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Fig. 7. The amount of C in HA, linked to Ca and free and linked to R2O3 by layers up to 400
cm according to the fertilization variants
Long-Term Mineral Fertilization and Soil Fertility
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2.3.3 Soil organic matter reserves in depth up to 60 cm
The systematic introduction of macro elements at different norms and ratios during a period of 40 years of cultivation of the trial field lead to formation of different reserves of total carbon in soil at depth up to 60 cm with well expressed differentiation (Fig 8). The long-term 2-field agricultural use of the trial field without mineral fertilization was characterized with lowest reserves of total C. The independent nitrogen fertilization with increasing norms
caused their increase according to the check variant with 12.6 %.This increase, however, was lower than the increase registered in all other variants. Highest reserve in absolute values at the moment of taking samples was found in the variants with 40-year fertilization with
N0P180K0 and N60P180K0. The main reason for this fact is that besides the variation in the content of total C, respectively humus, variation in the values of the other component was found when determining reserves – volume density of soil. According to Yankov (2007,
Personal Communication), highest values of volume density averaged for the 0-60 cm layer
were demonstrated by the variant with systematic introduction of phosphorus (180 kg/ha) –
1.43 g/m3, and lowest mean values – by the variant with N180P60K60 (1,22 g/m3). Over 36 %
of the total carbon reserves in soil at depth up to 60 cm were concentrated in the 20-40 cm layer, followed by the layer lying beneath (Table 12). Regardless of the low differentiation in the content of total C down the soil profile up to the 60th cm, the differentiation of the layers according to their reserves was very well expressed. Humus reserves in soil were highest in the 20-40 cm layer. The layer 10-20 cm have a negative C balance according to check variant.
The maximum increase according to the control in 0-10 cm and 10-20 cm layer was
established in the variants with N180P0K0 and N0P180K0 (136,3 and 135,6 %, respectively for 0-10 cm layer and 103,2 % and 105,3 % for 10-20 cm layer).
C kg/m2
10,837
12,690
(NoPoKo)
(N180P60K60)
13,076
12,056
(
(N60PoKo)
N120P120K120)
11,830
(N120PoKo)
12,810
(N60P180Ko)
13,256
12,204
(
(
NoP180Ko)
N180PoKo)
Fig. 8. Total Carbon reserves for layer 0-60 cm, C kg/m2
Combination between macroelements affected positively the humus reserves in soil. The
variant with balanced fertilization N120P120K120 contributed enrichement of soil carbon reserves in 40-60 cm layer with 37,1% according to the same layer in check variant. This
tendency was established also for long-term fertilization with N180P60K60.
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Soil depth, cm N0P0K0 N60P0K0 N120P0K0 N180P0K0 N0P180K0 N60P180K0 N120P120K120 N180P60K60
0
–
10
1,659
1,762 1,955 2,262 2,250 2,162 2,069 1,801
10 - 20
2,323
2,104
2,301
2,397
2,446
2,080
2,127
2,240
20 - 40
3,694
4,233
4,593
4,384
4,754
4,891
4,548
4,645
40 - 60
3,161
3,957
2,981
3,161
3,807
3,677
4,333
4,005
Table 15. Reserves by depth up to 60 cm according to fertilization, C – kg/m2
3. Conclusion
The systematic mineral fertilization for a period of 40 years with different norms and at different ratios between nitrogen, phosphorus and potassium had high effect on the
agrochemical condition of slightly leached chernozem (Haplic Chernozems) down the soil
profile.
The soil acidity forms, averaged for the investigated depth of the 0-400 cm profile, were significantly affected by the type of fertilizer combination. The depth was the factor with decisive effect in all forms of soil acidity. Influence of mineral fertilization was higher on exchangeable Al3+, Ca2+ and the ∑Са+Mg, and significantly lower - on the values of residual hydrolytic acidity and the rate of alkali saturation. The amount of exchangeable Mg2+ had a clear tendency toward increasing down the soil profile
Independent long-term mineral fertilization with 180 kg N/ha and with N180P60K60 caused
the occurrence of exchangeable Al3+ in the soil absorption complex in the surface layers 0-10
and 10-20 cm. It was not present further down the profile.
