Wednesday, September 20, 2006

But what of the lilies in the field?

Add the $20 hole to my long list of "conventional garden wisdom" that needs to be debunked.

You'll often hear about the importance of lavishing organic materials on the planting hole, amending the soil in a plant's future home with lots of organic material, bone meal, blood meal, kelp, fish heads, sacrificial virgins, etc. etc.

The idea makes sense, of course: We all know how important it is to have rich, humusy soil full of nutrients and capable of retaining moisture. So it makes sense, right, to mimic good soil by sparing no expense in improving the soil nature has provided with organic materials, etc.

However, as is usually the case with Conventional Wisdom, the seemingly-sound idea behind the $20 hole does not match reality. Quoting from the University of Florida IFAS-Extension literature:
A common procedure for transplanting container-grown plants involves amending the backfill around the root ball with an organic material such as peat. However, a significant amount of research over a range of irrigation schedules, plant materials, and soil types provides no evidence that this practice is beneficial. In fact, incidences where roots remain in the amended backfill soil and do not grow into the undisturbed field soil have been reported. The argument for amending backfill soils centers around the fact that peat increases the waterholding capacity of a sand soil. What actually happens is the water in the adjacent landscape soil is held at greater tensions than the water in the amended backfill. The result is a drying container root ball as water moves from the container medium and the amended soil into the adjacent landscape soil as it dries.
Plants transplanted in very rich holes do not, as expected, thrive; instead they are actually harmed by the over-abundance of organic material. This is clearly an example of too much of a good thing, and a waste of money, at that.

I've known about this debunking for some time, but came across it again today while exploring UF's extension website, which offers a lot of sound gardening advice backed by a good dose of empiricism. I don't enrich my planting holes much, usually removing a spadeful of Florida "dirt" and replacing it with compost from my pile, a fork or two of peat, some ground pine bark, granular, slow-release fertilizer (Osmocote), some Milorganite (for a weak but quick boost of NPK) and, lately, some water-absorbant crystals. No pretensions for science or consistency. Whatever suits my fancy and whatever is at hand.

Then, lots of mulch.

In annual beds, I pull the mulch back, empty a couple of wheelbarrows full of compost and spread it evenly. Then I plant my seedlings in the compost, heaping the compost up into little, low mounds. The mulch goes back, and that's it.

The mounding serves two purposes: It raises the plants higher, so the mulch doesn't cut their stems. And, since everything sinks in Florida over time, it prevents the plants from being "drowned" or rotting from too much water. (This is the nature of an inert sandy soil: organic materials decay, but the sand does not.)

At least once or twice a year, for all my bushes and new trees (particularly for roses), I pull back all the mulch from the beds and put down a thick layer of compost and shredded leaves and a sprinkle of Osmocote. Then I push back the mulch.

I think this slow and dispersed amending of the soil, from the surface down, avoids the pitfalls of the too-rich hole while still improving the general health of the soil.

1 comment:

Ki said...

We have the exact opposite condition with clay soil. Do dig a $20 hole or million dollar hole just don't add amendments. We have clay and if you add amendments you've just created a tub with the amendments acting like a sponge and drowning the plant. Back fill using the dug up clay.