Soil chemical, physical, and biological properties range from those highly favorable to plant growth to those highly unfavorable to plant growth. It is rare—especially in the tropics—to find a soil in its natural state in which all properties are highly favorable to plant growth. Nevertheless, as long as there is sufficient soil depth to provide an adequately deep and well-drained root zone, proper amendment and management of soil properties can result in almost any soil becoming suitable for plant growth. Even naturally infertile soils and soils with very low water-holding capacity can produce extraordinarily high crop yields with proper management and inputs.
Soil properties that negatively affect plant growth must be addressed by adding amendments, physical manipulation (tillage), changing soil management practices, or combinations of these three actions. For example, soil compaction can only be corrected by tilling the soil to break up compacted zones. However, addition of soil amendments (especially organic materials) and altering soil management practices may help to prevent formation of compacted zones. Unfavorable soil chemical properties require soil amendments, but proper soil management can slow down the recurrence of such problems. A biological problem, such as an infestation of soil-borne fungi or of parasitic nematodes, can be addressed quickly through chemical means, or more slowly and sustainably by changing crop and soil management practices.
The objectives of this document are to 1) define soil amendments, 2) discuss common soil problems that can be treated by adding amendments, and 3) describe common types of soil amendments.
Soil amendments defined
Soil amendments are not the same as chemical fertilizers. Fertilizers are added to soils specifically to add plant-available nutrients. Soil amendments, on the other hand, are used primarily to correct adverse chemical, physical or biological soil properties other than low nutrient availability. The distinction between amendments and fertilizers is not always clear-cut. For example, organic matter can serve as both fertilizer and soil amendment. In this paper, we will limit our discussion to amendments.
Common soil problems treated with amendments
A number of common soil problems can be treated by adding soil amendments. Some of these problems are described below.
Soil acidity and alkalinity
Soil pH is one of the most important and fundamental of soil properties. Soil pH controls the bioavailability of many essential nutrients (Figure 1, Page 1), determines the relative toxicity of various metals in soils, and strongly affects biological activity in soils. The pH is a measure of soil acidity and alkalinity. The pH scale ranges from 0 to 14; with a pH of 7 being “neutral”, while values below 7 are acidic, and values above 7 are alkaline. For most plants, a pH of about 6 is ideal, although some acid-loving plants will thrive in soils with a pH as low as 4 and alkaline-tolerant plants can survive in soils with a pH up to 10. Even within a plant’s “tolerable” pH range, plant health and yield can suffer at the outer limits of the range. Many plants grown in the humid tropics and sub-tropics are adapted to moderately acid soil pH values (Table 1), including rice, coffee, pineapple, and passion fruit. This is fortunate, because most tropical soils tend to be naturally acidic!
Soil acidity is a natural consequence of soil exposed to humid climates for long periods (e.g. centuries to thousands of years). Such conditions will result in losses of non-acidic soil elements such as calcium and magnesium. At the same time, acidic elements such as aluminum and iron persist in soils, become progressively more soluble, and react to produce acid. In cases where soils are acidic, it is important to consider crop acid tolerance before applying amendments to raise soil pH.
In highly acid soils (pH<5), bioavailability of N, P, and K is often low. At the same time, aluminum, iron, and other metals may become so soluble that they are toxic to plants. In fact, aluminum and manganese toxicity are very serious problems for plants growing in highly acid soils; plants that can withstand low soil pH are able to tolerate high concentrations of available aluminum and manganese in soil (Yost 2000). Soil acidity also adversely affects microbial activity, with bacteria becoming less active and fungi more active in highly acid soils. Growth of beneficial nitrogen-fixing bacteria is usually inhibited in highly acid soils.
Alkaline soils have a pH higher than 7. These soils are most common in semi-arid and arid regions, and where soils are salty because of salts in irrigation water and/or poor drainage. Alkaline soils can also occur in sub-humid regions where soils have formed on alkaline parent materials such as limestone. Soil pH values up to 8 rarely cause problems for plants (acid-loving plants are an exception). However, pH above 8 will often result in severe deficiencies of micronutrients such as iron, zinc, and manganese.
