SOIL AND PLANT NUTRITION 1.0 Introduction Plants need at least 16 elements for normal growth to be able to produce well. Some of the elements are needed in large quantities. Carbon, hydrogen and oxygen are supplied by air and water while 13 other elements are taken up by plants in inorganic form from the soil or must be added as fertilizers. Of the 13 elements, nitrogen (N), phosphorus (P), and potassium (K) are needed in larger quantities. These nutrients are referred to as primary nutrients, and are the ones most frequently supplied to plants as fertilizers. The three secondary elements, calcium (Ca), magnesium (Mg), and sulphur (S), are required in smaller amounts than the primary nutrients. Another group is that of micronutrients or also called trace elements, which consist of seven essential elements: boron (B), copper (Cu), chlorine (Cl), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn). Despite that these elements are needed by plants in smaller quantities, their importance to plants functions are the same as those of primary and secondary elements. The availability of nutrients to plants is determined by the form and chemical properties of the element, the soil ph, and interactions with soil colloids, microbial activity and soil physical conditions such as aeration, compaction, temperature, and moisture. Supplementing soil plant nutrition is not an innovative approach as it has been done for more than five centuries now. The 16 elements needed by plants are commonly referred to as essential nutrients. They are called essential because if one of them is not available, the plants cannot grow properly. Some of the nutrients are called macronutrients because the plants need them in significant quantities and others are micronutrients as they are needed in small quantities. The macronutrients are present in plant tissue in quantities from 0.2% to 4.0% (on a dry matter weight basis). Micro nutrients are present in plant tissue in quantities measured in parts per million (ppm), ranging from 5 to 200 ppm, or less than 0.02% dry weight. All the nutrients (macro and micro) have the same importance such that the nitrogen element that is needed by plants in the largest quantity, is not more important than Copper, an element required in smaller quantities. 2.0 Factors affecting plant nutrient absorption Plants nutrient absorption can be influenced by many factors that are grouped as chemical factors (ph, EC), physical factors (soil structure, texture) and biological factors (immobilization, fixation and mineralization processes). Nutrient uptake from the soil is achieved by cation exchange, where root hairs pump hydrogen ions (H + ) into the soil through proton pumps. These hydrogen ions displace cations attached to negatively charged soil particles so that the cations are available for uptake by the root. The root, especially the root hair, is therefore the most essential organ for the uptake of nutrients. South Sudan Agribusiness Development Programme Page 1
2.1 Chemical Factors Some of the chemical factors are the level of nutrients (EC) and the ph around the root zone. EC (electrical conductivity of the soil) shows the amount of nutrients dissolved in the soil solution ready for plants uptake. Good soil should have an EC < 2 and ph shows the degree of alkalinity or acidity of the soil. Good soil ph ranges from 5.5-6.9. ph: In chemistry, ph is a numeric scale used to specify the acidity or basicity(alkalinity) of an aqueous solution. ph stands for 'potential of Hydrogen'. A ph value is a number from 1 to 14, with 7 as the middle (neutral) point. Values below 7 indicate acidity which increases as the number decreases, 1 being the most acidic. Values above 7 indicate basicity (alkalinity) which increases as the number increases, 14 being the most basic (alkaline). This scale, however, is not a linear scale like a centimetre or inch scale (in which two adjacent values have the same difference). It is a logarithmic scale in which two adjacent values increase or decrease by a factor of 10. 2.1.1 Levels of nutrients Depending on levels of various nutrient elements in the root zone, the plants can be negatively or positively affected through element antagonisms or synergism processes. 2.1.2 Antagonism This occurs when the high level of one nutrient affects the absorption of others. For example, high level of calcium will reduce the absorption of phosphorus, potassium, magnesium, iron and zinc. 2.1.3 Synergism This is when the prominent level of one nutrient will help the absorption of others. For example, a high level of calcium helps the absorption of nitrogen and high levels of phosphorus results in increased absorption of sulphur. The ph around the roots zone is important because at a certain level of alkalinity, some elements create insoluble precipitates. For example, iron, manganese and copper will form insoluble precipitates at higher ph while at acidic conditions, higher concentrations of aluminium and iron ions will lead to phosphorus fixation on soil clay colloids. At lower ph, leaching of minerals is also very high. 2.2 Physical factors Poor soil structure and poor land preparation limits plant nutrient up-take as the roots, as it is for the leaves, need oxygen to carry out gaseous exchange and mineral absorption. Fine texture creates more contact between roots and the growing media thus, increasing nutrient absorption. 2.3 Biological Factors Biological factors, such as soil fauna and flora as well as soil micro-organisms such as bacteria and beneficial fungi activities, help in organic matter decomposition, fixation and nitrification process. Other factors that affect plant nutrient up-take are temperature, humidity, light, and type and crop development stage. South Sudan Agribusiness Development Programme Page 2
