Water and Nitrogen Regimes and Productivity of Shallow Organogenic Soils (1972)

The effective fertility of organogenic soils is primarily determined by their reserves of organic matter. It has been hypothesized that the correlation between crop yield and peat layer thickness stems from differences in water and nitrogen reserves. The validity of this premise required rigorous experimental verification.

1. Experimental Conditions and Methodology (1968–1971)

In 1968–1971, specialized field, lysimetric, and laboratory investigations were carried out at the Polesian Experimental Peatland Station (Brest region). The objective was to determine the water and nitrogen nutrition status of crops and evaluate their contribution to yield formation across varying moisture levels, fertilizer regimes, and peat layer thicknesses.

In the field trial, two water regime treatments were established for timothy, barley, and potatoes:

• Control (natural): groundwater tables fluctuated under natural drainage and meteorological conditions;
• Experimental (sub-irrigation / sluicing): groundwater levels were maintained at specified target depths from the soil surface via sluicing controls.

Two fertilizer backgrounds were superimposed on these treatments: recommended doses of phosphorus-potassium fertilizers, and phosphorus-potassium supplemented with nitrogen (PK + N₃₅ kg/ha). Peat depth ranged from 40 to 90 cm.

The experimental soils developed on wood-sedge-hypnum peat underlain by fine sand. Total nitrogen content in the plow layer ranged between 2.08–2.63%, phosphorus 0.32–0.44%, and potassium 0.10–0.13% by dry weight. Ash content was 17–37%, and peat decomposition was approximately 30%.

In lysimeters (2 × 2 m surface area) with uniform peat thickness (35 cm), different drainage standards were maintained: constant depths of 50, 70, and 100 cm, and variable depth of 100–180 cm. The test crop was timothy grass. Fertilizers applied were P₆₀K₁₅₀ kg/ha in the first two years, followed by complete mineral fertilization in subsequent seasons.

Laboratory tests encompassed agrochemical and physiological analyses to determine nitrogen and water status in plants. Soil mineral nitrogen (nitrates), transpiration intensity, and leaf sap concentration were tracked dynamically. Yield data were analyzed using statistical methods (analysis of variance, correlation, and regression).

Atmospheric precipitation varied across seasons: during the growing periods of 1968, 1970, and 1971, rainfall exceeded regional norms by 23%, 20%, and 11% respectively, whereas in 1969 precipitation was 25% below normal.

2. Groundwater Level Dynamics and Peat Layer Moisture

Groundwater tables fluctuated within defined ranges (Table 1). In 1969, groundwater on the sub-irrigated plot was elevated during critical periods to 85–98 cm below surface, compared to 134–146 cm on the control. Seasonal mean drainage norms for timothy grass were 110 and 135 cm, respectively. Similar trends were observed under barley and potatoes.

In 1970 and 1971, disparities between drainage standards were even more pronounced. Under timothy in 1970, seasonal mean water tables were 141 and 99 cm on control and sluiced plots, respectively; in 1971, values were 114 and 82 cm. This demonstrates that sub-irrigation via sluicing substantially raises groundwater levels in drained peatlands.

Soil moisture directly mirrored groundwater depths (Table 2).

In 1969, moisture in the 0–50 cm layer on May 31 stood at 46.4% on control versus 55.2% on the sluiced plot, corresponding to moisture reserves of 232 and 276 mm. In 1970–1971, moisture reserve disparities reached up to 100 mm. Plowing layer moisture differences under barley and potatoes were less pronounced.

Nitrate levels remained modest throughout the seasons, declining at certain growth stages to 30 mg/kg soil. No strictly systematic effect of fertilizer or water regime on plow layer nitrate concentration was detected, although moisture increases correlated with higher nitrification. A similar picture emerged in lysimeters: in 1971, nitrate content was 153 mg/kg at 50 cm drainage depth, 171 mg/kg at 70 cm, 162 mg/kg at 100 cm, and dropped to 120 mg/kg at deeper water table levels (>100 cm).

