Plant Nutrition and Fertilizers
The success of farming depends largely on the growth of crops. Crop growth is influenced by a number of factors of which plant nutrients are an important group. There are 16 nutrient elements considered essential for plant growth.
Essential Plant Nutrients
An essential plant nutrient element has the following characteristics
Nutrients Essential for Plant Growth and Forms in which Nutrients are Taken up by Plants
i. The completion of the life cycle of the plant cannot be achieved in the absence of such an element.
ii. Plays a specific role in the plant.
iii. Causes set back to growth of the plant showing visual symptoms when the plant is deficient in it.
Essential elements are inorganic in nature and they are grouped into
i. Primary nutrients – Required in larger quantities ii. Secondary nutrients – Needed in lesser amounts than primary nutrients iii. Micronutrients – Required in small quantities
These sixteen elements are listed Carbon, hydrogen and oxygen are obtained from air & water. The other thirteen elements are referred to as fertilizer elements and have to be obtained from the soil. Their addition in quantities necessary for plant growth will increase the growth rate, dry matter content and yield of the crop.
Plant nutrients are usually absorbed through roots. Roots have the ability to absorb nutrients selectively. Root absorption takes place both as active and passive absorption.
Active absorption takes place as an exchange phenomenon and requires energy. Most plant nutrients are absorbed in this manner. Passive absorption is part of the transpiration cycle (mass flow). Water and some dissolved solutes are absorbed by this process. l Gas exchange takes place through the stomata found in leaves. Carbon dioxide required for photosynthesis and oxygen required for plant respiration are exchanged through the leaves. The supply of an adequate quantity of a particular nutrient for crop growth depends on both the behavior of that nutrient in the soil and the ability of the crop root system to utilize it. When these elements are not available to the plant in quantities optimum for growth, the quantity and quality of yield is affected.
Sixteen chemical elements are known to be important to a plant's growth and survival. The sixteen chemical elements are divided into two main groups: non-mineral and mineral.
The Non-Mineral Nutrients are hydrogen (H), oxygen (O), & carbon (C). These nutrients are found in the air and water. In a process called photosynthesis, plants use energy from the sun to change carbon dioxide (CO2 - carbon and oxygen) and water (H2O- hydrogen and oxygen) into starches and sugars. These starches and sugars are the plant's food. Photosynthesis means "making things with light".
Since plants get carbon, hydrogen, and oxygen from the air and water, there is little farmers and gardeners can do to control how much of these nutrients a plant can use.
The 13 mineral nutrients, which come from the soil, are dissolved in water and absorbed through a plant's roots. There are not always enough of these nutrients in the soil for a plant to grow healthy. This is why many farmers and gardeners use fertilizers to add the nutrients to the soil.
The mineral nutrients are divided into two groups, Macronutrients and micronutrients.
Macronutrients can be broken into two more groups, primary and secondary nutrients.
The primary nutrients are nitrogen (N), phosphorus (P), and potassium (K). These major nutrients usually are lacking from the soil first because plants use large amounts for their growth and survival.
The secondary nutrients are calcium (Ca), magnesium (Mg), and sulfur (S). There are usually enough of these nutrients in the soil so fertilization is not always needed. Also, large amounts of Calcium and Magnesium are added when lime is applied to acidic soils. Sulfur is usually found in sufficient amounts from the slow decomposition of soil organic matter, an important reason for not throwing out grass clippings and leaves.
Micronutrients are those elements essential for plant growth which are needed in only very small (micro) quantities. These elements are sometimes called minor elements or trace elements, but use of the term micronutrient is encouraged by the American Society of Agronomy and the Soil Science Society of America.
The micronutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). Recycling organic matter such as grass clippings and tree leaves is an excellent way of providing micronutrients (as well as macronutrients) to growing plants.
Naturally occurring organic fertilizers include manure, slurry, worm castings, peat, seaweed, sewage, and guano. Green manure crops are also grown to add nutrients to the soil. Naturally occurring minerals such as mine rock phosphate, sulfate of potash and limestone are also considered organic fertilizers.
Manufactured organic fertilizers include compost, bloodmeal, bone meal and seaweed extracts. Other examples are natural enzyme digested proteins, fish meal, and feather meal. The decomposing crop residue from prior years is another source of fertility. Though not strictly considered "fertilizer", the distinction seems more a matter of words than reality.
Benefits of organic fertilizer
However, by their nature, organic fertilizers provide increased physical and biological storage mechanisms to soils, mitigating risks of over-fertilization. Organic fertilizer nutrient content, solubility, and nutrient release rates are typically much lower than mineral (inorganic) fertilizers.
- Organic fertilizers had released between 25% and 60% of their nitrogen content
- Controlled release fertilizers (CRFs) had a relatively constant rate of release
- Soluble fertilizer released most of its nitrogen content at the first leaching
Disadvantages of organic fertilizer
It is difficult to chemically distinguish between urea of biological origin and those produced synthetically. Like inorganic fertilizers, it is possible to over-apply organic fertilizers if one does not measure and distribute the required amounts according to the recommended amounts for the plot of land in question. Release of the nutrients may happen quite suddenly depending on the type of organic fertilizer used.
