Efficiency of nitrogen fertilizers
The efficiency of nitrogen fertilizers is associated with increased nitrogen use by plants and with reduced irreversible losses. The general solution is to optimize the conditions and regimes of nitrogen nutrition, improve the level of agronomic practice, and improve reclamation measures.
Coefficients of nitrogen fertilizer use
Efficient application of nitrogen fertilizers is possible only when their properties and the specific features of nitrogen transformation in soils are taken into account. All nitrogen fertilizers, except slow-acting forms, are highly soluble in water. Nitrate fertilizers migrate in the soil with soil moisture and are not bound by any type of absorption other than biological. Biological absorption occurs only in the warm season. Therefore, under conditions of a leaching water regime of the soil, nitrates can be washed out, primarily on light soils. At elevated rates on light-textured soils in fallow fields under conditions of excessive moisture or irrigation, losses of nitrate nitrogen can reach 10–25% of that applied.
Ammonia and ammonium forms, when they enter the soil, are absorbed by the soil absorption complex. In this form they lose mobility but remain available to plants and are not washed out, except in light soils with low absorption capacity. Under favorable conditions, as a result of nitrification they are transformed into nitrates, acquiring their properties. Urea behaves similarly in soil after its conversion into ammonium forms as a result of the activity of ureobacteria.
All nitrogen fertilizers initially or during nitrification accumulate in the soil in the form of nitrates. Nitrates are subject to denitrification processes, which are characteristic of almost all soils, and the main nitrogen losses are associated specifically with them. Nitrogen losses from denitrification for ammonium and amide forms are about 20%, for nitrate forms up to 30% of the amount applied. In clean fallow and with increasing rates, nitrogen losses increase to 50%.
From an ecological point of view, denitrification is positive, since it “frees” the soil from excess unused nitrates, preventing their entry into groundwater and water bodies.
In the soil, part of the fertilizer nitrogen, as a result of the vital activity of microorganisms, is transformed into organic forms unavailable to plants. As a result of immobilization, approximately 10–12% of nitrogen from nitrate and 30–40% from ammonium, ammonia, and amide fertilizers is fixed in organic form. The intensity of immobilization increases with the application of organic fertilizers with low nitrogen and high carbon content (stubble, straw, strawy manure).
It was previously assumed that plants in the first year of nitrogen fertilizer application use 60–70% of the nitrogen supplied. These data were obtained in field experiments using the difference method by comparing nitrogen removal in control variants (without fertilizer) and in variants with fertilizers. Later studies using labeled nitrogen atoms showed that under field conditions plants absorb 30–50% of nitrogen from fertilizers, but at the same time, in fertilized variants, plant use of soil nitrogen increases by 20–30%. Due to this, total nitrogen removal in fertilized variants increases by 20–30%, as a result of which the nitrogen use coefficients calculated by the difference method are 20–30% overestimated compared with actual values.
Nevertheless, for practical tasks, for example, calculating nitrogen balance and nitrogen fertilizer rates, the nitrogen use coefficient obtained by the difference method is used, because it characterizes the total nitrogen consumption by plants. Balance calculations performed in long-term experiments including several rotations of crop rotation confirm these conclusions. Nitrogen fertilizer use coefficients by the balance method are 60–70%.
Most of the nitrogen fertilizer applied to the soil is spent during the growing season on plant consumption, immobilization, denitrification, leaching, and erosion. Therefore, the aftereffect of nitrogen fertilizers is usually not taken into account.
Efficiency of nitrogen fertilizers
Balanced use of nitrogen fertilizers increases the content of vitamins and organic acids.
Optimizing the conditions of plant nutrition with nitrogen includes:
1. Application of optimal rates and forms of nitrogen fertilizers taking into account the biological characteristics of crops and properties of fertilizers, soil and climatic conditions, and results of plant nutrition diagnostics.
The determination of optimal nitrogen fertilizer rates is based on the results of field experiments conducted in various soil-climatic zones. These data are compared with the results of soil and plant diagnostics, on the basis of which recommendations are drawn up for fertilizer use, methods, and ways of adjusting rates depending on specific conditions.
