Agrochemical soil indicators and an effective fertilizer application strategy

How nutrients are distributed in the soil

Soils vary greatly in terms of nutrient reserves, acidity, and sorption capacity. How effectively applied fertilizers work depends on these properties. All nutrients in the soil are divided into three main groups, between which a dynamic equilibrium is maintained.

• Reserve forms of nutrients — 85–90%
• Exchangeable-adsorbed forms — 3–10%
• Water-soluble forms — 1–5%
• Nitrogen in the topsoil (0–25 cm) — 0.05–0.5%

It is important for an agronomist to understand in what form the nutrients are present in their fields. Readily available water-soluble substances make up only a small fraction of the total reserve, while the bulk is locked in organic matter and sparingly soluble minerals. As plants absorb dissolved salts, the reserves in the soil solution are replenished through the transition of elements from an exchangeable-adsorbed state, and subsequently from a reserve state.

Nitrogen nutrition diagnostics and fertilizer requirement calculation

Nitrogen content in the topsoil depends directly on the amount of humus. However, the gross nitrogen reserve does not reflect the real picture of plant nutrition, as almost all of it is bound in organic compounds. The availability of these reserves depends on the structure of the organic matter and is distributed by fractions as follows:

• non-hydrolyzable (most stable) — 60–80% in grey forest soils and 65–82% in chernozems;
• hardly hydrolyzable — 8–12% in grey forest soils and 5–10% in chernozems;
• easily hydrolyzable and mineral — the smallest part of the reserves (mineral forms in total rarely exceed 2%).

Mineral nitrogen in the form of nitrates (NO3-) and ammonium (NH4+) is formed during the decomposition of organic matter. The rate of this process constantly changes depending on temperature, soil moisture, tillage practices, and predecessors. Because of this dynamic, the content of available nitrogen fluctuates throughout the season.

It is especially important to control nitrogen levels when growing vegetable, fruit, and berry crops, for which high doses of organic and mineral fertilizers are traditionally applied. To ensure produce does not accumulate dangerous nitrates, nitrogen fertilizer doses must be calculated taking into account the actual reserves of mineral nitrogen in the root zone. To do this, conduct annual soil and plant diagnostics before the start of the season.

Nitrogen supply	Nitrogen content in soil layers, mg/kg		Fertilizer requirement
	Layer 0–40 cm	Layer 0–60 cm
By nitrate nitrogen (N-NO₃)
Very low	< 5	< 3	Very high
Low	5–10	3–8	High
Medium	10–15	8–12	Medium
High	> 15	> 12	None
By total mineral nitrogen (N-NO₃ + N-NH₄)
Very low	< 7	< 5	Very high
Low	7–15	5–10	High
Medium	15–25	10–20	Medium
High	> 25	> 20	None

Nitrogen: biological fixation and crop requirements

The soil nitrogen regime directly determines the productivity of vegetable and fruit-berry crops. For these plants, the optimal nitrate nitrogen content is 40–45 mg/kg. It is important for an agronomist to monitor this indicator, keeping in mind that the maximum permissible concentration (MPC) is 130 mg/kg. The main natural source for replenishing soil nitrogen remains grain legumes.

• Optimal nitrate nitrogen content — 40–45 mg/kg
• MPC of nitrates in soil — 130 mg/kg
• Share of available phosphorus — 1–3% of the total
• Share of exchangeable-adsorbed potassium — 1–2% of the total

Perennial legume grasses fix atmospheric nitrogen significantly more efficiently than annuals. Their period of active nitrogen fixation lasts 3–4 months, whereas annuals have only 1.5–2 months. As a result, annual crops almost completely consume the accumulated nitrogen to form their own yield and remove it at harvesting. Under favorable conditions (neutral pH, sufficient moisture, phosphorus, potassium, and micronutrients), nitrogen fixation by clover and alfalfa reaches 350 g/m², while grain legumes (peas, beans, broad beans) provide only 2–8 g/m².

Crop	Total amount of nitrogen fixed by plants (g/m²/year)	Change in soil nitrogen reserves after harvesting (g/m²)
Alfalfa	30 (up to 50–60)	+10 (up to 15–20)
Clover	15–16 (up to 25–30)	+7.5–10 (up to 12.5–15)
Lupine	Up to 15	About +3.0
Grain legumes	5–6	-0.5 (up to -1.5)

On average, due to symbiotic fixation, grain legumes are capable of providing 70% of their nitrogen requirements. When planning a nutrition program, keep in mind that white cabbage, red cabbage, Brussels sprouts, pumpkin, garden beet, and celery have an increased demand for this element. They respond well to the application of high doses of nitrogen fertilizers.

