Managing nutrients, water and cropping

Nutrients for a crop

A plant needs 16 nutrients to grow well. Air supplies carbon and oxygen. Water supplies hydrogen and oxygen. The soil supplies the other 13. These 13 run short when a field is farmed again and again.

• Macronutrients are needed in large amounts: nitrogen, phosphorus, potassium, calcium, magnesium and sulphur.
• Micronutrients are needed in small amounts: iron, manganese, boron, zinc, copper, molybdenum and chlorine.

A shortage of a nutrient harms the plant. It affects reproduction, growth and the plant's resistance to disease. So the soil must get back what the crop takes out. Farmers do this with manure and fertilisers.

Manure

Manure is made when animal waste and plant waste decompose. It holds a lot of organic matter and only a small amount of nutrients. The organic matter improves the soil. In sandy soil it helps the soil hold water. In clay soil it helps water drain, so the field does not become waterlogged.

There are three ways to make manure.

• Compost: farm waste such as animal dung, vegetable waste, straw, weeds and household waste is left to rot in pits.
• Vermi-compost: compost made faster with earthworms, which break down the plant and animal waste.
• Green manure: before sowing, the farmer grows a plant such as sun hemp or guar. Then the plants are ploughed into the soil, where they rot and enrich it with nitrogen and phosphorus.

Fertilisers

Fertilisers are factory-made plant nutrients, mainly nitrogen, phosphorus and potassium. They give a quick growth of leaves, branches and flowers, and so a healthy plant.

Fertilisers must be used with care.

• Apply the right dose at the right time. Too little does not help, and too much is a waste of money.
• Too much can wash into streams and ponds and pollute the water.
• Too much can harm the tiny living things of the soil, the microorganisms that keep it fertile.

Organic farming uses little or no chemical fertiliser or pesticide. It uses manure, recycled farm waste, cultures of blue-green algae as natural fertiliser, neem leaves or turmeric to keep pests away, and healthy cropping patterns.

More fertiliser, smaller gains

Set the dose of fertiliser below. The first doses raise the yield a lot. Each later dose raises it by less. At some point the extra grain is not worth the cost of the extra fertiliser.

Try it in Python

The program goes through doses of $20$ kg, one after another. It finds the last dose that still pays for itself.

Irrigation

Rain is uncertain, so farmers bring water to the fields. The main sources are these.

• Wells: a dug well collects water from the layers of soil and rock that hold water. A tube well taps water from deeper layers. Pumps lift the water.
• Canals: an extensive system. Water from reservoirs or rivers flows along a main canal, which divides into branch canals.
• River lift systems: where a canal does not carry enough water, or carries it only at times, water is drawn straight from the river.
• Tanks: small reservoirs that catch and store the run-off of a small catchment area.

Rainwater harvesting also helps. It collects and stores the rain for use in the dry months.

Cropping patterns

A farmer chooses which crops to grow, and in what pattern.

• Mixed cropping: two or more crops are grown together on the same piece of land. Wheat with gram, wheat with mustard and groundnut with sunflower are examples. If one crop fails, the other may still give a harvest.
• Inter-cropping: two or more crops are grown together in a definite pattern. A few rows of one crop alternate with a few rows of another. Soyabean with maize, and finger millet with cowpea, are examples. The crops are chosen to need different nutrients, so the soil is used fully. A pest of one crop also cannot spread to every plant.
• Crop rotation: different crops are grown one after another on the same land, in a planned order. With enough moisture and irrigation, a farmer can grow two or three crops in a year, each with a good harvest.

Practise with new numbers

Common mistakes

• "More fertiliser always means a much bigger harvest." Each extra dose gives less extra grain, and it costs money.
• "Manure and fertiliser are the same." Manure comes from rotted waste and adds organic matter. Fertiliser is factory-made nutrient.
• "Mixed cropping and inter-cropping are the same." Inter-cropping follows a definite pattern of rows. Mixed cropping does not.
• "A tank brings water from a river." A tank stores the run-off of a small catchment area. A river lift system draws water from a river.

Remember these

• A plant needs 16 nutrients, and 13 of them come from the soil. Six are macronutrients and seven are micronutrients.
• Manure adds organic matter. Compost, vermi-compost and green manure are kinds of manure.
• Fertilisers give nitrogen, phosphorus and potassium, and must be used in the right dose.
• Irrigation sources are wells, canals, river lift systems and tanks.
• Mixed cropping, inter-cropping and crop rotation are the cropping patterns.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many nutrients does a plant need, and how many come from the soil?

2. Name two macronutrients and two micronutrients.

3. Why does manure help both sandy soil and clay soil?

4. What is green manure?

5. A farmer grows soyabean and maize in alternate rows. Which cropping pattern is this, and why is it a good idea?