Methods for optimizing nitrogen fertilizer rates based on the content of mineral nitrogen in the soil are becoming widespread. Effective rates for various agricultural crops depend on soil and climatic conditions.
Table. Grain yield of winter wheat and efficiency of nitrogen fertilizers depending on the content of mineral nitrogen in soil (according to VNIIUA).
| Soil nitrogen supply group | Content of Nmin in the 0–40 cm soil layer, kg/ha | Average yield, t/ha (control) | Yield increase (t/ha) with nitrogen application at a rate of, kg/ha | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0+45 | 30 | 30+45 | 45 | 45+45 | 60 | 60+45 | 90 | 90+45 | 120 | 120+45 | LSD0.95, t/ha | |||
| I | 0–60 | 2.38 | 0.86 | 0.93 | 1.33 | 0.93 | 1.04 | 1.15 | 1.47 | 1.13 | 1.37 | 1.06 | 1.18 | 0.18 |
| II | 60–80 | 2.94 | 0.72 | 0.76 | 1.32 | 1.02 | 1.39 | 1.10 | 1.28 | 1.13 | 1.26 | 1.26 | 1.26 | 0.11 |
| III | 80–100 | 4.06 | 0.40 | 0.27 | 0.57 | 0.44 | 0.58 | 0.42 | 0.47 | 0.52 | 0.52 | 0.38 | 0.36 | 0.12 |
| IV | 100–130 | 4.32 | -0.02 | -0.06 | 0.05 | 0.11 | 0.06 | -0.02 | -0.02 | -0.07 | -0.08 | -2.3 | -2.7 | 0.22 |
| V | > 130 | 5.00 | -0.18 | -0.12 | -0.36 | -0.08 | -0.19 | -0.35 | -0.45 | -0.45 | -0.47 | -0.41 | -0.49 | 0.25 |
Depending on the supply of soils with mineral nitrogen, they are divided into 5 groups: from very low — group I, to high — group V. Soil supply with mineral nitrogen affects the efficiency of nitrogen fertilizers: very high on soils with nitrogen content less than 80 kg/ha, low at content above 130 kg/ha. The greatest increases in grain yield were noted when fertilizers were applied in two applications: the first in spring, the second at the booting stage.
For the regions of Western Siberia, depending on the content in a 0–40 cm soil layer, a scale of cereal crops need for nitrogen fertilizers has been developed.
Table. Need of cereal crops for nitrogen fertilizers depending on the content of N-NO3 in the 0–40 cm soil layer in autumn or spring (according to Kochergin).
| N-NO3 | Plant supply with soil nitrogen | Need for nitrogen fertilizers | Approximate rates of nitrogen fertilizers, kg/ha a.i. | |
|---|---|---|---|---|
| mg/kg soil | kg/ha | |||
| At low and medium plant supply with phosphorus (up to 100 mg P2O5 per 1 kg soil, according to Frantsesson) | ||||
| 0–5 | 0–25 | Very low | Very strong | 60 |
| 5–10 | 25–50 | Low | Strong | 45 |
| 10–15 | 50–75 | Medium | Medium | 30 |
| > 15 | > 75 | High | Absent | 0 |
| At high plant supply with phosphorus (150–200 mg P2O5 per 1 kg soil, according to Frantsesson) | ||||
| 0–10 | 0–50 | Very low | Very strong | 80 |
| 10–15 | 50–75 | Low | Strong | 60 |
| 15–20 | 75–100 | Medium | Medium | 45 |
| > 20 | > 100 | High | Absent | 0 |
These same methods, with changes and modifications, are also used in other regions of the country.
To optimize nitrogen fertilizer rates, balance-calculation methods are also used, based on nitrogen removal with the planned harvest.
2. Maximum shift of fertilizer application dates toward the period of intensive nitrogen consumption by plants, split application of the total fertilizer rate in several applications.