Phosphorus and potassium: nutrient availability and management

Phosphorus is involved in all key plant life processes; however, its availability in the soil is strictly limited. Crops absorb the element only in the form of soluble orthophosphoric acid salts (H₂PO₄^-, HPO₄^2-). These mobile compounds account for only 1–3% of the total soil phosphorus reserve. The content of available phosphorus forms naturally increases when transitioning from sod-podzolic and light-gray forest soils to gray, dark-gray forest soils, and chernozems.

To improve the soil phosphorus regime, it is necessary to regularly apply organic and phosphorus-containing mineral fertilizers. Chicory, head cabbage, cauliflower, pumpkin, late-maturing carrot, dill, and rhubarb show particular sensitivity to phosphorus deficiency. Timely provision of these crops with available phosphorus determines their future yield.

Potassium is a major source of plant nutrition, and its reserves in the soil largely depend on the parent material. In clay and loam soils, the potassium content reaches 2% or more, whereas in sandy, sandy loam, and peat soils, it drops to 0.1%. At the same time, plants can only absorb exchangeable potassium, which constitutes 1–2% of the total amount. The assessment of plant requirements for potassium and phosphorus is carried out based on the content of their mobile forms in the 0–25 cm soil layer.

In the soil, processes of mobilization (conversion of phosphorus and potassium into available forms) and immobilization (binding of elements) are continuously occurring. The intensity of nutrient release depends on growing conditions and the agricultural practices used. The speed of this process is determined by the following factors:

• volume of organic matter in the soil;
• biological characteristics of the cultivated crop;
• depth and method of tillage;
• irrigation regime;
• doses and forms of applied fertilizers.

            Degree             P2O5                       K2O
 Class
           of supply      By Kirsanov*      By Chirikov**           By Kirsanov    By Chirikov
 soil
   1      Low          <80         <100           < 80       <100
   2      Medium        81-150      101-150        81-120     101-150
   3      High-avg    151-200      151-200       121-170     151-200
   4      High       201-300      201-300       171-250     201-300
   5      Very high      >300       >300           > 250      > 300
          
       *For acidic soils, **for neutral soils

Phosphorus and potassium from soil reserves and applied fertilizers do not leach from the soil, so soil analysis for these elements, as well as calcium, magnesium, and microelements, can be performed not every year, but every two or three years.

Medium-late white cabbage, summer squash, radish, parsley, and rutabaga are characterized by increased sensitivity to the level of potassium nutrition. Garden beet, late garden carrot, celery, as well as pumpkin and chicory are particularly demanding of potassium. The need is less significant in tomato, cucumber, garlic, radish, spinach, and lettuce. Bean, pea, sorrel, lettuce, and green onion respond weakly to potassium fertilizers.

Fruit, vegetable, and berry crops react negatively to an insufficient amount of mobile calcium and magnesium in the soil. Low calcium content can be found in acidic soils and peatlands. Light sandy soils can be depleted of magnesium.

Vegetable crops such as cauliflower, Brussels sprouts, chicory, pumpkin, horseradish, late-maturing carrot, asparagus, and among berries – raspberry, which are distinguished by a significant need for magnesium, may respond positively to magnesium fertilizer even with an increased content of this element in the soil. For soil application, potassium magnesium sulfate can be used; for foliar top dressing of plants during the growing season – magnesium sulfate.

An important role in the cultivation of vegetable and fruit crops belongs to such microelements as boron, molybdenum, manganese, copper, zinc, and cobalt. The content of most of these elements in plants ranges from one hundred-thousandth to one-thousandth of a percent. Therefore, they are called microelements.

The criterion for plant supply with microelements (and consequently the necessity of applying micro-fertilizers) is their content in the soil. Moreover, it is not the total amount that is important, but their content in a mobile form accessible to plants.

The mobility of trace elements in the soil depends on the following factors:

• parent material, biological activity, and soil properties (soil reaction, carbonate content, particle size and mineralogical composition, humus content, sesquioxides);
• the application of a complex of agrotechnical measures, especially soil water and chemical reclamation;
• the application of organic and mineral fertilizers.

Based on the plants' need for trace elements, there are three groups of crops:

• Crops with low trace element uptake and relatively high absorption capacity: maize, grain legumes, potatoes.
• Crops with increased trace element uptake and high or medium absorption capacity: root crops, vegetables, grasses (legumes, cereals, herbs), sunflower, fruit crops.
• Crops with high trace element uptake: all the above-mentioned crops under conditions of a high agrotechnical background (irrigation, high fertilizer application rates, use of the best cultivars, timely tillage and plant care, etc.).

