11
Organic Fertilizers to Sustain Soil Fertility
ELIGIO MALUSÁ1,2*, LIDIA SAS-PASZT1, S³AWOMIR G³USZEK1 AND
JOLANTA CIESIELSKA1.
ABSTRACT
The maintenance of organic matter in the soil is crucial to assure its physical,
chemical and biological fertility. The continuous development of new processing
technologies, particularly for the treatment of industrial organic wastes, is
providing new potential to produce organic fertilizers. In this chapter we review
the production processes and the results of their field use of three groups of
products that are experiencing a new interest due to innovative processing
methods or application possibilities. Waste by-products of livestock, meat, leather
and other industries can be transformed in high value organic fertilizers,
particularly by enzymatic digestion. The presence of several bioactive molecules
in the extracts from seaweeds induce an increase of antioxidant amounts/
activities in the plant which are improving the nutritional value of fruit and
vegetables, thus enhancing the overall quality and marketable value of fresh
produce. On the other hand, their application improve the tolerance to both
biotic and abiotic stresses of the crops resulting in higher yield. By-products of
coal mining (lignite) or of the biogas production (biochar) applied as soil
amendments or fertilizers, after increasing their nutritional constituents,
positively affect the soil chemical physical and biological properties. These are
major example of the possibility of transforming key waste sources into resources
that can enhance soil fertility and fight against its degradation.
Key words: Animal hydrolyzed proteins, Biochar, Lignite, Seaweeds
extracts.
1
2
Research Institute of Horticulture–Skierniewice–Poland.
CRA-Centre for Plant-Soil Systems–Turin–Italy.
Corresponding Author E-mail: [email protected]
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INTRODUCTION
The management of soil fertility and plant nutrition shall aim to fulfil the
global demand for food and feed minimizing the negative impact on the
environment (Hayati et al., 2011; Lal 2000). The maintenance of organic matter
in the soil is crucial in this respect, assuring its physical, chemical and biological
fertility, which allow the soil to perform the functions within the agricultural
production and the environment (Izaurralde et al., 2001). To this aim and to
combat the progressive reduction in organic carbon in soils policies have been
developed to reduce chemical inputs in agriculture (e.g., the European Union
Directive 91/676) and in the meantime to foster the recycling of organic matter
deriving from different sources into fertilizers.
Organic farming is a specific crop system that relies only on organic
fertilizers. The use of organic fertilizers in organic farming is regulated by
provisions that, depending on the legal framework for the specific country, can
allow different kinds of products. In the European Union, according to the
Commission Regulation (EC) No 889/2008 on organic production they can be
used for maintaining the fertility of the soil or to fulfil specific nutrition
requirements of crops. Of the thirty three kinds of fertilizers and soil
conditioners included in the EU Regulation, seven are products of plant origin,
seven of animal origin, and three are of different origin (peat, mushrooms
culture wastes and stillage). In the US the standard is allowing the use of raw
and composted animal manure, raw and composted plant materials, and plant
or animal materials that have been chemically altered by a manufacturing
process, provided that, the material is included on the National List of synthetic
substances allowed for use in organic crop production (US Organic Foods
Production Act Provisions–National Organic Program).
Some constraints in the organic fertilizers sector derive from their legal
definitions and the establishment of clear quality standards, which are
particularly important for trade at global level. This holds true also for a specific
group of products that are defined as bio-stimulants or growth enhancers,
metabolic enhancers, etc. Nevertheless, the continuous development of new
processing technologies, particularly for the treatment of organic wastes, is
further opening new possibilities to produce new organic fertilizers. In this
chapter we review three groups of products that are experiencing a new interest
due to innovative processing methods or application possibilities: fertilizers
derived from animal livestock by-products, seaweeds-based products and
fertilizers derived from biochar or lignite.
2.
FERTILIZERS PRODUCED FROM ANIMAL LIVESTOCK BYPRODUCTS
Manure is considered the organic fertilizer for antonomasia; a legal definition
consider it as “any excrement and/or urine of farmed animals, with or without
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litter, or guano, that may be either unprocessed or processed (Regulation (EC)
No 889/2008). The amount of organic matter and nutrient elements in animal
manure may vary according to several factors: the genetics of the animals
(Table 1), their growth stage and diet (Table 2), the type of bedding used as
well as building and storage management.
Table 1: Average content of macronutrients in animal manures of different
origin (adapted from different sources).
Animal species
N%
P2 O5 %
K2O %
Dairy cow
Beef steer
Horse
Swine
Sheep/goat
Rabbit
Chicken
0.57
0.73
0.70
0.49
1.44
2.40
1.00
0.23
0.48
0.25
0.34
0.50
1.40
0.80
0.62
0.55
0.77
0.47
1.20
0.60
0.40
Table 2: Average composition of swine manure for different animal growth
stages (adapted from Girard et al., 2013).
Parameter
Dry matter %
Total N %
Phosphorous mg kg–1
Potassium mg kg–1
Calcium mg kg–1
Magnesium mg kg–1
Growth stage
Maternity
Nursery
Growing and Finishing
1.8
0.2
593
1049
697
213
2.7
0.3
762
1964
701
311
4.7
0.6
1690
3405
1700
674
Among the biodegradable fractions of the organic matter present in the
manure, those present in soluble form include volatile fatty acids (acetic, butyric
and valeric acids), monosaccharides (sugar) and alcohols and are easily utilized
by soil microorganisms (Boursier et al., 2005). The slowly biodegradable fraction,
composed of polymers, cannot be directly assimilated by microorganisms and
must first be hydrolyzed. The distribution of the organic matter among the
different fractions can vary depending on many factors such as the type of feed
and the manure storage time (Boursier et al., 2005).
