I just discovered a terrific blog:

http://www.robertkouriksgardenroots.blogspot.com/ (see blogroll).

On this site Robert Kourik explores many aspects of plant roots and soil science. He holds many views that are at odds with the status quo in landscape gardening today, but are nonetheless both intriguing and backed by solid research.

One such view is that plants can thrive with frequent, small amounts of water applied through drip irrigation. He argues that when drip is run once or twice a week, as is typical, a substantial amount of water is needed just to re-hydrate the soil so that plants can accept water. Kourik claims that watering a small amount daily gives plants better access to moisture. I’m not sure I agree with this theory as I have been a tireless advocate of deep watering to establish deep roots. Kourik alleges, however, that most of the microbial activity in the soil (nature’s fertilizer factories) takes place within the first couple inches of soil, and that frequent irrigation can provide the plants with the same hydration while using less water.

Another controversial view that Kourik holds is that amending planting holes is detrimental to plant health. Again, see the link for more details but the basic idea is that by amending the planting hole you discourage plant roots from spreading and adapting to the native soil. Instead, he claims, roots simply twist around themselves in the amended hole. I have planted thousands of plants in my career as a gardener, and I have utilized Kourik’s method, as well as other more traditional techniques that involve amending planting holes with compost to increase drainage and to encourage the growth of plant roots. Plants that I have planted without amendment have often struggled do to lack of drainage and tight soil that roots simply cannot penetrate. That said, plants in this group that have become established do seem to do better than traditionally planted neighbors. The fact is, every soil is different, and if you have a loose sandy loam, amendment may not be necessary, but if you have a adobe clay, you may have to mix in some organic matter to get anything short of bermuda grass to grow.

Lastly, Kourik warns of the dangers of symphylans, small soil organisms that eat plant roots. He claims that adding compost that is not fully decomposed and adding a layer of mulch over 2 inches can invite these nasty little creatures for which he says there is no know organic solution. I have often advocated a thick layer of leaf matter or the dump’s early mulch to control weeds and build topsoil, but this revelation is making me question the extent to which garden’s should be mulched, and with what. I have researched symphylans a bit and found some worrying studies, but they are mostly focused on agriculture. If you have any information about this little known pest, please post a comment.

Here is a comment on this post from my mom Carol Mitchel:

The root thing is debateable.  I agree with you that it all depends on what type of
soil you have.Here we have mostly sandy loam with some red adobe.  I have planted all
of my trees without any amendments. I add the amendments to the top of the soil.
I figure the nutrients get washed down to a certain extent.  I like the alfalfa hay
the best because it builds such beautiful soil while it fertilizes. I think legumes might
have other benefits to plants.  The research I read in Science News about the hairy vetch
actually turning on a couple of genes in tomato plants that caused them to be resistant to
certain diseases as well as live longer impressed me.  I have been using this in my garden
for the last 3 or 4 years with success.  I plant the tomatoes amongst the vetch in their
wall of waters.  Later I cut the vetch and use it for mulch around the tomato plants.  I use
this in conjunction with my bottles of red water.  The bottles provide a little extra heat
at night and give the plants the wavelength of light they like.