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Physical properties of media for container-grown crops. II. Peat mixes
Scientia Horticulturae, 10 (1979) 325--330 Elsevier Scientific Publishing Company, Amsterdam --Printed in The Netherlands
325
P H Y S I C A L P R O P E R T I E S O F M E D I A F O R C O N T A I N E R - G R O W N CROPS. II. P E A T M I X E S
M. PRASAD Horticultural Research Centre, Research Division, Ministry of Agriculture and Fisheries, Levin (New Zealand) (Received 6 November 1978)
ABSTRACT Prasad, M., 1979. Physical properties of media for container-grown crops. II. Peat mixes. Scientia Hortic., 10: 325--330. The effect of addition of sand, perlite, polystyrene and pumice in half and quarter proportions and in various particle sizes, on physical properties of peat mixes was studied. Coarse sand had no effect on air space (AS), while fine sand reduced AS and at the higher rate the value fell to an unsatisfactory level. At the higher rate of coarse sand addition, easily available water (EAW) fell to an unsatisfactory level. Fine sand increased EAW. Total porosity (TP) and water-buffering capacity (WBC) were generally reduced as a result of sand addition. The addition of coarse perlite at the higher rate increased the AS, while fine perlite at both rates and medium perlite at the high rate reduced AS. The effect of addition of coarse and fine perlite on EAW was similar to sand addition. Total porosity was reduced by coarse perlite and by medium perlite addition at the higher rate. The addition of polystyrene increased AS, but decreased EAW and the value fell to an unacceptable level at the high rate of application. Bulk density, WBC and TP were decreased by the addition of polystyrene. The addition of a standard grade of pumice had no effect on AS, but the EAW, WBC and TP were reduced. However, only in the case of WBC was the reduction below the critical level.
INTRODUCTION
S e c o n d a r y c o n s t i t u e n t s , including sand, perlite, p o l y s t y r e n e a n d p u m i c e , are o f t e n m i x e d with p e a t in the p r e p a r a t i o n o f growing-media. In N e w Zealand the additions, in m a n y cases, are n o t t o i m p r o v e the physical p r o p e r t i e s o f the c o m p o s t b u t t o save o n the a m o u n t o f peat, a relatively expensive material. Inf o r m a t i o n on the effects o f a d d i t i o n o f these materials on the physical properties o f g r o w i n g - m e d i a is f r a g m e n t a r y ( J o i n e r and Conover, 1 9 6 5 ; White and Mastalerz, 1 9 6 6 ; Boggie, 1 9 7 0 ; Brighton, 1 9 7 3 ; Bunt, 1 9 7 4 ; G o h and H a y n e s , 1977). The objective o f the p r e s e n t investigation was, t h e r e f o r e , t o s t u d y t h e e f f e c t o f additions o f sand, perlite, p o l y s t y r e n e and p u m i c e in d i f f e r e n t p r o p o r t i o n s t o peat, and in s o m e cases the e f f e c t o f d i f f e r e n t particle sizes, o n physical properties o f p e a t mixes.
326 MATERIALS AND METHODS Details o f the techniques used for the determination of physical properties of the mixtures used in this e x p e r i m e n t are given by Prasad (1979). Particle sizes of the secondary materials used are given in Table 1. All were added to the peat at the rate of '~ or 'h on volume basis and then mixed. In order to avoid bias in the control samples (peat with no additions), these were handled in the same way as the peat mixtures, so t ha t any b reakdow n of peat during mixing would also have occurred in the control samples. Before the samples were packed into containers for a moisture r et ent i on determination, t h e y were brought to a moisture c o n t e n t equivalent to pF 1.5 -- 2.0 by eye estimation. The peat pumice mixtures were packed only at one density, 0.5 kg cm 2, the o t her mixtures were packed at 0.5 or 0.1 kg cm -2.
