Soil Sciences Student s Book FET FIRST Level 3 J de Fontaine and F Mitchell
FET FIRST Soil Sciences NQF Level 3 Student s Book J de Fontaine and F Mitchell 2007 Illustrations and design Macmillan South Africa (Pty) Ltd 2007 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, photocopying, recording, or otherwise, without the prior written permission of the copyright holder or in accordance with the provisions of the Copyright Act, 1978 (as amended). Any person who does any unauthorised act in relation to this publication may be liable for criminal prosecution and civil claims for damages. First published 2008 08 10 09 07 1 3 5 7 9 10 8 6 4 2 0 Published by Macmillan South Africa (Pty) Ltd Private Bag X19 2096 Northlands, 2116 Gauteng South Africa Text design by Heather Brooksbank Cover design by Deevine Design Artwork by Mark de Lange and Ian Greenop Typesetting by Future Pre-Press Edited by Tony Lavine and Peta Jones The publishers have made every effort to trace the copyright holders. If they have inadvertently overlooked any, they will be pleased to make the necessary arrangements at the first opportunity. ISBN-13: 978 085320 892 1; eisbn: 97 8143102 067 6 WIP: 1972 M000 It is illegal to photocopy any page of this book without written permission from the publishers. The publisher would like to thank the following for permission to use photographs in this book: Science Photo Library: pages 9, 87, 172 The Bigger Picture: page 151
Contents Topic 1: Fertilisation of soils.............................. 1 Module 1: Essential plant nutrients in fertilisers and the principle of limiting factors.... 2 Unit 1: Essential plant nutrients............................................... 3 Unit 2: The principle of limiting factors........................................... 9 Unit 3: Symptoms of nutrient deficiencies in crops................................... 14 Unit 4: Mixed fertilisers and their application...................................... 21 Module 2: Organic and inorganic fertilisers............................ 28 Unit 1: Using organic fertilisers.............................................. 29 Unit 2: Green manures................................................... 44 Unit 3: Advantages and disadvantages of organic and inorganic fertilisers...................... 50 Module 3: Problem soils........................................ 61 Unit 1: Acidic, alkaline and sodic soils........................................... 62 Unit 2: Soil ph........................................................ 67 Unit 3: Management of problem soils........................................... 74 Module 4: Using soil and plant analysis to determine fertiliser requirements........ 80 Unit 1: Soil and plant sampling for analysis....................................... 81 Unit 2: Calculation of fertiliser requirements....................................... 93 Topic 2: Soil water and evapotranspiration................... 109 Module 1: Soil water and the process of evapotranspiration................. 110 Unit 1: Soil water...................................................... 111 Unit 2: Evapotranspiration................................................. 120 Module 2: Calculating irrigation requirements......................... 133 Unit 1: Irrigation requirements............................................... 134 Topic 3: Soil erosion and its prevention..................... 151 Module 1: Causes, forms and consequences of erosion.................... 152 Unit 1: Causes and forms of soil erosion......................................... 153 Unit 2: Indicators and impacts of soil erosion...................................... 162 Module 2: Prevention of soil erosion............................... 170 Unit 1: Preventative measures for soil erosion...................................... 171
Topic 1 Fertilisation of soils Overview In this topic you will identify the essential plant nutrients in fertilisers understand the principle of limiting factors describe the use of organic and inorganic fertilisers identify problem soils learn to use soil analysis to determine fertiliser requirements. 1
Module 1 Essential plant nutrients in fertilisers and the principle of limiting factors Overview In this module you will list the available sources of macro- and micro-nutrients explain the principle of limiting factors describe deficiency symptoms in crop plants explain the concept of mixed fertilisers and describe methods of applying solid fertilisers. 2 Topic 1: Module 1