Table. Use of nitrogen from ammonium sulfate depending on the timing of its application (according to VNIIUA).
| Experimental variant | Amount of nitrogen, % of applied | |||||
|---|---|---|---|---|---|---|
| under barley | under millet | |||||
| used | fixed | losses | used | fixed | losses | |
| PK + (15NH4)2SO4 before sowing | 58.2 | 22.4 | 19.4 | 54.8 | 28.9 | 16.3 |
| PK + (15NH4)2SO4 split | 68.2 | 16.9 | 14.9 | 63.2 | 25.4 | 10.4 |
3. Use of slow-acting and encapsulated forms of nitrogen fertilizers with controlled nitrogen release rates.
4. Use of nitrification inhibitors, as a result of which the nitrogen fertilizer use coefficient increases by 10–15%.
Unproductive nitrogen losses can be reduced by introducing intermediate and post-harvest crops.
Application of 1 kg of nitrogen from mineral fertilizers gives yield increases: 8–15 kg of grain, 50–70 kg of potatoes, 20–30 kg of meadow grass hay, 30–40 kg of root crops of sugar beet, approximately 3 kg of flax fiber.
In the zonal aspect, the efficiency of nitrogen fertilizers depends on moisture supply conditions and the level of natural soil fertility. High efficiency and stable action of nitrogen fertilizers have been noted in the Non-Chernozem zone on humus-poor sod-podzolic, gray forest soils, podzolized and leached chernozems, and the greater the leaching, the higher the efficiency. High efficiency is manifested on light soils, which are characterized by a constant nitrogen deficit. On drained peat-bog soils their action decreases, since potassium and phosphorus are in the first minimum. However, in the first years of development of peatlands in the central and northwestern regions of the Non-Chernozem zone, the efficiency of nitrogen increases.
When moving from north to south and from west to east within the European part of Russia, the continentality of the climate increases and precipitation decreases, which affects the efficiency of nitrogen fertilizers. The nitrogen supply of soils also changes.
Efficiency increases in the series sod-podzolic soils → gray forest soils → chernozems. A decrease is noted in steppe regions due to reduced moisture supply and more favorable conditions of nitrogen nutrition. Nitrogen fertilizers are also effective in the eastern regions of the country: in the forest-steppe of the Trans-Urals and Eastern Siberia it is higher; in the forest-steppe of Western Siberia with a more continental climate it is lower. In the Trans-Urals, 1 kg of nitrogen from mineral fertilizers gives an increase in grain yield of spring wheat of 10 kg/ha, in Eastern Siberia 11 kg/ha, in Western Siberia 5 kg/ha.
In the steppe regions of the European part of Russia on thick, ordinary, and southern chernozems, due to the high nitrogen content in the soils and moisture deficit, the efficiency of nitrogen fertilizers decreases and becomes unstable. This is even more pronounced on chestnut and light chestnut soils of the arid regions of the southeast.
Measures for accumulating and preserving soil moisture, as well as under irrigation conditions, make the efficiency of low rates of nitrogen fertilizers in these regions quite high and greater than that of phosphorus and potassium fertilizers.
Under the influence of nitrogen, starch hydrolysis, respiration intensity, and the activity of oxidative enzymes increase, which leads to premature consumption of the seed’s plastic substances. To eliminate the negative effect of nitrogen on the processes of substance transformation in the seed before the formation of the photosynthetic apparatus, some isolation of fertilizer nitrogen from the seed is necessary. One can count on a stable positive effect of band application of nitrogen fertilizers close to seeds only in relation to non-acidic soils saturated with calcium, sufficiently supplied with phosphorus available to the plant and in need of nitrogen, as well as in relation to plants whose seeds have large reserves of carbohydrates (wheat, oats, barley).
When nitrogen is added to the nutrient solution, the phosphorus content in the nitrogenous fractions of organophosphorus compounds increases significantly, primarily in the fraction of nucleoproteins — substances that play a role in the differentiation of meristematic tissues, that is, pathways of effective influence of fertilizers on morphogenetic processes in plants.