TABLE 1 Particle size ranges of secondary constituents added to peat Materials
Coarse perlite Medium perlite Fine perlite Coarse sand Fine sand Pumice Polystyrene chips'
% of material in the particle range < lmm
1--2mm
> 2mm
0 33 50 0 50 37 0
50 33 25 50 25 35 5
50 33 25 50 25 28 95
' 63% was greater than 3.18 mm and 32% greater than 4.4 mm. RESULTS AND DISCUSSION The o p t i m u m levels of air space (AS), easily available water (EAW), waterbuffering capacity (WBC) and total porosity (TP) were defined by De B o o d t and Ver d o n ck (1972) and these were taken as points of reference. There were shrinkage problems with the lighter packings, especially when fine materials were mixed into the peat. The results obtained from lighter packing were th er ef o r e omitted. In general, the results obtained from lighter packing gave similar trends to those obtained with heavier packing. A d d i t i o n o f sand t o p e a t . - - The addition of coarse sand (CS), with relatively
high AS, had little or no effect on AS (Table 2). Bunt (1974) f o u n d that increasing the percentage of grit in peat mixes from 25 to 50 had no effect on AS. The fine sand (FS) with a low AS reduced the AS of the peat mixes considerably and at the 50% addition rate the value fell to unsatisfactory levels.
327
TABLE 2 T h e e f f e c t o f a d d i t i o n of sand o n physical p r o p e r t i e s o f p e a t mixes. Values in t h e same c o l u m n f o l l o w e d b y t h e same l e t t e r s are n o t significantly d i f f e r e n t a t t h e 5% p r o b a b i l i t y level Treatment
Air space (% v o l u m e )
Easily available water (% v o l u m e )
WaterTotal Bulk buffering porosity density capacity (% v o l u m e ) (g/cc) (% v o l u m e )
Hauraki p e a t ~4 H a u r a k i + 1A CS 1/2 H a u r a k i + 1/2 CS ~A H a u r a k i + IA F S 1/2 H a u r a k i + 1/2 FS CS FS
30.5 27.8 31.4 21.9 12.3 23.9 5.8
25.8 23.0 17.4 32.9 33.5 7.6 20.1
4.3 3.5 1.9 3.1 1.8 0.9 1.9
d d d c b c a
d cd b e e a bc
e d b c b a b
88.1 76.4 64.1 80.8 68.5 36.5 35.3
f d b e c a a
0.150 0.506 0.909 0.568 0.995 1.780 1.758
a b d c e f f
These results agree with the findings of Boggie (1970), Bunt (1974) and Goh and Haynes (1977). EAW was reduced by the addition of CS (not significantly at ~Arate). At the higher rate of CS application, EAW fell to unsatisfactory levels. The addition of fine sand (FS) to peat increased the EAW. Bunt (1974) found that EAW was higher in mixtures of peat and fine sand than in peat and grit. Water-buffering capacity was reduced as a result of sand addition. Goh and Haynes (1977) also reported reduction of WBC as a result of addition of sand to peat. Total porosity was reduced b y CS and FS application; this reduction was greater at the higher rate of application. These results agree with the findings of Boggie (1970) and Bunt (1974). The results showed that when CS or FS was applied to peat at 1~ rate, a mix with favourable physical characteristics for container-grown plants resulted. In addition, due to an increase in weight, a greater stability resulted, which was advantageous to tall plants in windy situations. A d d i t i o n o f p e r l i t e to p e a t . ~ There was an increase in AS b y the addition of
coarse perlite (CP) at the higher rate; with medium perlite (MP) at the low rate there was no change, while at high rate the AS was reduced (Table 3). Fine perlite (FP) reduced AS. Joiner and Conover (1965) found that by increasing the volume of perlite from 58 to 83%, the drainable pore space was increased. Similarly, White and Mastalerz (1966) found that AS (pF 1.2) was increased as a result of addition of perlite (approx. size -+ 3 mm) to peat. There was a reduction of EAW as a result of addition of CP and this was more evident at the higher rate where the levels were below optimum. With MP addition there was a reduction in EAW only at the higher rate, while FP in-
