Does dry upper soil hurt established vegetables if water is available deeper?

Over the past number of years, I have added hundreds of cubic yards of wood chips to my large garden, along with a few hundred yards of composted leaves and grass clipping.  (I live in the forest, and I have lots of arborist friends who bring me chips year round.)   I compost some of the wood chips for a few years before adding them to my garden, and I have added a ton fresh to the garden by piling them feet deep between wide raised rows, and letting them compost in the garden.  After the chips are between the rows for two or three years (while being top dressed with grass clippings throughout the summer), I dig them out and add them to the raised rows.  After two or three years, the wood chips between the rows have turned into something that looks like coco fiber, and the only discernible pieces of wood are the texture of styrofoam.  By following this process over and over, my 48" wide raised rows keep getting higher, and the chip area between the rows keeps getting deeper and deeper.  

My raised rows are now 4+ feet deep of soil, well composted wood chips, and fully composted leaves and grass clipping.  I can dig up to my shoulder with just my hand, I have an almost disgusting population of worms, and I've grown some monster vegetables.    

Although summer precipitation here varies widely from year to year, the soil is always saturated in the spring due to the winter melt and spring rain.  My entire garden is above grade and I installed drain tile around the edges, so although it gets very wet 2+ feet deep, it doesn't tend to go anaerobic.  It's just deep, wet, and has an extremely high organic matter content.  The organic matter combined with the native clay makes my soil an incredible sponge. (The entire garden is built on top of a bunch of buried logs and tree stumps, so the water holding capacity is limitless for practical purposes.)  We had a very dry summer last year and reached what was called a stage 3 drought ("exceptionally dry").  I barely had a blade of green grass in the yard, but my garden never really dried out more than about one foot underground, even in areas that I did not water.  

Vegetables develop enormous root systems in the extremely loose and fertile soil that I have built up.  Okra and tomatoes put out roots at least 10-12 feet from the stalk and 4+ feet deep. Nevertheless, the upper 12 inches of soil seem to contain most of the roots, with far fewer reaching down below that.

Amidst the drought last year I was torn about whether I should water.  On the one hand, there was plenty of water and nutrients available deep underground, and once well established, my plants had access to that water and nutrients.  On the other hand, most of the roots were in the upper layer of soil, so if I let that layer dry out, those roots would be unable to absorb water and nutrients.   I am fairly certain that if I had never watered once last year, my well established plants with deep roots (tomatoes, okra, eggplant, corn, peppers, etc) would never have wilted from drought.  But on the other hand, I worried that the plants would not produce well if they were forced to rely only on their deep roots.  So I did give them a deep watering once a week throughout the drought.

Does anyone have any insight or experience about whether allowing well-established plants to go deep for water and nutrients will affect yield?  Any other pros/cons to watering the surface roots when water and nutrients are available deeper?

If they don’t wilt at all they are not thirsty is my opinion

I didn't make it myself, so take this with a pinch of salt.

In dryland farming, it is adviced not to irrigate the surface for the following reasons:
- It will promote weeds that will drain your water resources up and below.
- It will break the isolative crust, allowing deep humidity to escape through the breaks.

The only work that is needed is to scratch the surface after heavy rains (and after the surface has dried) so the crust remains homogeneous.

In your case, your surface is so full of organic matter that I don't know if the above still applies. Maybe you can rest in you hammock?

As you say, most of the feeder roots are in the top foot of soil. So I think you were right to water, to keep your production levels high. That's what I would have done.

In my sandy soil, I find that a bit of coarse straw mulch helps the top layer stay moist a little bit longer.

If you have enough space, it seems like you have the basis for an interesting experiment.

Never lived a dry season as much as you, but to my understanding, you probably have an immense population of fungi underground with all that carbonic matter, i would not worry since they probably contribute to take part of the water to your plants.

When you get your food from your own garden patch, you are heavily invested. It is not a hypothetical notion. You cannot walk away from all that labour, all that work, when the weather shifts.

The fact that the OP only needed to water once a week to maintain production, in a drought, indicates that his methods were a colossal success! Well done Jonathan!

Thanks for the ideas, everyone.  If we get a dry stretch this summer I may in fact water half a row to keep the surface layer moist, and not water the other half of the row.  I don't think that well-established, deep rooted veggies would wilt after a month of no rain in my soil.  But not wilting isn't enough; I want tons of vegetables, which is why I am curious about the necessity of moisture in surface layers.  I only plant 10 or 12 tomatoes and give each plant tons of space and lots of love.   I could probably get more production by planting 10 times as many plants in the same space, but I get my kicks out of monster plants loaded with tomatoes.  

This determinant variety grew 5-1/2 feet tall and produced 80+ pounds last year.  I wish I had taken a photo of the whole plant!




As for the fungus comment.... yes.  My soil looks a lot like the soil in the hardwood forest on my property.  It is a giant fungal mass of organic carbon and worm castings.

I have invested an inordinate (and totally inefficient) amount of time building feet deep of soil.  It's not a practical approach, but it's been an interesting years-long experiment that should pay dividends for the rest of my life.