Soil alkalinity most often is a result of one of two conditions. The first and most common is the occurrence of calcium carbonate (lime) in soils. Many dry-region soils have natural accumulations of lime, because this moderately-soluble mineral persists in soils. Wherever soils contain natural lime (calcareous soils), soil pH will stay above pH 7.0 and may be as high as 8.3. Soil alkalinity can also be caused by the presence of high amounts of sodium along with associated carbonates. Such cases usually results when sodium is added to soil (e.g. in irrigation water) accompanied by poor soil drainage, which prevents sodium from leaching below the root zone. Soils with high sodium content are referred to as sodic soils, and is a very serious soil condition that is difficult to correct (Figure 2).
Soil salinity and sodicity
Saline soils have accumulations of soluble salts in concentrations that can be harmful to plants. Just as with pH, soils have varying degrees of salinity, and plants have varying tolerance to soil salinity. Soil salinity is usually expressed as electrical conductivity (EC) or as TDS (total dissolved solids). A TDS value of 640 ppm approximately corresponds to an EC of 1 dS/m. Salinity adversely affects plants by reducing the availability of soil water to plants, since the salts pull water away from plants through osmosis. Most plants that are well adapted to the humid and sub-humid tropics cannot tolerate soil salinity and may be harmed by even modest amounts of soil salinity. On the other hand, there are plant species that can tolerate high amounts of soil salinity, though few of these tend to be crop species (Table 2). The salinity tolerance ranking shown in Table 2 is a general guide only, as plant tolerance to salinity also depends on growing conditions and other stressors. The negative effects of salinity can be mitigated—though not eliminated—through proper management of soil moisture, by keeping the root zone moist (not wet) at all times. A moist root zone will dilute salt concentrations. Overly wet root zones, however, are detrimental for plants. Drip irrigation is an especially good tool for managing salinity, because it is the best method for managing root zone soil moisture.
Salinity and sodicity often occur together, but sodicity is usually the more serious problem of the two because it is more difficult to correct. Sodicity is the accumulation of high amounts of sodium on soil clays, especially in the absence of adequate amounts of calcium. Sodicity does not harm plants directly, but causes soil structure to disintegrate and can make soils impermeable to water.
Amendments that neutralize soil acidity are bases, chemically speaking. “Lime” is a general term that encompasses several materials derived from naturally occurring limestone. Agricultural lime is the pulverized form of limestone (CaCO3) most commonly used as a liming material. Agricultural lime varies in its effectiveness depending on chemical purity and particle size (smaller particles are most effective). Hydrated lime (Ca(OH)2) and quicklime (CaO) are formed when agricultural lime is heated; they are usually more expensive than agricultural lime, and therefore less commonly used. Quicklime must be used with caution, as it can be caustic to humans and plants. Dolomitic lime (CaMg(CO3)2) is a naturally-occurring mixture of calcium and magnesium carbonates that can be used in place of agricultural lime. Marl is a soil material that contains high concentrations of calcium carbonate, but its use is not recommended unless no other liming materials are available, because of its low purity.
