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Ask the Soil Pros Archive

• Is there a particular type of soil I should use for bonsai trees?
• How does the pH level affect my plants?
• Why should I add soil amendments to my garden?
• How much compost do I need?
• What is the right way to pot plants in containers?
• Why use soil amendments?
• What else do soil amendments do?
• What kind of soil is best for pots?
• How do I amend my soil?
• Potting soil recipe
• Soil Microbes
• Reseeding your lawn
• Soil Texture
• Cation Exchange Capacity
• Germinating seeds
• Good Bonzai mixes
• Growing flowers in desert clay
• Fixing acid soil

I am new to the planting world. I recently purchased a bonsai tree and would like to replant it. Is there a particular type of soil I should use or will regular potting soil work?

Thanks for your question about potting soil for bonsai. Interestingly, the potting soil we make for Orchard Supply Hardware is a main ingredient in the bonsai mixes used in the world class bonsai collection at the National Arboretum in Washington D.C. I don't know if you are in California and have an Orchard Supply near you, but if you do a good bonsai mix would be OSH Potting Soil mixed equally with chicken grit (the small granite pebbles used to feed chickens– you should be able to find that at a pet/feed store). If you can't buy the potting soil we make, make sure to buy a high quality mix containing peat moss, perlite, and very evenly ground wood products to mix with the grit. The grit is added because bonsai requires very well-drained soil and the sand in most potting mixes isn't enough to fulfill that requirement. There are retail bonsai mixes available but representatives at the National Arboretum don't recommend them because they are too coarse. For more information on bonsai care, contact the National Bonsai Foundation.

How does the pH level affect my plants?

The pH of your soil is important to know because it often determines the amount and type of nutrients that are available to your plants. The pH scale of 0-14 measures the soil's acidity and alkalinity; the lower the number, the more acid the soil and the higher the number, the more alkaline or base. A pH of 7.0 is neutral, but most plants will grow in soils that range from 5.5 to 7.5 pH.

Some plants actually prefer an acid or alkaline soil, such as azaleas or rhododendrons which need an acid soil, but in general, the more neutral the pH level, the better.

Most nurseries sell small pH testing kits and many communities also have soil testing labs that can test your soil. Both are inexpensive and helpful, though you'll get more accurate results from a lab.

As a rule of thumb, remember that areas that receive a lot of rain, like the Pacific Northwest, usually have more acidic soils, and those that don't, like the Southwest, tend to have alkaline soils.

To make your soil more acidic, amend it with redwood soil conditioner or peat moss. In some cases, add iron sulfate or soil sulfur. To make it more alkaline, add lime.

Why should I add soil amendments to my garden?

Taking care of your soil is one of the simplest and most important things you can do to keep your garden healthy. Adding organic soil amendments during planting and periodically through a plant's life helps it develop strong roots and vigorous flowers and foliage.

The general rule of thumb is the finer the soil type, the courser the amendment and vice versa. Coarse, sandy soils benefit from finely textured amendments, such as mushroom compost. And fine clay soils need coarse amendments such as redwood, fir bark humus or gypsum (which should be used sparingly because it will raise the salt content and harm plants).

Be sure to find out if the redwood or fir bark is stabilized and enhanced with nitrogen, however, or it could actually pull nitrogen from your plants. If it isn't, add a source of nitrogen such as fish emulsion, bat guano, blood or bone meal.

And always use caution with animal manures. They are a good source of nutrients for any soil type but can burn plants if not properly aged.

Those with heavy clay soils, be forewarned: it could take up to five years of regular, generous amending (preferably with a rototiller) to improve it.

How much compost do I need?

In flowerbeds and in vegetable gardens, spread 2 to 4 inches over existing soil and thoroughly blend into the top 2 to 4 inches.

When seeding new lawns, till seedbed to a depth of 6 to 8 inches. Mix in compost and starter fertilizer prior to seeding. Smooth and firm seedbed, then sow seed in two directions perpendicular to each other. Cover seeds with 1/4" compost and lightly roll or rake in. Keep soil moist during germination period by applying frequent, light waterings.

Renovating established lawns: Remove weeds. De-thatch and aerate area to be renovated. Broadcast seed, spreading more seed on bare areas. Apply starter fertilizer and cover with 1/4" compost. Keep soil moist during germination period by applying frequent, light waterings.

You can also use compost as a decorative mulch that prevents weeds and moisture loss. In all garden beds, carefully apply a 3 to 4 inch layer of compost to prevent light from reaching soil surface.

To plant trees and shrubs, dig a hole for planting, making sure hole is twice as wide as plant's root ball and approximately 1 inch shallower than root ball. Place plant in hole. Mix an equal amount of compost with soil from the planting hole. Replace amended soil in hole and tamp down firmly. Water thoroughly.

What is the right way to pot plants in containers?

