Managing soil fertility factors and land reclamation methods

The yield of crops directly depends on how close the soil properties are to ideal and how promptly the factors limiting plant growth are eliminated. Often, an agronomist has to deal not with a single local problem, but with a whole complex of negative conditions. For example, saline-alkali soils combine high alkalinity, an excess of salts, and extremely unfavorable physical properties, which requires a comprehensive reclamation approach.

Elimination or minimization of limiting factors is achieved through reclamation and agrotechnological practices. The table below systematizes the main soil problems and proven ways to solve them in practice. These measures help restore the field to a productive state.

Optimal soil parameters for maximum yield

Based on the generalization of long-term field experiment results, physical and chemical soil properties were determined at which cereal crops are able to form a maximum yield. These include a strong granular or cloddy structure, loamy particle-size distribution, good aeration, and the ability to accumulate large reserves of available moisture. These benchmarks help to assess the real potential of a specific field.

• Soil density — ≈ 1.1 g/cm³
• Porosity — ≈ 60 %
• pH reaction — 6.0–7.5

The most important indicator of potential fertility remains the level of humus in the arable layer. To obtain high yields of cereal crops, the organic matter content must correspond to the soil type. Below are benchmarks by soil type for planning the fertilizer system:

• for light sod-podzolic soils — no less than 2.5–3.0 %;
• for loamy sod-podzolic soils — 3.5–4.0 %;
• for gray forest loamy soils — 4–6 %;
• for chernozems — 5–7 %.

For complete plant nutrition, the soil must contain a sufficient amount of macro- and microelements, as well as mobile compounds of nitrogen, phosphorus, and potassium. A high level of biological activity is no less important. Without this, even with an optimal soil structure, plants will experience nutrition deficiency.

Systematic monitoring of these indicators allows for timely adjustments to tillage technology. Timely application of reclamation practices protects investments in seed and fertilizer. This makes it possible to fully unlock the genetic potential of the cultivated crops.

Table 59 – Optimal values of soil-ecological indicators for field crops Sugar Indicators Wheat Maize Alfalfa Clover Potato Rice Buckwheat Sorghum Beet Humus horizon depth, cm 60–150 60–150 40–150 20–100 20–80 20–80 20–40 10–20 50–150 Humus content, % 3–8 3–8 3–8 2–4 2–4 2–4 1.5–4 1–3 3–8 Humus reserves, t/ha 250–600 250–600 250–600 200–350 200–350 150–400 100–300 50–100 250–600 pH 6.5–8.2 6.5–8.2 6.5–8.2 7–8.8 5.5–6.8 6–7 6.5 6.5–8 6.5–8.5 Density, g/cm³ 1.35–1.40 1.35–1.50 1.30–1.35 1.35–1.50 1.35–1.50 1.30–1.35 1.40–1.50 1.35–1.40 1.35–1.40 Particle-size distribution, Light-loamy Heavy-loamy Heavy-loamy Heavy-loamy Heavy-loamy Loamy Loamy particles < 0.01 mm 45–60 45–75 30–50 and sandy 45–60 45–75 30–45 30–45 40–60 30–45

      Solonetzic property, Na+, % of
                                       <5           <3           <6        Less than 7         <1         Less than 1        3–8           <1           3–8
      sum of exchangeable cations
      Salinity, dry residue, %
                                      < 0.5        < 0.3       0.3–0.7      0.3–0.7       < 0.2        < 0.2       0.3–0.8        < 0.2       0.3–0.8
      Permanent wilting point (PWP), k/mg*
                                     1.5 mg       1.7 mg       1.6 mg        1.3 mg       1.7 mg       1.8 mg       2.0 mg       1.7 mg       1.1 mg
      CaCO3 content, %                 0–6           0–5          0–5         0–10                      < 1.0         0–5           0-6          0-8
                                                                                       Not allowed
      Sum of positive temperatures
                                   2500–5500 3000–5000 2000–4000 2000–5000              800–5000     800–2300 4200–14000 900–3600 2000–12000
      above 10°C

PWP=k/mg, where mg is the content of maximum hygroscopic moisture in the soil; k is a coefficient depending on the plant. Determined in vegetation vessels.

