Organic Hydroponics (Part Two)

Introduction

The aim of our research was to compare the growth of lettuce in a nutrient film technique (NFT) hydroponic system using either an organic hydroponic solution, or a conventional inorganic fertilizer solution. The results should help to determine the viability, and potential of organic hydroponics.

Experiment One

The experiment was conducted in a 6 x 6m heated glasshouse at Massey University’s Plant Growth Unit (PGU).

There were two treatments, namely:

1) an organic “solution” derived from a 3:2 ratio of ‘Betta-Crops’ liquid fish and liquid seaweed;

2) A conventional hydroponic nutrient solution (Cooper, 1967).

Both solutions were maintained at a pH of 7.0 and a CF of 16 and the solution was recirculated.

Standard NFT channelling was used, 4.5m long with holes 0.2m apart, and three distinct lettuce cultivars – Cireo, Impuls, and Cortina were used in a randomised and replicated split plot design.

Lettuce seeds were sown into hydropots and the seedlings grown on for 14-days prior to placing out in the different nutrient solutions, where they were grown on for a further six weeks at which time harvesting commenced. Four harvests were made at weekly intervals and the fresh weight of the lettuce heads obtained.

Results

Fig. 1 shows that the conventional lettuces grew at a significantly faster rate than the organic lettuces. In fact the conventionally grown cultivars weighed over 200% more than their organic counterparts. By the third harvest there had been a dramatic change, by comparing the organic results from the second harvest to the third harvest, it can be seen that the weights decreased when they should still be increasing, as is the case with the conventional lettuces.

Fig. 1 represents the average lettuce head fresh weight differences between cultivars regardless of treatment, clearly illustrating that the Cortina cultivar grows significantly faster than the other two. Fig. 2 represents the effect of the treatments on the average fresh lettuce head weights regardless of cultivar, clearly showing the significant difference between the organic and conventionally grown lettuce.

Discussion

The reduction in weight of the organically grown lettuce for the final three harvests appeared to be caused by the roots dieing. This was believed to be due to a marked reduction in oxygen availability for these plants due to the presence of algae and of unbroken down organic matter in the nutrient solution.

Conclusion

The results showed that organic hydroponic production is possible. It is apparent that the cultivar of lettuce chosen is important if rapid growth is required. The red and oak leafed types grew much slower than the butterhead types such as Cortina both in both an organic solution or a conventional one.

The presence of organic matter in suspension is an issue that must be dealt with prior to using the solution to ensure that all the nutrients in the organic solution are available to the plants, and that the oxygen depletion problems will not occur.

Experiment Two

This experiment was carried out to determine why plants in the conventional solution grew better than those in the organic solution.

Materials and Methods

The experiment was conducted in a 6 x 6m-heated glasshouse at Massey University’s Plant Growth Unit (PGU).

The establishment procedure was the similar to the first experiment with seeds sown on June 19th, and the seedlings placed into the different treatments in the glasshouse on July 24th, where they were grown on for a further six weeks before the final harvest. Plants were also harvested each week for three weeks from July 31st to establish the growth rates of the young plants (using a technique called “growth analysis), in which leaf area, leaf dry weight, root dry weight, and stem dry weights are recorded. The rest of the lettuce plants were left until harvest on August 4th, at which time all the remaining lettuces were harvested.

Results

Relative Growth Rates (RGR), Net Assimilation Rates (NAR) and Leaf Area Ratios (LAR) were calculated but only the RGR’s are shown in this article (Fig. 3). These results show a dramatic change in growth rates between the lettuce plants growing in the organic solution, to those in the conventional solution with the RGR’s in the conventional solution averaging some 10% greater than the organic solution grown plants.

This is a huge difference as it is a rate and thus over time results in a large difference in fresh weight yields. (Table 1)

Table 1: Mean fresh weight (g) of harvested lettuce

Cultivars­ Organic­ Conventional­

Cireo­ 19­ 108­

Impuls­ 21­ 74­

Cortina­ 33­ 144­

These results are similar to those obtained in the first experiment.

Experiment Three

Because of the unsatisfactory performance of the organic solution in the previous two experiments it was decided to compare a number of different organic solutions with a conventional solution with the aim to find an organic solution that could produce a quality product, as rapidly as a conventional solution.

The new materials used were:

1) Cattle effluent a plentiful by-product that contains reasonable quantities of the right nutrients to support plant growth. The source of the material for this trial was the aerobic pond on the Massey University number four dairy farm.

