Soil, Mulch, & Nutrition
Soil
Use the following methods to find your soil type:
Ribbon Test:
- Take a small handful of soil that is damp but not too wet.
- Work it in your hands for a few minutes and squeeze a small amount between your thumb and index finger.
- 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:
- Carefully dig a fist-sized hole in your tree basin with a hand trowel.
- Fill the hole with water and watch how fast it drains out.
- Sandy soil will drain quickly while clay soil will drain slowly.
Here are some characteristics of the two extremes, sand and clay:
| Sand | Clay | |
| How does it hold water? | Poorly | Well |
| How fast does water drain? | Fast | Slow |
| How does it hold nutrients? | Poorly | Well |
| How easy is it for roots to grow? | Easy | Difficult |
Once you have found your soil type, you can find more information by using the following soil info charts:
Sandy Soil |
|
| Water | Water generously 2 or 3 times a week on a slow drip. We recommend 20 gallons a week for the first 2 years after planting, followed by half as much for the 3rd year. The water you put in runs through the soil quickly. It travels straight down and does not spread to the side. Therefore, it’s best to move your water source around in order to get all of the roots. When sand dries out too much, it can actually become hydrophobic, that is; it refuses to accept water. If this happens, a watering agent such as Water-In should be added to the water to allow the water to penetrate. Water-In is available at most garden stores. Do not use detergent as a substitute as this will be toxic to the plant. |
| 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 overdo it, it will just run off. That is why chemical fertilizers are not recommended – too much chemical runoff into 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 (wood chips or compost) at all times in the tree basin. 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) | Plants in the pea and bean family are nitrogen fixers; that is, they can draw nitrogen from the air and turn it into a usable form for plants. Other plants growing nearby can benefit from this. Annual plants in this family include climbing sweet pea or red clover (give the sweet pea something else to climb besides the tree!). Perennials include native lupine shrubs. Be sure to not put plants right against the trunk as this can cause crown rot. |
| Sidewalk Protection | Since roots can travel easily through sand and water tends to move downward quickly, sandy soils will have fewer problems with roots cracking the sidewalk. Of course it is better for the tree if you can remove as much concrete as possible. Local ordinances require at least 4’ of walking room on a sidewalk between a street tree and your house, not including your private property. In other words, if the sidewalk is only 6’ wide, your tree basin can only be 2’ wide. But you can expand it lengthwise. Please call the DPW Street Use and Mapping at 554-5810 to find out more about basin expansion. |
Clay Soil |
|
| Water | Water 10-15 gallons per week with a slow drip. The heavier the soil, the less water it may need. Watch to see how fast the water drains and avoid creating a swampy condition. Roots need oxygen as well as water. Try not to step on or dig in the soil when it is wet, because you can squeeze the oxygen out of it. If clay soil dries out too much, water may just puddle on the surface. Try to keep the soil evenly moist without flooding it. |
| 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. 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) | Since clay soil needs more oxygen, choose plants with tough roots to break up the clay. Some examples are chrysanthemums and mints, or tubers such as iris. Keep them from growing too close to the tree to avoid crown rot. |
| 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. Plant less aggressive species and smaller trees, have as large a tree basin as possible. PLease call DPW Street Use and Mapping at 554-5810 to find out more about how big you can expand your basin. (SF City law requires 4’ of sidewalk space for wheelchairs and strollers to pass a tree basin). You can also do preventative root pruning when the tree is young by taking a flat shovel and cutting around the border of the tree basin every 6 months or so. Using a "rootguard" for larger tree species or species with invasive roots may prevent or delay future sidewalk disruption, but works better in sandy soils. |
Multch
Mulch is organic matter that is usually partially decomposed. Compost is more fully decomposed organic matter, but it is sometimes used as a mulch. You probably just need a bucket full for the average size tree basin. The mulch goes on top of the soil, about 3" deep. Don’t pile the 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, 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.
Materials to use for multch: 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 on 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:
- St Mary's Urban Youth Farm, 600 block Alemany just past Ellsworth
Naomi Goodwin, Garden Coordinator, slugsf@hotmail.com - 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-noon. For weekday times, call 822-7686.
- Feline Pine Cat Litter can be spread as mulch after it has been used. For more info visit their site.