The breeding of livestock in specialized farms with a limited, or even nil,
land area for the distribution of the manure produced represents an
environmental concern. Therefore, the treatment of manure with either aerobic
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or anaerobic processes to produce compost or biogas, respectively, is considered
a method to reduce the environmental impact. The anaerobic digestion produces
also a good quality liquid fertilizer, since the liquid effluent of the digester
contains almost all nitrogen present in the raw material as ammonium
(Ortenblad 2000).
Composting farm manure is an excellent method to stabilize its nutrient
content and to reduce many of the drawbacks associated with raw manure use
(Table 3). A good compost of animal by-products is a safe fertilizer due to the
low content in soluble salts, the minimal load of plant and human pathogens,
the slower release of its nutrients, and the higher content of humus fractions
(de Bertoldi et al., 1996). To achieve a good composting process, an initial C:N
ratio of between 25:1 and 40:1 must exist for the initial matrices. This condition,
together with a well aerating system, assures that a temperatures between
55°C and 80°C is reached for a period of at least 15 days, which ensures the
stabilization of the product and reduction of pathogen population (Zucconi
et al., 1981).
Table 3: Average chemical characteristics of vermicomposts (adapted from
de Bertoldi et al., 1996).
Parameter
Total N%
N-nitrate (ppm)
P%
K%
Ca%
Mg%
pH
EC (mmhos/cm)
Average content
1.9–2.5
900–1000
0.4–0.6
0.7–0.9
4.4–4.6
0.4–0.6
6.8–7.0
11–12
The nutrient content of the compost varies according to the materials
utilized for the preparation of the initial mass. However, since mature compost
has undergone extensive microbial degradation and stabilization, little
mineralization of the remaining organic N is likely in the year of application.
Thus, the “plant available” N content of the compost is only a fraction of the
total N, which depends also on the C:N ratio of the compost (Chen and Inbar
1993; O’Keefe et al., 1986). When the C:N ratio of a compost is high (e.g., > 30)
immobilization of N can be expected, and therefore the availability coefficient
is nil. At low C:N ratios (e.g., 8 to 10), which are common in quality composts,
N availability coefficients may increase to approximately 0.5. As a general
rule, availability of P and K in composted manure is higher than N (around
0.8). Many commercially available organic fertilizers are based on composted
animal manures supplemented with rock powders, plant by-products, and
additional animal by products like blood, bone, and feather meals.
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Another method of composting manure as well as other materials of animal
and plant origin is utilizing various species of worms, specifically brandling
worms (Eisenia foetida) and red worms or red wigglers (Lumbricus rubellus).
The result of this process is a vermicompost, a product that can be further
processed to obtain extracts. Passing through the gut of the earthworm, recycled
organic wastes are excreted as castings, or worm manure, an organic material
rich in nutrients that looks like fine-textured soil. Secretions in the intestinal
tracts of earthworms, along with soil passing through the earthworms, make
nutrients more concentrated and available for plant uptake (Table 3).
Commercial producers of vermicompost normally are using dairy cow or
pig manure, sewage sludge, agricultural waste, food processing and grocery
waste, cafeteria waste, grass clippings and wood chips. Green waste should be
added in moderation to avoid excessive heating, while meat waste and dairy
products are likely to putrefy and therefore are not utilized. The pH, nutrient,
and microbial content of these fertilizers varies upon the inputs fed to worms.
Pulverized limestone, or calcium carbonate can be added to the system to basify
the pH.
Animal products, such as skin, wool, bristle, horns, feathers, hoofs, etc.,
are significant waste by-products of livestock, meat, leather and other
industries. If these waste by-products are not utilized or treated properly they
could pose serious environmental problems threatening the sustainability of
this production sector. However, they can be considered as possible raw
materials in the production of organic fertilizers (Tahiri and Guardia, 2009).
Nevertheless, in case of the European Union, all products of animal origin
must undergo specific treatments to exclude the risk of transmitting the prione
causing the bovine spongiform encephalopathy (BSE) (Commission Decision,
2001/9). The materials shall be treated under alkaline condition (pH > 11) and
pressure > 0.36 MPa, with a temperature > 140oC; furthermore, the peptides
obtained from the following hydrolysis process shall have a molecular weight
< 10 kDa.
The pre-tanning solid wastes originated from leather and livestock
industries include skin trimmings, keratin wastes (feather, hair, wool, bristle,
horns, hoofs, beaks, claws, etc.), fleshing wastes (Schramm 1997). The structural
fibrous proteins collagen and keratin are the main component of them. Collagen
is a fibrous protein characterized by a triple helical structure formed by three
polypeptide chains. Efforts for the utilization of this waste are connected with
washing with acids and a following extraction or pressing at high temperatures.
Keratins are proteins resistant to degradation by common proteolytic enzymes
and are characterized by long polypeptide chains which are cross-linked by
disulphide bonds that, together with the hydrophobic interactions, are thought
to be responsible for their stability and resistance to degradation of keratin
(Farag and Hassan, 2004; Safranek and Goos, 1982).
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Fleshing is an easily degradable material representing 10–15% of animal
skin and can be considered an organic N fertilizer, containing about 15% N,
with slow release since the final protein breakdown is accomplished with the
aid of soil microorganisms. It was shown it could be composted using the
earthworm Eisenia foetida (Ravindran et al., 2008), but also used directly as
organic fertilizer (Serrano et al., 2003).
Feather meal, an important by-product of the poultry processing industry
(amount to 7–8% of chicken weight), contains 75–85% proteins (almost 90%
being keratin), which gives a content of about15% N. Under laboratory
conditions at optimal temperature and moisture, changes in microbial
populations (both heterotrophic and proteolytic bacteria and soil fungi) with
time were determined in response to feather meal application as compared
with untreated soils, confirming that the recycling of C and N of the organic
fertilizer is microbially-mediated (Hadas and Kautsky, 1994).
