In order to keep the soil and plants in your polytunnel healthy while growing annual crops, it is important to try to operate a system of crop rotation. If you are new to polytunnel gardening then it may all seem a little complicated. Don't worry though, once you understand the issues involved, it is a relatively simple matter to work out a crop rotation plan for your polytunnel.
Usually, it is best to have a crop rotation system that will operate over three or four years. The items grown will be separated into either three or four groups. Each of these groups will be grown in one or three or four areas. Each year, you will switch around the groups so they are grown in a different growing area. By continuing to rotate the groups of crops around the different parts of your polytunnel, you will ensure that you do not grow any of the groups of crops in the same part of the tunnel for three or four years.
The groups of plants that it is most important to include in a crop rotation system are: legumes, brassica and tomatoes (and other crops in the nightshade family such as potatoes). Legumes do not actually require the moves themselves, but rather should be moved about in order to take advantage of their nitrogen fixing properties.
Legumes, such as peas and beans, will leave nitrogen in the soil which can be taken up by the next plants to occupy the space. Brassica will love utilising this additional nitrogen, as will other leafy plants. Brassica, such as cabbage, broccoli or cauliflower, can get diseases which will remain in the soil. Moving them year on year will help to reduce the incidence of such problems. Leafy vegetables should usually move into the bed vacated by your nitrogen fixing legumes.
Tomatoes and potatoes etc. also suffer from diseases that can stay in the soil. Growing them in the same bed year on year can lead to more such problems. Equally, since they can share diseases, it is best not to plant tomatoes just after potatoes, or vice versa. If you are planting beds for the first time, potatoes can be useful to break up the soil – followed by legumes and then brassicas for the simplest three year system.
When you choose to plant in polycultures (collections of more than one crop in each growing area), which is good for organic growing, complexities do creep in. But when it comes to crop rotation, you can still follow the same principles.