Peat bags

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Plastic bags filled with a peat-based medium are now generally available. Each peat bag, which measures 35 cm x 105 cm when flat and contains 42 L of fertilized peat (or a mixture of peat with vermiculite, perlite, or polystyrene), can support up to three tomato plants as long as regular watering and fertilizing through a drip irrigation system are provided.

Cover the greenhouse floor with polyethylene film (Plates 3,4) and lay the bags on it. Some growers use a double-layered polyethylene material as a floor covering, with a black bottom layer to prevent weed growth and a white top layer to reflect sunlight into the crop canopy Make three or four 4-cm slits in the sides of the bags to provide drainage after the medium is wet. The planting depth in the peat substrate is an important factor that affects later growth. The shallower the depth of peat, the more critical the planting depth becomes, especially if a permanent reservoir of water is present, making part of the peat bag unavailable for active root growth. This water reservoir is at the base of the peat bag and is developed by positioning drainage holes above it. A minimum substrate depth of about 10 cm, a planting depth of 2.5 cm, 5 cm of drained peat beneath the pot, and a water reservoir of 2.5 cm below the drainage level are recommended. Only two aspects of the general culture in peat bags are different from those of soil: watering and feeding. Watering crops grown in peat is easy, provided some basic rules are followed, the moisture content of the peat substrate is examined frequently, and appropriate action is taken when indicated. In fact, crops grown in peat may be easier to water than crops grown in soil because the moisture content of the latter is more difficult to assess, and the drainage characteristics of the soil and subsoil make decisions on watering less certain. Because peat bags contain only a small volume of growth medium, they offer a much lower water-holding capacity than most soils. Failure to apply water when it is needed can therefore have a more rapid detrimental effect on the crop than with soil-grown crops. The following rules are recommended for watering crops grown in peat bags:

  • Use a drained peat bag with a water reservoir beneath.
  • Provide additional irrigation outlets to areas that need extra water.
  • Maintain an efficiently operating irrigation system by preventing or clearing blockages as soon as they occur
  • Check the moisture level of the substrate frequently and modify the watering regime if necessary.
  • Vary the frequency of watering rather than the quantity applied each time, so that the substrate is aerated between waterings and a uniformity of moisture content is maintained from one bag to another.

Problems arising from a faulty watering program can be classified as waterlogging, excessive drying back, and excessive variation of moisture from bag to bag. Waterlogging is easy to detect, as it results in slow growth and thin plant heads. When this problem is more serious the plants develop yellow heads characteristic of iron deficiency. Waterlogging problems usually develop when the watering regime does not allow enough time for proper soil aeration between applications. An excess amount of water applied on one occasion may not matter, as the surplus drains to waste, but a second application made before the substrate has dried to its normal minimum water content reduces root action and starts the cycle of waterlogging. Regular and frequent checks to control water frequency help to avoid this problem. Once waterlogging has occurred and the plants are showing symptoms, correcting the problem is a slow process; hold back the water to the substrate until it has dried to its normal minimum level, however long this might take. Invariably, some crop yield is lost while the problem is being corrected.

The problem of excessive dryness is equally serious but just as easy to avoid, provided the irrigation system works effectively and sufficient time is allocated to manage the watering program. If the medium is often allowed to dry to below the normal minimum water level, when water can no longer be squeezed out by hand, plant growth will be impaired, especially if the salt content of the medium is high. Media that are frequently allowed to dry too much also cause a general stunting of growth and considerable yield loss. The remedy is easy-apply more water by increasing the frequency of irrigation. The initial recovery may take several days; nothing can be gained by applying large volumes of water at every single irrigation, as most of it will run off to waste.

The third potential problem is excessive variation in water content within the crop. The application of water can never be accurate enough to cover all variation within .a crop, and extreme imbalances can develop. In addition, fast-growing plants can produce their own localized water deficiency problems, and weak, diseased, or removed plants can precipitate local waterlogging. Where the problem is not extensive, rebalance a crop by occasional hose watering to top up dry areas and by temporarily removing one or more irrigation outlets from areas of waterlogging. Occasionally inducing waterlogging can prevent or correct large-scale water imbalance, but use the technique only on an actively growing crop with a strong root system. This practice is also valuable for leaching out excess salts from the substrate. As a general irrigation rule, apply water until the driest area of the crop has recovered its full water requirement at each application. In this way, water is prevented from building up, and areas of substrate with a lower water requirement drain off any surplus without danger.