2) Liquid vermicast, a product made by soaking vermicast in water for a specified period of time, then syphoning off the liquid that now contains the nutrients. The liquid vermicast contains a balanced range of nutrients but at a relatively low concentration.

Materials and Methods

The experiment was again undertaken in a glasshouse at Massey University’s Plant Growth Unit (PGU). There were two replicates of the four treatments under trial and the treatment solutions were recirculated. 4.5m long open channelling was used, with panda film inside that had slits cut place the plants roots through. (Figure 2) and in photo 10. Photo 11 shows a Cortina and Cireo cultivar lettuce growing in the liquid vermicast solution.

Seeds were sown in perlite in a plug tray on July 5th, 2001 and the seedlings planted out on July 21st, 2001. Growth analysis data (as in experiment two) was recorded.

Treatments

The four nutrient solutions used were:

1. 50% Liquid vermicast supplied by Worm Tech NZ (CF5). (LV)

2. Fish/seaweed mix as in experiment 1 & 2 + 20% by volume liquid vermicast (CF16). (F/S+LV)

3. Conventional NFT solution (CF16). (Convent)

4. 50/50 Cattle effluent from No. 4 Massey University dairy unit, and water (CF10) (CE)

Results

Only the Relative Growth Rate results are presented here, and these demonstrate that one organically derived solution (cattle effluent) will result in growth rates similar to a conventional nutrient solution. Once again the fish/seaweed solution performed badly, and the vermicast derived solution proved to be the worst.

3.4 Discussion

Since the trial was limit to three harvests, due to time restraints it was not possible to take the plants through to a final commercial harvest. However from the results presented it can be seen that the cattle effluent and conventional solutions had better growth rates than the others, with the liquid vermicast solution growth rates substantially lower.

Final Observations

After the third harvest there were a few plants left over in all the treatment channels (except for the liquid vermicast) and these were left to grow on.

All the plants grew well (although the conventional plants looked superior), and then at week seven the effluent lettuces started to develop leaf spots and had all collapsed by week eight.

Experiment Four

Effluent Aeration Trial

A sample of five litres of the effluent used in the third trial was constantly aerated using a fish tank aerator and 25ml samples were taken at intervals and stored in a deep freeze until all the samples were collected. They were then processed to determine the concentration of Nitrogen in the solution as NH4+, and NO3-, and how these concentrations change over time with aeration. The results are presented in Fig 5.

3.5.1 Discussion

The results are interesting because:

1. Since lettuce plants tend to grow faster under a higher NO3 environment if this can be achieved by simply aerating the solution before hand, then this could help provide a way of increasing the NO3 concentrations in some of the organic solutions trial in this research.

2. This trial ended on day 19 but it would be beneficial to re-run the experiment for a longer period.

Experiment Five

Tomato Observation Trial

Tomatoes were used for the observation trial to see how they would react to the organic solutions, particularly due at the lower levels of nutrition. Tomato hydroponic crops are typically run at a CF of 40-60 (4-6 mS cm-1), but as the trial organic solutions could not easily brought up to this level, the conventional solution was limited to CF 16.

4.2 Materials and Methods

The observation trial was conducted in a heated glasshouse at Massey University’s Plant Growth Unit (PGU). It involved four standard NFT channels, each supplied with a different recirculating solution.

All the plants had been grown in a potting mix, watered with a 3:2 (fish:seaweed mix, CF 16, pH6) for two weeks prior to transplanting on July 27th, 2001.

The four solutions used were:

1. 100% cattle effluent (CE) from No. 4 Massey University dairy unit (CF13)

2. 50/50 CE and water + 20% by volume liquid vermicast from Worm Tech NZ (CF8).

3. 50/50 CE and water (CF9)

4. Conventional NFT solution (CF16).

4.4 Discussion and Observations

Tomato plants generally require higher CF levels than lettuce to produce the quantity and quality crops over longer periods of time, for this reason it has been shown that they accomplish this best at CF40-60. In this study the CF of the conventional solution was only 16, the plants were strong and robust, while by comparison all the tomato plants growing in the organic solutions were spindly and lacking in nutrition. This is clearly demonstrated in the plant height measurements. (Fig. 6)

4.5 Conclusion

From the results shown it could prove difficult to come up with an organic alternative to the conventional inorganic solutions for growing tomatoes. The nutrient demands required by tomatoes is extremely high in comparison to a crop like lettuce, for this reason it makes sense for anyone wanting to try organic hydroponics to start with lettuce, and once they have mastered the lettuce crop then to move on to try tomatoes.