Violation of the optimal nitrogen-to-phosphorus ratio at the beginning of growth leads to disruption of the synthesis of amino acids and nucleoproteins, which determine the initial growth of plants. Therefore, it is proposed that on soils poor in nitrogen, small rates of nitrogen should be applied together with phosphorus fertilizer at sowing. On low-fertility soils with insufficient nitrogen content, it is recommended to apply, in rows at sowing, in addition to phosphorus-potassium fertilizers, nitrogen fertilizers at a rate of 5–10 kg/ha of nitrogen.
Increasing the efficiency of nitrogen fertilizers
The efficiency of nitrogen fertilizers depends on:
- zonal features;
- the complex of agronomic and reclamation measures used in crop rotation and for specific crops;
- the technology of nitrogen fertilizer application (timing and methods of application, rates, forms);
- methods of plant nutrition diagnostics.
In Russia, a set of agrochemical techniques has been developed that help increase the efficiency of nitrogen fertilizers:
1. Compliance with agronomic technology of nitrogen fertilizer application taking into account rates, forms, timing, and methods of application.
2. Optimal ratio of nitrogen and other macro- and microelements taking into account soil fertility and biological characteristics of the crop.
3. Optimization of nitrogen nutrition of crops during the growing season. Both the direct effect of fertilizers as a source of nutrition and the indirect effect associated with mobilization of nitrogen due to activation of mineralization of soil organic matter are taken into account. This is important because the amount of nitrogen formed from mineralization is the most difficult to account for by existing methods. Nitrogen from mineralization of organic matter can create an excess of nitrogen in the soil, which leads to lodging of cereals, deterioration of product quality, and negative impact on groundwater.
4. Use of nitrification inhibitors. Although this technique is temporary, at the stage of developing comprehensive measures to increase efficiency, inhibition of nitrification can help reduce nitrogen losses.
5. Improvement of slow-acting forms of nitrogen fertilizers: production technology of urea-formaldehyde forms (UFF) and encapsulated nitrogen fertilizers, as well as the search for new forms of mineral fertilizers with gradual transfer of fertilizer nutrients into the soil.
6. Application of liming in crop rotation with systematic application of nitrogen fertilizers on acidic soils, primarily sod-podzolic soils.
7. Application of agronomic techniques aimed at regulating the processes of nitrogen mobilization and immobilization and humification processes.
The mineralization of soil nitrogen upon application of nitrogen fertilizers is affected by:
- the degree of cultivation of sod-podzolic soils: in cultivated soils, easily hydrolyzable humus compounds predominate;
- the activity of soil microorganisms;
- increased absorption capacity of the plant root system;
- the form of nitrogen fertilizer, for example, ammonium forms promote better nitrogen uptake than nitrate forms;
- liming;
- application of organic fertilizers, which increase the amount of microflora.
Nitrate and ammonium forms of nitrogen can be immobilized as a result of interaction with soil organic matter, fixation by soil microorganisms, and fixation of ammonium forms by clay minerals. Immobilization absorbs 20–60% of the applied nitrogen; its magnitude depends on:
- forms of nitrogen fertilizers: amide and ammonium forms are fixed 1.5–2 times more than nitrate forms;
- fertilizer rate: with an increase in rate, the absolute amount of immobilized nitrogen increases, while the relative one — the share of applied — decreases;
- the amount of fixed nitrogen: in soils with high humus content, the nitrogen content is higher than in low-humus soils;
- the content of energy material, which simultaneously with mineral fertilizers increases immobilization of fertilizer nitrogen;
- the C : N ratio in soil: the more carbon and the less nitrogen, the more is immobilized.
Zonal features
In zones where high efficiency of nitrogen fertilizers is observed, per 1 ton of applied nitrogen an additional 10–15 t of grain, 30–40 t of sugar beet root crops, 5–6 t of raw cotton, approximately 2 t of flax fiber, and 20–30 t of meadow grass hay are obtained.