Vegetable crops are considered to have a high demand for trace elements:

• for boron: cauliflower, Brussels sprouts, kohlrabi, white cabbage, celery, table beet;
• for copper: leaf cabbage, table beet, carrot, spinach, green onion;
• for manganese: beans, peas, cucumber, lettuce, radish, black radish, spinach, and table beet;
• for molybdenum: cauliflower, lettuce, spinach;
• for zinc: vegetable beans.

A high demand for boron is observed in apple, cherry, and plum trees, and for zinc in apple trees, and less often in pear trees, especially during dry seasons and on limed soils.

For other vegetable crops, a fairly strong positive effect of microfertilizers can be observed on soils with a medium supply of mobile forms of trace elements; on soils with low supply, microfertilizers are more effective.

Tables 7 and 8 show the gradation of different soil types according to their trace element content.

Table 7. Grouping of soils according to the content of mobile forms of trace elements, determined by the Peive-Rinkis method, pH 4.8 (for acidic soils).

Trace elements: assessing soil supply and compensating for deficiencies

For the correct calculation of microfertilizer application rates, it is important to know the actual content of mobile forms of trace elements in the topsoil. This data is obtained through chemical analysis of soil extracts. Depending on the soil type and its acidity, different extractants are used.

Element	Extractant	Low supply (mg/kg)	Medium supply (mg/kg)	High supply (mg/kg)
Manganese	0.1 N H2SO4	less than 30	31–70	more than 71
Zinc	1 N KCl	less than 0.7	0.8–1.5	more than 1.5
Copper	1 N HCl	less than 1.5	1.6–3.3	more than 3.3
Cobalt	1 N HNO3	less than 1.0	1.1–2.2	more than 2.3
Boron	H2O	less than 0.33	0.34–0.7	more than 7.1
Molybdenum	buffer solution with pH 3.3	less than 0.1	0.11–0.22	more than 0.23

For neutral soils, a different assessment method is used — with an acetate-ammonium buffer solution at pH 4.8. This reagent allows for more accurate determination of mobile forms of metals. In this case, the gradation of supply levels is different.

Element	Low supply (mg/kg)	Medium supply (mg/kg)	High supply (mg/kg)
Manganese	less than 10.0	10–20	more than 21.0
Zinc	less than 2.0	2.1–5.0	more than 5.1
Copper	less than 0.20	0.21–0.50	more than 0.51
Cobalt	less than 0.15	0.16–0.30	more than 0.31

In the regional soils, there is a marked imbalance of elements. A low content of zinc and molybdenum is recorded almost everywhere. Chernozems often lack manganese, while the supply of cobalt, copper, and boron remains at a medium level. To compensate for this deficiency, it is necessary to use organic fertilizers, ash, and specialized microfertilizers.

Humus balance: the role of crop residues and organic matter

Humus is the main indicator of soil fertility and structural condition. It serves as the primary reservoir of plant nutrients: it contains 95–99% of soil nitrogen, up to 60% of phosphorus, 80% of sulfur, as well as potassium, calcium, magnesium, and trace elements. Organic matter glues soil particles into an agronomically valuable structure and serves as an energy source for beneficial microbiota. Furthermore, humus acts as a natural barrier, binding toxic compounds, heavy metals, and pesticide residues.

The humus content in the regional soils is distributed unevenly. It naturally increases when moving from grey forest soils to leached and podzolized chernozems, but decreases in typical and typical carbonated chernozems. Under field conditions, organic matter reserves constantly fluctuate under the influence of two opposing processes. On one hand, mineralization occurs with the release of nutrients, and on the other, the synthesis of new humic compounds.

The main source for restoring organic matter in fields where manure is not applied is crop residues. Different agricultural crops leave behind unequal amounts of organic matter. On average, across a crop rotation, this volume is 20–25 kg/100 m² per year. However, it must be taken into account that only 12–13% of this mass becomes humified.

• Perennial legumes and grasses leave up to 100 kg/100 m² of plant residues (depending on the green mass yield).
• Annual legume and grass mixtures, as well as grain crops, leave up to 25–30 kg/100 m² of residues.
• Row crops (potatoes and root crops) leave the least amount of organic matter: after sugar beet harvesting, no more than 7 kg/100 m² of plant residues remain in the soil.

