Soil, Mulch & Nutrition

Girl with muddy handsYour tree is more likely to thrive if you know what type of soil it’s planted in and if you mulch the basin properly. The information below will help you in both matters.

Soil

Use the following methods to find your soil type:

Ribbon Test:

  1. Take a small handful of soil that is damp but not too wet.
  2. Work it in your hands for a few minutes and squeeze a small amount between your thumb and index finger.
  3. Try to squeeze out a flat “ribbon” from the soil. If you can’t make a ribbon or it falls apart immediately, you have high sand content. Soils with as little as 1/3 clay will form ribbons very easily. You can also feel the texture of the various particles on your skin. Sand will feel very rough while clay will feel very smooth. Feeling the texture and measuring the size of the ribbon, you can work out an approximation of how much sand or clay you have.

Drainage Test:
Tree basin

  1. Carefully dig a fist-sized hole in your tree basin with a hand trowel.
  2. Fill the hole with water and watch how fast it drains out.
  3. Sandy soil will drain quickly while clay soil will drain slowly.

Here are some characteristics of the two extremes, sand and clay:

Soil CharacteristicSandClay
How does it hold water? PoorlyWell
How fast does water drain?FastSlow
How does it hold nutrients?PoorlyWell
How easy is it for roots to grow?EasyDifficult

Once you have determined your soil type, using the following information for guidance:

Sandy Soil

Sandy Soil

Water

Water runs through sandy soil quickly, so trees in sandy soil require more watering.  Get details about watering here.

Nutrients

It’s good to add high-nitrogen organic fertilizers every couple of months, especially before mulching. Organic fertilizers include fish emulsion, kelp powder and lawn clippings. Don’t pile the lawn clippings too thick; just sprinkle a thin layer and allow to dry thoroughly. Remember that the soil cannot hold onto these nutrients so don’t over-fertilize or the excess fertilizer will run off in the rain. That’s why chemical fertilizers are not recommended – too many of the chemicals end up in the Bay. If you must use chemical fertilizers, go for “slow release” fertilizers such as Ozmocote or stake-type fertilizers. Be sure to keep a good layer of mulch (see “Mulch” below) in the tree basin at all times. Replenish the mulch as often as needed to keep a thick layer. DO NOT PILE THE MULCH UP AGAINST THE TREE TRUNK as this can cause crown rot and kill your tree.

Companion Plants (if tree basin is 2’x3′ or larger)

For the first year, mulch is the best companion for your tree.  After the first year, small non-aggressive annuals may be planted in the basin, as long as the base of the tree is kept free of plants.  Some good options include strawberry, small succulents, and native wildflowers.  Stay away from rosemary, lavender, jade and other large or woody plants as these will compete with your tree for water and nutrients.

Sidewalk Protection

Because roots can travel easily through sand, and water tends to move downward quickly, sandy soils will have fewer problems with roots cracking the sidewalk. Get more info about sidewalk care here.


Clay Soil

Clay Soil

Water

Heavy clay soil requires less watering. Get details about watering here.

Nutrients

Since clay soil holds onto nutrients well, you shouldn’t need to add chemical fertilizer. An occasional top layer of organic fertilizer such as lawn clippings, fish emulsion or kelp powder can be helpful, but not necessary. Mulch is still an important part of tree care and soil management (see “Mulch” below). Replenish your mulch as often as needed to keep a thick layer. There is less of a need to add nitrogen when mulching than with sandy soil. DO NOT PILE MULCH UP AGAINST THE TREE TRUNK.

Companion Plants (if tree basin is 2’x3′ or larger)

For the first year, mulch is the best companion for your tree.  After the first year, small non-aggressive annuals may be planted in the basin, making sure that the base of the tree is kept free of plants.  Some good options include strawberry, small succulents, and native wildflowers.  Stay away from rosemary, lavender, jade and other large or woody plants as these will compete with your tree for water and nutrients.

Sidewalk Protection

Roots have a hard time trying to grow in clay soil. Therefore, they are more likely to grow close to the surface and cause sidewalk cracking. Avoid this by planting less aggressive and smaller trees, and have as large a tree basin as possible. Get more info about sidewalk care here.

 

Mulch

Mulch is organic matter that is usually partially decomposed. Compost is more fully decomposed organic matter, but it is sometimes used as mulch. Mulch goes on top of the soil, about 3″ deep. Don’t pile mulch around the trunk, as this can cause trunk rot and tree death. Leave a space around the trunk a few inches in all directions.

Benefits of mulch:

  • Holds moisture in the soil longer. A big benefit in our windy climate!
  • Suppresses weeds
  • Raises your tree basin to sidewalk level. When we cut the concrete, there is often a gap between the sidewalk level and the soil level. We plant the tree at sidewalk height to avoid it sinking too deeply. Depending on the size of the gap, you may need to add a bucket or two of native soil and then add the mulch.
  • Over time, mulch improves the soil. The mulch breaks down into organic matter over time, which benefits all soil types. Just keep refreshing the mulch layer periodically.
  • Looks nice.Mulch as a top dressing is much better than brick, stone, or grating. Those other materials add nothing to the soil and can wound the tree if not adjusted. For heavy clay soils, they can actually compact the soil, causing roots to grow under the sidewalk rather than in the soil.
Sidewalk garden

Plenty of mulch in this sidewalk garden on Lisbon Street in the Excelsior neighborhood.

Materials to use for mulch: Wood chips, shredded bark, and compost. Avoid sawdust and pine needles, as they take too long to decompose and can actually rob nitrogen from the soil.

Note: For new trees in sandy soils, it helps to add a little organic nitrogen fertilizer just before mulching. Sandy soil is already low in nitrogen, and mulch may temporarily lower nitrogen levels as it decomposes. This is not generally a concern for mature trees and plants.

Where to get mulch free or nearly free:

  • Your own yard. Grass clippings, chopped leaves and branches make great mulches.
  • Bayview Green Waste: 1300 Carroll Avenue, off 3rd Street. Open to the public on Saturday from 7:30 a.m. to noon. For weekday times, call 415-822-7686.

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