A soil amendment or conditioner is something that is added to or mixed with existing soil in order to improve soil tilth. Amendments can increase soil moisture, nutrient holding capacity, aeration, and water infiltration, among others. They can be either organic or inorganic materials. Organic matter is the most important amendment that can be added to soil and is extremely beneficial for soil health.
Unfortunately, some soil amendments, despite their promotion, have not been shown to provide any significant benefit. Other amendments may improve soil qualities, but are a risk to human health. There are also some common practices employed when growing plants that are counterproductive. These unproven amendments and counterproductive practices include:
These gels or crystals will absorb water and swell up to several times their original size. The gels then slowly release water over time into the soil. However, within two to five years, these gels degrade into substances such as acrylamide. Acrylamide is a neurotoxin and carcinogen, and can be absorbed through the skin or inhaled. Even in its original, intact state, polyacrylamide can cause skin irritation and mucus membrane inflammation.
Clay soils exhibit poor aeration, infiltration, and drainage because of the small size of the soil particles. Sandy soils, composed of larger soil particles, are well drained and aerated. The thought is that incorporating sand into clay soils will improve the soil texture, increasing aeration, infiltration, and drainage. However, these undesirable traits of clay soils can actually be magnified if sand is added in the wrong proportions. The small clay particles plug the larger pore spaces of the sandy soil creating a very heavy, dense soil. Half of the soil by volume would have to be sand before the benefits of the amendment could be seen.
Gypsum, which is hydrated calcium sulfate (a low-solubility salt), is effective in treating sodic soils, which are soils high in exchangeable sodium. The sodium between soil particles attracts water, causing the soil to disperse. The dispersed particles seal the soil surface, reducing infiltration. Addition of gypsum replaces the sodium on the exchange sites with calcium, which results in flocculation of the soil particles into soil aggregates. The resultant sodium sulfate can then be leached out of the soil. Although gypsum does improve structure in sodic soils, it will not soften clay nor loosen compacted soil.
Gypsum will not lower the pH of alkaline (basic) soils. A misunderstanding exists that the sulfate in gypsum will form sulfuric acid when combined with water. Gypsum is a neutral salt, and will not generate acidity and affect pH when dissolved in in soil water. Gypsum may lower the pH of sodic soils by forming sodium sulfate that will leach from the soil.
Gypsum will not increase the pH of acid soils. When liming with an amendment such as calcium carbonate, the carbonate combines with the acidity (hydrogen) to form water and carbon dioxide gas. Gypsum does not react in a similar manner; it does not combine with acidity to form a neutral compound. Due to complex reactions, gypsum may increase the pH of acidic soils high in soluble aluminum.
Some gardeners advise that gypsum be added to ameliorate salinity in saline soils (soils high in soluble salts). Gypsum itself is a low-solubility salt, and could actually increase the salinity of the soil. The basis for the claim that gypsum can be applied to successfully reduce soil salinity is the confusion that exists between saline and sodic soils. While gypsum should be applied to saline-sodic soils, it does not ameliorate soil salinity. The only way to approach saline soils is to grow salt-tolerant plants or to leach the salts out of the soil profile with water.
With the exception of inoculation of legume crops with rhizobia, little or no improvement in soil health and microbial populations has been observed with the addition of soil microbes. Even poor-quality or barren, infertile soils contain enough soil microbes such that proper management (such as additions of organic matter) will cause the existing microbial population to increase.
There are many sources that recommend placing coarse material, such as sand, gravel, or clay shards, in the bottom of a planter (below the finer-textured soil) to enhance drainage. This practice results in the opposite of the desired effect.
Placing coarse material below finer material/soil will decrease drainage. The soil the plants are growing in may actually reach near-saturation before water moves downward into the coarser material. This is because finer textured material holds water more tightly than coarser materials. The force by which soil holds water is termed the capillary force, or matric potential. The water content of the finer material may approach saturation before the capillary forces of the coarser soil/material can “pull” the water downward.