Dr. Dirt Q&A

Q: I'm a homeowner experimenting with seashore paspalum, a very salt tolerant grass. I also have a home in the Bahamas
and want to use paspalum and irrigate with salty water.

I got some soil samples and the soil is very high in calcium. The percent base saturation is about 94%. I know the paspalum can handle
the high pH and salts, but from what I've read, won't the soil almost turn to concrete when I begin irrigating with salty water--even if it's
only around 5,000 PPM.
Stewart

A: That is a good question. The answer is related to the type of salts present in the soil and water. As long as calcium and magnesium dominate, the soil structure will be maintained, and water infiltration will continue. I suggest you have the potential irrigation water source tested. Sodium is the ion that has the potential to cause damage and sealing of the soil.

If sodium is high in the water, continue a regular program of soil testing, and request the lab check the sodium adsorption ratio (a complex formula that compares the quantities of sodium, calcium, and magnesium). If the sodium adsorption ratio increases over time, the lab will likely recommend you add gypsum (calcium sulfate), or possibly sulfur since the soil is high in calcium (and natural processes will form gypsum).

Q: I have a moderate Iron Chlorosis problem in my garden, affecting peaches, rasberries, red currents and grapes.

Last spring I got a truck load of gypsum right from the mine [I live in Nephi, UT.], I broadcast the Gypsum with a shovel until it looked about right, around each plant or tree,then tilled it in. The idea behind this was the sulfate would break down over time leaving my soil more acidic.

Everything looked fine no problems. No great change in how green everything was either. When the grapes were ready to be picked, the outside of the skins were salty, I didn't know why and blamed it on air pollution. This year the Red Currants are getting ripe, and the outside of their skins are salty also. I haven't harvested any peaches or rasberries after doing this.

What have I done ?
Can this be fixed ?
Will any vegetables be affected ?
Is it safe to eat the fruits or vegetables after rinsing ?
Does adding Gypsum help with Iron Chlorosis ?
What about adding it around the Rose bushes in the flower beds ?

Ed

A: Gypsum is known as a neutral salt, which means it has little impact on the soil pH and nutrient availability. The primary use of gypsum as an amendment is with sodium-affected soils.

You can relax. You may have increased the salt content of your soil, but have probably done no permanent damage. If the plant accumulates excessive salts, one way it gets rid of them is to release them through the cell walls. Wash the fruits well, and enjoy them without anxiety.

If you want to lower the pH and improve iron availability, there are several possible solutions. Iron sulfate, sometimes marketed as Ironate,
will lower the pH and provide some iron to the plants. I often use about a half-cup in each of 3 or 4 holes within the drip line of the plant. You can use a shovel to open a small trench, add the iron sulfate, then close the trench again. The most effective acidifying agent is elemental sulfur. To be most effective, sulfur should be incorporated into the soil. A soil testing laboratory can test your soil and recommend appropriate rates. You can purchase either of these amendments at most quality lawn and garden supply stores. Aluminum sulfate is available in some places, and falls between iron sulfate and sulfur in its acidifying effect.

I suggest you collect some soil samples and send them to a lab. Use the results from the analysis to determine whether you need to amend your soil further.
See Dr. Dirt's webpage for suggestions on collecting soil samples.

Q: David Horsley, a local op ed columnist for the Amarillo Daily News-Globe wrote a column questioning the wisdom and utility of using redwood mulch in gardens since he had received conflicting information. The correspondence below addresses the use of mulch.

A: Briefly, a mulch has three basic purposes. The first two are most important in our climate (hot, sometimes windy, semi-arid, with summer rainfall - Texas Panhandle).

The most important is water conservation. The mulch limits evaporation from exposed soil surfaces - two to three inches of redwood chips or any other wood chip mulch on the soil surface are excellent for this purpose. Surface mulches are especially effective if drip irrigation or
soaker hoses apply water under the mulch. The mulch will absorb some water from sprinkler irrigation and rainfall, but the advantages of
mulches for water conservation far outweigh the disadvantages. This is a semiarid climate. The atmosphere always wants much more water than the soil receives in precipitation. It is not necessary to remove the mulch in the spring to prevent disease problems as is the case in some humid climates.

