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Nitrogen is the most widely used fertilizer nutrient. There are several myths about nitrogen fertilizers used to promote various agronomic concepts. For example, that “anhydrous ammonia makes the ground hard”.
This Crop File summarizes results of a twenty-year Kansas State University study that evaluated the impact of different nitrogen fertilizers on soil physical properties.
After ten and twenty years of measurement, nitrogen fertilizer use did not affect common soil physical properties. The measurements also did not differ between the different fertilizer products. Final results of the study are shown in Table 1.
¶ B. Materials and methods
Four study locations
Study established 1969; continued through 1988.
Ottawa, Manhattan, two at Powhattan)
Five replicated treatments at each study.
Soil textures were silt loam, silty clay loam.
Four nitrogen fertilizers and no nitrogen check.
Anhydrous ammonia (NH3 , 82-0-0)
Ammonium nitrate (NH4 NO3 , 34-0-0)
Urea (CO(NH2 )2 , 45-0-0)
Urea-ammonium-nitrate solution (UAN solution, 28-0-0)
Nitrogen applied annually.
Ammonia injected 6 to 8 inches deep.
Other sources were surface applied.
Rates: 100, 150, or 200 lb N/ac.
Tillage system varied.
Fall tillage: disk (≈ 4 in. deep) or chisel (≈ 6 in. deep)
Spring tillage: disk (≈ 4 in. deep) or field cultivator (≈ 3 in. deep)
Cropping system varied.
Powhattan location #1: continuous corn; average, 165 lb N/ac/year
Powhattan location #2: continuous grain sorghum; average, 165 lb N/ac/year
Ottawa location: continuous corn; average, 155 lb N/ac/year
Manhattan location: grain sorghum, winter wheat, soybean; average, 200 lb N/ac/year
Soil samples collected in 1978 and 1988 for analysis.
Collected from two soil layers.
Surface: 2.5 to 5.5 in
Subsurface: 8.3 to 11.5 in
Bulk density
Study showed no significant differences.
Bulk density is indicator of soil compaction.
Calculated as soil dry weight divided by volume.
Includes soil particle volume and pore volume among soil particles.
Typically expressed as “grams per cubic centimeter (g/cm3 )”. iii. Water has density of 1.00 g/cm3 and 2.43 lb/ft3
Higher density indicates more compaction.
Indicates fewer pore spaces, reduced aeration, lower water holding capacity
Clod density
Study showed no significant differences.
Clod density is transient property
Depends on numerous soil, climatic, and mechanical factors.
Clods formed by tillage are subsequently disintegrated by other mechanical manipulation and climatic influences.
Impact of raindrops plus water entry cause soil fragments or clods to "melt" and run together
Probe resistance
Study showed no significant differences.
Refers to “mechanical resistance”
Interacts with other soil properties such as bulk density, texture, moisture content and porosity
Measures amount of force required for standardized probe to penetrate soil sample
Saturated hydraulic conductivity (Ksat)
Study showed no significant differences.
Ksat is infiltration rate once the soil has reached 100% saturation and infiltration rate has become constant
Available water holding capacity (AWHC)
Study showed no significant differences
Quantity of water that soil is capable of storing for plant use
Considered to be amount of water between field capacity and permanent wilting point
Expressed as inches (or centimeters) of water per inch (or centimeter) of depth for each soil layer
Field capacity
Study showed no significant differences at 0.33 bar.
Soil water content after gravitational drainage over approximately one day.
Generally ranges from 0.10 to 0.33 bar (10 to 33 kPa)
“kPa” = kilopascals
Permanent wilting point
Study showed no significant differences at 15 bar
Amount of water held so tightly by soil that roots cannot absorb this water and plant will wilt
Usually occurs at 15 bar (1500 kPa).
Measurement: Bulk density Clod Density Probe Resistance Saturate Hydraulic Conductivity AWHC* “Field Capacity” (0.33 bar) Wilting Point (15 bar)
g/cm3 as lb/ft3
Expressed as: after: after: g/cm3 lb/ft3 lb/in2 in/ft in/ft in/ft in/ft in/ft
After (years): 10 yr 20 yr 10 yr 20 yr 20 yr 20 yr 10 yr 10 yr 10 yr 10 yr 10 yr 20 yr
Surface: 2.5 to 5.5 in
Check (no N) 1.31 1.6 81.8 99.9 1.5 93.6 3.04 16.3 1.98 3.84 1.86 1.5
Anhydrous ammonia 1.34 1.59 83.7 99.3 1.45 90.5 3.38 10.7 2.09 3.97 1.88 1.51
Ammonium nitrate 1.3 1.59 81.2 99.3 1.46 91.1 3.49 11.7 2.05 3.88 1.82 1.52
Urea 1.31 1.58 81.8 98.6 1.46 91.1 3.38 13.7 1.81 3.68 1.87 1.5
UAN solution 1.31 1.6 81.8 99.9 1.49 93 2.6 9.7 2.23 4.02 1.79 1.52
Significant difference?
No N check vs. fertilized no no no no no no no no no no no no
Between fertilizers no no no no no no no no no no no no
Subsurface: 8.3 to 11.5 in
Check (no N) 1.35 1.49 84.3 93 1.67 104.3 3.42 2.8 1.87 4.6 2.72 2.26
Anhydrous ammonia 1.34 1.51 83.7 94.3 1.66 103.6 3.93 2.5 1.63 4.4 2.77 2.27
Ammonium nitrate 1.35 1.52 84.3 94.9 1.65 103 4.24 1.8 1.57 4.4 2.83 2.29
Urea 1.35 1.5 84.3 93.6 1.68 104.9 3.49 2.5 1.64 4.42 2.77 2.3
UAN solution 1.36 1.49 84.9 93 1.67 104.3 3.49 2.5 1.87 4.48 2.6 2.24
Significant difference?
No N check vs. fertilized no no no no no no no no no no no no
Between fertilizers no no no no no no no no no no no no
* Available water holding capacity
Darusman, Stone, Whitney, Janssen, and Long. 1991. Soil properties after twenty years of fertilization with different nitrogen sources. Soil Sci. Soc. Am. Journ. 55:1097-1100.
Intrawech, Stone, Ellis, and Whitney. 1982. Influence of fertilizer nitrogen source on soil physical and chemical properties. Soil Sci. Soc. Am. Journ. 46:832-836
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