Unit 1: Essential plant nutrients Introduction In this unit you will learn about the nutrients that plants need in order to grow, develop and reproduce in a healthy way. You will learn about essential and non-essential plant nutrients and macro- and micro-nutrients. The unit deals with the sources and functions of each macro- and micro-nutrient. Plant nutrients Plants need nutrients in order to grow, develop and reproduce. Plant nutrients are chemical elements such as carbon (C) and nitrogen (N). Plant nutrients are sometimes called plant elements. Plants get carbon (C), hydrogen (H) and oxygen (O) from the air and water. However, plants get all of their other nutrients from the soil. In the soil, plant nutrients occur in the form of mineral salts. Mineral salts are inorganic ions that form when rock particles are broken down by the process of weathering. The mineral salts dissolve in the water in the soil. Plants are then able to absorb the mineral salts, and the nutrients they contain through their roots. Plant nutrients can be divided into two main categories: essential nutrients nutrients that plants must have in order to grow, develop and reproduce non-essential nutrients nutrients that plants do not need in order to grow, develop and reproduce, but which are still needed for special functions in the plant In order to be healthy, plants need all of the essential plant nutrients. In addition, plants need the correct amount of each essential nutrient. We divide essential nutrients into two types based on the amount of the nutrient needed by plants: macro-nutrients essential nutrients that are needed in large amounts by plants micro-nutrients essential nutrients that are needed in small amounts by plants chemical elements pure substances made up of only one kind of atom mineral salts compounds occurring in rocks or soil and made up of positive and negative ions inorganic ions charged ions of mineral origin weathering changes in rock and soil due to exposure to the weather absorb take in or swallow up??? Did Words & Terms you know? Macro means large. Macronutrients are called macronutrients not because they are large in size but because they are needed in large amounts. Micro means small. Micronutrients are called micronutrients not because they are small in size but because they are needed in small amounts. You will learn more about macro- and micro-nutrients in the next two sections. To make sure that a crop is healthy and gives a good yield, the farmer must check that the crop has each of the essential plant nutrients in the correct amounts. Macronutrients You have learnt that macro-nutrients are plant nutrients that plants need in large amounts. Let us look in more detail at the sources and functions of the different macro-nutrients. Unit 1: Essential plant nutrients 3
Carbon (C), hydrogen (H) and oxygen (O) Of all the macro-nutrients, carbon, hydrogen and oxygen are needed in the largest amounts by plants. Plants get carbon mainly from the carbon dioxide gas (CO 2 ) in the air. Plants get hydrogen and oxygen mainly from water (H 2 O) in the soil. Functions of carbon, hydrogen and oxygen Carbon, hydrogen and oxygen are used by plants to make the large organic molecules such as carbohydrates and lipids that make up the structure of the plant and supply energy. Carbon, hydrogen and oxygen are the only macro-nutrients that plants get from the air or water. Plants must absorb the rest of the macro-nutrients from the soil. Nitrogen (N) Nitrogen is the macro-nutrient that most affects plant growth and therefore crop yield. Functions of Nitrogen Nitrogen is needed for the growth of stems and leaves. This is called vegetative growth. Nitrogen is also needed to make proteins, enzymes and the pigment chlorophyll, needed for photosynthesis. There is usually a lot of nitrogen in soils. However most of the nitrogen in soils is in the form of organic nitrogen in the organic matter in the soil. Plants cannot absorb nutrients if they are in an organic form. Plants can only absorb nutrients in the form of inorganic ions. The inorganic sources of nitrogen in the soil that plants can absorb are ammonium ions (NH 4+ ) and nitrate ions (NO 3- ). Bacteria in the soil gradually change organic nitrogen into the inorganic ammonium and nitrate ions that plants can use by a process called mineralisation. Words & Terms organic living or dead part of an animal or plant, containing carbon in its molecules carbohydrate energyproducing organic compounds of carbon (C), hydrogen (H) and oxygen (O) e.g. sugars and starch lipids fats proteins a group of organic compounds containing carbon, hydrogen, oxygen, nitrogen (N) and sometimes sulphur (S) and phosphorus (P) enzyme a compound that facilitates or speeds up chemical reactions changes mineralisation changing an organic substance into a nonorganic substance Phosphorus (P) The main source of phosphorus in the soil is in the form of phosphate 