a garden plot.

NOTE: The four year cycle   evolved in 16th century Belgium  and involved rotating legumes, root   vegetables (e.g. a lot of turnips), grains,  and a fallow season of no planting. The modern  four year pattern for  backyard gardeners became popularized in the U.K. starting in the 1920's and  emphasized the importance of regularly incorporating  organic matter  into  soil to promote its  fertility.

What the modern  plan achieves is that 1) legumes replace depleted nitrogen sources, 2) root veggies  penetrate deep into the soil and move some nutrients upward,   and 3) the cycle provides  four years of usable crops with at least  a three year gap before a plot might be used again for the same crop minimizing the risk of a  crop's   exposure to  particular pathogens or insect infestations. Of course, adding compost to replace organic matter  is implicit for every year. An excellent resource about practical rotations is North Carolina State University's Center for Environmental Farming Systems publication, Crop Rotations on Organic Farms.

HINT -  Rather than pulling up bush beans, tomatoes, peppers etc. at the end of the growing season, cut them off at ground level and let their root systems decompose in the ground.  It will help keep the soil from getting compacted, leave some nutrients in the ground AND may save your back from spading...

Cover Crops

Cover crop in raised beds.
Source: paoniafarmandhome.com

For farmers, cover crops do a lot. They are used to suppress weeds, reduce erosion and compaction, add organic matter, improve soil structure and restore nutrients to name just a few benefits.  For  backyard gardeners, the benefits probably  focus mostly on adding nitrogen sources and extra organic matter.  

There are actually two seasons for planting cover crops. The first season spans early spring to early summer and  usually involves planting buckwheat, clover or a legume like a field pea. These crops grow fast allowing time for a late season crop to be planted. The second season spans late summer to early fall and frequently  involves sowing oats, field peas, or ryegrass to provide organic matter for the next season. These crops typically have deeper root systems than vegetables and bring nutrients up closer to the surface as well as adding organic matter.

I've only mentioned a few of the available cover crop options that farmers might select to improve soil.  For  backyard gardeners, using cover crops in late summer or early fall probably is the more practical option.   Either ryegrass (not winter rye) and buckwheat are  good choices  because both are annuals and can be  worked into the soil easily before they go to seed. Some gardeners  use winter rye, but it requires more effort to work it into the soil in spring after its sprouts.  For a lot more information on cover crops, consult Cornell University's Cover Crops for Vegetable Growers  website. 

Finally, for gardeners who might be interested in knowing what being an "organic  farmer" entails as well as useful information about organic farming practices, I recommend the U.S. Dept. of Agriculture's Guide to Organic Producers. This is set up as a workbook and checklist for  prospective organic farmers, but  backyard gardeners will learn a lot just be looking through it,  too...

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Source: earthwiseradio.org

About Animal Manures 

Applying animal manures to a garden can be  a  very effective way to sustain soil fertility. Organic and conventional farmers  frequently incorporate manures into their  field management strategies,  but for backyard gardeners with limited space and maybe limited access to manure sources it can be more challenging.  For a gardener with a small, e.g. 4' x 8', plot, a commercial bag of manure from a garden center etc. is attractive because the manure already has been  composted, sterilized and rendered safe from pathogens and viable weed seeds.

For a person with a larger vegetable garden, this approach is not especially economical and makes the offer of  getting fresh manure from a neighbor's pet chickens or rabbits  appealing, but it requires more thought than just saying, "Yes, I'll take all you've got."

Animal manure should be composted before being spread on the backyard garden.  If not composted, then its best to dress it either onto a fallow plot or to till it into a  bed at the end of current the   growing season so that it will decompose over the winter. In its fresh state manure can be simply too concentrated for plants to handle. For example, there may be too much nitrogen, it might be too acidic, it could contain lots of weed seeds or a pathogen (e.g. worse case: salmonella)... Or, maybe it'll be just too smelly for your backyard.  For both commercial as well as neighborly sources of  "brown gold," either check the label or ask about how "organic" the manure is.  Unless you are getting manure from an organic, or at  least, a hormone free source,  you could  be inadvertently  adding some extras like animal antibiotics to your garden's soil. 

Finally, the manure's animal source  is important to know because nutrient content varies.  Chicken manure is generally considered desirable because it contains relatively high amounts of nitrogen and potassium compared with other manures; on the other hand,  like cattle manure, the fresh variety also contains a lot of ammonia that potentially can damage plants if over applied.  Cattle manure contains a lot of organic matter and a wider range of nutrients  (as well as weed seeds), but it requires about a year to decompose. Horse manure is similar to cattle manure in content, but can take up to two years to decompose -- making it a slow release  fertilizer.  Simply do not use pet feces - the risks of exposure to parasites and harmful bacteria are too great.  If you plan to use animal manures in your garden, it's a good idea to follow the guidelines of the  National Good Agricultural Practices Program available from Cornell University's website.  It is very, very  important to know when to add manures and how to apply  them safely in your garden. 

And remember  (maybe)...

"Creating your own urban farm is as simple as planting your flowerbeds with edibles." ~Greg Peterson