Soil

Section 1: Soil Composition

Soil is a loose, crumbly, and porous material — an important resource for agricultural production. It is a complex system composed of solid, liquid, and gaseous components. The solid components include various minerals with different chemical compositions and organic matter. The liquid component is mainly soil water, which contains dissolved salts. The gaseous component is air. Water and air coexist in soil pores — more water means less air, and vice versa. Soil also supports a wide variety of microorganisms.

The minerals in soil are formed over long periods from rocks and minerals through the processes of wind, sun, rain, and biological activity. They contain many elements including aluminum, iron, calcium, magnesium, potassium, silicon, and phosphorus, and form complex compounds such as aluminosilicates and oxides.

The organic matter in soil is much less abundant than the mineral fraction. Soil organic matter comes mainly from plant and animal residues, applied organic fertilizers, humus, and microorganisms. It is primarily composed of the elements carbon, hydrogen, oxygen, and nitrogen, as well as phosphorus and sulfur. Under the influence of soil moisture, air, animals, and microorganisms, most of the organic matter is converted from complex organic compounds into simple inorganic compounds such as water, carbon dioxide, and ammonia, while a small portion is transformed into humus. During this process, the decomposition of organic matter releases nutrients that plants can absorb. Soil organic matter plays an important role in improving soil fertility.

Section 2: Soil Colloids

Soil colloids are composed of both inorganic and organic substances. The inorganic colloids in soil are mainly aluminosilicates and hydrated oxides of iron and aluminum. The organic colloids are mainly soil humus. In practice, these two types of colloids are often combined. The study of soil colloids is of great importance for understanding the nature of chemical phenomena in soil.

Electrical Charges on Soil Colloids

Soil colloidal particles have very large surface areas and can adsorb ions, thereby carrying electrical charges. In practice, soil colloids predominantly carry negative charges.

Ion Adsorption and Ion Exchange in Soil Colloids

Because soil colloids carry negative charges and have large surface areas, they can adsorb cations from the surrounding liquid. These adsorbed cations can undergo exchange with cations in the soil solution. For example, when lime is applied to acidic soil, it not only neutralizes the acidity of the soil solution but also exchanges the hydrogen ions and aluminum ions adsorbed on the soil colloids:

\[\text{Soil colloid}\ce{-H+} + \ce{Ca^{2+}} + 2\ce{OH-} \ce{<=>} \text{Soil colloid}\ce{-Ca^{2+}} + 2\ce{H2O}\]

\[\text{Soil colloid}\ce{-Al^{3+}} + 3\ce{Ca^{2+}} + 6\ce{OH-} \ce{<=>} \text{Soil colloid}\ce{-Ca^{2+}} + 2\ce{Al(OH)3 v}\]

\(\ce{Ca^{2+}}\) ions also have a flocculating effect on soil colloids, promoting the formation of gels and improving soil structure.

Similarly, when soluble fertilizers such as ammonium nitrogen fertilizers or potassium fertilizers are applied to soil, the concentrations of ammonium and potassium ions in the soil solution increase. The cations previously adsorbed on the soil colloids are exchanged:

\[\text{Soil colloid}\ce{-Ca^{2+}} + 2\ce{NH4+} \ce{<=>} \text{Soil colloid}\ce{-NH4+} + \ce{Ca^{2+}}\]

\[\text{Soil colloid}\ce{-Mg^{2+}} + 2\ce{K+} \ce{<=>} \text{Soil colloid}\ce{-K+} + \ce{Mg^{2+}}\]

In this way, ionic nutrients from applied fertilizers are adsorbed by soil colloids, preventing them from being lost before crops can absorb them.

When plant roots absorb ionic nutrients, the concentration of nutrient ions in the soil solution around the roots decreases. At the same time, because roots release organic acids and carbon dioxide, the hydrogen ion concentration in the soil solution increases. This exchanges the nutrient ions adsorbed on the soil colloids back into solution for plant uptake:

\[\text{Soil colloid}\ce{-NH4+} + \ce{H+} \ce{<=>} \text{Soil colloid}\ce{-H+} + \ce{NH4+}\]

\[\text{Soil colloid}\ce{-K+} + \ce{H+} \ce{<=>} \text{Soil colloid}\ce{-H+} + \ce{K+}\]

Therefore, the ion adsorption and exchange processes of soil colloids play an important role in storing and supplying nutrients to crops.