When potting plants, fill container with potting soil and water thoroughly if it contains polymers to aid their hydration. Push evenly to the sides,creating a hole slightly larger than the plant's root ball. Gently lift plant from nursery container and loosen compacted roots, if necessary. Place in hole, positioning the top of root ball slightly higher that the surrounding soil level. Gently pack potting mix around plant so it firmly touches the entire root ball. Water thoroughly. Don't put pottery shards or rocks in the bottom of the container to assist drainage. They actually create a "false bottom" that can make drainage more difficult.

Why use soil amendments?

Soil isn't just... well, dirt. Millions of hard-working microscopic creatures live there who are busy processing organic material like compost into available nutrients your plants can use. To do their best work, though, they like to have a big buffet to snack on. The more you amend your soil with organic matter, the more they eat and the more nutrients they produce for your plants. And that kind of nutritious environment is just what plants need to stay healthy and vibrant.

What else do soil amendments do?

Lucky gardeners have soil that's easy to dig in. It holds nutrients and water well, but drains easily. That's called "loam." The rest of us have either clay soil (which is as hard as a rock in summer and a mucky mess in the winter) or sandy soil (which dries out quickly and doesn't hold nutrients well). Soil amendments are like personal fitness trainers for your soil. They help bad soils shape up into healthy, well-conditioned soils. And they help good soils keep their strength and vigor.

What kind of soil is best for pots?

Think of potting soil as water in a tropical aquarium. If the water doesn't have the right pH level, the right balance of elements and nutrients, the fish die. The same is true for potted plants, which also have special needs for aeration, drainage and moisture. They need a rich, uniformly blended mixture with just the right ingredients like Sun Land Platinum Potting Mix. Both have fertilizers and a wetting agent for thorough irrigation. Remember: never use plain compost or garden soil in a containe.

How do I amend my soil?

Add soil amendments to your garden soil when it is moist for easiest handling. Never work soil when it is very wet because it will compact it and make it worse. Spread a three to six inch layer of amendment on top of soil surface and dig in thoroughly. Depending on the area to be covered, you can use a hand trowel, spade, shovel, pitchfork or rototiller.

I want a recipe for generic indoor gardening. I want to add amendments to a commercial potting soil, including vermiculite, perlite, peat moss, sand, charcoal, bonemeal and Osmocote pellets. Any suggestions?

A good commercial potting soil should be ready to plant in without you having to do any work to make it better and will already have many of the above ingredients you mentioned. Sun Land's potting soils contain redwood, fir, peat, sand, perlite and granular fertilizers, as well as water-holding polymers for our premium mix. We can't give away our secret recipe, of course, but if you like to experiment, try to remember the special qualities of each ingredient: peat moss is hard to moisten but retains water well. Peat Moss also lowers the pH level, so add a greater percentage if you want to grow things like azaleas and other acid loving plants. Redwood, fir and sand create open pore spaces for water, air and roots. Vermiculite and perlite "lighten" the mixture. Osmocote is a good, slow-release fertilizer, but you might want to add a starter fertilizer as well.

The best bet is to read potting soil labels carefully. And take a look at the product to see that it is uniformly ground and mixed. Potting soils differ widely in quality and ingredients -- and that can mean the difference between a thriving plant and an unhealthy one.

I'm interested in finding a source of microbes to use in amending the soil. Can you help? Also, I'd like a source for information on using microbes, pro and con.

It may come as a surprise to many people that their soil is alive - billions of microbes already live in it and the more there are, the better the soil is for your plant. Their presence is crucial because their activity helps convert the nutrients in the soil into a form the plants can use. In general, home gardeners can't just go out and buy microbes, the most important of which are bacteria, fungi and actinomycetes to add to their garden, but adding compost provides "food" for beneficial microorganisms and encourages microbial activity. Commercial growers can buy beneficial microbes that can suppress disease pathogens, but these are not readily available to the public.

One word of caution with any compost that has sewage sludge as an ingredient, as you say Compro has: in California, many counties have banned such products from farmlands because of environmental or health concerns. The farming industry worries that sludge may boost heavy metal content in the soil. Seeding a lawn with sludge is, of course, different than growing food with it. Still, you may want to do a little research at your end of the country to see what Virginia's position is on the issue before adding this type of amendment, especially if you have animals or children. There are many composts on the market that do not contain sludge, of course, that you can choose from.

I need to level and reseed my lawn. Should I bring in all Compro (a type of compost made with sludge) to level the lawn (6-8" in spots) or some mix of topsoil and Compro? I live in Northern Virginia, which has red clay soil.

While it's vital that your lawn be level, it isn't necessary to do it with 100% compost, which will be more expensive than a combination of topsoil and composted organic material. Since you have heavy clay soil, you should mix 50% compost with 50% topsoil and till in the area to be seeded to a depth of 6-8". Also till in a starter fertilizer prior to seeding. Smooth and firm seedbed, then sow seed in two directions, perpendicular to each other. Cover seeds with 1/4" compost and lightly roll or rake in. Keep soil moist during germination period by applying frequent, light waterings.

