Vegetable Crop Rotation

It is very important to practise the craft of crop rotation if one desires to grow good quality vegetables. In order to prevent the incidence and build up of certain pests and diseases it is necessary to follow a system of good gardening husbandry.

What is very important is to ensure that the same plants are not grown in the same place for two years running, this is very important for the leafy brassicas (cabbages, cauliflowers, broccoli etc.) but less so for the legumes (peas and beans). The brassicas are prone to club root disease, Plasmodiophora brassicae, which is evident by wilting leaves, especially on warm days; yellowing leaves and deformed roots of a club shape, hence the common name. In mild cases there be a few expanded nodules on some roots but severe cases will have most of the plant roots badly swollen which will lead to decay and the eventual death of the crop. Club root fungus will stay in the soil for around eight years or even up to ten years duration. Spores will easily be transferred to other parts of the garden or allotment rendering these areas unable to grow healthy brassicas as well. Transfer of spores is by way of footwear and the retention of plant debris.

Control and eradication of club root disease

• Only buy healthy young plants from a reputable grower and dip the roots pre-planting in a commercially available root dip
• Practise strict crop rotation
• Consider growing your own crop from seed
• Do not accept soil from another plot unless it is certified club root disease free
• Keep all footwear and cultivation tools clean
• Keep weeds and diseases under control
• Remove at once plant material that you suspect may be succumbing to the disease
• Maintain a soil pH of 7.5 for brassicas

If an outbreak of club root has occurred, liming may reduce or control the disease. The pH level of the soil should be at a reading of pH 7.5 for brassicas; this can be achieved by applying ground limestone, which is technically known as calcium carbonate. It is sold under various names such as carbonate of lime, garden lime, ground limestone or ground chalk. Soil testing should be carried out by using a soil test kit and following the instructions included.

The pH scale runs from pH 0 to pH 14, 7 being neutral and 6 – 6.5 is ideal for most plants. Ericaceous subjects, that are those plants that grow healthily in acidic conditions, include azaleas and heathers and at the other end of the scale the brassicas that prefer alkaline conditions.

To raise the soil pH to a reading of pH 7.5

For example: to raise a loamy soil from a pH of 5 to achieve a reading of 7.5 for brassicas, apply 800g of ground limestone per metre square, or 28 ozs per square yard.

Other diseases that can occur as a result of not practising crop rotation are carrots and parsnips that are susceptible to Carrot root fly, Psila rosae. Carrot root fly is very destructive to carrots and parsnips; first, it burrows around the outside, eventually tunnelling throughout the root and so ruining the crop. They are low fliers rarely going above 50cm so one method to control carrot root fly is to surround the crop with a polythene barrier about 60cm high. However, in order to help control these pests crop rotation should be practised. Another pest that can destruction is Onion fly, Delia antiqua, which attacks members of the onion family. The onion fly looks just like ordinary small houseflies, thin in body and long-legged. It lays its eggs during the spring and early summer near the base of onions and leeks, and also in areas close to the crop and sometimes in the leaf axis. After hatching, they feed for three weeks, overwinter and then tunnel into the onion or leek crop.

When lifting attacked plants ensure that all grubs are removed. There are chemical treatments available from Garden centres.

Vegetables crops should be planted together as family members, as these various family members require the same conditions for good growth.

• Members of the brassicas family include: cabbages, cauliflowers, brussel sprouts, broccoli, Swedes and turnips.
• Legumes and pods: peas, French beans, broad beans, lima beans, runner beans and okra.
• Alliums: onions, salad onions, shallots, leeks, garlic and bunching onions.
• Roots and tubers: carrots, potatoes, parsnips, beetroot, tomatoes and celery.

Cabbages, broccoli, brussel sprouts do best in a soil ph value of pH 7.5. whereas potatoes do best in more acidic conditions so these two groups need to be kept apart in the vegetable garden. Lime will probably have to be applied to your soil to grow a very healthy crop of brassicas, but potatoes will react by becoming susceptible to potato scab in alkaline conditions, so they should not directly follow a crop of brassicas. Peas and beans grow happily in a soil with a ph reading of around 6 .75 so will follow a brassicas crop of the previous year. Onions crops do well in a soil of pH 6 -6.5 so follow on well from the peas and beans. Potatoes grow best in more acidic conditions with an ideal pH of around 5, which should be near the pH value of the plot after liming for the brassicas in year one.

Vegetable crop rotation scheme