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Waste Management 28 (2008) 1432–1440

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Fertilization of maize with compost from cattle manure

supplemented with additional mineral nutrients
M.V. Gil, M.T. Carballo, L.F. Calvo *

Chemical Engineering, Institute of Natural Resources, University of León, Avda. Portugal, 41, 24071 León, Spain

Accepted 15 May 2007

Available online 10 July 2007

Abstract

An alternative approach for cattle manure management on intensive livestock farms is the composting process. An industrial-scale
composting plant has been set up in northwest Spain for producing compost from cattle manure. Manure composting involved an
increase in pH, electrical conductivity (EC), cation exchange capacity (CEC) and NO 3 AN concentration, and a decrease in temperature,
moisture content, organic matter (OM) content, NHþ 4 AN concentration and C/N ratio. Cu, Zn and Ni concentrations increased due to
the reduction of pile mass during the composting process. The resulting compost was applied to a field to study the viability of applying
this compost combined with a nitrogen mineral fertilizer as a replacement for the mineral fertilization conventionally used for maize (Zea
mays L.). The thermophilic phase of the composting process was very prolonged in the time, which may have slowed down the decom-
position of the organic matter and reduced the nitrification process, leading to an over-short maturation phase. The humification and
respirometric indexes, however, determined immediately after compost application to the soil, showed it to be stable. Compost applica-
tion did not decrease the grain yield. A year later, soil pH, OM content and CEC were higher with the compost treatment. Total P, K, Ca
and Na concentrations in compost-amended plots were higher than in mineral-fertilized ones, and no significant differences between
4 AN; NO3 AN, available P, Mg and B. Compost caused no heavy metal pollution

treatments were found in soil concentrations of NHþ
into the soil. Therefore, this compost would be a good substitute for the mineral fertilizers generally used for basal dressing in maize
growing.
 2007 Elsevier Ltd. All rights reserved.

1. Introduction can produce nitrogen losses by volatilization or leaching

into surface waters and groundwater. Indeed, Basso and
Nowadays, the amount of organic wastes produced by Ritchie (2005) found that more nitrate was leached as a
the cattle on intensive livestock farms is significant; further- result of untreated manure use than from compost applica-
more, they are produced at specific points and daily. In tion. An alternative approach for manure management is
such conditions, farmers do not have enough available composting, which implies organic matter stabilization,
agricultural land on which to dispose of the produced man- sanitization regarding weeds and pathogens, deodoriza-
ure in appropriate doses. Furthermore, there are many tion, improvement of handling of the product and possibil-
problems associated with the storage and use of raw man- ity of safe storage and transportation (Parkinson et al.,
ures, such as odour, emissions or leaching of hazardous 2004).
compounds and health risks, loss of nutrients and difficulty This type of manure management is implemented in a
of handling and application. The accumulation of manure company placed near the city of León in northwest Spain,
can therefore cause problems of environmental pollution. which operates a livestock farm where 13,500 cows are fat-
On the other hand, the agricultural use of raw manure tened per year, generating approximately 247 m3 day1 of
cow dung. Two other large-scale projects are now at the
*
Corresponding author. Tel.: +34 987 291 844; fax: +34 987 291 839. planning stage, and the plant may expand to 17,000 ani-
E-mail address: lfcalp@unileon.es (L.F. Calvo). mals. As composting seemed to be the most suitable way

0956-053X/$ - see front matter  2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.wasman.2007.05.009
 M.V. Gil et al. / Waste Management 28 (2008) 1432–1440 1433

