the Second World War and the Bengal famine in 1943, technological possibilities of agricultural development in India were explored and it was observed that ...
Chapter 22
Soil Fertility: Evaluation and Management BIJAY-SINGH AND YADVINDER-SINGH Upon handful of soil our survival depends. Husband it and it will grow our food, our fuel, and our shelter and will surround us with beauty. Abuse it and the soil will collapse and die, taking humanity with it. — Atharava Veda, the Sanskrit Scripture, 1500 BC.
22.1. Introduction Fertile and productive soils are vital components of stable societies because they ensure growth of plants needed for food, fibre, animal feed and forage, medicines, industrial products, energy and for an aesthetically pleasing environment. Fertility of the soil refers to its capacity to support the production of crops. It is a scientific discipline that integrates the basic principles of soil biology, soil chemistry, and soil physics to develop the practices needed to manage nutrients in a profitable and environmentally-sound manner. Thus, soil fertility can be defined as the capacity of the soil to supply sufficient quantities and proportions of essential plant nutrients required for optimal growth of specified plants as governed by the chemical, physical and biological attributes of soil. As pollution of soil, air and water must also be prevented while optimization of the nutrient status of soils for crop production, modern soil fertility practices take into account both environmental protection as well as agricultural productivity. A productive fertile soil can support optimal plant growth from seed germination to plant maturity by providing adequate soil volume for plant root development, water and air for root development and growth, chemical elements to meet the nutritional requirements of the plants, and anchorage for the resultant plant structure. These attributes of the soil can be distinguished as inherent or dynamic soil quality indicators. Inherent soil quality indicators cannot be manipulated in crop production and include soil texture, depth and mineralogy. Dynamic soil quality indicators include soil organic matter content, nutrient- and water-holding capacity, and soil structure. Among these, soil fertility is one of the most important dynamic soil parameters, which can be maintained or improved through appropriate management for enhancing productivity of agricultural soils.
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Soil fertility focuses on an adequate and balanced supply of nutrients to satisfy the needs of plants. As different plants have different needs for the essential nutrients and different tolerances of the toxic elements, soil fertility is plant-specific. For example, cassava is native to the humid tropics and grows well on strongly acid soils containing a large amount of soluble aluminium. On the other hand, wheat likes neutral or alkaline soils containing very little soluble aluminium. Thus, a soil can be fertile for cassava but will be infertile for wheat. To achieve production objectives, more nutrients are usually required than can be supplied by the soil. For example, a soil considered fertile in its natural state may be able to sustain wheat grain yields of just over 2 t ha -1. It will be necessary to raise the fertility of the field by adding nutrients in the form of organic manures, fertilizer or both to reach a yield level of 5 t ha-1. Thus soil fertility is a relative rather than absolute term. It is important to consider whether a particular soil will respond to the use of inputs to improve soil fertility and increase yields. It is this responsiveness to management that often constitutes a major criterion used by farmers for a fertile soil. Farmers may only become aware of the potential to improve the fertility of their soils when the effect of using improved germplasm and the addition of mineral fertilizers becomes visible. For crop production purposes, soil fertility should therefore, be viewed in the broader context of soil productivity, putting into perspective the chemical, physical and biological properties of the soil as they regulate supply of nutrients either inherently available in the soil or applied as manures and fertilizers to plants.
22.2. Historical Aspects of Soil Fertility Interest in soil fertility seems to be originated with the development of agriculture. By Roman times many soil fertility management practices were in use. These included manuring, liming, crop rotations, and fallowing to build up the supply of available nutrients. During the middle of nineteenth century to the beginning of twentieth century significant progress was made in the evaluation of soil fertility and its improvement with the addition of manures and fertilizers. Jean Baptiste Boussingault (1802-1882) quantified nutrients in manures applied to field plots and prepared balance sheets showing inputs and outputs in the harvested crop. At about the same time, Justus von Liebig (1803-1873) propounded the law of the minimum, which states that yield of a crop in the field was in direct relation to the essential plant nutrient present in the minimum quantity. By 1855, it was established that (i) crops require both phosphorus (P) and potassium (K), (ii) nonleguminous crops require a supply of nitrogen (N) for their growth and development, and (iii) soil fertility can be maintained by means of mineral fertilizers and manures. Later, many long-term fertilizer experiments established around the world convincingly showed remarkable benefits from the use of animal manures and fertilizers, fallowing and green manuring, and depletion of soil fertility with concomitant yield reductions due to continuous cropping without addition of manures or fertilizers. In India, since the ancient times soils have been classified according to nature (or fertility) of the soil as urvara (fertile) and usara (sterile). Fertile soils were further subdivided according to suitability to different crops. The earliest reference of manuring to enhance soil fertility can be traced as far as the times of Atharva-Veda (1500 – 500 B.C.). The natural fertility of Indian soils has been the one common feature noticed by all foreign travellers to India prior to 20th century. The Royal Commission on Agriculture appointed in British India in 1926 summarized the fertility status of soils in the country as: (i) red soils and black soils of peninsular India were deficient in N, P and organic matter but K and lime were not deficient, and (ii) contents of N, P and organic matter in alluvial soils were low and variable; K was adequate and P though not plentiful was less
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deficient than in other Indian soils (Abrol and Nambiar 2008). After the food crisis during the Second World War and the Bengal famine in 1943, technological possibilities of agricultural development in India were explored and it was observed that average yield of rice could be increased by 50%; 10% with the use of improved cultivars and 40 % through manuring. Similar increases were mentioned for other crops as well (Randhawa 1979). In the 1950s, a number of permanent manure plot experiments were initiated which convincingly showed that mineral fertilizers would not inhibit soil productivity if applied judiciously. Soil fertility research was strengthened in the 1960s with the introduction of high yielding varieties of rice and wheat. In the 1970s, emphasis was laid on nutrient balances and soil fertility management in multiple cropping systems. Micronutrient research received greater attention in 1980s, once zinc deficiency was detected in the intensive cropping systems (Abrol and Nambiar 2008).