Although liming materials are applied to soils primarily to neutralize acidity, they also add calcium (and magnesium, in the case of dolomitic lime), which is essential for plants and often deficient in acid soils. The relative effectiveness of liming materials is expressed as the calcium carbonate equivalent (CCE); pure calcium carbonate has a CCE of 100 (Table 3). The CCE depends on the chemical composition but also on the particle size. Large lime particles will react more slowly in soils than fine particles, and hence will take longer to have the same acid-neutralizing effect. In addition to neutralizing acidity, lime application to tropical soils may improve soil aggregation, porosity, and bulk density. Acid soils should normally be limed to pH 6.0 – 6.5. The correct amount of lime to add to an acidic soil depends on two factors—the pH of the soil and the soil buffering capacity, also known as “reserve acidity.” Soils with high clay content and high cation exchange capacity (CEC) tend to have high buffering capacity. The only way to accurately determine a specific soil’s lime requirement is to have the soil tested by a reputable soil testing laboratory. The laboratory will measure pH and buffering capacity and recommend a lime rate, usually in tons/ha; the rate will assume that the liming material has a CCE of 100%. If it is impossible to send a soil sample to a laboratory, lime requirement can be estimated by incubating samples of moist soil with differing amounts/rates of lime. After five days, the pH is measured (Sonon and Kissel 2015), and soil should be limed using the rate which resulted in a pH closest to 6.0 – 6.5. However, this incubation method should only be used if laboratory soil analysis is impossible, because the accuracy of the method has not been thoroughly evaluated.
There are no amendments to counteract soluble salts in soils or lessen their effects on plant growth. The only cure for soil salinity is adequate leaching of salts below the root zone with high-quality irrigation water. If this is impossible, maintain optimum soil moisture to minimize salt stress, use irrigation methods that minimize salt in the root zone (e.g. drip irrigation), and/or grow salt-tolerant crops. Applications of gypsum (CaSO4.2H2O) can help rid soils of sodium, but not soluble salts, so long as drainage is adequate. The amount of gypsum to add increases with amount of sodium and soil cation exchange capacity.
Acids to neutralize alkalinity
Amendments that neutralize soil alkalinity are acids, chemically speaking. Such amendments are used much less frequently than alkaline liming materials. There are several reasons for this. First, sodic soils are often alkaline, but applying gypsum and leaching sodium from soils will usually lower pH to desirable levels. Second, many alkaline soils are calcareous. The pH of calcareous soils cannot be brought below 7.0 unless all calcium carbonate is neutralized, which will usually require prohibitive amounts of acid amendments. For these reasons, neutralizing alkaline soils is usually impractical for annual crops. However, neutralizing the root zone of tree crops is possible by adding acid-forming amendments such as sulfur, thiosulfate, and ferrous sulfate. The appropriate application rate of acid-forming amendments will depend on soil pH and buffering capacity, and should be determined through soil testing. Some useful related information is provided by Mickelbart and Stanton (2012).
Organic amendments for soils are those of biological origin. There are a tremendous variety of organic amendments, including crop residues, manures, food or other wastes, organic fertilizers, biochar, compost, and others. The variety of organic amendments is almost endless; some common organics are shown in Table 4. In general, organic materials are not added to soils to control pH, but they can affect pH and plant response to pH. Organic amendments are normally added to soils to increase soil organic matter and/or to provide plant-available nutrients. Though a full discussion of organic amendments is outside the scope of this paper, I will make a few key points about organics.
One important property of organics is the carbon to nitrogen ratio (C:N). The carbon percentage of organics is fairly constant at 50-60% by weight. On the other hand, nitrogen percentage varies from well below 1% to more than 6%. Therefore, the C:N ratio of organics can be as low as 8 to as high as 200. In general, organic materials that are easily decomposed in soils have low C:N (i.e., they contain a high proportion of nitrogen) and are valuable as fertilizers. Manures, green crop residues, and food wastes are examples of such materials. When these materials are added to soils and microbial decomposition begins, they serve as nutrient sources for plant use. When these organic amendments decompose, the nutrients they provide are no more available or beneficial to plants than those from chemical fertilizers. However, the addition of organics to soils also includes the benefit of adding organic matter. The rate of organic matter decomposition is highly variable and depends on the chemical composition of organic material, C:N ratio, soil moisture, pH and temperature, and microbial populations. In general, microbial decomposition of organic materials is most rapid in moist, warm, and slightly acid soils.
