Fertilizer Seasonality: Why Fall Is the Only Time to Buy

A 38-year Purdue University study found that fall fertilizer prices were cheaper than spring prices in 36 out of 38 years — a 95% win rate that makes the fall buying window one of the most reliable seasonal patterns in any commodity market. The study covered five nitrogen products, DAP, and potash across Indiana retail markets from 1967 to 2009. The conclusion was unambiguous: farmers and dealers who pre-buy fertilizer in September and October consistently pay less than those who buy in March and April.

This article explains why the pattern exists, how it applies to all four fertilizer commodities tracked on the Commodity Seasonal Calendar — Nitrogen (Urea / NH₃), Phosphate (DAP / MAP), Sulfur, and Potash (MOP) — and what the optimal buying windows look like for each.

The Planting Cycle: Why Prices Peak in Spring

The fundamental driver of fertilizer seasonality is the Northern Hemisphere planting cycle. Corn, wheat, soybeans, and most row crops require nitrogen, phosphate, and potassium at planting. In the United States, Canada, and Europe, this means the primary application window runs from March through May. The demand surge is predictable, concentrated, and enormous in scale.

Fertilizer dealers begin restocking in January and February, ahead of the spring application season. Manufacturers and distributors respond by raising prices as demand builds. By March and April — peak planting season — retail fertilizer prices are typically at their annual high. The farmdoc Illinois Production Cost Report, which tracks weekly retail prices for anhydrous ammonia and urea across Illinois, consistently shows February through May as the most expensive period of the year.

The Purdue 38-year study quantified this premium precisely. Across all five nitrogen products studied, the average April price was 10 to 20 percent above the average October price. For anhydrous ammonia — the most widely used nitrogen fertilizer in the U.S. Corn Belt — the average fall discount was 12.35 percent. Potash showed a consistent fall discount of approximately 8 to 12 percent. DAP and MAP phosphate followed a similar pattern, though with somewhat less volatility given that phosphate production is not directly tied to natural gas feedstock costs the way nitrogen is.

Nitrogen (Urea / Anhydrous Ammonia)

Nitrogen is the most volatile of the four fertilizer commodities because its production cost is directly tied to natural gas. Anhydrous ammonia is synthesized from natural gas via the Haber-Bosch process, which means nitrogen prices carry both a seasonal demand premium (spring planting) and a natural gas price premium. When natural gas prices spike — as they did in 2021 and 2022 — nitrogen prices can move dramatically outside their normal seasonal range.

Despite this volatility, the seasonal pattern is remarkably consistent. The Purdue study found that anhydrous ammonia fall prices were cheaper than spring prices in 35 out of 38 years — a 92% win rate. The farmdoc Illinois data shows September prices averaging approximately 95% of the yearly average, making August and September the optimal entry window before fall application demand begins to build.

Month	Signal	Driver
Jan–May	SELL	Spring planting demand peak; prices 10–20% above fall average
June	NEUTRAL	Post-planting lull; prices declining
Jul–Aug	ACCUM / BUY	Summer low; minimal demand between seasons
Sep–Nov	BUY	Fall pre-buying window; 10–20% discount vs. spring
December	ACCUM	Prices firming; best of fall discount fading

Phosphate (DAP / MAP)

Diammonium phosphate (DAP) and monoammonium phosphate (MAP) follow the same planting-season demand cycle as nitrogen but with lower volatility. Phosphate production is not directly tied to natural gas, so the commodity does not carry the same energy cost premium. The primary price driver is the same agricultural demand cycle: spring planting creates a demand surge, fall is the pre-buying window.

The fall discount for phosphate is somewhat smaller than for nitrogen — typically 6 to 10 percent below the spring peak — but it is equally consistent. DTN weekly retail fertilizer price data confirms that September through November is the optimal buying window for DAP and MAP. One important distinction: phosphate has a meaningful export market, particularly to Brazil (which has a second major planting season in October–November) and India. When Brazilian or Indian demand is strong, it can partially offset the Northern Hemisphere post-planting price decline.