Section 3: Soil Acidity and Alkalinity

Crop growth has specific requirements regarding soil acidity and alkalinity. Some crops prefer acidic conditions, some prefer alkaline, but most grow best in neutral or slightly acidic to slightly alkaline soils. The pH range suitable for most crops is between \(6.5\) and \(7.5\). Excessively acidic or alkaline soils are unfavorable for crop growth. Therefore, understanding soil acidity and alkalinity and their relationship to crop growth is very important for agricultural production.

Why Does Soil Become Acidic?

Plant roots release \(\ce{CO2}\) during respiration, which reacts with water to form carbonic acid. The decomposition of soil organic matter produces various organic acids such as acetic acid and butyric acid. The humic acids in soil humus are also weak acids. All these substances make the soil acidic. Aluminum ions in soil are often adsorbed on soil colloids. Through ion exchange, they can enter the soil solution and undergo hydrolysis, producing \(\ce{H+}\) ions and making the soil acidic:

\[\ce{Al^{3+} + 3H2O <=> Al(OH)3 + 3H+}\]

Acidic soils can be improved by applying lime and other methods.

Why Does Soil Become Alkaline?

Soil alkalinity is mainly caused by the hydrolysis of salts such as \(\ce{Na2CO3}\) and \(\ce{NaHCO3}\) in the soil:

\[\ce{CO3^{2-} + H2O <=> HCO3- + OH-}\]

\[\ce{HCO3- + H2O <=> H2CO3 + OH-}\]

Alkaline soils can be improved by applying gypsum (\(\ce{CaSO4}\)) in combination with organic fertilizers and humic acid fertilizers.

Relationship Between Soil Acidity/Alkalinity and Crop Nutrition

Soil acidity and alkalinity have a significant effect on crop nutrition. For example, in acidic soils with high concentrations of \(\ce{Fe^{3+}}\) and \(\ce{Al^{3+}}\), when soluble phosphate fertilizers such as superphosphate are applied, these ions react with the phosphate to form insoluble phosphate salts, greatly reducing the effectiveness of the fertilizer:

\[\ce{Fe^{3+} + PO4^{3-} -> FePO4 v}\]

\[\ce{Al^{3+} + PO4^{3-} -> AlPO4 v}\]

In agricultural practice, lime is often applied to acidic soils before applying soluble phosphate fertilizers in order to neutralize the soil acidity.

Soil acidity and alkalinity also affect the availability of nutrients. Generally, nitrogen availability is highest at pH \(6\)–\(8\); potassium availability is highest above pH \(6\); and phosphorus availability is highest at pH \(6.5\)–\(7.5\).

The conversion of organic nutrients to inorganic forms that crops can directly absorb is carried out mainly by microorganisms (which secrete enzymes that decompose organic matter). Excessively acidic or alkaline conditions are unfavorable for microbial growth. For example, urine and urea fertilizer applied to soil require the enzyme urease produced by microorganisms for hydrolysis to produce ammonium carbonate. Neutral soil conditions favor this process; in acidic soils, the reaction proceeds more slowly:

\[\ce{CO(NH2)2 + 2H2O ->[\text{urease}][\text{hydrolysis}] (NH4)2CO3}\]

Soil acidity and alkalinity also directly affect plants. Strongly alkaline conditions dissolve the protoplasm in cells, damaging plant tissues. Strongly acidic conditions denature the protoplasm, affecting enzyme activity and nutrient absorption.

Section 4: Oxidation–Reduction Reactions in Soil

Oxidizing and Reducing Agents in Soil

A large number of oxidation–reduction reactions occur in soil. Many elements — including oxygen, carbon, nitrogen, hydrogen, iron, manganese, and sulfur — undergo electron transfer reactions. Many of these reactions occur in the presence of microorganisms, making them even more complex.

Oxidizing agents in soil include oxygen, iron oxides, manganese dioxide, and sulfates. Reducing agents include ferrous salts, manganese salts, sulfides, sugars, aldehydes, carboxylic acids, and organic matter easily decomposed by microorganisms. Among organic compounds, we already know that aldehydes and sugars are reducing agents. But why are carboxylic acids reducing agents in soil? Because under the action of soil microorganisms, carboxylic acids and easily decomposable organic matter are oxidized to carbon dioxide.

Oxidation–Reduction Reactions in Soil

When soil aeration is good, the pores within soil aggregates contain oxygen, and oxygen is also dissolved in the soil solution. Under these conditions, oxidation reactions proceed smoothly, oxidizing \(\ce{Fe^{2+}}\) to \(\ce{Fe^{3+}}\) and \(\ce{Mn^{2+}}\) to \(\ce{MnO2}\). At the same time, aerobic microorganisms are more active because oxidation reactions provide sufficient energy. The vigorous activity of aerobic microorganisms accelerates the conversion of soil organic matter to inorganic nutrients, speeding up nutrient release.

