Materials used in Agriculture

The lesson of the story: feed the soil what it lacks, in the right amount, and find out what it lacks by testing it. The rest of this unit explains each part of that.

A seed sprouts, grows into a plant, flowers, and its flowers become fruits with new seeds: the life cycle goes on and on. To complete it, a plant needs 17 elements, called the essential elements (or plant nutrients). Without any one of them, it can’t grow and reproduce normally.

The 17 essential elements are: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca), magnesium (Mg), boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni) and zinc (Zn).

Carbon, hydrogen and oxygen come from air and water: plants take carbon dioxide from the air and water from the soil, and make sugar from them by photosynthesis. The other 14 come from the soil, dissolved in the water that the roots take up.

The soil nutrients are grouped by how much of each a plant needs.

• a.Primary nutrients are nitrogen (N), phosphorus (P) and potassium (K). Plants need them in large amounts, so they run short first, and farmers must manage them carefully.
• b.Secondary nutrients are calcium (Ca), magnesium (Mg) and sulphur (S), needed in smaller, but still fair, amounts.
• c.Micronutrients are iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel. Plants need only tiny amounts of them, but without them, growth, development and the harvest are still held back.

In nature, the nutrients go round in a cycle. Leaves fall, plants and animals die, dung is dropped; all of it decays in the soil, and the nutrients in it are returned for new plants to use. But when a farmer harvests a crop and takes it away, the nutrients go with it. Farm the same field again and again, and the soil runs short. That shortage is made up with fertilizer.

A fertilizer is any substance that supplies plants with the essential nutrients they need. We add fertilizer to the soil to make up for nutrients it lacks. It increases the fertility of the soil (its ability to grow crops), helps plants grow and develop, and keeps them healthier and better able to resist disease.

There are two types of fertilizer: organic and inorganic.

a. Organic fertilizer

Organic fertilizer (or manure) is made from the dead bodies and wastes of living things, decayed by bacteria and fungi. Compost, farmyard manure, green manure, oil cake (pina), bone meal and wood ash are organic fertilizers.

Compost is made from decayed plant and animal waste: fallen leaves, straw, grass, leftover fodder, kitchen scraps, animal dung and urine, and dead animals. Dung of cows and buffaloes (gobar mal) is the most common organic manure in Nepal.

Green manure: before sowing the main crop, a farmer grows a fast-growing plant on the field, and then plows it into the soil while it is still green. There it rots, and becomes manure. Plants of the pea and bean family (legumes) are best for this, such as dhaincha and sunhemp: bacteria in small lumps (nodules) on their roots take nitrogen from the air, so when they rot, they add nitrogen (as nitrates) to the soil. In Nepal, the leafy branches of plants such as asuro (adhatoda), titepati (mugwort) and khirro are also cut and buried in fields as green manure.

Oil cake (pina), what is left after oil is pressed from mustard seeds, is rich in nitrogen. Bone meal (powdered bones) supplies phosphorus and calcium. Wood ash supplies potassium.

Advantages of organic fertilizers

1. They supply many nutrients at once, slowly, as they decay, so the soil stays fertile for a long time.
2. They add humus (decayed organic matter), which makes the soil soft and crumbly, lets air in, and helps it hold water.
3. They feed the earthworms and microbes that keep soil alive.
4. They don’t harm the soil even after many years of use, and they cause little pollution: they turn wastes into something useful.
5. Crops grown with them are as nutritious and tasty as any, and are free of chemical residues.

b. Inorganic fertilizer

When the soil lacks nutrients, they can also be supplied as chemicals. Inorganic fertilizers (also called chemical fertilizers) are inorganic compounds, made in factories from minerals and gases, that supply nitrogen, phosphorus and potassium to plants. For example, urea is made from ammonia and carbon dioxide:

$$2\text{NH}_3 + \text{CO}_2 \longrightarrow \text{NH}_2\text{CONH}_2 + \text{H}_2\text{O}$$

(Urea is in fact an organic compound, but as a factory-made fertilizer it is grouped with the chemical fertilizers.)

Chemical fertilizers dissolve easily in water, so plants take them up quickly and the effect shows within days. They are concentrated: a small amount contains a lot of one particular nutrient. By the main nutrient they supply, they are of three kinds: nitrogenous, phosphatic and potassium fertilizers.

