What Is My Soil Test Telling Me About Nutrient Needs for My Plants?
Soil tests provide necessary information about the nutrient presence and availability
in the soil. Mobility and availability of nutrients depend on soil pH, the nature
of the parent rock, vegetation and microbial activity in the soil.
Plants require 16 essential elements to grow and develop: hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, iron, carbon, boron, magnesium, chlorine, manganese, molybdenum, copper and zinc. A useful trick to help you remember the essential elements is to imagine a sign posted outside a diner that reads, “HOPKiNS CaFé, C.B., Manager. Closed Monday Morning. See you (Cu) Zoon (Zn)!”
The most important information gathered from a soil test is the status of the soil’s acidity, or pH. The concentration of hydrogen ions is marked as pH, representing a certain ratio between hydrogen (H+) and hydroxyl (OH-) ions in the soil. Soil pH influences the nutrient availability and uptake by the plants, the activity of soil microorganisms, and the organic degradation and mineralization.
It also can strongly influence the effectiveness of applied fertilizers by altering the reaction of the soil by acidifying it or alkalifying it. For example, acidic conditions can weather rocks, causing them to release calcium, potassium, magnesium and manganese. Likewise, the breakdown and decomposition of organic matter can lower the pH or rain can flush ions from the soil profile making it more alkaline.
Many of the developmental problems that plants or trees have, such as low vigor, bitter pit or the appearance of rough bark with brown streaking in the cambium layer (known as “measles”), could be traced back to changes in pH values. Low pH indicates higher acidity, which in turn makes elements, like manganese, and heavy metals, like aluminum, more readily available to the plants causing phytotoxicity. Meanwhile, the other nutrients become chemically bound, thus unavailable.
The best way to keep everything in balance is to keep the soil pH around 6.5. This is true for most plants, except blueberries. Blueberries prefer acidic soil with a pH between 4 and 5, with the optimum range from 4.2 to 4.5.
Because the most commonly used nitrogen fertilizers will lower the pH over time, a liming recommendation will be given if the soil is too acidic. The type of lime (calcareous or dolomitic) will depend upon the presence and availability of magnesium. If the magnesium level is low, dolomitic lime is preferred since it is a conglomerate of calcium and magnesium.
The best way to apply lime is by spreading it on the surface and incorporating it into the soil by tillage or simply by shovel, hoe or any other utility tool. It takes about two years before the liming takes effect and changes soil pH. Frequency of applications is determined by crop management practices (particularly nitrogen management) and by cation exchange capacity.
Higher soil pH (6.5 and above) may reduce the availability of certain elements, such as micronutrients boron, copper, iron, manganese and zinc. The chart below is a visual presentation of the correlation between the pH and nutrient availability in soil (Figure 1).
Interpreting a WVU Soil Testing Laboratory report
The report received from the WVU Soil Testing Laboratory contains a wealth of information regarding your soil, if you know how to interpret the values (Figure 2).
Below is a list of the values you should pay particular attention to, as well as what they mean:
- MEQ/100 refers to the cation exchange capacity and is expressed in milliequivalents/100 grams of soil.
- %SAT is the percent of nutrient saturation.
- BS refers to the percent base saturation (sum of individual percent nutrient saturation).
Cation exchange capacity
Cation exchange capacity is the ability of a soil to release, absorb and retain positively charged elements, like magnesium, potassium and calcium. These positively charged cations are being exchanged on the surface of the soil particles binding to them because the soil is negatively charged. If potassium binds to the soil, it pushes out calcium releasing it into the water solution making it available for uptake by the roots. Most of the calcium, magnesium and potassium comes from the exchange sites (Figures 3A and 3B).
Percent base saturation
Percent saturation is yet another indication of the soil’s ability to attract, hold and exchange cations to provide necessary elements to satisfy nutritional needs. Percent saturation shows how many cation exchange sites on the soil particle are occupied by the individual nutrients (percent nutrient saturation) or by the three major cations that have basic or alkaline reaction (potassium, manganese and calcium) expressed in percent base saturation.
Other cations that are present in the solution but not always measured include sodium, iron, manganese, zinc, copper, aluminum, etc. The fact that they are present but not measured allows for the sum to be either less than 100% or above 100%.
Figure 1. The influence of soil pH on nutrient availability
Figure 2. Sample of a WVU soil testing report.
Figure 3A. Graphic representations of cation exchange on soil particles.
Graphic credit: M. Danilovich
Figure 3B. Simplified version of the cation exchange capacity.
Graphic credit: M. Danilovich
Mirjana Bulatovic-Danilovich, Consumer Horticulture Specialist, WVU Extension Service Agriculture and Natural Resources
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