a reprint of a report
by G.M. Hitchon, D.A. Hall, and R.A. K. Szmidt
Department of Horticulture
West of Scotland College
Auchincruive, Aye
Scotland

Abstract

The development of the hydroponic perlite culture system of protected crop production at the West of Scotland College has been based, hitherto, on coarse, horticultural grade expanded perlite which has 90%~ by volume in the range 1-5 m. Recent work has compared crop performance of glasshouse tomatoes in horticultural-grade perlite with that in two Sardinian plaster-grades which have much finer particle-size distributions, one with 90% by volume in the range 0.6-1.4 nm, the other 90% by volume <1 mm.

Cumulative yields of fruit were similar in each of the three grades of perlite to the end of August in 1987 and to the end of September in 1988. Hence, although the air-filled porosity of the Sardinian plaster-grade perlite was lower than either the medium or horticultural-grade material, oxygen availability at the root surface did not limit root function.

1. Introduction

The perlite culture system of protected crop production was originally developed at the West of Scotland College (Hall et al., 1984). It is based on a relatively coarse grade of expanded perlite which has 90% by volume of particles in the range 1 to 5 m and a very high air-filled porosity (AEP) of around 60%, as measured by the method of Bragg and chambers (1988).

Recent evidence from trials with plants grown in peat suggests that provided the AFP is greater than 10%, oxygen availability at the root surface will not limit plant growth and development (Bunt, 1988). Even the fine, plaster-grade perlites have AFPs in excess of this value and should, therefore, form the basis of an excellent rooting substrate for hydroponic crop culture. Hence, the work reported in this paper was designed to study the influence of particle-size grade of perlite on the cropping performance of tomatoes.

2. Materials and methods >p> Three grades of expanded perlite and two nutrient solution formulations were tested over two years in a gulley-reservoir culture system (Hall et al., 1988).

The 'fine' and 'medium' grades of perlite were ?provided by British Gypsum Ltd. from their Sardinian mine; the 'coarse grade was a Milos perlite. Table 1 shows data relating to their relevant physical properties.

Table 1 - Some physical properties of three perlites used as rooting substrate.

Physical property	Fine	Medium	Coarse
Particle-Size (mm)	(% by volume)
4.75-2.36	1.2	6.0	61.6
2.36-1.18	8.2	71.9	27.9
1.18-0.60	22.7	17.7	5.5
0.6 -0.3	32.3	2.8	0.3
<0.3	35.4	1.6	3.7
Loose bulk density (kg/m3)	96	162	85
Air-filled .porosity (% v/v)	17	49	59

The nutrient solution formulations are given in Table 2. Equal volumes of stock solutions A and B were diluted as necessary to maintain the conductivity of the nutrient solution in the growing bags between 2500 and 3000 uS/cm. Control of alkaline drift in the rooting substrate was achieved either by acidifying the input feed with a 1:1 (v/v) mixture of concentrated nitric and phosphoric acids (Table 2, Feed 1) or by using the appropriate ammonium: nitrate nitrogen ratio (Table 2, Feed 2; Hall, 1983).

1987 Experiement

On 6 May 1987, six-week old tomato plants cv 'Shirley' were set out in a gulley reservoir culture system based on 30 liter, 3-plant growing bags. There were four, replicate, 36-plant plots of each treatment. The 24 plots were arranged in a randomized block design in a Venlo-type glass house fitted with computerized irrigation and environmental controls. After the initial wetting up of the perlite, equal volumes of nutrient solution were applied to all plants.

Fruits were harvested three times a week until 4 October, then twice a week until 20 October when the experiment was terminated. Samples of perlite were taken for analysis every two weeks as an aid to managing crop nutrition.

Table 2 - Composition of stock solution used to prepare main season nutrient feeds

Fertilizer		kg/100 liters
Feed 1
Stock A	Calcium nitrate (prilled) Potassium nitrate	6.4 6.2
Stock B	Monopotassium phosphate Magnesium sulphate Trace element mix*	2.2 3.0 0.3
Feed 2
Stock A	Calcium nitrate	5.9
Stock B	Potassium nitrate Monopotassium phosphate Ammonium nitrate Magnesium sulphate Trace element mix	7.9 1.7 0.6 2.5 0.3

* Trace element mix: Iron EDTA (13% Fe) 228g; manganese sulphate (32% Mn) 12.0 g; borax 22.8 g; zinc sulphate (23% Zn) 30.3 g; copper sulphate (25% Cu) 6.0 g; sodium molybdate 1.0 g.

1988 Experiment

In the second year, tomato plants of the cv 'Compacto' were planted on 28 April 1988, following the cultural system used in 1987. Fruits were harvested three times a week until 22 August, then twice a week until 3 November when the experiment was terminated. Samples of perlite were taken for analysis every three weeks as an aid to managing crop nutrition.

3. Results

1987 Experiment Plants grew well in all treatments. No visual symptoms of nutrient disorders or water stress were recorded. The first ripe fruits were picked on 23 June. There were no significant differences in fruit yield from plants given the two different nutrient solution formulations.

Table 3 shows the cumulative yields of fruit from each of the three perlite grades. There were no significant differences in fruit yield to the end of August from plants grown in the three grades of perlite.

