**4. Soil N availability and other plant nutrient availability enhancement**

Nitrogen plays an important role in yield determination when relatively adequate levels of other agricultural factors exist. Continued use of inorganic fertilizers has not only altered the soil p H, soil structure, and texture, but has also disrupted niches for micro- and mesofauna, which are essential for nutrient recycling [49]. Alternatively, under systems of organic farming management, when industrial N fertilizer is not used, organic matter origin-N, after biological degradation, is converted into mineral N forms, ammonium and nitrate, and becomes a major factor in plant production. However, as the mineral N content in soils increases beyond the capability of plants to take it up, it will cause N leaching and increase other kinds of N losses into the environment. In this case, it is important to understand the N cycle and soil N balance within an agroecosystem.

As common knowledge, the origin of all kinds of N is air N2, 79% by volume of the earth's atmosphere. Soil microorganisms, free-living or associated with legumes, fix atmospheric N. This complex biological process begins with air N and ends with organic N. After organic matter decays, NH4 + , which is ready to be used by higher plants, is released. NH4 + can be converted into NO3 – by nitrifying bacteria and generally most NH4 <sup>+</sup> is modified into NO3 – in the soil. Thus, over 90% of soil N is typically NO3 – , not NH4 + , although NH4 <sup>+</sup> can be formed from NO3 – through the process of denitrification in soils [50].

The denitrification process starts with NO3 – and converts it to NH4 + , N mono-oxides (NOx, greenhouse gases), and N2. Denitrification of nitrate produces about 90% N2 and 10% NOx. However, the natural N balance has been affected by industrial N fixation since the green revolution. Symbiotic N amounts to about 100-175 million tons each year in the 1970s world‐ wide, with industrial fixation of 3.5 million tons, and lightning may fix 10 million tons of N, a value that has probably not changed over time [51] . In 1989, industrially fixed N increased to 80 million tons in response to the needs for high-yielding crops [22].

By the year 2050, the world population is expected to double from a level of more than 5 billion. It is reasonable to expect that the need for fixed N for crop production will also at least double. If this is supplied by industrial sources, synthetic fertilizer N use will increase to about 160 million tons of N per year [51]. Consequently because of its relatively low plant uptake level, generally around 50% or less, several major environmental reasons exist to seek alternative fixed N fertilizers, including the fact that it affects the balance of the global N cycle, pollutes groundwater, increases the risk of chemical spills, and increases atmospheric nitrous oxide (N2O), a potent greenhouse gas. The global budget for N2O appears to be out of balance, exceeding sinks by 30-40% and increasing at 0.25% each year [7]. In this case, biological N fixation should receive more attention because about 2 tons of industry-fixed N is needed as fertilizer for crop production to equal the effects of 1 ton of N biologically fixed in a legume crop [51].

In cultivated land, the soil N balance is a complex system covering serial biological processes and physical and chemical processes. It includes plant N uptake, N fixation, organic N mineralization, nitrification and denitrification, nitrate leaching, and other losses, as N2 or NOx, released into the atmosphere [52]. In the case of organic farms, soil available N is primarily from legumes as green manure and organic fertilizers. SOM reportedly supplies most of the N and S and half of the P uptake by plants within an organic farming system [53,54]. The plant tissues of green manure contain most of the micro or macro plant nutrients, including N, potassium (K), P, and S. Phosphates, K, calcium (Ca), magnesium (Mg), S, and other micro plant nutrients are accumulated by cover crops during the growing season. Hoyt indicated the nutrients content of cover crops; see **Table 3** [55]. These nutrients are maintained in the residues of green manure plants; later, they become available to successive crops after incorporation into the soil.


**Table 3.** Green manure biomass productivity and nutrient content (kg ha–1) [55].

During the composting process of green manure, some carbonic and other organic acids are formed as by-products of microbial activities. These organic acids react with insoluble mineral rocks and phosphate precipitates, releasing phosphates and exchangeable nutrients [29]. Gardner and Boundy found that wheat intercropped with white lupin (*Lupin albus L.*) has access to a larger pool of P, Mg, and N than wheat grown in monoculture [30]. The former two nutrients were probably mobilized by exudates of organic acids from the lupin root and then taken up by wheat roots.

