DIVISION S-8—FERTILIZER TECHNOLOGY AND USE

The Influence of Soil-Fertilizer Geometry on Nitrification and Nitrite Accumulation1

D. F. BEZDICEK, J. M. MACGREGOR, AND W. P. MARTIN2

ABSTRACTNine granules (330 ppm N) of either urea, (NH4)2SO4, or

diammonium phosphate (DAP) were spaced in a square pat-tern at distances of 0, 0.5, 1.0, or 1.5 cm on 30 g of soil. Sampleswere watered, incubated, and extracted with 1 N KC1. Theextracts were analyzed for NH4

+ , NO2~, and NO3~. Gener-ally NO2"~ accumulation for the three N carriers increased withgranule spacing; the geometric effect was DAP > (NH4)2SO4>urea. Nitrite varied from 140 to 265 ppm N. Increased NO2~accumulation with increased granule spacing was accompaniedby a corresponding increase in NH4+ oxidation rate with littlechange in NO3~. Geometric effect with four granules (55ppm N) spaced up to 2.5-cm apart showed that both NO2~ andNH4

+ decreased with increased granule spacing accompaniedby increased NO3~. Single-granule studies suggested that thegreater geometric effects of DAP resulted from the lower solu-bility and relatively low mobility of phosphate ions. The subse-quent periphery pH decrease minimized outward movement ofNH4

+ . These effects were less for (NH4)2SO4 and for urea.

Additional Key Words for Indexing: ammonia toxicity, nitriteoxidizing organisms, urea, diammonium phosphate, ammoniumsulfate.

OPTIMUM PH RANGE for nitrification in soil occurs frompH 7.5 to 9.0 (6, 8, 12); however, Fuller (8) has

reported NO2~ and NO3~ formation at pH 13 which indi-cated tolerance to high OH~ concentrations. Nitrite accu-mulation occurs in both alkaline and neutral soils followingammonium fertilization (4, 7, 8), in acid soils treated withurea (5, 14), near decomposing alfalfa residues (13), andat low temperatures and moisture (10). Warren (17) foundthat most organisms can tolerate large concentrations of

ionized ammonium salt. However, NH3 toxicity increaseswith pH as the fraction of the unionized from (NH3) in-creases. Nitrite accumulation in soil is usually caused byNH3 toxicity to Nitrobacter agilis (1, 2), although NH3at higher concentrations is also slightly inhibitory to Nitro-somonas europeae (2). Nitrite inhibition of its own oxida-tion has been shown to occur at concentrations as low as 4mmole NO2~/ml. Tyler and Broadbent (16) implicatedself inhibition of NO2~ in acid soils but suggested thatequivalent inhibition in calcareous soils would require aone hundredfold NO2~ increase.

Many laboratory studies involving N transformationsare conducted using either solutions or finely divided crys-tals with little regard to localized pH and salt effects fromthe fertilizer granule. When commercial fertilizers areadded to soil, the environment surrounding the granule isgreatly influenced by the chemical and physical propertiesof each granule. In water-soluble carriers, the rate of solu-bilization is often greater than the rate of biological trans-formation; consequently, granule environment becomes afactor both in initial and in continuing N transformations.Hauck and Stephenson (9) have shown that the nitrogengranule can either stimulate or retard nitrification depend-ing upon granule-site pH and NH4

+ concentrations. Solidurea granules nitrified at a lower rate in Marden silt loam

998 SOIL SCI. SOC. AMER. PROC., VOL. 35, 1971

than liquid urea since local areas of high pH and NH4+

concentrations accompanied the granules. Nitrification inan acid soil was stimulated by a variety of carriers con-taining either diammonium phosphate, urea, or formamidewhen granule environment exceeded a pH of 8.0 withhigher N fertilization rates.

The granule proximity of three N salts on NH4+ , NO2~,

and NO3~ accumulations is considered in this paper.

MATERIALS AND METHODSThe Ap horizon of an Ulen fine sandy loam of lacustrine

origin (sandy, mixed, frigid family of Aerie Calciaquolls),which developed near the shorelines of glacial Lake Agassizin northwestern Minnesota, was used for this investigation.After sampling, the soil was air dried, passed through an8-mesh sieve and stored in sealed polyethylene bags. Somephysical and chemical properties of this soil are shown inTable 1.

Diammonium phosphate (DAP) granules, (NH4)2SO4granules, and urea prills were obtained from the TennesseeValley Authority, Allied Chemical Company, and the SpencerChemical Division of the Gulf Oil Company, respectively.Granules or prills were passed through appropriate screens toobtain the desired granule size. Their composition and solu-bilities are also included in Table 1.

