After the growing medium is mixed, it is usually placed in a container. When soilless mixes were first developed, a wooden trough (15-20 cm deep and lined with polyethylene) was the most common container used. A drainpipe laid along the centre of the trough drained the water and acted as a duct for steam during sterilizing (Fig. 131. A layer of gravel provided general drainage and protected the polyethylene during cultivation. Since soilless mixes are naturally low in nutrients, fertilizers must be added to promote optimum plant growth. Two major methods have been used for supplying fertilizers to crops grown in peat media: the simplest is to add all the nutrients required by the crop when the peat mix is prepared (see Table 11).
0.5-mm polyethylene liner
15 mm gravel
0.5-mm polyethylene liner
15 mm gravel
50 x 25 mm supporting stakes at 2-m centres
Fig. 13 Trough culture of tomatoes.
Table 11 Ingredients for a complete soilless mix based on peat moss and vermiculite (1.0 m3)
Horticultural vermiculite Ground limestone (dolomitic) Gypsum (calcium sulfate 1 Calcium nitrate Superphosphate 20% Epsom salts (magnesium sulfate)
Osmocote 18-6-12 (9 months) Fritted trace elements
(FTE 503) Chelated iron (NaFe 138 or 330)
0.5 m3(2 compressed bales of 0.17 m3) 0.5 m3 (4.5 bags of 0.11 m3) 7.5 kg 3.0 kg 0.9 kg 1.5 kg 0.3 kg
* Expansion of compressed bales is estimated to be 50% above original volume.
The success of this technique depends on slow-release fertilizers to provide a continuous supply of nitrogen, phosphorous, and potassium throughout the growing season. The calcium requirements of the crop are met mostly by the calcitic limestone normally added for pH adjustment of the peat. Micronutrients are also available and are added as fritted trace elements, which ensures their slow release over the cropping season. The main advantage of this approach is that regular feeding throughout the cropping season is not required unless the presence of nutrient deficiencies indicates a need. Some of the most serious disadvantages of this procedure are the low level of nutrition control and the potential for crop failure caused by excessive salts that result from rapid breakdown of the delayed-release fertilizer at very high to medium temperatures.
A more popular approach to nutrition involves combining a base application of soluble fertilizer when the peat is mixed (Table 12) and adding soluble fertilizers at regular intervals through the irrigation system (see Table 131.
The vigor of a crop, and the balance between vegetative growth and fruit development, can be adjusted to some extent by the composition of the feed. For example, high-potash (1:0:3.5) feed is normally used to control growth for a short time after planting, when light conditions are poor. Conversely, high-nitrogen (1:0:1) feed is used to maintain adequate vigor throughout much of the summer, when light and productivity are high. A major difference between peat-grown crops and soil-grown crops in feeding requirements is the need for a regular supply of phosphate; this nutrient is readily leached from peat and has to be replaced to maintain adequate levels. Alternating a phosphate-containing feed (e.g., 1:0.5:2) with a standard feed such as 1:0:2 supplies phosphate throughout the season. Phosphorus can also be supplied continuously in the form of a special phosphate-containing feed, but this system necessitates supplying calcium or magnesium in separate feeds. Always remember that concentrated solutions containing calcium that come in contact with phosphate-containing solutions can result in insoluble calcium phosphate, which blocks the irrigation system. Likewise, magnesium sulfate should not be mixed in high concentrations with phosphate-containing feeds. Minor elements are generally provided in peat substrates as glass-fritted mixes
Table 12 Ingredients for a base mixture of peat moss and vermiculite (lm3)
Horticultural vermiculite Limestone (pulverized FF) Superphosphate 20% Potassium nitrate Magnesium sulfate Chelated iron 10% Borax (sodium borate) Fritted trace elements (FTE 503 or FTE 302)
0.5 m3 (2 bags of 0.17 m3)* 0.5 m3 (4.5 bags of 0.11m3) 5.9 kg 1.2 kg 0.9 kg 0.3 kg 35 g 35 g 110 g
Expansion of compressed bales is estimated to be 50% above original volume.
Table 13 Fertilizer feedings for crops grown in peat troughs
Type of feed
1:0:1 (potassium nitrate 65}
(calcium nitrate 45} 250:0:250:85:0 (ammonium nitrate 28}
Note: Fertilizer rates are in kilograms per 1000 L of stock solution. Stock solutions have been formulated on the assumption that a fertilizer injector with a feeding ratio of 1:100 (one part stock per 100 parts of water) is used to incorporate one stock solution at a time to the irrigation water. Alternatively, the recommended fertilizers in each feed can be dissolved in 100 000 L of water for direct application to the crop.
Medium potash with magnesium 1:0:2
Medium potash with phosphate 1:0.5:2
Medium potash 1:0:2 with calcium
High nitrogen 1:0:1 with magnesium
High nitrogen 1:0.5:1 with phosphate
High nitrogen with calcium that release their nutrients slowly over a cropping season. Trace-element deficiencies can be corrected by the application of chelated trace element mixes.
Chelates are applied either continuously in the liquid feed or as a foliar spray for corrective action. The rate used depends on the product. It is usually best to follow the manufacturer's recommendations. In general, the feeding guidelines given in this publication should be adequate for crops grown in peat substrate throughout the season. However, if nutrient levels in the substrate become too high or too low, it is possible to reduce or increase the strength of the liquid feed to compensate accordingly. Ideally, an injection system that is equipped with a continuous-monitoring and feed-back control unit should be available for optimum water and nutrient supply to the substrate in accordance with the needs of the crop, (see Plate 2).
The higher conductivity and potassium levels in the suggested ranges apply to the early part of the cropping season, when light is limited. If an initial peat-substrate analysis shows nutrient levels outside the ranges given, the medium may still be suitable for vegetable growing, with some modification to the feeding program to bring the nutrient status back within acceptable limits. After attaining an optimum analytical range for the peat substrate, devise a feeding program that maintains optimum nutrient levels in the substrate. In general, apply liquid feed at every watering, using a medium potash feed (1:0:2) that contains potassium at a nominal strength of 300 ppm.
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