The action of nitrogen fertilizers can manifest differently within large agricultural regions of the country. For example, in the Non-Chernozem zone, applying 1 kg of nitrogen at optimal rates allows additionally obtaining 8–15 kg of grain, 50–70 kg of potatoes, 3.5 kg of flax fiber, 70–100 kg of silage maize. The high effect of nitrogen fertilizers in this zone is manifested on sandy loam and sandy soils, where a nitrogen deficit is almost always observed. Under conditions of a leaching water regime, due to nitrogen losses in the autumn-winter-spring period, spring application is significantly more effective than autumn application.
Nitrogen fertilizers are more effective on sod-podzolic, gray forest soils, podzolized and leached chernozems, and on all irrigated soils. With improved moisture supply, the efficiency of nitrogen fertilizers increases on all types and varieties of soils. On podzolized and leached chernozems of the forest-steppe zone of Ukraine, the return on nitrogen fertilizers is greater than in the right-bank forest-steppe, but less than in the left-bank forest-steppe.
In different areas of the steppe zone, for example, in Moldova, large yield increases from nitrogen fertilizer application are obtained on typical chernozems, smaller increases on ordinary and carbonate chernozems. Per 1 kg of applied nitrogen, up to 6 kg of winter wheat grain, up to 7 kg of maize grain, 2.5–3 kg of sunflower seeds, and 40–60 kg of sugar beet root crops are additionally obtained.
In the steppe regions of Ukraine on ordinary chernozems, nitrogen fertilizers are effective on winter wheat, sugar beet, and maize crops. Their action weakens from west to east. In the steppe of the European part of Russia, a positive effect is noted on ordinary and carbonate chernozems of the Kuban, in the foothills of the North Caucasus, and on the North Azov chernozems. On carbonate chernozems of Rostov Oblast and ordinary chernozems of the Volga region, the action of nitrogen fertilizers decreases.
The action of nitrogen fertilizers on chestnut soils with low humus content is manifested in regions with good moisture, for example, in Ukraine, Transcaucasia, and mountainous regions of the North Caucasus. Under the conditions of a strongly arid climate of Stavropol, Rostov Oblast, the Volga region, and Northern Kazakhstan, the action of nitrogen fertilizers on chestnut soils is more often weak. The same is characteristic of ordinary and southern chernozems and chestnut soils of the Asian plain part of Russia.
Influence of agro-reclamation measures on the efficiency of nitrogen fertilizers
The efficiency of nitrogen fertilizers is associated with timely and high-quality implementation of a complex of agronomic, reclamation, and soil-protective measures, with a high standard of farming: absence of weeds, provision of optimal soil regimes, sowing of highly productive varieties and hybrids, and an integrated plant protection system. All measures for reproducing fertility and cultivation contribute to increasing the efficiency of nitrogen fertilizers and greater returns. The balance of soil organic matter should be positive or deficit-free.
Organic fertilizers reduce the negative effect of high rates of mineral nitrogen and contribute to its more efficient use. Mineral nitrogen should be in the soil in an optimal ratio with other nutrients corresponding to the requirements of the crop being cultivated.
Liming of acidic soils significantly increases the efficiency of nitrogen fertilizers due to better nitrogen use, increased mobilization, and improved phosphorus nutrition. In arid steppe and dry-steppe regions, a positive effect of nitrogen fertilizers is possible under irrigation with optimal rates and irrigation regimes.
Under conditions of risk of erosion processes, the efficiency of nitrogen fertilizers depends on the soil-protective complex and anti-erosion soil tillage system, for example, contour-reclamation plowing across the slope, combined plowing, slitting of slopes, ridging, furrowing of fallow, which reduce water runoff and soil washout.
References
1. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. B. A. Yagodin. Moscow: Kolos.
2. Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. Mineev V. G. Moscow: Publishing House of VNIIA named after D. N. Pryanishnikov.
3. Evtefeev Yu. V., Kazantsev G. M. (2013). Fundamentals of Agronomy: A Textbook. Moscow: FORUM.
