Issue 117: Comparing Growing Media

The effect of various growth mediums on the development and production of plants in hydroponics systems.

By Ben Safronovitz

In a trial conducted at Camdeboo Farm near Fourways, Johannesburg, South Africa, one basic question was posed. Are there any differences in the overall performance of plants grown in different growing mediums? …The reason for the trial is the general uncertainty (at least in South Africa) regarding growing mediums. Many growers are confused or reluctant to use some mediums due to a lack of experience, lack of performance information, and/or an ignorance regarding the financial implications of using the medium in the short- and long-term.

The main objective of a grower is to invest the minimum possible in infrastructure and achieving the maximum yield, which in financial terms means increased profit. Therefore, in the short-term, investing in an inferior growing medium is a reality. The fact is that a larger initial investment will in the long-run return an increased yield per growing cycle to warrant the investment, and yield a greater profit.

In South Africa sawdust as a hydroponic medium is used extensively; it is inexpensive, readily available from a nearby mill, and due to its low-cost, can be discarded after each growing cycle.

The importance of understanding the characteristics of a growing medium and the long- and short-term affect on the plant is essential to the overall success or failure of a crop, therefore the trial targeted the most commonly used growing mediums in the country, each on its own (to serve as a control batch) and also in combination with other growing mediums. Without elaborating on the physical and chemical differences among the tested growing mediums, each has characteristics, which make it a usable medium – for example: bulk density, porosity, aeration, ability to contact water with high holding capacity, highly hygroscopic, create capillary water and nutrients dynamics, etc.

The mediums used for the purpose of this trial were:
•    Palm pith (Coir) 100%
•    Perlite 100%
•    Vermiculite 100%
•    Sawdust 100%
•    Palm pith 50% / Perlite 50%
•    Palm pith 70% / Perlite 30%
•    Palm pith 40% / Perlite 30% / Vermiculite 30%
•    Perlite 50% / sawdust 50%
•    A locally produced light compost pot soil 50%  / Perlite 50%

Growing medium trials at Camdeboo Farm near Fourways, Johannesburg.

A standard tunnel 30m x 10m was selected for the purpose of the trial, positioned north-south with excellent light cover during the day. A white/black ground cover plastic sheet was used to eliminate weed growth and increase light reflection at the tunnel floor level, thus increasing plant photosynthesis.

Day one – seedlings transplanted into medium.

Medium consists of 50% palm pith and 50% Perlite.

Seedlings in 100% sawdust.

Seedlings grown in 50% pot soil and 50% Perlite.

Seedlings grown in 100% Perlite.

Seedlings grown in 100% Vermiculite.

Perlite 50% Sawdust 50%

The trial principles of using one crop (tomato), one cultivar (Malory – indeterminate variety produced by Mayford-Sakata seeds), under the same climatic conditions and nutrient feed and irrigation cycle regime across the various mediums, allowed a fair and  unbiased evaluation of the plants’ performance during a complete growing cycle.

The hydroponics method used was quite revolutionary, which is in the process of patent registration. In essence, the method uses growing containers with a slit on one side at a certain level. This method allows routine irrigation (four times daily) of approximately 800 cubic centremetres of water and nutrients per plant on average, which has enormous implications on cost in terms of saving water and nutrients. The growing system was an open system (not re-circulated) and the drain kept to 10% of the total volume.

The floor plan was organised to allow for eight rows of 48 containers each. One block of 48 containers served as a control, representing each of the tested mediums. There were 432 containers with two plants per container, a total of 864 plants. Each medium covered a block of 48 containers situated along the tunnel width to ensure fair distribution of light or shade across the eight rows, eliminating the possibility of false results due to uneven exposure to light.

Seeds were sown on 18 September 2010 in a specific seedling medium (Promix).  Germination occurred between the 27th and 30th of September and seedlings were transplanted into the various mediums in the experimental tunnel on the 16th  and 17th of October. Seedlings were irrigated four times a day at intervals that were changed as plants demonstrated vegetative growth and after consideration of weather/seasonal changes during the growing phase.

Plants showed a strong vegetative growth during the first 30 days (15-20 cm per week), and the first flower clusters appeared during the first and second week of November. Plants were constantly pruned to eliminate side shoots, and were tied to trellises from the third week.

The growing medium trials after one month.

During the first 60 days first and second flower clusters started setting fruit when definitive characteristic of the various growing media started showing differences. Plants grown in palm pith showed a very pronounced and strong vegetative growth, especially leaves. Plants became bushy while fruit developed quickly into full fruit clusters (6-7 fruits) with significant fruit size. Plants in Perlite developed thinner, less pronounced leaf density, but larger fruit clusters. In both mediums – palm pith and Perlite – plants did not develop significant height.

First cluster in a palm pith medium.

Typical tomato cluster grown in palm pith.

Tomato cluster grown in palm pith.

