Table 1. Examples of common root substrates and their physical propertiesz.

 Root substrate

 Bulk density (g.cm-3)

 Total pore space (% v/v)

 Air-filled pore space (% v/v)

Water-holding capacity (%v/v)

80 peat: 20 perlite

0.108

79

10

68

70 peat:30 perlite

0.101

80

11

69

80 peat: 20 bark

0.090

83

13

70

80 peat: 20 PBH

0.098

83

12

72

80 coir: 20 perlite

0.090

81

8

73

70 peat: 30 PBH

0.098

86

20

85

50 peat:
50 pine bark

0.130

80

22

58

50 coir: 50 pine bark

0.140

78

13

65

50 peat:
25 vermiculite:
25 perlite

0.110

86

24

62

50 coir:
25 vermiculite:
25 perlite

0.100

77

12

65

40 peat:
30 vermiculite:
30 bark

0.160

82

28

54

40 coir:
30 vermiculite:
30 bark

0.120

84

21

63

z Properties are estimated for substrates in 4-inch containers. Air-filled pore space and water-holding capacities will depend upon container size.

Table 2. Purposes and Commonly Recommended Rates for Amendments Used in Root Substrates

Amendment

Purposes

Commonly used rates

Calcitic limestone

Increases pH and provides calcium

10 – 15 lbs.yd^-3

Dolomitic limestone

Increases pH and provides calcium and magnesium

10 – 15 lbs.yd^-3

Hydrated lime

Rapidly increases pH

0.5 – 1.0 lbs.yd^-3

Aluminum sulfate

Lowers pH and provides aluminum and sulfur

0.5 – 1.0 lbs.yd^-3

Iron Sulfate

Lowers pH and provides iron and sulfur

0.5 – 1.0 lbs.yd^-3

Superphosphate

Provides phosphorus

2.0 lbs.yd^-3

Treble superphosphate

Provides phosphorus

2.0 lbs.yd^-3

Calcium sulfate (gypsum)

Provides calcium and sulfur without increasing pH

1.5 – 2.0 lbs.yd^-3

Magnesium sulfate (Epsom salts)

Provides magnesium and sulfur. Sometimes used in nutrient starter charges.

0.5 – 1.0 lbs.yd^-3

Calcium nitrate

Provides nitrogen and calcium. Sometimes used in nutrient starter charges.

1.0 lbs.yd^-3

Potassium nitrate

Provides nitrogen and potassium. Sometimes used in nutrient starter charges.

1.0 lbs.yd^-3

Ammonium nitrate

Provides nitrogen. Sometimes used in nutrient starter charges.

1.0 lbs.yd^-3

Urea

Provides nitrogen. Sometimes used in nutrient starter charges.

1.0 lbs.yd^-3

Microelement fertilizers

Provides micronutrients.

Follow label rate.

Wetting agents

Improves wettability of substrate

Follow label rate.

^Z 2.5 to 3.0 lbs.yd^-3 of calcitic or dolomitic limestone will generally result in approximately a 0.5 unit increase in pH. The exact change in pH will depend on the properties of both the limestone and the peat.

^Y Avoid using hydrated lime to rapidly increase pH if the substrate contains manures or significant concentrations of ammoniacal nitrogen as this can result in the evolution of phytotoxic ammonia.

^xAluminum sulfate reacts rapidly, iron sulfate has a moderate reaction rate while elemental sulfur reacts slowly. Rates given for these three materials are rates that will generally result in a pH decrease of 0.5 to 1.0 units depending upon the properties of the substrate.

^w Different commercial products contain different concentrations of mineral elements and may react differently in the substrate. Always follow label directions for all products. In some cases growers may choose to incorporate microelements at half the recommended rate in the unused substrate and apply additional microelements through the fertilization program.

^v All wetting agents are not alike. Different materials may react differently in the substrate. Always follow the label directions.

^U The rates listed in this table are commonly recommended rates are meant to be informative and not to serve as specific recommendations. This is because the properties of the specific amendment, the substrate characteristics and the environmental conditions will all impact the exact response obtained from the amendment. Always experiment with amendments before using them on a large scale.

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