3- ions (PO 4 ). The availability of phosphorus from the soil is strongly dependent on the ph or acidity of the soil. At a ph lower than 5.5, the phosphorus in the soil tends to react with iron (Fe) in the soil to form a compound that cannot dissolve in water and therefore cannot be absorbed by plants. At a ph above 6.0, this reaction is reversed and the phosphorus becomes available for plants to absorb. The best soil ph in terms of phosphorus availability is between 6.0 and 7.0. Phosphorus availability can also be influenced by other inorganic ions in the soil. For example, phosphorus combined with aluminium or iron cannot be absorbed by plants, but phosphorus combined with calcium or magnesium can be absorbed by plants. Functions of Phosphorus Phosphorus is needed for root growth and the formation of the reproductive parts of the plant, such as flowers, fruits and seeds. Phosphorus is also needed to make cell membranes, enzymes, nucleic acids (the carriers of genetic information) and energycarrying molecules. 4 Topic 1: Module 1
Potassium (K) The source of potassium in the soil is the potassium ion (K + ). Functions of potassium Potassium has many functions, but is very important for fruit development. It is also needed for regulating water absorption and for promoting resistance to diseases and difficult environmental conditions like frost and drought. Magnesium (Mg) Magnesium is absorbed in the form of magnesium ions (Mg 2+ ) from the soil. Functions of magnesium Magnesium is needed to make chlorophyll and for getting enzymes to work. Calcium (Ca) Calcium is absorbed in the form of calcium (Ca 2+ ) ions. Functions of calcium Calcium is needed for the growth of the tips of roots and stems storage of food pollen formation and to prevent leaves from falling off the plant too soon. for the structure and functioning of cell walls and cell membranes. Sulphur (S) 2- Sulphur is absorbed in the form of sulphate ions (SO 4 ) from the soil. Functions of sulphur Sulphur is needed for the formation of certain important proteins and enzymes. Assessment activity 1 Summarising the function of macro-nutrients in the plant 1. Use the information in the text to complete the following table for summarising the function of macro-nutrients in plants. Table 1: The sources and functions of plant macro-nutrients Macro-nutrient Symbol Sources Function in the plant Carbon C Hydrogen Oxygen Nitrogen Phosphorus PO 4 3- (phosphate ion in the soil) Potassium Unit 1: Essential plant nutrients 5
Magnesium Formation of chlorophyll; functioning of enzymes Calcium Sulphur Micro-nutrients Unlike macro-nutrients, micro-nutrients are needed in very small amounts. Micro-nutrients are sometimes called trace elements because plants only need trace amounts of them. However, just because micronutrients are needed in small amounts they are not less important than macro-nutrients. Micro-nutrients are essential nutrients and plants cannot survive without them. Words & Terms trace very small quantity Micro-nutrients including iron (Fe), manganese (Mn), copper (Cu), boron (B), zinc (Zn), cobalt (Co), molybdenum (Mo), sodium (Na) and chlorine (Cl) are essential to some plants too. Most micro-nutrients are involved in the functioning of enzymes in the plant. We still do not know exactly all of the functions of each micro-nutrient. Table 2 shows the sources of micro-nutrients in the soil and the known functions of each micro-nutrient. TABLE 2: The sources and functions of plant micro-nutrients found in the soil Micro-nutrient Source Function Iron (Fe) Fe 2+ (ferrous ions); Fe 3+ (ferric ions) Formation of chlorophyll; functioning of enzymes Manganese (Mn) Mn 2+ (manganese ions) Functioning of enzymes Copper (Cu) Cu 2+ (copper ions) Functioning of enzymes Boron (B) BO 3-2- and B 4 O 7 (borate ions) Absorption of calcium; growth of shoot tips Zinc (Zn) Zn 2+ (zinc ions) Formation of plant hormones; functioning of enzymes Cobalt (Co) Cu 2+ (copper ions) Functioning of enzymes Molybdenum (Mo) MoO 4 2- (molybdenum oxide ions) Functioning of enzymes needed for nitrogen based reactions Sodium (Na) Na + (sodium ions) Regulation of osmosis and the maintenance of salt water balance Chlorine (Cl) Cl - (chloride ions) Regulation of osmosis and the maintenance of salt water balance 6 Topic 1: Module 1