Figure 1: Poor land preparation affects nutrients availability to plants 3.0 Plant Nutrition Diagnosis Nutritional diagnostic is very useful. It helps to understand the nutrient levels of soils for a good crop nutrition program. Soil, irrigation water, leachate and foliar material analysis are mostly used for nutritional diagnostics. However, when diagnosing for plant nutrition deficiencies, we need also to understand the weather, level of irrigation and pest control managements. Sometimes nutrition deficiencies appear because of some other factors that hinders the absorption of nutrients and not the real nutrient deficiency in the soil or poor nutrition programs. 3.1 Nutrients Analysis 3.1.1 Soil Analysis This type of test helps to know the level of nutrients, ph, EC, organic matter, texture, etc. It is very important before growing a crop to know the status of the soil such that in case the soil needs amendments it should be corrected. South Sudan Agribusiness Development Programme Page 3
Parameter Normal Optimal Range Risk Range Preventive or Range corrective actions ph 6.0 8.0 5.5 6.5 > 7.5 - Addition of acids to the solution - Addition of lime to the soil EC (ds/m) 0.5 5.0 < 2.0 > 3.0 - Frequent irrigations and adequate drainage Calcium (mm) Up to 20.0 4.0 10.0 < 4.0 - Addition of calcium - Addition of magnesium Magnesium Up to 15.0 2.0 5.0 < 2.0 - Addition of (mm) magnesium - Addition of calcium Sodium (mm) Up to 40.0 As low as possible Potassium (mm) Up to 1.0 It s a nutrient to be added based on requirements Bicarbonates (mm) Sulphates (mm ) Chlorides (mm Up to 40.0 As low as ) possible > 8.0 - Addition of calcium - Addition of organic matter and sulphur - Frequent irrigations and adequate drainage If exceed the crop requirements - Adjust to the crop requirements Up to 10.0 0.5 1.0 > 3.0 - Addition of acids to the solution Up to 20.0 2.0 5.0 < 1.0 - Addition of sulphr or sulphates > 8.0 - Frequent irrigations and adequate drainage Boron (mg/l) Up to 3.0 0.1 0.4 > 0.7 - Frequent irrigations and adequate drainage - Addition of calcium - Addition of nitrogen Figure 2: Levels of different chemical parameters in the soil. 4.0 Essential Elements Nitrogen Nitrogen (N) is an essential nutrient used in relatively large amounts by all living organisms. It is a necessary element for formation of amino acids, the building blocks of protein. It is also essential for plant cell division, vital for plant growth and it an important part of chlorophyll. Nitrogen is also necessary component of vitamins and it influences energy reactions in plants The deficiency of nitrogen in plants is characterised by stunted growth and chlorosis. Since plants can remobilize N from older tissue to provide N to younger tissue, chlorosis usually appears on the lower leaves first while the upper leaves remain green Figure 3: Nitrogen deficiency symptoms on leaves South Sudan Agribusiness Development Programme Page 4
Phosphorus It is an essential element in early development stage of plant growth despite that it is needed in low quantities compared to major elements. It stimulates young root formation and early fruit formation. It is essential in several biochemical processes involved in photosynthesis, respiration, cell division as well as plant growth and development processes. It is also an essential element in seed formation. Phosphorous also helps to increase plant water-use efficiency. The deficiency is characterised by small leaves purple in colour. Phosphorus trans-locates from older tissue to new, actively growing tissue readily, so discoloration tends to appear on older leaves. With phosphorous deficiency, the plants suffer from reduced fruit and seed production. Dependent on ph range, excess of foliage with no flowers can also indicate lack of phosphorus. Figure 4: Phosphorus deficiency symptoms on leaves Potassium Potassium is needed by plants in similar quantities as nitrogen. It is very essential for enzymatic activities, it helps to enhance photosynthesis, and it is a key component in protein synthesis. Potassium improves plant disease resistance, drought tolerance and decrease lodging. Potassium also promotes flowering and fruit formation. Potassium deficiency leads to stunted growth, poor root development, older leaves may look scorched around the edges and/or wilted. The leaves will also show interveinal chlorosis (yellowing between the leaf veins) and yellowing of the leaves starting on older leaves and progressing upwards. The symptoms on fruits are depicted by poor colour (pale) when ripening. The plants will also show little resistance to heat, cold and disease problems. K deficiency can result in defoliation, thus reduced photosynthesis, and ultimately low yields. South Sudan Agribusiness Development Programme Page 5
