Comparison of Soilless Compost Mixes

Comparison of soilless compost mixes

Kinsealy composts

These were developed by the Glasshouse Crops Department at the Horticultural Research Centre of the Agricultural Institute, Kinsealy, Dublin, Eire, after the UC composts had been found unsatisfactory. The present formulae for the Kinsealy composts are based on peat alone. Three basic composts are given, two specific tomato composts and a general potting compost. In addition, there is a tomato seedling compost, with a quarter of the plant food of the tomato potting compost. This division between the tomato and general composts would not appear to be too important because the nutrient content of both follow similar lines to composts from other organizations, the main difference lying in the proportion of readily available to slow-release nitrogen.

It is interesting to note that the Kinsealy formulae are the only ones to specify the use of kieserite as the source of magnesium. The other formulae used dolomite lime as the magnesium source, which also neutralizes the compost. Hence if kieserite is used, the rate of addition of ground limestone is higher than in a dolomite lime-ground limestone mixture, although the total amount of lime used will be similar.

Tomato seeds are sown in the seedling compost. The seedlings are then pricked out into the propagating compost where they remain until planting out. The tomato growing-on compost has a higher rate of potassium and magnesium addition and the magnesium source has been altered from kieserite to dolomite limestone which is available over a larger period of time. Owing to the high rate of addition of dolomite limestone, no ground limestone or chalk is required.

General potting compost was formulated after a wide range (over 70 species) of plants had been grown in it, including cucumbers. The basic formula is the tomato propagating compost, with half the rate of superphosphate, twice the amount of calcium ammonium nitrate and no source of slow-release nitrogen, so that the compost can be stored after mixing. Although higher rates of superphosphates have been tried, there has been no benefit reported by Kinsealy. It is interesting to note that this compost is similar in nutrient level to the GCRI compost as described.

Difficulty may be experienced with weighing out and evenly mixing the low trace elements in these formulae. The problems may be reduced by the replacement of these individual trace elements by 400g /m3 (10oz) of trace element frit 253A per cubic yard of compost.


GCRI mixes

These were first formulated by A. C. Bunt at the Glasshouse Crops Research Institute at Littlehampton in Sussex, England. There were three composts recommended at first, all adaptations of UC formulae: a seed-sowing or propagating compost based on equal volumes of peat and fine sand, and two 75 peat/25 fine sand composts for pricking out or potting on.

In the seed-sowing compost the only source of nitrogen was that readily available from potassium nitrate as seedlings are not in the compost long enough to require slower-release nitrogen. Most growers now use proprietary bases as recommended by manufacturers.

The two potting composts varied in the nitrogen reserve. The compost known as High N Reserve Compost used urea formaldehyde as the source of slow-release nitrogen and must be used fairly soon after mixing as it will not store. The organic nitrogen level is varied according to the season; the readily available nitrogen level is higher than in the comparable UC compost and, to give a balanced growth, the phosphate and potash levels are also higher.


GCRI Low N Reserve

Compost was developed as a compost that can be mixed and stored before use. It has a low level of readily available nitrogen and no slow-release nitrogen. Because of this, liquid feeding should commence earlier after potting up in this compost than in the High N Reserve Compost.

The GCRI potting composts use mixtures of dolomite lime and ground limestone for neutralizing the compost to a pH of 5.5, the dolomite lime supplying the necessary magnesium. They also contain fritted trace elements to rectify deficiencies of boron, zinc and manganese in the peat, as well as supplying copper and molybdenum.


Cornell lightweight composts (Peat-lite)

These were developed for the container growing of bedding plants at Cornell University, although subsequently they have been used for potting plants. Standardization was the main theme and this was achieved by including 25-50% vermiculite or perlite with the moss peat. This gave rise to the name of peat-lite for these composts, mix A containing vermiculite and mix B perlite.

The seed mix supplies nitrogen and phosphate as the major elements and boron and iron as trace elements, plus lime of course to neutralize the peat. Mixes A and B for growing on are fertilized with potassium nitrate and superphosphate but the potassium nitrate may be substituted by 1.6-8kg/m3 (2-12Ib per cu yd) of a slow release fertilizer. It must be remembered that the natural levels of potash and magnesium in vermiculite are high to supplement the added fertilizers but calcium content is variable. Because of this, ground limestone is used in preference to dolomite for the neutralizing of the compost to provide calcium. These mixes may be stored.


Peat-lite composts

These have undergone considerable modification in recent years and are now based on varying proportions of peat and vermiculite or perlite, usually 50-75% peat to 50-25% vermiculite or perlite, with slow release base fertilizers added at varying rates according to manufacturers’ recommendations, plus lime at 3oz per bushel (4 lb cu yd/2.4kg/m3), split between ground limestone and magnesium limestone.

If there are no trace elements in the base fertilizer these are added at 14g (1/2oz) per bushel/28g (10oz) per cu yd/ 400g/m3. (Frit 253A or WM255)

Problems may arise from the good drainage characteristics of the peat-lite composts, the water tending to run through the compost before it is completely wetted. Watering should therefore be done only lightly initially. Generally the formulae suggested for this compost seem rather vague.


