eChapters

Labile fractions of soil carbon (C) refer to the fraction of soil carbon that are easily altered by microbial activities. These fractions serve as sensitive indicators to changes in management induced soil quality and are grouped under active soil C pool. The method described by Weil et al. (2003)estimates changes in biologically active soil C using a dilute KMnO . This is an improvement over the method used by Blair et al (1995), where a very high concentration (0.333 M) of KMnO₄ was used.

This is one of the commonly used method to estimate organic carbon content in soil. In this wet digestion method, soil organic matter is oxidised with a mixture of K₂Cr₂O₇ and concentrated H₂SO₄ utilizing the heat of dilution of H₂SO₄. The unused K₂Cr₂O₇ is back titrated with ferrous sulphate or ferrous ammonium sulphate.

In dry combustion technique, the combustion of soil sample (about 250 mg and mm sieved) is carried out at high temperature (about 1000 ºC) through feeding in an inert boat in the electric furnace. The CO₂ produced from the oxidation of carbon present in the soil sample at high temperature is carried towards non dispersive infrared detector (NDIR) by a carrier gas where it is quantified and total carbon content is determined.

Wet sieving procedure separates soil into different size aggregates. However, for comparing the aggregate size distribution among management treatments and soil types, it is convenient to use indices. Mean weight diameter (MWD) and water stable aggregate percentage are some of the common indices used in aggregate stability studies.

Mean Weight Diameter (MWD)

This index was proposed by Van Bavel (1949) and is being used widely. The entire soil sample is passed through an 8 mm sieve prior to analysis and is subjected to wet sieving procedure as described before. The parameter which Van Bavel (1949) called the MWD is equal to the sum of the product of (a) the mean diameter (d ) of each size fraction and (b) the proportion of the total sample weight (w ) occurring in the corresponding size fraction. The summation is carried out over all ‘n’ size fraction including the one that passes through the finest sieve.

Thisprocedure estimates aggregatestabilitytoimpact ofraindropsand water erosion. Wet sieving procedure was originally proposed by Yoder, 1936. Later, the procedure was modified byKemper and Rosenau, 1986.

Equipment

1. Yoder type sieve shaking electrically operated machine (3/4 inch stroke at 30 strokes per minute)
2. Concentric sieves of various sizes of pores to fit properly into the machine.
3. Weighing balance

Aggregation is one of the basic physical characteristics of soil. Under natural conditions, primary soil particles, particularly clays, tend to join together to form secondary units called aggregates. Size distribution of soil aggregates is important because the size of the aggregates determines their susceptibility to water and wind erosion. Also, their size is important in determining the pore size distribution and regulating pore space in cultivated soils.

There are a variety of procedures being in use to test the stability of aggregates and aggregate size distribution. However, choice of the method for analysis of aggregate size distribution is determined by the purpose of analysis. The developed measures are relative and purpose specific. This chapter discusses dry aggregate analysis.

Aggregation is a direct indicator of soil structure. Ahealthy soil is considered to have well aggregated structure. Though not routinely followed during soil testing and chemical analysis, aggregation is one of the major indicators of soil physical health. Wet sieve method (Yoder, 1936) is usually followed for soil aggregate analysis. This involves gently raising and lowering a nest of sieves in water to assess the stability of aggregates to rapid wetting. However, the Yoder’s wet sieving method has been modified to make it simpler and currently, the method of Kemper and Rosenau, 1986 is being commonly used in research laboratories. One of the pre-requisites in these methods is the availability of wet sieving apparatus and further, these methods are laborious and time consuming. On the other hand, the method described by Cambardella and Elliott (1993) is considered very ideal because of its simplicity and is being widely used for aggregate bound soil carbon fraction study. The soil sample is wet sieved with a series of three sieves (2000, 250 and 53 micron size).

Soil bulk density (BD) or apparent density is determined in -situ by core method. Stainless steel or metallic cores of cylindrical shape with known height and diameter are inserted into the soil till the rim of the core. The soil core is then excavated and the soil sample is oven dried to get the dry weight. The ratio of the oven dry weight to the total volume of the core gives the bulk density of the soil.