Figure 5: Potassium deficiency symptoms on fruits. Figure 6: K deficiency symptoms on leaves Calcium Calcium is a structural component of plant cell walls. It is important for plant cell division and formation. It reduces plant respiration during stress. It helps in translocation of photosynthetic materials from leaves to fruiting organs. It increases fruit set and stimulates microbial activities in the soil. Calcium also enhances uptake of nitrate- NO3. Calcium deficiency results in poorly developed root systems and relatively little fruits of inferior quality. New leaves are distorted or hook shaped. The growing tip may die. Calcium deficiency contributes to blossom end rot in tomatoes, tip burn and brown/black heart of lettuce. This element is absorbed for the plant through the transpiration process. If temperature is too high and humidity is too high or too low the deficiency will appear. Figure 7: Blossom end rot as calcium deficiency symptom South Sudan Agribusiness Development Programme Page 6
Magnesium Magnesium is a key element for chlorophyll formation and it improves mobility and utilization of phosphorus as well as increases iron utilization in plants. Magnesium is also an activator and component of many plant enzymes and is associated with the ability to influence uniform maturity of crops and it also plays a key role in protein synthesis. Under deficient supply of magnesium plants will exhibit slow growth characterised with yellowing of leaves especially bottom older leaves. In cereal crops, Mg deficiency symptoms include interveinal chlorosis on the lower leaves. Leaf edges may also show a thin hint of red or purple. On broad-leaved crops, the lower leaves of Mg-deficient plants may initially show interveinal chlorosis and become purplish-red with green veins. Figure 8: Magnesium deficiency signs on leaves Figure 9: Mg deficiency signs on leaves South Sudan Agribusiness Development Programme Page 7
Iron Iron is important in the activation of enzyme systems in plants including: fumaric hydrogenase, catalase, oxidase, and cytochrome. A shortage of Fe also impairs chlorophyll production. Iron is associated with the synthesis of chloroplast protein (formation of chlorophyll). It acts as an oxygen carrier in plants and it is useful in cell division and growth of plants. Deficiency of iron occurs frequently in fruit trees and in acidic soils. New leaves are the most symptomatic and when the condition is severe they can be all yellow or white but with green veins. Figure 10 and Figure 11: Iron deficiency signs on tomato leaves South Sudan Agribusiness Development Programme Page 8
Zinc Zinc availability is greatly affected by soil Ph. As soil ph increases, Zn availability decreases. The reverse is also true. Zinc availability to plants is lower in soils with high organic matter contents. Plant zinc uptake can also be negatively affected in excess of P. Zinc is important in plant nutrition and different enzymatic functions; it helps in stabilizing cytoplasmic ribosomes, oxidation processes, transformation of carbohydrates and synthesis of auxin and indole acetic acid. It is a critical component for chlorophyll production and helps in seed formation. Yellowing between veins of new growth. Terminal (end) leaves may form a rosette. Figure 12 and Figure 13: Deficiency of zinc on leaves Manganese Manganese availability in the soil is mostly affected by soil ph, organic matter and soil moisture, with ph having the greatest effect. Manganese is most available at low ph, hence Mn toxicity in acidic soils that can easily be alleviated by liming to above Ph 6.5. Manganese has an antagonistic interaction with Fe. As Mn concentrations in plant tissues increase, Fe concentrations decrease and the reverse is true. Manganese functions as part of enzyme systems and helps in chlorophyll synthesis. Manganese also helps to increase phosphorus, calcium and nitrogen uptake in plants. It is a constituent of some enzymes involved in respiration and protein synthesis. Manganese deficiencies are most common on sandy, acidic mineral soils. The plants with manganese deficiency show stunted growth and yellowing of leaves particularly young ones, vividly seen between the veins. The leaves may also develop dark or dead spots, leaves, shoots and South Sudan Agribusiness Development Programme Page 9