of recycling this manure, the company has recently built a crop, and/or the compost would not supply sufficient quan-
composting plant that processes about 90,000 m3 of cattle tities of nutrients at the right moment. Bazzoffi et al. (1998)
manure per year. It is hoped that the finished compost found that urban refuse compost produced a lower maize
could be sold as organic fertilizer for agricultural or horti- grain yield than mineral fertilization, as did Businelli
cultural purposes and gardening. However, before the et al. (1990), who observed a decrease of maize yield with
compost will be applied to soil for crop production, its compost compared with mineral fertilization. A combina-
nutritional value and possible negative effects need to be tion of compost application with a nitrogen mineral fertil-
assessed (Zhang et al., 2006). Soumaré et al. (2003a) recom- izer meeting N needs was therefore proposed for maize, a
mend the characterization of composts for use in agricul- high nutrient-demanding crop, to replace the conventional
ture with regard to their agronomic value (availability of mineral fertilizer. Thus, the objective of this work was to
elements) and heavy metal contents. prove the feasibility of using bovine manure compost
Manures and other organic wastes, such as municipal instead of chemical basal fertilizer for maize grown under
solid wastes and agricultural or forest residues, have long semiarid conditions, in order to launch this organic fertil-
been used in Spain for fertilizing crops and maintaining soil izer commercially.
fertility. With the advent of chemical fertilizers, organic The aim of this study is to ascertain whether the compo-
wastes were gradually replaced by mineral products, sting of cattle manure on an industrial scale is an appropri-
because of their lower cost and easier transportation and ate way of managing manure on intensive livestock farms
application (Pomares and Canet, 2001). The massive use and if the final product can be sold and applied to the soil
of chemical fertilizers in intensive agriculture has greatly as a fertilizer and/or amendment without risks. For
increased concern for the declining fertility of soils. Soil answering this question, the following objectives were
nutrient depletion is the result of increasing pressure on solved: (i) to establish whether the composting process pro-
agricultural land, resulting in higher nutrient outflows that duced an agronomically valuable compost, with chemical
are not compensated for (Wopereis et al., 2006). Organic characteristics fulfilling the requirements of Spanish and
inputs are required to ensure that intensive systems do European legislation; and (ii) to assess the agronomic
not threaten the sustainability of land use. However, small potential of the cattle manure compost thus obtained for
farmers are reluctant to use organic wastes or composts growing maize, comparing the effect of this compost sup-
due to uncertainty as to their benefits and safety. plemented with mineral fertilizer with the conventional
Several works have shown beneficial effects of compost mineral fertilization on maize yield and on soil chemical
application for crop production. In this regard, Aggelides properties and nutrient and heavy metal concentrations
and Londra (2000) assessed the effects of compost pro- in soil.
duced from municipal solid waste (MSW) and sewage
sludge on soil physical properties. Borken et al. (2002) 2. Materials and methods
studied the effects of compost from organic household
waste on soil properties in degraded forests. The effects 2.1. Composting process
of composted cotton-gin trash and composted garden
waste properties were examined by Bulluck et al. (2002). The compost used was produced from bovine manure
Debosz et al. (2002) evaluated the effects of sewage sludge (faeces and straw bed) in the composting plant mentioned
and household compost on the physical, chemical and above. Here, the average temperature is 1.82 C in January
microbiological properties of soil. Vagstad et al. (2001) and 19.92 C in July, with an average daily minimum of
studied the effects of paper sludge compost on barley and 1.66 C in January and an average daily maximum of
wheat crops. Cuevas et al. (2003) studied the effects of var- 23.78 C in June (Spanish National Meteorological
ious composted sewage sludge rates on a maize crop. Sta- Service).
matiadis et al. (1999) applied compost from green wastes, The cow dung is collected daily from the sheds and
cow manure and spoiled hay to a broccoli field, and Sou- transported to the compost plant in trailers. The manure
maré et al. (2003 b) used municipal solid waste compost is scraped up from the slightly sloping concrete floor with
on soils with a ryegrass crop. Basso and Ritchie (2005) used a shovel-loader hitched to a small tractor. The composting
dairy manure compost in maize crop. process consists of two stages. The first phase of intensive
Cattle manure compost was applied to the soil as a fer- decomposition is performed by means of mechanical turn-
tilizer for maize growing. Nevertheless, compost alone ing and forced aeration, in a completely roofed silo
could not work as effectively as chemical fertilizers because (45 · 60 m) with eight silo units (each 4.5 m wide and
its mineralization time is generally unknown and therefore, 49 m long). The dung is dumped from the transport vehicle
the availability of nutrients to plants is also uncertain. directly into the silos, which are filled up to a height of
Sometimes, compost application as a substitute for the 2.1 m. Daily turning is carried out with a diesel-driven
conventional mineral fertilization is problematic because Backhus 9.45 mobile silo turning machine, specially
some crops have high nutrient needs or punctual needs designed for composting biological waste and sludge in lin-
throughout their growth cycle, and large quantities of com- ear or tunnel plants. It is equipped with a crawler drive fit-
post would be necessary to satisfy the overall needs of the ted with rubber chains, and runs along the silo walls, which
1434 M.V. Gil et al. / Waste Management 28 (2008) 1432–1440