22.3. Soil Fertility Evaluation Nutrient management practices formulated to achieve economically optimum plant performance as well as minimal leakage of plant nutrients from the soil-plant system can only be optimized after soil fertility evaluation. Thus, soil fertility evaluation is a central feature of modern soil fertility management. The fundamental purpose of soil fertility evaluation is to quantify the ability of soils to supply nutrients for plant growth. Soil fertility evaluation can be carried out using a range of field and laboratory diagnostic techniques and a series of increasingly sophisticated empirical and/or theoretical models that quantitatively relate indicators of soil fertility to plant response. The diagnostic techniques include chemical and biological soil tests, visual observations of plant growth for nutrient deficiency or toxicity symptoms and chemical analysis of plant tissues. New approaches include passive or active optical sensing technologies and geographical information systems that facilitate landscape scale site-specific assessment of soil fertility and can better describe and address the temporal and spatial variability of soil fertility. In view of the need to balance productivity and environmental protection for a wider and more diverse range of land uses, soil fertility evaluation is more complex today as illustrated conceptually in Figure 22.1.
22.3.1. Soil Testing The four basic components of soil testing are soil sample collection and handling, soil analysis, interpretation of results, and recommendations for actions. For successful soil fertility evaluation each component must be conducted properly. A soil sample is the basic entity which is used for evaluation of soil fertility and for giving advice to the farmer for a profitable manipulation of soil fertility. Thus, it is important that soil sample should be truly representative of the field. If a field is too heterogeneous, as it may appear from the undulating nature or knowledge about the previous crop cover, several samples from the parts of soil which are apparently more homogenous, should be collected. It has been observed that the error in sampling a field is larger than the error in laboratory analysis. In fact, a soil test is no better than the sample on which it is performed. Ideally, samples should be taken prior to seeding and before applying any amendments. In order to make an intelligent use of periodical soil tests, careful recording of inputs and outputs is essential. The fundamental tenet of soil testing is that only a proportion of the total quantity of a nutrient element becomes available for assimilation by plants. Thus, a soil test extracts via complexation, dissolution, desorption, exchange or hydrolyzation a percentage of
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Figure 22.1. Conceptual summary of the process of soil fertility evaluation (Modified from Thomas Sims and McGrath 2011)
total soil nutrient pool that is proportional to the quantity that will become available to the plant. A large number of different empirical soil tests are currently in use to measure available nutrients, even for a single element. These can be categorized as: •
Neutral salt solutions, e.g. 0.01M CaCl2 (for P and K), 0.01M Ca(H2PO4)2 (for S), 2M KCl (for mineral N) • Anion or cation exchange resins (for P, S, K, Mg and Ca) • Acids or alkalis of varying strength, e.g. 0.005M H2SO4 (for P), 0.5M NaHCO3 (for P) • Complexing or chelating agents, e.g. 0.005M DTPA (for Cu, Zn, Mn and Fe) The neutral salt solutions represent mild extractants and these tend to reflect the intensity factor of the nutrient supply of the soil. The acids and complexing agents represent severe extractants and are dominated by the quantity factor of nutrients in the soil. As majority of soil tests are based on theoretical consideration of some aspects of plant nutrient availability such as intensity, quantity, buffer power, and diffusion of nutrients in soil, isotopic dilution techniques have been used to label the plant-available
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nutrient in the soil and then trace its availability through crops, thus avoiding the artefact effects of soil extractants. It has been tried for phosphorus in greenhouse or field. Irrespective of whether the quantity or intensity factor (or both) is (are) measured, a soil test can be used for evaluating soil fertility of a group of soils based on correlation with crop response. All soil test values need to be related to the nutrient status of the soil and sufficiency for a specific crop in order to apply a corrective dose of the nutrient through fertilizer or manure. The numerical value of the soil test has no meaning unless information is generated to evaluate (1) what the soil test value means concerning growth and/or yield level in relation to the amount needed to maximize growth or yield, (2) whether crop growth or yield will be greater when the nutrient is added to the soil and how much greater, and (3) the amount of nutrient needed for the crop to attain better growth or yield in different soils and for different crops at different test levels. A combination of correlation and calibration studies is necessary to generate information needed to answer these questions. Interpretation of soil test for agricultural system is based on establishment of a statistical correlation between soil test value and some aspect of plant response such as yield of dry matter, grain or nutrient, preferably in the field. Although greenhouse studies are usually the first step to determine whether there is a relationship between plant uptake of a nutrient or yield and the amount of nutrient extracted by a particular soil test, ultimately field experiments are conducted to determine how accurate the soil test will be under normal growing conditions. In some soil test-crop response correlation procedures, soils are grouped according to clay content, cation exchange capacity, or soil pH, and separate calibrations are established for each group of soils (Dalal and Subba Rao 2006). Sometimes the relationship can be established for only one nutrient and one crop, and on a particular group of soils. In such cases the soil test should only be used for those specific conditions. For example, in some regions useful correlations have been established between the soil P test and percent of maximum yield only for specific crops. These are helpful in determining the adequate soil test P level for maximum yields. As shown in Figure 22.2, crops such as wheat, rice, and soybean vary in their response to the amount of P in the soil. While yields of both rice and soybean change rapidly with small differences in soil test P, wheat requires higher levels of soil P to attain maximum yields. The next step in soil fertility evaluation using soil tests is calibration, which aims
Figure 22.2. Response of different crops to a range of available P levels in the soil
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categorization of soil test levels in terms of the probability of economic response to nutrient applications. Figure 22.3 shows a general relationship between relative yield (defined as the yield obtained without application of the nutrient under consideration divided by the yield recorded when no other factors are limiting) obtained by applying a nutrient and the availability of the nutrient as determined by the soil test. The calibration procedure consists of mathematically modelling the relationship between the soil test level and plant response to nutrient additions. As these relationships are almost always nonlinear, curvilinear regression models are used to identify the point where plant performance is optimal – usually associated with 93-95% of the maximum attainable yield. Soil test value associated with optimum yield is referred to as the critical value (Figure 22.4). Data pertaining to relative yield from field experiments in a given region are combined and plotted against the soil test values to determine the critical soil test level mathematically or graphically. Use of relative yield minimizes the influence of uncontrolled variables and thus helps interpretation of data collected over years, locations, soils, climates and management settings. After field correlation-calibration experiments have been conducted, soil test levels of a given nutrient can be placed into categories
Figure 22.3. Crop yield response to a low and high rate of a given nutrient as related to the original soil nutrient level
Figure 22.4. Generalized illustration of the principles of soil test correlation and calibration using curvilinear model
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Table 22.1. Probability of a crop yield increase due to nutrient application as fertilizer Nutrient index level as per the soil test
Meaning of the index level for crops
Very low Low Optimum High
Probability of application of the nutrient to be beneficial will be 80% Probability of application of the nutrient to be beneficial will be 65% Applying the nutrient has about 50% chance of being beneficial in growth or yield Applying the nutrient will be beneficial less than 1% of the time
related to the magnitude and probability of yield response. These categories give quick insight to fertilizer decisions. Their general meaning is given in Table 22.1 in terms of the probability of a yield increase due to fertilizer application. This procedure to develop fertilizer recommendations is particularly suitable for nutrients such as P and K, generally considered immobile in the soil.