Anyone considering peat substrate culture of greenhouse tomatoes for the first time is aware that watering requires considerable managerial effort and a dependable irrigation system, and might well decide that the risk of mistakes does not justify the change from soil to peat substrates. However, the fact that water management errors in substrate culture are quickly manifested into visible symptoms makes peat substrate and other soilless culture systems attractive. In soil culture, incorrect watering usually becomes obvious only after the crop has changed its growth habit significantly. Consequently, although soil has a greater water-holding and buffering capacity, greater crop losses can still be incurred without the grower's awareness of any mistakes having been made. A competent grower in substrate can see potential errors in irrigation when they first appear in the peat mixture and correct them before they have any effect on the plants. The recent development of computerized irrigation controllers equipped with properly adapted soil water tensiometers has made the scheduling of irrigation of crops grown in peat bags much simpler and has resulted in significant water and nutrient savings while minimizing excessive nutrient leaching into the environment.

Nutrition is the other major area in which peat-grown tomatoes differ significantly from tomatoes grown in soil. Peat substrates have a much lower buffering capacity than most soils in relation to both major and minor elements. The grower therefore needs to monitor continually the nutrient availability in the substrate and to adjust accordingly the composition of the feed applied to it. This work requires a rapid and reliable analytical service, and a dependable and accurate technique for frequent application of fertilizers. The results of peat substrate analysis enable corrective action to be taken for an optimum root environment before any adverse symptoms are observed in the crop; to depend on crop symptoms alone for determining a necessary change in the feeding program greatly increases the risk of yield loss. As important as the analytical service is the ability of the grower to interpret the results and take any corrective action needed. Although the initial nutrient levels in peat substrates vary according to the supplier, the source, and type of peat used, the ranges in Table 14 can be regarded as normal and should be used only as a guide.

The results of peat analysis vary according to sampling and analysis procedures. The comparability of any peat-sample analytical results with the guidelines in Table 14 therefore depends on the following conditions:

  • A peat sample should be taken from the full depth of substrate in the bag.
  • The sampling point should be near a growing plant and should extend through the rooting zone.
  • Several samples should be taken throughout the greenhouse area to be tested and mixed together to supply at least 0.5 L of substrate for analysis.
  • Samples should not be taken immediately after watering or from areas that are clearly wetter or drier than the average for the house.
  • Peat substrate samples taken as described above must be brought to a uniform water content by either adding distilled water to them or allowing them to dry out as needed before proceeding with analysis. The release of a small amount of water after a handful of peat has been squeezed moderately indicates a desirable water content.
  • All analytical tests are performed on an aqueous suspension of the peat substrate sample, at a peat-to-distilled-water ratio of 1:1.5, by volume. Experimental and commercial evidence suggests that the peat substrate can be recycled without a reduction in yield. However, the cropping potential of recycled peat substrate can be influenced by the following factors:
  • the level and uniformity of nutrients in the peat
  • the salt level in the medium
  • the pest and disease status of the substrate.

Table 14 Desirable nutrient levels in the substrate of peat bags, based on a substrate-to-water dilution of 1:1.5

Nutrient Concentration

If growers plan to reuse the bags they should reduce the strength of the fertilizer feed by half, starting about 6 weeks before the planned termination of the first crop, and should apply plain water during the last 2-3 weeks. This extended period of gradual nutrient leaching allows the nutrient levels in peat substrates to be reduced. The degree to which the nutrient levels are reduced varies with the ease with which they are leached. For example, in a well-leached substrate the nitrate level is very low, the phosphate and potassium levels are low, and the calcium and magnesium levels remain high. To minimize the problems caused by a lack of uniformity in the nutrient content of reused peat bags, sterilize the leached peat medium in bulk. After sterilization, analyse the peat medium and add base fertilizers as needed before rebagging. The principles of steaming are similar for both soil and peat. The objective is to destroy harmful organisms while preserving most of the beneficial organisms and nutrients, without allowing salts to build up. Excessive steaming should therefore be avoided; raising the temperature through the substrate to 82°C for 20 min is all that is needed. As with soil, the peat should not be too wet or too dry at steaming, otherwise the cost of the operation is unnecessarily high or the efficiency of the operation unnecessarily low. Because of the small amount of peat substrate used in a greenhouse, compared with soil, both the energy and labor expended in steaming peat are considerably less.

Nitrogen (nitrate)





Acidity (pH)

Electrical conductivity (|j.S/cm)

30-80 20-50 140-400 140-200 25-35 5.5-6.6 1000-2500

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