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Influence of sustained fertilization on the amount of humus and effective fertility of leached chernozem.

INTRODUCTION

Soil fertility plays vital role in agricultural industry. Its maintenance and increase is the main way to enhance agricultural productivity. According to a number of scientists [1-4] in addition to its natural fertility associated with its character and genesis any soil has artificial fertility which is formed due to anthropogenic activity, i.e. agricultural methods used by a man (soil treatment, fertilization, melioration measures and etc.). Their combination is represented as effective (economic) fertility which is measured by volume of yield. Its part which is presented by the yield of a current year is called effective fertility. It is characterized by labile forms of nutrients. The other parts which is left to form the further yield of crops, soil potentiality is characterized by content of humus and gross forms of nutrients.

In modern agriculture maintenance and large-scale soil fertility recovery is an important issue. This problem has come up due to the fact that intensification of farming is accompanied by increase in yields of crops, at this nutrients removal from soil is growing, humus decomposition is intensified and content of humus and soil fertility are reduced. Such a conclusion has been made by many researchers [1, 4-11].

Besides humus is a critical feature of soil fertility, that is specified by all scientists. Its content and reserves influence on agricultural values of soil. It is a source of plant nutrition, first of all, with nitrogen. It significantly increases absorbing capacity of soil, enhances soil structure and water and air balance and physical and mechanical properties (in the course of decomposition it discharges carbon dioxide necessary for photosynthesis, metacarbonic and organic acids accelerating dissolution of hardly soluble mineral salts and enriching soil with nutrients). It enhances thermal properties, encourages development of soil microbial flora, increases biological activity of soil and has a favorable effect on plat growth.

Humus accumulates great amount of nutrients which in the course of mineralization with the help of microorganisms pass into soil solution where they are absorbed by plant roots. Humus is especially important for nitrogenous nutrition of plants since it stores 80-90% of all soil nitrogen. When soil fertility and its biological activity increase, the significance of soil nitrogen for yield formation relatively grows [8]. Rate of biological and biochemical processes which are responsible for accumulation of nutrients necessary for plants, depends on the contents of humus. In soils with high content of humus a thick humus horizon with high absorbing capacity is formed.

Thickness of a humus horizon and amount of humus proves soil fertility. Soils with high content of humus have better physical properties, are easier for treatment, are less subject to firming, have better water-retaining capacity. The humus horizon is black which helps promotes active absorption of solar energy. That is why these soils have better thermal environment [4].

Usually loss of soil fertility is related to decrease of humus in soil. This results from the fact that in the course of humus formation humus is not only accumulated but also decomposes. In case of low farming standards, when soil doesn't receive enough vegetable matter (manure, crop and root residues, straw and etc.), decomposition process prevails over accumulation. Apart from this loss of humus is caused by water and wind erosion and by frequent hoeing.

A number of researchers give data with regard to the loss of humus in different soils. Ktsoev [9] notices that <<by the last 20 years the rate of humus decrease in a plough layer has reached 0.05% a year>>. The same researcher states that according to N.F. Korobsky since the expedition of V.V. Dokuchaev soils has lost up to half of their humus in a plough layer (here the soils of North Caucasus are meant). Making reference to Professor B.H. Fiapshev, he writes that as a result of 25-years' use of common chernozem in Kabardino-Balkaria the content of humus in a layer 1 meter thick has reduced by 90 tons/hectare, which is 21% of its initial amount. Shaposhnikova and others [12] give data according to which without fertilization over 15 years the content of humus in the plough layers has reduced by 0.22%, i.e. by 0.015% a year.

Thus, in case of sustained agricultural exploitation without fertilization soil potentiality gradually reduces as a result of decreasing humus.

Presence in soil of available for plants forms of nutrients (first of all, nitrogen, phosphorus and potassium) which are typical for effective soil fertility, is of primary importance for direct formation of yield of crops.