Soil texture refers to the relative distribution of sand, silt and clay on a sample of <2 mm size. It is an intrinsic soil property and basically refers to the particle size distribution in a soil. A number of physical characteristics and chemical properties of a soil are influenced by texture or particle size distribution. To estimate the content of sand, silt and clay particles, the soil samples are first brought into a dispersed state by mechanical and chemical treatments so that different binding agents are removed. In the dispersed soil samples, the soil particles settle down at a differential settling rate according to their size. In laboratory, the common methods for analysis for particle size distribution are : Buyoucos hydrometer method and International pipette method. In this chapter, the procedure of Buyoucos hydrometer method is described.

Principle

This method is based on the principle of variable rate of settling of particles as per their size and as described by Stokes’Law. Large particles shall settle fast. The measurement of density of the suspension is done at different times using a hydrometer (ASTM 152H-Type hydrometer is preferred). The measurement in this hydrometer is based on a standard temperature of 20 _oC and a particle density of 2.65 Mg m_-3 and the units are expressed in grams of soil per liter.The principal source of error in this procedure is the incomplete dispersion of soil clays as the clay particles are cemented by agents such as organic matter, CaCO and iron oxides. Incomplete dispersion results in low values for clay and high values for silt and sand. Thus, soils with higher organic matter (>1.0%) should be first treated with 6% hydrogen peroxide (H₂O₂) for removal of organic carbon. The rate of sedimentation is also affected by temperature and the density of the dispersing solution.

Exchanges sodium (Na+) is animportantparameter forsalt affected soils. This is estimated by extraction with ammonium acetate and then measuring the extract inflamephotometer.

Reagents

1. Neutral normal ammonium acetate solution (1 N CH₃COONH₄ of pH 7): Dissolve 77.09 g of ammonium acetate in distilled water and make up the volume to one litre. Adjust the pH of the solution to 7 with ammonia solution or acetic acid.
2. Standard solution of Na (500 mg/L Na): Dissolve 1.271 g of NaCl in distilled water and make up the volume to one litre.
3. Working standard solution of Na (100 mg/L): Dilute 200 ml of 500 mg/L stock solution to 1000 ml.

Ion exchange is a reversible process by which a cation or anion held on the solid phase is exchanged with another cation or anion in the liquid phase. Soil mineral and organic colloidal particles have negative valence charges that hold dissociable cations. The power to retain cations at the surface of soil colloids is referred to as the cation exchange capacity (CEC). Cation exchange is a reversible reaction in soil solution, dependent upon negative charges of soil components arising from permanently charged or pH-dependent sites on organic matter and mineral colloid surfaces. The CEC is the sum of cations held by a kilogram of soil and is expressed in Cmol (p₊)/kg soil. In most agricultural soils, the cation-exchange capacity (CEC) is generally higher than the anion exchange capacity (AEC).

Electrical conductivity (EC) is a useful indicator of soil salinity. Relationships have been developed between EC and salinity classes for a 1:1 soil: water suspension. With increase in salt content in a soil, the EC increases. Thus, for salt affected soils, measurement of EC is vital as accumulation of salts in the root zone is detrimental to plant growth. EC is a measure of the ability of a soil solution to conduct electrical current.

Equipment and Reagents

1. Digital Conductivity meter
2. 0.01 M potassium chloride (KCl) solution: Dissolve 0.7456 g dry KCl in distilled water and make up the volume to 1.0 litre. This solution has an electrical conductivity of 1.413 dS m_-1 at 25ºC.

Soil pH is the measure of hydronium ion (H₃O⁺) or more commonly hydrogen ion (H⁺) activity in the soil suspension. Soil pH can be measured in soil-water suspension, saturation extract or in 0.01 M CaCl₂ solution. Saturation extract is used to identify specific soil problems such as salinity or sodicity. Measurement of pH using 0.01 M CaCl₂ solution gives more reproducible results than measured through soil-water suspension. It also provides a good approximation of field conditions as the electrolytic composition of 0.01 M CaCl₂ is considered similar to that of soil solution. However, among the three methods, soil-water suspension is most commonly used in research laboratories as well as for fertilizer recommendation as it is simpler and more convenient.

Soil sample collection is the first and most important step for soil quality analysis. A soil sample collected with due care and proper procedure is essential for getting reliable result of the analysis. The sample should represent the field and for practical purposes, an area of about 0.5 hectare is considered as one sampling unit. In case of conspicuous variation observed in the field in terms of texture, colour, slope etc, separate samples should be collected from areas differing in these attributes. In other words, sample should be collected from an area in the field with similar characteristics as seen through normal visual observation. Sampling should thus be avoided from areas like near trees, permanent vegetations, manure pits, manure piles or recently fertilized fields.

