Journal of the NACAA
ISSN 2158-9429
Volume 13, Issue 2 - December, 2020

Soil Myth Busting for Extension Educators: Reviewing the Literature on Soil Nutrition

• Excessive nutrient levels. Based on decades of research on both crop and ornamental species, scientists have determined optimal levels of macro- and micronutrients required for normal plant growth. While nutrient deficiencies can be common in intensive agricultural production, nutrient toxicities are unusual and as a result have received little research attention until recently. The relatively young field of urban soils research has begun to document the occurrence of nutrient toxicities as a result of improper fertilizer use and/or organic material additions. Phosphate toxicity is perhaps the best example of this phenomenon, but excessive levels of other nutrients such as calcium and magnesium are also commonly seen in garden and landscape soil tests, possibly due to underlying soil conditions or the improper use of gypsum and Epsom salts, respectively.

Myth 2: “Use acidic and alkaline amendments to change soil pH”

The pH of any soil is determined by soil type, the plants naturally growing in the region, and climate factors such as rainfall.  Forests tend to predominate in wet climates and nutrients are tied up in the organic matter of the forest system, which efficiently recycles those minerals so that they are not lost from the underlying soils. Organic matter supplies soluble organic acids, which lower soil pH (Richie and Dolling, 1985); therefore, regions where forests naturally predominate tend to have more acidic soils. In arid regions, evaporation exceeds rainfall and in these soils salts accumulate in the upper layers of soil. While nutrients can be abundant, salinity levels may be high and may impair plant growth. The abundant levels of positively charged nutrients such as potassium, sodium, calcium, and magnesium bind with acidic components and create more alkaline soil conditions.

Measuring soil pH can be done with a soil test, with a portable meter, or with pH strips (Figure 4). However, it is also helpful to know nutrient content of the soil, and this can only be found through soil testing by a university or commercial lab (which will also report the pH). Soil test kits sold to homeowners are notoriously inaccurate and pH can be seriously under- or over-estimated by them (Faber et al., 2007).

Figure 4. Readily available test strips can be used to measure soil pH with varying degrees of accuracy. Photo courtesy of Wikimedia.

Because many environmental factors determine soil characteristics, including pH, it is difficult if not impossible to modify underlying soil chemistry in any permanent way. As discussed earlier, organic matter applied to soil will disappear over time, as will its effects on soil pH.  Additions of lime (which contains calcium) can raise soil pH, while elemental sulfur and ammonium products can acidify the soil. But this is a temporary measure (especially in sandy soils) and potentially creates nutrient toxicities if those elements are already sufficiently available in the soil. Furthermore, a landscape soil system is so vast that a great deal of amendment would need to be added to have a measurable effect on underlying pH. Finally, the addition of such amendments requires significant tillage, which we have recommended against elsewhere (Chalker-Scott and Downer, 2019).

Myth 3: “Garden and landscape plants require routine fertilization”

Organic, inorganic, slow release, soluble, granular, complete or micronutrient - there are many fertilizer options available for gardeners at garden centers. Gardeners generally apply fertilizers on a regular basis for “maintenance,” often in the complete absence of any actual plant nutrient deficiency. Unfortunately, most gardeners do not want to take the time to pursue soil analysis and determine exactly what minerals are deficient - so they just apply a complete fertilizer (which is simply a balanced NPK formulation) on a routine basis. Unnecessary fertilizer application has negative consequences on plant and soil health, resulting in changes to vegetation structure, food availability, and other phenological properties in ways that can harm wildlife populations such as birds (Lepcyzk et al., 2004).

Some fertilizers offer plant specific formulations, such as rose food, citrus food (Figure 5), azalea fertilizer (or more generically acid-loving plant fertilizer). However, there is no published research that prescriptive fertilizers will enhance growth of the intended plant any better than generic fertilizer products. In fact, just fertilizing with nitrogen – the most commonly deficient nutrient - may solve many perceived deficiencies in garden plants.

Figure 5. Specialty fertilizers are heavily marketed for specific crops, but there is no published evidence they are any different from generic fertilizers. Photo by A.J. Downer.

Frequent fertilizer application is often used in an attempt to produce faster growth – but fertilizers are not growth regulators and do not cause plants to grow faster than normal. But lusher growth of landscape plants, due to uptake of nutrients in excess of what’s needed for normal growth, can predispose them to some diseases, such as powdery mildew. Nor does adding nutrients to trees and other woody plants improve their resistance to insect pests (Herms, 2002). Plants that suffer from poor drainage, insufficient light, and other environmental factors that limit plant growth are not assisted by increased fertilizer use. A sustainable approach to landscaping (Figure 6, which includes judicious use of woody mulch (Chalker-Scott, 2015,) reduces use of fertilizers (Hartin et al., 2014).