• Humus from perennial grasses — up to 6 kg/100 m² per year
• Humus from grain crops — 4 kg/100 m² per year
• Humus from row crops — 2 kg/100 m² per year
• Humus from 1 t of semi-rotted manure — 50–60 kg

To assess the potential of a specific field, it is necessary to know exactly the class of soil humus supply. This is a basic indicator upon which both natural soil fertility and the efficiency of planned mineral nutrition rates depend. Below is a detailed grouping of the main soil types according to their organic matter content level:

Soil	I very low	II low	III medium	IV increased	V high
Light-gray forest soils	≤ 2.5	2.6–3.0	3.1–4.0	4.1–5.0	>5.0
Gray forest soils	≤ 3.5	3.6–4.0	4.1–5.0	5.1–6.0	>6.0
Dark-gray forest soils	≤ 4.5	4.6–5.0	5.1–6.0	6.1–7.0	>7.0
Podzolized chernozems	≤ 6.0	6.1–7.0	7.1–8.0	8.1–9.0	>9.0
Leached, typical chernozems	≤ 7.0	7.1–8.0	8.1–9.0	9.1–10.0	>10.0
Ordinary chernozems	≤ 6.0	6.1–7.0	7.1–8.0	8.1–9.0	>9.0
Alluvial soils	≤ 2.0	2.1–4.0	4.1–7.0	7.1–8.0	>9.0

However, in practical work, it is important to consider not only the percentage concentration but also the physical reserve of elements in the root zone. In the 0–50 cm layer, reserves of humus and nitrogen fluctuate significantly depending on the soil and climatic zone of the region. Actual average indicators for soils of various zones of the Republic of Bashkortostan are presented in the table below:

Zone / Soil type	Humus reserves, t/ha	Nitrogen reserves, t/ha
Northern forest-steppe zone
Gray forest	141	12.0
Dark-gray forest	270	14.9
Podzolized chernozems	463	24.3
Southern forest-steppe zone
Gray forest	139	9.9
Dark-gray forest	268	14.0
Leached chernozems	395	21.0
Typical chernozems	384	20.8
Northeastern forest-steppe
Gray forest	147	8.2
Dark-gray forest	308	15.6
Podzolized chernozems	289	19.4
Pre-Ural steppe zone
Podzolized chernozems	478	28.9
Leached chernozems	380	20.0
Typical chernozems	372	22.8
Typical chernozems (carbonated)	300	20.0

• Critical pH for nitrogen fixation — 4.5
• Depth of the assessment layer — 0-50 cm
• Maximum humus reserve in the Pre-Ural region — 478 t/ha
• Maximum nitrogen reserve in the Pre-Ural region — 28.9 t/ha

Influence of soil acidity on nutrient availability and root growth

The reaction of the soil solution (the ratio of hydrogen H+ and hydroxyl OH- ions) directly governs root system development and the availability of nutrients. For crops, both high acidity and excessive alkaline exposure are equally dangerous. A deviation of pH from optimal values can completely block the effect of expensive mineral fertilizer applications.

Soils with increased acidity predominate in the northern, northeastern, and mountain-forest zones of Bashkortostan. These include sod-podzolic, gray forest, podzolized, and partially leached chernozems. In the southern forest-steppe and the Trans-Ural steppe zone, the soil solution reaction is more favorable for plants — typical and leached chernozems prevail there.

With increased soil acidity, an agronomist faces the following negative factors:

• Impaired growth and branching of the plant root system.
• Reduced permeability of cell membranes, which blocks the uptake of nutrients into tissues.
• Deep disturbances in the carbohydrate and protein metabolism of crops.
• Disruption of generative organ formation, leading to a drop in yield.
• Active proliferation of pathogenic soil fungi.
• Increased solubility of aluminum and manganese compounds that are toxic to plants.
• Increased dispersion of humus, causing organic matter to leach faster.
• Available phosphates are bound into insoluble salts with iron and aluminum.
• Decreased mobility of an important micronutrient — molybdenum.

To develop a strategy for chemical land improvement, fields are classified by the pH value of a salt extract (exchangeable acidity) and hydrolytic acidity (Hg). These indicators allow for the precise calculation of liming requirements:

Soil class	Salt extract pH (pH KCl)	Degree of acidity	Need for liming	Hydrolytic acidity (Hg)
I	4.0 and below	Very strongly acidic	Very strong	more than 6.0
II	4.1–4.5	Strongly acidic	Strong	5.1–6.0
III	4.6–5.0	Moderately acidic	Medium	4.1–5.0
IV	5.1–5.5	Slightly acidic	Weak	3.1–4.0
V	5.6–6.0	Near neutral	Very weak	2.1–3.0
VI	6.0 and above	Neutral	None	less than 2.0

Agricultural crop response to soil acidity

Most cultivated plants and beneficial soil microorganisms show the best productivity in a slightly acidic and neutral environment. However, sensitivity to pH deviations from the optimum level varies significantly among different crops. To plan an effective nutrition system, an agronomist must take into account the individual requirements of each plant for the soil environment.