Case Study 1 Hydroponics Hydroponics is a method of growing plants without soil. The plants are grown in stable growth media, such as gravel or sawdust. The nutrients the plants need are dissolved in distilled water to form a nutrient solution. The nutrient solution is supplied to the plants as irrigation at regular intervals. High value crops are suited to hydroponic production to offset the high cost of this production system. In South Africa, tomatoes, cucumbers, peppers, lettuce and herbs are popular choices for hydroponic production. Tomatoes are usually grown in a drain to waste or open bag hydroponic system. In this system, plants are grown in open containers. The nutrient solution is supplied about 12 times a day by means of a dripper. Excess nutrient solution drains out of the containers as waste. Herbs are often grown in a gravel-flow hydroponic system. In this system, the nutrient solution is re-circulated so that the roots of the plants are covered in a thin film of the nutrient solution at all times. There are many types of nutrient solutions that can be used for hydroponics. One of the more commonly used solutions is called a modified Hoagland solution. Assessment activity 2 Case study on hydroponics 1. Read the case study on hydroponics on the previous page. 2. Table 3 shows the components of a modified Hoagland nutrient solution commonly used in hydroponic systems. Complete the table by determining which nutrients are supplied by each component. TABLE 3: Sources of nutrients in a modified Hoagland solution Component of solution Chemical formula Source of which nutrient/s? Ammonium phosphate NH 4 H 2 PO 4 Nitrogen, phosphorus Potassium nitrate KNO 3 Calcium nitrate Ca(NO 3 ) 2 Magnesium sulphate MgSO 4 Iron phosphate FePO 4 Boric acid B(OH) 3 Manganese chloride MnCl 2 Manganese, chlorine Copper chloride CuCl 2 Zinc chloride ZnCl 2 Molybdenum oxide MoO 3 Ammonium molybdenum oxide (NH 4 ) 6 Mo 7 O 24 3. List the macro-nutrients and micro-nutrients available in a modified Hoagland solution. Unit 1: Essential plant nutrients 7
Assessment activity 3 Short test on macro- and micro-nutrients 1. Define the following terms: a) plant nutrients b) essential plant nutrients c) macro-nutrients d) micro-nutrients. 2. Are macro-nutrients more or less important than micro-nutrients for plants? Explain. 3. Complete the following table: Macronutrient Source Function nitrogen (N) phosphorus (P) potassium (K) 4. Why is magnesium important for plants? 5. Which two micro-nutrients are needed for a plant to maintain its salt water balance? Assess yourself Assess your performance in the following activities: 1 2 3 4 Summarising the function of macro-nutrients in the plant Case study on hydroponics Short test on macro- and micro-nutrients 1 = not achieved; 2 = not yet competent; 3 = competent; 4 = outstanding Unit summary This unit deals with the nutrients that plants need for healthy growth, development and reproduction. Plant nutrients that are needed for plants to survive are called essential plant nutrients. Essential plant nutrients are classified as macro-nutrients or micro-nutrients. Macronutrients (C, H, O, N, P, K, Mg, Ca and S) are those nutrients that are needed in large amounts by plants. Micro-nutrients (Fe, Mn, Cu, B, Zn, Co, Mo, Na and Cl) are those that are needed in small amounts by plants. The unit deals with the sources and functions of each macro- and micronutrient. 8 Topic 1: Module 1
Unit 2: The principle of limiting factors Introduction This unit deals with a very important idea. It is the idea or principle of limiting factors. The unit covers the basic concepts behind the principle of limiting factors. The emphasis is on the role of plant nutrients as limiting factors. Every organism needs certain things in order for it to survive. For example, the basic things that plants need include sunlight, carbon dioxide, water, nutrients, a growth medium like soil, and a favourable temperature and ph. If an organism does not have enough of any one of these factors, the organism will struggle to survive, even if all the other things are present in the right amounts. For example, a plant may have the right amounts of sunlight, carbon dioxide, nutrients, soil, and a favourable temperature and ph, but if it does not have the right amount of water, it cannot survive. In this case, water is the limiting factor. Water is limiting the survival of the plant. The Principle of Limiting Factors states: The functioning of an organism is limited by the essential environmental factor that is present in the least favourable amount. Words & Terms principle general law organism any living thing Figure 1.1: Tomatoes being grown under hydroponic conditions. The hydroponic system provides everything the plant needs in optimum amounts. As a result, limiting factors never affect plant growth Unit 2: The principle of limiting factors 9
