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Plant Answer Line Question

Keywords: Soils, Soil amendments, Mulching, Compost

Help! My clay soil is stunting the growth of my plants. I've amended the soil with compost and manure. Is there a another method of conditioning the soil that you can recommend?


First and most important, it appears mulching is the best organic solution for conditioning clay and heavy soils. Organic soil conditioners include compost, well-rotted animal manures, and natural fertilizers. Planting green manures such as clover, rye grass, or vetch are also effective for breaking up large clods in clay soil over time.

Sheet composting - laying compost over the entire area to be worked and using a fork (or rototiller) to work it into the soil to a depth of 2-4 inches - is cited by the resources listed below as an efficient method of soil conditioning. Both books listed below recommend repeating this process at least twice a year, in early spring and in late fall.

Secrets to Great Soil [by Elizabeth P. Stell, 1998, (pbk)] and
The Gardener's Guide to Better Soil [by Gene Logsdon, 1975, (pbk)]

The Saving Water Partnership (the City of Seattle and other government entities) has a website full of information about improving soil.

The site includes Growing Healthy Soil.

Current thinking contradicts the notion of working compost or other amendments into the soil, as explained in a March 31, 2010 Garden Professors blog post by Professor Linda Chalker-Scott of Washington State University Extension Horticulture. She specifically takes issue with the "Growing Healthy Soil" information linked above. Here is an excerpt:
"Not only will extensive digging or rototilling destroy any soil structure you might have, it will also take out the roots of any desirable plants in the vicinity). [...] improper soil amendment can cause serious problems such as soil subsidence, perched water tables, and nutrient overloads. This last point is especially important to anyone living near aquatic ecosystems, since excess nutrients always end up in the water. Before you plant this year, find out what your soil needs before amending it. And remember that mulching is the natural (and sustainable) way to add organic matter to the soil."

Date 2019-08-14
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Plant Answer Line Question

Keywords: Soil amendments

I have very bad rocky, clay soil. To dig in this is like prison work. We rototilled the area and put in topsoil and now it is like quicksand. I am going to build up my beds but want to break some up to get trees and other plants to take root. What are you supposed to use or do with hard clay? I love plants, and would like to get gardening, but I can't think of how to solve this problem.


There is no immediate solution. It may take a few years of adding good amounts of organic matter and compost to improve the soil quality. Here is some information that may help you along the way.

A Seattle Post-Intelligencer article, 'The well-made bed: Pile on the compost' by Ann Lovejoy, provides some tips for improving your soil with compost.

Washington State University Extension horticulture professor Linda Chalker-Scott has a cautionary tale on how NOT to amend clay soil, plus tips on improving it slowly over time.

Fine Gardening has an online article on improving clay soil.

Lastly, a Home and Garden Television article entitled, "How to plant in rocky soil" may help you overcome the rockiness of your yard.

Date 2019-09-27
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Plant Answer Line Question

Keywords: Prunus lusitanica, Soil testing, Soil amendments

Some of my Prunus lusitanica (Portugal laurel) shrubs are changing the color of the foliage and stems. Normally the leaves should be dark green and the stems are a dark cranberry red. The soil here at the coast is very sandy. I have put composted manure (the type from bags), fertilized them, and added a bit of lime to the soil around the trunk and close to the root zone. I have not seen much of a response. Do you know what is the optimal pH for Prunus lusitanica? I am concerned about these shrubs because I just planted them last summer.


Prunus lusitanica tolerates a wide variety of pH and moisture levels in soils. See California Department of Forestry SelecTree webpage about this plant.

According to the webpage of a local Seattle garden writer, the leaves do change color slightly, acquiring a bluish tinge in late fall to winter. She also says that Prunus lusitanica does not like wet feet.

What colors are the leaves turning? You might consider testing the soil, to make sure things are not out of balance. Here is a link to the Miller Library website's links about soil testing.

Is it possible that the bagged manure was still hot, that is, not fully aged? If so, that could cause problems.