Organic materials resistant to microbial decomposition and with high C:N ratios can serve as soil amendments but do not make good fertilizers. Brown crop residues and woody materials are examples. When they are added to soils and start to decompose, they may actually “consume” available nutrients for several weeks or months. These materials with a high C:N ratio resist decomposition, so they will persist in soils for a long time. They become an important part of humus, which contains stored carbon and which helps give soil its structure. Forming humus also encourages storage or “sequestration” of carbon in soils.
In general, adding organic materials to soils will yield positive results. However, the exact results and the length of time to achieve them will depend on the nature of the organic inputs. Compost is an organic soil amendment made by piling and decomposing organic materials, resulting in a stable end product that will decompose slowly in soils (Evanylo 2011). Finished compost has a low C:N ratio but decomposes slowly in soils because it is chemically complex. Compost is normally not rich in nutrients and hence does not increase plant available nutrients in the short term. However, compost is a superb soil amendment. Compost can be made from virtually any organic material, as explained in a recent ECHO West Africa note (Gouba 2017). Long-term application of properly prepared compost to soil can improve its water-holding capacity, aggregation, fertility, and can encourage growth of beneficial microbes.
Biochar has received much recent attention as a soil amendment. Biochar adds carbon to soil, can improve soil chemical and physical properties, and application of biochar can increase crop yield (Major, 2010). A recent ECHO Asia Note contained an extensive discussion of biochar (Shafer, 2018), and the reader is encouraged to consult this document for more details. Biochar can be a valuable soil amendment, but it is not a “magic potion” to solve all soil ills.
Microbes are not usually considered soil amendments, although they may be added to soils to change specific soil biological properties. For example, Rhizobium bacteria are helpful inoculants for leguminous crops, fixing nitrogen that can nourish the plants. Likewise, mycorrhizael fungi can be added to support crops growing on degraded soils, for crops which can grow symbiotically with these fungi. Trichoderma fungi have also been added to soils as beneficial microbes to combat pathogenic organisms and for plant growth enhancement (Shelton 2018). Additions of microbes to soil is most beneficial to address specific situations, such as when planting a legume crop in a field for the first time, or adding mycorrhizael fungi to soils degraded by erosion or nutrient depletion.
A final group of soil additives are referred to as “stimulants”, a term that is very broad and ill-defined. Manufacturers of such additives may make numerous claims, such as improving soil fertility, unlocking soil nutrients, improving plant tolerance to stress, to be soil probiotics, reduce or eliminate the need for fertilizers, and more. The large variety of such additives makes it impossible to provide specific recommendations. Some soil additives are legitimate products, with scientifically known and demonstrable modes of action, and with documented success. However, many so-called soil additives do not fit this description, so a few words of caution are in order. The well-known phrase, “If it sounds too good to be true, it probably is,” applies very well to soil additives. Soils are not magical realms; a product claim that sounds magical is probably not legitimate. For example, soil microbial communities are very complex—additives that are purported to create sweeping changes in soil microbial communities are not likely to live up to their promises, unless the soil is highly degraded.
My best advice is to investigate the claims made about a soil additive. First, do the promised benefits seem reasonable, or do they sound magical? Second, can the promised benefits be explained scientifically? Third, have the benefits been verified through research by a neutral third party? Finally, is the vendor willing to answer questions and provide supporting information? If the answers to these questions are “no,” then “Let the buyer beware”!
Soil amendments are added to soils to correct adverse chemical, physical or biological soil properties other than low nutrient availability. A thorough understanding of soil chemical properties through soil sampling and analysis is necessary to determine the type and rate of soil amendment that is necessary in a specific situation. Common types of soil amendments include inorganic amendments such as liming materials and gypsum, and organic materials such as crop and animal wastes, composts, and biochar. Additions of organic materials will usually improve soil properties and should be encouraged. When evaluating soil additives, consider whether or not they act in soils according to known modes of action that can be explained scientifically.
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Evanylo, G. and M. Goatley, Jr. 2011. Chapter 9: Organic and Inorganic Soil Amendments. In Urban Nutrient Management Handbook. Virginia Cooperative Extension.
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