22.3.2. Plant Testing Soil fertility evaluation can be carried out not only by soil tests but also by characterization of growth and nutrient composition of plants. Nutrient deficiency symptoms, in-field evaluation techniques, analysis of plant tissues in the laboratory and remote sensing constitute the plant-based soil fertility evaluation techniques. The underlying premise in all the techniques is that the content of a nutrient in the plant is proportional to the availability of the nutrient in the soil.
22.3.2.1. Nutrient Deficiency Symptoms The appearance of a plant has long been a clue to its nourishment or lack of it. When soil is not able to supply adequate amounts of one or more plant nutrients, plants start showing starvation signs called deficiency symptoms. These symptoms are nutrientspecific and show different patterns in different plants. For example, P deficiency typically causes stunted growth and N deficiency an overall yellowing of leaves and stunted growth. Interveinal chlorosis due to Fe deficiency is associated with high soil pH. Potassium deficiency shows as marginal ‘spotting’ that develops into complete necrosis of the leaf margins. It is a good tool to detect deficiencies of nutrients in the soil. However, one must develop diagnostic proficiency through practice and close observation to identify nutrient deficiencies in different plants. The deficiency symptoms in many cases are not always clearly defined and in cases, the symptoms can be common to other causes or may be masked by other nutrients. Some typical examples are given below: • • • • • •
N deficiency can be confused with S deficiency. Calcium deficiency can be confused with B deficiency. Fe deficiency can be confused with Mn deficiency. Leaf stripe disease of oat can be confused with Mn deficiency. Effect of virus, little leaf etc. can be confused with Zn deficiency and/or B deficiency. Brown streak disease of rice can be confused with Zn deficiency. Deficiency symptoms always indicate severe starvation and therefore the crop may have suffered before the deficiency symptoms appear. Many crops start losing yields well before deficiency signs start showing. This yield limiting condition is called hidden hunger which refers to a situation in which a crop needs more of a given nutrient and yet has not shown any deficiency symptom. The nutrient content is above the deficiency
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symptom zone but still considerably below that needed for optimum crop production. In this case, significant responses can be obtained with application of nutrients even though no recognizable symptoms have appeared. To overcome such conditions, it is always recommended to confirm deficiency problem with other diagnostic techniques and the most common is soil testing.
22.3.2.2. In-field Evaluation of Nutrient Status of the Plant The use of rapid tissue tests of plant saps are one of the common in-field diagnostic techniques. Since soil tests provide semi-quantitative information on plant nutritional status, these can only be used as a guide rather than as the sole basis for nutrient management recommendations. Recently, small hand held spectrometers called chlorophyll meters are being used for field scale plant N testing in intact leaves. Chlorophyll meters direct a beam of light corresponding to the wavelength absorbed by the chlorophyll molecule through a plant leaf. The meter essentially measures leaf greenness which is a measure of leaf chlorophyll content. Similarly leaf colour charts have also been employed to estimate leaf colour. Both chlorophyll meters and leaf colour charts are particularly suited to the irrigated crops because short supply of water can also lead to chlorosis. Chlorophyll meters and leaf colour charts constitute an important advancement in field scale management of N nutrition in agronomic crops. Due to its ability to characterize spatial variability down to resolution of a few meters and to identify nutrient deficiencies in real time, remote sensing can help overcome the shortfalls of traditional plant and soil analysis. Although, remote sensing is of limited use in agriculture due to cost and time associated with collection and processing of data generated by satellites, recently portable active plant sensors have become commercially available. These sensors emit their own light and can be hand-held or mounted on equipment and have not only been shown to reliably predict crop yields but also can control variable rate N applications in real time.
22.3.2.3. Plant Analysis Composition of plants or a portion of the plant with respect to elements essential for growth as worked out by plant analysis can be interpreted by two widely used methods: (i) critical nutrient concentration and (ii) the diagnosis and recommendation integrated system (DRIS), which deals primarily with nutrient ratios. As with soil testing, critical nutrient concentration approach consists of correlation and calibration studies, because a significant relationship exists between the nutrient concentration in a plant and plant response to addition of the nutrient (Figure 22.5). At the critical concentration plant performance changes from suboptimum to optimum. Below the critical concentration an increase in the supply of the nutrient leads to an increase in yield. Plant analysis is useful in identifying hidden hunger in plants and in locating soil areas where deficiencies of one or more nutrients occur. Critical nutrient concentration depends upon growth stage and plant part that is sampled. To be of use in deficiency diagnosis, critical concentrations must be determined for individual crops, when no other nutrient limits growth. Under these conditions the critical value should be independent of soil type. Nevertheless, soil water supply to the plant has a significant influence on nutrient concentrations, with concentrations usually being lower under dry conditions, even when the soil supply of the nutrient is adequate. Thus, when plant nutrient concentrations are being evaluated against critical values, soil water availability must be
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Critical nutrient deficiency range
Figure 22.5. Illustration of critical nutrient range approach for plant analysis interpretation
monitored. Plant analysis is particularly useful for fruit crops because of their perennial nature and having extensive root systems. The optimum values of nutrient content in plants are pre-standardized so as to make recommendation after sample analysis. The DRIS approach focuses on nutrient ratios because within the plant these are more important than the concentration of any individual nutrient. As nutrient ratios in plants tend to be more constant throughout the growing season, use of DRIS allows greater flexibility in the time of plant sample collection. The DRIS system first establishes norms for all nutrient ratios associated with maximum crop yield either by widespread sampling of a crop in a given region or from scientific literature. All factors that affect crop yields are also measured at the time of plant sample collection so as to establish an integrated relationship between the DRIS norms, crop yield and other growth limiting factors. Interpretation of plant analysis by the DRIS system can be done graphically or mathematically by the calculation of DRIS indices. Computer programmes are now available for rapidly calculating DRIS indices for wider use of DRIS by plant analysis laboratories.
