Crop Rotations discussed

Are formal rotations necessary?

If anything would seem to be a fundamental part of horticulture, it is crop rotation. It is therefore with some hesitation that I challenge the value of the traditional three and four year formal rotation. If you read the popular gardening press it appears to be mandatory  for anyone who wishes to grow vegetables. Whenever I pick up a beginners gardening book there are complicated diagrams of how to precisely manage your plot!

I wonder how many allotment growers actually formally rotate. Personally I have always paid lip service to the notion of not following one crop with another of the same kind and more specifically not growing in succession plants from the same family. For example if I wanted to grow turnips, they would not immediately precede or follow cabbages. Actually if I was to be really truthful I don’t like turnips and delicious swedes (which we call turnips in the north) are best grown by a farmer!

As the years pass I find myself less and less a slave to the idea of rotation. I grow numerous brassicas and although I vaguely try not to follow one with another, I frequently fail. I practice a lot of inter-cropping and frequently one crop immediately follows another and for example I might even sow my broad beans under the canopy of not yet completely consumed sprouts (a rather unorthodox practice). I frequently plant in double rows or blocks and my layouts are more designed to efficiently use my enviro-mesh to keep off the pigeons, discourage the mice or keep the carrot fly away. If my only choice of planting site is near my wretched cherry tree when it is really dry in summer or planting one brassica after another the latter choice wins!

 I had a wonderful early crop of broad beans, but now soil under the cherry tree is severely dehydrated!

I have only just discovered wonderful enviro-mesh

Rotations as an advance in agricultural history

Rotation is a respected agricultural tradition. We all remember ‘Turnip’ Townsend’s four course rotation from our history lesson at school. It was a major leap forward in farming three hundred years ago.The four year rotation continued to be used for a very long time. No longer relevant in modern agriculture, much longer rotations that last many years still have a place and farmers who insist on mono-cropping have their own special problems.

The four course rotation was an important  innovation as intensive agriculture started to develop. Before that land was often left fallow from one crop to another. 

It was long before fertilisers had been invented. I imagine it likely that although artisans appreciated the value of animal manures on their own plots they were not initially much used in the new technology of farming. What an advance when nitrogen fixing clover was introduced and both turnips and clover brought browsing cattle and perhaps rootling pigs onto the land to add droppings and urine. 

Clover, peas and beans in a rotation all facilitated bacteria to fix atmospheric nitrogen and enabled this most valuable of nutrients to enter the ground in an available form.

Weed control was primitive and barely existed. Some crops such as peas were considered  ‘dirty’ and allow prolific growth of weeds, other crops like potatoes could be used as cleaning crops because of their strong weed smothering growth.

A rotation of crops probably helped to efficiently recycle organic debris and nutrients. 

Different crops have different nutrient requirements and might make varying nutritional demands of the land. In those days there was great merit in following nitrogen fixing legumes with nitrogen hungry brassicas. It makes a huge difference to the yield of brassicas to have an ample supply of nitrogen.

Some soils over the years can become acid and adding lime can sometimes increase yield. It was sensible in olden times that where lime was needed, it was best to apply it immediately before brassicas. Soil alkalinity reduces the severity of clubroot, a severe scourge of these plants.

If there was (and still is) a single major advantage of rotation it is reducing the infection and build up of soil borne pest and disease.

So why is a formal rotation irrelevant now?

A very significant disadvantage is that it commits the gardener to growing more of a particular crop than he needs. Worse it encourages an annual cycle with what many of we minimum cultivators regard as the decadent practice of leaving an empty, fallow, dug-over  plot to be leached, denuded and eroded by wind and rain throughout the Winter. Best for both the kitchen needs and soil fertility to grow vegetables in every season. 

Vegetables such as overwintered leeks, sprouting broccoli and sprouts will be in the wrong place when a new cycle of rotation commences.

On a similar theme, most modern vegetable growers grow more than one crop in a year. Many fashionable vegetable crops such a ‘salad leaves’ need to be resown several times over to achieve continuity. Many gardeners these days wish to explore the possibilities of inter-cropping, companion planting and green manuring. It’s very inconvenient and technically limiting to insist on growing similar plants together.

No rotations are needed for perennial vegetables such as asparagus and rhubarb. Not the best example of intercropping here

Some traditional reasons for rotation are just out of date! Good gardeners whatever their precise methods maintain good weed control all the year round. Many of us try to practice zero tolerance of weeds setting seed. None diggers refuse to bring ancient buried weed seed to the surface! My own peas and beans are just as clean as my potatoes! As a user of glyphosate I have no perennial weed to smother!