You might also bring in photographs or sample leaves to a Master Gardener Clinic for diagnosis. You can locate a Master Gardener Clinic within Washington State at this website.

Date 2019-07-17
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Plant Answer Line Question

Keywords: Soil amendments

I have typical Seattle clay soil and I want to amend it before planting. I'm re-grading a large (~2000 sq. foot) area and have already added copious amounts of mulch, compost, overturned sod, etc. I'm planning on planting a cover crop of clover this summer, but before doing so thought it'd be a good idea to till in some sand. I was thinking about a level inch over the whole yard before tilling. Is there a particular chemical composition to use or avoid? How about grain size?


You may find this information from Colorado State University Extension on soil amendments useful. Excerpt: Don't add sand to clay soil--this creates a soil structure similar to concrete.

Professor Linda Chalker-Scott of Washington State University also debunks the idea of adding sand to improve clay soil.

For good advice on amending the soil, see these links:

Building Fertile Soil and Choosing and Using Cover Crops for the Garden and Orchard from the University of California, Santa Cruz Agroecology Program.

Growing Healthy Soil from Seattle Public Utilities.

It sounds to me as if you have already taken great steps toward improving your soil, and adding sand not only will not be necessary, but would not be a good idea.

Date 2019-08-02
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Plant Answer Line Question

Keywords: Organic fertilizers, Soil amendments

I was wondering if you could provide me with a more or less exhaustive list of organic fertilizers and soil amendments, their nutrient profile, and what they are good for, etc.


When you say 'organic' fertilizer and soil amendments, do you mean those items which are allowed under current organic regulations? I ask because there is considerable difference of opinion over what is meant by the word 'organic' in this context. Sewage sludge which contains heavy metals could be said to be organically derived, but might not pass muster in an organic garden, for example.

If you mean products which are on the Organic Materials Review Institute list of permitted soil amendments, here is a link to their lists.

If you need a truly exhaustive list, I recommend looking at some of the books available in the Miller Library on this subject. Rodale's Illustrated Encyclopedia of Gardening and Landscaping Techniques edited by Barbara Ellis (Rodale, 1990) has a very easy-to-use (but probably not exhaustive) guide to soil amendments and fertilizers. Below are are some other titles:

Soils : the right way to use fertilizers, composts, soil conditioners, soil testing/problem soils 1986

Let nature do the growing : the fertilizer-free vegetable garden / by Gajin Tokuno c1986

Fertilizers and soil amendments / Roy H. Follett, Larry S. Murphy, Roy L. Donahue

Fertility without fertilizers : a basic approach to organic gardening / Lawrence D. Hills

Feeding plants the organic way / Jim Hay

Growing green: animal-free organic techniques / Jenny Hall and Iain Tollhurst

The following links may be helpful to you:

Colorado State University Extension - Organic Fertilizers

National Sustainable Agriculture Information Service - Sources of Organic Fertilizers and Amendments

University of California Santa Cruz Agroecology Program - Building Fertile Soil

Washington State University Extension - Organic Fertilizers

Oregon State University - Improving Garden Soils with Organic Matter

Utah State University Extension

Date 2019-08-21
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Plant Answer Line Question

Keywords: Juncaceae (Rush family), Carex, Soil amendments, Lawns and turfgrasses, Compost

We would like to put in a new lawn around a home where there were mostly weeds. The soil is very a heavy silt because it is river bottom land. I have access to free sand; however, I've heard conflicting advice regarding adding sand to clay -- some say yes, others no. I also have access to a large supply of free horse shavings/manure from a horse stable. Would those shavings be good to add to the soil to help lighten it and add nutrients? I don't want to go to the expense of bringing in topsoil if I don't have to. What are your suggestions.


Adding sand to clay soil is not recommended as a way of lightening the soil, as it "may create a concrete-like structure", according to the booklet Ecologically Sound Lawn Care for the Pacific Northwest by David K. McDonald. Linda Chalker-Scott addresses the reasons for this in depth in "The Myth of Soil Amendments Part II".

