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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:

Cation exchange: The ‘soil cations’ essential for plant growth include ammonium, calcium, magnesium, and potassium. There are three additional ‘soil cations,’ which are not essential plant elements but affect soil pH. The additional ‘soil cations’ include sodium, aluminum and hydrogen.

The major distinguishing characteristic of cations is their positive charge. Just like a magnet, a positive charge is strongly attracted to a negative charge. When soil particles have a negative charge, the particles attract and retain cations. These soils are said the have a cation exchange capacity. Although most soils are negatively charged and attract cations.

The ‘soil cations’ are further divided into two categories. Ammonium, calcium, magnesium, potassium, and sodium are known as the ‘base cations,’ while aluminum and hydrogen are known ‘acid cations.’ Unlike the other base cations, sodium is not an essential element for all plants. Soils that contain high levels of sodium can develop salinity and sodicity problems.

The words ‘base’ and ‘acid’ refer to the particular cation’s influence on soil pH. As you might suspect, a soil with a lot of acid cations held by soil particles will have a low pH. In contrast, a highly alkaline soil predominately consists of base cations.

Cations in the soil compete with one another for a spot on the cation exchange capacity (Table 42 and Fig 18). However, some cations are attracted and held more strongly than other cations. In decreasing holding strength, the order with which cations are held by the soil particles follows: aluminum, hydrogen, calcium, potassium and nitrate, and sodium.

The essential plant nutrients, or essential elements, are listed in the Table 41. It enumerates 16 chemical elements of which the essentiality for plant growth and reproduction has been established according to the criteria formulated by D.I. Arnon and P.R. Stout, 1939. This listing has existed since 1954 with chlorine as the latest addition (cited by Hopkins 1999).

These plant nutrients are classified into various groups:

1.    Major elements and trace elements : Of the 16 listed, 9 are major elements or macronutrients and 7 are trace elements or micronutrients. Major elements are those which are needed by plants for their growth and reproduction in relatively large amounts while trace elements are needed in smaller amounts compared to the former. The major elements are further sub classified into structural elements (carbon, oxygen and hydrogen), primary macronutrients (nitrogen, phosphorus and potassium), and secondary macronutrients (calcium, sulfur, and magnesium). Micronutrients are Zn, Cu, Mn, Fe, Mo4 and boron (Table 41).

Water quality standards (Classification)

Some countries have established national standards and criteria for evaluation and classification of water quality. The classification of irrigation waters has been made according to one or more of the following criteria.

1.    Salinity or total concentration of soluble salts (EC)
2.    Sodicity hazards (SAR)
3.    Alkalinity hazards (RSC)
4.    Combined evaluation on the basis of EC and SAR by USSL diagram
5.    Combined evaluation on the basis of EC and Adj. SAR
6.    Combined evaluation on the basis of EC, SAR and RSC

The basic philosophy of nutrient management should be to apply fertilizers at rates to ensure high fertilizer use efficiency so that the amount of unutilized fertilizers is reduced to environmentally acceptable levels. Leaching of nitrate into public drinking water supplies has become a national problem. There is no realistic alternative but to use increasing quantities of inorganic fertilizers for maintaining food supplies in order to feed the increasing population. Used properly, fertilizers can, in fact, lead to better environment because greater vegetative growth and higher crop yields leave large quantities of residues in the soil, thus reducing erosion; absorb more carbon dioxide, a major source of green house effect and lessen the need to bring marginal and forest land into cultivation. The declining or stagnating yield trend in India has been attributed to multiple nutrient deficiencies and imbalance of nutrients. Therefore, the most widely recognized alternative is integrated nutrient management (Trehan et al., 2008).

Soil test based fertilizers is the maintenance of soil fertility and plant nutrient supply to an optimum level for sustaining the desired productivity through optimization of the benefits from fertilizers, organic manures, green manures, bio-fertilizers, non-conventional sources and crop residues. Soil test based fertilizers  aims at maximization of the use efficiency and minimization of the avoidable losses of nutrients from all the sources such that triple objective of maximization of crop yields, sustenance of soil, water and air quality and improvement of socio-economic conditions of farming community is accomplished.

Irrigation water always contains some soluble salts irrespective of its source. The suitability of waters for a specific purpose depends on the types and amounts of dissolved salts. Some of the dissolved salts or other constituents may be useful for crops. However, the quality or suitability of waters for irrigation purposes is assessed in terms of the presence of undesirable constituents, and only in limited situations is irrigation water assessed as a source of plant nutrients. Some of the dissolved ions, such as NO₃^- are useful for crops.

Collection of water samples

A representative water sample (500 ml) is collected in a glass or polyethylene bottle, which should be properly washed/rinsed with the same water that is being sampled. Floating debris or any other contaminant should be avoided while collecting the sample. After proper labelling (e.g. source of water, date of collection, and type of analysis required), the sample should be sent to the laboratory without undue delay.
Some of the anions such as SO₄ and NO₃ may be quite low in irrigation waters. Hence, the large volume of the sample has to be first concentrated by evaporating it to about 100 ml in order to obtain their detectable amounts.