It can be assumed that in the course of sustained systematic fertilization in crops rotation the content of humus and labile forms of nutrients responsible for soil fertility is somehow changed.

The objective of studies consists in monitoring dynamics of the stated values within the period of 43 years when different combinations and doses of NPK, manure+NPK, and defining the optimal variant which will allow reaching the best productivity of crop rotation providing fertility of leached chernozem at the depth of 80 cm or more under-laid by gravel is maintained and increased.

Procedure:

The main object of the study is leached chernozem a narrow line of which spreads upon a flatland part of foothills along Caucasian mountains and occupies the southern part of foothill plains (Kabardian, Ossetian and Chechen) at the height of 400-600 m above sea level. It is a forest-steppe natural zone which is characterized by sufficient moistening (650-700 mm of annual rainfall). Bedding is represented by calcareous loess loams and clays of different origin (alluvial and glacial) as well as gravel formations at various depths. Leached chernozems differ according to their texture, thickness of humus horizons and depth of gravel bedding.

Our studies cover leached chernozem with gravel bedding at the depth of 80-90 cm, that is why the thickness of humus horizons reduces up to 20-30 cm. The texture is heavy clay loam and light loam-rocky at the depth.

Leached chernozem has poor structure, fluffy and medium dense consistency and good water-air balance. Density of a plough layer is rather low (bulk weight is 1.01-1.20 g/[cm.sup.3]) and increases up to 1.4-1.5 g/[cm.sup.3] with depth. The soil is characterized by satisfactory air exchange and high water permeability especially when gravel bedding is not deep. Overall, leached chernozem has good physical properties [8].

According to gross chemical analysis data leached chernozems contain plenty of silica and sesquioxides; there occurs a significant amount of calcium, magnesium and sulfur in the whole soil body.

Content of humus in the plough layer varies from 3.5 to 7.5%, but more often is equal to 4.5-6.0%, at this with depth it evenly decreases to 1.2-2.0%. Amount of humus varies within 380-570 tons/hectare. In humus humic acids (lime humates) prevail, with depth amount of fulvic acids increases. Humus is rich with nitrogen (5-6%). Active and exchangeable acidity is not high - pH aqua = 6.2-6.4, pH salt = 5.8-6.0, hydrolytic acidity = 2.1-2.8 mg-eq per 100 g of soil. Weakly acidic reaction of the soil is rather good for plants. Degree of base saturation is equal to 94-98%, which results from high content of humus, enrichment with montmorillonite minerals and calcareousness of parent rock material. Calcium prevails over magnesium in the composition of exchangeable cations. Their amount in the plough layer is equal to 33-37 mg-eq per 100 g of soil.

Leached chernozems are noted for high gross amounts of nutrients; in particular, the amount of gross nitrogen is equal to 0.24-0.45%, phosphorus to 0.3, potassium to 1.6-2.3%. In a half-meter layer amount of nitrogen is equal to 21, phosphorus to 10-30, potassium to 94-115 tons/hectare. According to Turin-Kononova in the plough layer the amount of easy hydrolysable nitrogen is from 4 to 10, labile phosphorus-12-14 according to Chirickov, exchangeable potassium 13-15 mg per 100 g of soil according to Chirickov, i.e. availability of labile phosphorus and nitrogen is low and medium and of exchangeable potassium medium and sometimes increased [8].

Studies conducted in the first department of training and experimental farm "Federal State Budgetary Educational Institution of Higher Professional Education "Gorsky state agricultural university" within the framework of a stationary field experiment started in 1971. There at different times crops of field crops rotation, including clover, fall wheat, postharvest maize, grain maize, potato (hemp), postharvest maize for silage or green crop and fall wheat are rotated.

