What kind of oak?

Oaks have a tendency to be allelopathic; i.e., they retard growth below them.

Oaks are not tough competitors for citrus; oaks get their food and water from deep tap roots; citrus get their food and water primarily from the first 18 inches; so they don't compete directly. If your planting site is not directly beneath the oak; and if it gets a reasonable amount of light/sun, your citrus should do okay. Soils beneath and around oaks tend to be acidic; and citrus like that.

how far away is it? how close is the dripline?

Not sure what type of oak and it is about 20 feet away. I will have to run new drip line so I can put it wherever needed

John, you are mistaken about the location of oak tree roots, at least the ones that absorb water and nutrients. Those are ALL located in the upper 12 inches of the soil surface. Some oaks send anchor roots down deep into the soil but they have nothing to do what the 'feeder' roots do. As a matter of fact, 12 inches would be deeper than any I've ever seen.

The extremely wide spreading, fibrous mat of feeder roots is normal physiology for most deciduous trees. It not only sustains the trees in terms of water and nutrients, but holds the trees in place. I would fully expect that a mere twenty feet away from this oak, the soil system would be packed full of the fibrous roots of that oak.

These roots extend very far beyond the reach of the (so called) drip-line. Depending upon the health of the tree, those roots may be found a hundred feet away from the trunk.

So why so shallow? Because these trees need an enormous amount of water, of dissolved minerals, and...OXYGEN. All three will be found (especially 02) in the greatest quantities in the top few inches of soil.

It's also something of a myth (2nd myth) that the soil under oak trees is acidic. FRESH oak leaves might be somewhat acidic, but dead ones are not. That's even true with the evergreen oaks so common to the South.

Stick your shovel into the soil and tell us what you find about two or three inches down. That's about where the mat of feeder roots will start.

By the way, if you can post some pictures of the leaves...even fallen leaves....I'll be able to identify it for you. Leaves AND acorns would be a plus.

Rhizo... as we say in my country, Me rindo... in English, I surrender.. to your obviously greater knowledge of oaks and other species, of which I am only a casual observer.

I do know, from my casual observations, that oaks have roots as deep as the tree is tall; but that observation only applies to the oaks I have known in California.

John, thank you for your grace "under fire ", so to speak. This is topic is a very good teaching tool that I will pursue for just a bit.

First, let me say that the belief that tree root systems are a mirror image of their above ground structure is prevalent. False, but common. Yes, some of the big structural roots enter the deeper areas of a soil system but, as I have said, anchor roots are not the major bulk of a root system and do not participate in the
physiological support of the tree body.

If you think about how tall a nature oak tree can become....roots could not grow nor thrive in depths of 60 to 100s of feed. You're familiar with that phrase "folllow the money "? With plant roots, it's "show me the oxygen ".

Another long lived myth is that of tap roots. Many tree species begin life with tap roots but will lose them in favor of the more successful fibrous root system.

I'm not saying that we can't grow stuff under the canopy or within the persistent root competition of large trees. But it can be a real challenge as the new plants try to compete for water, nutrients, oxygen, and physical space in an area that can reach 2 to 3 times the radius of the canopy and just below the surface of the soil.

Rhizo, thanks for correcting those two myths, both of which I have held as gospel most of my life..

Luckily, my future depends only on Meyer lemons about which I know quite a lot, but still learning. We have 15,000 trees now; and an aggressive 5 year plan for 50,000 producing 15 million fruits per year.