For plant analysis to be meaningful, collection of particular plant part(tissue) at the right stage of growth is very important. A research worker in soil fertility is generally interested in finding out the nutrient uptake by crop at different stages. The stage at which nutrient uptake is to be studied will vary with different crops. One may also be interested in collecting a plant sample when some abnormality is observed in the plants and a nutrient deficiency or toxicity is suspected. In such cases no time should be lost in collecting the sample otherwise secondary reactions that often take place in the mineral content of plants will conceal the real cause of deficiency or toxicity. In such cases only the affected plants are to be sampled as compared to general sampling technique described below.

For research projects each plot receiving differential fertilizer or some other treatment is to be sampled separately. For general advisory purposes one sample would be good enough for the entire field (not exceeding 2 hectares). One should, however, take care that the sample comes from a field or a part there of having a uniform stand. If a field is patchy and differences in stand and plant growth are obvious, each such area should be sampled separately.

Estimation of organic carbon in soil (Walkley and Black, 1934)

Principle: Soil organic carbon is oxidized to CO₂ by heat of dilution obtained when a known amount of soil is treated with excess of standard K₂Cr₂O₇ in the presence of concentrated H₂SO₄. The excess of K₂Cr₂O₇ not reduced by the organic matter is back titrated with standard ferrous ammonium sulphate in the presence of phosphoric acid or sodium fluoride using diphenyl indicator. At the point colour changes from violet to blue or bright green.

Apparatus: 500 ml conical flasks, Pipettes of 10 and 20 ml, Burette, Balance, Measuring cylinder, Asbestos sheet.

Introduction

Many of the globe’s major food crops viz., wheat, rice and vegetables have reached “yield stagnation” or “yields plateau”. This can be attributed to the land degradation and soil diminishing capacity to supply nutrients at the required rate due to over mining of essential nutrients. Further, imbalance use of fertilizers and lesser use of organic manures over the years have badly affected the soil health. To bring back life to the soil and relieve it from nutrient stress; fertilizers and manures/bio-fertilizers have to be applied in a scientific manner. At the same time, their indiscriminate use can result either in their under utilization and toxicity or leaching to the lower horizons, leading to underground water and create pollution problems. The soil is also the main source of sixteen mineral elements identifies in view of their importance for plant growth. These are N, P, K, S, Ca, Mg, Na, Fe, Mn, Zn, Cu, Mo and Cl (Tisdale et al., 1985). Nitrogen, potassium and phosphorus have been designated as major nutrient elements in order of their removal by the plant; sulphur, calcium and magnesium as secondary nutrients while zinc, copper, iron, manganese and molybdenum are called micro nutrients as these are taken by plant in traces. 
Nutrients need of vegetables depends upon the yield response to applied nutrients from fertilizers, agro-climatic conditions, variety and nutrient availability from the soil pool. The nutrient status of the soil further depends upon the quantity of total and available nutrients present in the soil pool. The soil tests helps in selecting the proper manuring schedule for various crops in a sequence by knowing about the nutrient build up or depletion in the soil profile. The adequacy of soil testing methods to correctly predict the fertilizer needs varies with the soil type, climate/soil moisture and crop species. The rainfall also influences the efficiency of soil testing methods, especially nitrogen and potassium when it becomes imperative to have tissue analysis for modify the fertilizer recommendations. Plant tests also help to identify the nutrient deficiency in standing crop and in the event of deficiency of a particular nutrient helps in taking corrective measures (Sud 
et al., 2008).

Soil pollution is any physical or chemical change in soil that adversely affects the health of plants and other organisms living in and on it.

Soil pollution is important not only in its own right but because so many soil pollutants tend to move into surface water, groundwater, or air.

The chief soil pollutants are salts, petroleum products, heavy metals, and agricultural chemicals. Point source releases of industrial solvents (both LNAPLs and DNAPLs) and feedstock chemicals (e.g., for making plastics) often occur as the result of LUSTs (leaking underground storage tanks).

Salinization, a common problem in irrigated arid and semiarid regions, makes soil unfit for growing most crops. It is extremely difficult to remove excess salts from salinized soils.

A variety of techniques are used for soil remediation, which is cleaning up contaminated soil.

1. These include physical techniques such as
   1. Soil vapor extraction (SVE), moving air through the soil.
   2. Thermal conduction heating, heating the soil before SVE.
2. Bioremediation methods include
   1. Landfarming, fertilizing, liming, and repeatedly plowing contaminated soil to stimulate bacterial decomposition of high molecular weight hydrocarbons like those in diesel oil.
   2. Bioventing, pumping air at low rates through soil to provided O₂.
   3. Biosparging, pumping air into aquifers to provided O₂.
   4. Phytoremediation, particularly useful for removing heavy metals.