During the field experiment different doses and combinations of NPK, three levels of NPK-compounds, comparative effect of mineral and organic fertilizers (manure) have been studied. Single dose of NPK for each crop is equal to the recommended one according to results of short-term experiments and is within the range of N20-70P40-60K40-90. In the manure variant manure was applied in amount of 30 tons/hectare for a rotation of 5-field crop rotation in the form of semi-decomposed cow manure in autumn before plowing. The variants with manure + NPK and N2P2K2 are leveled out with regard to amount of nutrients, i.e. they are equivalent. In the estimated variant fertilizers doses have been calculated by the balance method for a certain planned yield of each crop of rotation, proximal to the maximum one possible in these soil and environmental conditions.

In autumn before fall ploughing manure, phosphate and potassium fertilizers were applied and before fall wheat--a part of nitrogen. In spring before preplanting cultivation nitrogen was applied for spring crops. In the course of planting all crops P10 was applied in the form of granulated superphosphate. Nitrogen and less often phosphate-potassium fertilizers (for clover) were applied as fertilization. Generally ammonium nitrate, urea, granulated superphosphate and potassium salt were applied. The main fertilizer were applied manually, preplanting and fertilization were applied mechanically. Fertilization of close-growing crops--manually, topdressing with the variants with a triple dose of nitrogen (fall wheat)--with a shoulder sprayer by 15-20% solution of urea. Manure has been applied for potato and maize manually.

The experiment has been conducted in dryland conditions. A plot area is equal to 100 [m.sup.2], replication is quadruple and sequence of the variants is regular.

Before starting the experiment and after each crop rotation at separate plots soil profile cuts have been made. There samples of soils from genetic horizons have been taken. In the samples taken a number of agrochemical parameters have been measured, in particular, humus according to the method of Turin, absorbed ammonium according to Konev, nitrate nitrogen according to Grandval-Lajoux, labile phosphorus and exchangeable potassium according to Chirickov. During the vegetation period samples from different soil layers have been taken for further analysis with regard to content of labile forms of nutrients according to the above stated methods.

The essential part:

The studies conducted for 43 years have shown that due to systematic fertilizers application leached chernozem has undergone noticeable changes with regard to both types of its fertility.

Data in table 1 demonstrate that without fertilization during the stated period of monitoring the content of humus in the plough layer has gradually reduced till 2003, and for the next 10 years it has slightly increased, though in average it has decreased in comparison to the initial value and is equal to 3.2%. At the beginning of monitoring in a subsurface layer A1 (30-40 cm thick) there were by 1.29% less humus than in the plough layer but further its amount increased, possibly due to migration of soluble humus from the plough layer to the subsurface layer. And at the end of monitoring it increased by 0.41% in comparison to the initial value. As far as the lower layers B1 and B2, there the stated trend of increasing amount of humus is also observed; it is most clearly manifested in layer B1. In case of rather significant amount of rainfall high filtration capacity of soil with closely located gravel bedding obviously stimulates removal of easy soluble humus substance from the plough layer to the lower layers.

Altogether it can be said that within the monitored period the amount of humus in the soil layer 0-40 cm thick has hardly changed. This may be only caused by the fact that crop yield on the non-fertilized variant has been relatively low and humification of crop and root residues has made up to some extent for the loss of humus related to its decomposition.

Our data do not agree with results of studies conducted by Shaposhnikova and others [12], according to which amount of total carbon in common light-clay chernozem has been gradually decreasing in all soil layers within 15 years of monitoring. Ktsoev [9] also notes the similar reducing of humus in the plough layer of leached chernozem of North Ossetia-Alania from 7.10 in 1963 to 6.15% in 1983 and 5.79% in 1993.

According to the studies of Bizhoev and others [6] after sustained fertilization and irrigation of common calcareous heavy-clay chernozem within the framework of a stationary field experiment started in 1948 in dryland conditions within 3 crop rotations the amount of humus in a soil layer 0-20 cm thick in a control variant has dropped, then changed insignificantly and represented the most low value in the experiment. In another long-term field experiment started in 1979 on the same soil with the same sequence of crops in crops rotation it has been found out that the main changes in content of humus in a layer 0-20 cm thick take place within the first 912 years, then this process has slowed down and the amount of humus has settled at a certain level typical for each studied agrosystem.