Liebig s Law of the Minimum The Principle of Limiting Factors developed from a law in agricultural science called Liebig s Law of the Minimum. Liebig s Law of the Minimum was formulated in 1840 by a German scientist called Justus von Liebig. Liebig s Law of the Minimum states: Growth is controlled not by the total of resources available, but by the scarcest resource. Liebig s Law was originally developed from crop growth studies. It is based on the observation that adding more of the plant nutrients that were already present in high amounts did not lead to an increase in plant growth. Plant growth only increased when more was added of the plant nutrient that was present in the lowest amount. Assessment activity 1 Liebig s barrel Liebig used a barrel to explain his Law of the Minimum. Imagine a barrel made with pieces of wood that are different lengths. Each piece of wood represents one essential nutrient that a plant needs to grow. The length of wood represents the amount of nutrient available. The volume of water the barrel can hold is limited by the shortest piece of wood. In the same way, the growth of a plant is limited by the nutrient that is in shortest supply. Figure 1.2: Liebig s barrel illustrating his Law of the Minimum 1. In a group, suggest other ways that you could demonstrate Liebig s Law of the Minimum. 2. Choose one suggestion and present it to the class. 10 Topic 1: Module 1
Tolerance limits Liebig s Law of the Minimum only dealt with limiting factors as those factors that were present in too little amounts. However, too much of a factor can also limit an organism s survival. For example, too much heat can kill a plant. Too much water can drown a plant. Too much of a certain plant nutrient can become toxic for a plant. This brings us to the idea of tolerance limits. Each organism has an optimal range for each environmental factor within which the growth and development of the organism is optimal or best. For example, each crop has an optimal range for the level of nitrogen in the soil (Figure 1.3). In the optimal range, the crop plants show the best growth. There are minimum and maximum values called optimal limits that define the lower and upper values of the optimal range. Below the lower optimal limit, the crop will have a deficiency of the nutrient. The crop will become stressed. It will show symptoms of deficiency and its growth will decrease. Eventually there is a limit of tolerance beyond which the crop cannot survive and growth drops to zero. Above the upper optimal limit, the nutrient level is so high that it becomes toxic to the crop. The crop becomes stressed. It shows signs of toxicity and its growth decreases. Above the maximum limit of tolerance, the crop cannot survive and growth drops to zero. deficient sufficient toxic Figure 1.3: Graph showing a certain crop s range of nitrogen tolerance Unit 2: The principle of limiting factors 11
Assessment activity 2 Interpretation of graphs of tolerance Study the graph in Figure 1.3 and answer the following questions: 1. What is the optimal range of nitrogen for this crop? 2. What is the upper limit of tolerance? 3. What is the lower limit of tolerance? 4. What is the lower optimal limit? 5. What is the upper optimal limit? 6. What is the maximum growth that the crop is capable of, and what is the nitrogen value at this point? 7. At a nitrogen level between 4 ppm and 8 ppm, what happens to the growth of the crop plants? Why does this happen? 8. At a nitrogen level between 19 ppm and 23 ppm, what happens to the growth of the crop plants? Why does this happen? 9. Why does the growth of the crop reach zero below 4 ppm nitrogen and above 23 ppm nitrogen? Conditions affecting limiting factors There are various things that can affect the limiting factor in a field at a certain time and place. These include: Seasonality Limiting factors can be seasonal. In other words, an environmental factor may be the limiting factor at a certain time of the year but not at other times of the year. For example, in winter, temperature may become the factor limiting growth. But in the warmer months of spring and summer, the temperature is no longer the limiting factor and other factors like water or nutrients may become the limiting factor. Water is a seasonal factor because rainfall varies from month to month. Light is seasonal because day length and light intensity varies from season to season. Type of crop Different crops and cultivars have different nutrient requirements. This is because they have different growth rates, root structures, and efficiencies of nutrient use. As a result, in one field, plants from Crop A will be use up one factor faster than plants from Crop B. The nutrient becomes the limiting factor for Crop A but not for Crop B. Growth stage of crop Limiting factors may also change depending on the growth stage of the crop. Crops have different nutrient requirements at different stages of their life cycle. 12 Topic 1: Module 1