Multiple-Granule Studies

Nine granules (20.5 mg N) of each of the three N-carrierswere added to 30 g of the Ulen soil in separate, 100-ml pyrexbeakers. Granules were placed in a square pattern at 0.5-cmintervals from 0 to 1.5 cm. Measurements were taken from thecenter of the granules. Zero spacing (tangential intersection)is hereafter referred to as 0-cm spacing. The spaced granules,covered with an additional 30 g of soil, provided 330 ppm ofadded N. Deionized water was surface applied to a final mois-ture content of 15% water. Comparative, nongranular mix-tures were also prepared in which equal amounts of the threegranular N-carriers ground to powder in a mortar, were incor-porated as a "mix" throughout 60 g of soil. Each treatmentwas replicated three times. All samples were placed in anenclosed water bath to minimize evaporation losses, and tem-perature was maintained at 24±2C.

Entire samples were periodically removed from the incuba-tion chamber and extracted with two 50-ml portions of INKC1. Filtered extracts were analyzed for NH4+ and (NO2~ +NO3~) by the MgO-steam distillation method of Bremner(3). Nitrite was determined colorimetrically by the modifiedGriess-Ilosvay method described by Bremner (3). Nitrate wasobtained by subtracting NO2~ from the (NO2~ + NO3~) val-ues. In the mix treatment, pH was determined on a saturatedpaste using a Beckman Expandomatic meter prior to the KC1extraction.

Four-granule geometric studies were made with the threeN-carriers using 180 g of soil in 250-ml beakers. Granules con-taining 10 mg N were spaced on the surface of 90 g soil at

Table 1—Some properties of Ulen fine sandy loam and thefertilizer salts

Soil properties

SandSiltClay1/3 atm. tension1/10 atm. tensionpHOrganic matterTotal NExtractable PExchangeable K

77. 3%10.3%12.4%13. 0% H2022. 5% H2O8.34.0%0.16%9 ppm

45 ppm

Fertilizer propertiesN source NH4+N Solubllltyf

g/100 ml HjO

(NH4)jSO4 20.5% 75.4(NHj)jHPO4 20. 5% 68CO(NH2)2* 42.0% 108

0.5-cm intervals from 0 to 2.5 cm outward from the centergranule and covered with 90 g of soil. The four granules sup-plied 55 ppm added N (!/6 of the N concentration of thenine-granule study). Deionized water was added as previouslydescribed in the nine-granule study. The samples were peri-odically extracted with two 100-ml portions of IN KC1. Theextracts were filtered and analyzed. Statistical analyses ofmeans were determined by Tukey's honest significant differ-ence (15).

Single-Granule Studies

Single-granule studies were conducted with each of thethree N-carriers using granules containing 40 mg N. Therespective fertilizers were added to 10-mm sections of 6-mmdiameter plastic tubes and compacted at 9,000 psi in a Car-ver press. The plastic and excess fertilizer material were re-moved to the desired size of approximately 5 by 6 mm.

Reaction cells were constructed of three plexiglass rings(8.2-cm diameter by 2-cm high) taped together to form a6 by 8.2-cm cell. This assembly was taped to a 10-mesh nylonscreen on a plexiglass base. Chilled soil was mixed with snowin a cold room to provide a uniform, 10% moisture-equivalent.Approximately 250 g of the soil-snow mixture was then addedto each cell, and the appropriate granule was placed in thecenter. An additional 250 g of the soil-snow mixture was thenadded. This gave a concentration of 89 ppm -of added N pergram air-dry soil. No-granule control cells were similarly pre-pared. Duplicate samples were made in a cold room for eachof the three N-carriers and placed in the previously describedincubation chamber for 5.5 days. The end rings of each cellwere then removed, and the middle 2-cm section was sampledat 0.5-cm lateral intervals from the center to the cylinder wall.The pH of each sample was determined as previously de-scribed; the samples were extracted with two 50-ml \N KC1leachings which were analyzed for NH4 + , NO2~, and NO3~.Extractable N for each sampled zone was calculated on a drysoil-weight basis.