The data we have received (Table 1) illustrate that in the fertilized variants the amount of humus has changed differently over the time. So in case of single dose of NPK changes in the plough layer 0-30 cm thick were similar to those in a non-fertilized control variant, while in a 30-40 cm layer a reduction trend was observed, also typical for the lower layers. In case of double dose NPK in the layer 0-30 cm thick insignificant increase of the amount of humus was noted, while it was more noticeable than in the control variant. In case of triple dose N3P3K3 in the layer 0-30 cm thick the amount of humus were gradually reducing and within 43 years it reduced almost by 0.5%. The same occurred in the layer 3040 cm and the reduction was equal 1.06%. In the estimated variant in the layer 0-30 cm the content of humus reduced by 0.22% within 16 years and then settled at the level of 5.30%, i.e. in comparison to the initial value it reduced by 0.26%. From the above stated we conclude that in case of application of medium doses of mineral fertilizers the loss of humus in the layer 0-40 cm is relatively small and significantly increases in case of increased and high doses. In our opinion it results from the fact that in the latter case plants being sufficiently provided with moisture form higher crop yield which is achieved by consumption of nutrients not only from mineral fertilizers, but also from soil resources, in particular, nitrogen. Mineral forms of nitrogen appear as a result of mineralization of soil organic matter, first of all, humus, that is why its amount significantly reduces.

A variant of organomineral fertilizer manure + NPK is of certain interest. According to this variant manure in the amount of 30 tons/hectare was applied once in 5 years and in other years mineral fertilizers equivalent N2P2K2 were applied [8]. The data received show that in the layer of 0-30 cm the amount of humus has slightly reduced within the first 5 years in comparison to the control variant, then started to grow and by the end of monitoring was by 0.43% higher than that of the control variant. The same trend is typical for the layer of 30-40 cm where this value has increased by 0.22%. Consequently, regular manure application has stimulated enrichment of the plough and subsurface layers with humus as distinct from variants with only mineral fertilizers application. Other researchers also give the similar data [6, 8, 12, 13].

An issue concerning change of nutritive regime of soil in the course of sustained exploitation without fertilization and with their regular application is of great interest for science and agricultural industry. This issue is especially vital for leached chernozem with gravel bedding at the depth of 20-80 cm.

Mineral nitrogen consisting of ammonium and nitrate forms is the main source of plant nutrition. It is well known that ammonium nitrogen in the form of ammonium cation is easily absorbed by soil colloids in the course of exchange reactions in soil solution and is prevented from removal into the deep soil layers. Nitrate nitrogen in the form of N[O-.sup.3] is not absorbed, can be easily removed through the soil body and reaching the gravel bedding be lost for plants. That is why it hardly can be accumulated in soil, however due to occurring nitrification certain amount of it is always found in leached chernozem. Nitrate nitrogen together with ammonium nitrogen can characterize nitrogen regime of soils. As far as labile phosphorus and exchangeable potassium are concerned, they poorly migrate through the soil profile, that is why their accumulation in soil is quite possible. On the basis of these considerations we have set a problem to monitor the dynamics of these nutrients during the whole period of monitoring crops rotation.

The received results are shown in Table 2. Data from this table show that in the control variant without fertilization the amount of mineral nitrogen in the layer of 0-20 cm has slightly changed over 40 years, however there is a trend for its reduction by the middle of the period and further stabilization at 3.3 mg per 100 g of soil. In the course of regular application of a double dose of NPK the stated parameter has significantly exceeded that of the control variant at all stages of monitoring, in particular within the last 10 years: the margin is equal to 1.1 mg per 100 g of soil. The combination of manure with NPK matches with the double dose of NPK and exceeds the control variant at all periods of monitoring. Other authors [6, 8, 14-19] also states the increase of the amount of nitrates and ammonium in a layer of 0-30 cm of common and leached chernozem and chestnut soil in the course of regular fertilization in crop rotation. Most of them register obvious exceeding of fertilized variants over the control variant and note high level of NPK and the combination of manure with NPK.

