Nutrient Deficiencies of Field Crops

Guide to Diagnosis and Management

Prakash Kumar, Manoj Kumar Sharma

• Publisher: CAB International
• Year: 2013
• ISBN:
  9781789244274
• Topic: Technology & Engineering
• Subtopic: Agronomy
• Index: Technology & Engineering

Nutrient imbalance in soils is an emerging threat to sustainable agriculture: intensive cultivation, use of poor quality groundwater, depletion of soil organic matter and excessive use of fertilizers are major reasons for poor soil fertility worldwide. This necessitates correct diagnosis of plant nutrient deficiencies to avoid further use of pesticides in cases where pests or pathogens that are not in fact the cause of poor crop health. Richly illustrated with 600 colour photographs, this book is a visual field identification guide for symptoms of most common nutrient deficiencies in field crops, covering all their stages of occurrence. Detailed descriptions and suggested for management practices are given with each entry.

1 An Introduction to Plant Nutrition

Manoj Kumar Sharma

The biggest challenge for agriculture over the coming decades will be to meet the world’s increasing demand for food in a sustainable manner. Therefore, our goal will be to produce more to feed the growing population. In order to achieve this uphill task, there are two options before us. The first option is to bring more land under cultivation and the second option is to increase production per unit cultivated area by adopting intensive cultivation. Because of the continuous increasing demand on land for other developmental activities, the scope for increasing cultivated area is limited. Accordingly, greater attention will have to be paid to increase the production per unit area of cultivated land by adopting exhaustive use of agricultural inputs.

Deteriorating soil fertility and improper management of plant nutrients have further aggravated the problem. Large increases in productivity cannot be attained without ensuring that plants are supplied with adequate and balanced nutrition. Soils are the storehouse of most of the plant nutrients essential for plant growth and development and the way in which nutrients are managed will have a great impact on plant growth, soil fertility and agricultural sustainability. Plant growth is considered the result of a complex process by which the plant synthesizes food by using solar energy, carbon dioxide, water and nutrients from the soil.

Essential Plant Nutrients

Plants require a number of elements for their normal growth and development. Nearly 90 elements are taken up by plants but all are not essential. To differentiate between essential and non-essential elements, Arnon and Stout (1939) proposed the following criteria of essentiality:

1. The plant must be unable to grow normally or complete its life cycle in the absence of the element.

2. The element is specific and cannot be replaced by another.

3. The element plays a direct role in plant metabolism.

But it seems that an element would have to be considered essential even if it does not fulfil the second criterion of essentiality. For example, vitamin B₁₂ is known to be essential for many bacteria, diatoms and other algae, but the essentiality of cobalt by itself has not been demonstrated. According to this criterion, molybdenum and chlorine cannot be considered as essential because molybdenum can be replaced by vanadium and chlorine can be replaced by halides, however, they are functional in plant metabolism.

In order to overcome the above difficulties, Nicholas (1961) presented a more accurate definition of essential elements and used the term ‘functional or metabolic nutrient’ to include any mineral element that functions in plant metabolism, whether its function is specific or not.

The chemical elements that are now considered to be essential for higher plants are presented in Table 1.

Classification of Essential Nutrients

The essential nutrients are the chemical compounds necessary for the growth and metabolic activities of an organism. The essential plant nutrients may be divided into macronutrients (primary and secondary nutrients) and micronutrients depending on their concentrations in plant tissues.

Table 1. The list of chemical elements essential for higher plants.

Carbon (C)	Potassium (K)	Zinc (Zn)
Hydrogen (H)	Calcium (Ca)	Molybdenum (Mo)
Oxygen (O)	Magnesium (Mg)	Boron (B)
Nitrogen (N)	Iron (Fe)	Chlorine (Cl)
Phosphorus (P)	Manganese (Mn)	Sodium (Na)
Sulphur (S)	Copper (Cu)	Silicon (Si)
Cobalt (Co)

The elements sodium, silicon and cobalt have not been stated as essential elements for all higher plants. Vanadium has been established as an essential element for some microorganisms.

Macronutrients

Macronutrients or major nutrients are found and required in plants in relatively higher amounts than micronutrients. The content of a macronutrient in plant tissue can be a thousand times greater than the content of a micronutrient. These include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium. Carbon, hydrogen and oxygen constitute 90–95% of the plant dry matter weight and the remaining six macronutrients are further sub-divided into primary and secondary nutrients.

• Primary nutrients: Nitrogen, phosphorus and potassium are called primary nutrients because they are the major constituents of commercial fertilizers that are used in large amounts to correct their widespread deficiencies.

• Secondary nutrients: Calcium, magnesium and sulphur are called secondary nutrients because they are required in moderate quantities by plants, their deficiencies are localized and they can be supplemented through carriers of the primary nutrients. For example, single super phosphate is a phosphatic fertilizer and contains both calcium and sulphur. Likewise, ammonium sulphate is a nitrogenous fertilizer that also supplements sulphur.

Micronutrients

Micronutrients are required by plants in relatively small quantities but are as essential as the macronutrients. These elements are also known as trace elements. They are further classified into micronutrient cations (iron, manganese, zinc and copper) and micronutrient anions (boron, molybdenum and chlorine).