are 2.3 m high and 30 cm thick, and made of concrete. The Table 1
rotor, which is mounted horizontally, is equipped with Main characteristics of the compost used in the field experiment (means of
three replicates) (dry weight)
throwing racks and tearing blades, and the rotor drive is
effected hydraulically via the rotor sword. During the turn- Parameter Compost
ing process, the rotting material is transported in a longitu- Moisture content (% f. w) 29.1
dinal direction and thrown backwards via the rotor and pH, H2O 9.6
Electrical conductivity (dS m1) 3.35
deposited loosely. When the linear turner arrives at the Organic matter (%) 56.96
input side, the rotor is brought into transport position by Total nitrogen (Kjeldahl) (g kg1) 21.3
using special retraction kinematics. The machine drives NHþ 4 –Nðg kg Þ
1
0.8
1
without an additional transport alliance manually from silo NO3 –Nðg kg Þ 1.1
to silo via ramps. Forced aeration accelerates the decompo- Total phosphorus (mg kg1) 10400
K (mg kg1) 21700
sition and works with a feedback control in order to main- Ca (mg kg1) 23700
tain temperature below 80 C and oxygen content above Mg (mg kg1) 5680
5–10%. The forced aeration system consists of fans to pull Na (mg kg1) 4480
air through the composting mass from perforated piping, B (mg kg1) 21
connected to the fans, which deliver the air. The pipes Fe (mg kg1) 1030
Mn (mg kg1) 176
are installed along the ground of the silos. This system Cu (mg kg1) 53.8
makes possible the removal of excess heat and the use of Zn (mg kg1) 82
biofilters to treat the air. After the first phase of compo- Cr (mg kg1) 2.6
sting inside the silo, about two weeks, the material leaves Ni (mg kg1) <5
the silo. In another roofed area, measuring 200 m · 55 m, Pb (mg kg1) 1.5
Cd (lg kg1) 88
the compost is maintained in trapezoidal piles (2.0 m high Hg (lg kg1) <100
with a 3 m · 4 m base) and turned daily with a turning
machine during the maturation phase for an additional
eight weeks. The turning improved both the homogeneity
of the composting substrate and the oxygen supply. Water considerable seasonal thermic oscillations. The mean
was added regularly by aspersion to avoid moisture level annual rainfall is about 556 mm. The average annual tem-
dropping below 35–40%. perature is 10.9 C, with an average daily minimum of
Three windrows were processed simultaneously and 0.8 C in January and an average daily maximum of
monitored. Temperature was monitored at three depths: 27.2 C in July (Spanish National Meteorological Service).
0.25, 0.5 and 1.0 m. The organic material in the three win- The purpose of the experiment was to study the possibil-
drows was processed in the silo for the first 20 days and ity of using bovine manure compost as a substitute for the
then moved to the trapezoidal piles, where it was kept until usual basal dressing in maize growing. Two treatments
day-75. After that, the composted material was stored for were used: compost plus mineral application and conven-
about 15 days without turning until sieved (<5 mm mesh) tional mineral fertilization as a control. The total surface
in the composting plant. Representative samples were of soil used in this experiment was 5200 m2. The experi-
taken approximately every two weeks by mixing 10 subs- mental set-up was a completely randomized design with
amples taken from the whole profile of the silo or pile. A three replicates per treatment, i.e., six subplots of
part of each sample was immediately frozen (20 C) and 23 m · 28 m were used. The plantation frame consisted of
kept for NHþ 4 AN analysis and the other part was air dried 0.8 m between lines with a distance of 0.4 m between
and ground to 2 mm for other analyses. plants. The compost dose was determined by its nitrogen
content, on the basis that it only releases 50% of its organic
2.2. Field experimental set-up N in the first year of cultivation (Urbano, 1995; Zublena
et al., 1996), which resulted in the application of 8.94 t ha1
The end product was used for land application. Charac- of compost (dry basis) as a basal dressing plus 650 kg ha1
teristics of this compost are shown in Table 1. The heavy of calcium ammonium nitrate (27% of N) top dressing.
metal concentrations were within the levels set out in the Likewise, mineral fertilizer was applied according to the
legislation in force (Royal Decree 824/2005 of the 8th July usual practice of the grower, 750 kg ha1 of N, P, K (N,
concerning fertilizers in Spain, and the European Commis- P2O5, K2O): 6, 11, 22 fertilizer as a basal dressing plus
sion’s Working Document about Biological Treatment of 650 kg ha1 of calcium ammonium nitrate (27% of N)
Biowaste, Second Draft, of 2001). The field experiment top dressing. Both basal treatments were effected in April
was conducted in collaboration with a farmer in the prov- 2002, compost being spread on the soil surface and imme-
ince of León in northwest Spain. The experimental field diately raked evenly for each subplot and then rototilled
was established on a sandy clay loam soil near Quintana into the soil to a depth of about 20 cm. The top fertilization
de Rueda at 4234 0 5100 N and 515 0 2400 W, where the crop was applied in June 2002. The grain was harvested in April
grown was maize (Zea mays L.). The climate is continen- 2003, and the top 15–20 cm of soil were sampled and ana-
tal, characterised by cold winters and hot summers, with lysed at the beginning of May 2003.
 M.V. Gil et al. / Waste Management 28 (2008) 1432–1440 1435