22.3.3. Biological Tests 22.3.3.1. Field Tests The field plot technique essentially measures the crop response to nutrients. Treatments consisting of nutrient levels are randomly assigned to an area of land, which is representative of the conditions. Several replications are used to obtain reliable results and to account for variations in the soil. The data generated are used to establish equations to work out fertilizer recommendations that will optimise crop yield for maximum profitability. When large numbers of tests are conducted on soils that are well characterized, recommendations can be extrapolated to other soils with similar characteristics. Field tests are expensive and time-consuming, but these are valuable soil fertility evaluation tools. These are used in conjunction with laboratory and greenhouse studies for calibration of soil and plant tests. 22.3.3.2. Greenhouse Tests The greenhouse techniques utilize small quantities of soil to quantify the nutrientsupplying power of a soil. Generally, soils are collected to represent a wide range of soil chemical and physical properties that contribute to the variation in availability for a
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specific nutrient. Nutrient level treatments are applied to the soils and a crop is planted that is sensitive to the specific nutrient being evaluated. Crop response to the treatments can then be determined by measuring total dry matter or nutrient yield.
22.3.3.3. Laboratory Tests The Neubauer seedling method consists of measuring uptake of nutrients by a large number of plants grown on a small amount of soil. The roots thoroughly penetrate the soil, exhausting the available nutrient supply within a short time. The total nutrients removed are quantified and tables are established to give the minimum values of macro and micronutrients available for satisfactory yields of various crops. 22.3.3.4. Microbiological Methods Certain micro-organisms exhibit behaviour similar to that of higher plants when exposed to environments deficient in one or more plant nutrients. For example, growth of Azotobacter or Aspergillus niger reflects nutrient deficiency in the soil. The soil can be rated from ‘very deficient’ to ‘not deficient’ in different nutrients, depending on the extent of colony growth. In comparison with methods that utilize higher plants, microbiological methods are rapid, simple and require little space.
22.4. Soil Fertility Maps Soil testing provides information which enables farmers to make profitable use of fertilizers and manures for growing crops. However, soil testing services are not adequately equipped to perform this job on millions of fields across the length and breadth of a region. In the absence of a specific soil test for all and individual farms, the available soil test data can be converted to a single value or index for each nutrient. The nutrient index values are then used to prepare fertility maps so as compare the levels of soil fertility of one area with those of another and to make generalized fertilizer use recommendations for different agro-eco-regions. Parker et al. (1951) introduced the concept of Nutrient Index Value (NIV) to describe the fertility status of soils for the purpose of mapping. It is computed as: NIV = (NL+2×NM+3×NH)/(NL+NM+NH), where NL = Number of samples falling in low category of nutrient status. NM = Number of samples falling in medium category of nutrient status. NH = Number of samples falling in high category of nutrient status. Separate indices are calculated for different nutrients like N, P and K. The NIVs have been classified as: less than 1.5 indicates low nutrient status, between 1.5 and 2.5 medium, and more than 2.5 high nutrient status. Motsara (2002) computed nutrient index values and prepared a soil fertility map for N, P and K using 3.65 million soil analysis data collected from 533 soil testing laboratories representing 450 districts in India. The level of soil fertility emerged is shown in Table 22.2. The soil fertility maps with district boundaries may be superimposed on the soil map of the state/district, so that nutrient management recommendations may go along with other soil management recommendations, thus improving soil quality and ultimately leading to the fertility capability classification of soil.
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Table 22.2. District-wise soil fertility classification based on nutrient index values calculated using 3.65 million soil analysis data Nutrient
Nitrogen Phosphorus Potassium
Percent of districts as Low
Medium
High
63 42 13
26 38 37
11 20 50
Source: Motsara (2002)
There are always differences among farmers in the level of application of fertilizers and manure. Hence, differences in soil fertility in an agricultural tract are bound to develop in an unpredictable manner. The effects of fertilizer use in an agricultural area over a period of time may either deplete or build-up fertility. Nutrients like N and K, which are subject to leaching losses, and over large areas are generally applied in amounts less than their removal, tend to deplete with time. On the other hand, nutrients like P and Zn, which interact with the soil and are usually applied in amounts much larger than their removal, tend to build-up in the soil. Such changes contribute to variability in nutrient supplies in an agricultural area. A periodical updating of soil test summaries and soil fertility maps is, therefore, necessary.
22.5. Progressive Appearances of Nutrient Deficiencies in Indian soils There is an increasing concern about the sustainability of Indian agriculture because of deterioration in soil fertility. The data from soil testing laboratories and published literature were analysed to determine the trend in fertility status of agricultural soils of India since 1967, the start of Green Revolution era (Pathak 2010). Based on the soil test values of N, P and K, soil samples were classified into three categories i.e., low, medium and high, and nutrient indices were worked out for soils of different states. In West Bengal, Gujarat, and Tamil Nadu, N fertility increased, while it declined in Orissa and Kerala. In the remaining states the N fertility status remained almost same from 1967 to 1997. An increasing trend in P status of soil was observed in Assam, Karnataka and Kerala. In the rest of the states it remained unchanged. Potassium fertility either remained same or decreased. Available information indicated that the soil organic carbon content either remained static or increased in certain regions of India. Thus, contrary to the general perception, there has not been much depletion of soil fertility of agricultural soils of the country over the years. It was in 1966 that field scale occurrence of khaira disease of rice was reported for the first time in India. Deficiency of Zn was the cause. Almost around same time appeared the reports of Zn deficiency in Mexican wheat varieties from Punjab. Reports of Fe and Mn deficiencies from coarse textured low organic matter alluvial soils in late seventies and early eighties have been followed by reports of the micronutrient deficiencies from different parts of the country. Fallouts of the strides made in food grain production, accomplished through growing of high yielding varieties, use of high analysis NPK fertilizers, increase in irrigated areas and increase in cropping intensity catalysed the depletion of the finite reserves of micronutrients in soil. As a consequence, decline in micronutrient fertility of Indian soils have been on the rise one by one in the country over last four decades (Figure 22.6).