Other formulations

Above are given the better known of the compost formulations but there arc many more, particularly from the various Experimental Horticultural Stations scattered throughout this and other countries. However, these are all rather similar to the GCRI type compost and it would serve no purpose to list them here.


Mixing composts — ready-made bases

The formulae given above can be rather bewildering for the commercial grower, let alone the amateur. Production of these composts requires the accurate weighing out of comparatively small quantities of fertilizers which have to be evenly mixed with large quantities of compost. This is not easily done and many of the composts used by the commercial growers are made from ready mixed compost fertilizers containing the major plant foods nitrogen, phosphates and potash with in some cases trace elements also. Most also include a balance between readily available and slow-release nitrogen. As a great simplification this reduces the chance of error, with usually a single weighing out of the fertilizer plus a further weighing out of the lime.


Lime in soilless composts

Lime is essential in a soilless compost and it performs two functions: it neutralizes the acidity of the peat and supplies the nutrient calcium to the compost.

Sphagnum moss peat recommended for soilless compost has a pH of between 3.3 and 4.3 approximately, but commonly in the narrower range of 3.8 to 4.1. For a 3:1 peat-sand compost, the pH recommended is 5.5, ie. approximately one pH unit lower than is recommended for a soil-based compost. If only the peat and sand are mixed together, the resultant pH will be about 4.3, assuming the use of an acid sand. To increase the pH to 5.5, 4.5g/litre (6oz) of ground limestone will have to be added per bushel of compost or approximately 4.8kg/ m3 (8lb per cu yd).

Ground limestone is chemically calcium carbonate. Chalk is also calcium carbonate but it is white and softer than limestone. Ground limestone is to be preferred for compost work as it will dissolve more slowly and maintain a reasonably consistent pH over the life of the compost. The grading (or size range) of the ground limestone is important: small particles will raise the pH initially but coarser particles (up to 1.5mm/1-1/16in) are needed to maintain the pH with time, Generally about ten days is required for the pH of the compost to reach 5.5. If the lime contains many fine particles, as in dolomite lime, the pH may reach 5.5 in only three days but the coarser particles must still be provided to maintain this level.

In some of the compost formulae, mixtures of ground limestone and dolomite are used. The reason for this is that dolomite provides magnesium, which may not be present in the other plant foods added to the compost. Dolomite is usually sold as a fine dust rather than as a graded material like ground limestone and it will therefore raise the pH of the compost initially quite quickly. All the formulae give mixtures of dolomite and ground limestone since ground limestone is necessary for the calcium. Both materials have similar neutralizing powers, so that in a formula containing 5.5k (91b) ground limestone per m3 (cu yd) of compost, 3.1kg/m3 (5lb) ground limestone and 2.4kg/m3 (4lb) dolomite may be used instead if no other source of magnesium is included in the compost-5.5g/litre (7-1/2oz) per bushel.

Finally, the adjustment of the pH in an all-peat compost is a simple matter if the pH of the peat is known. It is known that 600g/m3 (1lb) of limestone will raise the pH of 1 cu yd of peat by 0.3 of a pH unit. For example, if the peat has a pH of 3.5 and the compost pH required is 5.0 (the most suitable pH for an all-peat compost), the pH is to be raised by 5.0-3.5 = 1.5. Therefore the amount of ground limestone required per cu yd of peat is 1.5/0.3 or 3.1kg/m3 (5lb per cu yd) of peat. If a smaller quantity of peat is being mixed, 3.1kg/m3 (5lb per cu yd) is equivalent to 3g/litre (4oz) per bushel (Guideline figures only: peat pH varies greatly as do peat substitutes).


Reduction in compost volume of peat/sand mixtures

When peat and sand are mixed, there is a reduction in the volume of compost produced. For example, if a normal 3 peat/1 sand mix is being made and say 9 litres or bushels of peat and 3 litres or bushels of sand are mixed together, 12 litres or bushels of compost will not be produced since the particles of sand will tend to be absorbed into the spaces in the peat, resulting in an average reduction of one-sixth in volume. In the example quoted above, probably around 10 litres or bushels of compost will be produced and it is this volume that is used to calculate the addition of lime and fertilizer. If 4.5g/litre (6oz) ground limestone and 3g/litre (4oz) compound fertilizer were being added to each bushel of compost, then the mix above would require 10 x 4.5 (45g) 10 x 3 (3oz) 10 x 3 (3oz) of fertilizer. Unless the reduction in volume is allowed for, too much lime and fertilizer will be added, causing damage to the plants.

Therefore for the first mix of compost, the peat and sand should be measured out and then mixed. The volume of the compost thus produced should be measured and the amounts of lime and fertilizer to be added calculated. These same amounts may then be added to further mixes without measurement of the final compost volume as this will be similar for the same volumes of peat and sand initially.


06. April 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Organic Gardening, Soil Cultivation | Tags: , , | Comments Off on Comparison of Soilless Compost Mixes


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