Calculation of NH3 Concentration

Calculations from each zone were made on a volume basisof water; a constant soil moisture of 10% was assumed. Itwas also assumed that no NH3 was lost during sampling. Thefollowing calculations were used where the equilibrium con-stant for NH4OH formation from NH3 and H2O equals:

K = [NH4+] [OH-]/ [NH3] = 1.8 X 1Q-6

Combined with the auto dissociation constant of water

([OH-] [H+] = 10-14),K1 = [NH4 + ]/[H + ] [NH3] = 1.8 X 109

The total analytical ammonia was defined as

*NH3 = [NH4+] + [NH3]

By combining [2] and [3],

- [NH3][NH3

' After hydrolysis with urease. t 200.

Solving for [NH3],

[NH3] =1.8 X 10° [H + ] + 1

[1]

[2]

[3]

[4]

[5]

BEZDICEK ET AL.: SOIL-FERTILIZER GEOMETRY & NITRIFICATION 999

The FNH3 for each sampling zone was converted to moles/1after calculation for the volume of water based on the assump-tion that 1 ml H2O = 1 g. From [5], [NH3] was calculated,whereas, NH3 (Mg N/g soil) was determined by

Table 2—Recovered NH4 + , NC>2~, and NO3~ after incubatingnine DAP granules (330 ppm N) placed at various geometric

intervals in 60 g of Ulen fine sandy loam8

[NH3]FNH,

X total analytical NH4+ (/*g N/g soil) [6]

RESULTS AND DISCUSSION

Multiple-Granule Studies

Granule spacing, time, and recovered NH4+, NO2~, andNO3~ for nine DAP granules are shown in Table 2. After 4days the only significant change in all three ions occurredbetween the mix and granular treatments. The finelyground DAP in intimate contact with soil promoted rapidnitrification of the NH4

+ with minimum accumulation ofNO2~. After 7 days a much larger percentage of NH4

+

from DAP had been converted to NO2~ and NO3-. Increas-ing the granule spacing beyond 1.0 cm significantly in-creased disappearance of NH4

+ whereas NO2~ productionincreased significantly beyond the 0.5 cm-spacing. At 10days granule-spacing effect on NH4

+ disappearance andNO2~ accumulation was similar. Relatively little NH4+ orNO2~ remained after 14 days. After 4, 7, and 10 days,NO3~ concentrations were significantly greater with thegreater contact surfaces of the mix treatment, but after14 days of incubation there were no differences in NO3~concentrations between the different spacings.

The relatively large accumulations of NO2~ during thefirst 10 days may be attributed to the toxicity of free NH3to nitrite-oxidizing soil organisms such as Nitrobacter un-der the alkaline regime. Several investigators (1, 2) havereported free NH3 toxicity to Nitrosomonas. Ammonia tox-icity to Nitrosomonas would explain the increased rate ofNH4

+ oxidation (and increased NO2~ accumulation) withincreased geometric spacing since maximum free NH3concentration would probably occur at zero spacing. Theeffect of zero granule spacing would approximate a large,single granule. Hauck and Stevenson (9) have shown thatincreasing the granule size increased NO2~ production pre-sumably by an increase in free ammonia concentrationwithin the granule site. When fertilizer granules were pul-

IncubationtimeDays

4

7

10

14

Granulespacing

cm1.51.00.50.0mixBSD, .051.51.00.50.0mixBSD, .051.51.00.50.0mixBSD, .05

1.51.00.50.0mixBSD, .05

Added N extractedNB.+

67.567.267.262.748.010.421.523.327.527.725.06.06.98.6

15.016.810.17.02.72.43.24.42.4ns

NOj"Cf

11.510.97.27.32.64.6

41.740.432. 129.00.67.4

36.628.620.220.20.08.6

13.44.84.30.00.06.8

NO3~

0.61.11.01.84.92.8

15.313.820.916.225.08.2

28.635.736.326.543.311.361.371.567.773.062.5ns

* Statistical differences between means were evaluated by Tukey's honest significancedifference (BSD). Nonsignificant (NS).

verized (mix treatment), free NH3 concentrations werelikely minimized; thus, nitrification would proceed rapidlyto completion with little NO2~ accumulation.

The (NH4)2SO4 granule-placement data (Fig. 1) showa rapid decrease in NH4

+ with approximately 67% (230ppm) of the added N present as NO2~ after 10 days. Al-though geometric effects > 1 cm were not studied, NO2~production from the (NH4)2SO4 was greater than from theDAP granules. The same geometric trend was apparent,but the differences were not significant. The rate of nitrateproduction was much slower than from DAP granules withonly 30% N recovered as NO3~ after 14 days. This wasprobably due to greater salt solubility and activity of(NH4)2SO4. The greater salt-soil contact area of the mixtreatment produced significantly more NO3~, althoughgeometric pattern resulted in no significant NO2~ accu-mulation differences.

