Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityMarch, 1997
Special Soil Tests
Keywords: landscape, micronutrients, nursery, nutrition, pH, plant health, soil/media testing
I n early 1996, I wrote two articles on the interpretation of standard mineral soil tests for the
nursery and landscape. Standard soil tests usually include soil pH, buffering capacity, CEC,
available phosphorus, exchangeable calcium, magnesium, potassium and base saturation. In this
column I'd like to cover what we know (or don't know!) about interpreting elective soil tests that
are usually available from soil testing labs for an additional fee. My emphasis is crop nutrition,
not human health, - so I won't cover heavy metals.
The micronutrients. The Research Extension Analytical Laboratory at Ohio State offers
tests for boron (B), manganese (Mn), zinc (Zn), copper (Cu), and iron (Fe). Quite honestly, we
are at a disadvantage in interpreting soil tests for these minerals because there hasn't been much
research that correlates soil availability of micronutrients with growth of many of our important
ornamental crops. The current recommendations for the micronutrients that follow rest heavily
on research with agronomic or fruit crops,- not ornamentals.
The availability of most micronutrients tends to decrease with increasing pH because the solubility
of these elements decreases. Also, high phosphorus levels in soils can antagonize micronutrient
uptake, particularly zinc. In Ohio soils, it is more common to find that micronutrient availability is
limited by soil pH rather than lacking in the soil. Iron deficiency occurs quite frequently in Ohio
in slightly acid to neutral soils for this reason. Classic examples of ornamental plants that suffer
from pH-induced iron chlorosis include rhododendrons, azaleas, white pine, and pin oak. Testing
soil for iron is of little value since the amount of iron in the soil (sometimes as much as 10% of the
soil, by weight!) has not been correlated with deficiency symptoms. Therefore I have no
recommendations for available iron in Ohio soils. Iron deficiency can be definitively
diagnosed by foliar analysis coupled with a soil pH test. However, familiarity with the symptoms
of iron chlorosis and the identity of the plant are usually all that's required by the professional to
correctly diagnose this problem.
Manganese deficiency also may be induced by high soil pH. This deficiency commonly occurs in
red maple. The symptoms of manganese deficiency in red maple are similar to iron deficiency.
An intervenal chlorosis (yellowing between leaf veins) is first observed in the youngest leaves.
Some Ohio soils have been found to be low in Mn, Zn, and B. Soils that are very high in organic
matter tend to have lower levels of available Mn and Zn. In contrast to Mn and Zn, some forms
of B leach readily, and thus deficiencies in this element are more likely to occur in low organic
matter, coarse soils. Currently, accepted threshold values for soil available Mn, Zn, and B are 10
PPM, 1.5 PPM , and 0.5 PPM, respectively. Currently there is no recommendation for available
Cu in Ohio.
Soil tests for Mn, Zn, and B should be coupled with foliar analysis for a definitive diagnosis.
Recommended foliar micronutrient concentrations for woody plants follow: B: 20 to 150 PPM;
Fe: 50 to 250 PPM; Mn: 30 to 250 PPM; Cu: 5 to 25 PPM; Zn: 20 to 150 PPM. These ranges
are quite broad and can only serve as a general guide. Since there are literally hundreds of types
of ornamental plants, it's quite possible that sufficiency ranges for some ornamentals will fall
above or below these ranges. The chelate or sulfate forms are the most common types of
fertilizers used to supply Fe, Mn, Cu, or Zn. Chelates are generally the most soluble and plant-
available form. Sodium borates are commonly used to supply B.
Percent soil organic matter. Soil organic matter (SOM) is a very important test that
should be used more often. SOM is a key component of a 'healthy' soil; it supports a wealth of
macro- and microbial organisms that contribute to soil fertility and structure. Organic matter is
also a slow-release source of nutrients to plants, thus soils that are relatively high in SOM require
less frequent fertilization than low SOM soils. In natural plant communities, nitrogen and other
key nutrients are recycled and made available to plants through the process of SOM
decomposition.
Five percent or greater is a good target for SOM. Growing cover crops is the traditional way of
increasing SOM. Incorporation of composted organic materials (composted leaves, composted
yardwaste, composted sludge, etc.) is also an excellent way to increase SOM. In field research
over the last two years, I found that incorporating a 2" layer of compost nearly doubled SOM in a
soil with a starting SOM of 6%. Retesting for SOM every few years, or at least on a rotational
basis, is a good practice since SOM is lost over time by oxidation and mineralization. In essence,
SOM is used up by soil microbes for food unless more organic matter is added to the soil.
Soil Nitrates. I don't recommend using the soil nitrate test. Nitrates are made available
from ammonium forms of nitrogen by the process of nitrification. Once in nitrate form they are
rapidly taken up by plants but also rapidly leached from soil. Using a soil test for nitrate is
analagous to taking a picture of a moving target! This test is useful in certain situation where a
constant, high level of nutrition must be maintained. Greenhouse soil-based media are the prime
example. Of course, the use of soil in greenhouse is becoming rare. The mineral soil test should
not be used for soilless media.
Soil soluble salts. This is another valuable test that should be used more often. Soluble
salts (SS) are the summation of all of the fertilizer salts that are available in the soil solution. If
you suspect a problem with overfertilization or deicing salt-contamination, this is the test to
choose. The standard soil test for P, K, Ca, etc. will not provide the information you need to
determine whether there is potential for root injury, - the SS test will. The test also gives an
indication of the general fertility level of the soil.
Soluble salts dissolved in solution conduct electricity, and this is the underlying principle for
laboratory measurement of soluble salts. Soluble salts are expressed in units of electrical
conductivity. The units that the R.E.A.L lab uses for SS are Mhos x 10^-5/cm. Other labs may use
related conductivity units, such as millimhos/cm. Many different conductivity units are currently
in use, and this can lead to confusion.*
Soluble salts in well-fertilized field soil may range between 100 to 200 Mhos x 10-5/cm . More
often, I see values in the 50 to 100 Mhos x 10-5/cm range. Salt sensitive crops may be at risk
when soluble salts exceed 175 Mhos x 10-5/cm.
To summarize, there are a number of special soil tests available from soil labs in addition
to the standard mineral soil test. The soluble salts test is useful for the landscaper or grower as a
diagnostic tool whenever over-fertilization is suspected. The organic matter test is useful for
growers to determine how much effort is required to build up the soil between field rotations.
The micronutrient tests may be of use in special cases; however, where micronutrient deficiencies
are likely, the first steps are foliar analysis and the soil pH test.
___________________________________________
* Some common units of conductivity:
100 Mhos/cm x 10-5 = 1 mMhos/cm = 1 mS/cm = 1 dS/m = 1000 mMhos/cm.
mMhos = millimhos. mMhos = micromhos. mS = millisiemens. dS = decisiemens.