Maize plants in the middle 20 rows in each subplot were was calculated: Q4/6 = A472/A664, which is often called
harvested separately to estimate the grain yield. A random the humification index. The respiration rate of the final
subsample of each fraction was oven-dried at 60 C (Wope- compost was determined with the Oxitop measuring sys-
reis et al., 2006), ground to pass through a 0.5 mm mesh tem (Veeken et al., 2003), which measures the drop of oxy-
and then analysed. Soil samples were also taken from the gen pressure in a closed system while the carbon dioxide
20 inner rows of each treatment subplot. Each sample con- produced by the respiration is trapped in a NaOH solu-
sisted of a mixture of ten cores randomly collected with an tion. The oxygen consumption is directly related to the
auger from each subplot. They were taken from the top 15– measured pressure drop. Thus, cumulative oxygen uptake
20 cm of soil, allowed to air dry and sieved through a 2 mm (COU) during 96 h and the oxygen uptake rate or respira-
mesh before analysis. tion rate (OUR) were calculated.

2.3. Analytical methods 2.4. Statistical analyses

The protein content of the maize was determined by Data were evaluated by one-way ANOVA. Tukey’s test
acid digestion (Kjeldahl method), followed by titration. was used for means comparison and statistical significance
Temperature during composting process was measured of hypotheses was assessed at a = 0.05. All statistical anal-
with a temperature probe. Bouyoucos’ densimeter method yses were performed using SPSS v. 11.0 software.
was used to determine soil texture, which was obtained by
fitting the percentages of sand, silt and clay fractions to 3. Results and discussion
the USDA soil texture classification triangle (MAPA,
1994). The moisture content was determined by drying a 3.1. Evolution of the composting process
sample at 105 C until constant weight was reached; pH
was measured in ratios of 1:2.5 soil/water and 1:25 com- The temperature was measured at depths of 0.25, 0.5
post/water; and electrical conductivity (EC) was measured and 1.0 m. The temperatures reached about 70 C after
in ratios of 1:25 compost/water (MAPA, 1994). The 20 days of composting, and they decreased over time
organic matter (OM) content of the composting samples (Fig. 1a). The composting material was kept at 55 C or
was analysed by loss on ignition at 430 C for 24 h (Nav- above for a minimum of two weeks, which contributed to
arro et al., 1993). Soil organic matter content was deter- the hygienization and sanitization of the end-product
mined by the Walkley–Black wet digestion method through pathogen, weed and seed reduction (Paredes
(Walkley and Black, 1934). The cation exchange capacity et al., 2005). The high temperatures of composting indicate
(CEC) was determined in BaCl2 extracts by ICP–AES that further research could be undertaken about compo-
spectrophotometry, according to Hendershot and sting as a detoxification process, which is outside the scope
Duquette (1986). Total nitrogen concentrations were of this paper. During the process, temperatures were higher
determined according to the Kjeldahl method (MAPA, at depths of 0.5 and 1.0 m than at 0.25 m, because the heat
1994). Ammonium–N concentrations were determined in loss through diffusion is greater at lower depths. From day-
KCl extracts using a pH ion-meter coupled with an 20 to day-40, higher temperatures were recorded at 0.5 m
ammonium ion selective electrode (APHA et al., 1992). than at 1.0 m depth, but the difference disappeared around
Nitrate–N concentrations were determined in CaSO4 the middle of the process. This can be explained consider-
extracts using a UV–vis spectrophotometer, according to ing that 0.5 m is the most active zone since the concentra-
the method described by Sempere et al. (1993). Available tion of oxygen is usually lower at the bottom of the pile due
phosphorus was determined in HCO3Na extracts using a to the higher compaction of the material (Cayuela et al.,
UV–vis spectrophotometer (Olsen method). Concentration 2004). A temperature of 80 C as a control value for the
of total P was determined by ICP–AES spectrophotome- forced aeration system may be too high because it can
try after digestion in HNO3 65% in a pressurized micro- reduce the microbial activity, the waste being decomposed
wave. Concentrations of Ca, Mg, K and Na were slowly, although thermal organisms may be also present.
determined in ammonium acetate extracts by atomic Thus, a good threshold temperature would be 65 C; how-
absorption spectrometry; concentrations of Fe, B, Zn, ever, 80 C was the temperature used in the composting
Cu and Mn were determined in DTPA and CaCl by plant to control aeration. The thermophilic phase was very
ICP–AES spectrophotometry; concentration of B was long, with temperatures around 50 C up to day-60 of the
determined by ICP–AES in water extracts; and concentra- process, perhaps owing to the high initial temperatures,
tions of Ni, Cr, Pb, Cd and Hg were determined by ICP– which slowed down the decomposition of the organic mat-
MS spectrophotometry after digestion in HNO3 65% in ter. It could mean that an insufficient maturation phase
pressurized microwave. The humification index in the final occurred. Finally, the temperature of the pile decreased
compost was determined in NaOH extracts by means of to about 38 C and was homogeneous. The moisture con-
measuring the absorbance at k = 472 nm (A472) and tent of the pile (Fig. 1a) tended to decrease throughout
664 nm (A664) (Sapek and Sapek, 1999). The following the composting process, reaching about 30% in the final
absorbance ratio indicating the degree of humification compost.
1436 M.V. Gil et al. / Waste Management 28 (2008) 1432–1440

