Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityMarch, 1996
Interpreting the OSU Horticulture Soil Test: What the Numbers Mean
Keywords: soil/media testing, pH, soils, landscape, nursery, nutrition
T he Horticulture Soil Test is available through the Research Analytical Extension Laboratory (R.E.A.L. for short) at the Ohio State University. Since I've come to Ohio State I have taken over the job of interpreting the soil tests for nurseries. In this article I'd like to explain how I interpret your tests (and hopefully not put myself out of a job when you see how easy it is!)
The Horticulture Soil Test is used appropriately for field soils as well as growing media that contain a large fraction of soil (greater than 20%). (Those media that contain a smaller fraction or no mineral soil should be tested using a soilless extraction method, such as the saturated medium extract procedure (SME) used by Ohio State.) The Horticulture Soil Test uses the same analytical procedures that are used in most soil labs throughout the country, however, not all labs report results in the same units. Recall from last month's L.A.N.D. column (February) that the R.E.A.L lab changed reporting units in pounds per acre to parts per million (PPM) as of January, 1996. To convert PPM to pounds per acre, multiply the PPM value by two.
A facsimile of the table provided in the Horticulture Soil Test report appears with recommended values in Table 1. The first two columns, plow depth and lime applied within two years, contain information supplied by the grower. This information is important for calculating the lime requirement, which is typically printed at the top of the report (not shown in table one). Plow depth is used in the lime calculation because it determines how much soil volume is involved in the lime reaction.
Soil pH. The next column provides the soil pH. A pH range from 5.5 to 6.5 is considered optimum for a wide variety of plants. However, this is not a hard and fast rule, as many plants will tolerate or even thrive at pH values outside of the range. Ericaceous plants such as rhododendrons and azaleas prefer soils in a range about one unit lower. The desirable pH for highly organic soils also is lower, between 5.0 and 6.0; however, in Ohio, vegetable
production predominates on these 'muck' or organic soils.
Lime test index and the soil lime requirement. The lime test index is a value that is used in the lime requirement calculation. As the lime test index decreases below 69, the lime requirement increases. The lime requirement provided by the Horticulture Soil Test is calculated to bring the soil pH up to 6.5. No lime recommendation will appear if the pH 6.5 or the lime test index exceeds 68. When the grower indicates that an acid-soil preferring species is grown, the target pH is 5.2.
Phosphorus. Typically, phosphorus (P) is present in extremely low concentrations in the soil solution. Much of the P in soil is 'unavailable' P which become available over a long period of time. The test for available phosphorus uses an extractant (Bray 1 extractant) that removes both P in solution and P stored in the soil that is 'in equilibrium' with the soil solution. In other words, available P includes that in solution plus that which may readily move into solution. Recommended P values are 25 PPM for landscape maintenance, and 50 PPM for nursery production. Much higher values may be found in some well-fertilized soils. Phosphorus levels as high as 300 PPM will not harm plants but may be of environmental concern.
Potassium (K), Calcium (Ca), Magnesium (Mg), and the cation exchange capacity (CEC). Potassium, Ca, and Mg are the so-called basic cations that are held on soil cation exchange sites. These exchange sites are negatively charged, whereas the exchangeable ions that can occupy the sites are positive charged (a positively charged ion is called a cation). The sum of the cation exchange sites in the soil is referred to as the Cation Exchange Capacity (CEC). Mineral clays have a great number of negative charge sites, thus clay soils have a high CEC, whereas sandy (coarse) soils have a low CEC. Organic matter also has negative charge sites and may contribute to the CEC, but in most Ohio soils, the CEC is primarily determined by the clay fraction. Table 2 gives approximate CEC values for soil textures.
Laboratory measurement of the CEC and exchangeable Ca, Mg, and K is accomplished by displacing all ions from the cation exchange sites with ammonium acetate. When added in excess, the ammonium ions completely supplant the Ca, Mg, and K on the CEC. These ions are measured individually, then summed to estimate the total
number of charge sites. The CEC is the sum of these charge sites, expressed in millequivalents per 100 grams of soil. Hydrogen also resides on the CEC and is accounted for in this determination.
Target values for landscape soils are 125 PPM K and 75 PPM Mg, and for nursery production 200 PPM K and 125 PPM Mg. A wide range in Ca is permissible, but there should be a minimum of 400 PPM Ca; it is not unusual to observe 10 to 20 times as much Ca in the heavy clay soils of central Ohio! Lime addition is the primary means of supplying Ca and Mg. Potassium fertilizers as well as N-P-K fertilizers are used to supply K.
