Fruit trees struggling in rocky soil - how can I help them out?

March 8, 2018 8:44 AM

I have some fruit trees that were planted in hard, rocky soil. They're getting by, but certainly not flourishing (or providing much fruit). In an ideal world we would have improved the soil before the trees had been planted. Is there anything we can do now, though? I have lots of great compost, but I don't know if layering it on top of the rocky soil will do much if it can't penetrate. Thanks!
posted by malhouse to Home & Garden (7 answers total)

Best answer: How rocky is rocky? Did you soak the ground when digging, and if so, how fast did the water penetrate the soil? I'm familiar with very rocky, dense clay soil, where water will just sit in a hole. Plants in those situations generally don't grow very much (but we planted those for an ornamental screen, not to produce fruit, so we didn't try much more than a bit of mulch on the surface).

You could lay down a couple inches of compost on top of the ground*, at least the distance of the tree canopy if not 1.5 times that distance. Make a little moat around the edge of that, so when you water the tree, the water and compost stays in the area and (hopefully, eventually) penetrates the ground. And what kind of roots do your trees have? Are they surface spreaders, or deep rooters? Depending on the tree, you may be able to dig the compost into the ground a bit.

* Here's a discussion on mulching on top of trees - I should also note that you should also avoid over-mulching on top of young tree roots.

You'll probably get better advice from a local nursery or local botanical groups who know the local soils and weather and can suggest locally appropriate solutions.
posted by filthy light thief at 9:16 AM on March 8, 2018 [1 favorite]

Best answer: This is where a soil auger comes in handy. With an auger, you can drill a series of holes evenly spaced under the tree's canopy, then fill them with a soil amendment and/or fertilizer. The depth and diameter of the hole depends on the tree size and what you plan to put in it. A one-inch diameter auger bit going down 6-8 inches is perfect for adding granular fertilizer. For adding soil amendments, you'd want a minimum 2-inch diameter hole, though a two-and-a-half or three inch hole would be easier to pour the amendment down. Depth - at least eight inches, but more is better.

Spacing: start a foot away from the trunk and work in concentric circles out to about a foot beyond the tree's canopy. Spacing - 12-14 inches apart would be great, but it may be too much work, and remember - you can make this a project spanning two-three years. So if you need to space more widely this year, don't fret. Remember, anything you do for them will be an improvement.

If you have a reasonably sturdy drill, you can buy auger bits at many hardware stores. Or you can rent a soil auger, though they usually come with much wider diameter bits, since they're using for digging post holes and footings.

I've done this with many of my trees over the decades, and it does make a difference. Pockets of soil amendments improve drainage and airflow, so even though it's not nearly as good as properly preparing the soil beforehand, it does help improve poor soil situations. And the best way to feed a tree is to get the fertilizer down in the root zone, below the root zone of the lawn or groundcover.
posted by Lunaloon at 11:48 AM on March 8, 2018 [2 favorites]

Lunaloon's plan sounds good, but I'd add that long term success depends on the soil being permeable by roots and water. I know of a tree planted in a bowl hacked out of shale. It might as well have been planted in a bucket and never did thrive.
posted by SemiSalt at 1:38 PM on March 8, 2018

Also, watch the trees to see if they're water-stressed; if so, drip or soaker watering will help (get a steady but not drowning quantity of water down to their limited roots).

Maybe keep them pruned small until the soil is better, exactly as you would if you were growing them in containers on purpose. This will depend on the tree species and maybe climate, look for local extension advice?

Not too much mulch at any one time, but keep an eye on it and add more when it's thin. Some will wash or blow away, some will be metabolized into the air, but a little bit will get carried into the soil and improve it. A year doesn't show much. A decade of steady mulching can transform the soil.
posted by clew at 2:07 PM on March 8, 2018

The thing you want to be wary of with adding compost/mulch on top of the soil / near the surface is that your tree will develop a shallow root system and do really badly if there's a dry period.

+1 to asking your county Master Gardeners / Extension. They'll know more about the common soil types in your area and will be able to ask you lots of questions about your trees (what kind? how old? how big? symptoms besides low fruit yield?) to give you good advice.
posted by momus_window at 4:13 PM on March 8, 2018

Your local National Soil Conservation Service is a good resource, too. I have a rocky soil base and just planted an orchard in a high tunnel. With the NRCS soil scientist, I took soil samples and he told me what to add before planting. One of the recommendations was planting a combination of white dutch clover and oats. The oats die when it freezes, but the clover is a perennial and is a permanent source of nitrogen for the trees. Some of the other things he had me add to the soil before planting were lime, fish bone fertilizer, and K-Mag (I'm growing organically).
posted by summerstorm at 10:18 PM on March 8, 2018

Rocky soil isn’t as big a deal as poor drainage. If it drains well and has a reasonable water holding capacity then that’s a bonus. Poor drainage is a bigger problem than the rocks themselves. If you have heavy sand and gravel, then you will need to water the trees more often.

If you have clay soil then one option is to have part of the tree ball exposed and not fully in the ground. Then cover the part of the ball that is exposed above ground with a really good quality soil that is very high in organic matter.

Adding a couple of inches of soil to the base of the tree and making a small bowl shape can help with watering.

What else could cause a lack of fruit?

Soil conditions are important to successful fruit trees but pollination is a key ingredient. Although some fruit trees are self-pollinators, others are not. Planting the right variety of fruit trees in a close proximity to each other increases cross-pollination resulting in a higher chance of fruit yield.

Adding bee-friendly plants and flowers to your garden to interest the pollinators can be useful as well. Be careful not to spray insecticides and pesticides over your fruit trees when they are in bloom as you are bound to kill off pollinators in the process.

If you’re not sure where to find the perfect pollination partner for your fruit tree, this pollination checker can help you choose the right variety.
posted by joebailey at 2:57 AM on December 3, 2018