a 80 80 advantage in acidic soils, although the high pH could also

70 70 cause a reduction in the availability of some nutrients in
other soils. The increase in pH is due to complex chemical

Moisture (%)
60 60
T(ºC)

50 50
and biological reactions occurring during the mineraliza-
tion of organic matter (Cayuela et al., 2004), such as the
40 T (0.25 m) 40
T (0.50 m)
degradation of acid-type compounds like carboxylic and
30 30
T (1.00 m) phenolic groups or the mineralization of compounds, such
Moisture (%)
20 20 as proteins, amino acids and peptides, to ammonia. The
0 10 20 30 40 50 60 70 80
Composting time (days) solubilization of the ammonia led to the formation of
ammonium and an increase in pH.
10 6 EC also increased during the process (Fig. 1b), as
b
9 5 reported by other authors (Sánchez-Monedero et al.,
2001), who stated that EC increases during composting

EC(dS/m)
4
8 owing to the concentration of ions as the weight of a pile
pH

3
7 decreases and also because of the increased concentration
2
of nutrients, such as nitrate. A decrease in EC, instead,
6
pH 1 could have been explained by losses due to leaching. The
EC (dS/m)
5 0 addition of water to the pile was well controlled and the
0 10 20 30 40 50 60 70 80
loss of soluble salts by leaching seemed to be avoided.
Composting time (days)
On the other hand, EC values were not too high and no
problems of soil salinization were foreseen.
c 5000 1500

1250 There was a major decrease in OM content during the

4000
N-NO3(mg/kg)
N-NH4(mg/kg)

first stage of composting (Table 2), after which it remained

1000
3000 practically constant until the end of the process. This may
750
2000
be due to the mineralization of labile organic compounds
500 which mainly occurs during the thermophilic phase,
1000
N-NH4 (mg/kg) 250 whereas, humification prevails over mineralization during
0
N-NO3 (mg/kg)
0 the maturation stage. The OM content of the final compost
0 10 20 30 40 50 60 70 80 was higher than the minimum value of 35% established by
Composting time (days)
the Spanish compost legislation. The CEC value (Table 2)
Fig. 1. Evolution of main parameters during composting of cattle increased slightly during the composting process, during
manure: (a) temperature and moisture content, (b) pH and electrical the OM humification.

conductivity (EC) and (c) NHþ
4 –N and NO3 –N concentrations. Total N concentration decreased during the high tem-
perature phase (Table 2), owing to the mineralization of
On the other hand, odours were not a major problem the organic N, to remain constant thereafter. The
during the process, probably owing to the relatively high NHþ 4 AN concentration increased at the beginning of the
initial composting temperatures and also to the biofilter process (Fig. 1c), when OM degradation was more intense
located in the ventilation system. When composting tem- and ammonium was produced through the organic N min-
peratures are high, microbes probably utilize the bound eralization, but decreased later, because of either volatilisa-
oxygen from the biomass more effectively, which reduces tion losses or nitrate formation. In this experiment, the
the need for atmospheric oxygen and keeps the level of very high initial ammonium concentration fell during the
odorous compounds down. composting process, but the final NHþ 4 AN concentra-
There were an increase in pH from 7.3 to 9.6 (Fig. 1b), tion was double the 400 mg kg1 value established as a
mainly during the first half of the process. This could be an compost maturity index (Zucconi and de Bertoldi, 1987).

