Healthy Soils, Healthy Plants

Julian Bell, West Mains Allotment
Presented at SGM, November 2015


In my day job, I work for an agricultural college (SRUC) and spend my time advising arable farmers with rather large ‘allotments’! Since starting on the allotment at West Mains Road three years ago I have noticed many common problems and thought I would share some of my thoughts and current research thinking on;
– The biology and control of soil borne pests and diseases which I believe are one of the biggest threats to the sustainability of crop production on the allotment (as they are in conventional farming).
– Soil health and structure.

There are a number of reasons why soil borne pests and diseases can be a problem on allotments.  This makes it important to develop a plan to control them and so maintain the long term productivity of your plot.
– Allotments have been intensively cropped with vegetables without a break for 100 yrs plus.
– The use of crop rotations is inconsistent and the length of time between crops within the rotation is almost always too short.
– Plot holders regularly share plants and the soil they are growing in spreading pests and diseases between plots. While good will between neighbours is always welcome care has to be taken to avoid introducing new pests.
– Plot holders home-save plants and tubers particularly potatoes.
– Plot holders don’t test their soil to find out what they have.
– Plot holders may not be aware of the problem and the solutions – that’s why I have prepared this information.

Overview of the main soil borne pests and diseases

In this section I outline the key issues with the most important soil borne pests and diseases, my own experience of these diseases at West Mains Road and links to more detailed information.

In terms of control measures I have rated measures as follows;
(1) Good control under all conditions,
(2) Good to moderate control under favourable conditions, risk of strong losses under some conditions
(3) Limited or no control

Brassica Clubroot

Common name Club root
Scientific name Plasmodiophora brassicae

Plants affected Brassicas i.e. Cabbage, Cauliflower, Brussel sprouts, Swede, Turnip and wild relatives (Shepherd’s purse)
Main symptoms Swollen and distorted roots, stunted growth, wilting in hot weather.
Caused by Fungus-like organism

Timing Mid-summer to late autumn

Clubroot affected brassicas
Clubroot affected brassicas – Source: SRUC, other


(1) Rotation – a minimum of 6 years between brassica crops. To be fully effective this approach also requires control of brassica weeds such as Shepherds purse between brassica crops as these weeds will also propagate the disease. Brassica weeds are common on my allotment and they can be hard to fully control. Nonetheless lengthening the rotation is the only long term means to reduce the clubroot burden in the soil and must be the main strategy.

(2) to (3) Liming and adequate drainage– prevent acid soil conditions through adequate liming to ensure the soil ph is above 6 which has been proven to reduce the severity of the infection. Anecdotally on the allotment this appears to be quite effective especially if the planting hole itself is lined with a handful of lime at planting.

(2) to (3) Resistant varieties – varieties have been bred to resist specific strains of the clubroot organism. Generally these varieties are only resistant to one strain of clubroot. They are useful as part of wider strategy including crop rotation but are not a substitute for it. In situations without the target strain of clubroot they will be no more resistant than other varieties. Resistant varieties include Brussels Sprouts, Cabbage (Kiloton), Cauliflower (Clapton), Swede. I have grown these varieties and have found that they can still be infected so clearly multiple strains of clubroot are found at WMR.

(2) to (3) Growing plants in large pots before planting out to ensure they have a strong root system before contact with the pest should help but I have not tried this myself.

For more details see –

Potato Eelworm – Potato Cyst Nematode (PCN)

Common name Potato Eelworm, Potato Cyst Nematode
Scientific name Two main species; Globodera rostochiensis (Golden Eelworm), Globodera pallida (White Eelworm)

Plants affected Potatoes, Tomatoes, Aubergine
Main symptoms Yellowing lower leaves and stunted growth from around early July, small tubers
Caused by Nematode
Timing Mid-summer to late autumn

Potato Cyst Nematode at WMR (variety Romano)
Potato Cyst Nematode at WMR (variety Romano) – Source: Julian Bell

(1) Use only certified seed potatoes – This is the single most important step you can take to limit the spread of PCN on your plot. Given the long history of potato growing at WMR it is likely that you already have some PCN on your plot, but it may be localised in just a few spots. PCN cannot on its own travel far in the soil, it needs carried. If however, you use your-own home saved seed you are highly likely to spread it across your entire plot within a crop or two as the pest will be attached to your seed potatoes. Even worse is using home-saved seed from a neighbours plot as this may mean carrying in the pest from further afield including the risk of bringing in one of the two species you don’t yet have. This is why using purchased seed is so important as it must be certified as PCN-free under EU regulation. Before they can be sold seed potato crops must undergo three levels of rigorous government testing to ensure they are PCN free; the soil they are growing in, the crop itself and the harvested potatoes.