The classification of nutrients into macro- and micronutrients seems to be arbitrary because there are many cases in which the differences in contents of the nutrients are not clearly defined so as to categorize them into macro- or micronutrients. For example, in many cases, the iron or manganese content in plant tissues is sometimes as high as the content of sulphur or magnesium.

Nutrient Mobility in Soil

The movement of nutrients in soil varies greatly and largely influences their availability to the plants. Knowledge about the mobility of nutrients in soil is very important from the nutrient management point of view when deciding the methods, time and frequency of nutrient supplying sources. On the basis of their mobility in soil, nutrients can be broadly categorized as: mobile, less mobile and immobile.

• Mobile nutrients: These nutrients are highly soluble and their large fraction is found in soil solution. Because of their high mobility, they become readily available to plants and are very prone to leaching losses. Such nutrient ions are NO₃⁻, SO₄²⁻, Cl⁻ and H₂BO₃⁻.

• Less mobile nutrients: These are also soluble but are found in lower quantities in soil solution as they are adsorbed on clay complexes and easily get released into soil solution. Their availability to plants is moderate. These include NH₄⁺, K⁺, Ca²⁺, Mg²⁺ and MoO₄²⁻ ions.

• Immobile nutrients: These nutrients are very tightly held by soil particles and are not easily released into the soil solution. Therefore, the availability to plants is low. They are Fe²⁺, Mn²⁺, Zn²⁺, Cu²⁺, HPO₄²⁻ and H₂PO₄⁻ ions.

Table 2. Nutrient elements and their uptake forms. (Developed on the basis of information given by Ramamoorthy and Velayutham, 1976; Tisdale et al., 1985; Mengel and Kirkby, 2001; and many others.)

Nutrient Mobility in Plants

The appearance of deficiency symptoms in plants chiefly depends upon the extent and the rate of retranslocation of nutrients from older to younger tissues within plants. Nutrients vary greatly in their mobility within plants. On the basis of their mobility, the nutrients are classified into two categories: mobile nutrients and immobile nutrients.

• Mobile nutrients: Mobile nutrients are those that are capable of being translocated from older to younger tissues within the plant. When the plant becomes deficient in these nutrients, as a consequence the deficiency symptoms are observed on the older leaves. These include nitrogen, phosphorus, potassium and magnesium.

• Immobile nutrients: Immobile nutrients are those that are not capable of being translocated from older to younger tissues within the plant. When the plant becomes deficient in these nutrients, the deficiency symptoms are thus evident on the younger tissues. These include calcium, sulphur, iron, manganese, zinc, copper, boron and molybdenum.

Functions of Nutrients in Plants

Carbon, hydrogen and oxygen (C, H and O)

Carbon, hydrogen and oxygen constitute about 90–95% of the dry matter of the plant. They are the constituents of organic components in plants and are involved in many enzymatic processes. Carbon and oxygen mainly take part as components of the carboxylic group; hydrogen and oxygen are involved in oxidation—reduction processes. Carbon is taken up in the form of CO₂ from the atmosphere and possibly in the form of HCO₃⁻ from the soil solution. Carbon is assimilated by plants as CO₂ in the carboxylation process. Hydrogen is taken up in the form of water from the soil solution. During photosynthesis H₂O is reduced to H, which then passes through a series of steps and is transferred to an organic compound resulting in the reduction of NADP⁺ to a reduced form (NADPH). This coenzyme is very important in oxidation—reduction processes in which H can be transferred to a large number of compounds.

Nitrogen (N)

The uptake of nitrogen by the plant occurs in the form of nitrate ions (NO₃⁻) or ammonium ions (NH₄⁺) from the soil solution or as gaseous ammonia or gaseous nitrogen (N₂) from the atmosphere. The fixation of atmospheric molecular N₂ is dependent on the presence of specific nitrogen-fixing microorganisms in symbiotic association with higher plants. The total content of nitrogen varies from 2 to 4% in plant dry matter. Nitrogen is an essential constituent of amino acids, nucleic acids, nucleotides and chlorophyll. It promotes rapid growth. It is also involved in enzymatic processes because all enzymes and coenzymes contain nitrogen. Both NH₄⁺ and NO₃⁻ forms can be taken up and metabolized by plants. The NO₃-N is assimilated by the processes of reduction and amination. The NH₄-N is also assimilated by the amination process.

The uptake of NO₃⁻ and NH₄⁺ forms is very sensitive to pH. The best uptake of NH₄⁺-N takes place in neutral conditions while rapid uptake of NO₃⁻-N occurs at low pH values. The NO₃⁻ uptake at high pH values may be reduced due to the competitive effect of hydroxide (OH⁻) ions.

Almost all of the NH₄⁺-N absorbed is assimilated in the root tissues and redistributed as amino acids. The NO₃⁻-N can be translocated as such to the upper parts of the plant but this depends upon the nitrate reduction potential of the roots. Thus, nitrate and amino acids are the main forms in which nitrogen is translocated in the vascular system of higher plants. NO₃⁻-N may also contribute to maintaining cation—anion balance and osmoregulation.

Sulphur (S)

The total content of sulphur varies from 0.2 to 0.5% in plant dry matter. Plants absorb sulphur mainly in the form of SO₄²⁻ ions from the...