Table 2
Evolution of main parameters during composting (dry weight)
Composting time (days) OM (%) CEC (meq/100 g) Total N (g kg1) C/N Total P (mg kg1) K (mg kg1)
0 71.36 c 103.70 a 25.5 b 16.27 b 9310 a 21621 a
10 59.32 b 101.89 a 21.3 a 16.19 b 11600 ab 23959 a
21 59.19 b 111.09 b 20.1 a 17.12 b 11300 ab 26817 ab
32 50.49 a 105.80 ab 22.0 a 13.34 a 13800 ab 27799 ab
47 50.26 a 111.30 b 22.3 a 13.10 a 16750 b 32490 b
62 55.29 ab 107.20 b 21.5 a 14.95 a 15500 b 29988 b
75 56.96 ab 109.30 b 22.3 a 14.85 a 10400 a 21700 a
OM: organic matter, CEC: cation exchange capacity. Values followed by the same letter in the same column are not statistically different according to the
Tukey’s test at 5% probability level.
 M.V. Gil et al. / Waste Management 28 (2008) 1432–1440 1437

It is therefore of major importance to ascertain the behav- OUR was 5.3 mmol O2 kg1 VS h1, which was lower than
iour of this compost into the soil. the 15 mmol O2 kg1 VS h1 value proposed for stable
The NO 3 AN concentration also increased up to about composts by Veeken et al. (2003). Moreover, the OUR
day-50 of composting (Fig. 1c), to remain constant thereaf- can be assumed to be the dynamic respiration index
ter, when there was less ammonium available to the nitrify- (DRI), proposed by the Working Document mentioned
ing bacteria. Therefore, nitrification occurred during most above. Its value was 168 mg O2 kg1 VS h1, which was
of the composting process, which is significant from an lower than the 1000 mg O2 kg1 VS h1 established in that
agricultural point of view. An inexplicable peak in the document for stable biowastes. Therefore, the compost can
NO 3 AN concentration occurred on the 20th day of the also be considered stable according to the respirometric
process. Usually, during the nitrification process in the final indexes. These results can be due to the fact that the respi-
stages of composting, nitrifying bacteria lower the pH rometric measurements were realized immediately after the
owing to the liberation of hydrogen ions. In this experi- application of compost to the soil, which was carried out
ment, in the final days of composting the nitrification pro- approximately 1 month after the end of the monitoring of
cess was reduced, with no noticeable effect on pH. On the the composting process, after it had been sieved at the com-
other hand, nitrification occurs mainly under mesophilic posting plant.
conditions (Mathur et al., 1993), and therefore, an inade- In relation to the total P and K concentrations (Table 2),
quate maturation-mesophilic phase, indicated by the high there is a limited increase both of total P and K from the
temperatures, could explain the reduction of the nitrifica- beginning to day-47, followed by a decrease to levels
tion process and the high pH value. Despite possible insuf- slightly higher than the initial ones by day-75, although
ficient maturation by day-75 of the process, the company not significantly different from them. This slight increase
finished the composting, probably because it works in a can probably be explained by the concentration of P and
continuous process and the material of the next group of K due to the reduction in pile weight.
silos had to be processed. They stored the final compost No major change occurred in the concentrations of Cr,
in order to sieve it and further maturation could, however, Pb, Cd and Hg over time (Table 3). The concentrations of
have occurred during storage. Cu, Zn and Ni seemed to increase during the process
The C/N ratio is an index of the degree of OM stability. (Table 3), also perhaps because of the concentration effect
In this experiment, it decreased (Table 2), to remain con- caused by the reduction in pile mass. However, final heavy
stant in the final stage of the composting process, indicat- metal values were within the safe levels for plant growth, as
ing the stabilization of the OM. The C/N ratio of the set out in the legislation (the Royal Decree and the EC
final compost was slightly higher than the 12 value estab- working document mentioned earlier).
lished as a maturation index (Bernal et al., 1998), as Cayu- The compost obtained had a high OM content and total
ela et al. (2004) also found in their olive mill waste N, total P, K, Ca and Mg concentrations compared with
composts. However, according to Allison (1973), materials those found by Soumaré et al. (2003a) in farm and MSW
4 AN; NO3 AN, K, Mg, Fe, Cu

with C/N < 15 do not alter the microbiological equilibrium composts. Total N, NHþ
of the soil. The maximum limit of C/N ratio established by and Zn concentrations in the final compost were higher
the Spanish legislation for compost is 20. than those found by Guerrero et al. (2001) in MSW com-
The humification index, Q4/6 ratio, was calculated in the post. Also the final compost had a higher OM content
4 AN; NO3 AN concentrations, and a