Nitrification of urea (Fig. 2) was slower than that ofeither DAP or (NH4)2SO4. After 10 days, approximately80% of the added N was in the form of NO2~. No appre-

HSD(.Q5)

Q 100

12 14

Fig. 1—Extractable NH4 + -, NO2~-, and NO3--N as a functionof time where nine (NH4)2SO4 granules (20.5 mg N) wereplaced at various geometric patterns in 60 g of Ulen soil.

12 14

Fig. 2—Extractable NH4 + -, NOa~-, and NO3~-, N as a func-tion of time where nine urea granules (20.5 mg N) wereplaced at various geometric patterns in 60 g of Ulen soil.

1000 SOIL SCI. SOC. AMER. PROC., VOL. 35, 1971

ciable geometric effect occurred during the first week.Geometric effects were evident at 10 and 14 days, withsignificant geometric effects on NO2~ occurrence at 14days. After 14 days, geometry had resulted in no signifi-cant differences in NO3~ concentrations either as granulesor mix. Single granule studies to be discussed showed thatNH4

+ mobility (or as free NH3) was maximized by alka-linity produced by the decomposition of urea granules.Thus, in addition to the high NO2~ levels produced byapparent free ammonia, the geometry effects were mini-mized by the mobility of NH4+ or free ammonia aroundurea granules.

Four-Granule Studies

Figure 3 shows recovery of the three ions after four DAPgranules were incubated for 7 days in the Ulen soil. Incontrast to the nine-granule data shown in Table 2, NO2~significantly decreased with greater granule spacing. Thecombined decrease of NH4

+ and NO2~ was accompaniedby rapid increase in NO3~. Lower fertilizer salt concen-tration of the four-granule study apparently reduced tox-icity (NH3 implied) to the NO2~ oxidizers with increasedgranule spacing. Total extracted N was erratic but increasedsignificantly with spacing; this suggests volatilization lossand/or precipitation-product formation at lesser spacings.Hauck and Stephenson (9) postulated precipitation ofCa(NH4)2(HPO4)2 • H2O at the granule site of DAP. Ifsuch products form, they would probably not be measuredby the steam distillation analysis method employed.

Extractable N for four (NH4)2SO4 granules after 7 daysof incubation is shown in Fig. 4. Although geometric effectbeyond 1.5 cm was not studied, it is evident that increasedspacing decreased extractable NH4

+ with little effect oneither NO2~ or NO3~ concentrations. Nitrate comprised35% or more of added N contrasted with negligibleamounts in the nine-granule study (Fig. 1). Evidently,lower salt concentration increased nitrification. Nitrate con-

aLU

5cczs

70

60

50

40

30

20

10

- Total Extractable N

HSDC01)

-11.2

-12.4

NO;

0 05 1.0 1.5 2.0 2.5GRANULE DISTANCE, (cm)

Fig. 3—Effect of granule distance on extractable N at 7 dayswhere four DAP granules (10 mg N) were added to 180 g ofUlen soil.

centration approximately equalled that of NO3~; whereas,in the nine-granule study maximum NO2~ comprised 70%of the added N. Total extractable N was not significantlyeffected by spacing, but concentration was higher thanfor DAP.

Four-granule studies were also conducted with urea.Resulting data were essentially the same as with (NH4)2-SO4 and are therefore not reported.

Single-Granule Studies

Single-granule studies were conducted to more ade-quately explain geometric effect around individual fer-tilizer granules resulting from variations in solubility, hy-drolysis, ion migration, and from effects from medium pH.Declining concentrations of NH4

+ and pH of the soil atincreasing distances from a single granule of each of thethree N-salt sources after 5.5 days are shown in Figs. 5 and6. The lower solubility of DAP (Table 1) and slower mi-gration rate of phosphate ions resulted in relatively highNH4

+ concentrations within 1 cm of the granule relativeto NH4+ of the other two N salts. The relatively high NH4+concentration in the outer sampling periphery of the ureagranule suggests some solution and lateral diffusion asurea, as (NH4)2CO3 after hydrolysis, or as free NH3. The(NH4)2SO4 granules, having higher initial solubility, wouldionize most rapidly. The lateral movement of the NH4

+

through the moist Ulen soil would be initially slower from(NH4)2SO4 since it hydrolyzed to an acid environment;therefore, movement as free NH3 would have been mini-mized.