At the beginning in the variant without fertilization the amount of labile phosphorus in the plough layer of soil has reduced insignificantly as compared to the initial value, but then dropped more dramatically, in two times. There can be several reasons. They are fixation of labile phosphates in the form of hardly soluble calcium, magnesium, aluminum and ferrum salts, consumption of phosphorus by plants, partial migration through the soil profile and etc. The similar dynamics is also observed in fertilized variants, though it is less evident due to applied phosphate fertilization. Anyway labile phosphorus has been accumulated in the plough layer of soil as compared to the control variant though the initial amount is not achieved.

Almost the same can be stated with regard to exchangeable potassium: without fertilization its amount in leached chernozem has reduced in the course of time and over 40 years it has dropped by 6.3 mg per 100 g due to annual removal of potassium by yield of planted crops and possibly to certain migration of its cations through the soil layers. In fertilized variants the similar dynamics is observed. The only difference is that they exceeded the control variant by 2.8-3.1 mg per 100 g of soil. At this it should be noted that the equivalent variants of double dose of NPK and manure + NPK are almost similar [20].

Conclusions:

1. The considered data of studies show that in the course of sustained agricultural exploitation without fertilization leached chernozem fertility demonstrate the obvious trend for reduction, though it can't yet be considered disastrous. It is manifested in reduction of humus in a plough layer of soil.

2. In the course of mineral and organic fertilization the negative trend is smoothed, in particular with regard to humus which amount is not reduced but alternatively increased. In order to maintain a balanced amount of humus on leached chernozems with gravel bedding it is required to apply from 7 to 12 tons/hectare of manure during the 1 rotation of crop and 2-3 tons/hectare after the 3 one annually [8].

3. Without fertilization the amount of mineral nitrogen in the plough layer of soil is unstable and slightly varies. In the course of regular fertilization it gradually increases.

4. Without fertilization the amount of labile phosphorus and exchangeable potassium gradually reduces, while in the course of regular fertilization it reduces to a lesser extent, exceeding values of the control variant. At this organic-mineral combination and a double dose of NPK have almost similar effect.

References

[1.] Ganhgara, N.F., 2001. Soil Science. Textbook. Moscow: Publishing House Agroconsult, pp: 392.

[2.] Dzanagov, S.H., 2013. Basics of Soil Science and Agrochemistry. Vladikavkaz: Publishing house of the Gosky state agricultural university, pp: 272.

[3.] Kaurichev, I.S., 1982. Soil Science. Textbook. Moscow: Kolos, pp: 496.

[4.] Kurbanov, S.A. and D.S. Magomedova, 2012. Soil Science with Basics of Geology. Saint-Petersburg-Moscow-Krasnodar: Publishing house "Lan", pp: 288.

[5.] Bizhoev, V.M., 2005. Changes in fertility of common chernozem in the course of 50-years' fertilization and irrigation. Nalchik: Publishing house "Private Enterprise "Polygraphia", pp: 199.

[6.] Bizoev, V.M., T.P. Lifanenkova and S.H. Dzanagov, 2006. Dynamics of humus in chernozem in the course of sustained fertilization and irrigation. Plodorodie, 6(33): 32-34.

[7.] Grishina, L.A. and V.A. Kovda, 1973. Soil Organic Matter. Basics of Soil Sciences. Book 1. Moscow: Publishing house "Nauka", pp: 296-321.

[8.] Dzanagov, S.H., 1999. Efficiency of fertilizers in crop rotation and soil fertility. Vladikavkaz: Publishing house of the Gorsky State Agricultural University, pp: 363.