Assessment activity 3 Short test on limiting factors 1. Write the Principle of Limiting Factors in your own words. 2. State Liebig s Law of the Minimum. 3. What do the following terms related to tolerance limits mean? (a) optimal range (b) upper optimal limit c) lower limit of tolerance. Assess yourself Assess your performance in the following activities: 1 2 3 4 Liebig s barrel Interpretation of graphs of tolerance Short test on limiting factors 1 = not achieved; 2 = not yet competent; 3 = competent; 4 = outstanding Unit summary This unit deals with the concept of limiting factors. The Principle of Limiting Factors states that the functioning of an organism is limited by the essential environmental factor that is present in the least favourable amount. This principle, developed from Liebig s Law of the Mimimum, states that growth is controlled not by the total of resources available, but by the scarcest resource. Each organism has upper and lower tolerance limits for each limiting factor. Unit 2: The principle of limiting factors 13
Unit 3: Symptoms of nutrient deficiencies and toxicity in crops Introduction In this unit, you will learn about the different symptoms or signs that indicate that a plant is deficient in one or more plant nutrient. If a plant does not have enough of a particular nutrient, the plant usually shows certain symptoms. Symptoms can be a change in the colour or shape of the plant s leaves. If a farmer knows and can recognise such nutrient deficiency symptoms, then the nutrient that is in short supply can be increased by the application of the appropriate fertiliser. The use and limitations of nutrient deficiency symptoms Nutrient deficiency symptoms which can be seen with the naked eye can be a powerful tool for evaluating whether plants have enough of all the essential plant nutrients. They allow the farmer to identify the lacking nutrient and quickly take action to remedy the situation. The farmer does not have to rely on other people or specialists or wait for the results of tests. Words & Terms symptom a condition that indicates the existence of something serious deficient not having enough Words & Terms evaluate to judge the value of visual based on the use of sight However, visual nutrient deficiency symptoms can present problems. By the time the symptoms appear, the crop has already been damaged and so growth and yield will be decreased. Many of the deficiency symptoms are similar for more than one nutrient. So, it can be difficult to determine exactly which nutrient is lacking. Plants may show the same deficiency symptoms for other problems such as disease, drought, salt damage, heat, chemical damage or water logging. This can lead to the incorrect diagnosis of the problem. Bearing in mind the limitations of nutrient deficiency symptoms, farmers should make use of other diagnostic tools. Such tools include soil analysis and plant analysis, if possible. Soil analysis is recommended for identifying nutrient deficiencies for annual crops. Plant analysis is recommended for testing nutrient deficiencies in perennial crops. You will learn more about soil analysis in Module 4. Nutrient deficiency symptoms are still very often the first clue or sign that there is a nutrient problem in a field. A farmer with knowledge and experience of the field s history can interpret deficiency symptoms quite accurately. 14 Topic 1: Module 1
Types of deficiency symptoms and terminology Chlorosis Chlorosis refers to a yellowing of the leaf. Chlorosis is directly caused by a lack of chlorophyll, the photosynthesis pigment. A lack of chlorophyll can in turn be caused by a deficiency of any of the nutrients involved in chlorophyll production and photosynthesis. We refer to plant tissue that lacks chlorophyll as chlorotic. It is important to identify the parts of the plant that show chlorosis, because this can point to which nutrient is deficient. For example (Figure 1.4): Words & Terms photosynthesis the process by which a plant produces sugars from carbon dioxide and water, by action of light (radiant energy) on chlorophyll pigment a substance which gives colour in plant or animal cells Overall chlorosis refers to a general yellowing of the leaves as is caused by nitrogen deficiency. Interveinal chlorosis refers to yellowing of the leaf tissue between the veins, but the veins themselves stay green as caused by iron deficiency. Marginal chlorosis refers to the yellowing of the margins of the leaf chlorosis as caused by calcium deficiency. Figure 1.4: (a) overall chlorosis, (b) interveinal chlorosis and (c) marginal chlorosis. The colour plate on the inside back cover shows photographs of leaves with chlorosis. Necrosis Necrosis is the term used to describe the signs of death in any living tissue. Parts of the plant or the whole plant may die as a result of nutrient deficiency. We call dead tissue that is still attached to a living plant necrotic tissue. As with chlorosis, necrosis can take many forms. Necrotic tissue can be white, grey, light brown, dark brown or black. Necrosis can be seen in leaves, stems or roots. Necrosis can be interveinal, veinal, marginal or it may affect only the tips of leaves. Words & Terms tissue a group of specialised cells in an organism veinal of veins, i.e. the network of tubes in leaves containing (and transporting) sap Unit 3: Symptoms of nutrient deficiencies in crops 15
