With the climatic conditions in the Territory and in particular Darwin, solarisation of a 25cm-high mound of potting mix appears to be a distinct alternative to the currently available methods of steam pasteurisation or sterilisation or soil fumigants. High temperatures, constant sunshine and intense solar radiation are in abundance in the Darwin area and as far south as Katherine for most of the year. Investigations in Darwin have shown that solarisation using two layers of clear plastic was far more effective than a single layer of clear plastic at raising the temperature of a 25cm mound of potting mix to levels lethal to three commonly found soilborne pathogens, Pythium myriotylum, a tropical pathogen which grown well at 40°c, Sclerotium rolfsii and Phytophthora nicotianae. Potting mix was mounded onto nursery trolleys that had a layer of foam to insulate the mounds from heat loss from the metal base of the trolley.
The temperatures generated during hot sunny days under the two layers of clear plastic, from September to mid-May, ranged from the mid-40°C to a high of 51°C, with mean maximum temperatures ranging from 42°C to about 48°C. The wet season would be the only exception, which experiences a great deal of overcast rainy weather, reducing temperatures by 4-5°C below those experienced during clear summer days. These temperatures effectively eliminated all three pathogens within a relatively short period of time, 3-9 days. The shorter period was a result of consistent fine hot weather, particularly during the build-up. The longer period resulted from overcast rainy weather, as experienced during the wet season, and the period at the start of the dry season (May) and post dry season (September).
Solarisation was not completely effective during the winter (dry season) period, mid-May through to September, temperatures only reaching as high as 41°C. However it could still be possible to adapt these findings in future experiments. By reducing the height of the mound to 10-15cm, it should then be possible to use this technique to generate temperatures that are high enough to pasteurise the potting mix, however this has not been fully investigated. It would be more appropriate to stockpile treated potting mix in bins or containers for the winter period until the return of ideal solarisation weather. The solarisation process was designed for the trolley system, due to its mobility. It would be of most use to the smaller nursery that only use small volumes of potting mix at anyone time. Each trolley can hold approximately ½m3 of mix and can be solarised within 3-9 days depending on the weather. However mounds of potting mix could just as easily be treated on the ground as long as there is an impervious layer of material separating the potting mix from the bare earth. This impervious layer could be heavy duty plastic, concrete or even bitumen. Larger nurseries with vacant land in full sun could just as easily carry out this process. Perhaps not on a regular basis due to the small volumes of mix being solarised and the large volumes needed by the nursery, but occasionally during the spring/summer months. This would reduce the reliance on fumigants, reducing nursery costs.
Solarising potting mixes in plastic bags could be used as an alternative means of solarising bulk mix under the second layer of plastic. This would be appropriate for retail outlets selling potting mix in bags as is the case with a few local nurseries. Its use in the field situation in Darwin is also a distinct possibility, as the most appropriate time to conduct solarisation in Darwin, spring/summer, is when there are few or no crops grown in the ground. If the land was solarised in strips, then the plastic could be left in place and painted and the seedlings then planted directly through the plastic into the treated soil. A comparison of plant growth and yield could then be conducted in such a situation.
With the ever-increasing demand for the control of pesticides, this soil treatment technique is an alternative. It is relatively cheap, it utilises the natural elements, requiring no pesticides for this environmentally friendly product.
DATE: November 1992
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