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Figure 22.6. Progressive expansion in occurrence of nutrient deficiencies Source: Katyal and Rattan (1995)
22.6. Soil Fertility and Soil Poductivity Soil productivity is a measure of the ability of soil to produce a particular crop or sequence of crops under a specified management system. Optimum nutrient status alone will not ensure soil productivity. Factors such as soil moisture and temperature, soil physical conditions, soil acidity and salinity and biotic stresses (disease, insects, and weeds) can reduce the productivity of even the most fertile soils. Thus, soil productivity is affected by a range of factors. Some, such as climate (including rainfall, evaporation, solar radiation, temperature and wind) are beyond farmers’ control. Others however, such as soil fertility, are more influenced by farmers’ past and present activities. Soil fertility affects as well as gets affected by the choices that farmers make regarding agricultural production, fertilization, and soil and water conservation regimes. Soil productivity, therefore, encompasses soil fertility plus all the other factors affecting plant growth, including soil management. Soil fertility connotes primarily the combined effect of chemical and biological properties, and is probably the most important single soil factor affecting productivity. Thus all productive soils are fertile for the crops being grown, but many fertile soils are unproductive because they are subjected to drought or other unsatisfactory growth factors or management practices. When the soil tests high with respect to different plant nutrients, it is classified as fertile. If highly fertile fields are not yielding up to expectations, the answers to poor performance lie not in trying a new soil test method but in understanding how nutrients and water move to and into plant roots. Root interception, diffusion, mass flow are the mechanisms that move nutrients to plants and are largely responsible for desired plant performance. These mechanisms of nutrient uptake work best in well structured, aerated, deep soils, high in organic matter, with low bulk density, adequately drained and having proportioned micro- and macro- pore sizes. These are characteristics of a productive soil. A well-structured soil allows for a large unrestricted root system to develop.
22.7. Managing Fertilizers to Maintain Soil Fertility For millennia, the loss of soil fertility caused by crop production was restored by applying organic manures to fields, by alternating crops that increase soil fertility (such as legumes,
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which add N to soil by fixing atmospheric N into organic compounds) with other crops, and by leaving the fields fallow. This changed with the advent of the Green Revolution. Advances in plant genetics and breeding and other agricultural technologies (such as irrigation) resulted in increased agricultural production. But, higher crop yields mean greater depletion of soil nutrient supplies, which eventually must be balanced by increased nutrient input to maintain the fertile soils needed by our societies. Thus a hallmark of high-intensity agriculture is its dependence on mineral fertilizers to restore soil fertility.Where the supply of nutrients in the soil is adequate, crops are likely to grow well and produce large amounts of biomass. Fertilizers are needed when nutrients in the soil are lacking and cannot produce healthy crops and sufficient biomass. There are four management objectives associated with any practical farm level operation, including management of fertilizers. These are productivity, profitability, cropping system sustainability, and a favourable biophysical and social environment (Figure 22.7). Sustainability refers to the medium- and long-term effects of fertilizer management options to maintain or increase the productivity and profitability of the cropping system. Best management practices for fertilizers support the realization of these objectives in terms of cropping, soil fertility and the environmental health. A strong set of scientific principles guiding the development and implementation of fertilizer best management practices has evolved from a long history of agronomic and soil fertility research. When seen as part of the global framework, the most appropriate set of fertilizer best management practices can only be identified at the local level where the full context of each practice is known. Nutrient stewardship is the efficient and effective use of plant nutrients including those supplied by fertilizers to achieve economic, social and environmental benefits with
Figure 22.7. Fertilizer best management practices based on nutrient stewardship principles support the four management objectives of productivity, profitability, cropping systemsustainability, and a favourable biophysical and social environment Source: Bijay-Singh and Ryan (2015)
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engagement from farmers and other stakeholders. This concept essentially describes the selection of the right source of nutrients for application at the right rate, at the right time, and in the right place. The scientific principles that apply to these four areas of management are applicable at the farm level, but may differ widely depending on the specific cropping system, the particular region and the crop combination under consideration. As a practice, nutrient stewardship is dynamic and evolves as science and technology expands our understanding and opportunities; practical experience helps to decide as to what practices work or do not work under specific local conditions (Fixen 2007). Decision-support systems guiding the adoption of fertilizer best management practices require a dynamic process of local refinement (Figure 22.8). To ensure adoption of fertilizer best management practices by small scale farmers, as in India, it is also important to consider farmer’s knowledge, experiential learning, and social capital.
Figure 22.8. Decision support for the development, evaluation and adaptation of fertilizer best management practices as carried out by a dynamic process of local refinements (Modified from Fixen 2007)
Fertilizer source, rate, timing and placement are interdependent, and are interlinked with the set of agronomic management practices applied in the cropping system. The best fertilizer management practices are consistent with the processes mechanisms specific to the scientific disciplines of soil fertility, plant nutrition, soil chemistry, hydrology and agrometeorology, and recognize interactions with other cropping system factors such as cultivar, planting date, plant density and crop rotation. Interactions among nutrient source, rate, time and place are also considered in managing fertilizers for controlling soil fertility. For example, a controlled release source need not be applied with the same timing as a water soluble source. Fertilizer best management practices are worked out to avoid detrimental effects on plant roots, leaves and seedlings. For example, amounts banded near seedlings need to be kept within safe limits, recognizing ammonia and/or biuret content and overall salt index of the source. Besides specific costs and potential returns for each practice, influence on crop quality is also taken into account. For example, nitrogen fertilizers influence not only yield but also the protein which is important in animal and human nutrition, and influences bread making quality in wheat.