The second is preventing growth of undesirable plants (weeds) under the mulch in a garden bed. Some of the compounds in redwood mulch have allelopathic properties. (See the next Q & A for more information.) This means they act as natural pre-emergent herbicides, preventing germination of seeds or killing seedlings as they germinate. These compounds generally have little effect on established plants, though there may be a few sensitive species. Controlling weeds is another way to conserve water for the desired plants. Most organic compounds used as a mulch will cause the soil to get darker. The darker soils result from the accumulation of decomposing organic materials: leaves, bark, roots, etc. In our region, the accumulation of organic matter in the soil is always a good thing. Over time, the decomposition of these organic materials releases weak acids that gradually lower the soil pH.
In our semiarid climate, the soil pH is often too high for optimal nutrient availability, and lowering it a bit helps. Unless too much irrigation water is applied, the chemicals released as the bark is decomposed should never leach downward to groundwater in this climate. It would also be unusual for them to accumulate in surface waters unless the mulch is carried away in runoff. Proper construction of a garden bed with a border should limit water runoff.

The third purpose of mulch is temperature insulation. In our climate, with its dramatic temperature swings in the fall and spring, this mediates
the differences in soil temperature. A mulched soil will cool more slowly in the fall, and warm more slowly in the spring. Two to three inches of mulch will not prevent the soil from freezing during an extended period of cold weather.

So your question was: To use, or not to use? Consider your application. Why are you using a mulch? If your primary concern is aesthetics,
redwood chips are as good as any. If you are concerned that redwood chips will be harmful to the environment or to your bed of perennials, relax. Avoid mixing the wood chips into the soil, and DO NOT use sawdust as a mulch. Sawdust decomposes too quickly and uses all available nitrogen, leaving none for the plants. If you notice a plant beginning to pale after placing the mulch, apply a spot-feeding amount of your favorite house-plant fertilizer, like Peter's Professional or Miracle-Gro. If the plant does not improve within a week, rake the mulch away, add some extra water to remove toxins from the root zone, and fertilize it again. If it still does not improve, it was not the mulch.

My suggestion is mulch. Free sources are more economical than purchased ones. You can even use the wood chip mulch available free from the City of Amarillo (and many other municipalities). It is not as uniform, and is more likely to have seeds, but is still an effective water conservation tool.

Q: Hello Dr. Dirt. I just read your Q & A on redwood mulch, but I still have a question about turpentine. I have been using the redwood mulch that Backyard Adventures have been kind enough to give away, but one of the local garden centers said that it contains turpentine and will eventually kill all the plants in my flower bed unless I scrape it out and replace with another mulch. Is this correct?

Thanks for the input.

A: Backyard Adventures uses only redwood with no chemical treatment, as you can find on the link below.

http://www.backyardadv.com/features.html
http://www.askmar.com/Redwoods/Masters_Thesis.html

This second link is from an M.S. Thesis on Redwoods. One portion talks about allelochemicals present in growing redwoods. A combination of some of these allelochemicals with natural resins is known as turpentine. These chemicals make the trees resistant to insects, bacteria, fungi, and other disease vectors, which is the reason decomposition of redwood is slower than that of other woods. The thrust of the discussion is that the allelochemical content is higher in the outer layers of the tree, not the heartwood that is used in making the lumber. The allelochemical composition is greater in older trees, and no old trees are used in current logging operations for redwood lumber. Additionally, these same chemicals, when leached from the wood during decomposition, may or may not exhibit allelopathic properties (properties that would restrict the growth of microbes, insects, or plants).

Old wives' tales and conventional wisdom often have a basis in historic observations, though the understanding of what caused the observation is often incomplete. So, it is likely that someone somewhere historically observed a decrease of growth when using a redwood mulch (old wood, bark and outer layers), that has been generalized to include all redwood lumber mulches used anywhere.

Until someone shows me an example of a problem with redwood mulch in this region, I will continue to advocate its use. In our climate (rainfall and temperature), the decomposition rate of the redwood should be slow. If water is applied through drip irrigation (not sprayed on the surface), little leaching of these chemicals into the soil will occur. Seedlings and transplants are much more susceptible to allelochemicals than established plants.

General suggestions for the use of redwood mulch for best results:

Only apply to established plants (not seedlings or transplants)
Apply irrigation water through buried drip or soaker hoses, not through aboveground sprinkler systems
DO NOT Overwater - this will accelerate decomposition.

