Polytunnels tend to be cultivated intensively; huge crops are expected from a small space and overcrowding can be the rule. All this puts strain on the soil, but there are solutions that can improve matters, short of the huge task of removing all soil and importing a fresh batch.
It’s best to keep soil in good health on an ongoing basis rather than waiting for the signs of exhausted soil, or a disease problem that has got out of hand.
1. Practise crop rotation, as outlined on the previous page. Just a little planning will give plants a better chance.
2. When removing crops, try to clear all debris and leave soil completely bare. One squashed tomato can produce a batch of seedlings that will carry disease forward into a new crop. Gather up all dried and diseased leaves and pull up roots. Don’t cherish legume roots in the polytunnel, even though they may add a little nitrogen.
3. Allow as much sunlight as possible into the structure, as this can have a cleansing effect. Wash down the inside of the polythene with a dilute Citrox solution to kill off spores without harming plants. Remember that the frame also needs scrubbing down.
4. Add manure and compost. Both of these provide an obvious nutrient boost and help restore depleted soils. Dig in manure in the autumn, so that it has time to break down completely before you plant spring and summer crops. Compost contains microbes which compete with organisms in the soil that are responsible for some plant diseases. You can make compost tea by adding a shovelful of compost to a bucketful of water. Stir well and leave this to stand overnight before pouring it on to the soil. Water the whole soil’s surface in this way, if possible.
Compost tea
5. Add some seaweed to cleanse the soil. Use fresh seaweed as a mulch, make a liquid feed or scatter powdered meal over the surface of the soil. Fresh seaweed appears to have the perfect balance of minerals, nutrients and microorganisms needed to help restore depleted and diseased soils. Seaweed can be bought as a powder or a concentrated liquid. Both of these will be of great nutritional benefit, although they obviously don’t have the same microbial action as the fresh weed.
Seaweed is great for soil health
6. Alternate drought and flooding to confuse bugs. Allow empty borders to completely dry out. Leave them this way for two weeks, and then flood to saturation point. Repeat this process at intervals over the winter months. Few inactive pests and diseases can tolerate such extremes, especially if the top layer of soil gets powder dry. Earthworms will be safe enough, since they move to deeper levels in the soil or simply shift sideways to a better spot. This is not a cure-all, but it helps!
Seawater can work miracles if the state of the soil is really bad. Use this to flood the soil instead of water from the tap.
7. Keep the surface of the soil moving. During the cold wet months of the year, soil can develop a green crust. Keep hoeing to break this up. Aeration of the soil’s surface is a very old rule of gardening. It used to be called ‘tickling the soil’ and is of real benefit in combating the stagnant conditions that can develop in a polytunnel.
8. Encourage wildlife! Frogs eat a variety of pests, as do ground beetles. Thank heavens for ladybirds and hoverflies! Don’t indiscriminately kill all living things just because it’s hard to tell which ones are friends and which are foes.
Frogs are friends and should be encouraged
9. Choose disease-resistant varieties. Search in the organic sections of seed catalogues: there are varieties bred for all kinds of resistance to disease. Growing these will help break the cycle and will reduce the build-up of problems in the soil.
10. Buy a soil tonic such as Revive, Garden N or Rootgrow. These are packed full of microorganisms to help plants grow better even in a poor soil.