Figure 1.5: (a) interveinal necrosis, (b) veinal necrosis (c) marginal necrosis and (d) tip necrosis Changes in colour Certain nutrient deficiencies cause the plant or parts of the plant to change colour from their normal green colour. Colours that indicate different nutrient deficiencies include white, light green, yellow, orange, red, purple and very dark green. For example, in maize very dark green leaves can indicate phosphorus deficiency; leaves with brown stripes can indicate magnesium deficiency; and purple stems can indicate iron or phosphorus deficiency (Figure 1.6). 16 Topic 1: Module 1
Figure 1.6: Some colour based signs of nutrient deficiencies in maize (a) leaves with brown stripes can indicate magnesium deficiency and (b) purple stems can indicate iron or phosphorus deficiency Abnormal growth and development Nutrient deficiencies can lead to abnormal shapes and sizes of stems and leaves. Stems may be very thin or very thick. Leaves may be small, rolled, brittle, puckered, torn, wilted or square at the tip (Figure 1.7). Figure 1.7: Some abnormal leaf growth (a) rolled leaves and (b) puckered leaves Stunting Stunting refers to reduced growth. Stunted plants are shorter than normal and do not reach their full potential height. Unit 3: Symptoms of nutrient deficiencies in crops 17
Symptoms of deficiency of the 13 essential nutrients Macro-nutrient deficiency and toxicity symptoms TABLE 4: Macro-nutrient deficiency symptoms Macro-nutrient Deficiency symptoms Toxicity symptoms Nitrogen (N) Phosphorus (P) Potassium (K) Calcium (Ca) Magnesium (Mg) Sulphur (S) Chlorosis. Older leaves change from normal green to pale green, then yellow and then yellowish white. The whole leaf is affected. Young leaves stay green but get much paler. Stunted growth. Small leaves. Weak stems. Stunted growth. Slow growth can make phosphorusdeficient plants look like very young, healthy plants. Stems, petioles and underside of leaves turn yellow and then a reddish purple colour. Note: Phosphorous deficiency symptoms are not very distinct and can be difficult to identify. Chlorosis. Starts with marginal chlorosis. Develops into interveinal chlorosis. The veins stay green. Necrosis. Interveinal chlorosis develops into interveinal necrosis which appears as dry brown burn marks developing from the margin inwards to the midrib. The veins stay green. Leaves eventually roll and crinkle. Note: In some plants such as potatoes and beans, the first symptom can be small white spots on the leaves. Necrosis. The growing tips of shoots and young leaves die, leaving soft dead tissue. Sometimes the petioles develop but the leaves do not, so there is only a bit of necrotic tissue at the tip of each petiole. Tips of leaves appear hook-shaped. Margins of leaves start to roll downwards so that leaves look like upside-down cups. Plants wilt easily. Chlorosis. Begins with mottled chlorotic areas in the interveinal tissues of older leaves. Veins stay green. Interveinal tissue expands and becomes puckered. Leaf margins roll upwards or downwards Necrosis. Interveinal chlorotic tissue dies. Chlorosis. General chlorosis of all leaves. Old leaves turn light green. Young leaves turn yellow. Young leaves and petioles turn a pale pinkish red or purple colour. Necrosis. Eventually brown necrotic spots may form on the petioles. Leaves can become twisted and brittle. Nitrogen toxicity Plants turn dark green Plants grow tall weak stems Shoot development is poor Vegetative buds form instead of reproductive buds Ammonium toxicity Plants stunted Lesions occur on stems and roots Leaf margins roll downward Reduces uptake of Fe, Mn and Zn (see deficiency symptoms for each of these nutrients) Reduces uptake of Mg and Ca (see Mg and Ca deficiency descriptions) Leaf distortion Marginal chlorosis Marginal necrosis Mg, B and K deficiencies (see deficiency symptoms of these nutrients) Ca and K deficiencies (see deficiency symptoms of these nutrients) Interveinal chlorosis of new leaves Premature death or dropping of leaves Reduced K, Mg, Ca, Mn (See deficiency symptoms of these nutrients) 18 Topic 1: Module 1