Nitrogenous fertilizers

Nitrogenous fertilizers are the compounds used to supply nitrogen to plants. Examples:

Fertilizer	Formula	Nitrogen in it
urea	NH₂CONH₂	46%
ammonium nitrate	NH₄NO₃	35%
ammonium sulphate	(NH₄)₂SO₄	21%

Why plants need nitrogen: nitrogen is part of every protein and of chlorophyll. It makes plants grow fast and leafy, raises the rate of photosynthesis, helps build protein and protoplasm, and increases the harvest.

Effects of nitrogen deficiency (signs of too little nitrogen): the plant grows slowly and stays small, and the harvest falls. The leaves turn pale yellow, starting with the older, lower leaves (the plant moves its scarce nitrogen to the young leaves). Flowers, fruits and seeds are small.

Phosphatic fertilizers

Phosphatic fertilizers are the compounds used to supply phosphorus to plants. Examples:

Fertilizer	Formula
ammonium phosphate (as DAP, diammonium phosphate, the most used in Nepal)	(NH₄)₂HPO₄
single superphosphate (calcium superphosphate)	Ca(H₂PO₄)₂ mixed with CaSO₄
triple superphosphate	Ca(H₂PO₄)₂
ammonium metaphosphate	NH₄PO₃

Why plants need phosphorus: it helps roots grow and develop, and helps flowers, fruits and seeds form and ripen on time. Phosphorus is part of DNA and of the molecules that carry energy in every cell. It also helps plants resist disease.

Effects of phosphorus deficiency: the roots stay small. Cell division slows, so the plant is thin and stunted. Leaves are dull dark green, often with a purplish tinge. Fruits and seeds ripen late and the seeds are shriveled. The plant becomes weak against disease.

Potassium fertilizers

Potassium fertilizers (potash fertilizers) are the compounds used to supply potassium to plants. Examples: potassium chloride (KCl, called muriate of potash, the most used), potassium sulphate (K₂SO₄), potassium nitrate (KNO₃) and potassium carbonate (K₂CO₃). Wood ash contains potassium carbonate, which is why ash is good for the soil.

Why plants need potassium: it makes stems and roots strong, so plants don’t fall over, and helps them resist disease and drought. It helps flowers and fruits develop, and helps the plant make and move sugar, starch and protein.

Effects of potassium deficiency: the plant makes less protein and starch, and cell division slows, so it grows weak. The leaves stay small and fall early, and the edges of the older leaves turn yellow and then brown and dry, as if scorched. Stems are weak and fall over easily, and plants may dry up and die.

NPK fertilizer

A fertilizer that supplies all three primary nutrients, nitrogen, phosphorus and potassium, is called an NPK fertilizer (a complex or mixed fertilizer). Its bag carries three numbers, such as 20-20-10: the percentages of nitrogen (N), phosphate (written as P₂O₅) and potash (written as K₂O). So 100 kg of a 20-20-10 fertilizer supplies 20 kg of N, 20 kg of P₂O₅ and 10 kg of K₂O. Urea is 46-0-0 and DAP is 18-46-0. An NPK fertilizer gives a balanced feed in one application, so a farmer doesn’t have to buy and spread three fertilizers separately.

Advantages of chemical fertilizers

• a.They contain exactly the nutrients plants need, in concentrated form, so they quickly increase the fertility of the soil and the harvest.
• b.They dissolve easily in water, so plants absorb their nutrients quickly.
• c.They are easy to use, to transport and to store, and can be made in large amounts.

Disadvantages of chemical fertilizers

Chemical fertilizers are useful, but using too much of them, or using nothing else, does great harm.

• a.They don’t break down into humus the way manure does (exam answers often say they are not biodegradable). What plants don’t take up is washed by rain into rivers, ponds and wells. There it pollutes the water: nitrates in drinking water are harmful, especially to babies, and the extra nutrients make algae grow so thickly in ponds that fish die.
• b.They change the acidity of the soil: ammonium fertilizers and urea, used year after year, make soil more acidic; some others make it more basic. Crops then grow poorly.
• c.They supply nutrients but no humus. Used alone, year after year, they leave the soil hard and lifeless, with fewer earthworms and microbes. Then the soil gives less and less unless more and more fertilizer is added, and the farmer comes to depend on it.
• d.Too much nitrogen makes plants soft and leafy, easy prey for insects, and can lower the taste, quality and keeping time of fruits and vegetables.