Figure 6 shows that urea hydrolysis and ammonificationproduced little change in the natural soil pH (8.25 at 5.5days), whereas both DAP and (NH4)2SO4 produced amarked pH depression to 7.65 immediately adjacent to

.the granule. This pH depression had essentially disap-peared beyond 1.5 cm from the DAP granule. The closerelation of NH4

+ concentration with soil pH is probablydue to low mobility and movement of the P ions from thegranule site. Precipitation of slightly soluble compounds,such as Ca(NH4)2(HPO4)2 • H2O (9) may partially

'0 0.5 1.0 1.5GRANULE DISTANCE,(cm)

Fig. 4—Effect of granule distance on extractable N at 7 dayswhere four (NH4)2SO4 granules (10 mg N) were added to180 g of Ulen soil.

BEZDICEK ET AL.: SOIL-FERTILIZER GEOMETRY & NITRIFICATION 1001

1.000

CO

1.0 1.5 2.0 2.5 3.0 3.5 4.0DISTANCE FROM GRANULE,(cm)

Fig. 5—Distribution of NH4 + -N around single DAP, (NH4)2SO4, and urea granules (40 mg N) in 450 g of Ulen soil at5.5 days.

account for high concentrations of NH4+ near the granule.

The large changes in soil pH and NH4+ adjacent to thegranule probably explain the larger geometric effects ob-served for DAP in the nine-granule study. The more dif-fuse pH and NH4

+ patterns which occurred for both ureaand (NH4)2SO4 probably minimized the granule effect.

Because of the implication of free NH3 toxicity in NO2~oxidation, the undissociated NH3 was calculated in eachsampling zone on the basis of soil pH and total NH4

+ (Fig.7). Concentration of NH3 for urea was highest of all car-riers in all zones from the granule site. However, the NH4

+

concentration within the granule site was lowest from urea(Fig. 5). The relatively high pH for urea is largely respon-sible for the high NH3. The order of NH3 concentrationwas urea > DAP > (NH4)2SO4 which was proportionalto the granule pH; thus carriers hydrolyzing to higher pHproduced more NH3.

By comparison of total NO2~ accumulation from thegeometric studies of the three N-carriers (Table 2 and Fig.1 and 2), urea, which produced the greatest NH3 alsoproduced the largest amount of NO2~. However, (NH4)2-SO4, which hydrolyzed to the lowest pH and producedthe lowest NH3 level, produced considerable NO2~. Ap-parently, factors other than high pH and high NH3 wereoperative in inhibiting the soil NO2~ oxidizers. Salt effectsfrom sulfate could be questioned.

It is interesting to note the comparatively lower NO2~levels for DAP granules in Table 2 since DAP producedconsiderable NH3 as shown in Fig. 7. However, total NH4

+

and NH3 was largely restricted to the immediate granulesite; this may account for the lower total NO2~ as well as

1.0 1.5 2.0 2.5 3.0 3.5 4.0DISTANCE FROM GRANULE,(cm)

Fig. 6—Distribution of soil pH around single DAP, (NH4) 2SO4,and urea granules (40 mg N) in 450 g of soil at 5.5 days.

"10 1.5 2.0 2.5 3.0 3.5 4.0DISTANCE FROM GRANULE, (cm)

Fig. 7—Distribution of calculated NHs around single fertilizergranules at 5.5 days.

the greater geometric effects observed. High NH3 levelswithin and beyond the urea granule probably causedgreater inhibition of NO2~ oxidation and minimized gran-ule geometric effects.

SUMMARY

These studies have shown that NO2~-N can accumulateup to 260 ppm with certain N carriers in the alkaline soilstudied (pH 8.3). Urea which hydrolyzed to the highestpH also produced the greatest amount of NO2~ and freeNH3. Nitrogen carriers intimately mixed with soil nitrifiedfaster than their respective granular forms. Such trendwas most evident for DAP. Increasing the distance be-tween nine granules promoted NH4

+ oxidation with subse-quent increase in NO2~ for DAP. Nitrite accumulation withincreased granule spacing was less evident for (NH4)2SO4and urea. Single-granule studies suggested that the greatergeometry effects obtained for DAP were because NH4

+

was confined largely to the granule site; whereas for(NH4)2SO4 and urea, NH4

+ was more diffusely distributed.

ACKNOWLEDGMENTThe authors wish to express appreciation to Dr. C. L. W.

Swanson, Agronomist, Texaco Inc., and to Texaco Inc. for thefinancial support received.

1002 SOIL SCI. SOC. AMER. PROC., VOL. 35, 1971