Introduction

In classification of irrigation water, it is assumed that the water will be used for average conditions with respect to soil texture, infiltration rate, drainage, quantity of water used, climate and salt tolerance of the crop. Large deviations from the average for one or more of these variables may make it unsafe to use what, under average conditions, would be a good water; or may make it safe to use what, under average conditions, would be a water of doubtful quality. The relationship to average conditions must be kept in mind in connection with the use of any general method for the classification of irrigation water.

However, the quality of irrigation water is evaluated, mainly by the quantity of dissolved salts, proportion of anions, monovalent and divalent cations, sodium adsorption ratio, residual sodium carbonate, potential salinity, specific ion toxicity, proportion of magnesium, calcium in water etc.

Introduction

It is necessary to ascertain the quality of irrigation water so as to gauge the possible effects of this water on the soil. The irrigation water often results in water logging, salinity and alkalinity problems. It is therefore very important to know its quality. Following are the basic criteria or characteristics on which irrigation water quality is judged.

1. Presence of total soluble salts in irrigation water.
2. Proportion of carbonates and bicarbonates in relation to calcium and magnesium in irrigation water.
3. Proportion of sodium in relation to other cations in irrigation water.
4. Concentration of specific ions in irrigation water.

Several parameters are employed to judge the quality of irrigation waters No parameter in complete in judging the quality of irrigation water. Further the quality of water should be judged by considering the crop and soil factors. Any judgment without considering the above factors may not be useful. Researchers devised many parameters by following above said criteria. Some of the commonly adopted parameters used to judge the quality of irrigation water are:

The crop growth and productivity is conditioned by many factors of which, the nutrient status (Content) of plant parts such as leaf, stem, etc play a critical role. Moreover the leaf and stem are considered as the indicator parts of plants for assessing the nutrients content of plant. Each crop plant requires the essential element at a specific concentration at different growth stages and it is known as ‘critical level’. When the nutrient content of plant depletes below the critical level the plants may exhibit some symptoms. The requirement or otherwise the availability of nutrients can be assessed by i) plant diagnosis ii) soil analysis and iii) plant analysis by two methods a) by qualitative test and b) by quantitative estimation. Based on the plant or soil tests, the required nutrients can be applied for crops to sustain the growth and rectify the deficiency disorders. The rapid tissue test would pave way for rectifying the nutritional problems for quick recovery, however the quantitative estimation of both plant and soil for nutrients concentration will be more useful and economic for applying fertilizers either as basal or foliar and would be the long term strategy to cope up with nutritional problems.

For rapid tissue test to assess the nutrient status, different parts of plant should be taken as indicator tissue and some of the representative crops are furnished below:

Leaf analysis and its importance

Improved fertilizer management for crops is important in view of today’s need to reduce production costs, conserve natural resources, and minimize possible negative environmental impacts. These goals can be achieved through optimum management of the fertilizer applied. Understanding the crop nutrient requirements and using soil testing to predict fertilizer needs are keys to fertilizer management efficiency.

Plant tissue testing is another tool for use in achieving a high degree of precision in fertilizer management. Timely tissue testing can help diagnose suspected nutrient problems or can simply assist in learning more about fertilizer management efficiency. Leaf analysis (also called stem leaf analysis, tissue analysis or foliar analysis) is the most precise method of monitoring plant nutrient levels. While soil analysis reveals the levels of essential soil nutrients, leaf analysis shows the grower exactly what the plant has successfully absorbed. Leaf analysis is especially helpful in detecting nutrient deficiencies before they affect plant health and yield.

Chemistry of vegetable crops

Vegetables play an important role in human nutrition supplying some of the materials, which other food materials are lacking. Vegetables are important in neutralizing the acidity formed during digestion. They also serve as roughages in human nutrition and thus helping digestion.

They are the important sources of minerals like Ca, P and Fe. They also form an important source of vitamins, especially vitamin A. Green and yellow vegetables like carrot, turnip, spinach, beans, etc. contain appreciable amounts of vitamin A. Direct seed of beans, peas, and legumes contain appreciable amounts of proteins. Vegetables also contain sufficient amounts of vitamin C and also appreciable quantities of thiamine, niacin and folic acid. Tomatoes and potatoes contain fairly high amounts of vitamin C.

There are four major tracts where salt affected soils are commonly met within India. These are:

1. The Semi-arid Indo-Gangetic alluvial tracts (mainly in Punjab, Haryana, Uttar Pradesh and a part of Bihar)
2. The arid tracts of Rajasthan and Gujarat.
3. The arid and semi arid tracts of southern states, particularly of the irrigated rigor (Vertisols) soils.
4. The coastal alluvium

Earlier it was estimated that about 7 million hectares of land have been affected by salinity/sodicity conditions in India. The area under these soils increased and reached to the level 10 million hectares at present.