Integrated Plant Nutrition Systems (IPNS)
Definition of IPNS l Integrated Plant Nutrition System is a holistic approach to plant nutrition by obtaining the nutrients from both inorganic and organic sources to maintain and sustain soil fertility and enhance crop productivity in a framework of an ecologically compatible, socially acceptable and economically viable situation. Need for integrated use l Organic manures sustain soil fertility at a low level of production. l Chemical fertilizers have concentrated forms of nutrients. Application results in leaching, fixation and build-up of certain nutrients at the expense of others, resulting in nutrient imbalances. l Fertilizer use efficiency (FUE) is low in all chemical fertilizers and organic manure when used singularly. l Combined use of organic manures and chemical fertilizers increases FUE Objectives of IPNS l Increasing the fertilizer use efficiency. l Increasing the return to investment on fertilizers. l To use a balanced system of crop nutrition management. l To account for the different amounts of plant nutrients, in a budget sheet giving the following: 57
- Available in the soil.
- Available from organic sources.
- Available in crop residues.
- Derived from biological fixation.
- Derived from fertilizers and their residual effects.
When preparing a budget, a whole cropping system is looked into than a singular crop, over a long term. What has gone wrong in fertilizer use? l Before chemical fertilizers were introduced, organic forms were used. l Green manures, crop residues and FYM were used in large quantities. l A season of farming was followed by a fallow period for replenishment of soil fertility. l Chemical fertilizers supplied sufficient plant nutrients for obtaining high yields. These fertilizers did not provide the crop requirement of all nutrients. l The continuous use of chemical fertilizers destroyed the soil structure, turned the soil acidic and brought about nutrient imbalances resulting in nutrient interactions. l The organic matter in the soil did not improve with application of chemical fertilizers resulting in erosion and loss of top soil. l The yield increases obtained during the early years of chemical fertilizer use evened out and addition of extra fertilizers did not bring the desired benefits. Farmer practices of IPNS l Vegetables, potato and green leafy vegetables are cultivated using both organic and chemical fertilizers. l Here farmers obtain higher yields than farmers using either chemical fertilizers or organic manures. l Extension of this practice to other crops has resulted in increases of yield and sustainability of production at a higher level. Chemical fertilizers or organic manures l To sustain crop yields, the rate of removal has to be balanced by added amounts. Use of chemical fertilizers is necessary for supplying the nutrient requirement but without recycling of crop residues, yields will suffer. l If only organic manure is used land will benefit but yield will be lower. l Availability of nutrients from organic manure is slow but long lasting. l Use of organic manure improves the physical, chemical and biological conditions of the soil. l Singular use of chemical fertilizers has an adverse effect on the soil structure. If the organic matter is low the bulk density increases. l Use of chemical fertilizers along with organic manure gives a soil rich in nutrients with good physical and microbiological properties. This will increase the availability of nutrients. 58 l Application of organic manure results in formation of water stable aggregates which could resist erosion. l Combined use of chemical and organic fertilizers increases cation retention and improves nutrient availability. l Addition of manure widens the C:N ratio in soil. Singular use of chemical fertilizers narrows this ratio. l Addition of mineral N fertilizers results in instant increases of N but organic manures increases the available N. l Fixation of fertilizer P could be reduced and effectiveness of fertilizer K can be increased when chemical fertilizer is combined with organic manures. l High analysis fertilizers have low contents of micro nutrients, but combined use with organic manure makes these nutrients available to plants. l Combined use of manure and chemical fertilizers results in higher return to investment and better cost-benefit ratios. l Thus it is beneficial to use both manure and chemical fertilizers for crop production.
Organic farming or IPNS l Organic farming systems do not use any synthetic form of plant nutrient.It also avoids the use of pesticides and weedicides. Organic manure of animal origin should not have artificial ingredients fed to animals and animals should not be treated with antibiotics or steroids as growth promoters. l Obtaining green manure continuously from trees results in removal of plant nutrients from soils, which are not easily replenished. This results in poor growth of the green manure trees itself, which could result in a short fall of supply of the material. Thus a limitation to resource management occurs. l Organic farming maintains the productivity at a low level. Where demand for food is becoming more increasing organic farming per se cannot meet the required demand. l Plants obtain nutrient sources in the ionic form. Whether they come from organic or inorganic forms, the type of ions is the same. l It is best to combine the use of both manure and mineral fertilizers rather than manage the singular use of manure. Managing a nutrient balance in IPNS l IPNS attempts to keep a balance between crop removal and nutrient addition to the soil. In this respect a book keeping exercise has to begin with the first step of how much the soil already has. l Soil analysis for evaluating the nutrient availability and its limitations that could reduce the productivity, has to be done at the beginning. l The yield goal of the cropping system of single or multiple crops that would be grown over time (say one year) has to be evaluated to arrive at the nutrient removal.
The quantity of mineral fertilizers to be applied has to be decided on soil test values for P and K. l The addition of organic manures to supplement the requirement of the minor and micro nutrients has to be evaluated. l Nitrogenous fertilizers to be used on crop demand rather than on blanket recommendations has to be practiced. l Limitations to FUE by drought, surface erosion, poor drainage are to be avoided. l Correct application practices in basal and top dressing applications of both organic and chemical fertilizer have to be adopted. l Crop residues have to be recycled to supplement the nutrient pool of the cropping system. l If legumes are included in the cropping system, credit has to be given for possible biological nitrogen fixation (BNF).