Table 60 – Optimal values of soil-ecological indicators for perennial plantations Sugar Date Indicators Apple Sweet Cherry Plum Grape Tea Currant cane palm

      Root-inhabiting layer depth, cm     150–250       150–250      100–200       50–150         50–100         40–80       100–200        50–150
      Humus horizon depth, cm                                                                                                    Not appli-
                                            40–150         40–150       40–100        20–40          20–30         20–40        50–80          cable
      Humus content, %                                                                                                           Not appli-
                                              3–8           3–8           3–8        2,5–3,5          2–4           3–5          3–4           cable
      Humus reserves, t/ha                                                                                                       Not appli-
                                            250–600       250–600      250–400       250–300        100–150        50–100       50–200         cable
      pH                                     6,5–8,2       6,5–8,2      6,5–8,2       6,9–8,5         3–4          6,5–8,2      6,5–8,2       8,0–8,8
      Density, g/cm3
                                           1,35–1,45       < 1,35      1,40–1,50    1,35–1,40      1,35–1,40      1,35–1,40    1,35–1,45     1,35–1,45
      Particle size distribution, par-
      ticles less than 0.01 mm                               Sandy loam and Heavy loam
                                        Heavy loam                           Medium loam Loamy        Loamy         Loamy        Loamy
                                                         light loam    and clay
                                          35–60                              30–45       30–50        30–60         30–60        30–60
                                                         < 25          45–75
      Alkalinity (solonetzicity), Nа+, %
      of total exchangeable cations                                                                 Alkalinity
                                                                                                    not possi-
                                              <4             <3           <5           3–7          ble            <3           <3            5–12
      Salinity, dry residue, %                                                                      Salinity
                                              0,20          0,15         0,50       0,40–0,60    not possible      0,20        0,20          0,5–1,5
      Stoniness, % of volume                  <5            <5           <5           5–10             <5           5–10       Less than 5   5–40
      Permanent wilting point (PWP),
      k/mg*                                   1,4 mg        1,5 mg       1,6 mg        1,1 mg         1,8 mg        1,8 mg       1,6 mg        1,3 mg
      CaCO3 content, %
                                            0,3–10,0       0,3–2,0     0,3–10,0      0,3–40,0     Inadmissible    0,3–10,0     0,3–10,0      1,0–50,0
      Sum of active temperatures
      above 10°C                           2000–4000     2200–4500    2000–4500     3600–6000      4000–7000     1200–3600    6000–14000   6000–14000

Directions for the development of soil fertility. Changes in soil properties over a certain period can also lead to a transformation of fertility relative to the initial level. Consequently, during a growing season, annual cycle, crop rotation cycle, or a specific period, the change in fertility can manifest as incomplete, simple, or expanded reproduction.

Types of reproduction:

• The formation of soil fertility below the initial level means incomplete reproduction.
• Returning soil fertility to the initial level means simple reproduction.
• Creating soil fertility above the initial level represents expanded reproduction.

One of the tasks of expanded reproduction of soil fertility is the optimization of the most agronomically valuable soil properties using various methods and techniques of regulation. T.N. Kulakovskaya (1980), when assessing fertility and the degree of soil cultivation based on agrochemical properties, proposes using a complex agrochemical score, which reflects the degree of compliance of main soil properties with the requirements of cultivated crops.

V.A. Kovda (1998) emphasizes that the expanded reproduction of soil fertility should always be a fundamental principle of agriculture. In his opinion, the practical solution to this task cannot be limited to the mobilization of natural soil resources but must be based, first of all, on the return and compensation of the utilized portion. It is also necessary to provide agroecosystems with increasing amounts of additional energy and to improve conditions for increasing the productivity of photosynthesis.

A typical regional management system for soil fertility should provide for:

• measures to combat erosion and drought;
• reduction of humus and nutrient reserves in soils;
• wide introduction of soil reclamation;
• creation of an optimal biochemical, chemical, and physicochemical environment in soils;
• complex use of all known ways to increase soil fertility.

In addition, it is necessary to develop new techniques and methods for increasing soil fertility and the yields of crops.

One of the most important indicators of soil fertility is its humus content. During long-term cultivation of agricultural crops, as shown by the long-term studies of A.M. Lykov (1976), there is a predominance of decomposition (mineralization) processes over formation (replenishment), and a gradual decrease in soil humus content occurs.

Basic methods of regulating the humus balance:

• The primary method for regulating the balanced carbon cycle, the source of humus formation in agroecosystems, is the application of fertilizers (manure).
• The periodic cultivation of perennial grasses in crop rotation is of great importance for stabilizing the soil's humus status, as they enrich the soil with plant residues to a much greater extent than other crops.
• On acidic soils with low buffering capacity, a high positive role of liming in reducing the mineralization and mobility of humus has been identified.
• Systematic application of only mineral fertilizers in a crop rotation, as a rule, does not prevent humus loss, but only reduces it due to the input of a large quantity of root and harvest residues into the soil and their inclusion in the humification process.

