Law of supply

Topic: Theory of the Firm, Supply and Perfect Competition · NCERT: Class 9, Ch 9 "The Price Puzzle: What Drives the Market"

Meaning

The law of supply says that, other things staying the same, the price of a good and the quantity supplied move in the same direction. When the price rises, sellers offer more. When the price falls, they offer less.

This is why the supply curve slopes upward [2]. It is one half of the demand–supply model, which explains how market prices are set and why prices of perishables such as tomato and onion swing so sharply in India.

Explanation

How the law works

• Quantity supplied: the amount a seller is willing and able to sell at a given price, in a given period.
• Supply: the full list of these amounts at different prices. It can be shown as a supply schedule (a table) or a supply curve (a graph).
• Supply curve: price is on the vertical (Y) axis and quantity is on the horizontal (X) axis. In most cases it rises from left to right [2].
• Class 9 example (mango seller A):

Price (₹/kg)	Quantity supplied by A (kg)
50	1
100	2
150	3

• As the price doubles from ₹50 to ₹100, A's supply doubles from 1 kg to 2 kg. Plot these points and you get an upward-sloping curve (Fig 9.4).

Why quantity supplied rises with price

• Reason 1: higher profitability, so existing sellers produce more.
• The Class 12 logic behind this is rising marginal cost (MC). MC is the extra cost of making one more unit.
• As output grows, MC rises, so an extra unit is worth making only if the price is higher.

• A competitive firm produces where price = MC, on the rising part of its MC curve. So a higher price gives a higher profit-maximising output.

• Reason 2: new firms enter.

• A higher price attracts new firms into the market, so total market supply rises.

• Class 12 link: under perfect competition, a firm's supply curve is the rising part of its MC curve above minimum AVC (average variable cost, the running cost per unit). Below minimum AVC the firm shuts down and supplies zero. That price is called the shut-down price.

"Other things staying the same": movement vs shift

• The law holds only when the non-price factors stay fixed. These are the number of sellers, technology, production costs, sellers' price expectations and prices of related products [2].
• Movement along the curve: only the good's own price changes. This is a change in quantity supplied.

• Example: mango price rises from ₹50 to ₹100 → the market moves up the same curve.

• Shift of the curve: a non-price factor changes. This is a change in supply [2].

• Example: more mango sellers arrive → the whole curve shifts right.

• Seasonal mango story (Class 9):

• Start of season: few mangoes arrive → supply is low → mangoes are costly.
• Mid-season: many mangoes arrive → supply shifts right → prices fall.
• This is a shift, not the law of supply failing.

From individual supply to market supply

• Individual supply: the quantity one seller offers at each price.
• Market supply: the total quantity from all sellers at each price. It is found by horizontal summation: fix a price, then add the quantities. You add along the X-axis, never the prices.
• Class 9, Table 9.2:

Price (₹/kg)	Seller A	Seller B	Seller C	Market supply (kg)
50	1	3	2	6
100	2	4	6	12
150	3	7	8	18

• Identical firms: market supply = n × one firm's supply. NCERT Ex. 24 has 3 identical firms, so market supply is 0, 0, 6, 12, 18, 24, 30, 36, 42.
• Kinked market supply (Class 12):
• Firm 1: S₁ = p − 10 for p ≥ 10 (0 below). Firm 2: S₂ = p − 15 for p ≥ 15 (0 below).
• Market: 0 for p < 10, p − 10 for 10 ≤ p < 15, and 2p − 25 for p ≥ 15.
• Worked check at p = 20: S₁ = 10, S₂ = 5, so market supply = 15, and 2(20) − 25 = 15.
• Why the kink: the firms have different shut-down prices (10 and 15). Above 15 the second firm joins, the slope jumps from 1 to 2, and the curve becomes flatter.

Limits and exceptions

• Fixed-supply goods (land, antiques, stadium seats): quantity cannot rise with price, so the supply curve is vertical (perfectly inelastic).
• Perishables (tomatoes, fish): they cannot be stored, so by evening sellers dump stock at almost any price.
• Speculative holding back: if sellers expect prices to rise further, they may supply less now even as the current price rises. In the model this is a leftward shift caused by price expectations [2].
• Backward-bending labour supply: above a high wage, workers may choose more leisure and work fewer hours, so the labour supply curve bends back.

In India

• Perishables and post-harvest losses:
• The Ministry of Food Processing Industries (MoFPI) had NABCONS (NABARD Consultancy Services) study post-harvest losses in 2022. The study covered 54 commodities, 15 agro-climatic zones and 292 districts [8].