328 TABLE3 The effect of addition of perlite on physical properties of peat mixes. Values in the same column followed by the same letters are not significantly different at the 5% probability level Treatment
Air space (% volume)
Easily available water (% volume)
WaterTotal Bulk buffering porosity density capacity (% volume) (g/cc) (% volume)
Hauraki peat 3AHauraki + 1ACP 1/2 Hauraki + 1/2 CP 3AHauraki + ~AMP 1/i Hauraki + 1/2MP 3AHauraki + ~ FP 1/2 Hauraki + 1/2 FP CP MP FP
32.8 33.4 35.0 31.9 26.1 28.4 24.6 30.5 19.1 13.6
22.6 17.4 12.6 23.6 19.4 26.5 24.3 8.3 17.8 23.5
3.3 2.6 2.3 4.4 2.8 5.3 5.8 4.1 5.3 3.6
ef fg g ef c d c de b a
e c b ef d g f a cd ef
b ab a de ab ef f cd ef bc
84.6 77.6 70.9 85.4 72.1 83.8 81.9 59.4 63.4 68.5
f e cd f d f ef a b c
0.134 0.159 0.173 0.163 0.173 0.163 0.180 0.204 0.206 0.208
a b c b c b c a a d
creased EAW. J o i n e r and C o n o v e r {1965) f o u n d t h a t there was a r e d u c t i o n in available w a t e r as the v o l u m e o f perlite in p e a t was increased. T h e y d o n o t m e n t i o n the particle size o f their perlite. White and Mastalerz {1966) also f o u n d t h a t v o l u m e o f w a t e r at c o n t a i n e r c a p a c i t y {water held at s u c t i o n p F 1.2) was decreased as a result o f perlite application. W a t e r - b u f f e r i n g c a p a c i t y was r e d u c e d b y the a d d i t i o n o f CP at the high rate and increased b y the a d d i t i o n o f MP at 1A rate and FP at 1A and ~ rate. T o t a l p o r o s i t y was n o t a f f e c t e d b y perlite a d d i t i o n e x c e p t w h e n CP was a d d e d at b o t h rates and MP was a d d e d at ~/i the rate, in w h i c h case t h e r e was a reduction. These results indicate t h a t CP m a y be used t o increase AS o f p e a t mixes. This m a y be desired if c r o p s are being g r o w n in p e a t mixes d u r i n g w i n t e r m o n t h s and w a t e r e d b y capillary irrigation. It m a y also be useful f o r r o o t i n g cuttings. On the o t h e r h a n d , FP m a y be used t o increase EAW in p e a t mixes. This w o u l d be useful in e x t e n d i n g the time p e r i o d b e t w e e n w a t e r applications a n d w o u l d be p a r t i c u l a r l y useful in the case o f house-plants. A d d i t i o n o f p o l y s t y r e n e to peat. -- The a d d i t i o n ot~ p o l y s t y r e n e increased the AS, especially at the higher rate {Table 4). P o l y s t y r e n e h a d a reverse e f f e c t on EAW, and this was less t h a n a d e q u a t e at the higher addition. WBC, TP a n d bulk d e n s i t y were decreased b y p o l y s t y r e n e a d d i t i o n and the latter t w o were r e d u c e d to u n a c c e p t a b l e levels at the higher rate o f application. I m p r o v e m e n t o f aeration as a result o f p o l y s t y r e n e a d d i t i o n t o p e a t has been m e n t i o n e d b y C o o k {1971), B r i g h t o n {1973) and B u n t (1974). P o l y s t y r e n e w o u l d be a useful a d d i t i o n t o peat w h e n plants are g r o w n in relatively small
329 TABLE 4 T h e e f f e c t o f a d d i t i o n of p o l y s t y r e n e o n p h y s i c a l p r o p e r t i e s o f p e a t mixes. Values in t h e same c o l u m n f o l l o w e d b y t h e same l e t t e r s are n o t significantly d i f f e r e n t at t h e 5% p r o b a b i l i t y level. Analysis o f p o l y s t y r e n e was n o t carried o u t because it was n o t possible t o s a t u r a t e it with water Treatment
