Determinants of supply: technology, input prices, unit taxes and shifts

Detail

1. Core idea: why "anything that moves MC shifts supply"

• Supply is the quantity of a good that a firm is willing and able to sell at each price.
• Determinants of supply are all the factors, other than the good's own price, that change how much firms supply.
• Marginal cost (MC) is the extra cost of producing one more unit. Formula: MC = ΔTC / ΔQ (change in total cost ÷ change in output).
• The firm's supply curve is part of its MC curve.
• Short run: the supply curve is the rising part of the SMC curve, at prices at or above minimum AVC (average variable cost).
• Long run: the supply curve is the rising part of the LRMC curve, at prices at or above minimum LRAC (long-run average cost).

• A perfectly competitive firm produces where P = MC. So at any price, the MC curve tells you the output.

• The key rule: if something changes MC at every output, it moves the MC curve. Moving the MC curve moves the supply curve.

• Market supply is the horizontal sum of all firms' supply curves. So it also depends on the number of firms.

2. Technological progress

• Technological progress is an innovation that lets the same capital and labour produce more output, or lets a given output be produced with fewer inputs. NCERT Class 12 calls this an organisational innovation.
• The chain:
• Fewer inputs are needed per unit, so MC falls at every output.
• The MC curve shifts down (right).

• The supply curve shifts right. The firm supplies more at every price.

• Worked example:

• Before the new technology, MC at 100 units = ₹20. At P = ₹20 the firm supplies 100 units.
• After the new technology, MC at 100 units = ₹16, and MC reaches ₹20 only at 130 units.

• So at the same P = ₹20, the firm now supplies 130 units. The supply curve has shifted right.

• Class 9 examples:

• Drip irrigation (water is delivered drop by drop to the plant roots) and weather sensors raise crop output from the same land.

• Cold storage stops mangoes from spoiling, so they can reach distant markets. This raises market supply in those markets.

• Class 9 exercise: drip irrigation cuts water use by 40% and raises yield by 30%.

• The farmer spends less on water and gets more output, so cost per unit falls.
• The farmer is willing to supply more at each price.

• If many farmers adopt it, market supply shifts right.

• Official data on drip irrigation:

• Per Drop More Crop (PDMC) is a part of the Pradhan Mantri Krishi Sinchayee Yojana (PMKSY). It promotes micro irrigation (drip and sprinkler systems) to raise water-use efficiency on farms [5][6].
• Economic Survey figures quoted by PIB: water savings of 20–48%, energy savings of 10–17%, labour cost savings of 30–40%, fertiliser savings of 11–19%, and yield increases of 20–38% [5]. The NCERT exercise figures (40% water, 30% yield) fall inside these ranges.
• The government pays 55% of the installation cost for small and marginal farmers and 45% for other farmers [5].
• More than 115 lakh hectares had been brought under micro irrigation by July 2026 [5].
• This makes PDMC a technology shift plus an input subsidy. Both move farm supply right.

3. Input prices

• Input prices are the prices a firm pays for its inputs, e.g. the wage rate (price of labour), rent, and the prices of raw materials, fuel and fertiliser.
• The chain when input prices rise:
• Each unit costs more to make, so AC rises, and MC usually rises, at every output.
• The MC curve shifts up (left).

• The supply curve shifts left. The firm supplies less at every price.

• When input prices fall, the chain runs the other way, and supply shifts right.

• Worked example:
• A tailor needs 2 hours of labour per shirt. The wage rises from ₹100 to ₹150 per hour.
• Labour cost per shirt rises from ₹200 to ₹300. So MC rises by ₹100 at every output.

• At the old price, the tailor now supplies fewer shirts.

• Class 9's list of similar supply shocks:

Shock	Effect on cost	Supply
Discovery of an alternative (cheaper) input	Cost falls	Right
Depletion of a resource (e.g. groundwater, a mine)	Cost rises	Left
Bad weather, natural disasters	Output lost, cost per unit rises	Left

4. Other determinants (Class 9)

• Prices of related goods in production. These are other goods the same farmer or firm could produce with the same land, labour or machines.
• Example (Fig 9.6): chickpea prices are high and wheat prices are low. So the farmer grows chickpeas next season.
• Wheat supply falls (shifts left) even though wheat's own price did not change first.
• Lesson: the supply of a good depends on how profitable the other options are.