Analysis of numerous results of agrochemical research conducted in our country and abroad shows that the reproduction of the nitrogen pool of soils is possible exclusively through the systematic application of organic fertilizers. The application of only mineral fertilizers, as a rule, only slows down nitrogen losses from the soil, providing for the achievement of a balanced equilibrium of this element in an agroecosystem only in isolated cases.

Under the influence of phosphate fertilizers, the total reserves of mineral phosphorus compounds in the soil increase, and their mobility intensifies. The latter also affects the underlying soil horizons, which is associated with the redistribution of phosphates in the soil profile due to the activity of root systems and the increased mobility of exchangeable bases, sesquioxides, and humus substances as a result of acidification caused by physiologically acidic fertilizers, mainly nitrogenous ones.

When potassium fertilizer application to the soil is carried out over a long period, potassium is fixed in the root zone in exchangeable and non-exchangeable forms, while remaining available to plants. The interaction of these two forms of potassium with its water-soluble form creates a dynamic system that prevents leaching and ensures potassium plant nutrition.

The effect of fertilizers on soil fertility largely depends on the soil buffering capacity. Being a highly effective means of increasing the stability of an agroecosystem, fertilizer, when applied in an unbalanced manner, can turn into a factor that degrades soil fertility. One of the main reasons for the so-called "hidden" negative effect of physiologically acidic nitrogen fertilizers lies in the negative response of plants and soil microflora to increased acidity. A high concentration of hydrogen ions in the soil solution inhibits the vital activity of most crop plants and microorganisms involved in nitrogen mineralization and nitrogen fixation. Excessive application rates of mineral fertilizers cause a disruption of the soil microbiocenosis structure. It leads to a significant increase in the number of saprophytic fungi and actinomycetes that synthesize phytotoxic substances. Acidification leads to the undesirable development of other soil processes that reduce the stability of colloids and their absorption capacity, and also increases the formation of poorly soluble iron and aluminum phosphates. Another reason for the "hidden" negative effect of increased application rates of mineral fertilizers on soil fertility can be the accumulation of ballast elements in it, such as chlorine, fluorine, and heavy metals, whose high concentrations have a toxic effect. To reduce the levels of heavy metal uptake from the soil into plants, the application of manure and lime is recommended, as they reduce the mobility of these elements. Therefore, regulating soil acidity within optimal limits is an indispensable condition for the sustainable functioning of an agroecosystem under conditions of long-term mineral fertilizer application on soils with low buffering capacity.

When developing measures to improve soil fertility, it must be assumed that the factors of plant life are equivalent; therefore, it is necessary to act on this complex of factors simultaneously. The reproduction of fertility is carried out in two ways: physical and technological.

The physical path includes the application of fertilizers, pesticides, and soil amendments, i.e., it is associated with significant material costs. The second path is associated with changing soil properties, for example, as a result of improving methods of tillage. The costs are low, but the effect is short-term; therefore, sustainable improvement of soil fertility is necessarily associated with higher or lower costs. According to A.I. Gorbyleva (2002), these measures must meet the following requirements.

They should contribute to the gradual approximation of soil property indicators to their optimal parameters:

• The leading and fast-acting factor in this direction is fertilizers and soil amendments. Their combination, calculated for a positive balance of nutrients, should be accompanied by the inclusion of such methods and application techniques that improve soil properties without polluting the environment or causing secondary acidity.
• Great importance is attached to the choice of tillage methods, which should not degrade water-physical properties in connection with the soil's structural state due to compaction, but should ensure the best conditions for aeration and water supply to plants.

The next mandatory condition for increasing soil fertility is the introduction of crop rotation, which is the main means of combating soil toxicosis and soil exhaustion. Monoculture, in most cases, leads to a decrease in yield and deterioration of soil properties, usually associated with dehumification.

Methods for activating the biological cycle and improving soil properties

High yields and active soil microflora force plant roots to work intensively, involving nutrients from deep soil horizons in the biological cycle. For this process to be continuous and lead to expanded reproduction of fertility, the field should not lie fallow. It is important for an agronomist to maximize crop density and use the soil's natural resources throughout the entire growing season.

Since soil properties differ even within the boundaries of a single natural zone, there are no universal templates. In each specific case, the basic technology must be supplemented with special techniques to improve the water-physical properties of the soil. This task must be solved in a complex manner: carry out land development works, increase the proportion of grain legumes in crop rotations, and introduce hydraulic reclamation with dual regulation of the water regime.