How to Choose Proper Soil

Soil is a mixture of minerals, organic matter, gases, liquids, and organisms that together support life. If you have watched the movie "the Martian" (2015), you may remember Matt Damon used Martial dirt (minerals)  fertilized with human waste, which contains bacteria, and water to grow potatoes on Mars. Let's look at the components to have better idea of soil.

Typical Soil Profile

1. Minerals

Minerals include sand (size > 63 µm), silt (size > 2 µm) and clay (size < 2 µm). Generally, sand helps to drain water, and clay helps to retain water. The ratio of them will determine the soil texture. You may have heard "loam soil". Loan has sand-silt-clay of  40–40–20% by weight. Loam is considered ideal for gardening and agricultural uses because it retains nutrients well and retains water while still allowing excess water to drain away. And typical loam soil constitutes: Water (25%), Gases (25%), Sand (18%), Silt (18%), Clay (9%), Organic matter (5%).

In general, the soil in bay area has high content of clay, which makes the soil cannot drain well. In order to plant succulents and cacti, you may consider to amend the soil (consult with amendment supplier), or replace some soil and design slopes to improve the drainage. Raised garden is also desired.

2. Organic matter

Soils supply plants with nutrients, most of which are held in place by particles of clay and organic matter. You may add compost or amendments to increase the organic matter. However, adding too much compost may burn your plants.

3. Gas

Soil has porous structure. During watering, water replaces some air. Then the water content decreases due to surface evaporation and plant root suction. Air is also essential for some bacterial in soil. Everything should be in good balance. In order to aerate soil, you can add lava amendment or perlite. Perlite is those lightweight white particles in soil you see when you visit nursery.

4. water

To be more accurate, it is soil solution after it dissolve some minerals. It carries the nutrition. As I mentioned in another post, too much watering may kill your cacti or succulents. A good way to avoid over-watering is using moisture meter. Some moisture meter can also measure pH, which indicates how acidic the soil is. Most cacti and succulents prefer slightly acidic soil.

How to pick the proper soil?

When you walk into Home Depot or Orchard Supply, you may be confused by all different brands and types of soil. Here is a brief introduction:

1. Garden soil or nursery soil: it is usually cheaper than other soil. You may amend it for your purpose. For instance, you may add compost and perlite for potting.

2. Potting soil: it is most expensive soil, well balanced, designed for potting. It may also contain slow release organic fertilizer.

3. Raised garden soil: light-weight well-drained soil, designed for raised garden. You can mix it with compost to grow vegetables.

4. Vegetable mix: rich, well aerated soil to grow vegetable. You may just amend 1 or 3 to do the work.

5. Top soil: sandy loam for really good drain. Use top soil when you need to raise grades or fill in areas over four inches deep.

6. Essential soil landscape mix: supposed to resist compaction. I doubt it.

7. Soil for citrus: you don't really need it. Amending your soil to better drain and slightly acidic will do the work.

8. Soil for Cacti & Succulents: expensive, but make things easier if you don't want to make your own.