Figure 6. An arid climate landscape. Photo by A.J. Downer.

Before any application of fertilizer, soils should be analyzed by a university or commercial soil lab to determine actual mineral contents (Figure 3). While plant appearance can indicate foliar nutrient deficiencies, this does not necessarily mean that soils are likewise limited. Excessive levels of some soil nutrients can interfere with availability and uptake of other nutrients; the discussion of excessive phosphorus use later in this article is a good example. Poor root health, due to improper planting (Chalker-Scott and Downer, 2019, 2018) also restricts nutrient uptake, resulting in foliar deficiency symptoms. Additionally, plants that suffer from root rot can show nutrient deficiency symptoms, as their roots are not able to absorb the necessary nutrients. Finally, some plants may not be well adapted to the site or climate in which they are growing (which may manifest as poor root health).

Routine fertilization is a problem for plants, soils, and the surrounding environment. In arid climate soils, inorganic fertilizers add more salts to already salty soils, thus making physiological drought stress from salinity more of a problem for plants. In wetter climates, excessive nutrients from unnecessary fertilization leach into streams and other aquatic systems, impairing water quality. Many landscape plants require little or no fertilization to perform adequately in landscapes (Figure 7). Fertilizing plants that don’t require it is expensive, unnecessary, and damaging to plant, soil and environmental health.

Figure 7: These Salvia plants did not respond to routine fertilization with a soluble complete fertilizer. The clay soil in which they are growing already contained sufficient nutrients. Photo by A.J. Downer.

Myth 4: “Add phosphate fertilizer at transplant to enhance root growth”

Phosphorus, the “P” in the NPK label (Figure 8) found on fertilizers, is a macronutrient required for the synthesis of essential macromolecules including nucleic acids (like DNA) and energy transfer compounds (like ATP). It is one of the “big three” nutrients (along with nitrogen and potassium) that is often deficient in intensively managed agricultural fields. Phosphate is the major, and sometimes only, component of so-called “transplant” fertilizers, which are widely advertised as root-growth stimulators for new trees and shrubs. But residential and other nonagricultural soils are rarely deficient in phosphate. Additional phosphate, applied routinely without knowledge of existing soil phosphate concentrations, can lead to phosphate toxicity and associated mineral imbalances (Chalker-Scott and Olmsted, 2009).

Figure 8. Bone meal fertilizer recommended for transplanting.  Note high P level in formulation (3-15-0). Photo by Linda Chalker-Scott

Phosphorus toxicity can often be visualized as interveinal chlorosis of leaves (Figure 9). The chlorosis is due to foliar deficiencies of iron and/or manganese linked directly to excess soil phosphorus. Research has documented this problem over the last several decades (e.g., Delmas et al., 1957; Fox and Warner, 1971), yet this information does not make its way into Extension recommendations for home gardeners.

Figure 9. Interveinal chlorosis. Photo by Linda Chalker-Scott.

When plants are exposed to increased levels of phosphorus, iron phosphates are precipitated out of solution both in the soil and within plant tissues. This form of iron cannot be taken up by roots, nor can it function in its biochemical role as an enzyme cofactor. The lack of functional iron in leaves causes interveinal chlorosis (Figure 9). Chalker-Scott and Olmsted (2009) not only found an increase in Rhododendron leaf chlorosis, but also saw oedema, root damage, and death with increasing soil phosphorus levels (Figure 10).

Figure 10. Leaf chlorosis as a result of increasing phosphate fertilizer (C=control, no fertilizer; ½X = half of recommended application; 1X = recommended application; 2X = twice recommended application). 1= Normal (green leaves), 2= Light chlorosis in young leaves, 3= Moderate interveinal chlorosis, 4= Severe chlorosis, young leaves white (from Chalker-Scott and Olmsted, 2009).

In addition to these induced nutrient deficiencies, excessive phosphorus also inhibits mycorrhizae, whose presence is crucial to the health of most ornamental plants (Chalker-Scott, 2017). Phosphorus is also the principal nutrient responsible for aquatic eutrophication and the link between the use of phosphorus-containing fertilizers and fish kill (Figure 11) is well known (Oliveira and Machado, 2013). Before any phosphorus-containing fertilizers are added to garden and landscape soils, a soil test should be performed to verify the need. The potential for soil, plant, and environmental damage is too great to ignore.

Figure 11.  Eutrophication results from excessive levels of phosphorus, which stimulates algal growth and eventually algal death and decay. The decay process by microbes depletes oxygen in the water, leading to the death of fish and other aquatic life. Photo courtesy of Colourbox.

Myth 5: “Add Epsom salt to garden and landscape soils as a general fertilizer”

Epsom salt (Figure 12), also called magnesium sulfate, contains two plant macronutrients: magnesium and sulfur. Sulfur is a part of certain amino acids, while magnesium plays a role as an enzyme cofactor – a chemical that can assist in chemical reactions by accepting or donating an electron.  While both of these are essential elements, neither is needed in excess. Yet many garden and landscape soils, as mentioned earlier, are improperly amended with Epsom salt.