Sulfur (Elemental / Sulfuric Acid)

Sulfur is the most unusual of the four fertilizer commodities because it is primarily a by-product of oil refining and natural gas processing rather than a manufactured product. Approximately 90 percent of global sulfur supply comes from hydrodesulfurization — the process of removing sulfur from petroleum and natural gas to meet environmental standards. This means sulfur supply is largely inelastic in the short term: it is produced whether or not demand is strong.

The demand side, however, follows the same agricultural cycle as the other fertilizers. Sulfur's primary agricultural use is as a feedstock for sulfuric acid, which is used to produce phosphate fertilizers (DAP and MAP). When phosphate fertilizer production ramps up for the spring planting season, sulfuric acid demand rises, pulling elemental sulfur prices higher. 888Chem and the IMARC Group both identify spring (March–May) and fall (September–October) as the two peak demand periods for agricultural sulfur. The optimal buying window is the summer lull — June through August — when agricultural demand drops sharply between the spring and fall application seasons.

Potash (MOP / KCl)

Muriate of Potash (MOP, or potassium chloride) is the most geopolitically concentrated of the four fertilizer commodities. Global production is dominated by a small number of producers — Nutrien (Canada), Mosaic (Canada/U.S.), and Belarusian Potash Company — which gives the market an oligopolistic structure that can amplify or dampen seasonal price moves depending on producer pricing strategy.

Despite this concentration, the seasonal pattern is consistent. The Purdue 38-year study used October as the benchmark fall price for potash and found it consistently below the April spring price. DTN data confirms March through May as the most expensive months for potash. The fall pre-buying window — September through November — is the optimal entry point, with August as an early accumulation opportunity as prices approach the seasonal low.

The 12-Month Fertilizer Seasonal Calendar

The following table summarizes the seasonal signals for all four fertilizer commodities by month:

Month	Nitrogen	Phosphate	Sulfur	Potash
January	SELL	SELL	SELL	SELL
February	SELL	SELL	SELL	SELL
March	SELL	SELL	SELL	SELL
April	SELL	SELL	SELL	SELL
May	SELL	SELL	SELL	SELL
June	NEUTRAL	NEUTRAL	ACCUM	SELL
July	ACCUM	ACCUM	ACCUM	NEUTRAL
August	BUY	ACCUM	BUY	ACCUM
September	BUY	BUY	BUY	BUY
October	BUY	BUY	SELL	BUY
November	BUY	BUY	NEUTRAL	BUY
December	ACCUM	ACCUM	ACCUM	ACCUM

Note that sulfur diverges from the other three in October — this is because sulfur demand peaks in fall as phosphate fertilizer production ramps up, making October a sell window for sulfur even as it is a buy window for nitrogen, phosphate, and potash.

Override Factors: When the Pattern Breaks

The fertilizer seasonal pattern is among the most reliable in commodity markets, but several factors can disrupt it. Natural gas price shocks are the primary override for nitrogen: when natural gas prices spike sharply — as in the European energy crisis of 2021–2022 — nitrogen production costs rise so dramatically that the seasonal pattern is overwhelmed. Geopolitical supply disruptions can affect all four fertilizers; Russia and Belarus are major producers of potash and nitrogen, and sanctions or export restrictions can create supply shocks that override the seasonal calendar. Chinese export policy is a secondary risk for phosphate and nitrogen, as China periodically restricts exports to prioritize domestic supply.

Viewing Fertilizer Seasonality on the Commodity Seasonal Calendar

The Commodity Seasonal Calendar at comcalend.com tracks all four fertilizer commodities — Nitrogen (Urea / NH₃), Phosphate (DAP / MAP), Sulfur, and Potash (MOP) — under the Fertilizers category filter. Each monthly cell shows the seasonal signal (BUY, SELL, ACCUM, or NEUTRAL), the historical rationale, and the data sources behind the signal. All four fertilizer rows are available free, with no registration required.

For traders and investors building a seasonal accumulation strategy across the fertilizer complex, the September–November window offers a structurally sound entry point backed by nearly four decades of statistical evidence and a clear agricultural demand mechanism. The planting cycle does not change. The spring demand surge does not change. And the fall discount — documented in 36 out of 38 years — is as close to a free lunch as commodity markets offer.

Supported by Walmer Portal.