Total nitrate and moisture stocks within a 1-meter profile naturally scaled with peat layer thickness: deeper peat inherently stores larger aggregate water and nitrogen reserves.

3. Physiological Indicators of Plant Water Availability

Plant water availability varied according to drainage depth and soil moisture regimes (Tables 3 and 4).

Elevating the water table markedly enhanced water availability for timothy grass: average transpiration rates over three years increased from 1.08 to 1.34 g/g · hr, while leaf sap osmotic concentration declined.

In lysimeters, water availability was optimal at a 70 cm drainage norm. A sharp decline occurred when groundwater receded deeper than 100 cm: transpiration rate dropped from 1.33 to 1.06 g/g · hr, while sap concentration increased from 7.4 to 8.6%. Over four years, a groundwater fluctuation range of 50 to 100 cm proved fully permissible, with minimal variation in crop physiological health and yield (Tables 3–5).

4. Crop Productivity and Lysimetric Trials

In field trials, keeping groundwater tables within 100 cm of the soil surface boosted timothy grass yields (primarily second-cut hay) by an average of 18% over three seasons (Table 6). In lysimeters, a 50–70 cm drainage standard likewise generated maximum perennial grass yields (Table 5).

While elevating water tables slightly improved water metrics in barley and potatoes, it did not translate into increased yields (Tables 3 and 6). In barley, grain yield actually decreased on sluiced plots by 4 c/ha (12%). Thus, water was not the limiting factor for these crops, or artificial sub-irrigation via sluicing imposed physiological drawbacks such as soil cooling or temporary oxygen deficiency.

5. The Role of Nitrogen Fertilizers in Leveling Peat Thickness Disparities

Nitrogen fertilizer application yielded substantial yield gains in timothy grass (Table 6). Applying N₃₅ kg/ha ammonium nitrate in spring together with PK fertilizers increased first-cut hay yield by almost 1.5 times. Summer nitrogen top-dressing (1971 data) allowed harvesting nearly identical second-cut hay volumes, whereas the unfertilized control produced negligible second-cut biomass.

For barley and potatoes, nitrogen application produced moderate gains (8–12%), peaking at up to 20% for barley on moist plots.

6. Scientific Conclusions and Practical Recommendations

1. Crop water availability on shallow organogenic soils directly depends on groundwater table depth. For timothy grass, water availability was optimal at a seasonal mean drainage norm of 70 cm, with an operational tolerance range of 50 to 100 cm. Lowering groundwater deeper than 100 cm severely impaired plant hydration and reduced hay yield by 18–34%. Sluicing did not benefit potato and barley yields, demonstrating that moisture was not the primary limiting factor or that sub-irrigation introduced negative agrophysical side effects.
2. Maximum efficiency on shallow peatlands with low-to-moderate decomposition degrees is achieved by applying nitrogen fertilizers to perennial grasses. Concurrently optimizing water tables and nitrogen nutrition exerted an additive positive effect on timothy yield, delivering gains of up to 85%. Barley and potatoes responded moderately to nitrogen; maximum responses (up to 20% and 10% gains, respectively) occurred under drainage norms shallower than 100 cm.
3. Nitrogen fertilizers eliminate yield disparities across organogenic soils with variable peat depth, with fertilizer return being inversely proportional to peat layer thickness. Improving soil moisture alone had a much weaker leveling effect. Consequently, natural fertility differences on shallow wood-sedge-hypnum peats (decomposition rate ~30%) are governed primarily by available nitrogen pools and secondarily by soil moisture reserves.

7. References

1. Gusev N. A. Some Methods for Studying Plant Water Regimes. — Leningrad, 1960.
2. Dadykin V. P. Peculiarities of Plant Behavior on Cold Soils. — Moscow, 1952.
3. Markovich I. I., Lositskaya R. V., Tishkovich A. V. Peat Utilization in Agriculture: Collected Papers. — Minsk, 1971.
4. Skoropanov S. G. et al. Proceedings of the Academy of Sciences of the BSSR, Agricultural Series, No. 4, 1970.