(2) Rotation – a minimum of 6 years between potato crops. To be fully effective this approach also requires control of volunteer potatoes between potato crops as they will also propagate the pest. This requires careful harvesting of all the small potatoes when lifting the main crop. It also requires digging up any volunteer potatoes that appear in the year or two after the potato crop. Also be careful to clean soil off any tools when working between different blocks within your rotation (or from other allotments) as this can spread the pest too.

(1 to 3) Resistant varieties – potato varieties are available that offer 100% resistance to the individual species of the pest. For example Maris Piper is completely resistant to G Rostochiensis and as long as this is the only species present the potato crop will be unaffected by the pest. However if any G Palllida is present Maris Piper has no resistance and growing this variety will result in a rapid increase in G Pallida with subsequent yield decline.

On my allotment, I suffered patches of severe PCN in the first year, since then I have switched largely to varieties resistant to G Rostochiensis and generally have seen less sign of damage though some infestation continues. From this it appears that the PCN on my allotments is mainly Rostochiensis however I don’t know for sure as one supposedly resistant variety appeared to be attacked. I will be submitting some soil samples to a crop lab this winter for testing to try and determine if G Pallida is also present.

There are potato varieties resistant to both PCN pest species but these are not commonly available though this is likely to change. In commercial potato production, new varieties are available resistant to both species, mainly originating from Holland where PCN is a severe problem.

There are a good selection of Rostochiensis resistant varieties such as Maris Piper, Nicola, Markies all of which I grow. However many of the older ‘tasty’ varieties are not resistant such as Sharpes Express and Duke of York which I continue to grow. These are early varieties which under light infestations of PCN tend to be less affected as they are lifted before the pest can fully complete its life cycle. Main crop susceptible varieties such as Romano, Kerrs Pink, Pink Fir apple are likely to be more severely affected by PCN.

For detailed information on potato varieties check the following database;

Onion Whiterot

Common name Onion white rot
Scientific name Sclerotium cepivorum

Plants affected Onions, garlic, leeks and other alliums
Main symptoms Yellow and wilting foliage
Caused by Fungus
Timing Mid-summer until early autumn

Onion Whiterot
Onion Whiterot – Source: RHS, other

This is a very persistent disease lasting up to 20 years. After suffering this disease in the first year on the allotment I have stopped growing onions. I still grow leeks which do not appear to be severely affected.


Given that there are no absolute cures for any of the common soil borne pests and diseases the No1 strategy to reduce the severity of the problem over time is to implement an effective crop rotation.

The key principles are;
-Implement a rotation ideally of at least 6 years, shorter rotations remain beneficial but will not prevent the steady worsening of soil borne pest and disease.
-Mark out your plot clearly into discrete parcels of equivalent size, keep good records.
-Practise good soil hygiene (minimise the transfer of soil around your plot, clean your tools between plot).
-Control volunteer crop seedlings (potatoes) and related weed species (shepherds purse) in between crops.

Some plot-holders may be concerned that introducing a lengthy rotation will curtail the amount they can produce of their staple crops such as potatoes, brassicas and leeks and onions. In the short term this may be an issue but in the longer term lengthening the rotation is the only way to ensure sustainability of production. There are also a number of benefits including;
-Each crop will be grown over a smaller area but over time crop yields will rise to the point where overall production will be the same or higher. Maintaining a larger area of poorly performing crop may produce little and will be a lot more work.
-A wider range of crop types can be grown including things you can not buy easily in the shops; Jerusalem artichokes, Scorzonera.
-The use of break crops will reduce the work load, increase soil organic matter and structure, provide habitat and food for a wide range of beneficial insects and birds.

On our allotment I have adopted a six year rotation as follows;
Year 1 – Potatoes
Year 2 – Pulses – peas, beans
Year 3 – Brassicas – broccoli, cauliflower, swede
Year 4 – Leeks, carrots, parsnips
Year 5 – Break crop year 1 (plus sweetcorn, courgette, squash)
Year 6 – Break crop year 2

Break crops – these include rye grass, cereals (oats, rye grain, wheat), phacelia, red clover, lupins. I avoid mustard as this is a brassica and would merely propagate clubroot.

