Written by Spirekassen 27 Sep 2023 08:13

Reusing Soil for Winter Cultivation

As you may have noticed that autumn is upon us. The once-bright summer evenings have grown dimmer, and the tomatoes in the greenhouse are taking longer to ripen, with dew forming on the greenhouse glass. Autumn has officially begun.

If you haven't yet tried winter cultivation in your greenhouse, now is the perfect time to start. It's a fairly straightforward process. Just because winter is approaching doesn't mean your cultivation has to end; you'll simply need to focus on growing cold-resistant crops.

You probably have some leftover growing bags or other soil from your summer cultivation. Most of this soil can be easily reused for winter cultivation.


Quality matters most

It's important to note that all cultivated soil will break down over time, and the soil structure will gradually deteriorate as organic matter decomposes. You can almost see the soil sinking a couple of centimetres each year with the naked eye due to natural decomposition. Therefore, it's best to reuse high-quality soil. Quality soil is characterized by its stable structure over an extended period of time, which is crucial for successful plant cultivation across multiple seasons.

  • A stable soil structure ensures that nutrients are available to the plants.
  • It provides sufficient oxygen for plant roots to breathe.
  • It ensures there is enough accessible water for the plant roots.

Price and quality are closely linked when purchasing soil in the spring. Cheap, low-quality soil often loses its structure during the summer and isn't as suitable for recycling, as it's typically designed for short-term cultivation.


Soil recycling

If you're not cultivating in raised garden beds in the greenhouse, there's a good chance you're using grow bags, pots, or self-watering containers. Regardless of the container, a large portion of the soil can be reused. At Spirekassen, we've been using soil from summer cultivation for winter planting for several years. After removing the greenhouse tomatoes, we loosen the soil and remove larger roots. Then we place the soil in seed trays and pots, where we cultivate Asian cabbage, cold-resistant salads, and chicory. Recycling the soil and cultivating during the winter is an eco-friendly choice.


Cultivating in self-Watering boxes

If you're using self-watering boxes, here's what you can do: Cut off the old tomato plants, cucumbers, chili peppers, etc., at the roots, leaving the growing bag on the box. After that you can simply sow, for example, salad seeds around the old roots of tomatoes and chili peppers. The salad seeds will quickly sprout, and you can almost be self-sufficient until the new year. If you choose the right salad varieties and sow them in the autumn, the plants will easily tolerate several degrees of frost without any issues. Avoid using the self-watering function in the container during autumn and winter; instead, water your salad from above.


pH-levels in recycled soil

Most purchased soil typically has a pH-level of 6, which is slightly acidic. However, during the summer season with cultivation, the pH-level often shifts to around pH 7 or higher, which is normal. To lower the pH level to a range that plants prefer, you should water with rainwater during the winter. Most rainwater is naturally slightly acidic with a low pH-value. Over time, the soil will regain its balance benefiting your plant growth.


Winter cultivation doesn't require fertilisation

It's not crucial for the recycled soil to have a high level of nutrients for winter cultivation. During the winter, plants have reduced needs for fertilisation, as other factors like light, warmth, and nutrients become limiting factors. The Law of the Minimum for plant growth dictates that the growth factor in the shortest supply limits the utilisation of other growth factors.

These growth factors include light, water, CO2, oxygen, and nutrients (vitamins and minerals). Throughout the winter, we experience deficits in both light and warmth in an unheated greenhouse. Therefore, winter-cultivated plants do not benefit from fertilisation when light is limited, and temperatures are low.


Valuable tips for recycling soil for winter cultivation:
  • Use high-quality soil for recycling.
  • Irrigate with rainwater.
  • Use less water than during the summer.
  • Avoid using fertiliser.