22.7.1. Fertilizer Source The nutrients applied through fertilizers must be in plant available forms or in a form that converts timely into a plant available form in the soil. The fertilizer source should
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suit soil physical and chemical properties. For example, nitrate application to flooded soils, or surface applications of urea on high pH soils should be avoided. Synergisms among nutrient elements and sources should be recognized. Examples include the P×Zn interaction, N increasing P availability, fertilizer complementing manure. Blending compatibility of fertilizer materials must be recognized because uniformity of application of blended fertilizer materials can be limiting, particularly when certain combinations of fertilizer sources attract moisture when mixed. Similarly, granule size should be similar to avoid product segregation. Benefits and sensitivities to associated elements in fertilizers also need to be appropriately recognized. Most nutrients have an accompanying ion that may be beneficial, neutral or detrimental to the crop. For example, the chloride accompanying K in muriate of potash is beneficial to maize, but can be detrimental to the quality of tobacco and some fruits. Some sources of P fertilizer may contain plant available Ca and S, and small amounts of Mg and micronutrients. Fertilizer sources should also take into account the effects of non-nutritive elements. For example, natural deposits of phosphate rock are enriched in several metals, including cadmium. The level of addition of these elements should be kept within acceptable thresholds.
22.7.2. Fertilizer Rate Adequate methods to assess soil nutrient supply are used to work out fertilizer application rates. Practices used include soil and plant analysis and response experiments. Quantity and plant availability of nutrients in all available indigenous nutrient sources such as manure, composts, biosolids, crop residues, and irrigation water need to be assessed. Since yield is directly related to the quantity of nutrients taken up by the crop until maturity, the selection of a meaningful yield target attainable with optimal crop and nutrient management and its variability within fields and season to season thus provides important guidance on the estimation of total crop nutrient demand. To work out fertilizer application rates, nutrient use efficiency should also be taken into account because some loss of nutrients from soil-plant system is unavoidable. If the output of nutrients from a cropping system exceeds inputs, soil fertility declines in the long-term may occur. This aspect also needs to be considered while working out fertilizer application rates. For nutrients unlikely to be retained in the soil, the most economic rate of application is where the last unit of nutrient applied is equal in value to the increase in crop yield it generates (law of diminishing returns). For nutrients retained in the soil, their value to future crops should be considered. Assess probabilities of predicting economically optimum rates and the effect on net returns arising from error in prediction. The exact relationship between crop yield and nutrient rate will vary for different crops and nutrients but the general shape of this relationship is relatively consistent for many crops and nutrients and is depicted in Figure 22.9. The yield is severely affected when a plant nutrient is deficient. When the nutrient deficiency is corrected by applying fertilizer, yield increases rapidly (Zone A) until the critical range of plant nutrient concentration is reached and yield is maximized. Nutrient sufficiency occurs over a wide concentration range, where yield is unaffected (Zone C). Increases in nutrient concentrations by fertilizer application above the critical range indicate that the plant is absorbing nutrients above that needed for maximum yield, commonly called luxury consumption. Elements absorbed in excessive quantities can reduce plant yield directly through toxicity or indirectly by reducing concentrations of other nutrients below their critical ranges (Zone D). The minimum amount of fertilizer required to maximize crop yield is called the agronomic optimum rate and it is located within Zone C.
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Figure 22.9. Relationship between crop yield and essential nutrient application rate
As adequate nutrient supply from the soil or applied fertilizer or manure, is vital to soil fertility and crop production, a limited supply of even one of the essential nutrients can limit crop yield. The concept that a certain sufficiency level of a nutrient will limit plant growth or yield to a certain level independently of levels of other nutrients or growth factors is known as the law of the minimum. On the other hand, insufficient supply of other nutrients (such as P and K) tend to limit growth or yield to a certain proportion of the potential maximum depending on sufficiency levels of other growth factors. Therefore, how different nutrients behave according to these principles generally influence the degree and type of interactions between nutrients and with other growth factors. Although N is usually the first limiting nutrient for non-legume crops, without adequate supply of other nutrients, N use efficiency is adversely affected. For example, increased N uptake and utilization with adequate K means improved N use efficiency and higher yields. Figure 22.10 shows how wheat yield and N use efficiency were increased by fertilizer K application to a deficient soil, resulting in improved economic and environmental benefits.
Figure 22.10. Potassium application improves yield response to N fertilizer and N use efficiency
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22.7.3. Fertilizer Timing Fertilizer nutrients should be applied to match the seasonal crop nutrient demand, which depends on planting date, plant growth characteristics, and sensitivity to deficiencies at particular growth stages. Phosphorus, for example, is critical for early cell division and multiplication when the amount absorbed is very small. Therefore, supplying sufficient P early in a crop’s growing season is likely to be more important than during the middle of the growing season. However, late in the growing season, nutrients accumulate in the grain rather than in the leaves or stalk. Therefore, mid-season nutrient application may increase both quality and grain yield if other plant requirements such as water are met. For example, N top-dressed at tillering has been found to increase both yield and protein of wheat, especially at low soil N levels. Therefore, it is important to understand nutrient needs and timing of nutrient uptake for each crop. Fertilizer timing is also governed by dynamics of soil nutrient supply. For example, mineralization of soil organic matter supplies a large quantity of some nutrients, but if the uptake need of the crop precedes its release, deficiencies may limit productivity. Similarly timing of weather factors influencing nutrient loss will govern fertilizer timing. For example, in South Asia, leaching losses tend to be more frequent during monsoon season. Logistic of field operations will also dictate the timing of fertilizer application. For example, multiple applications of nutrients may or may not combine with those of crop protection products and nutrient applications should not delay time-sensitive operations such as planting.