Figure 12. Epsom salt appeals to gardeners, though its use is rarely necessary. Photo courtesy of Needpix.com.

The internet is full of unsubstantiated claims for Epsom salt application to gardens and landscape, including:

• Cures blossom end rot;
• Deters pests;
• Helps seeds germinate;
• Increases chlorophyll production;
• Produces more flowers; and
• Improves bloom quantity and quality of roses (Figure 13).

Figure 13: Despite the assertion of Rosarians, there is no published evidence supporting use of Epsom salt in rose culture. Photo by A.J. Downer.

There are no documented benefits to support these claims (Chalker-Scott and Guggenheim, 2018), and they should be ignored. Instead, a soil test should be performed at an appropriate soil testing lab to determine if there is magnesium deficiency (Figure 14). If a deficiency is present, follow soil lab guidelines for appropriate additions. If there is no deficiency, do not add this chemical to your garden or landscape.

Figure 14. In this soil, magnesium (391 ppm) far exceeds that which is recommended (50-120 ppm).   Nothing containing magnesium should be added. Test results from University of Massachusetts soil testing lab.

Do not rely on leaf symptoms that appear to indicate a soil magnesium deficiency (Figure 15). Chlorosis, or leaf yellowing, is a common result of many environmental stresses, including drought, cold, and nutrient imbalances. Adding magnesium when there is no soil deficiency can interfere with calcium uptake and possibly other nutrients (Chalker Scott and Guggenheim, 2018a). Only a soil test can identify a deficiency of magnesium or any other nutrient.

Figure 15. Magnesium deficiency presents as chlorosis, but many other factors can also cause leaf yellowing. Photo courtesy of Wikimedia.

Why does this myth persist, especially in the complete absence of any scientific literature?  Speculation suggests that there is an emotional appeal behind the name “Epsom salt,” conjuring images of soothing spa treatments. It is doubtful whether marketing this product as simply magnesium sulfate would generate the enthusiasm found for Epsom salt.

Myth 6: “Gypsum will create better soil by modifying structure, texture, and pH”

Gypsum is calcium sulfate and has a long and confusing history of use in gardening. Retail packaging often asserts that gypsum will modify soil texture, improve soil structure, increase water penetration and drainage, loosen compacted soils, and neutralize salts (Figure 16). Although not suggested on most labels, many gardeners mistakenly believe gypsum reduces soil pH due to its sulfur content (sulfur content often is a marketing point for gypsum products). Regardless of its perceived acidification ability, adding gypsum does not change soil pH (Chalker Scott and Guggenheim, 2018); soil pH is governed by climate and environmental factors as discussed earlier.

Figure 16: This product claims to “loosen hard soil” – a misleading statement. Photo by A.J. Downer.

Before gypsum is added, it is crucial to determine if a calcium deficiency is present in the soil (Figure 17). If it is not deficient, gypsum should not be added. Additions will simply create nutrient imbalances which are difficult to correct.

Figure 17. In this soil, calcium (2269 ppm) far exceeds that which is recommended (1000-1500 ppm). Nothing containing calcium should be added, including gypsum. Test results from University of Massachusetts soil testing lab.

If there is a deficiency in calcium as confirmed by a soil test, the addition of gypsum will address that deficiency. Past this, the other claimed benefits are dependent on soil characteristics. Gypsum has documented benefits as in the following cases:

• Sodic soils. Sodic soils contain sodium and will not allow water to easily enter or drain from them. Adding gypsum to these soils will allow soil particles to bind together or flocculate. This is a chemical reactions where calcium (which has two positive charges) replaces sodium (with one positive charge) on clay particles (which have a negative charge). Sodic clay particles are covered with a “wool” of sodium ions which can only bind to one clay particle. Thus, the clay particles cannot bind to one another. Calcium has two positive charges, meaning that it can bridge two clay particles and pull them together. However, sodic soils are extremely uncommon in most gardens.
• Heavy clay soils. Many gardeners claim they have a clay soil simply because it doesn’t drain well. Most of the time this is due to soil compaction or textural discontinuities (Chalker-Scott and Downer, 2019). A heavy clay soil is one that contains >40% clay, which must be verified by laboratory analysis. (An approximation can be made by a method described elsewhere [Chalker-Scott and Downer, 2019].) If a soil contains high clay content but is low in calcium, gypsum can increase water infiltration and drainage (Chalker-Scott and Guggenheim, 2018) through flocculation as described above. This is especially important in poorly drained clay soils, where additions of gypsum can control root rot diseases caused by Phytophthora (Messenger 2000).