Conversely, under waterlogged conditions or anaerobic fermentation, reducing agents cause \(\ce{SO4^{2-}}\) to be reduced to \(\ce{S^{2-}}\), \(\ce{Fe^{3+}}\) to \(\ce{Fe^{2+}}\), and \(\ce{MnO2}\) to \(\ce{Mn^{2+}}\). Under these conditions, anaerobic microorganisms become dominant. Not only is nutrient release slower, but intermediate decomposition products accumulate, which is often unfavorable for plant growth. For example, when \(\ce{Fe^{2+}}\) or \(\ce{H2S}\) accumulate to certain concentrations, they become toxic to plants.

\(\ce{H2S}\) harms plants by interfering with nutrient and water absorption, particularly potassium uptake. \(\ce{Fe^{2+}}\) harms plants by impeding the absorption of phosphorus and potassium, and also affects nitrogen uptake.

Relationship Between Soil Oxidation–Reduction Reactions and Plant Nutrition

Under appropriate oxidation–reduction conditions, soil organic matter is continuously decomposed by microorganisms — either accumulating as humus or converting organic nitrogen and phosphorus into inorganic forms available for direct plant uptake. If oxidation conditions are too strong, the mineralization of organic matter proceeds too rapidly, and the released nutrients may be lost before plants can absorb them, leaving the soil deficient in available nutrients. If reducing conditions are too strong, mineralization is slow, providing insufficient available nutrients, and toxic inorganic substances such as \(\ce{Fe^{2+}}\) and \(\ce{H2S}\), as well as organic toxins like butyric acid, may accumulate and harm plant growth.

The oxidation–reduction status of soil is closely related to plant nutrition. Therefore, various agricultural practices are needed to regulate soil oxidation–reduction conditions. For upland fields, tilling the soil promotes aerobic (oxidizing) conditions. For paddy fields, draining and sun-drying also promote oxidation. Conversely, irrigating upland fields and applying organic fertilizers, or flooding paddy fields, promote reducing conditions.

Section 5: Transformations of Nitrogen, Phosphorus, and Potassium in Soil

Transformations of Nitrogen in Soil

Nitrogen in soil exists in both inorganic (mineral) and organic forms. Soil organic matter, humus, and applied organic fertilizers are all sources of organic nitrogen, with the main nitrogen-containing organic compounds being amino acids, proteins, and nucleic acids. Inorganic nitrogen includes both applied nitrogen fertilizers and nitrogen converted from organic forms, present as \(\ce{NH4+}\), \(\ce{NO3-}\), \(\ce{NO2-}\) ions, molecular nitrogen, and others. Plants can directly utilize \(\ce{NH4+}\) and \(\ce{NO3-}\) ions. The nitrogen-fixing bacteria in the root nodules of leguminous plants can utilize atmospheric nitrogen.

Organic nitrogen — such as nitrogen in proteins and other organic compounds — cannot be directly utilized by most crops. Only after microbial decomposition into simple amino acids or amides, and further breakdown into ammonia or hydrolysis into ammonium salts, can crops absorb and use it. For this reason, organic nitrogen is called slow-release nitrogen.

Most important nitrogen fertilizers currently in use, with the exception of urea and a few others, are ammonium-type or nitrate-type nitrogen. Urea (\(\ce{CO(NH2)2}\)) is an amide-type nitrogen that is converted to ammonium nitrogen (\(\ce{(NH4)2CO3}\)) in soil through microbial action. Therefore, the main forms of inorganic nitrogen in soil are \(\ce{NH4+}\) and \(\ce{NO3-}\).

Ammonium nitrogen is in the reduced state (nitrogen oxidation state \(-3\)); nitrate nitrogen is in the oxidized state (nitrogen oxidation state \(+5\)).

Ammonium nitrogen in soil is oxidized to nitrous acid and its salts by nitrifying bacteria (specifically, Nitrosomonas):

\[\ce{2NH3 + 3O2 ->[\text{nitrifying bacteria}] 2HNO2 + 2H2O}\]

Nitrous acid and its salts are further oxidized to nitric acid and its salts by nitrifying bacteria (Nitrobacter):

\[\ce{2HNO2 + O2 ->[\text{nitrifying bacteria}] 2HNO3}\]

When soil aeration is poor or when excess organic matter is present, denitrifying bacteria become active, converting nitrate nitrogen to gaseous nitrogen that escapes, causing nitrogen fertilizer losses. Denitrification is a reduction process because nitrogen goes from the \(+5\) oxidation state in nitrate to the \(0\) oxidation state in molecular nitrogen.