Residual hair is formed of about 82% protein, with nitrogen content of
13%, and might well be used to develop organic fertilizers (Serrano et al., 2003).
Raw wool is a slow release nitrogen fertilizer that is also positively affecting
the soil physical properties (Zheljazkov et al., 2009).
However, the nutritional properties of all these highly proteic by-products
are better exploited when they are processed to obtain soluble proteins, peptides,
and free amino acids through hydrolysation. Indeed, plants are able to absorb
and assimilate amino acids not only from soil but also directly from the leaves,
thus making such hydrolysates suitable for foliar application (Ashmead et al.,
1986). Even though the rate of foliar absorption of nitrogen is negatively
correlated with the molecular weight of the amino acids, such a relationship is
not always valid: indeed, the absorption rate of nitrogen from arginine and
L-lysine was significantly higher compared with other amino acids having the
same molecular weights (Furuya and Umemiya, 2002).
Protein hydrolysates are obtained from the single or combined thermal,
enzymatic or chemical (with strong acid or alkali) hydrolysis of by-products
deriving from livestock industry, particularly leather manufacture. However,
the different process produces materials with diverse suitability for plant
nutrition purposes. Thermal hydrolysis processing requires that raw materials
are cooked in water at 90–140oC for 36 h without modifying the pH of the
suspension nor adding any chemical. This treatment produces poorly hydrolysed
long peptide chains of limited availability for plant nutrition. Enzymatic
hydrolysis produces rather homogeneous mixtures of polypeptides, showing
good availability for plants. When chemically hydrolysed, the raw materials
are treated at 80–100oC for few hours in a solution of compounds such as
sulphuric acid or calcium hydroxide and at the end of the treatment the pH is
buffered at about 7. Acid hydrolysis for the solubilisation of proteins from skin,
bone and cartilage wastes using phosphoric acid was developed by Chakarska
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et al. (2006). Acid hydrolysis is highly efficient in producing hydrolysates which
composition is, in case of some amino acids, made of a racemic mixture of Dand L-forms. However, a recent study on several protein hydrolysates obtained
with different industrial processes has pointed out that the formation of a
racemic mixture occurs independently of the hydrolysis procedure employed
(Corte et al., 2013). Although plants are able to absorb both D- and L-amino
acids, the latter configuration seems to be preferred by plants for direct
assimilation (Vranova et al., 2011). Alkaline hydrolysis is similar to acid
hydrolysis, but is less effective, unless associated with a thermal treatment
(Gousterova et al., 2008).
In enzymatic hydrolysis, raw materials are treated in water at 40–55oC
for few hours using proteolytic enzymes of different origin. Enzymatic hydrolysis
produces rather homogeneous mixtures of polypeptides, showing good
availability for plants. Keratinases are the enzymes mostly used in this process.
They are mostly serine or metallo proteases, with a wide temperature and pH
activity range (Gupta and Ramnani, 2006), that can be used for enzymatic
hydrolysis. They are produced by different species fungi (Farag and Hassan,
2004; Friedrich et al., 1999; Moreira et al., 2007), bacteria (Atalo and Gashe,
1993; Allpress et al., 2002; Gessesse et al., 2003; Giongo et al., 2007; Suzuki
et al., 2006) and actinomycetes (Böckle et al., 1995; Chao et al., 2007; Gousterova
et al., 2005). These microorganisms or their enzymes (Kida et al., 1995; Veselá
and Friedrich, 2009) can be utilized for the enzymatic process; however, when
microorganisms are directly used for the biodegradation of keratin wastes, a
lower amount of soluble proteins is recovered in comparison to hydrolysis carried
out with the use of keratinases, since the microorganism itself consumes some
of the biodegraded products. For example, the maximum amount of soluble
protein obtained after growing a keratinolytic bacterium Vibrio sp. for 5 days
was around 2.5 g/L on feather waste (Grazziotin et al., 2007), while with the
use of an enzyme extracted from Paecilomyces marquandii about 4–5 g/L were
obtained in only 5 hours (Veselá and Friedrich 2009). When the hydrolysis
was preceded by steaming the solubilization of the nitrogen from keratin meal
was about 98%, two-fold in comparison to that obtained without steam
treatment (Veselá and Friedrich, 2009). Kida et al. (1995) performed hydrolysis
of powdered feather meal with the use of a commercial bacterial proteinase
from Bacillus subtilis, obtained 86.5 to 89% digestion ratio of the substrate
under optimal conditions. Among microbial-derived enzymes, a heat stable
alkaline protease was found to be produced by the fungus Paecilomuces lilacinus
(Chakraborty and Sarkar, 1998) and a protease produced by a bacteria was
able to digest leather fleshings (Ghosh et al., 2004). Hydrolysation of fleshings
with pancreatic enzymes at pH 8.5 allowed to obtain, 80.0 mg/ml total protein
content, 10.64 mg/ml free fatty acids and 72.86 mg/ml collagen in the
hydrolysate supernatant (Kumaraguru et al., 1998). Production of hydrolyzed
proteins from raw hide split was obtained by using papain and neutrase
separately. The optimum hydrolysis conditions, yielding the highest amount
of proteins were 70 C, pH 6–7 and 40–50C, pH 6–7, for papain and neutrase
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respectively (Damrongsakkul et al., 2008). Bajza and Vrcek (2001) have
developed a method which requires to subject the raw material to a pretreatment with increased temperatures, adding then the proteolytic enzyme,
without the need to cool the reaction mixture.
The content of free amino acids in hydrolysed protein fertilizers is typically
5 to 40% (Cavani et al., 2003). Glycine is generally the most abundant amino
acid in HPs, accounting for about 26–50% of the total; other amino acids found
to be particularly abundant are proline and hydroxyproline ornithine and
glutamine (Corte et al., 2013).