The higher the CEC, the greater the soil's capacity to store nutrients and prevent their loss by leaching. Theoretically, soils with higher CEC may require less frequent fertilization. A soil with a CEC of 7 to 10 meq / 100 grams has the capacity to store nutrients for about one year of plant growth. Much of the soil in Lake Co., Ohio falls below this range, thus it is obvious that low CEC values do not prevent intensive production. Where the CEC falls below 7 to 10 meq/ 100 g, we need to apply fertilizer more frequently, but at a lower rate to avoid nutrient loss through leaching. Incorporation of organic matter (compost, green manure) is usually recommended to improve low-CEC soils because over time organic matter can increase the CEC as well as water-holding capacity of coarse-textured soils.
Base saturation. The base saturation expresses the quantity of Ca, Mg, and K ions that reside on the CEC as a percentage of total CEC. The ratio of these ions to one another affect the plant's ability to take them up in the proportion and quantity required for optimal growth. The calcium to magnesium ratio should be considered when lime is added to the soil. If the ratio of Ca to Mg is less than 1:1 (more Mg than Calcium), use a lime that is low in Mg. Where the Ca to Mg ratio exceeds 10:1, use dolomitic lime (high in Mg). The Mg to K ratio should be 2:1 or greater.
What about Nitrogen? Nitrogen (N) will not appear in the standard mineral soil test, although a nitrate test is an option. The absence of an N test may seem odd since N is the element that most frequently limits the growth of plants. Researchers have attempted to come up with soil N tests that predict growth, but have failed. Much of the N in soil is locked up in organic matter that must be mineralized or transformed into inorganic N. Since we can't easily predict mineralization rates for organic N, it doesn't do much good to measure it. Inorganic N (especially nitrate-N) is the most important form taken up by plants; however its concentration fluctuates widely in Ohio's humid climate due
to leaching and denitrification. Since inorganic N is here today and gone tomorrow, we rarely measure it except in greenhouse soil-containing media.
Lacking an effective N test for Ohio's soils, we assume that plants in the nursery will require yearly nitrogen addition. Typical N rates for nursery production of deciduous trees and shrubs are 200-250 pounds N per acre per year. Narrow-leaf evergreens require somewhat less than deciduous plants; broadleaf evergreens require only half as much. Plants in the landscape may or may not require yearly applications, depending on their growth rate and the nature of the soil. The higher the organic matter in the soil, the less frequent the need for additional N. Typical rates for landscape maintenance are 2 to 3 pounds N per thousand square feet per year.
Putting it all together. Ideally, field soils should be sampled every 3 to 5 years, or at least with every crop rotation. Pre-plant testing is highly recommended, because this is the only practical time to alter soil pH. When I look at the soil tests, I look for deficiencies, and write the recommendation to bring the soil closer to optimal levels. Where only one element is deficient, I recommend a fertilizer that will supply that element. When both P and K are deficient, a balanced N-P-K fertilizer is recommended. Other than this case, I do not make an N recommendation but assume that the grower applies N as a standard nursery practice.
It is not always possible to bring a soil in line with optimum values in one fell swoop. For example, 300 pounds of actual K per acre is a maximal rate because in excessive concentrations this element can increase soil soluble salts to injurious levels (greater than 180 mhos x 10-5/cm). In contrast to K, large amounts of P may be added to most soils in one application without concern for plant injury. However, keep in mind that low CEC soils have a reduced capacity for storing nutrients, thus splitting up fertilizer applications over the course of a year (typically, spring &
fall) is recommended to improve fertilizer efficiency in these soils.
Wow! I'm really on a roll here with my favorite subject, plant and soil fertility. But since readers may not share my passion, I think I'll leave off here and save micronutrients for another L.A.N.D. column!
Table 1. Ideal values for the Horticulture Soil Test. Where a range is given for PPM (parts per million) P, K, Ca, and Mg, the upper value is recommended for nursery production and lower value for soils in the landscape.
Standard test results
Sample information
Base saturation
plow depth
lime applied
lime
test
Phos-phorus
Potas-sium
Calcium
Mag-nesium
Cation
exchange
in
last 2yrs
pH
ind-
P
K
Ca
Mg
capacity
%
%
%
inches
Tons/A
ex
PPM
PPM
PPM
PPM
meq/100g
Ca
Mg
K
grower
supplies
this
grower
supplies
this
5.5-6.5
*
25 - 50
125 - 200
400 plus
75 - 125
7-10 or higher
40% - 80%
10% - 40%
3% -
5%
Table 2. Typical Cation Exchange Capacities (meq/100 g soil) for three classes of soils
Soil type CEC
Fine texture (clay) 20-30
Medium texture (silt) 5-20
Coarse texture (sand) 1-5
