Reusing Potting Soil

When replanting in containers, should the old potting soil be recycled?

Reusing potting soil can work for a while; it can be rejuvenated each time by adding some nutrients and perhaps some organic matter. If a pathogen, however, whether fungal, bacterial or nematode, is introduced by way of an infected plant, the whole pot is contaminated and subsequent plantings are at risk of being infected. Have you ever planted seeds in a container and nothing comes up?  This is one reason why. 

Sanitation is important both for commercial operations as well as for the backyard. Buying new potting mix each time is the best solution. Since this can become costly, the question arises, “Can I clean up or sterilize used soil?” Here are three methods:

1) Solar sterilization, putting clear plastic over a pile of soil and allowing it to bake for 4-6 weeks, is a good idea in some locals since warm weather is required. 2) Steam sterilization is quite effective, but one must have access to steam. 3) Lastly, and perhaps the most practical for small quantities of mix is the oven. Here are the directions: place slightly moistened soil mix in a covered pan and bake in the oven at 250 degrees F for about 45 minutes.  The center of the mix needs to reach a temperature of 180 degrees F for at least 30 minutes; use a candy or meat thermometer to check the temperature.  This process may produce an unpleasant odor.  

Used potting soil can be thrown in a garden compost pile. Alternatively, a designated pile of used potting soil only can be created. When the process is complete, the soil is able to be reused for container purposes. This procedure will be relatively safe if either compost pile heats to proper temperatures, and the heating is uniform. Ideal temperatures range from 104 F. for 5 days to 130 F. for 4 hours. 

If potting soil is reused, I would recommend a type of crop rotation. Whether vegetables or annual flowers, replanting with the same species of plant or even in the same family is not advisable. For the next planting, rejuvenate the soil and plant with a different family of plants. In conclusion, gardeners have been successful in reusing soil mixes; nevertheless, the above information is useful for making an informed decision.

Ohia Decline on the Big Island of Hawaii

This is a phenomenon that has been observed as far back as 1906. In the late 1960’s a major decline of ohias took place.  In 1986 the USDA Forestry Service published a lengthy article on the subject   http://www.fs.fed.us/psw/publications/documents/psw_gtr086/psw_gtr086.pdf

The decline is complex and not totally understood. We do know that older trees are not as vigorous as they once were and are more susceptible to stress. The stress may come as a prolonged period of rain or the opposite, drought; poor draining soils will compound the problem. Other factors may include vog (volcanic emissions), low soil nutrients, bulldozers and perhaps dense stands of invasive species.

                                           

These stresses alone may cause some type of decline. But what usually happens in the next phase is the invasion of a root rotting fungus or perhaps a tree boring beetle. Fungal organisms are often found infecting the roots of declining trees but are not thought to be the primary cause.   

Young ohia trees can grow well for many years on shallow soils.  Eventually they may decline when they become large trees, and the shallow soil simply cannot support their growth, particularly under drier conditions.

What can be done?  Actually, not much.

·        If the property contains several older ohia trees, planting young ones will diversify the age group.

·         Keeping the trees healthy is important; fertilize if needed.

·         It is impractical to water a stand of ohia trees during a drought, but watering a few trees around the house may be feasible. 

Photos by Forest and Kim Starr

Planting Cocoa, Coffee and Macadamia Nut Trees at Higher Elevations

The cacao tree evolved as an understory shade tree in tropical rainforest regions. For commercial production, cacao is best adapted for hot, humid tropical areas with evenly distributed rainfall. This would mean year-round temperatures at about 68°F/20C and no freezing temperatures. For growing cacao trees around the home, these limitations can be stretched within the range of 65F to 90°F (18C to 32C). Temperatures below 50°F/10C may damage or kill the plant; defoliation and dieback will occur between 40F - 46°F  (4C - 8C).  Flowering and fruiting only occurs at 68°F/20C or above. Temperatures in excess of 90°F/32C may limit plant growth. As a reference, the warm subtropical climate in South Florida is very marginal for growing cacao.

Cacao is a shade plant and grows best in about 25% shade.  It is often planted with other commercial crops that protect it. In some regions it is grown in full sun, although shade is used during establishment.  For homeowners it can be planted under the canopies of tall, overhanging trees or next to buildings or structures. Cacao does not tolerate windy conditions and should be planted only in wind-protected areas.

On the other hand, macadamia nut trees can withstand colder temperatures than cacao. Although they are adapted to warm, subtropical conditions, mature trees can withstand winter temperatures as low as 25F - 26°F (-3.8C to -3.3C) for short periods with minor foliar damage. Young trees, however, are killed by temperatures near freezing. Temperatures below 28°F/-2.2C will cause damage to flowers and young fruit thus reducing production. In the tropics, macadamias are better adapted to medium elevations of 2100 to 3600 feet.  In Hawaii, commercial macadamia trees are not planted above 2500 feet.

The optimum growing conditions for coffee include high humidity, protection from wind and temperatures from 59F to 75°F (15C to 24C). Coffee plants are damaged or killed by freezing temperatures, while constant temperatures at or below 41°F/5C may cause leaf drop and tree decline.

In the tropics or warm subtropics, coffee is grown at high altitudes up to 3,500 feet; temperatures there are moderate and never freezing. In a few places like Kenya and Columbia, coffee is grown at elevations as high as 7,000 ft.