When applied to the soil, only about 5% of HPs are directly absorbed by
roots, with the rest being primarily metabolized by soil micro-organisms and
then made available to plants (Schiavon et al., 2008). It was demonstrated
that the plant can uptake without previous digestion by micro-organisms either
amino acids (Kielland, 1994) and proteins (Paungfoo-Lonhienne et al., 2008).
Furthermore, mycorrhizal fungi alre also able to uptake and transfer to the
plant these kind of compounds. Wheat colonized by G. mosseae absorbed organic
N in the form of N-glycine by 0.2% and 6% at low and high level of N-fertilization,
respectively (Hawkins et al., 2000). HP obtained by using chemical and
chemical-enzymatic hydrolysis proved to be effective as growth enhancers and
biostimulants of plant metabolism when applied to leaves in fruit and vegetable
crops (Ertani et al., 2013; Zhang et al., 2006). Proline and hydoxyproline increase
resistance of plants to worst climatic conditions; alanine, valine, and leucine
improve the quality of fruits; several amino acids have a chelating effect on
micronutrients, which ease the absorption and transportation of micronutrients
inside the plant (Koksal et al., 1999).
Fertilizer manufactured from partially hydrolysed hair can be obtained
by reductions or oxidation as well as extensive hydrolysis (Kamal et al., 1998).
An hydrolysed product obtained through alkaline hydrolysis at high
temperature and pressure of wool wastes was characterized by 75–80% of water
soluble materials that included peptides, amino acids, salts, dyes, lipids, some
carbohydrates, and potassium ions (Gousterova et al., 2003). This product,
applied to the soil, positively influenced microbial soil populations and growth
of ryegrass (Nustorova et al., 2005).
A comprehensive study on the characteristics and safety of HPs was carried
out by Corte et al. (2013). The different material used to produce the HP could
be traced through electro phoretic analysis: those obtained from fleshing were
predominantly composed of peptides with MW less than 10 kDa, while those
obtained from shaving contained bigger peptides (MW between 10 and 25 kDa).
However, differences in the protein size distributions were observed. Products
with high protein concentration were characterized by a mixture of partially
hydrolized proteins (MW between 75 and 50 kDa) and of smaller peptides (MW
about 10 kDa). On the other hand, the electrophoretic pattern of products with
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lower protein content were characterized by a mixture of peptides of MW
between 10 and 20 kDa. The safety of HPs deriving from the processing of
different raw materials with various hydrolysis treatments was verified by
analysing the changes in growth kinetics induced by them at three different
concentrations (10, 103 and 105 ppm, w/v) on growing yeast cells. No significant
alteration was caused by HPs at the two lowest concentrations, which are
similar to the dilutions normally applied in the field. The fertilizer effect on
soil microorganisms activity was measured up to about one month after their
application. Interestingly, neither the degree of degradation nor the kind of
raw materials seemed to influence the protein availability to soil microorganisms.
Other products or by-products of animal origin that are utilized for the
production of organic fertilizers include blood meal and fish meal. The former
is a dried slaughterhouse waste containing about 12% nitrogen. Fish meal
contains about 10% nitrogen, along with about 6% phosphate. The content of
nutrient elements depends on the production process utilized: acid-digested
fish emulsions usually have a NPK content around 4-4-1, while enzyme-digested
fish emulsions usually have a title for NPK of 4-1-1.
3. SEAWEEDS AND THEIR EXTRACTS
Marine algae have traditionally formed part of the oriental diet, but their major
use in Western countries has traditionally concentrated on the extraction of
compounds, many of which found to be useful functional ingredients with
numerous health benefits (Wijesinghe, Jeon, 2012), used by pharmaceutical,
cosmetics, and food industries (Løvstad Holdt and Kraan, 2011). However,
seaweeds have been since long time exploited also as organic fertilizers either
directly or after composting with straw or other organic wastes to improve the
productivity of crops (Craig, 2010). Studies on the fertilization properties of
seaweeds are dating back to the 60’s (Caiozzi et al., 1968; Francki, 1960a, b).
However, the development of extracts from different seaweed species has
increased the role of this organic fertilizer, also exploiting the ability of liquid
extracts to maintain in a soluble form the microelments mixtures (Cu, Co, Zn,
Mn, Fe, Ni as well as Mo and B) (Davis et al., 2003).
The algae utilized as fertilizers belong mainly to the brown seaweeds, a
group comprising about 2,000 species, growing on the shores of the temperate
zones. Among them, Ascophyllum nodosum, Ecklonia maxima, Fucus spp.,
Laminaria spp., Sargassum spp. and Turbinaria spp. are used to produce
organic fertilizers or biostimulators (Craigie, 2010; Hong et al., 2007; Nabti
et al., 2009). About 15 million metric tonnes of seaweed products are produced
annually (FAO 2006), but products relating to agriculture (soil conditioners,
fertilizers, biostimulants and animal feeds) represent less than 1% of the overall
value of the current seaweed industry (Craigie, 2010).
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Most seaweed fertilizers are produced using kelp as raw material that is
dried and grounded. Kelp meal is suitable for application directly to the soil; it
contains about 1% N and 2% K, along with minimal amounts of P, Mg, S and
numerous microelements, and is most commonly used only on high-value crops
at a rate of 25–50 kg/ha.
The seaweed extracts (SWE) are aqueous preparations characterized by
different colour, odour, viscosity, and particulate matter contents, depending
on the production process. Extract preparation can be carried out by different
methods which are normally covered by patent. An alkaline seaweed extraction
method was already patented at the beginning of the XX century (Penkala,
1912). However, the current most common method of extraction is performed
with alkaline solutions at ambient or high temperatures (hot pressurized
alkaline process, Milton, 1952), obtaining a dark coloured product of pH 7–10.