Instead of adding sand, David McDonald recommends trying to till in compost. At least two inches of compost tilled into the upper six to eight inches of soil is recommended, but four inches tilled into the upper twelve inches is preferable . Try to avoid doing this when the soil is waterlogged, as it may damage the soil structure.

Composting the horse manure and shavings you have access to could be a feasible way to obtain the compost to till into the soil. The Guide to Composting Horse Manure by Jessica Paige of Whatcom County WSU Extension discusses how to compost and use horse manure. She recommends curing such compost at least a few weeks before application, and adds that one to three months is a good, typical composting time in summer or three to six months in winter.

Alternatively, according to David McDonald, if there are a few months of warm weather between autumn and seeding time, you could simply till the fall leaves and grass clippings into your soil. Depending on your planned schedule, this could be very easy. (You can find McDonald's full booklet "Ecologically Sound Lawn Care for the Pacific Northwest: Findings from the Scientific Literature and Recommendations from Turf Professionals" online as a very large PDF.)

Another option might be to consider some sort of groundcover if you discover that establishing a lawn is an excessively extensive project. Carex species or possibly Juncus phaeocephalus phaeocephalus are more naturally adapted to heavy soils in wet areas than lawn grasses and so may be less work in the end. Though they would not be appropriate for a heavy traffic area, they would be grasslike in structure. Sagina subulata might be more amenable to heavy traffic.

Date 2019-10-10
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Plant Answer Line Question

Keywords: Soil amendments, Compost

I bought compost from the city of Port Angeles and in a sifted wheelbarrow of compost I got three gallons of pencil diameter "twigs". These are not composted. They break/snap and are green inside. The compost was supposed to be tilled into the garden and flower beds but somewhere in the back of my mind I sort of remember that this will take nitrogen out of the soil to compost down. Is that correct or should I not be concerned?


It sounds like screening your compost was a good place to start; Mike McGrath recommends removing the "odd original ingredient" from compost this way in his Book of Compost (New York, NY : Sterling Pub. Co., 2006). Woody material should definitely be removed, he says. If your compost does not seem otherwise 'off' (an ammonia smell, a sulfurous smell, very odd color), sieving off the woody material is often sufficient.

Twigs compost more slowly than other material, and you could, if you like, simply re-compost them, according to the King County Solid Waste Division.

There is some debate over the effects of inadequately decomposed material such as your woody twigs in compost and mulch. Linda Chalker-Scott addresses the question of less-than-fully composted yard waste in her May 2003 myth. She agrees that inadequately decomposed yard waste has a reputation of removing nitrogen from the soil, but writes that the way the yard waste is used affects the way it interacts with the soil. As a mulch (a layer over the soil to prevent weeds or retain moisture), it does not significantly reduce soil nitrogen, but as a compost (incorporated into the soil), it may reduce nitrogen in the soil.

If you are still concerned about the quality of your compost, Stu Campbell suggests using the following techniques on municipal compost in his Mulch It! (Pownal, Vt. : Storey Books, 2001) First, test the pH, and, if it is off, store and turn the compost for several months before using it. Mature compost should have a pH between 6 and 8, which you can test using a soil test kit or some of the other options listed on Cornell University's composting pages.

Date 2019-10-10
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Plant Answer Line Question

Keywords: Gypsum, Soils, Soil amendments

Part of my yard has clay close to the surface. In addition to heavy and yearly amending with compost and other organic materials, I have heard that adding gypsum is helpful for breaking up heavy clay. Any thoughts or suggestions on this?


One of the most reliable resources I've found on how to amend soil is Prof. Linda Chalker-Scott, of Washington State University Extension. According to her very useful website on horticultural myths, gypsum does improve clay soil but is not the best choice for most home gardens. Here is an excerpt:

"With the exception of arid and coastal regions (where soil salts are high) and the southeastern United States (where heavy clay soils are common), gypsum amendment is just not necessary in non-agricultural areas. Urban soils are generally amalgamations of subsoils, native and non-native topsoils, and--in home landscapes--high levels of organic and non-organic chemical additives. They are also heavily compacted and layered (and gypsum does not work well on layered soils). In such landscapes, it is pointless to add yet more chemicals in the form of gypsum unless you need to increase soil calcium levels. This nutrient deficiency can be quickly identified by any soil testing laboratory for less than a bag of gypsum costs. (If you need to improve sulfur nutrition, it's wiser to use ammonium sulfate). To reduce compaction and improve aeration in nearly any landscape, application of an organic mulch is more economically and environmentally sustainable."