Understanding the transformations of nitrogen in soil is of great significance for studying nitrogen fertilizer application methods. For example, ammonium nitrogen fertilizers should be applied deeply in paddy fields to be effective. Otherwise, if ammonium nitrogen accumulates in the surface layer of the paddy soil — where the irrigation water contains dissolved oxygen — nitrifying bacteria become active and oxidize the ammonium nitrogen to nitrate nitrogen. Nitrate nitrogen is not readily adsorbed by soil colloids and easily washes away with water. Even if it does not wash away but moves to the deeper reducing layer of the paddy soil, denitrifying bacteria convert it to gaseous nitrogen that is lost. In recent years, nitrification inhibitors have been applied together with ammonium fertilizers to slow the conversion and reduce nitrogen losses.

The nitrogen content of soil is increased primarily through fertilization, decomposition of plant root and stem residues, nitrogen fixation by rhizobial and free-living nitrogen-fixing bacteria from the atmosphere, and small amounts brought in by rainfall and groundwater. Common agricultural practices to increase soil nitrogen include growing leguminous crops, returning crop residues to the field, applying microbial inoculants, organic fertilizers, and nitrification inhibitors, and managing irrigation and soil aeration to promote beneficial microbial activity.

Transformations of Phosphorus in Soil

Phosphorus in soil also exists in two forms: inorganic and organic. Inorganic phosphorus is mainly composed of phosphate salts of calcium, magnesium, iron, and aluminum. These can be classified as: poorly soluble phosphate salts that plants cannot easily utilize, such as fluorapatite (\(\ce{Ca5(PO4)3F}\)); weakly acid-soluble phosphate salts that plants can use but that must first dissolve, such as \(\ce{CaHPO4}\); and water-soluble phosphate salts that plants can readily use, such as \(\ce{Ca(H2PO4)2}\). Organic phosphorus mainly refers to phosphorus-containing organic compounds in plant, animal, or microbial matter, such as nucleoproteins, nucleic acids, and phospholipids. Most organic phosphorus cannot be directly absorbed by plants; it must be gradually decomposed into phosphoric acid and its salts by microorganisms before becoming available as plant nutrition. Organic phosphorus and poorly soluble phosphate salts are collectively called slow-release phosphorus, while soluble and weakly acid-soluble phosphate salts are called readily available phosphorus.

Soluble phosphate salts can be partially converted in acidic or alkaline soils into phosphate forms that plants cannot easily use — this is called phosphorus fixation. In neutral soils with high organic matter content, phosphorus fixation is weaker and phosphorus availability is higher.

Because of phosphorus fixation, the mobility of phosphorus in soil is less than that of nitrogen. Therefore, phosphate fertilizers must be applied near crop root zones for good effectiveness.

Transformations of Potassium in Soil

Soil generally contains more potassium than nitrogen or phosphorus. However, potassium in soil exists mainly as complex potassium aluminosilicates, such as orthoclase and hydrous mica minerals. These aluminosilicates are poorly soluble in water and can only gradually transform into soluble potassium compounds through weathering processes. Therefore, potassium aluminosilicates are slow-release forms.

\[\ce{2KAlSi3O8 + H2O + 2H2CO3 -> Al2(Si2O5)(OH)4 + 2KHCO3 + 4SiO2}\]

This weathering process can be accelerated in the presence of microorganisms.

Potassium in soil exists in three main forms: mineral-bound potassium (accounting for over \(98\%\) of total soil potassium, not readily available to plants — slow-release potassium), potassium adsorbed on soil colloids, and potassium in the soil solution. The latter two forms can be directly utilized by crops and are classified as readily available potassium.

During soil weathering, under conditions of high temperature and heavy rainfall, potassium is relatively easily leached away by rainwater. Therefore, generally speaking, soils in northern China (where rainfall is less and weathering is weaker) tend to have higher potassium content, while soils in southern China (where rainfall is heavy and weathering is intense) tend to have lower potassium content. Common agricultural practices to increase soil potassium include applying potassium fertilizers, plant ash, organic fertilizers, and green manures.

Nitrogen, phosphorus, and potassium are all essential nutrient elements for crops. To achieve scientific farming and high yields, in addition to applying nitrogen and phosphorus fertilizers based on soil nutrient status and crop requirements, potassium fertilizer should also be applied in appropriate proportions to achieve a balanced N:P:K ratio.