[9.] Ktsoev, B.K., 1997. Soil Fertility and Fertilizers Efficiency in Pre-Caucasian Region. Moscow: Publishing house of the Moscow State University, pp: 166.

[10.] Lykov, A.M., 1985. Humus and Soil Fertility. Moscow: Publishing house "Moskovsky Rabochiy", pp: 192.

[11.] Orlov, D.S., 1985. Chemistry of soils. Moscow: Publishing house of the Moscow State University, pp: 374.

[12.] Shaposhnikova, I.M., A.I. Garmashev, V.I. Zhurba, P.L. Lebedeva, L.Y. Tsarev and E.A. Shamarakova, 1990. Productivity of grain-fallow crop rotation and fertility of common chernozem depending on systematic organic and mineral fertilization. Agrochemistry, 12: 11-23.

[13.] Yagodin, B.A., Y.P. Zhukov and V.I. Kobzarenko, 2003. Agrochemistry. Ttextbook. Moscow: Publishing house "Mir", pp: 584.

[14.] Basiev, A.E., 2005. Efficiency of a stage of field crop rotation and agrochemical properties of leached chernozem depending on a fertilization system, Synopsis of a thesis of a Candidate of Agricultural Sciences, Vladikavkaz, pp: 24.

[15.] Gizoev, V.S., 1980. Efficiency of a fertilization system in a stage of crop rotation on calcareous chernozems o the Pre-Caucasian Region, Synopsis of a thesis of a Candidate of Agricultural Sciences, Ordzhonikidze, pp: 20.

[16.] Dzhanaev, Z.G., 2004. Soil and agrochemical estimation of fertility of soils of North Caucasus. Moscow: Publishing house of the Moscow State University, pp: 759.

[17.] Lazarov, T.K., 2001. Fertilizers influence on productivity of a stage of field crop rotation and agrochemical properties of leached chernozem of the forest-steppe zone of the Republic of North Ossetia-Alania, Synopsis of a thesis of a Candidate of Agricultural Sciences, Nalchik, pp: 23.

[18.] Hekilaev, A.B., 1994. Productivity of a stage of crop rotation depending on fertilizers on chestnut soils of the Republic of North Ossetia, Synopsis of a thesis of a Candidate of Agricultural Sciences, Vladikavkaz, pp: 18.

[19.] Kanukov, Z.T., 2009. Influence of ferlizers on yield of crops, quality of crops, productivity of a stage of crop rotation and agrochemical properties of leached chernozem of the Republic of North Ossetia-Alania, Synopsis of a thesis of a Candidate of Agricultural Sciences, Vladikavkaz, pp: 25.

[20.] Khadikov, A.Y., Z.T. Kanukov, A.E. Basiev, T.K. Lazarov, S.H. Dzanagov, 2012. Influence of different fertilizer doses on agrochemical parameters, nutritive regime of leached chernozem and soybean crop yield in conditions of the forest-steppe zone of the Republic of North Ossetia-Alania. Bulletin of the Gorsky State Agricultural University, 49, Part 3. Vladikavkaz, pp: 31-37.

Sozyrko Khasanbekovich Dzanagov, Taimuraz Konstantinovich Lazarov, Aslan Eseevich Basiev Zaurbek Tamerlanovich Kanukov, Arthur Yurievich Khadikov

Federal StateBudgetary Educational Institution, "Gorsky State Agricultural University", The Russian Federation, 362040, Vladikavkaz, 37 Kirov street,

Received: 25 June 2014; Received: 8 July 2014; Accepted: 25 July 2014; Available online: 20 August 2014

Corresponding Author: Sozyrko Hasanbekovich Dzanagov, Federal State-Funded Educational Institution, "Gorsky State Agricultural University", The Russian Federation, 362040, Vladikavkaz, Kirova street, 37
Table 1: Changes in the content of humus in leached
chernozem of the Republic of North Ossetia-Alania
with sustained fertilization in the course of field
crop rotation, %.