Insects such as caterpillars, aphids, stem borers and grain weevils eat a large share of the world’s crops. To kill or control them, farmers use insecticides: chemicals that kill or control the harmful insects that damage crops. They are dusted on as a powder or sprayed as a liquid. For example, malathion is used to kill the insects that attack vegetables and fruits.

Insecticides are of two kinds: bio-insecticides and chemical insecticides.

a. Bio-insecticides

Bio-insecticides are insecticides made from natural things, mostly the parts of plants that insects dislike. They are made locally, from plants that grow around us. Jholmol is a bio-insecticide made in Nepal. Other examples from Nepali farms:

• the ash that farmers in villages sprinkle on plants;
• branches of asuro (adhatoda) laid in the field before planting potatoes;
• neem (nim) leaves kept in stored grain;
• juice of titepati (mugwort) and neem used to kill bedbugs.

The careless use of chemical insecticides pollutes the land, water and living things. Bio-insecticides can be used instead. They repel insects and protect plants from disease, and they don’t harm the soil, other living things or human health.

b. Chemical insecticides

Chemical insecticides are compounds made in factories on a large scale. Examples: DDT (dichlorodiphenyltrichloroethane), BHC (benzene hexachloride), methoxychlor, aldrin and dieldrin, and malathion.

Advantages of insecticides: they kill the insects that harm crops and help to increase the harvest. But they do great harm to the environment as well. Farmers often use chemical insecticides not because they want to but because they feel they must, to save their crop. These chemicals are poisons and must be stored safely.

Disadvantages of insecticides:

1. Residues of insecticides left on fruits and vegetables enter our bodies with our food and harm our health.
2. Breathing in sprays, or getting them on the skin, can harm the lungs, the nervous system and the glands (hormone system); badly handled, they poison and even kill the people who spray them.
3. They pollute the air, water and soil.
4. They kill not only harmful insects but also useful ones: bees that pollinate crops, and insects that eat pests.
5. They upset the ecosystem. DDT, for example, washes into water. Fish and other water animals in polluted water die, or those that survive have fewer young. DDT also builds up in body fat as it passes up the food chain, so birds and fish that eat other animals carry the most, and their growth and breeding are harmed.

Precautions while using chemical insecticides

1. Read the instructions on the label carefully before use, and use only the amount recommended.
2. The person spraying should wear protective clothing: gloves, long sleeves, boots, a mask and goggles.
3. Never let the insecticide touch the eyes or mouth; don’t eat, drink or smoke while spraying, and wash well afterwards.
4. Spray with the wind (the wind blowing from behind you), so the spray is carried away from you, not into your face. Don’t spray on a windy day.
5. Store what is left over safely: in its own labeled container, locked away from children, food and animals.
6. Don’t wash the sprayer and other equipment near a water source (a stream, pond, well or tap).
7. Wait the number of days given on the label before harvesting a sprayed crop, so the poison has time to break down.

To keep getting good crops, a farmer must keep the soil in good health. Using too many chemicals to push up the harvest can spoil it. Soil quality can be kept up in these ways:

a. Test the soil

Testing the soil from time to time shows which nutrients it lacks, and how acidic it is, so the farmer adds only what is needed. It also shows which crops suit the soil.

b. Use suitable farming methods

• crop rotation: growing different crops on a field one after another (rice, then wheat, then a legume such as lentils, which adds nitrogen), so that the same nutrients aren’t used up year after year;
• mixed cropping: growing two or more crops together, such as maize with beans;
• agricultural lime on acidic soil;
• plenty of compost and manure, and as little chemical fertilizer as needed;
• suitable irrigation, neither too little nor too much water.

c. Use the land properly

A field left unplowed and bare for years slowly loses its fertility. Burning dry leaves and crop stubble on the field kills the useful microbes and earthworms in the soil. On steep slopes, rain washes the fertile topsoil away, so farmers build terraces: flat steps of land, each a little higher than the one below, that hold the soil and water.

1. Choose the correct answer

2. Differentiate between

3. Give reasons

4. Answer the following questions