The process generates compounds that were not present in the original
seaweeds, which nature and quantities depend on the composition of the raw
material and on the processing conditions.
The disruption of the seaweed structure can also be obtained by high
pressure collecting the soluble cytosolic components in the filtered liquid (Hervé
and Percehais, 1983). An alternative method allows to physically disrupting
the seaweed by low temperature milling to give a “micronized” suspension of
fine particles (Hervé and Rouillier, 1977), which are characterized by a greenish
to greenish-brown colour and acid pH. The different methods that can be utilized
in the production process, together with the possibility to add in the extract
micronutrients to take advantage of the chelating properties, provide for a
range of commercial products that can perform differently when applied to
agricultural crop species.
The effect on plant growth and yield of SWE has been demonstrated on
different crops (Craigie, 2010; Khan et al., 2009). Just to mention few examples,
tomato plants treated with low concentration (0.4%) of Ecklonia maxima extract
as a soil drench showed an increase in plant height (Crouch and van Staden,
1992). An increase in yield (about 60%) was achieved in grape treated with
SWE from A. nodosum consistently over a 3-year period in comparison to regular
crop management, due to increase in berry size and weight (Norrie and
Keathley, 2006). Increased plant growth as well as yield and berry quality was
shown in strawberry (Masny et al., 2004; Spinelli et al., 2010). SWE reduced
the yield oscillation on apple trees showing a biennial bearing behaviour
(Spinelli et al., 2009).
SWE induce an increase in the concentration of bioactive molecules,
including antioxidants, in the cells of treated plants. These feature has a double
positive effect: from one side the high antioxidant amounts/activities following
their application are improving the nutritional value of fruit and vegetables as
well as prolonging their shelf life, thus enhancing the overall quality and
marketable value of fresh produce. On the other hand, such effect has been
proved to improve the tolerance to both biotic and abiotic stresses.
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For what concern the quality of produce, application of SWE was found to
increase endogenous antioxidant activity in plants due to non-enzymatic
antioxidant compounds (a-tocopherol, ascorbate, b-carotene and phenolics) and
enhanced the activities of antioxidant enzymes such as ascorbate peroxidase,
glutathione reductase and superoxide dismutase (Allen et al., 2001; Fan et al.,
2011).
SWE are effective in increasing the tolerance of plants to abiotic stress
conditions such as frost events (Rayorath et al., 2009), drought (Zhang and
Ervin, 2004; Zhang and Schmidt, 2000), salinity (Mancuso et al., 2006), heat
tolerance (Zhang and Ervin, 2008), as well as under nutrient deficiency
conditions (Beckett and Van Staden, 1989; Nelson and Van Staden, 1984).
Plants treated with seaweed extract have been reported to show increased
nutrient uptake (Caiozzi et al., 1968; Crouch et al., 1990; Mancuso et al., 2006),
probably also due to the observed increased total volume of the root system
(Slàvik 2005) and changes in the root system architecture (Sivasankari et al.,
2006). Even thought SWE contain macronutrients such as Ca, K, P, and
micronutrients like Fe, Cu, Zn, B, Mn, Co, and Mo (Khan et al., 2009), their
amount is generally limited; for example a commercial extract obtained from
Ecklonia maxima contained only 400 mg/l N, 300 mg/l P and 7000 K mg/l, and
considering the dilution in the working solutions, only few milligrams are
applied to the plant (Papenfus et al., 2013). The increase in growth and seedling
vigour is, on the contrary, more prominent under nutrient deficiency conditions,
which, together with the relatively low application rates (< 15 L/ha), prompts
the conclusion that their effect can be mainly attributed to the plant growth
regulator compounds present in the extracts. Indeed, SWE contain growth
promoting substances such as auxins and cytokinins (Crouch and Van Staden,
1993; Durand et al., 2003; Stirk et al., 2004; Tay et al., 1985), gibberellins
(Wildgoose et al., 1978), betaines (Blunden et al., 1991) and the polyamines
putrescine and spermine (Papenfus et al., 2012). However, it is possible that
these components exhibit synergistic activity (Vernieri et al., 2005).
The bio-stimulatory effect is supported by the fact that SWE they are
bioactive at concentrations even lower than 1 × 10–3 (Crouch and van Staden,
1993) and that their effect can vary depending on the concentration applied
(Vinoth et al., 2012). Results obtained on seed germination are also underlining
such activity: extracts from Gracilaria gracilis, Cystoseira barbata and Codium
tamentorum improved germination of tomato, pepper and aubergine (Demir
et al., 2006). SWE of Gracilaria edulis and Sargassum wightii enhanced seed
germination and induction of rooting and development of multiple shoots from
in vitro cultures of tomato particularly at low concentration, giving better results
than synthetic hormones (Vinoth et al., 2012).
Among the non-hormonal compounds found in SWE, kahydrin, alginic acid,
betaine and betaine-like compounds have shown biostimulatory effects.
Kahydrin is a derivate of the vitamin K and its exogenous application induced
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the acidification of rizosphere (Luthje and Bottger, 1995) which can result in
positive effects on the solubilisation of insoluble nutrients. Alginic acid can
form cross-linked polymers, which increase the soil water-holding capacity
(Moore, 2004) and significantly stimulates hyphal growth (Kuwada et al., 2006)
and the colonization of roots by arbuscular mycorrhizal fungi (Kuwada et al.,
1999). Betaines function mainly as omoprotectants, thus enhancing the plant
resistance to drought and salinity stress (Huang et al., 2000) which can account
for the positive effects observed after application of SWE under abiotic stresses
conditions. Organic materials reach in betaines were found to enhance the
root colonization by fluorescent Pesudomonads (Urashima et al., 2005), a finding
observed also after soil application of SWE (Spinelli et al., 2010). Bacteria
belonging to the Pseudomonas genus are known to be plant growth promoting
rhizobacteria (Bardi and Malusà, 2012), which could further widen the effect
of SWE soil application.