final sieved compost and its value was 3.92. Typical values and total N, K, NHþ
of the Q4/6 ratio for humified material are usually <5 (Gie- similar P content, than those found by Bar-Tal et al.
guzyńska et al., 1998), which shows that, according to this (2004) in cattle manure compost, which was also applied
stability index, the compost can be considered stable. COU to the soil.
can be assumed to be equivalent to the AT4 (respiration
activity after four days) and its value was 6.5 mg O2 3.2. Field experiment
kg1. The European Commission’s Working Document
mentioned above establishes the maximum value of AT4 The grain yield was not significantly different between
as 10 mg O2 kg1 for a stabilized material. The maximum treatments, since the maize grain harvested was
Table 3
Evolution of heavy metal concentrations during composting (dry weight)
Composting time (days) Cu (mg kg1) Zn (mg kg1) Cr (mg kg1) Ni (mg kg1) Pb (mg kg1) Cd (lg kg1) Hg (lg kg1)
0 8.2 a 56.0 a 5.36 ab 15.2 a 2.0 79.1 a <100
10 10.9 a 61.2 a 9.27 b 17.7 a 2.0 108.2 b <100
21 11.7 a 60.5 a 3.45 a 14.8 a 2.0 80.5 a <100
32 10.7 a 63.5 a 4.85 ab 19.1 a 1.1 103.0 b <100
47 11.2 a 96.0 b 5.37 ab 27.7 b 2.6 120.9 b <100
62 27.9 b 94.9 b 2.70 a 22.6 b 1.5 109.3 b <100
75 53.8 c 82.0 b 2.60 a – 1.5 88 a <100
Values followed by the same letter in the same column are not statistically different according to the Tukey’s test at 5% probability level.
1438 M.V. Gil et al. / Waste Management 28 (2008) 1432–1440

7580 kg ha1 on the plots treated with mineral fertilizer application of organic amendments, in comparison with
and 7700 kg ha1 on those treated with compost plus min- mineral fertilization.
eral. The Ca, Mg, Fe, Mn, Cu, Zn and B concentrations Neither treatment significantly changed the total N con-
were analysed in the grain (data not shown), but no signif- centration (Fig. 2), but both treatments reduced concentra-
4 AN and NO3 AN to similar values (Fig. 3).

icant differences were observed between the two treatments tions of NHþ
except in Fe concentration, which was higher when com- Thus, the inorganic N concentration into the soil after har-
post plus mineral was applied (36.0 mg kg1 as opposed vest was similar after both treatments, which would not
to 22.6 mg kg1 for mineral fertilization). Significant differ- indicate a deficiency in this nutrient with compost fertiliza-
ences were also observed in the protein content of the grain tion in relation to mineral fertilization. It is thought, how-
for both treatments; it was higher with the application of ever, that it would be useful to measure the plant’s N
compost, 7.7% as opposed to 6.8% for mineral fertilization. uptake to lend more validity to the conclusions.
Analysis of the heavy metal (Cr, Ni, Pb, Cd and Hg) con- The available P concentration significantly increased to
centrations of the maize grain showed these elements to be the same extent with both treatments (Fig. 3). Total P con-
below the analytical detection limit. centration at the end of the experiment was higher with the
Both treatments significantly increased the soil pH, but application of compost plus mineral (Fig. 3), revealing a
this increase was greater with the application of compost greater amount of organic P in the soil after this treatment.
(Fig. 2). The increase of soil pH with compost shows that Available K significantly increased with the compost,
it could be useful for increasing the pH of acid soils and whereas, its concentration did not change with mineral fer-
it could also avoid the decrease that sometimes happens tilization (Fig. 3).
after successive applications of mineral fertilizers. Paino Compost increased Ca concentration in the soil more
et al. (1996) also reported that soil pH rose when compost significantly than mineral fertilization did (Table 4). Na
was used. concentration also significantly increased with compost
The organic matter content into the soil significantly application, but did not change with mineral fertilization
increased after compost application in relation to initial (Table 4). Mg, Cu and Zn concentrations, on the other
value (Fig. 2), and was finally significantly higher in plots hand, did not change significantly with either treatment
treated with compost than in mineral-fertilized soils.
Increased soil OM improves the physical characteristics
of soil, such as soil water retention and movement, soil b
350 a
structure and porosity (Adegbidi et al., 2003). The compost Initial soil
300
increases the humus content of the soil, which is particu- Mineral fertilizer

larly important for the establishment of the carbon cycle 250 Compost + Mineral
b
(Leirós et al., 1993). The increase of organic matter content 200
b b
a a
into the soil by compost application is a positive effect, 150
although it does not change much over such a short period 100
and the impact of the compost should be assessed over 50 b
a a b a
a a
more than one year after successive applications of organic 0
materials. Both treatments also increased the CEC of the N-NH4 N-NO3 Available P Total P K (g/kg)
(mg/kg) (mg/kg) (mg/kg) (mg/kg)
soil, but the final value was higher with compost (Fig. 2).
The results are also in agreement with Bulluck et al. Fig. 3. Nutrient concentrations of maize soil initially and after the first
(2002), who found higher CEC values in the soil after the harvest. Values followed by the same letter in the same parameter are not
statistically different according to the Tukey’s test at 5% probability level
(n = 3).