• MSP (Minimum Support Price) is the price at which the government promises to buy a crop. An MSP announcement works as a relative-price signal. It pushes farmers towards crops with better assured returns.

• Number of sellers. More sellers shift market supply right. Fewer sellers shift it left.

• Producer expectations. This is what producers think prices will be in the future.
• Potato wholesalers who expect prices to rise in the peak season hold back supply now.
• Current supply shifts left. They sell later at the higher price.
• This links to hoarding, which means storing goods to sell later at a higher price. It can cause artificial shortages.

5. Movement along the curve vs shift of the curve

Change	Cause	Term
Movement along the curve	Change in the good's own price only	Extension (price up) / contraction (price down) of quantity supplied
Shift in the supply curve	Technology, input prices, taxes/subsidies, related goods, number of firms, expectations	Increase (shift right) / decrease (shift left) in supply

• Exam tip: "quantity supplied" goes with price, which means a movement along the curve. "Supply" goes with the other factors, which means the whole curve shifts.

6. An NCERT imprecision to correct

• Class 9 says that when there are more sellers, supply "exceeds demand" and prices fall.
• More precisely:
• More sellers shift the supply curve right.
• The new supply curve meets demand at a lower equilibrium price and a higher quantity.

• Excess supply (quantity supplied > quantity demanded) exists only briefly, at the old price, before the price adjusts.

• In the new equilibrium, supply equals demand again. The market does not stay in excess supply.

7. Unit tax

• A unit tax (also called a specific tax) is a fixed amount of tax charged per unit sold, whatever the price.
• NCERT example: a tax of ₹2 per unit on 10 units sold = ₹20 of tax. Total tax = t × Q.
• Effect on costs:
• The tax is a cost for every unit. So LRAC and LRMC each rise by exactly t at every output.

• LRAC⁰ becomes LRAC¹ = LRAC⁰ + t. LRMC⁰ becomes LRMC¹ = LRMC⁰ + t.

• Effect on supply:

• The supply curve shifts from S⁰ to S¹. It moves left, and up by exactly t, and the shift is parallel.

• The shut-down price (the lowest price at which the firm will still produce, equal to minimum LRAC in the long run) rises by t.

• Worked example:

• Minimum LRAC = ₹50, so the firm's shut-down price is ₹50.
• The government imposes a unit tax of t = ₹5.
• The new minimum LRAC is ₹55. The firm now needs at least ₹55 to stay in the market.

• At any quantity, the firm now asks ₹5 more than before.

• Indian example: central excise on petrol and diesel (a tax in ₹ per litre, not a %):

• Basic excise duty was cut by ₹2 per litre from 4 October 2017 [4].
• It was cut by ₹13 per litre on petrol and ₹16 per litre on diesel in two steps, in November 2021 and May 2022 [3].
• It was raised by ₹2 per litre on both in April 2025. This rise was not passed on to consumers [3].
• It was cut by ₹10 per litre on both on 27 March 2026, to protect consumers and oil marketing companies from a global oil price shock [2].
• In supply terms, each cut shifts the fuel supply curve down by exactly the size of the cut. Each rise shifts it up by that amount.

8. Specific vs ad valorem tax (beyond NCERT)

• An ad valorem tax is a tax charged as a percentage of the price (from Latin, "according to value").

	Specific (unit) tax	Ad valorem tax
Base	₹ per unit	% of price
Indian example	Central excise on petrol and diesel (₹ per litre) [2][4]	GST (%)
Supply shift	Parallel upward shift by t	Pivot: the gap widens at higher prices

• Worked example (ad valorem tax of 18%):
• At a pre-tax price of ₹100, the tax is ₹18.
• At a pre-tax price of ₹200, the tax is ₹36.

• So the gap between the old and new supply curves grows as price rises. The curve pivots; it does not shift in parallel.