The increased growth and yield observed after the application of SWE
could also be indirectly accounted for to the several mechanisms that seem to
be triggered by these products, which increase plant tolerance to pathogens.
Two algal polysaccharides, laminarin and carragenan, were shown to trigger
signalling pathways mediated by ethylene, jasmonic acid and salicylic acid
and to induce locally the expression of defence-related genes encoding chitinases
and proteinase inhibitor in tobacco leaves (Mercier et al., 2001). Up-regulation
of various PR protein genes as well as enhanced activities of different defence
enzymes (chitinase, glucanase, polyphenol oxidase, fenyl-alanin ammoniolyase
and lipoxygenase enzymes) were also observed in carrots following SWE
application. As a result, the carrot plants showed a lower occurrence of disease
symptoms due to Alternaria radicina and Botrytis cinerea in comparison to
plants treated with salicylic acid or untreated (Jayaraj et al., 2008). The disease
control observed in SWE-treated carrot plants was attributed to the elicitor
activity of oligosaccharides present in the seaweed extract. The treatment of
plants with an extract of Ulva sp. reduced the infection of Medicago truncatula
by Colletotrichum trifolii, which was paralleled by enhanced expression of a
broad range of genes involved in phytoalexin, PR protein and cell wall protein
production (Cluzet et al., 2004). Pepper plants treated with an extract of
A. nodosum accumulated the highest capsidiol concentrations in leaves
(a phytoalexin with fungistatic action on the development of Phytophthora
capsici), when compared to the control and showed enhanced foliar resistance
to this pathogen (Lizzi et al., 1998). When an extract of the same seaweed was
incorporated into the planting medium, it delayed and reduced the incidence
of Verticillium wilt in pepper plants (Garcia-Mina et al., 2004).
4.
LIGNITE AND BIOCHAR
A peculiar group of organic fertilizers is formed of products derived from energy
industry: by-products of coal mining (lignite) or of the biogas production
Hydrogels for the Release of Fertilizers
267
(biochar). Lignite is a brown carbonaceous sedimentary rock with woody texture,
composed of accumulated layers of partially decomposed vegetation, which was
formed during the Cretaceous period (Kasi«nski, 1989).
Carbon, amounting to about 63–78% of total mass, is the principal element
of lignite; other elements contained in a relevant amount are calcium and
magnesium (0.1–0.2%), while several microelements are present in traces
(Table 4). The chemical composition of lignite is based on a complex of humic
substances: humic, hymatomelanic and fulvic acids, humins and bitumins
(Thomas 2012). The concentration of total humic acids varies between
43– 53% of the total mass. Humic acids fractions represent about 25% of organic
carbon (Corg) while, fulvic acids fractions represent 11.5% of Corg and humates
are constituting 50% of Corg. (Hoffmann and Huculak-M×aczka, 2011). Such
amounts are very much similar to those found in other organic materials used
for the production of organic fertilizers rich in humic substances (Fong et al.,
2006).
Table 4: Average content of nutrient elements in lignite (adapted from
Maciejewska 1998).
Macroelements
Total N
Mineral N
Phosphorus
Potassium
Sulphur (as sulphate)
Calcium
Magnesium
mg/kg
Microelements
mg/kg
420–590
30–75
15–30
15–150
15–30
2250–2750
450–750
Iron
Manganese
Zinc
Boron
Molybdenum
15–45
15–30
3–7
3–7
0.7–1.5
Lignite can also be utilized as a component of a plant organic fertilizer,
but, due to the high degree of polymerization and consolidation of the lignite’s
organic substance, requires to be processed through chemical extraction,
physical modification and/or biological decomposition (Hoffmann and Hoffmann,
2007; Maciejewska, 1998). Nitrification with nitric acid (20% by weight) and
further ammonization of oxidized lignite at high temperatures for a period of
2 hours allowed to produce a fertilizer that contained 7–13% of nitrogen,
depending on the temperature used in the process (Coca et al., 1984). The
conversion of ammonia into readily available nitrate occurs only for 45% of the
nitrogen present in the fertilizer. This rate is similar to other organic fertilizers
(e.g., based on peat and humates), but lower to inorganic ones, such as urea
and ammonium nitrate (Coca et al.,1984). Therefore, such organic fertilizers
can serve as a nitrogen slow release product.
The treatment of lignite with alkali allows to extract humic acids and to
produce humates (Quigley et al., 1988). The extraction with a 1M KOH solution
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Fertilizer Technology Vol. 1: Syntheis
allowed to extract about 20% of humic acids from raw lignite, an amount that
was increased up to 80% when adding to the process a pre-treatment with 10%
nitric acid (Fong et al., 2006).
Another technological approach in the processing of lignite is through biosolubilisation, which can be utilized to produce fertilizers suitable also for
organic farming. Biological decomposition involves the use of bacteria and fungi
that can degrade aromatic compounds typical of lignin, which macromolecular
structure resembles that of lignite (Hofrichter et al., 1999; Polman et al., 1994).
Among the bacteria, several Pseudomonas and Bacillus species have been found
able to decompose lignite: Pseudomonas cepacia DLC-07 strain was successfully
used to depolymerize lignite (Gupta et al., 1990), and Pseudomonas putida
efficiently degraded its humic acids (Machnikowska et al., 2002). A consortium
formed by Bacillus cereus, Bacillus pumilus and Bacillus subtilis strains
solubilised oxidized lignite with an efficiency comparable to that of the fungus
Cunninghamella (Maka et al., 1989). However, the most efficient degradation
is obtained with white and brown rot fungi, the primary degraders of lignin in
nature (Hammel, 1996). Hypocrea lixii can degrade carboxyl and hydroxyl
groups making oxidized lignite soluble (Tao et al., 2010). Trametes versicolor
(Fakoussa and Frost, 1999), Phanerochaete chrysosporium (Ralph and
Catcheside 1997), Penicillium sp. (Yuan et al., 2006) and Neosartorya fischeri
(Igbinigie et al., 2008) are known to degrade enzymatically lignite.