The nutrients content of different legumes can be estimated by the mathematical formulation described by Peet [56]:

Rye: N = 0.0194 × biomass – 17.4

**4. Soil N availability and other plant nutrient availability enhancement**

within an agroecosystem.

+

298 Organic Fertilizers - From Basic Concepts to Applied Outcomes

The denitrification process starts with NO3

the soil. Thus, over 90% of soil N is typically NO3

matter decays, NH4

converted into NO3

–

from NO3

crop [51].

Nitrogen plays an important role in yield determination when relatively adequate levels of other agricultural factors exist. Continued use of inorganic fertilizers has not only altered the soil p H, soil structure, and texture, but has also disrupted niches for micro- and mesofauna, which are essential for nutrient recycling [49]. Alternatively, under systems of organic farming management, when industrial N fertilizer is not used, organic matter origin-N, after biological degradation, is converted into mineral N forms, ammonium and nitrate, and becomes a major factor in plant production. However, as the mineral N content in soils increases beyond the capability of plants to take it up, it will cause N leaching and increase other kinds of N losses into the environment. In this case, it is important to understand the N cycle and soil N balance

As common knowledge, the origin of all kinds of N is air N2, 79% by volume of the earth's atmosphere. Soil microorganisms, free-living or associated with legumes, fix atmospheric N. This complex biological process begins with air N and ends with organic N. After organic

– by nitrifying bacteria and generally most NH4

–

greenhouse gases), and N2. Denitrification of nitrate produces about 90% N2 and 10% NOx. However, the natural N balance has been affected by industrial N fixation since the green revolution. Symbiotic N amounts to about 100-175 million tons each year in the 1970s world‐ wide, with industrial fixation of 3.5 million tons, and lightning may fix 10 million tons of N, a value that has probably not changed over time [51] . In 1989, industrially fixed N increased to

By the year 2050, the world population is expected to double from a level of more than 5 billion. It is reasonable to expect that the need for fixed N for crop production will also at least double. If this is supplied by industrial sources, synthetic fertilizer N use will increase to about 160 million tons of N per year [51]. Consequently because of its relatively low plant uptake level, generally around 50% or less, several major environmental reasons exist to seek alternative fixed N fertilizers, including the fact that it affects the balance of the global N cycle, pollutes groundwater, increases the risk of chemical spills, and increases atmospheric nitrous oxide (N2O), a potent greenhouse gas. The global budget for N2O appears to be out of balance, exceeding sinks by 30-40% and increasing at 0.25% each year [7]. In this case, biological N fixation should receive more attention because about 2 tons of industry-fixed N is needed as fertilizer for crop production to equal the effects of 1 ton of N biologically fixed in a legume

In cultivated land, the soil N balance is a complex system covering serial biological processes and physical and chemical processes. It includes plant N uptake, N fixation, organic N mineralization, nitrification and denitrification, nitrate leaching, and other losses, as N2 or

through the process of denitrification in soils [50].

80 million tons in response to the needs for high-yielding crops [22].

, which is ready to be used by higher plants, is released. NH4

, not NH4

and converts it to NH4

+

, although NH4

+

–

+ can be

<sup>+</sup> can be formed

– in

<sup>+</sup> is modified into NO3

, N mono-oxides (NOx,

Hairy vetch: N = 0.0409 × biomass – 3.1

Crimson clover: N = 0.0204 × biomass + 13.8

Austrian winter pea: N = 0.0402 × biomass – 9.2

Caley peas: N = 0.0426 × biomass – 6.1

Subclover: N = 0.0280 × biomass + 2.9,

where "biomass" is the dry weight in kg ha–1, and the N content, N, is also in Kg acre–1.

For legumes, on average, pounds of K = pounds of N, and pounds of P = 10% of N.

The study of cowpea/maize intercropping shown that cowpea had used atmospheric N for crop growth and also fixed the nutrient into the soil for subsequent crop. The soil residual mineral N was increased by 82% compared with initial soil N. This demonstrated that biological N2 fixation by cowpeas replenished the available N to both crops and also for subsequence crop [57].