Crop	Favorable pHKCl range	Crop	Favorable pHKCl range
Table beet	7.0–7.5	Lettuce	6.0–7.0
Cabbage	7.0–7.4	Sunflower	6.0–6.8
Cucumber	6.4–7.5	Radish	5.0–7.3
Onion	6.4–7.5	Carrot	5.6–7.0
Soybean	6.5–7.5	Tomato	5.0–8.0
Pea	6.0–7.0	Potato	4.5–6.3
Bean	6.4–7.1	Lupin	4.6–6.0

All vegetable crops, based on their response to soil acidity and responsiveness to liming, are divided into five main groups. The most sensitive ones require mandatory soil deacidification even at minor indicator deviations. Other plants, by contrast, tolerate excess calcium poorly, so soil amendments for them are applied in reduced rates.

• Threshold pH for liming beet and cabbage — 5.8–6.0
• Threshold pH for liming pea and cucumber — 5.6
• Threshold pH for liming eggplant and melon — 5.5
• Threshold pH for liming potato and tomato — 5.1

• Most sensitive to acidity (liming is necessary at pH 5.8–6.0 or lower) — beet (table and forage), mustard, cabbage (head cabbage), onion, garlic.
• Sensitive to high acidity (optimal reaction pH 6–7, liming is advisable at pH 5.6 or lower) — pea, sunflower, cucumber.
• Tolerant to moderate acidity (respond positively to liming, the procedure is advisable at pH 5.5 or lower) — cauliflower, eggplant, melon.
• Low sensitivity to excessive acidity (tolerate excess calcium poorly, lime is applied in reduced rates on medium and strongly acidic soils at pH 5.1 or lower) — potato, tomato, radish, carrot, summer squash, pumpkin.
• Low sensitivity to acidity (practically do not require liming) — lupin, sorrel, chicory.

In addition to general favorable growth ranges, there are specific optimal values of the soil solution reaction for vegetable crops. At these indicators, plants develop most effectively and form maximum productivity. These parameters serve as a guideline when planning crop rotation and crop placement.

pHKCl 5.0	pHKCl 5.5	pHKCl 6.0	pHKCl 6.5
Chicory	Carrot	White cabbage	Asparagus
Radish	Pumpkin	Cauliflower	Beet
Radish (daikon/black)	Tomato	Cucumber	Celery
	Kohlrabi	Melon	Lettuce
	Rhubarb	Eggplant	Garlic
		Horseradish	Onion
			Pepper

Liming as a Basis for Soil Environment Management

Chemical reclamation of acidic soils is based on changing the composition of adsorbed cations, mainly by introducing calcium into the soil adsorption complex. The main method of neutralizing excessive acidity remains liming — the application of calcium or magnesium in the form of carbonates, oxides, or hydroxides. This technique does not just change the soil environment reaction; it comprehensively improves the soil's agrochemical, biological, and water-physical properties.

Lime application has a multifaceted positive effect on the plough layer. The mobility of aluminum, manganese, and iron, which are toxic to plants, is reduced in the soil. Simultaneously with acidity neutralization, soil colloid coagulation occurs, which prevents them from being leached out. This leads to the formation of water-stable aggregates, improves water-air and temperature regimes, and also facilitates tillage.

Chemical reclamation activates the life activities of beneficial soil microorganisms and suppresses the development of pathogens, reducing the risk of crop diseases. Liming also affects the availability of micronutrients to plants. For instance, molybdenum compounds transition into more accessible forms. Boron and manganese compounds, by contrast, become less mobile and require special control.

Lime application rates. When liming soils, it is considered sufficient to lower its acidity to pH 5.8–6.0. For industrial purposes, lime application rates are determined by the salt extract pH value, taking into account the soil texture to achieve optimal acidity levels.

Table 14. Estimated lime (CaCO3) application rates depending on the salt extract pH and soil texture, kg/m2.

                                      Salt extract pH
  Soil texture           4.5
                                    4.6    4.8    5.0    5.2    5.4    5.5
                       and less
            1               2         3      4      5      6      7      8
 Light loam               0.47      0.45   0.40   0.35   0.30   0.27   0.25

1 2 3 4 5 6 7 8 Medium loam 0.57 0.55 0.50 0.45 0.40 0.37 0.35 Heavy loam 0.77 0.76 0.65 0.55 0.50 0.47 0.45 Clay 0.85 0.80 0.75 0.65 0.60 0.50 0.47