Plant Doctor Archive

Arsenic damaging garden?

Question EARLIER this year I decided to raise my vegetable garden by 40 cm. I did this by using H4 treated timber. Despite large amounts of natural manures and compost the soil is dead. This I attribute to the ARSENIC in the treated timber. There seem to be no worms or micro-organisms , and the soil is not friable and has green algae. Moisture also will not drain through it.

Answer I ASKED the advice of Reg Lewthwaite, one of our horticulture lecturers, and an expert in both toxins and compost. Reg said:

I have seen many gardens growing in raised beds which were constructed with treated timber. There might be human health questions about arsenic, but it has never been a problem with plant growth.

The reasons plants might not do well include:

1. The compost being too fresh. Composts (including animal manures) which are fresh are still in the process of breaking down. During this phase the soil bacteria use up the free nitrogen in soil. The plant can’t get enough nitrogen to grow. The solution is to wait a while before planting.

Some composts release high levels of toxic chemicals during breakdown.

These disappear as the compost matures. The answer is to wait until the compost is mature before planting out.

2. Too high a proportion of compost to soil. Compost should be mixed with soil — not be the main ingredient. I have inspected vegetable gardens where plants have been killed by growing in pure compost. The reason is that as the compost breaks down it releases nutrients. In a pure compost these can get to a toxic level and damage roots.

The answer to this is to mix in more soil or to wash out the nutrients with a heavy watering.

3. Another reason develops from the last comment. In dry conditions the plants have to fight to get enough water. Make sure the plants are fully watered in dry conditions.