Colorado State University in an article no longer available says the following about gypsum as a soil additive:

"The belief persists that adding gypsum can 'break up' [...] compact clay soils [...] Calcium sulfate or gypsum when added to our clay soils only increases the already high calcium content. Since plants growing in our soils already have all the calcium they need, the added gypsum does nothing to improve plant growth. Spending money and time to add gypsum to a soil that is already high in calcium is a waste of money and effort.
"As far as relieving soil compaction, gypsum has no effect. Loosening soils is a physical process, not a chemical one. The way to break up tight, clay soils is through adding and mixing in organic amendments. The amendment holds the clay particles apart creating more space for the air so critical to plant root growth."

It sounds to me as if your practice of amending the soil with compost and organic matter is the best approach.

Date 2019-09-19
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Plant Answer Line Question

Keywords: Cupressus, Tree planting, Transplanting, Soil amendments, Conifers

I am going to take my 6-foot tall Wilma Goldcrest out of the giant pot it is currently in, and plant it in the ground. I am seeking some sort of consensus on how to prepare the hole into which the tree is going. Someone said that I should not put compost in the hole because that will encourage the roots to just stay in the area of the hole. If that's the case, then shouldn't the "no compost" rule apply to all new plantings (which, of course, it does not)? Also, when should I fertilize the tree and what kind of fertilizer should I use? I always use organic fertilizers. What about putting some bone meal in the planting hole to feed new root growth?


I refer you to the following information from Washington State University Extension horticulturist, Professor Linda Chalker-Scott, who discusses planting procedures in her book, The Informed Gardener (University of Washington Press, 2008). She says that the planting hole need only be the depth of the root system, but should be twice the width. She advises against amending the planting hole in any way: Backfill the hole with native soil, not a soil amendment. The idea is not to 'spoil' the plant by putting rich compost just in the hole, which will deter the roots from spreading out into the surrounding area.

Her debunked gardening myths may also be found online. This one addresses soil amendments and planting. She also addresses the use of bone meal as a planting amendment.

'Wilma Goldcrest' is a cultivar of Cupressus macrocarpa, or Monterey cypress. The University of California's Garden Information publication on "Pines and Other Conifers"(including Monterey cypress)says:
"Pines and conifers require less fertilizer than most other trees and shrubs. Heavy fertilizing can promote rank, unsightly growth, destroying their natural, symmetrical, picturesque form." If you do wish to use fertilizer, a dilution of something like seaweed or fish fertilizer would probably not be harmful.

Here is more about fertilizing conifers from University of Minnesota Extension Horticulture.
"Why Fertilize?
The plant itself will often indicate when it needs fertilizer. If growth rate and needle color are normal for a particular variety, fertilization is not necessary. If new growth is sparse or slow, or the needles are not a healthy color, or are shorter than normal, you should probably fertilize. Keep in mind, however, it is not unusual or abnormal for newly transplanted evergreens to exhibit slow growth until they're re-established.
Regular fertilization may be recommended if you are trying to grow evergreens in a less than ideal site, such as very sandy or heavy clay soil, or if the plant has suffered damage from insects or disease. You might also wish to fertilize to encourage more rapid growth in relatively young evergreens."

Local garden writer Ann Lovejoy says the following in her book, The Handbook of Northwest Gardening (Sasquatch Books, 2007): "I rarely feed plants directly, preferring to feed the soil with what are called 'feeding mulches,' made of materials such as compost, seed meals, kelp, and fish meals."