22.7.4. Fertilizer Placement Roots of crops explore soil progressively over the season. Placement needs to ensure nutrients are intercepted as needed. An example is the band placement of P fertilizer for wheat, ensuring sufficient nutrition of the young seedling, increasing yields substantially even though amounts applied and taken up are small. Fertilizer placement should take into account that soils vary in nutrient supplying capacity and nutrient loss potential. Logistic of soil preparation also needs to be recognized and fertilizer placement should fit needs of tillage system. In conservation tillage systems, subsurface fertilizer applications need to ensure that soil coverage by crop residues is maintained.
22.8. Balanced Crop Nutrition and Soil Fertility The exact fertilizer application schedules to be followed for different crops are invariably based on soil testing, field trials and nutrient balances for specific soil crop situation. As soil testing which is deployed to assess the soil fertility is not static but a dynamic concept, every time a crop is grown, all the nutrients should not be applied in a particular proportion. Rather fertilizer application should be tailored to the crop needs, keeping in view the capacity of the soils to supply different nutrients. Thus balanced fertilization does not mean a certain definite proportion of N, P, K or other nutrients to be added in the form of fertilizers and organic manures but it has to take into account the availability of nutrients already present in the soil, crop requirement and other factors. Balanced crop nutrition ensures an optimum supply of all essential nutrients to crop plants. It promotes synergetic interactions and keeps antagonist interactions out of crop production system. It discourages lopsided application of any nutrient or over-fertilization. A large number of long-term fertility experiments conducted in India during last few decades have shown that continuous use of fertilizer N alone produced the highest decline in yield and had deleterious effect on soil fertility and sustainability in terms of exhibiting deficiency of other major and micronutrients. Even when N, P and K were applied, the
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deficiencies of micronutrients and secondary nutrients became yield limiting factors after a number of years and their application became necessary to sustain high yield level. Balanced and integrated use of optimal dose of N, P, K and farmyard manure resulted in sustainable high yield levels. The soil-crop management system that would ensure efficient use of fertilizer nutrient and maintain crop yield and soil productivity will be nothing but balanced fertilization. There are several causes of the declining or lower crop responses to applied fertilizers or efficiency of fertilizer applications in several developing countries. One major cause of this decline is the continuous nutrient mining of the soils (particularly P, K, S and micronutrients) resulting from unbalanced fertilization practices which eventually lead to unhealthy soils and plants. Balanced fertilization implies the minimum supply of nutrients from any source, which at the same time is sufficient to meet the requirement of crop and maintain soil fertility. It further implies the efficient use of plant nutrients, which is achieved by sitespecific recommendations as well as by adopting best agriculture practices. It also recognizes the existence of natural soil processes, which make some losses of nutrients unavoidable. The balanced fertilizer use should not only aim at increasing crop yield and quality, and profits but also correct inherent soil nutrient deficiencies, avoid loss of nutrients to the environment and restore fertility of the soil that may have been degraded due to exploitive activities in the past. Scientifically, sound soil test methodology forms the key for ensuring a successful, efficient, economic and balanced use of fertilizers in agriculture.
22.9. Integrated Nutrient Management and Soil Fertility The use of organic manures as source of nutrients and its general benefit to the soil dates back to the beginning of settled agriculture, although at that time there was no understanding of how such manures were beneficial. Following the introduction of highyielding cereal varieties and widespread use of mineral fertilizers that provided N, P, and K as the primary plant nutrients, organic manures were thought of as a secondary source of nutrients. However, with increasing awareness about soil fertility and sustainability in agriculture, organic manures and many diverse organic materials, have gained importance as components of integrated nutrient management strategies. Consequently, major focus in sustainable agricultural systems is on the management of soil organic matter and plant nutrients through integrated use of mineral fertilizers with organic inputs such as animal manures, biological N fixation, crop residues, green manures, sewage sludge, and food industry wastes. The basic concept underlying integrated nutrient management is the maintenance and possible improvement of fertility of the soil for sustained crop productivity on long-term basis and use fertilizer nutrients as supplement to nutrients supplied by different organic sources available at the farm to meet the nutrient requirement of the crops to achieve a defined yield goal. Despite the characteristic pattern of soil organic matter depletion, there is seldom a stage of complete exhaustion, even under tropical conditions which favour rapid organic matter decomposition. Heavily cultivated soils tend to attain a steady state at a lower equilibrium limit. In virgin or previously uncultivated soils, soil organic matter levels are the highest for that particular environment. Cultivation invariably reduces soil organic matter levels to an extent that depends on management and inputs. In well managed cultivated soils, soil organic matter fluctuated between a low steady state value of soil organic matter in the heavily cultivated soil and the highest value observed in the uncultivated soil; cultivation alone tended towards the lower equilibrium soil carbon levels, but the addition of manures with fertilizers reduced the extent of soil organic matter decline with cultivation.
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Deficiency of soil organic matter is widespread in tropical soils and particularly those under the influence of arid, semiarid and sub-humid climates due to the controlling influence of climatic factors on primary productivity and biomass decomposition. Since sustainability is not possible without equalizing the nutrient removals and additions, soil fertility and productivity without fertilizers declines steadily. In temperate regions, crop residues are routinely incorporated into the soil, but the practice of returning residues to the soil is practically non-existent in tropical regions. Thus, in the tropics, low organic matter additions as well as accelerated degradation and loss due to year-round prevalence of biologically active temperature and moisture regimes leads to rapid reductions in soil organic matter levels and consequently reduced soil fertility and health due to the absence of the beneficial effects of the organic matter. In general, the soil organic matter content of tropical soils, when brought under cultivation, can fall to as low as about 30% of the original value of the uncultivated indigenous state, but most reports indicate about a 60% reduction after 10 years of cultivation. In long-term experiments initiated in a virgin soil, soil organic matter remained stable for 10 years after fertilizer application, but subsequently fell to about 40% of the initial value during the next 3 years. However, when manures and fertilizer were applied, the soil organic matter level was stable for 25 years, thus illustrating the value of integrated use of organic and inorganic nutrient sources in stabilizing and maintaining soil organic matter in cropping systems and ensuring sustainability regardless of the cropping system. Under irrigated conditions and regular application of recommended application rates of NPK fertilizers, productivity stagnated or declined after initially increasing for 5–6 years. It was the combined application of fertilizers and farmyard manure that unfailingly sustained soil fertility and productivity (Katyal et al. 2001). In addition to its physical and biological functions in soils, the application of organic manures can help reduce or eliminate the emergence of micro- and secondary nutrient deficiencies and prevent a fertilizer-induced drop in pH of poorly buffered acid soils. Even under rainfed conditions, in which fertility and productivity of soils was depleted to such an extent that fertilizer application was indispensable for getting an immediate boost in yield, the addition of organic manure was necessary to sustain the yield rise thus obtained. Thus, integrated nutrient management based on fertilizers and organic manures is essential to sustain high productivity and good soil health.