• Subsidies work in reverse. A subsidy is a payment from the government that lowers a producer's cost. It acts like a negative tax.

• Fertiliser subsidy:
  • It lowers the farmer's input cost, so farm supply shifts right.
  • Under the Nutrient Based Subsidy (NBS) scheme, the government pays a fixed subsidy based on the nutrient content of phosphatic and potassic (P&K) fertilisers [7].
  • The Cabinet approved NBS rates for Kharif 2026 (1 April to 30 September 2026). The estimated cost is about ₹41,533.81 crore, covering 28 grades of P&K fertilisers, including DAP [8].

• PLI (Production Linked Incentive):

  • It pays firms an incentive of 4–6% on incremental sales (sales above a base year) of goods made in India, for five years [9].
  • This acts like a negative unit tax on extra output, so supply shifts right.
  • Scale: an outlay of ₹1.91 lakh crore across 14 sectors [10].
  • Results by September 2025: incremental production/sales of more than ₹18.7 lakh crore, and more than 12.6 lakh jobs (direct and indirect) [9].
  • Results by 31 December 2025: 836 applications approved, cumulative sales above ₹20.41 lakh crore, and more than 14.39 lakh jobs [10].

• Who finally pays a tax (tax incidence) depends on the elasticities of demand and supply. It is covered in the notes on taxation and on market equilibrium and price controls.

Prelims Hooks

• A firm's supply curve is the rising part of its MC curve above minimum AVC (short run) or above minimum LRAC (long run). So any factor that changes MC shifts supply.
• Technological progress shifts supply right. A rise in input prices (e.g. the wage rate) shifts supply left.
• A change in the good's own price causes a movement along the supply curve (extension or contraction), not a shift. This is a classic "which of the following" trap.
• Unit tax of ₹t: LRAC and LRMC both rise by exactly t. The supply curve shifts up in parallel by t, and the shut-down price rises by t.
• Specific tax = ₹ per unit (e.g. central excise on petrol and diesel), which gives a parallel shift. Ad valorem tax = % of price (e.g. GST), which pivots the curve.
• PLI pays 4–6% on incremental sales over a base year, for five years, across 14 sectors, with an outlay of ₹1.91 lakh crore [9][10]. It works like a negative unit tax.
• Per Drop More Crop is a part of PMKSY. Subsidy is 55% for small and marginal farmers and 45% for others. Reported yield gain is 20–38% and water saving 20–48% [5].
• NBS gives a fixed subsidy per nutrient for P&K fertilisers only. Urea is outside NBS [7][8].
• If producers expect higher prices in future, current supply falls (hoarding). The effect is a shift left, not a movement along the curve.

Mains Points

• Taxing fuel pushes up costs across the economy.
• Excise on petrol and diesel is a specific tax, so every change moves the fuel supply curve by exactly that amount per litre.
• Fuel is an input for transport and farming, so fuel taxes also shift supply curves of other goods left.
• This is why the government uses excise cuts (e.g. ₹10 per litre, March 2026) as a tool against inflation [2].

• The trade-off is between revenue and cost-push inflation (inflation caused by rising costs of production).

• Supply-side policy vs demand stimulus.

• PLI, micro-irrigation subsidies and fertiliser subsidies all lower MC, so they shift supply right. This can raise output and lower prices together.
• A demand stimulus that raises output also tends to raise prices.

• The costs to watch are the fiscal burden, subsidies captured by large firms, and distorted input use (e.g. unbalanced NPK fertiliser use) [5][9].

• Relative-price signals in farming.

• MSP for a few crops (mainly wheat and rice) makes growing those crops more profitable than growing alternatives.
• This shifts supply towards water-hungry crops and away from pulses and oilseeds.

• A GS-III answer can argue for crop-neutral price signals combined with technology shifts such as drip irrigation, which raise supply without draining groundwater.

• Expectations and hoarding.

• Supply of perishables (potato, onion) shifts left when traders expect prices to rise.
• Cold storage (technology) and buffer stocks make supply steadier.
• Stock limits tackle the symptom, but they can discourage investment in storage.