Date 2019-09-11
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Plant Answer Line Question

Keywords: Soil amendments, Mosses

I have a small rooftop plant bed that's full of moss. Is that an indication of sour soil, and if so, can it be sweetened with Dolomite lime?


Moss is often simply an indication of a shady site or compacted soil, but can also be an indicator of low soil pH (i.e., acid soil). I wouldn't recommend adding lime without doing a soil test for pH (you can buy an inexpensive kit at most garden centers), and without considering the pH needs of the plants you have in the bed. You would not want to increase the alkalinity of the soil if your plants are acid-loving.

You may find this link about moss growing in garden beds (from Oregon State University) of interest. Here is an excerpt:
"Mosses grow in garden areas for the same reasons they grow in lawns: for example, deep shade, high acidity, poor drainage, and soil compaction. As in the lawn, mosses do not compete with other plants. Rather, they establish in bare areas where conditions are favorable (Cook and Whisler, 1994).
Mosses have not been shown to hinder the growth of garden plants or trees. Reasons for removal are generally aesthetic. But aesthetics are in the eye of the beholder, and mosses are commonly viewed as positive features in landscaping. For example, traditional oriental gardening holds distinctive roles for mosses (Japanese Garden Society of Oregon 1996; see also Encouraging Mosses). Furthermore, in some situations mosses may help reduce moisture loss and crusting on soil surfaces."

Date 2019-05-17
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Plant Answer Line Question

Keywords: Tomatoes--Diseases and pests, Soil amendments, Plant-water relationships

It's summer and my husband wants to add lime to our soil because some of our tomatoes have blossom-end rot. He thinks this will correct the problem. I think we would do better to make sure our tomatoes aren't drying out, and then work in soil amendments next time around. Also, couldn’t we use eggshells for calcium instead of lime? We have a ready supply from our backyard chickens!


I think you are on the right track. Blossom End Rot (BER) is a physiological disorder that tends to affect larger tomatoes rather than smaller ones. According to Craig LeHoullier’s book Epic Tomatoes (Storey Publishing, 2015), some varieties are especially susceptible: "Roma/paste varieties, and some of the longer indeterminate sauce types like Opalka and Speckled Roman. Adverse growing conditions [such as drought stress or low calcium levels] can make it problematic for many other varieties as well."

All print and web sources I consulted mention environmental conditions as a cause of this problem. It starts through the supply of water and calcium in the developing fruits. The effects may be seen on plants exposed to a period of drought during rapid growth; root damage; heavy, wet, or cold soil; excess salt in the soil.

A soil test is the ideal starting point to make sure the pH is adequate for tomatoes. Epic Tomatoes recommends amending the soil with lime if necessary. (Washington State University Extension says the time to do this is 2-4 months before planting, not in the middle of summer!) Mulch around the base of the plants to conserve soil moisture during hot spells and water regularly. Another reason a soil test is a good idea is mentioned by British Columbia author Linda Gilkeson in her book Backyard Bounty (New Society Publishers, 2011):
"Your soil might have enough calcium, but it isn't available, or the plant can't take it up fast enough. This is often because the movement of calcium inside the plant has been inhibited by drought stress, possibly from irregular watering. This is often seen on tomatoes in containers that experience alternating dry and wet soil. It can also be caused when plants grow too fast as a result of too much nitrogen fertilizer."

Since you prefer not to use lime (the dry powdery texture of calcium carbonate can be irritating to the skin and eyes), you will be pleased to know that the use of eggshells is mentioned by Mike McGrath in You Bet Your Tomatoes! (Rodale Press, 2002). At planting time you can "put some crushed-up eggshells into the planting hole." Make sure the eggshells are ground to a fine consistency.

Don't forget that those damaged tomatoes can still be used--just cut off the bad parts!

Date 2020-01-31
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Garden Tip

Keywords: Soils, Soil testing, Plant-soil relationships, Soil amendments, Fertilizers

Successful gardeners know that healthy soil translates to healthy plants. See the guide to soils and fertilizers for the home gardener from WSU Cooperative Extension.

Date: 2007-07-12
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