22.10. Summary and Emerging Trends in Soil Fertility Evaluation Soil fertility evaluation is an integral and vital component of agricultural production systems and it plays an increasingly important role in controlling environmental degradation problems associated with plant nutrients. Successful application of principles and practices of soil fertility evaluation and management to maintain soil fertility of agricultural soils is going to increase the profitability as well as minimize the environmental impacts of nutrient use – the fundamental goal of soil fertility management. Approaches to assess soil fertility evaluation and management will continue to evolve. Capability to make sound nutrient management decisions through improved soil fertility evaluation will further enhance through appropriate integration of newly emerging technologies with current practices. Two important challenges ahead will be: (i) how should newly emerging technologies will be assessed and integrated with current practices, and (ii) how can results of environmental soil tests be applied to assess the value of best management practices intended to protect or restore the environment or reduce soil-related human health risks? Precision agriculture or site-specific management and remote sensing are emerging as the technologies that have potential to significantly
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alter soil fertility evaluation and management. Precision agriculture technologies allow precisely mapping soil fertility in a field through the use of Global Positioning System (GPS), use variable rate application equipment, and generate GPS-based maps of crop yields. Remote sensing devices acquire electromagnetic energy that is emitted or reflected from plants and convert this energy into data that can be used in soil fertility evaluation. The ultimate value of these increasingly sophisticated technologies, however, lies in our ability to interpret the results.
Study Questions 1. Name some advantages and disadvantages of deficiency symptoms, plant analysis, and soil testing for detecting plant-nutrient needs. 2. How do you differentiate soil productivity and soil fertility? What are the situations when a soil testing high with respect to all the essential plant nutrients is not productive and vice versa? 3. What do you understand by soil fertility evaluation? What are the two most prevalent diagnostic techniques for soil fertility evaluation? Name some new upcoming approaches for assessment of soil fertility. 4. What are the basic components of soil testing? How soil tests can be used to work out fertilizer recommendations for a nutrient in a given region? 5. How do you differentiate between nutrient deficiency symptoms and colour of the leaf as tools for soil fertility evaluation? Explain in detail the usefulness of leaf colour in managing fertilizer nitrogen in cereals. 6. How can plant analysis be interpreted in terms of assessing nutrient supplying capacity of the soil? 7. Define nutrient stewardship. Explain in details the components of the best fertilizer management practices. 8. How can fertilizers and manures be judiciously managed together to maintain and improve soil fertility? 9. Describe the role of balanced and integrated nutrient management in maintaining fertility of the soils. 10. What is the role of soil organic matter in the maintenance of soil fertility? How does fertilizer management strategies influence soil organic matter status in the soil?
References Abrol, I.P. and Nambiar, K.K.M. (2008) Fertility management of Indian soils – A historical perspective. Asian Agri-History 12, 3-18. Bijay-Singh and Ryan, J. (2015) Managing Fertilizers to Enhance Soil Health. International Fertilizer Industry Association (IFA), Paris, France. pp 23. Dalal, R.C. and Subba Rao, A. (2006) Fertility Evaluation Systems, Chapter 134, Encyclopedia of Soil Science, Second Edition, CRC Press, Boca Raton, FL., USA. Fixen, P.E. (2007) Can we define a global framework within which fertilizer BMPs can be adapted to local conditions? In Fertilizer Best Management Practices. International Fertilizer Industry Association (IFA), Paris, France, pp. 77 86. Katyal, J.C., Rao, N.H. and Reddy, M.N. (2001) Critical aspects of organic matter management in the Tropics: the example of India. Nutrient Cycling in Agroecosystems 61, 77-88.
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Katyal, J.C. and Rattan, R.K. (1995) Genetic variations in tolerance to nutrient deficiencies. In Genetic Researchand Education: Current Trends and the NextFifty Years (B. Sharma et al., Eds), Indian Society of Genetics and Plant Breeding, New Delhi, pp. 468-479. Motsara, M.R. (2002) Available nitrogen, phosphorus and potassium status of Indian soils as depicted by soil fertility maps. Fertiliser News 47(8), 15-21. Parker, F.W., Nelson, W.L., Winters, E. and Miles, I.E. (1951) The broad interpretation and application of soil test Information. Agronomy Journal 43, 105-112. Pathak, H. (2010) Trend of fertility status of Indian soils. Current Advances in Agricultural Sciences 2, 10-12. Randhawa, M.S. (1979) A History of the Indian Council of Agricultural Research (1929-1979). Indian Council of Agricultural Research, New Delhi. Thomas Sims, J. and McGrath, J. (2012) Soil fertility evaluation. In Hand Book of Soil Sciences – Resource Management and Environmental Impacts, Second Edition (Huang, P.M., Li, Y., and Sumner, M.E., Eds.). CRC Press, Boca Raton, FL., USA, pp.1-36.
Further Suggested Readings Havlin, J.L., Tisdale, S.L., Nelson, W.L., and Beaton, J.D. (2013) Soil Fertility and Fertilizers – An Introduction to Nutrient Management. 8th Edition, Prentice Hall Brady, N.C. and Weil, R.R. (2007) The Nature and Properties of Soils, 14th Edition. Pearson Education Inc., Upper Saddle River, NJ, USA 4R Plant Nutrition: A Manual for Improving the Management of Plant Nutrition, (2012) International Plant Nutrition Institute. Soil Testing in India, Methods Manual (2011) Department of Agriculture & Cooperation, Ministry of Agriculture, Government of India, New Delhi.