• Losses in fruits and vegetables came mainly from inefficient harvesting, handling, storage and transportation [8]. So farmers cannot spread supply across the year, even when prices are high.

• Price Stabilisation Fund (PSF), a buffer to shift supply right:

• The PSF keeps a strategic buffer of onion, potato and pulses. Stock is released in a calibrated way to moderate price volatility and discourage hoarding and speculation [7].
• Onion buffer: 2.08 lakh tonnes (LMT) of Rabi-2021 onion was procured in 2021-22, and the 2022-23 target was 2.50 LMT [5]. In 2025, 3 lakh tonnes was procured, with calibrated releases expected from September 2025 [6].
• Releases go to cities or states where prices are rising, and to source mandis (wholesale markets) [5].

• In supply-curve terms: government stock enters the market → the market supply curve shifts right → lean-season prices ease.

• Operation Greens, to smooth perishable supply:

• It was announced in Union Budget 2018-19 with ₹500 crore, and MoFPI launched it in November 2018 for Tomato, Onion, Potato (TOP) [3][4].
• Short-term tools: a 50% subsidy on transport and storage. Long-term tools: grants of 35–70% of project cost, up to ₹50 crore per project, for value-addition projects in production clusters [3][4].
• The MIEWS portal (Market Intelligence and Early Warning System) tracks TOP prices and arrivals and sends alerts when intervention is needed [9].
• Its scope was widened to 22 perishable crops (including mango, banana and apple) for 2021-26 [4].

Don't confuse with

• Law of demand: price and quantity demanded move in opposite directions, so the demand curve slopes downward. The law of supply is a direct relationship, and the supply curve slopes upward.
• Change in supply: this is a shift of the whole curve, caused by a non-price factor such as technology, input cost, number of sellers or expectations. The law of supply only describes a movement along the curve when the good's own price changes.
• Elasticity of supply: this measures how much quantity supplied responds to price. The law of supply tells only the direction of the response. A vertical curve (land, antiques) has zero elasticity.
• Market supply vs individual supply: individual supply is one seller's schedule. Market supply is the horizontal sum of all sellers' quantities at each price, never the sum of prices.

Prelims Hooks

• Law of supply: other things being equal, price and quantity supplied are directly related, so the supply curve slopes upward. In a supply graph, price is on the Y-axis and quantity is on the X-axis [2].
• Movement vs shift: a change in the good's own price causes a movement along the curve. A change in technology, input cost, number of sellers, expectations or related-goods prices causes a shift [2].
• Market supply = horizontal summation. With n identical firms, market supply = n × firm supply. For S₁ = p − 10 (p ≥ 10) and S₂ = p − 15 (p ≥ 15), market supply for p ≥ 15 = 2p − 25. The kink comes from different shut-down prices.
• Trap: the mid-season fall in mango prices is a rightward shift in supply. It is not an exception to the law of supply.
• Vertical supply curve (perfectly inelastic): land, antiques, stadium seats.
• Operation Greens: MoFPI, Budget 2018-19, ₹500 crore, TOP crops, widened to 22 perishables for 2021-26 [3][4]. MIEWS is its price-monitoring portal [9]. The PSF keeps buffers of onion, potato and pulses [7].

Mains Points

• Supply shocks in perishables and food inflation (GS-III):
• TOP crops have gluts in some months and shortages in others, so prices swing sharply.
• Weak cold storage and high post-harvest losses (NABCONS 2022 study) mean supply cannot be spread across the year [8].

• Operation Greens (transport and storage subsidy, value-addition grants) tries to move supply from glut months to lean months [3][4].

• Buffer stocks as a supply tool, and their trade-offs:

• PSF releases (2.08 LMT onion in 2021-22; 3 lakh tonnes in 2025) shift supply right when prices spike [5][6].

• Holding a buffer costs money, and the stock can rot. Badly timed releases can also push down the prices farmers get.

• Expectations, hoarding and entry of firms:

• When sellers expect higher prices, they hold back stock, which breaks the simple law of supply. This is why stock limits under the Essential Commodities Act and calibrated buffer releases are used against speculation [7].
• More sellers shift market supply right. So reforms that make entry easier, such as ease of doing business or a single national farm market like e-NAM, can raise supply and keep prices in check.

Related concepts

• Individual supply
• Market supply curve

Read more

• Law of supply, individual supply and market supply