Compost mixes

Materials alone or in combination are prepared and mixed to achieve a rooting environment that is free from pests and disease organisms and has adequate air-filled porosity, easily available water, and suitable bulk density for the plant to be grown. While lightweight mixes are usually advantageous, 'heavier' composts are sometimes formulated to give pot stability for taller specimens. This should not be achieved by compressing the lightweight compost, but by incorporating denser materials such as sand. Quick-growing plants are normally the aim and loosely filling containers with the correct compost formulation, consolidated with a presser board and settling it with applications of water obtain this. Firming with a rammer reduces the total pore space whilst increasing the amount of compost and nutrients in the container. The reduction in air-filled porosity and available water with an increase in soluble salt concentration leads to slower growing, harder plants (see conductivity).

The addition of nutrients must take into account not only the plant requirements, but also the nutrient characteristics of the ingredients used. Most loamless composts require trace element supplements and many, including those based on peat, need the addition of all major nutrients and lime. The Glasshouse Crops Research Institute developed general purpose potting composts based on a peat/sand mix (see Table 22.2). They contain different combinations of nutrients and consequently their storage life differs. One of the range of composts has a slow release phosphate, removing the need for this element in a liquid feed (see phosphorus). The GCRI seed compost contains equal parts by volume of sphagnum peat and fine, lime-free sand. To each cubic metre of seed compost is added 0.75 kg of superphosphate, 0.4 kg potassium nitrate and 3.0 kg calcium carbonate. Variations on these mixtures are

Table 22.2 GCRI composts

Potting composts

Urea formaldehyde types*

Constituents

Seed composts

Winter use

Summer use

(per cent by volume)

50:50

75:25

75:25

75:25

75:25

Base dressings (kg/m3)

Ammonium nitrate

Nil

0.4

Nil

Nil

0.2

Urea formaldehyde

Nil

Nil

0.5

1.0

Nil

Magnesium ammonium phosphate

Nil

Nil

Nil

Nil

1.5

Potassium nitrate

0.4

0.75

0.75

0.75

0.4

Superphosphate

0.75

1.5

1.5

1.5

Nil

Ground chalk

3.0

2.25

2.25

2.25

2.25

Ground magnesian limestone

Nil

2.25

2.25

2.25

2.25

Fritted trace elements (WM225)

Nil

0.4

0.4

0.4

0.4

  • Composts containing urea formaldehyde should not be stored longer than seven days.
  • For longer term crops where there is a risk of phosphorus deficiency and liquid feeding with phosphate is not desired, use commercial magnesium ammonium phosphate. This also contains 11 per cent K2O.
  • Composts containing urea formaldehyde should not be stored longer than seven days.
  • For longer term crops where there is a risk of phosphorus deficiency and liquid feeding with phosphate is not desired, use commercial magnesium ammonium phosphate. This also contains 11 per cent K2O.

formulated with alternatives to peat, taking into account their different properties particularly with regard to their particle size, water-holding capacity and final air-filled porosity.

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