Variant      Layer,       1971   1976   1987   2003   2014   Average
              cm

Control     A[PI] 0-30    5.57   5.44   5.30   5.25   5.42    5.39
            A1 30-40      4.28   4.30   5.00   4.89   4.98    4.69
            B1 40-59      1.72   1.73   3.59   3.48   3.11    2.73
            B2 60-80      1.50   1.50   1.88   1.72   1.68    1.66

N1P1K1      A[PI] 0-30    5.57   5.41   5.30   5.20   5.41    5.38
            A1 30-40      4.28   4.37   4.41   4.36   5.09    4.50
            B1 40-59      1.72   1.34   2.50   2.18   1.92    1.93
            B2 60-80      1.50   1.58   1.88   1.61   1.54    1.62

N2P2K2      A[PI] 0-30    5.57   5.33   5.38   5.15   5.52    5.42
            A1 30-40      4.28   4.26   5.00   4.38   4.87    4.56
            B1 40-59      1.72   1.60   3.41   2.78   2.38    2.38
            B2 60-80      1.50   1.42   2.40   2.04   2.12    1.90

N3P3K3      A[PI] 0-30    5.57   5.48   5.32   5.18   5.09    5.33
            A1 30-40      4.28   4.06   3.88   3.46   3.22    3.78
            B1 40-59      1.72   1.68   1.74   1.70   1.66    1.70
            B2 60-80      1.50   1.46   1.49   1.43   1.42    1.46

Estimated   A[PI] 0-30    5.57   5.45   5.35   5.28   5.31    5.39
            A1 30-40      4.28   4.38   4.88   3.22   3.36    4.02
            B1 40-59      1.72   1.73   1.67   1.82   1.80    1.75
            B2 60-80      1.50   1.54   1.98   1.88   1.72    1.72

Manure      A[PI] 0-30    5.57   5.40   5.50   5.74   5.85    5.61
+NPK        A1 30-40      4.28   4.31   5.05   5.26   5.20    4.82
            B1 40-59      1.72   1.71   2.67   1.98   1.82    1.98
            B2 60-80      1.50   1.42   1.72   1.64   1.68    1.59

Table 2: Changes in the amount of labile forms of
nutrients in a layer of leached chernozem
of 0-20 cm depending on a fertilization system,
mg per 100 g of soil, average for the period from
1972 to 2012.

Parameters   Years     Variants

                        Without      [N.sub.2]   Manure
                     fertilization   [P.sub.2]    +NPK
                                     [K.sub.2]

N-           1971         2.9           2.9       2.9
N[H.sub.4]   1976         2.9           2.9       3.9
+ N-N[O      1987         2.3           4.1       3.3
.sub.3]      2003         3.4           4.3       4.4
(sum         2012         3.3           4.4       4.6
total)

[P.sub.2]    1971        15.6          15.6       15.6
  [O.sub     1976        15.3          12.9       13.5
  .5]        1987        14.8          15.9       16.1
             2003         8.1          10.0       11.5
             2012         7.5           9.4       9.6

[K.sub.2]    1971        19.1          19.1       19.1
  O          1976        17.4          11.8       10.5
             1987        13.4          15.5       14.7
             2003        15.8          16.8       16.9
             2012        12.8          15.9       15.6

Note: N-N[H.sub.4]-ammonium nitrogen, N-N[O.sub.3]-nitrate
nitrogen, [P.sub.2][O.sub.5]-labile phosphorus, [K.sub.2]
O-exchangeable potassium.
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Title Annotation:Research Article
Author:Dzanagov, Sozyrko Khasanbekovich; Lazarov, Taimuraz Konstantinovich; Basiev, Aslan Eseevich; Kanukov
Publication:American-Eurasian Journal of Sustainable Agriculture
Article Type:Report
Geographic Code:4EXRU
Date:Jun 1, 2014
Words:4790
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