Air space (% v o l u m e )
Easily available water (% v o l u m e )
WaterTotal Bulk buffering porosity density capacity (% v o l u m e ) (g/cc) (% v o l u m e )
Irish p e a t 3A Irish p e a t + 1A polystyrene 1A Irish p e a t + 1A polystyrene
23.3 a
32.9 c
5.3 c
94.4 c
0.104 c
27.4 b
24.6 b
3.9 b
82.9 b
0.095 b
33.6 c
14.9 a
2.0 a
68.0 a
0.063 a
pots and watered by capillary irrigation, especially in winter. The higher rate of polystyrene addition would be useful for epiphytic plants. Under conventional watering-systems, a more frequent watering would be required in peat-polystyrene mixtures than in pure peat. The low bulk density for peat-polystyrene mixtures could cause problems if light-weight pots are used under outdoor windy conditions. A d d i t i o n o f p u m i c e . - - The addition of pumice had no effect on AS, but re-
duced EAW, WBC and TP. However, only WBC was reduced to below the critical value (Table 5). The bulk density of peat mixes was increased as a result of pumice addition. TABLE 5 T h e e f f e c t o f a d d i t i o n of p u m i c e o n physical p r o p e r t i e s o f p e a t mixes. Values in t h e same c o l u m n f o l l o w e d b y t h e same letters are n o t significantly d i f f e r e n t at 5% p r o b a b i l i t y level Treatment
Air space (% v o l u m e )
Easily available water (% v o l u m e )
WaterTotal Bulk buffering porosity density capacity (% v o l u m e ) (g/cc) (% v o l u m e )
Hauraki peat sA H a u r a k i + 1A pumice 1/2 H a u r a k i + 1/2 pumice Pumice
32.3 b
31.3 c
5.6 c
96.8 c
0.140 a
30.1 b
25.1 b
3.0 b
85.5 b
0.249 b
30.6 b 20.1 a
25.8 b 15.0 a
3.0 b 1.5 a
83.0 b 58.8 a
0.401 c 0.569 d
330 CONCLUSION T h e results s h o w e d t h a t m i x t u r e s w i t h d i f f e r i n g p h y s i c a l p r o p e r t i e s c o u l d be p r e p a r e d , using a range o f m a t e r i a l s a d d e d to p e a t in d i f f e r e n t p r o p o r t i o n s a n d particle sizes. T h e e f f e c t s on p h y s i c a l p r o p e r t i e s c o u l d be u s e d b y t h e g r o w e r a c c o r d i n g t o his p a r t i c u l a r needs; t h a t is a c c o r d i n g to t h e t y p e of p l a n t t o be g r o w n , w h e t h e r the m i x t u r e is used f o r g r o w i n g o n or f o r r o o t i n g , t y p e o f irrigation p r a c t i s e d - - w h e t h e r capillary or drip, as well as availability and cost.
REFERENCES Boggle, R., 1970. Moisture characteristics of some peat--sand mixtures. Sci. Hortic., 22" 87--91. Brighton, C.A., 1973. EPS improves the aeration, drainage and temperature of composts. Nurserymen and Garden Centre, pp. 392--395. Bunt, A.C., 1974. Some physical and chemical characteristics of loam-less pot-plant substrates and their relationship to plant growth. Acta Hortic., 37: 1954--1965. Cook, C.D., 1971. Plastoponics in ornamental horticulture. Gard. Chron., pp. 20--22. De Boodt, M. and Verdonck, O., 1972. The physical properties of the substrates in horticulture. Acta Hortic., 26: 37--44. Goh, K.M. and Haynes, R.J., 1977. Evaluation of potting media for commercial nursery production of container-grown plants. I. Physical and chemical characteristics of soil and soilless media and their constituents. N.Z.J. Agric. Res., 20: 363--370. Joiner, J.N. and Conover, C.A., 1965. Characteristics affecting desirability of various media components for production of container-grown plants. Proc. Soil Crop Sci. Soc. Fla., 25: 320--328. Prasad, M., 1979. Physical properties of media for container-grown crops. I. New Zealand peats and wood wastes. Scientia Hortic., 10: 317--323. White, J.W. and Mastalerz, J.W., 1966. Soil moisture as related to container capacity. J. Am. Soc. Hortic. Sci., 89: 758--765.
