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D. L. N. Rao et al.
Africa have experimented with innovative agricultural practices for centuries. In
one such practice in eastern India for direct seeded rice, cow-dung is powdered and
mixed thoroughly with the soil after broadcasting of rice seeds and then land surface
is levelled with a wood log. This helps in hastening nitrogen mineralization from the
dung to meet the plant demand by the young plants due to improvement in water
holding capacity of the soil [21].
Generally, when manure is applied at rates adequate to meet the N requirements
of crops, P and K are more than adequately supplied as well. But that is not always
the case as the manure may not be available in sufficient quantities for application, in
many situations, or the manure may not have adequate P content. So, organic manures
alone may not be able to supply sufficient amount of P for optimum crop growth.
The organic manures are known to decrease P adsorption/fixation and enhance P
availability in P-fixing soils. Organic anions formed during the decomposition of
organic inputs can compete with P for the same sorption sites and thereby increase
P availability in soil and improve utilization by crops. Reddy et al. [77] observed
higher apparent P recovery by soybean-wheat system on Vertisol with a combination
of fertilizer P and manure (Table 3). On acid soils, as are common in tropical areas,
addition of manure or easily degradable plant material to the soil also increases the
availability of P to plants through competition between organic anions and phosphate
anions at ligand exchange sites.
2.2.3 Synchronization of Nutrient Release
To achieve plant adapted fertilization in organic farming, it is most important to match
nutrient release pattern from organics with nutrient uptake pattern of crops to achieve
as closed a system as possible and nutrients are not lost by leaching or volatilization.
One approach is to manipulate plant demand by controlling planting date and duration
of crop growth, another is to control the quality and quantity of organic inputs applied,
as for example by applying a material that immobilizes N during fallow or early crop
growth, and then releases it later when crop demand is greater [81]. Nitrogen release
from good quality materials (low C:N ratio of 12–14:1) is very fast,in green manures,
about 53, 73 and 92% of the total N was mineralized after 4, 8 and 16 weeks of
laboratory incubation in a Vertisol. From poultry manure, the corresponding values
were 43%, 63% and 84%, respectively. Addition of intermediate C:N ratio (30–
35) materials such as farmyard manure, soybean and chickpea straw resulted in
immobilization of N during the first 6 weeks followed by release of mineral N from
8 to 16 weeks of incubation. Addition of material with wide C:N ratio (80:1) such as
wheat straw resulted in a marked decrease in mineral N due to its immobilization. The
total amount of N mineralized ranged from 95% in Gliricidia to ~28% from added
wheat residues. Such information is useful to decide on combinations of different
proportions of organic materials of different quality to be applied so as to achieve
proper synchrony of N release with crop demand. The location of released nutrients
with respect to plant roots is also part of the synchrony concept.
D. L. N. Rao et al.
Africa have experimented with innovative agricultural practices for centuries. In
one such practice in eastern India for direct seeded rice, cow-dung is powdered and
mixed thoroughly with the soil after broadcasting of rice seeds and then land surface
is levelled with a wood log. This helps in hastening nitrogen mineralization from the
dung to meet the plant demand by the young plants due to improvement in water
holding capacity of the soil [21].
Generally, when manure is applied at rates adequate to meet the N requirements
of crops, P and K are more than adequately supplied as well. But that is not always
the case as the manure may not be available in sufficient quantities for application, in
many situations, or the manure may not have adequate P content. So, organic manures
alone may not be able to supply sufficient amount of P for optimum crop growth.
The organic manures are known to decrease P adsorption/fixation and enhance P
availability in P-fixing soils. Organic anions formed during the decomposition of
organic inputs can compete with P for the same sorption sites and thereby increase
P availability in soil and improve utilization by crops. Reddy et al. [77] observed
higher apparent P recovery by soybean-wheat system on Vertisol with a combination
of fertilizer P and manure (Table 3). On acid soils, as are common in tropical areas,
addition of manure or easily degradable plant material to the soil also increases the
availability of P to plants through competition between organic anions and phosphate
anions at ligand exchange sites.
2.2.3 Synchronization of Nutrient Release
To achieve plant adapted fertilization in organic farming, it is most important to match
nutrient release pattern from organics with nutrient uptake pattern of crops to achieve
as closed a system as possible and nutrients are not lost by leaching or volatilization.
One approach is to manipulate plant demand by controlling planting date and duration
of crop growth, another is to control the quality and quantity of organic inputs applied,
as for example by applying a material that immobilizes N during fallow or early crop
growth, and then releases it later when crop demand is greater [81]. Nitrogen release
from good quality materials (low C:N ratio of 12–14:1) is very fast,in green manures,
about 53, 73 and 92% of the total N was mineralized after 4, 8 and 16 weeks of
laboratory incubation in a Vertisol. From poultry manure, the corresponding values
were 43%, 63% and 84%, respectively. Addition of intermediate C:N ratio (30–
35) materials such as farmyard manure, soybean and chickpea straw resulted in
immobilization of N during the first 6 weeks followed by release of mineral N from
8 to 16 weeks of incubation. Addition of material with wide C:N ratio (80:1) such as
wheat straw resulted in a marked decrease in mineral N due to its immobilization. The
total amount of N mineralized ranged from 95% in Gliricidia to ~28% from added
wheat residues. Such information is useful to decide on combinations of different
proportions of organic materials of different quality to be applied so as to achieve
proper synchrony of N release with crop demand. The location of released nutrients
with respect to plant roots is also part of the synchrony concept.