Issue 19: In Search of Alternative Substrates

Issue 19
November/December -1994
Story Title: In Search of Alternative Substrates
Author: Roger Fox

The difficulty of disposing of hydroponic wastes is a major issue facing the industry world-wide. Research in the UK is examining the potential of alternative substrates which are biodegradable or reusable.

In Europe’s large scale hydroponic industries, the disposal of substrate waste has become a major concern. Research into the problem at Horticulture Research International (HRI) Stockbridge House in the UK, is exploring a number of alternative media options and yielding some interesting results.

The research project has been sponsored by the UK’s Horticulture Development Corporation (HDC) and to date has centred mainly on cucumber production. Currently, over 80% of long season cucumber crops in the UK are grown in hydroponics, and by far the most common substrate is rockwool. Rockwool has been used as a horticultural growing medium for nearly 20 years end its air and water holding properties make it excellent for cropping.

The success of the medium has, however, now led to disposal problems, as it is made from spun molten rock and does not break down. The slabs care usually dumped in landfill sites after use, but now, in areas of intensive hydroponic cropping, sites are becoming unavailable and costs of disposal have risen enormously. Accordingly, HRI are exploring two possible solutions to the problem: one is to grow in a substrate that is reusable for many years, and the other is to use a biodegradable substrate that will break down.

While it is possible to sterilise rockwool between crops and use it for two or more seasons, its structure tends to break down and it needs to be handled with care. One alternative being examined is polyurethane foam, which is available in slabs that ore claimed to have a useful life of 10 years or more. These have many properties similar to rockwool, but their lower water holding capacity means that they need treating in a slightly different way, if growth and yield care to be optimised. Irrigation should be applied more frequently than for rockwool but in smaller volumes, particularly for new slabs. If slabs dry out, they can be hard to wet again. But the foam’s strong rubbery texture and free-draining properties make it a very suitable material for steam sterilisation and resleeving.

Trials with cucumbers have shown that when slabs care re-used after sterilisation, their water retaining qualities are better and the structure is improved by the presence of old roots within the slab. In the trials, yields from new foam slabs equaled those from rockwool. Slabs that have been used once and then sterilised produced higher yields and the irrigation of the crop was easier to manage. The researchers believe that this substrate has great potential and it has already been taken up by some growers in the UK and northern Europe. In addition, the waste material can be incinerated at the end of its useful life.

Organic Substrates
Trials were carried out over two years to evaluate the suitability of a range of organic substrates for hydroponic cucumber production, and how best to manage their irrigation. Long season heated cucumber crops were propagated in rockwool and then planted onto the organic substrates in the cropping house. All nutrients were supplied to a standard hydroponic recipe, as used for rockwool.

In 1992, coir, shredded pine wood (Hortifibre), wood waste, peat, bark and straw were compared with rockwool as a standard. Reasonable yields were produced from all the substrates, but plants grown on Hortifibre performed particularly well in terms of yield and fruit quality.

From the first year’s trials, Hortifibre, straw and wood waste were selected for further development. Then in 1993, these three substrates were compared with rockwool using two different irrigation regimes.

In both years of the trial, Hortifibre was the most successful substrate. The nutrition was easy to manage and the substrate did not lose its structure. The product is made from loose fir wood fibres (Pinus sylvestris), held together in a fine net and sleeved. The wood is treated to remove the resins, tannins and turpenes which could be phytotoxic. The structure of Hortifibre encouraged the production of a strong root system and water holding capacity was good. The material is already widely used in France and is becoming more common in other North European countries.

In the straw slabs, microbial activity led to locking up of nitrogen and slab breakdown. This was detrimental to the crop in terms of reduced leaf size and fruit production.

Wood waste, largely sawdust, cilso performed well. During the first year of the trial, a fine particle size made initial wetting and root penetration slow, but this was improved by using a coarser grade material.

On initial wetting up, the slightly hydroponic nature of some organic substrates meant that it was better to maintain a shallow reservoir for a short time by not slitting the polythene wrappers to the bottom. Once saturated, however, the water holding capacity improved and run-off from these substrates was generally less than from rockwool.

These trials showed that long season cucumber crops could be grown successfully in all the substrates tested.

The most successful organic substrates were those that did not break down readily. A coarse open structure was better for maximum rooting. Irrigation and subsequent nutrient run-off was lower from organic substrates than for standard rockwool crops. Shredded pine wood (Hortifibre) gave the best fruit production results.

In terms of disposing, all of the substrates were disposed of by soil incorporation after use, but they would also be suitable for composting. In the case of Hortifibre, equipment has been developed by the French manufacturer to separate the wood fibres from the non-degradable plastic wrapping and netting, to make disposal after use as simple as possible.

Run-to-waste hydroponic production systems care the centre of controversy around the world, and particularly in Europe, over their potential for pollution. Currently, it is standard practice to feed approximately 30% above the plant’s theoretical requirements, in order to allow for inaccuracies in irrigation systems. The excess, containing high levels of nutrients, runs to waste together with any fungicides which may have been applied to the crop. This liquid runs into the greenhouse soil and may then reach underground water courses and rivers.

Ongoing research at HRI Stockbridge House is looking at developing practical recirculation systems for rockwool grown greenhouse salad crops, thereby reducing pollution risks. Both long-season tomatoes and cucumber crops were evaluated in the first year.

A simple collection system was developed whereby standard slabs were placed on polystyrene bridges in rigid polythene gutters. This allowed run-off solution to drip from the slabs and run down the channels below the bridges without contaminating slabs further down the row, thus minimising the risk of root disease spread from infected plants.

The solution was returned to the irrigation mixing tanks after collection, adjusted for pH and conductivity and then In used to irrigate the crop again. However, differential nutrient uptake by the plants can lead to build-up of certain elements in the solution, such as calcium and sodium. This was monitored by full weekly analysis of slab and drip nutrient levels. Corrections were made by adjustment of stock tank recipes, or by release of controlled amounts of solution from the system and replacement with fresh water.

In the tomato trial, a standard run-to-waste system was compared with two recirculation treatments. In the first recirculation system, nutrient levels and solution conductivity were controlled by adjustment to stock recipes and solution dumping as described. At times it was necessary to release considerable amounts of nutrient solution into the soil, thereby reducing the benefits of the collection and recirculation system.

The second recirculation system claimed to eliminate all solution dumping, and nutrient levels were controlled without the release of nutrient solution from the closed system.

The final results showed a slight yield loss for the recirculation systems, compared with the run-to-waste system, but no differences between the two recirculation treatments. Build-up of elements such as sodium was observed, but this did not reach critical levels, due to the very clean water supply at Stockbridge House.

Future trials will aim to maintain nutrient levels closer to the optimum and eliminate the slight yield loss seen in the recirculation systems.

Although tomatoes have been successfully grown in recirculating nutrient film technique (NFT) systems for many years, long-season cucumber crops have not been successful in such systems. There was therefore some doubt about how a recirculation system would perform.

In the first year, the trial simply compared a run-to-waste system, with a crop in recirculation as previously described. Solution was released from the system when considered necessary by monitoring the results of solution analysis. The crop was sown in January, planted in February and grew through until September. Surprisingly, there were no observable differences in plant growth between the two systems throughout the season and final yields showed an increase for the plants on the recirculation system. The trial will be repeated to further investigate this unexpected result.

(Information courtesy of Horticulture Research International, Stockbridge House, North Yorkshire UK.)