325
P H Y S I C A L P R O P E R T I E S O F M E D I A F O R C O N T A I N E R - G R O W N CROPS. II. P E A T M I X E S
M. PRASAD Horticultural Research Centre, Research Division, Ministry of Agriculture and Fisheries, Levin (New Zealand) (Received 6 November 1978)
ABSTRACT Prasad, M., 1979. Physical properties of media for container-grown crops. II. Peat mixes. Scientia Hortic., 10: 325--330. The effect of addition of sand, perlite, polystyrene and pumice in half and quarter proportions and in various particle sizes, on physical properties of peat mixes was studied. Coarse sand had no effect on air space (AS), while fine sand reduced AS and at the higher rate the value fell to an unsatisfactory level. At the higher rate of coarse sand addition, easily available water (EAW) fell to an unsatisfactory level. Fine sand increased EAW. Total porosity (TP) and water-buffering capacity (WBC) were generally reduced as a result of sand addition. The addition of coarse perlite at the higher rate increased the AS, while fine perlite at both rates and medium perlite at the high rate reduced AS. The effect of addition of coarse and fine perlite on EAW was similar to sand addition. Total porosity was reduced by coarse perlite and by medium perlite addition at the higher rate. The addition of polystyrene increased AS, but decreased EAW and the value fell to an unacceptable level at the high rate of application. Bulk density, WBC and TP were decreased by the addition of polystyrene. The addition of a standard grade of pumice had no effect on AS, but the EAW, WBC and TP were reduced. However, only in the case of WBC was the reduction below the critical level.
INTRODUCTION
S e c o n d a r y c o n s t i t u e n t s , including sand, perlite, p o l y s t y r e n e a n d p u m i c e , are o f t e n m i x e d with p e a t in the p r e p a r a t i o n o f growing-media. In N e w Zealand the additions, in m a n y cases, are n o t t o i m p r o v e the physical p r o p e r t i e s o f the c o m p o s t b u t t o save o n the a m o u n t o f peat, a relatively expensive material. Inf o r m a t i o n on the effects o f a d d i t i o n o f these materials on the physical properties o f g r o w i n g - m e d i a is f r a g m e n t a r y ( J o i n e r and Conover, 1 9 6 5 ; White and Mastalerz, 1 9 6 6 ; Boggie, 1 9 7 0 ; Brighton, 1 9 7 3 ; Bunt, 1 9 7 4 ; G o h and H a y n e s , 1977). The objective o f the p r e s e n t investigation was, t h e r e f o r e , t o s t u d y t h e e f f e c t o f additions o f sand, perlite, p o l y s t y r e n e and p u m i c e in d i f f e r e n t p r o p o r t i o n s t o peat, and in s o m e cases the e f f e c t o f d i f f e r e n t particle sizes, o n physical properties o f p e a t mixes.
326 MATERIALS AND METHODS Details o f the techniques used for the determination of physical properties of the mixtures used in this e x p e r i m e n t are given by Prasad (1979). Particle sizes of the secondary materials used are given in Table 1. All were added to the peat at the rate of '~ or 'h on volume basis and then mixed. In order to avoid bias in the control samples (peat with no additions), these were handled in the same way as the peat mixtures, so t ha t any b reakdow n of peat during mixing would also have occurred in the control samples. Before the samples were packed into containers for a moisture r et ent i on determination, t h e y were brought to a moisture c o n t e n t equivalent to pF 1.5 -- 2.0 by eye estimation. The peat pumice mixtures were packed only at one density, 0.5 kg cm 2, the o t her mixtures were packed at 0.5 or 0.1 kg cm -2.