Q: One thing I need clarification on: you said not to mix the mulch into the soil. I've been doing this for years with the city's chipped mulch, with no apparent harm. Do you mean that the redwood mulch shouldn't be mixed into the soil? Why not? Every spring when I till the garden, I churn in all of last year's mulch--most of which is starting to decompose anyway. If redwood does something bad to the soil, I could always scrape it aside before tilling. Please advise.
David

A: This issue is related to the nitrogen requirement of the organisms decomposing the wood chips. A problem can occur with any wood chips or high carbon content material, not just redwood chips. Wood has 500 or more times as much carbon as nitrogen. If the decomposition proceeds too rapidly (as on sawdust which has a tremendous amount of surface area), the microbes use all the nitrogen available in the soil and leave none for the plants. If you have been adding a nitrogen source (fertilizer, manure, or compost), you will not likely have a nitrogen deficiency develop. Sometimes, problems will not develop with wood chips since they are large, and have little surface area available for decomposition. Incorporating the wood chips will eventually improve the organic matter of the soil, and improves aeration. I generally advise against incorporating wood chips to avoid potential nitrogen deficiency problems for those who prefer not to add commercial nitrogen fertilizers.

It sounds like your system provides enough nitrogen and the wood chips decompose slowly enough that there are no problems.

Q: I live in Boston, MA  and have a question concerning worms. I am trying to amend the soil in some terrace planters and noticed recently that I could not find a solitary worm in a planter that's probably about 150 cu. ft.....
Is there an optimum number of earthworms that you could recommend for this volume of earth ... or perhaps some sort of ratio I could refer to ....

A: I do not know of any specific recommendations, but earthworms in natural environments occur in ranges of 10 to 1000 in 10 sq ft, 6 in deep (5 cu ft).
So, if you started on the low end, you might put 10 for every 5 cu ft, and thus about 300 in your planter. Some things you should consider include:

How well insulated is the planter to temperature extremes (especially in winter)?
Does the planter have adequate drainage (earthworms need oxygen in the soil)?
Is the planter connected to "ordinary" soil?
How often and greatly is the soil in the planter disturbed (tillage or cultivation - earthworms do not like disturbance)?
Some pesticides might effect numbers of worms.

If the earthworms like the environment, their numbers will reach an equilibrium in time.

Q: What is the normal range of total phosphorus (P) in soils in the western United States. I have samples from the bottom of a reservoir with ranges from 6,000 to 10,000 mg kg^-1 (ppm), and I am trying to put that into context. I am having difficulty finding any information on this.

A: The amount of total phosphorus (P) in mineral soils ranges from 200 to 5,000 mg kg^-1 according to the S. Kuo (Ch. 32 in Methods of Soil Analysis, Part 3: Chemical Methods, SSSA Book Series: 5). It is not surprising that materials taken from the bottom of a reservoir would have higher total P concentrations. Reservoirs receive runoff water which often includes sediments. These sediments may be organic material or clays, both of which often bring adsorbed P with them. These sediments will accumulate in the bottom of the reservoir through time, elevating the total P concentration. They may or may not have a great impact on the solution P that will be found in the water, depending on several chemical equilibria factors: pH, other elements present, etc.

The reason it was difficult for you to find information on total P levels in soils is that the greater issue of concern is soluble P. Total P encompasses P in soil minerals (e.g., apatite), in freshly deposited and crystallized precipitates (e.g., aluminum or calcium phosphates, depending on pH), organic residues and/or soil organic matter, and solution. Soil scientists generally define 3 pools of soil phosphorus: solution, labile and nonlabile.

Solution P is comprised of soluble phosphates and/or orthophosphates present in the soil water (specie is dependent on pH).
Labile P is comprised of phosphates and orthophosphates adsorbed to clay or soil organic matter, and readily mineralizable organic P compounds. When P is removed from the soil solution, it is restored from the labile P pool.
Nonlabile P is comprised of primary and secondary minerals, and of resistant organic P compounds.

The total P is not necessarily an indicator of the solution P because of all the complex chemical equilibria and biological reactions that affect P solubility.

In your work, soluble P is going to be more important, and will certainly be affected by the TMDL. However, of that influx of total P, not all will end up in solution. Characterization of the various P pools would be more meaningful to interpret the results you have.

Q: I am so interested in soil science since I began my garden. Where do I begin? Which publication would you start me off that I could understand, but with a some level of challenge? I want more than what I find in small local libraries, but still-I'm not a scientist...yet.

A: The first book I recommend is
http://www.barnesandnoble.com/

Soil Science Simplified
Helmut Kohnke, D.P. Franzmeier, D. P. Franzmeier

Paperback - REV, November 1995

Our Price:	$9.95
Barnes & Noble Member Price: $9.45


	Usually ships within 24 hours.

I recommend it for our doctoral students who do not have a background in soils. If this one is too elementary, please contact me again, and I will recommend one that moves you to the next level.