Potting Composts and Plant Growing Mixtures



Plants in Pots

Potting Composts and Plant Growing Mixtures Growing mixtures for plants in pots are confusingly called ‘composts’, although they bear no relation to garden compost from a compost heap. These potting composts need special properties if they are to do their job well.

  • They must hold water, yet also drain freely enough to allow air to reach the plant roots. This balance is provided by the spaces or ‘pores in the mixture.
  • They must be stable enough to retain these pores even when they are packed into pots and continually watered from above.
  • They must contain sufficient nutrients in a small volume to support plants over as long a period as possible. However, too many readily available nutrients can cause toxicities and imbalances, leading to deficiencies and disorders.
  • Ideally, organic potting mixtures should be living mixtures, ie. they should contain some form of composted organic matter which will supply the bacteria and fungi needed for the continual release of plant nutrients.

If you are buying a compost mixture, then check what it is meant to be used for. A ‘potting’ compost should contain more nutrients than a ‘multi-purpose’ compost, which is for both sowing and potting. ‘Seed’ composts contain few nutrients. You will need a potting compost to support cropping plants, such as tomatoes or fruit trees. The alternative is to make your own compost, bearing in mind the properties it needs.

Most successful mixtures contain a combination of one or more stable base materials, such as leaf mould, to provide the structure and hold air and water, plus nutrient-rich materials, such as worm compost, the main function of which is to provide plant foods. Materials commonly included in DIY potting mixtures are listed in the chart on the post about green manures for the greenhouse. Several mixture recipes are also given above, although these should be considered as a guideline only. Try your own variations to suit your particular ingredients, your plants, and your growing conditions. In particular, test the pH of any mixture containing new ingredients: aim for a value of about 6.0. Also, be aware that different mixtures may require different watering and feeding regimes.


Sterilizing soil for potting mixes

The aim is not to kill all life in the soil but to heat it to a sufficient temperature for long enough to kill harmful organisms and weed seeds. The temperature and time are quite critical. If you make a lot of your own soil-based compost, then you might consider buying a small steam sterilizer. A 10 litre bucketful of moist soil takes twenty-five minutes to sterilize. To sterilize small quantities of soil, the following methods have been suggested, although not scientifically tried and tested.

  • Electric or gas ovens – Place moist soil in a thin layer on a baking tray and cook for thirty minutes at 80°C (this is a smelly process!). It is easy to overbake it.
  • Microwave oven – Place moist soil in a large bowl, loosely covered. Heat on full power: 0.9kg for two and a half minutes, 4.5kg for seven minutes. Thoroughly clean the oven and door seal afterwards.
  • Saucepan – Pour 330m1 water into a 3.5 litre saucepan. Heat and, immediately it boils, fill the saucepan with dry soil to within 2cm of the top; put on a lid and boil for seven minutes. Remove from heat, keep the lid on, and stand for seven minutes.

Whatever method you use, spread the soil out in a thin layer to cool as soon as the process is complete.


Storing organic composts

When potting mixtures containing organic matter or organic fertilizers are stored, the micro-organisms continue to work. In airless conditions, this can cause the build-up of substances that are toxic to plants. To avoid problems:

  • Make up your own composts not more than two or three weeks in advance.
  • Store them in a cool place.
  • Ensure that the bag is not totally sealed.
  • Air compost that has been stored for a month or more by spreading it out in a thin layer and leaving it for a day or two.


Other growing systems

Growing bags

Like pots, these give plants a limited root run and dry out very quickly. However, they can be extremely convenient. If you cannot buy organic growing bags, then you can make them yourself by filling a plastic sack with a potting compost mixture.


Ring culture

This is a system used commonly for tomatoes. They are grown in bottomless pots (rings) of potting compost sitting on a 15-20cm layer of clean gravel into which the plant roots grow. This avoids the problem of disease build-up in the soil and gives plants more root run to take up water.


Hot bed

This is a raised bed of manure and soil traditionally used for cucumbers and melons. It provides extra heat as well as plant foods.


Straw bales

Both tomatoes and cucumbers can be grown in straw bales which have been soaked with water and given a high nitrogen fertilizer to start them decomposing. If you try this method, then make sure that the straw you use has not been treated with a chemical weedkiller near to harvest, as this can harm your plants.


Potting compost recipes

In all recipes, add ground limestone, dolomite, or  calcified seaweed to bring the pH to 6.0.

  • 2 parte matured garden compost, 1 part loam, 1 part bark.
  • 3 parts comfrey leaf mould, 2 parts three-year-old leaf mould. To every 10 litres, add 33g hoof and horn, 33g seaweed meal, and 33g bone meal.
  • 4 parts loam, 2 parts leafmould or coir. To every 10 litres, add 40g seaweed meal and 20g bonemeal.
  • 3 parts leaf mould, 1 part worm compost.


28. November 2010 by Dave Pinkney
Categories: Greenhouse Gardening | Tags: , | Comments Off on Potting Composts and Plant Growing Mixtures

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