Micro-nutrient deficiency symptoms TABLE 5: Micro-nutrient deficiency and toxity symptoms Micro-nutrient Deficiency symptoms Toxicity symptoms Iron (Fe) Manganese (Mn) Boron (B) Molybdenum (Mo) Zinc (Zn) Copper (Cu) Chlorosis. Begins as interveinal chlorosis in the younger leaves, followed by general chlorosis of entire leaf, causing leaves to appear white. If iron fertiliser is added, the veins become bright green before the rest of the leaf. Necrosis. Chlorotic areas develop necrotic spots. Chlorosis. Starts with interveinal chlorosis of the young leaves. Veins of young and old leaves are visible as dark net patterns when viewed against the light. Leaves develop a grey to purple shine. Necrosis. New leaves have dark necrotic spots. In grain crops, the necrotic spots are grey and they elongate and join up until the whole leaf dies. Chlorosis. There is a light general chlorosis. Necrosis. Growing tips turn brown and die. Necrotic spots develop on fruits. Leaves become crinkled and brittle. Petioles become dark and brittle. Sometimes a syrupy liquid is released from the base of the leaf. Stems may be hollow or roughened. Chlorosis. Begins with mottled spots of chlorotic tissue and develops into larger interveinal chlorotic areas. Leaves cup upwards to resemble saucers. Chlorosis. Young leaves become yellow. The veins stay yellow. Mature leaves develop pits on upper surface of the interveinal tissue. Leaves and stems are stunted. Guttation. Drops of water are forced out along the margins of the leaves. Necrosis. Chlorotic and pitted tissues turn dark and die. The veins stay green. Chlorosis. Young leaves show general light chlorosis. Mature leaves show green veins with grey to white-grey interveinal areas. Leaf edges look burnt and curled. Petioles bend downwards. Some leaves develop sunken necrotic spots Tiny brown spots on lower leaves starting from the tip Chlorosis of whole leaves with leaves turning from orange-yellow to bronzebrown Necrotic spots older leaves surrounded by chlorotic circles New leaves have necrotic edges and spots, may be malformed and stunted Chlorosis followed by necrosis beginning at the leaf tip and margins and spreading toward the midrib Eventually older leaves appear scorched and fall prematurely Stunted growth Leaves turn yellow-brown Leaves turn dark green Inteveinal chlorosis develops Fe deficiency (see Fe deficiency symptoms) Displaces iron (Fe) from the plant, causing chlorosis and other Fe deficiency symptoms, such as stunted growth (See Iron deficiency symptoms) Cobalt (Co) Little is known about its deficiency symptoms. Little is known about its toxicity symptoms Chlorine (Cl) Chlorosis. Occurs in smooth flat depressions in the interveinal area Wilting of young leaves. Bronze colouration on the upper side of the mature leaves. Leaves may have abnormal shapes. Salt injury and leaf burn Unit 3: Symptoms of nutrient deficiencies in crops 19
Assessment activity 1 Assignment on identifying deficiency symptoms in crops 1. Draw a table to summarise the deficiency symptoms of plants. The table should be suitable for you to use as a field reference. 2. Use your table to identify possible nutrient deficiencies in at least two crops in your college or local area. You may also need to ask local farmers about the history of the development of symptoms to help identify the deficiency. 3. Write a report outlining the following points: a) crops studied b) description of deficiency symptoms (in the order in which they appeared or developed) c) conclusions about which nutrient/nutrients are deficient. Assessment activity 2 Short test on deficiency symptoms 1. a) What is chlorosis? b) Is chlorosis alone a good indicator of which nutrient is deficient? Explain with examples. 2. Describe the symptoms you would expect to find in plants that are deficient in the following nutrients: a) nitrogen d) iron b) phosphorus e) copper c) potassium 3. Which symptoms would you use to distinguish between: a) sulphur and phosphorous deficiency b) potassium and magnesium deficiency. 4. What would you do if you could not clearly diagnose a nutrient deficiency using visual deficiency symptoms? Assess yourself Assess your performance in the following activities: Assignment on identifying nutrient deficiency symptoms 1 2 3 4 Short test on nutrient deficiency symptoms 1 = not achieved; 2 = not yet competent; 3 = competent; 4 = outstanding Unit summary 1 2 3 4 This unit focuses on the visual symptoms that can indicate nutrient deficiencies in a plant. The deficiency symptoms for both the macronutrients and micronutrients are described. 20 Topic 1: Module 1