Composting of lignite with N-rich agro-wastes can be a method to accelerate
the fungi decomposition process. The enrichment of lignite with whey and
molasses resulted in efficient lignite decomposition by Pleurotus ostreatus and
Lentinus edodes, two species of white rot fungi (Stêpieñ et al., 2012). Oxidation
of lignite with nitric acid, as pre-treatment to further microbial decomposition,
enhanced the biodegradability of the organic compounds, resulting in about
90% of lignite solubilisation, and improved the nutritional characteristics due
to a high nitrogen content (Machnikowska et al., 2002).
The first attempts to use lignite as a fertilizer (lignite breeze in quantities
of 1.4–40 t/ha) were made during the interwar period (Kissel, 1931). As a result
of its use, yields of wheat, potatoes and sugar beet significantly increased. The
positive effect of lignite on the growth, yield and health status of crop plants
has been demonstrated recently by several studies (Ayuso et al., 1996; Jones
et al., 2007; Sharif et al., 2002; Ulukan 2008; Wang et al., 1995). This effect can
be induced by the improvement of soil physical-chemical properties from the
stable fractions of humus in the soil (D×ebska et al., 2002). However, it has been
shown that humates and humic acid from lignite or other oxidized coal products
(leonardite), particularly with a low molecular mass, are taken up by plants
and actively modify the plant metabolism, promoting nutrient uptake or acting
as hormone-like substances (Nardi et al., 2002). In case of nutrients acquisition,
they induce higher rate of nutrient utilization by plants, particularly for
potassium, phosphorus, nitrogen, copper, manganese, iron and sodium (Ayuso
et al., 1996; Sharif et al., 2002; Stevenson, 1991; Tahir et al., 2011).
Hydrogels for the Release of Fertilizers
269
Biochar is the solid residual resulting from the low temperature pyrolysis
of renewable biomass feed stocks during bioenergy extraction (Lehmann et al.,
2006; Lehmann 2007). Biochars can be produced from an array of cellulosecontaining feedstocks such as biomass (Özçimen and Ersoy-Meriçboyu, 2010;
Spokas and Reicosky, 2009) and municipal wastes (Ryu et al., 2007), and by a
variety of processes yielding bioenergy and chemical co-products such as biooil and syngas (Bridgwater and Peacocke, 2000). The chemical and physical
characteristics of biochars vary depending on the conditions of the thermalchemical conversion applied to the biomass: indeed, biochars created from the
same biomass under similar pyrolysis conditions but in different units can
result in a different final products (Spokas et al., 2012b). The final product can
be formed of a material (named soot) that contains no residual relic structures
of the original feedstock material or of char and charcoal, materials that contain
relic structures as combustion residues (Spokas, 2010). These materials are
characterized by a different oxygen to carbon ratio (from 0 to 0.6) (Hedges
et al., 2000). The physical structure is affecting the organic and inorganic
composition: the pH can vary from 5.6 to 13.0, the C content from 33.0% to
82.7%, N content ranges from 0.1 to 6.0%, and the C : N ratio varies from 19 to
221 (Jha et al., 2010; Spokas et al., 2012a). Biochar can contain also appreciable
quantities of P, K, Ca, Mg, and micronutrients (Cu, Zn, Fe, Mn) with ashes
accounting to 5–60% of the weight, depending on the source of the biomass and
pyrolysis conditions (Cheng et al., 2008; Enders et al., 2012).
Biochar is considered to be a potentially beneficial soil amendment (Glaser
et al., 2002): it improves water infiltration (Ayodele et al., 2009), soil water
retention, ion exchange capacity and nutrient retention (Laird et al., 2010;
Lehmann et al., 2003), pH (van Zwieten et al., 2010). The greater proportion of
aromatic carbon in comparison with the biomass feed stock makes the
macromolecular structure of biochar more recalcitrant to microbial
decomposition than other kinds of organic matter (Baldock and Smernik, 2002).
However, it has been shown that mycorrhizal fungi use biochar as a habitat
and interact with it (Warnock et al., 2007).
It was shown that biochar influences the soil nitrogen cycle and availability
(Clough and Condron, 2010), affecting the soil microbial nitrification and
denitrification reactions (Ball et al., 2010; Spokas et al., 2010). The application
of biochar in different doses modified biological N fixation rates in bean (Rondon
et al., 2007): the proportion of BNF increased of about 50% with 90 g kg–1
biochar added to the soil. However, the total N derived from the atmosphere
was much increased with doses of 30 and 60 g kg–1 biochar (by 49% and 78%,
respectively) than with the highest one (only 30% increase). As a consequence,
bean yield increased by 46% and biomass production by 39% in comparison to
the control at 30 and 60 g kg–1 biochar respectively, while biomass production
decreased with the 90 g kg–1 dose.
Biochar is also affecting other soil and rhizosphere bacteria. Temporal
increase in abundance or a reduction of the magnitude of loss in bacterial family
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after biochar application included strains of Bradyrhizobiaceae (~8%),
Hyphomicrobiaceae (~14%), Streptosporangineae (~6%) and Thermomonosporaceae (~8%). On the other hand, biochar had have a negative effect
on bacterial family abundance for Streptomycetaceae (about –11%) and
Micromonosporaceae (about –7%) (Anderson et al., 2011). The effect of biochar
on soil microorganisms was also positive for phosphate solubilising bacteria,
and resulted also in an increased abundance of bacterial families that can
degrade more recalcitrant C compounds (Anderson et al., 2011).