The balanced take-up of nutrients where various crops have different requirements now fails to impress. Organic gardeners, especially the none digging kind, maintain high level of soil organic matter which provides an excellent store of nitrogen and all necessary plant nutrients. Growers like myself do the same and are also prepared to supplement plant nutrient requirements with fertilisers. In whatever order we grow our plants they will not go short of nutrients and leaching of nutrients will be no less or greater than if we were to formally rotate.

The cycle in a rotation defined as ‘roots’ is fundamentally flawed and still supports the ‘baby-talk’ that roots require extra phosphate. There are so many different families of plants that develop fleshy roots (or swollen stem bases such as kohl rabi and some fennels).  Carrots, swedes, parsnips, beetroot all belong to very different families with distinct nutrient requirements. Turnips and swedes are also brassicas and are both susceptible to clubroot.

Does it count as a root or can you grow fennel anywhere? er, not in the lawn!

In fact many of the constituents of the formal vegetable groupings are rather a rag bag of disparate types. Even the RHS on its website admits that for many of the extended range of vegetables we grow these days it does not really matter about rotating. Plants such as sweet corn, cucurbits, french beans, runner beans, salads (a huge group) and peppers can by common consent be grown anywhere. When I think about that list it includes almost all of the vegetables I grow!  

My biggest objection to formal rotations is that they often fail in their main purpose of reducing pest and disease. In very small gardens the different sections are just too close together to properly isolate them. Rotations are too short to prevent clubroot which can survive in the soil for as much as twenty years. I know many gardens where clubroot has efficiently been dispersed across the whole plot on soil attached to plant roots - not necessarily brassicas - as they have been rotated around.

At least the crops are isolated here!

Club root

The only way this fungus disease will infect your crop is from soil. On dirty spades, boots and principally plants. There is no other way it will enter your system. Sad to say many gardeners inherit a vegetable garden completely contaminated and unless their name is Methuselah they will never see the end of it! It used to be such common practice to buy a few brassica plants at the market and - bingo, you had introduced this disease.

If you are lucky enough to be club root free, always propagate your own plants and do so in uncontaminated compost. In the past I have had some near misses when so called friends have insisted they give me a wonderful new brassica variety. After they have gone they go on the fire!. They must find me extremely incompetent when on their next visit their plants are not to be seen!

My allotment in Bolton Percy was clubroot free for the twenty years I cropped it. I cannot say the same of the adjacent plot that I took over from another gardener. Fortunately he did not have the plot very long and he had not had time to rotate clubroot around!. I knew the exact location of the infected soil and never grew brassicas there. It might have helped that I don’t dig and don’t do any cultivations that involve shifting soil. I have no idea whether there was any degree of spread from my numerous worms! I supposed it helped that the plot was long and thin! The fact that I did not have a compost heap and weeds were left to die in situ was not insignificant. If you move contaminated compost around you might very well spread this debilitating disease.

Can soil on a trowel spread clubroot around?

The good news is that you can get a very good brassica crop on infected soil if you use lime and raise the soil pH to 7.5. It’s not that brassicas really like to be so alkaline it’s just that club root hates it! This involves doing the complete opposite of rotation as pH 7.5 will be far too high for any crop that follows. By mono-cropping you might still enjoy all those lovely Winter greens! 

I use lime in the form of dolomitic limestone which provides both calcium and magnesium

Nice cauliflowers even with clubroot if pH is high - but the soil must be fertile

Closing thoughts

I have run out of time! I wanted to explore each of the following.

1.Mono-cropping of tomatoes.

 Little and Large of tomatoes Albenga (Mole seeds) and Sweet Million (Marshalls). Both are more pest and disease resistant than similar Marmande and Gardener’s Delight. Too much of a challenge for even these fine varieties as the soil has not been changed for five years!

But badly grown tomatoes taste very nice

2.My peppers now looking good resown and planted in the same soil for four years now.

3.Considerations when replanting  perennial crops such as raspberries, strawberries and blackcurrants.

My Autumn Bliss have been in place seven years now. When I replace them I will choose a new site

4.Specific replant disease.

5.Whether there is a need for rotation in the ornamental garden.

6.How some different plants do specially well and others particularly badly, when one follows another.

7.Whether garden and natural ecological successions are inhibited or promoted by pest and disease. 

8.Gardens I know where it is now impossible to grow delphiniums and trilliums because of soil contamination. 

9.Not to mention brother-in-law Dave who has white rot contaminated soil and has had to stop growing onions.