I am an art student from The One Academy, Malaysia. I am doing my research about soil management, searching for information about soil texture.

Soil texture refers to the mixture of different sized particles in the soil that provide storage space for air, roots, nutrients and roots. Depending on their size, soil particles are classified as sand, silt and clay. Sand is the coarsest-sized particle. Sandy soils usually absorb water rapidly and have good aeration, but don't hold water or nutrients well.

Silt, the medium-sized particle, feels smooth when dry and slippery but not sticky when wet. Because smaller sized particles have smaller pore spaces between them, silt's water intake is slower than sand, but its water retention rate is higher.

Clay is the smallest sized particle. Clay soils often have drainage and aeration problems, but a certain amount of clay is desirable because of clay's ability to hold nutrients well.

The optimum soil texture is called "loam" and is a mixture of about 45% sand, 40% silt and 15% clay. It holds water and nutrients well and drains well, too. The best way to achieve this mix if it doesn't occur naturally is to add organic matter such as compost.

I am doing a report for a soils class and I am looking for information on "Cation Exchange Capacity."

Cation Exchange Capacity (CEC) is a measure of the soil's potential to hold negatively charged nutrient ions (of potassium, calcium, magnesium, sulfur and most trace elements.) The higher the number, the more fertile the soil. Clay soils have a naturally high CEC of about 25; sandy soils are low, about five. Adding organic material such as compost increases the number. Many nutrients are "locked up" in the soil and aren't in a form that is available to be used by the plant. As organic matter decomposes, its negatively charged ions interact with other positively charged nutrient ions and convert them into a form the plants can use -- it's like having a slow-release fertilizer in your soil. So, in fact, Cation Exchange Capacity is "the key" to unlocking your soil's nutrients.

We need some soil for seeds to germinate on. What kind of product should we use?

A good seedling mix has a combination of peat moss, perlite and vermiculite. First, it's critical seeds do not dry out during the germination period and that's why peat moss and vermiculite are used - they maintain needed moisture. However, seedlings are also prone to diseases that are present in overly wet, low aerated situations and perlite and vermiculite provide good drainage. Sometimes a layer of sand can be applied to the top of the mix to ensure the mix doesn't stay too wet at the surface. "Damping Off" disease is a common hazard and making sure there is good drainage at the surface can make a difference. If you can't find a premade mix at your local nursery, you can definitely find all the ingredients there to make your own.

I am looking for a soil especially for Bonzai that is free-draining and coarse. What do you recommend?

It's good you know how important drainage is for Bonzai. The reason is that the drainage properties of any container is very dependent on the container's depth. The gravitational pull and the "weight" of the water provides the downward movement of the water through the mix, but it slows as it approaches the bottom of the container, creating a "Saturation Zone." All containers have a layer at the bottom that stays wetter than the rest of the container. Since Bonzai pots are typically very shallow, a good Bonzai mix should have very good drainage properties or the possibility of overwatering, or not enough air, is great. Look for a potting mix that is a uniform blend of redwood and/or fir products, peat moss, sand and perlite. Sun Land makes such a mix (See Retail Products page) but even with a mix like ours, it's advisable to add extra sand to ensure excellent drainage.

What should we do to make high desert red clay into soil that will sustain flowering plants?

Clay soils have high nutrient levels but are difficult to grow in because they are too compacted. Because clay particles are so fine and closely spaced, we recommend using a very coarse soil amendment such as Sun Land Redwood Soil Conditioner (See Retail Products page) to break them apart and increase drainage and aeration. Desert soils can also contain a lot of sodium, so you should buy an inexpensive soil test kit at a local nursery to see if that is part of your problem. If so, add gypsum to your soil as well. And be patient - clay soils require repeated, heavy applications of soil amendments to become good, workable soil. For the best results, it's also important to choose flowering plants appropriate to your environment.

My soil is too acidic. What should I do?

Acidic soil refers to soil that has a low pH – strictly speaking below the neutral pH level of 7.0. The pH scale of your soil is important to know because it determines the amount of available nutrients for your plants to use. In general, the more neutral the soil, the more amount of available nutrients to the widest variety of plants. Acid soil generally has a pH level of 5.5 or lower and is easy to determine with a simple, inexpensive test kit available at most nurseries. Most plants grow well in a slightly acid soil (camellias, rhododendrons and azaleas love it, in fact) but to decrease soil acidity, you can add ground dolomitic limestone. It raises the pH level and also supplies magnesium and calcium, which may deficient in acid soils. Add limestone carefully, though, so you don't raise the pH level too high and make your soil alkaline.

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