TABLE 1 Particle size ranges of secondary constituents added to peat Materials
Coarse perlite Medium perlite Fine perlite Coarse sand Fine sand Pumice Polystyrene chips'
% of material in the particle range < lmm
1--2mm
> 2mm
0 33 50 0 50 37 0
50 33 25 50 25 35 5
50 33 25 50 25 28 95
' 63% was greater than 3.18 mm and 32% greater than 4.4 mm. RESULTS AND DISCUSSION The o p t i m u m levels of air space (AS), easily available water (EAW), waterbuffering capacity (WBC) and total porosity (TP) were defined by De B o o d t and Ver d o n ck (1972) and these were taken as points of reference. There were shrinkage problems with the lighter packings, especially when fine materials were mixed into the peat. The results obtained from lighter packing were th er ef o r e omitted. In general, the results obtained from lighter packing gave similar trends to those obtained with heavier packing. A d d i t i o n o f sand t o p e a t . - - The addition of coarse sand (CS), with relatively
high AS, had little or no effect on AS (Table 2). Bunt (1974) f o u n d that increasing the percentage of grit in peat mixes from 25 to 50 had no effect on AS. The fine sand (FS) with a low AS reduced the AS of the peat mixes considerably and at the 50% addition rate the value fell to unsatisfactory levels.
327
TABLE 2 T h e e f f e c t o f a d d i t i o n of sand o n physical p r o p e r t i e s o f p e a t mixes. Values in t h e same c o l u m n f o l l o w e d b y t h e same l e t t e r s are n o t significantly d i f f e r e n t a t t h e 5% p r o b a b i l i t y level Treatment
Air space (% v o l u m e )
Easily available water (% v o l u m e )
WaterTotal Bulk buffering porosity density capacity (% v o l u m e ) (g/cc) (% v o l u m e )
Hauraki p e a t ~4 H a u r a k i + 1A CS 1/2 H a u r a k i + 1/2 CS ~A H a u r a k i + IA F S 1/2 H a u r a k i + 1/2 FS CS FS
30.5 27.8 31.4 21.9 12.3 23.9 5.8
25.8 23.0 17.4 32.9 33.5 7.6 20.1
4.3 3.5 1.9 3.1 1.8 0.9 1.9
d d d c b c a
d cd b e e a bc
e d b c b a b
88.1 76.4 64.1 80.8 68.5 36.5 35.3
f d b e c a a
0.150 0.506 0.909 0.568 0.995 1.780 1.758
a b d c e f f
These results agree with the findings of Boggie (1970), Bunt (1974) and Goh and Haynes (1977). EAW was reduced by the addition of CS (not significantly at ~Arate). At the higher rate of CS application, EAW fell to unsatisfactory levels. The addition of fine sand (FS) to peat increased the EAW. Bunt (1974) found that EAW was higher in mixtures of peat and fine sand than in peat and grit. Water-buffering capacity was reduced as a result of sand addition. Goh and Haynes (1977) also reported reduction of WBC as a result of addition of sand to peat. Total porosity was reduced b y CS and FS application; this reduction was greater at the higher rate of application. These results agree with the findings of Boggie (1970) and Bunt (1974). The results showed that when CS or FS was applied to peat at 1~ rate, a mix with favourable physical characteristics for container-grown plants resulted. In addition, due to an increase in weight, a greater stability resulted, which was advantageous to tall plants in windy situations. A d d i t i o n o f p e r l i t e to p e a t . ~ There was an increase in AS b y the addition of
coarse perlite (CP) at the higher rate; with medium perlite (MP) at the low rate there was no change, while at high rate the AS was reduced (Table 3). Fine perlite (FP) reduced AS. Joiner and Conover (1965) found that by increasing the volume of perlite from 58 to 83%, the drainable pore space was increased. Similarly, White and Mastalerz (1966) found that AS (pF 1.2) was increased as a result of addition of perlite (approx. size -+ 3 mm) to peat. There was a reduction of EAW as a result of addition of CP and this was more evident at the higher rate where the levels were below optimum. With MP addition there was a reduction in EAW only at the higher rate, while FP in-
