How To Grow in Bags Using Soil as The Media

How to grow in bags using soil as the media/how to treat the soil/what kind of fertilizer systems

Potting mix

The available components for a potting mix may include coco peat, manure/compost, pumice/sand and soil. These can be mixed in equal ratios. Coco peat is meant to increase the water-holding capacity of the mix, pumice/sand improves drainage and soil increases the weight which reduces container tip-over while manure enriches the soil. The potting bag should have perforations at the side bottom to allow drainage. The soil used in the pots should be preferably collected from a virgin land or land that has been fallow for some period. The portion should not have had solanaceae crops grown on it.

Potting mix sterilization

In case the source of soil is suspected to be infected, then the potting mix can be sterilized through steam sterilization. This is meant to kill weed seeds, diseases and pests in the media.

Heat treatment of soil or potting mix involves reaching a target temperature for a specific minimum period of time. In general, lower temperatures require a longer heating time to achieve the same degree of kill of plant pathogens. Moist heat is also more effective than dry heat at any given temperature.

The entire mass of soil or potting mix needs to reach the desired temperature for the minimum treatment time. Begin timing when the coolest area of the treated mix reaches the target temperature. Uniformity of heating can be affected by a variety of factors, including variation in compaction, moisture, clods, bin shape, and method of heating. The coolest part of the heated mix may be at the center or near the edge, depending on these factors.

Target temperatures needed to kill specific organisms for moist soil or potting medium heated to the target for a minimum of 30 minutes:

Moist soil, 30 minutes at Organisms killed
120F (49C) water molds (oomycetes)
145F (63C) most plant pathogenic fungi, bacteria, and viruses, worms, slugs, centipedes
160F (71C) plant pathogenic bacteria, soil insects
180F (82C) weed seeds
212F (100C) heat resistant plant viruses and weed seeds


The greenhouse floor should be covered well with a polythene paper, if possible double polythene to avoid contamination from the soil. The potting bags should be spaced to achieve the correct spacing of tomatoes in greenhouse of 60cm between the rows and 40cm between plants preferably arranged in a zigzag manner.

The polythene pots used measure 12 inches (30cm) in diameter and 16 inches in height. Therefore to achieve the required spacing, the tomatoes are planted at the center of the pots thereby allowing 15cm on either side of the pot. Therefore if the pots are spaced 10 cm apart, the spacing of 40cm will be achieved.

Before transplanting the seedling, make a small hole with your finger on the spot where you want to plant.  It should not be too deep, try to mimic the exact size of the plug. Then gently put the seedling inside, press down softly. Don’t cover the plug with some more substrate

One tomato plant is planted per potting bag. Drip lines to be used should have their water emitters inside the bags. They should also be raised to be at the same level with the potting bags. Alternatively extended drippers can be directed from the main drip line into the potting bags, in this case the main drip line does not have to be raised.

After planting you need to establish a program for weekly application of fertilizer based on the soil analysis



Take a soil sample before transplanting and have the soil tested. The fertilizer program can then be governed by the results of the soil analysis. The optimum soil pH (acidity) is between 5.5 – 6.8. Soils below 5 should be limed to raise the pH and to supply the necessary calcium tomatoes need.

Some soils have a high pH (6.5 – 6.8) but are low in calcium. Regular agricultural lime would raise the pH too high and could possibly tie up other nutrients, especially the micronutrients. On these high pH soils, use gypsum or CaSO4. It will not raise the soil pH, but it will supply the needed calcium.

Application of fertilizer through fertigation will not be difficult as it will be chanelled through drip lines, however one has only to ensure that the EC and pH are right, that is EC range of between 1.5 and 3 and pH range of between 5 and 6.8. However for granular fertilizer, then calculations have to be done starting from the total amount of particular fertilizers required for the whole green house, then narrowed down to per square meter and then finally to a sing le plant. After this an appropriate measure can be devised to apply the required amount of fertilizer per plant.


  • It is important to always refill the pots with fresh soil at the beginning of each season for better results. Diseases that are likely to be controlled by potting include bacterial wilt, fusarium wilt and root-knot nematodes among others.
  • With the banning of plastic paper manufacture and usage in Kenya which may also affect potting poly bags, we shall need to discuss other containers that shall take their place keeping in mind factors like availability, affordability and durability.


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