I tend to mix a wide variety of species together to create a diverse mix which is good for;
-ease of establishment (if one species fails or is slow to establish another one will fill the gap).
-soil nutrient capture (legumes trap nitrogen but will leach it out in the winter when they die back but if ryegrass is also included this will capture nitrogen in the winter as it keeps growing then).
-soil structure (different rooting depths).
-biodiversity (different flowering times to provide food for bees and insects longer).

Break crops at WMR – Phacelia, lupins, oats
Break crops at WMR – Phacelia, lupins, oats – Source: Julian Bell

Soil structure

One key benefit of including break crops in the rotation is giving the soil a break from cultivation. This increases soil organic matter and improves soil structure.

What is good soil structure?

Left undisturbed soil will develop a block like porous nature made up of horizontal and vertical cracks and layers due to the action of roots, invertebrates and natural contraction and expansion of soil through drying, wetting and temperature changes. The longer soil is left undisturbed the greater this structural diversity becomes. The benefits of a well structured soil include;
-Increased infiltration of rainwater – heavy rain will be quickly absorbed without running off and without puddling and will infiltrate the sub soil making it available for use later by plants. This greatly reduces the need to water plants in the summer. It also reduces the loss of nutrients washed away with fine soil particles.
-Ease of access for plant roots – without any hard layers or pans, plant roots will grow quickly and will penetrate deeply into the soil where they can access nutrients and water in times of drought. This greatly reduces the need to water plants in the summer.
-Greater diversity of soil invertebrates – particularly of predators which often have a longer life cycle than pest species and are more vulnerable to disturbance.
-Surface layers are more free draining allowing planting and surface cultivation more quickly after rain.

Soil types

Source SRUC.


Why should you dig soil less?

Heavy cultivation and digging of soil;
-is harmful to soil structure and will result in poorer water and nutrient retention and retarded root development.
-is harmful to soil organisms particularly earthworms.
-buries a lot of organic matter deep in the soil where it is of little value to the crop – organic matter is most beneficial where it is close to or at the surface.
-is also hard work!

For these reasons it is important to keep cultivation to the absolute minimum though there are some practices which require some cultivations namely
-the need to control weeds where hoeing is used.
-when growing root vegetables.

Nonetheless setting out to cultivate the soil less will start to bring longer term benefits. This can be achieved as follows;
-restrict cultivations to the surface layers.
-only cultivate strips where crops are to be grown rather than the whole bed.
-Where soil is compacted break through the soil layers by ripping with a narrow implement. I have found an old ice-axe to be very good for this job!
-do not mix surface and deep subsoil layers.
-leave crop roots in the soil as they will form natural drainage channels when they rot down.
-spread composts and mulches on the surface around growing crops rather than digging them in. Here they will protect the soil and worms and other soil organisms will break them down and pull them into the soil.
-If you can bear a bit of untidiness don’t be in a rush to clear all the spent vegetable crops in the autumn but instead leave them over the winter to clear in the spring. They will act as a break crop, trap nutrients, protect the soil and provide food and homes for insects and birds.

To be avoided at all cost is double digging which is extremely damaging and self defeating. What is the sense in burying the most biologically active upper layer of the soil deep underground killing most of the organisms and removing the valuable organic matter from the surface root zone? Also why bring up sub-soil effectively to be the medium of growth? Also most crucially it is extremely hard work – so if it is hard work and damaging why do it?

I appreciate many if not all gardening books recommend double digging which may be a historical artefact. In Victorian times deep digging was fashionable but at that time so were paid gardeners needing to justify their existence and also most gardens had access to copious amounts of horse or farmyard manure. Adding high levels of organic matter can offset excessive cultivation and help protect soil structure. On a modern allotment, manure is harder to come by so more care needs to be taken of the soil.

I hope I have given you some food for thought however, my intention is not to give you all the answers. I continue to experiment and learn on my own allotment; for every success there is a failure, things that work one year don’t always work the next. Please treat gardening books with a pinch of salt, for every sensible recommendation there is at least one that is complete nonsense based on tradition rather than science. Instead try and understand the principles and work things out for yourself; that way you never know what you might discover!

Happy gardening,

Julian Bell

October 2015