328 TABLE3 The effect of addition of perlite on physical properties of peat mixes. Values in the same column followed by the same letters are not significantly different at the 5% probability level Treatment
Air space (% volume)
Easily available water (% volume)
WaterTotal Bulk buffering porosity density capacity (% volume) (g/cc) (% volume)
Hauraki peat 3AHauraki + 1ACP 1/2 Hauraki + 1/2 CP 3AHauraki + ~AMP 1/i Hauraki + 1/2MP 3AHauraki + ~ FP 1/2 Hauraki + 1/2 FP CP MP FP
32.8 33.4 35.0 31.9 26.1 28.4 24.6 30.5 19.1 13.6
22.6 17.4 12.6 23.6 19.4 26.5 24.3 8.3 17.8 23.5
3.3 2.6 2.3 4.4 2.8 5.3 5.8 4.1 5.3 3.6
ef fg g ef c d c de b a
e c b ef d g f a cd ef
b ab a de ab ef f cd ef bc
84.6 77.6 70.9 85.4 72.1 83.8 81.9 59.4 63.4 68.5
f e cd f d f ef a b c
0.134 0.159 0.173 0.163 0.173 0.163 0.180 0.204 0.206 0.208
a b c b c b c a a d
creased EAW. J o i n e r and C o n o v e r {1965) f o u n d t h a t there was a r e d u c t i o n in available w a t e r as the v o l u m e o f perlite in p e a t was increased. T h e y d o n o t m e n t i o n the particle size o f their perlite. White and Mastalerz {1966) also f o u n d t h a t v o l u m e o f w a t e r at c o n t a i n e r c a p a c i t y {water held at s u c t i o n p F 1.2) was decreased as a result o f perlite application. W a t e r - b u f f e r i n g c a p a c i t y was r e d u c e d b y the a d d i t i o n o f CP at the high rate and increased b y the a d d i t i o n o f MP at 1A rate and FP at 1A and ~ rate. T o t a l p o r o s i t y was n o t a f f e c t e d b y perlite a d d i t i o n e x c e p t w h e n CP was a d d e d at b o t h rates and MP was a d d e d at ~/i the rate, in w h i c h case t h e r e was a reduction. These results indicate t h a t CP m a y be used t o increase AS o f p e a t mixes. This m a y be desired if c r o p s are being g r o w n in p e a t mixes d u r i n g w i n t e r m o n t h s and w a t e r e d b y capillary irrigation. It m a y also be useful f o r r o o t i n g cuttings. On the o t h e r h a n d , FP m a y be used t o increase EAW in p e a t mixes. This w o u l d be useful in e x t e n d i n g the time p e r i o d b e t w e e n w a t e r applications a n d w o u l d be p a r t i c u l a r l y useful in the case o f house-plants. A d d i t i o n o f p o l y s t y r e n e to peat. -- The a d d i t i o n ot~ p o l y s t y r e n e increased the AS, especially at the higher rate {Table 4). P o l y s t y r e n e h a d a reverse e f f e c t on EAW, and this was less t h a n a d e q u a t e at the higher addition. WBC, TP a n d bulk d e n s i t y were decreased b y p o l y s t y r e n e a d d i t i o n and the latter t w o were r e d u c e d to u n a c c e p t a b l e levels at the higher rate o f application. I m p r o v e m e n t o f aeration as a result o f p o l y s t y r e n e a d d i t i o n t o p e a t has been m e n t i o n e d b y C o o k {1971), B r i g h t o n {1973) and B u n t (1974). P o l y s t y r e n e w o u l d be a useful a d d i t i o n t o peat w h e n plants are g r o w n in relatively small
329 TABLE 4 T h e e f f e c t o f a d d i t i o n of p o l y s t y r e n e o n p h y s i c a l p r o p e r t i e s o f p e a t mixes. Values in t h e same c o l u m n f o l l o w e d b y t h e same l e t t e r s are n o t significantly d i f f e r e n t at t h e 5% p r o b a b i l i t y level. Analysis o f p o l y s t y r e n e was n o t carried o u t because it was n o t possible t o s a t u r a t e it with water Treatment