Soil Sampling procedure

1. For a particular field unit, one composite soil sample is prepared out of the samples collected from 7-8 spots distributed across the field unit. The entire field is covered by moving in a zig-zag manner from each sampling spot.
2. For each sampling point or spot, scrapping off the surface litter such as grasses or crop residues is done. Then, a “V” shaped pit of 15-20 cm length and of equal width is made. Depth of sampling depends upon the requirement and on the type of crop. For field crops like paddy, wheat etc, samples are collected from 15 cm depth. From the “V” shaped pits, take outslice of ½ inch thickness from both sides of the exposed pit surfaces from top to bottom.
3. The soil samples so collected from the 7-8 points are mixed thoroughly by hand and then is reduced to about 500 g by quartering process. The quartering process consists of a procedure in which the entire soil mass is divided into four quarters; the two opposite ones are discarded and the remaining two are remixed. This process is repeated till about 500g soil is obtained.

Potato plays a vital role in food security for ever increasing world population (Thiele et al., 2010; Scott and Sourez, 2011; Scott and Sourez, 2012). It is highly capital and labour intensive crop (Kushwah and Singh, 2011). Presently, India is the second largest potato producer in the world. India is producing about 45-48 million tonnes of potato annually from about 2 million hectare of area. However, India would require 125 million metric tonnes of potato from 3.6 million hectare area in 2050 (Vision, 2050). The stagnating growth rates of cereals’ productivity, large scale diversion of food grains to feed & bio-fuel and expected steep rise in per capita consumption of pulses, edible oil, fruits, vegetables, milk, sugar and non-vegetarian food in the regime of steadily rising population is bound to put pressure on existing cultivable land. Since, cultivable land is expected to remain more or less constant in the next 40 years, the role of crops like potato having higher production potential per unit land and time will become imperative. In this context potato crop has very high probability of making crucial contribution to the future national food security agenda. The perceived changes in Indian socio-economics in the medium and long term are expected to enhance per capita food consumption of fresh potatoes.

Fertilizer and nutrient recommendations for different crops are mostly region specific and very general in nature which results into a blanket application of fertilizer leading to its poor efficacy and wastage of costly resources as well as of environmental pollution. In addition, this has caused severe imbalance of nutrients in soil. The spatial variability of available nutrients properly mapped along with other relevant parameters will help to develop site specific nutrient management practices with increased productivity and profitability apart from checking nutrient imbalance and environmental pollution.

Maximum potato yield occurs when sufficient P is available during early vegetative development and the entire period of tuber growth. Total plant P uptake increases rapidly during tuber initiation, levels off to a constant rate during tuber bulking, and ceases with plant maturation occurs primarily through the transfer of P reserves from the vine and roots. Phosphorus uptake by potatoes is relatively low compared with uptake of potassium or nitrogen but similar to uptake of sulfur. The amount of P in the soil solution that is readily available for plant uptake is very small compared with the total amount of P in the soil.

A general blanket application of fertilizer leads to its poor efficacy and wastage of costly resources as well as of environmental pollution. This becomes more relevant in case of P as unlike nitrogen applied P remains in soil and adds to its build up. In addition, this has resulted in severe imbalance of nutrients in soil (Kumar et al., 2009).

Keeping this in view, a multilocation experiment was conducted in the major potato growing states of India under the All India Coordinated Research Project on Potato to evaluate suitable dose of P based on soil test on P economy in potato (table 52). Exact quantification of phosphorus fertilizer based on soil test is very important.