Plants in sawdust developed to various heights with impressive first cluster fruit size, but significantly, the medium compacted and drain analysis suggested a low nutrient uptake compared to other mediums. Drain flow was higher than other mediums (i.e. the medium did not hold the water effectively during irrigation with the majority of input water and nutrients draining away). As a result, plants experienced longer stress periods, which affected the upper fruit clusters and produced less fruit per cluster and smaller in size.

The plants in Vermiculite performed the best in terms of vegetative growth. They reached the 2-metre trellis cable inside 60 days. Interestingly, the flowering and fruit-set stage was slow with less flowers and fruit on mature clusters (average three to five), but fruit size was impressive. Compared to the sawdust medium, Vermiculite’s water retention capacity is high and over-draining of water was due more to the saturated medium rather than lower water retention capacity. This may have affected the overall uptake of nutrients due to low capillary dynamics, which affected fruit formation.

From the mixed mediums, the combination of palm pith 50% / Perlite 50%, and the same mediums at 70% / 30%, produced the best balanced vegetative development as well as perfect flower clusters, fruit number and size.

The mixed palm pith 40% / Vermiculite 30% / Perlite 30% performed well, but due to the water retention characteristics of Vermiculite and palm pith, the medium had a more soggy texture. The vegetative plant development was impressive but fruit set and size differed slightly from the previous medium, but not significantly.

The pot-soil 50% combination with 50% Perlite did not perform well. The pot-soil was structured from large particles of organic matter with minimal water-holding capacity and fermented (similar to sawdust) into a very moist environment. The Perlite improved the water-holding capabilities of the mixed medium, but contributed to increased moistness and expedited the fermentation process. The medium became more acidic and compact over time, which is the reason for the development of tall spindly plants and a significant drop in fruit set and low number of fruit per cluster (two to three) – the smallest fruit size when compared to the other mediums.

Being a soil derivative the pot-soil was susceptible to soil-borne diseases such as Early Blight and Late Blight – a few sawdust containers also developed similar fungus maladies. About 40% of that specific block of plants grown in pot-soil developed fungus problems, which was rectified quickly, mainly with copper hydroxide to prevent it spreading.

Sawdust 50% / Perlite 50% performed slightly better than sawdust on its own. The Perlite added better water-holding capacity to the medium and slowed the drain of water and nutrients, causing plant growth and fruit formation to be better than sawdust only, but still poorer than Perlite on its own, specifically when comparing fruit size and number of fruits per cluster. From an economic point of view, it is a less expensive combination.

Fruit clusters grown in 50% sawdust and 50% Perlite.

Tomato cluster grown in a combination of sawdust & Perlite.

It is important to note that the data was collected on a daily basis and harvesting will be a decisive element of this experiment to analyse overall yield and quality per medium started on 15 December 2010 (90 days from sowing). The total harvest to date of 1,631 kg of ripe tomatoes is a trend, not a final indication of medium performance. Nevertheless, some conclusions can be made to predict the overall results due to various trends in plant performance.

The largest tomato was 402g grown in sawdust.

From all the tested mediums, palm pith and Perlite performed best in terms of production per plant, fruit size and weight. The combination mediums performed slightly behind with the exception of the palm pith / Vermiculite / Perlite mix, which was not far behind. The sawdust as a one-off medium performed reasonably well, but with considerably lower fruit size and weight. The pot-soil / Perlite mix performed worst in all categories.

The primary consideration as to which medium to use is economic, taking into account that some mediums are re-usable. The trial will test later the long-term effect on crops grown in re-usable media. It’s important to note that results will vary, depending on the hydroponics method used.

Ben Safronovitz

About the author
Ben Safronovitz graduated from agriculture school in Israel and has practised horticulture in South Africa since 1992. He specialises in soilless culture and has undertaken several experiments with various growing media and cultivars in South Africa and Israel. Ben is a lecturer in hydroponics at the Lifestyle Centre in Johannesburg, South Africa.

A special thanks to Camdeboo Farm, Fourways, Johannesburg, for providing the site and infrastructure and hosting the trials.

•    W.R.Carlile, The use of composted materials in growing media, International Symposium on Growing Media, ISHS Acta Horticulture.
•    D.Maragatham Jeyaseeli and Samuel Paul Raj, Physical Characteristics of coir as a function of its particles size to be used as soilless medium, American-Eurasian J. Agric & Environ. Sci.,8(4):431-437,2010 ISSN 1818-6769, IDOSI Publ.2010.
•    Palm Pith Manufacturer data sheet.
•    Dr David A. Hall, Role of Perlite in hydroponic culture, Pershore College Horticulture, UK.
•    Horticultural uses of Perlite and Vermiculite.
•    Growing media characteristics and overview.
•    Evaluation of coco coir as compared to sawdust for tomato crop production.
•    Sakata- Malory, Indeterminate tomato.  Ω

PH&G March-April 2011 / Issue 117