10.0 c Initial soil

Table 4
Mineral fertilizer
c b Properties of maize soil initially and after the first harvest (means of three
8.0 b Compost + Mineral
a replicates) (dry weight)
6.0 a Parameter Initial soil Mineral fertilizer Compost + mineral

4.0
Ca (mg kg1) 1002 a 1293 b 1707 c
Mg (mg kg1) 63 a 88 a 84 a
2.0 a a
b
a
Na (mg kg1) 9a 9a 14 b
a a
B (mg kg1) 0.3 a 0.9 b 0.7 ab
0.0 Fe (mg kg1) 45.3 b 43.7 b 36.0 a
pH OM (%) CEC Total N Mn (mg kg1) 22.4 c 10.5 b 6.6 a
(meq/100g) (g/kg) Cu (mg kg1) 0.6 a 0.7 a 0.6 a
Fig. 2. Chemical properties of maize soil initially and after the first Zn (mg kg1) 0.7 a 0.5 a 0.5 a
harvest. Values followed by the same letter in the same parameter are not Hg (lg kg1) <100 <100 <100
statistically different according to the Tukey’s test at 5% probability level Values followed by the same letter in the same row are not statistically
(n = 3). different according to the Tukey’s test at 5% probability level.
 M.V. Gil et al. / Waste Management 28 (2008) 1432–1440 1439

120
Initial soil safe and agronomically useful product is obtained.
100 Mineral fertilizer a Answering to the first objective, the compost studied had
a a
Compost + Mineral higher organic matter content and lower C/N ratio and
80 heavy metal concentrations than the values established by
60 Spanish and European legislation for compost or stabilized
biowaste. The manure composting process involved an
increase in pH, EC and NO 3 AN concentration and a
40
a a
a
decrease in OM content, NHþ 4 AN concentration and C/N
a a a a a a
20
ratio, giving a final product with characteristics similar
0
Cr (mg/kg) Ni (mg/kg) Pb (mg/kg) Cd (ppb) to those of other composts used for soil application by
other authors. The humification (A472/A664) and respiro-
Fig. 4. Heavy metal concentrations of maize soil initially and after the
first harvest. Values followed by the same letter in the same parameter are metric indexes (COU, OUR), determined after the applica-
not statistically different according to the Tukey’s test at 5% probability tion of the compost to soil, indicated that the compost was
level (n = 3). stable. Moreover, the results confirmed that this compost
would be a good substitute for the conventionally used
basal fertilization for maize, because it was successfully
(Table 4). Nor is there any clear variation in the concentra-
applied as a fertilizer combined with additional nitrogen
tion of B in the soil after the application of compost, and in
mineral fertilizers to complete the overall needs for crops
any event, concentration did not differ significantly from
with high requirements, in this case maize. There were no
when mineral fertilization was used (Table 4).
phytotoxicity symptoms in the plants treated with com-
Soil concentrations of Fe and Mn were lower after com-
post. The grain yield after compost application did not
post application than with mineral fertilization (Table 4).
decrease in relation to conventional fertilization, while
The lower Fe and Mn concentrations with the application
there was an increase in the protein and Fe contents of
of compost may be due to the higher pH and organic mat-
the harvested grain, indicating the fertilising value of the
ter content reached when compost was applied, as the sol-
compost. This compost is therefore a good basal fertilizer
ubility of these elements greatly depends on pH and
for maize growing. Its application improved the chemical
organic matter content decreasing as they increase (Domı́n-
properties and nutrient status of the soil in relation to the
guez, 1997), although perhaps these elements may be less
mineral fertilization and it did not increase the soil levels
abundant in the soil also because of partial absorption by
of heavy metals over dangerous limits. The application of
maize plants.
compost could also be a means of correcting the low
Higher concentrations of K, total P, Ca and Na were
organic matter content of most Spanish agricultural soils,
found after compost application than after mineral fertil-
which would mean an improvement in their fertility. In this
ization, which showed that the compost contributed more
way, there are soil benefits from an organic amendment,
of these nutrients than the mineral fertilizer. Bulluck
while at the same time, useless and sometimes dangerous
et al. (2002) found higher concentrations of Ca and K, as
wastes are recycled with no decrease in crop yield or grain
Soumaré et al. (2003 b) did of K and P after the application
quality.
of organic amendments in comparison with mineral fertil-
izers. Adegbidi et al. (2003) also found higher concentra-
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