Phosphorus is one of the key nutrients required for higher and sustained productivity of potato and its influence on tuber yield is very well established. Phosphatic fertilizers are expensive and in developing countries like India, they are either imported or manufactured using imported raw material. Due to increase in cost in the recent past, there has been a decreasing trend in the amounts of P fertilizer applied in India (Sundara and Natarajan, 1997 and Sundara, 1988). At the same time, soils contain substantial reserves of total P, most of it remains relatively inert, and only less than 10% of soil P enters the plant–animal cycle (Kucey et al., 1989). Upon addition to the soils, soluble phosphates react with the constituents of the soil and form compounds that are less soluble. This conversion depends upon the soil type. In acid soils, the reaction products are aluminium and iron phosphates and in the predominantly calcareous soils, the reaction products are calcium phosphates. As a result most of the P applied (often as much as 90%) is rendered unavailable for crop uptake but is retained in insoluble form. This is the reason that the potato-based cropping systems in different potato growing pockets generally show positive P balance resulting into its build up (Singh et al., 1994). Thus, soils commonly have large reserves of ‘fixed’ P that could support long-term crop requirements if it could be mobilized through appropriate soil management including use of P-solubilizing microbes. The ability of phosphate solubilizing bacteria (PSB) to convert insoluble forms of phosphorus to an accessible form is an important trait in sustainable farming for increasing plant yields (Singh and Kapoor, 1994). The beneficial effects of PSB on crop productivity have been widely described. These soil phosphate solubilizing bacteria stay near the roots and make the phosphorus available to plants from soil.

Factors for deterring the Fertilizer Schedule are two factor

Soil Supplying Power: Growing different crops during different seasons alters the soil nutrient status, estimated by soil analysis at the beginning of the season. The soil supplying power increases with legume in rotation, fertilizer application and addition of crop residues. The available nitrogen and potassium in soil after groundnut are higher to initial status of the soil. But after pearl millet, only potassium status in the soil is improved and no changes in P.

Nutrient Uptake by Crops: The total amount of nutrients taken by the crops in one sequence gives an indication of the fertilizer requirement of the system. The balance is obtained by subtracting the fertilizer applied to crops that nutrient taken by the crops.

Residual Effect of Fertilizers: The extent of residues left over in the soil depends on the type of fertilizer used. Phosphatic fertilizer and FYM have considerable residue in the soil, which is useful for subsequent crops. The residues left by potassium fertilizers are marginal. Legumes add nitrogen to the soil in the range of 15 to 20 kg/ha. The amount of nitrogen added depends on the purpose. Green gram grown for grain, contributes 24 and 30 kg N respectively to the succeeding crop. Inclusion of leguminous green manures in the system add 40 kg to 120 kg N/ha. The availability of phosphorous is also increased by incorporation of green manure crops.

Biofertilizers are preparations containing micro-organisms, with capability of mobilizing nutritive elements from non usable form to usable form through biological processes. Bio-fertilizers have definite advantage over chemical fertilizers. Chemical fertilizers supply over nitrogen whereas bio-fertilizers provide in addition to nitrogen, certain growth promoting substances like hormones, vitamins, amino acids, etc, crops have to be provided with chemical fertilizers repeatedly to replenish the loss of nitrogen utilized for crop growth. Bio-fertilizers application in agriculture will have greater impact on organic agriculture and also on the control of environmental pollution, soil health improvement and reduction in input use. Farmers try to increase yields of vegetable crops by mean of heavy nutrition’s. The use of chemical nitrogen and phosphorus fertilizers at high levels had an adverse effect on the accumulation of NH4+, NO3-, NO2- and PO4- in vegetable product tissues. Therefore, clean agriculture recently depends upon using biofertilizers as well as organic in order to produce high yields with the best commodity quality without contamination and less accumulation with heavy metals.

The application of high input technologies such as chemical fertilizers, pesticides, herbicides have improved the production but there is growing concern over the adverse effects of the use of chemicals on soil productivity and environment quality. Thus, integrated nutrient management has become an accepted strategy to bring about improvement in soil fertility and protecting the environment. This strategy utilizes a judicious combination of fertilizers, organic manures and bio-fertilizers. Biofertilizer is a natural product carrying living microorganisms derived from the root or cultivated soil. So they don’t have any ill effect on soil health and environment. Besides their role in atmospheric nitrogen fixation and phosphorous solubilisation, these also help in stimulating the plant growth hormones providing better nutrient uptake and increased tolerance towards drought and moisture stress. A small dose of biofertilizer is sufficient to produce desirable results because each gram of carrier of bio-fertilizers contains at least 10 million viable cells of a specific strain. Biofertilizers means the product containing carrier base (solid or liquid)

In order to get maximum benefit from manures and fertilizers, they should not only be applied in proper time and in right manner but any other aspects should also be given careful consideration. Different soils react differently with fertilizer application. Similarly, the N, P, K requirements of different crops are different and even for a single a crop the nutrient requirements are not the same at different stages of growth. The aspects that require consideration in fertilizer application are listed below: