Fall Fertilizer Considerations

Once the combine, digger or picker clears the field, it’s tempting to call the season done. But every bushel, ton or bin that leaves the field takes nutrients with it.

Post-harvest fertility is an opportunity to replace what was removed, rebuild soil reserves where needed and prepare for the next crop.

Fall can offer favorable field conditions and a less hectic application window, but not every nutrient belongs in a fall program. Soil conditions, crop needs, and fertilizer source all influence what should be applied now and what may be better saved for spring.

Growers typically use the fall window to:

• Replace nutrients based on crop removal and soil-test results
• Rebuild P and K where needed
• Support nutrient reserves in perennial crops
• Reduce pressure during spring field operations
• Prepare soil fertility for the next crop

While phosphorus (P) and potassium (K) are often the foundation of fall fertility, sulfur (S), calcium (Ca) and magnesium (Mg) also deserve consideration as part of a balanced nutrient management program.

Fall Fertility at a Glance

Spring is often ideal for most nutrients, including phosphorus and potassium, but logistically and soil moisture wise, that is not always an option. Fall provides a window when schedules are less hectic and temperatures drop offering favorable conditions for key macros, like P and K to safely bind with soil.

Which Nutrients Should Be Applied in Fall?

• Phosphorus and potassium are often good candidates for fall application because they are relatively immobile in many soils and can be removed in significant quantities at harvest.
• Sulfur, calcium, and magnesium may also fit depending on soil tests, crop needs, soil texture, and fertilizer source.
• Fall nitrogen requires more careful management because of greater loss potential. If cover crops are planted after harvest they can capture fall applied nutrition, then release nutrients as they decompose for the next crop.

Fertility Tip: Nitrogen-free, multi-nutrient fertilizers can offer flexibility in fall fertility programs. Polysulphate® supplies low-salt, low-chloride K, S, Ca and Mg in one granule, while ICL PKpluS® co-granulated phosphorus and Polysulphate offers 5-in-1 nutrition.

Key Macronutrients for Fall Fertility

Which Macronutrients Are Most Important for Fall Application?

Potassium (K): Replacing Harvest Removal

Potassium is often the highest-priority fall nutrient because many crops remove significant amounts at harvest. It supports water regulation, carbohydrate transport, stress tolerance, stalk strength, and crop quality.

Because K is relatively immobile in many medium- and fine-textured soils, fall can be an effective time to replace removal and rebuild soil-test levels. A few factors can influence that decision:

• Soil texture and CEC: Sandy, low-CEC soils have less capacity to retain K and may warrant different rates or timing.
• Crop sensitivity: Chloride-sensitive crops may not be ideal candidates for high rates of MOP (0-0-60).
• Fertilizer source: Consider chloride, salt index, and any additional nutrients supplied.
• Considerations: Crop demand and nutrients removed at harvest are important considerations. In almonds, for example, roughly 80 lb. of K₂O can be removed for every 1,000 lb. of harvested kernels, making post-harvest K replenishment important where soil and tissue tests indicate a need.

ICL Solution: Polysulphate® supplies K along with S, Ca, and Mg in a low-chloride mineral fertilizer. ICL PKpluS® adds phosphorus to that mix, providing an N-free option for balanced fall fertility. In orchards, before leaf drop, a foliar like Nova FINISH® can be a strong choice, or for irrigated systems Nova PeKacid® is also a fit for fall fertigation.

Phosphorus (P): Building Soil Reserves for Spring

Phosphorus supports energy transfer, root development, and early plant growth. Because P is relatively immobile in soil, fall can be an effective application window when rates are guided by soil tests, crop removal, and soil conditions.

• Soil Chemistry: Soil chemistry can also affect P availability, making source and placement important considerations. It can bind tightly to soil particles (specifically Ca, or Fe and Al). Ideally this helps it stay put over the winter, slowly breaking down into plant-available forms by spring. But in high fixation soils or when crops face cold spring soils, fall-applied phosphorus may not meet early season demand, meaning starter nutrition still plays an important role.
• Fertilizer source: While MAP and DAP are common fall P sources, they also supply nitrogen, which may not always be desirable in a fall program. Looking at lower salt or nitrogen-free sources like PKpluS can help build a more flexible program
• Considerations: In perennial crops, post-harvest P may good fit where soil or tissue testing indicates a need. In fertigated systems, acidifying P sources can provide additional benefits for nutrient availability and irrigation-water management. For broadcast programs, nitrogen-free multi-nutrients can offer greater flexibility.

ICL Solution: ICL PKpluS® is a dry co-granulated phosphorus that supplies nitrogen-free P plus K, S, Ca, and Mg in a single low salt granule. For fertigation, Nova PeKacid® and Agrolution pHLow® supply P and K through fertigation while providing acidifying benefits.

Should Nitrogen Be Applied in Fall?

Nitrogen requires more careful fall management because once converted to nitrate, it becomes vulnerable to leaching and denitrification.

• Where fall N is locally recommended, university guidance in many northern production regions advise waiting until soil temperatures are near 50°F and continuing to decline.
• N source, soil conditions, drainage, crop rotation, and inhibitor use can all affect loss risk, so follow local Extension recommendations.

Many row-crop growers instead split N between pre-plant and in-season applications to better match crop uptake. Perennial crops are different: trees may continue taking up and storing N after harvest while leaves and roots remain active, so crop nutrient status and tissue analysis should guide post-harvest applications.

Why Consider Sulfur, Calcium and Magnesium in Fall Fertilizer Programs?

P and K get most of the attention, but S, Ca, and Mg are also important components of balanced crop nutrition. Not every field or orchard needs a separate fall application of these nutrients, but multi-nutrient products can supply several needed nutrients in a single application without adding unnecessary complexity.

Soil tests, tissue tests, crop demand, soil characteristics, and fertilizer source should drive the decision.

The same applies for certain micronutrients, zinc, and boron, for example. Micronutrient foliars can be absorbed through remaining leaves and stored for spring and also promote uniform leaf drop.

• Sulfur (S) supports protein synthesis, enzyme activity, and nitrogen-use efficiency. A crop can take up N but cannot efficiently convert it into proteins when S is limiting. With cleaner air regulations reducing atmospheric sulfur deposition, sulfur deficiencies have become increasingly common across many production systems, making sulfur management an important part of modern fertility programs.
• Calcium (Ca) is important for cell-wall structure, membrane function, root growth, and crop quality. In the soil, adequate Ca can also support aggregation, soil structure and water infiltration, particularly where soil chemistry limits physical soil function.
• Magnesium (Mg) is the central component of chlorophyll and is essential for photosynthesis. High K levels can interfere with Mg uptake, making soil testing and source important when managing these nutrients together.

ICL Solution: Sulfate-S is readily available but can leach in coarse-textured soils, while elemental S must oxidize before plants can use it. Polyhalite-based fertilizers such as Polysulphate® and ICL PKpluS® supply sulfate-based S, Ca and Mg in addition to key macros with a naturally gradual release that helps extend nutrient availability.

Real-World Results

• Corn | Waseca, Minnesota University of Minnesota research found fall-applied Polysulphate improved sulfur uptake, increasing corn grain yield by +5.4 bu/A.
• Almonds | Parlier, California Fall-applied Polysulphate, used post-harvest alongside MAP and SOP, improved nutrient use efficiency and increased kernel yield by +4.8%.

Agronomist Takeaway

Post-harvest fertility is an opportunity to rebuild soil reserves and set the stage for next season’s yield.

While P and K often form the foundation because of their relatively low mobility and significant removal in high-yield systems, secondary macronutrients such as S, Ca and Mg can round out the program to protect future yields.

Multi-nutrient, gradual-release sources like Polysulphate® and ICL PKpluS® can help growers replenish several nutrients in a single, flexible pass, whether the best application window is immediately after harvest, later in fall or early spring.

Ready to build your post-harvest fertility program? Connect with an ICL agronomist to see how our fall-applied crop nutrition solutions fit your operation.

Source: This article draws ICL agronomy expertise, webinars and ICL field trial data in addition to information from university extensions:

https://crops.extension.iastate.edu/cropnews/2009/09/getting-ready-fall-fertilization

https://www.canr.msu.edu/news/fertilizer_practices_in_the_fall