The variant with independent nitrogen fertilization with 180 kg/ha has also the lowest
∑Са+Mg and the lowest value of sorption capacity. Intensive mineral fertilization with
N180P60K60 caused decreasing of degree of saturation with bases.
The value of the pH, sorption capacity, acidity on strongly acid positions and degree of
saturation with bases showed a tendency of decreasing at the end of 40th year of trail beginning comparing the end of 30th year. In the same way independently of fertilizer
variant the acidity on the slightly acid positions is strongly increased.
The mineralization ability down the soil profile was affected to a maximum degree of
significance by the mineral fertilization and the incubation. Depth had decisive effect on the value of the index. The maximum effect of this factor was registered after 28-day incubation
– 92.3 %. The role of mineral fertilization on nitrogen mineralization according to the
incubation period was significantly less expressed – 9.2 %, 3.2 5 and 4.0 %, respectively. The amount of nitrified nitrogen increased with the longer incubation periods with 52.7 % (28
days) and with 118.8 % (56 days), respectively, in comparison to 14-day incubation. The
long-term mineral fertilization with N180P60K60 had the highest values of mineralization
ability for all three incubation periods. The established strong effect of systematic mineral fertilization regardless of the norm and ratios of nitrogen, phosphorus and potassium on the mineralization ability of soil in comparison to the check variant was highest at depths 0-100
cm and 300 – 400 cm.
Systematic mineral fertilization carried out for 40 years in two-field crop rotation (wheat –
maize) affected the content of Ctotal deep down the profile of the slightly leached chernozem Long-Term Mineral Fertilization and Soil Fertility
117
soil. Systematic use of N180P60K60 contributed most for the increase of its content at average depth 0-400 cm. Fertilizer variants N180P60K60 and N120P120K120 led to lower amount of HA-Ca, below the level of the check and all other investigated fertilization variants. There was a well expressed tendency towards increase of carbon in HA-Ca as a result from the
independent phosphorus and nitrogen fertilization, regardless of the nitrogen norm.
The ratio CHA/CFA putted under average for the 400 cm profile to distinct differentiation between the fertilization variants.Variation was within a wide range: from 1.72 to 3.75. The long-term systematic nitrogen fertilization with 180 kg N/ha in combination with low
fertilization norms of phosphorus and potassium determined the type of humus as fulvic-
humic, averaged for the investigated depth 0-400 cm. In all other fertilization variants, regardless of the norms and ratios between the macro elements and in the check variant, the values of this ratio were above 2, which determined the type of humus as humic.
Independent nitrogen fertilization, especially with annually applied high norms, had a
strong negative effect on the mobility of the organic substance and caused serious decrease of the percent of carbon in the insoluble residue.
As a result from the systematic mineral fertilization in the 20-40 cm layer, higher reserves were formed by the layers lying above and below. Triple NPK combinations (N120P120K120
and N180P60K60) enriched organic mater reserves in 40-60 cm layer.
These results showed that regardless of the intensive agricultural activities and changes of some agrochemical characteristics, slightly leached chernozem soil (Haplic chernozems) in Sough Dobrudzha region in Bulgaria preserved its main genetic characteristics at the lower depths of the soil profile.
The effect of long-term fertilizer treatments on detail nutrient balances, technological quality of crops, concentration of available forms of macro elements and trace elements in soil and plant biomass dynamics and many other aspects of this experiment await detailed analysis.
4. Acknowledgment
The author is deeply indebted to Prof Maria Petrova for the initiated of this long term trail in 1967. The author thank to Dobrudzha Agricultural Institute and staff of Agrochemistry lab for correct support.
5. R