Air space (% v o l u m e )
Easily available water (% v o l u m e )
WaterTotal Bulk buffering porosity density capacity (% v o l u m e ) (g/cc) (% v o l u m e )
Irish p e a t 3A Irish p e a t + 1A polystyrene 1A Irish p e a t + 1A polystyrene
23.3 a
32.9 c
5.3 c
94.4 c
0.104 c
27.4 b
24.6 b
3.9 b
82.9 b
0.095 b
33.6 c
14.9 a
2.0 a
68.0 a
0.063 a
pots and watered by capillary irrigation, especially in winter. The higher rate of polystyrene addition would be useful for epiphytic plants. Under conventional watering-systems, a more frequent watering would be required in peat-polystyrene mixtures than in pure peat. The low bulk density for peat-polystyrene mixtures could cause problems if light-weight pots are used under outdoor windy conditions. A d d i t i o n o f p u m i c e . - - The addition of pumice had no effect on AS, but re-
duced EAW, WBC and TP. However, only WBC was reduced to below the critical value (Table 5). The bulk density of peat mixes was increased as a result of pumice addition. TABLE 5 T h e e f f e c t o f a d d i t i o n of p u m i c e o n physical p r o p e r t i e s o f p e a t mixes. Values in t h e same c o l u m n f o l l o w e d b y t h e same letters are n o t significantly d i f f e r e n t at 5% p r o b a b i l i t y level Treatment
Air space (% v o l u m e )
Easily available water (% v o l u m e )
WaterTotal Bulk buffering porosity density capacity (% v o l u m e ) (g/cc) (% v o l u m e )
Hauraki peat sA H a u r a k i + 1A pumice 1/2 H a u r a k i + 1/2 pumice Pumice
32.3 b
31.3 c
5.6 c
96.8 c
0.140 a
30.1 b
25.1 b
3.0 b
85.5 b
0.249 b
30.6 b 20.1 a
25.8 b 15.0 a
3.0 b 1.5 a
83.0 b 58.8 a
0.401 c 0.569 d
330 CONCLUSION T h e results s h o w e d t h a t m i x t u r e s w i t h d i f f e r i n g p h y s i c a l p r o p e r t i e s c o u l d be p r e p a r e d , using a range o f m a t e r i a l s a d d e d to p e a t in d i f f e r e n t p r o p o r t i o n s a n d particle sizes. T h e e f f e c t s on p h y s i c a l p r o p e r t i e s c o u l d be u s e d b y t h e g r o w e r a c c o r d i n g t o his p a r t i c u l a r needs; t h a t is a c c o r d i n g to t h e t y p e of p l a n t t o be g r o w n , w h e t h e r the m i x t u r e is used f o r g r o w i n g o n or f o r r o o t i n g , t y p e o f irrigation p r a c t i s e d - - w h e t h e r capillary or drip, as well as availability and cost.
REFERENCES Boggle, R., 1970. Moisture characteristics of some peat--sand mixtures. Sci. Hortic., 22" 87--91. Brighton, C.A., 1973. EPS improves the aeration, drainage and temperature of composts. Nurserymen and Garden Centre, pp. 392--395. Bunt, A.C., 1974. Some physical and chemical characteristics of loam-less pot-plant substrates and their relationship to plant growth. Acta Hortic., 37: 1954--1965. Cook, C.D., 1971. Plastoponics in ornamental horticulture. Gard. Chron., pp. 20--22. De Boodt, M. and Verdonck, O., 1972. The physical properties of the substrates in horticulture. Acta Hortic., 26: 37--44. Goh, K.M. and Haynes, R.J., 1977. Evaluation of potting media for commercial nursery production of container-grown plants. I. Physical and chemical characteristics of soil and soilless media and their constituents. N.Z.J. Agric. Res., 20: 363--370. Joiner, J.N. and Conover, C.A., 1965. Characteristics affecting desirability of various media components for production of container-grown plants. Proc. Soil Crop Sci. Soc. Fla., 25: 320--328. Prasad, M., 1979. Physical properties of media for container-grown crops. I. New Zealand peats and wood wastes. Scientia Hortic., 10: 317--323. White, J.W. and Mastalerz, J.W., 1966. Soil moisture as related to container capacity. J. Am. Soc. Hortic. Sci., 89: 758--765.