fruit diminished in size and failure blooming. Manganese deficiency symptoms typically include interveinal chlorosis with dark-green veins. Under severe deficiency, leaves develop brown speckling and bronzing, in addition to interveinal chlorosis. In maize, deficient leaves are light green with parallel yellowish stripes. Crops most sensitive to Mn deficiency include: beans, lettuce, onions, peas, potato, and spinach. Figure 14 and Figure 15: Manganese deficiency on leaves Boron Boron is essential for plant growth, germination of pollen grains and growth of pollen tubes. It is also essential for seed and cell wall formation, necessary for sugar translocation. It is also involved in transportation of nutrients, (N and sugars) form the leaves to the fruit, in the formation of amino acids and proteins and in the utilization of absorbed phosphorus and water. Boron deficiency is characterised by poor stem and root growth. Terminal (end) buds may die. Youngest leaves may change colour to red or bronze. Stems may be stiff; lateral shoots may develop, giving plants flat tops. Leaves are highly tinted purple, brown and yellow, bushy foliage and excessive shedding of fruit. often appear on the young tissue first. Figure 16 and Figure 17: Deficiency of boron on leaves South Sudan Agribusiness Development Programme Page 10
Copper Copper is required by plants in small concentrations. It plays a significant role in plant enzymes and enzymatic systems. It catalyses several plant processes and has a major function in photosynthesis, a major function in reproductive stages and helps in increasing sugar contents. It also intensifies colour and improves flavour of fruits and vegetables Copper deficiencies are most likely to occur on alkaline soils, poorly drained soils and sandy soils. The deficiencies of copper in plants can affect photosynthesis, respiration, carbohydrate distribution, N metabolism, cell wall metabolism, water relations, seed production and disease resistance. The deficiency is characterized by chlorosis, necrosis, leaf distortion (limp, curl, or drop), small leaves with necrotic (dead) spots and brown areas near the leaf tips, resetting of the leaves and dieback of terminal shoots as well as stunted growth of the entire plant. occur first in the young shoot tissues growth. Crops most sensitive to Cu deficiencies include wheat, alfalfa, oats, barley, millet, sunflowers, carrots, onions, lettuce and spinach.. Figure 18 and Figure 19: Copper deficiency symptoms South Sudan Agribusiness Development Programme Page 11
Molybdenum The element is required in the formation of enzyme which reduces nitrates to ammonium in plants and helps in the formation of legume nodules Molybdenum deficiencies often occur in acidic soils, in coarse textured soils and soil low in organic matter. Of all the plant micronutrients, Mo is the least needed by plants Mo deficiency is characterised by yellowing of older leaves, narrow and distorted leaves Figure 20: Molybdenum deficiency symptoms South Sudan Agribusiness Development Programme Page 12
Sulphur It is an integral part of amino acids and it helps in the formation of plant enzymes and vitamins. It also helps in the formation of root nodule in legumes. Sulphur is a very important element in chlorophyll formation process and photosynthesis reactions. Together with Mg, S plays a role in the formation of oils within the seeds. New growth turns pale yellow but the leaf veins remain green, older growth remains green. The plants become spindle with slander stalks but dwarfed. Sulphur deficiency may look similar to that of N, only that N deficiency symptoms are vividly seen at the plant bottom while that of S are mainly seen at the top. Figure 21: Deficiency signs for Sulphur South Sudan Agribusiness Development Programme Page 13
Soil ph and plant Nutrient availability The ph readings are used to determine the nutrients availability to the plants. As we can see in the figure bellow, most of the nutrients are available when the ph is 5.5 6.5. Figure 22: Effect of soil ph on plant nutrients availability South Sudan Agribusiness Development Programme Page 14
ph Soil Classification Effects < 4.5 Extremely acid Very unfavourable conditions. 4.5 5.0 Very strongly acid Aluminium Toxicity. 5.1 5.5 Strongly acid Excess of Cu, Fe, Mn and Zn. Deficiency of Ca, K, N, Mg, Mo, P and S. Poor bacteria activity. 5.6 6.0 Moderately acid Adequate for most of the crops. 6.1 6.5 Lightly acid Maximum availability of nutrients. 6.6 7.3 Neutral Minimum toxic effects. 7.4 7.8 Lightly basic Decrease the availability of Phosphorus. 7.9 8.4 Moderately basic Cu, Mn and Zn deficiencies. 8.5 9.0 Strongly basic High Iron deficiency. > 9.0 Extremely basic Presence of sodium carbonate, low bacteria activity. Figure 23: Effects of soil ph on plant nutrients availability Fertilizers Element % Composition UREA N 46 Sulphate of Ammonia (SA) N 21 Calcium Ammonium Nitrate (CAN) N 26 Mono ammonium phosphate (MAP) P, N 61 (P) 12 (N) Di-ammonium phosphate (DAP) P, N 46 (P) 18 (N) Phosphoric Acid P 60 Rock phosphate (Minjingu) P 3-35 Potassium chloride (MOP) K 60-62 Potassium sulphate K, S 50(K) 16 (S) Potassium, magnesium sulphate K, Mg, S 22 (K) 11 (Mg) 22 (S) Potassium nitrate K, N 44 (K) 13 (N) Calcium Nitrate Ca, N 27 (ca) 15.5 (N) Magnesium sulphate Mg, S 31.7 (S) 16 (Mg) Boric acid B 18 Figure 24: Common fertilizers for plant nutrient supply. South Sudan Agribusiness Development Programme Page 15