Table 3 - Total yield (kg/m2) of fruit on four harvest dates from tomato plants grown in three particle-size grades of perlite.

Particle-size grade	Total Yield of fruit (kg/m2)
	July 31	Aug 31	Sept 28	Oct 20
Fine:Sardinian	7.6	11.9	15.1b	17.1b
Medium:Sardinian	7.5	12.3	16.2b	18.2b
Coarse:Sardinian	7.5	12.1	15.8a	17.9a
SE (differenc)	+/- 0.19	+/- 0.22	+/- 0.27	+/- 0.35

1988 Experiement

Plants grew well and displayed no visual symptoms of nutrient disorders or water stress. The first ripe fruits were recorded on 20 June. There were no significant differences in fruit yield from plants supplied with the two different nutrient solution formulations, beyond the end of June, after the first five of forty-nine picks. Table 4 shows the cumulative yields of fruit for each of the three perlite grades.

Table 4 - Total yield (kg/rn2) of fruit on four harvest dates from tomatoes grown in three particle-size grades of perlite.

Particle-size grade	Total Yield of fruit (kg/m2)
	June 29	July 29	Aug 29	Sept 29	Oct 31
Fine:Sardinian	1.7	6.8b	9.2	12.0	13.4
Medium:Sardinian	1.9	7.3b	9.9	12.8	14.6ab
Coarse:Sardinian	2.0	7.9a	10.4	13.6a	15.3a
SE (differenc)	+/- 0.12	+/- 0.35	+/- 0.47	+/- 0.62	+/- 0.63

Cumulative yields of fruit from plants grown in fine, medium and coarse grade perlites were not significantly different at the end of June, August and September. Nevertheless, although yields from fine and coarse perlites were significantly different (p <0.5) at the end of July and October, yields between fine and medium and between medium and coarse were not.

4. Discussion One of the key features of perlite culture is the ease with which a near-constant, readily available supply of moisture can be provided, irrespective of time of day, weather conditions or stage of growth of the crop. As long as a shallow depth of free nutrient solution is maintained in the basal layers of the substrate, perlite's capillarity will lift water to replace that removed by the crop, thereby maintaining a steady moisture profile in the growing bags at all times (Hall et al., 1989).

Inextricably linked with water supply to roots is the provision of oxygen. Recent evidence from trials with plants grown in peat suggests that provided the AFP is greater than 10%, oxygen availability will not limit plant growth and development (Bunt, 1988). Since inert substrates support a lower microbial population than peat, it may likewise be supposed that crop productivity in perlite will not be limited by lack of oxygen if the AFP is 10% or higher (Hall et al., 1989). This supposition is backed up by the data in Table 1 which shows that even the fine grade perlite had an AFP of 17%.

This implies that oxygen availability ought not to have been a factor limiting plant growth and development. However, it seems likely that the slight, end-of-season, depressions in yield, from plants grown in fine grade perlite, at various times in 1987 and 1988 may have been due to lack of sufficient oxygen at the root surface. Oxygen flux through the upper layers of unsaturated fine perlite is less than in medium or coarse grade and could have adversely influenced the oxygen concentration in the basal reservoir.

When fine grade perlite is to be used as a hydroponic substrate, we recommend, therefore, that the depth of the basal reservoir should be less than the 40 mm be used in the experiments reported here.

Used in this way, the high water-holding capacity and strong capillarity of fine perlites are desirable characteristics enabling a near-constant moisture status to be readily maintained around the roots of protected crops. The results confirm the view that plaster- grade perlite, currently not widely used in horticulture, is a suitable substrate with potential for hydroponic crop production.

Acknowledgements

The authors would like to acknowledge financial support for this work from British Gypsum Ltd., Gotham Works, Nottingham.

References

Bragg, N.C. and Chambers, B.J., 1988. The interpretation and advisory application of compost air-filled porosity (AEP) measurements. Acta Horticulture.

Bunt, A.C., 1988. In: Media and mixes for container-grown plants. Unwin Hyman, London.

Hall, D.A., 1983. oThe influence of nitrogen concentration and salinity of recirculating solutions on the early-season vigor and productivity of glasshouse tomatoes. Journal of Horticultural Science 58, 411-415.

Hall, D.A., Wilson, G.C.S. and McGregor, A.J., 1984. Scots grow tomatoes in perlite. Grower, 17 May 1984, pp 23-24.

Hall, D.A., 1-litchon, G.M. and Szmidt, R.A.K., 1988. On the way to a smaller bag. Grower, 17 November 1988, pp 15-16.

Hall, D.A., Szmidt, R.A.K. and Hitchon, G.M., 1989. Glasshouse crop production in inert rooting media. Aspects of Applied Biology 22, 333-339.

HYDROPONICS--- Hydroponic growing systems are based on a new generation of superb rooting media - inert, sterile, uniform materials such as the mineral perlite and rockwool. These products act merely as supports for a complete nutrient solution on which the plants depend entirely for their water, mineral nutrients and oxygen. In the purest form of hydroponics, there is a complete absence of solid substrate, the nutrient solution itself acting as the rooting medium.

Tomatoes growing hydroponically in Perlite