The influences on nitrogen biogeochemistry, probably due to the surface
group chemistry of biochar, can account for the negative and positive agronomic
effects that have been recorded following its application (Gaskin et al., 2008;
van Zwieten et al., 2010). From a review of the published results emerged that
only fifty percent of the studies reported yield increases after biochar addition,
while the remaining recorded decreases to no significant differences (Atkinson
et al., 2010). However, the long term effect of biochar is not always evaluated.
Maize grain yield did not significantly increase in the first year after a single
application of 8 or 20 t ha–1 of biochar, but it increased in the following three
years of 28, 30 and 140%, respectively. The increase was attributed mainly to
the changes in soil chemical parameters (increased pH and several fold higher
availability of Ca and Mg) (Major et al., 2010). The changes in soil chemical
properties and the behaviour in yield increase can be considered parallel to
that registered by Maciejewska (1998) with single application of similar amount
of lignite. Considering the positive effect of lignite in the parameters related to
water capacity, a similar outcome could be expected also for biochar, as it has
been already found in some specific soil environments (Asai et al., 2009; Chan
et al., 2007). Hardwood biochar (black carbon) produced by traditional methods
(kilns or soil pits) possessed the most consistent yield increases when added to
soils (Spokas et al., 2012b).
The capacity of biochar to react with various nitrogen compounds
(Singoredjo et al., 1993) can be exploited to produce a nitrogen fertilizer. The
adsorption of ammonia on to biochar can be obtained at ambient temperature
and pressure in the presence of carbon dioxide and water (Clough and Condron,
2010). The rate of adsorption was observed to vary from 0.2 to 10 mg g–1 of
biochar, depending on the biomass matrix used in the process leading to the
formation of biochar, the pyrolysis process conditions and the concentration of
ammonia introduced (Clough and Condron, 2010; Taghizadeh-Toosi et al., 2012).
The application of this product to the soil increased the soil nitrogen availability
and, consequently, plant growth (Taghizadeh-Toosi et al., 2012).
The application of biochar as an amendment or nitrogen fertilizer can
support the reduction of the environmental footprint deriving from food, feed
and renewable energy productions, still maintaining the original role as a
potential carbon sequestration tool (Lehmann and Joseph, 2009). Yet, there is
still a need to define a common quality standard for the product (Bourke et al.,
Hydrogels for the Release of Fertilizers
271
2007; Brewer et al., 2009) and to understand the mechanisms behind the
interactions between biochar and the fertilizers derived from it, from one side,
and plant and soil microorganisms on the other (Brewer et al., 2011). This
needs resemble those occurred for quality composts in the 90’s, when these
materials began to be produced and traded. The simple chemical or physical
characteristics were, in that case, considered not sufficient to assess the quality
of the compost, obtaining also important information from simple bio-essays.
This could also be the case for biochars, since polyaromatic hydrocarbons and
growth-inhibiting organic compounds were detected in water extracts of
gasification biochars (Rogovska et al., 2012). Furthermore, there are significant
differences in stability between biochars (Spokas, 2010) and its properties
change over time in soil, changes which may also be affected by the initial
properties of the biochars (Joseph et al., 2010). Therefore, a combination of
analytical methods, including elemental contents, thermal degradation and
physical parameters, might be more explanatory to assess and predict the
behaviour of biochar after the application to soil (Joseph et al., 2009).
5.
CONCLUSIONS
Besides the three groups of products described, other wastes can be utilized to
produce organic fertilizers. For example, stillage, the main residue from the
starch-to-ethanol fermentation process of cereal grains, as well as fruit pomace,
the residues of fruit processing industry, can be an important source of raw
materials to produce organic fertilizers. In both cases the chemical composition
of these raw materials, as well as of the fertilizers produced thereof, is influenced
by the type and cultivar transformed and, in case of the stillage, by the efficiency
by which starch is converted to alcohol.
There is considerable potential to explore the added value benefits, in
terms of addition of organic matter to the agricultural soil system, of fertilizers
produced from by-products or wastes of different industries. The three groups
of products considered in the chapter are a major example of the possibility of
utilisation of key waste sources in a suitable and feasible way, thus avoiding
land fill and environmental contamination. The improvements in soil structure
and nutrient availability, the reduction of leaching and improvement of the
water holding capacity, as well as the promotion of soil microorganisms, are
important features common to all these organic fertilizers that pursue the
enhancement of soil fertility. A soil rich in organic matter is key for the
development of a sustainable agriculture fostering land use efficiency and
striving to meet the challenges deriving from changes in climate and increasing
global population. Considering the need of combating against soil degradation
processes such as salinization, desertification and loss of organic matter, the
availability of new sources of organic fertilizers which are particularly improving
the soil physico-chemical characteristics is critical; biochar and lignite can play
a role in this respect. However, organic fertilizers obtained with innovative
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microbial-based technologies, such as animal hydrolyzed proteins or seaweeds
extracts, can also be applied within new production systems using fertigation
or hydroponics (floating systems), thus widening their original scope as simple
soil inputs.
The evidence at hand establishes that relatively simple industrial
processing protocols provide a practical and economical solution to the disposal
of potentially difficult waste, leading to manufacture of high value added
products which benefits are indisputable in agriculture. The aim in developing
such organic fertilizers should be seen as an effort in achieving the Zero Waste
goal: an ethical, economical, efficient and visionary mission, designing and
managing products and processes to systematically conserve and recover all
resources (Palmer, 2005).
6. ACKNOWLEDGEMENTS
The work has been supported by a grant from the EU Regional Development
Fund through the Polish Innovation Economy Operational Program, contract
N. UDA-POIG.01.03.01-10-109/08-00.
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