Understanding grain nutrient removal for 4R management of phosphorus and potassium

The maintenance component of a “build and maintain” strategy also includes allowing crops to use readily available nutrients in the soil when soils test excessively high and draw down soil test values. All components require evaluating crop nutrient removal, which is defined as the amount of nutrients removed from the field in harvested crop biomass and grain. Because these nutrients leave the system and are not returned to the soil, they must be accounted for when developing fertilization programs aimed at sustaining soil test levels over time. When all aboveground biomass is removed, such as forages, corn silage, or crop residue, total nutrient removal increases further, reinforcing the need for accurate replacement strategies.

Keeping up with nutrient replacement continues to be an important consideration for U.S. producers. Recent North American soil test summaries show that a growing share of state soil test levels for P and K have slipped to or below critical over the past two decades (TFI, 2021). In other words, more fields are drifting toward the range where low soil test levels can begin to limit yield, which underscores the value of matching fertilizer applications to crop removal. This article walks through what crop nutrient removal really means and how you can use it to fine-tune your P and K fertility program. It also looks at the main factors that determine how much P and K leave the field at harvest.

Crop nutrient removal: What it is and how it is estimated?

The importance of accounting for crop nutrient removal has grown in recent years, largely due to increasing crop yields. While nutrient removal per unit of yield (for example, per bushel) has remained relatively stable or even decreased in some cases due to nutrient dilution, total nutrient export per acre has increased with increasing crop yields (Figure 1). This relationship is well established in the literature, as increases in yield are generally associated with greater total nutrient removal (Jones et al., 2021; Preza Fontes et al., 2025).

Although increases in nutrient removal can translate into greater amounts of P and K that must be replaced, fertilizer decisions should be based on a clear understanding of how crop nutrient removal is estimated. Nutrient removal is typically calculated using coefficients derived from grain nutrient concentration and yield. In the Midwest, these values are commonly provided in state fertilizer guidelines for each crop (Villamil et al., 2019), which are developed from large datasets of grain nutrient concentrations collected across a wide range of fields and growing conditions. Values are state-dependent and commonly represent the median or 75th percentile of the supporting database. For example, the median P and K removal values for corn grown in Minnesota are 0.28 lb P₂O₅ and 0.19 lb K₂O per bushel (Kaiser, 2024). So, for 250 bu/ac corn, the expected P removal is roughly 70 lb P₂O₅/ac and 48 lb K₂O/ac. In Illinois, the P and K removal values for corn are 0.37 lb P₂O₅ and 0.23 lb K₂O per bushel (Kaiser, 2024). So, for 250 bu/ac corn, the expected P removal is roughly 93 lb P₂O₅/ac and 58 lb K₂O/ac. These differences show regional patterns in removal coefficients and the effect of using median vs. 75th percentile values.

Rather than using the average value, the median value or the 75th percentile is typically selected as the book value to better account for the variability that exists among crops, environments, and management practices. However, many farmers and agronomists are increasingly interested in developing field-specific values based on their own data. While this is a valid and precise agronomic approach, it can also lead to errors if not done carefully (Figure 2).

Grain nutrient removal is calculated using four main components: harvested area, weight of moist grain harvested, grain moisture content, and grain nutrient concentration (Murrell, 2008). Using these variables, nutrient removal can be estimated as:

While this equation captures all variables involved, most are indirectly accounted for when yield and grain nutrient concentration are known. As a result, it is not practical for routine use. In most production systems, wet grain weight, moisture, and nutrient concentration are not measured for every field. Instead, advances in yield-monitoring technology and laboratory analysis have simplified this process. Today, nutrient removal is commonly estimated using yield data from yield maps together with measured grain nutrient concentrations, resulting in a more practical and streamlined approach.

In some cases, laboratories may provide nutrient removal coefficients per bushel, which can be directly applied to yield maps, further simplifying the process for farmers. It is important to consider how grain nutrient concentration is reported. Some laboratories express P and K concentrations on a near 0% moisture basis. In those cases, values should be adjusted to standard grain moisture (e.g., 15% for corn and 13% for soybean) to avoid overestimating nutrient removal.

Finally, nutrient removal values should be expressed as P₂O₅ and K₂O when making fertilizer recommendations. If grain nutrient concentration is reported as elemental P and K, convert them to P₂O₅ and K₂O by multiplying by 2.29 and 1.20, respectively (to go the other way, divide P₂O₅ by 2.29 to calculate elemental P, and K₂O by 1.20 to calculate elemental K). Failing to apply these conversions can result in incorrect fertilizer rates. It is important to note that oxide forms of P as P₂O₅ and K as K₂O do not exist chemically in soils or plants but have nonetheless been commonplace in fertilizer terminology since the 19th century. 

Factors affecting crop nutrient removal

Questions are often raised about whether factors such as soil test levels or fertilizer rates influence grain P and K removal. In general, any factor that affects yield will influence the total amount of P and K removed per acre. However, these factors typically marginally affect nutrient removal per bushel.

To illustrate this concept, a case study was analyzed using data from long-term P and K trials. These trials allowed for evaluation of nutrient removal across a range of soil test values and fertilizer rates. As shown in Figures 3 and 4, total P₂O₅ and K₂O removal significantly increases with higher soil test levels and fertilizer rates, primarily as a result of increased yields under those conditions. While soil test levels and fertilizer rates can influence nutrient removal per acre through their effects on yield, they influenced small deviation on nutrient removal per bushel. This relative stability is one reason why statewide and regional removal values can be applied across a broad range of conditions.

In some cases, higher soil test levels do not result in additional yield response. Once soil test values reach the optimum range (e.g., In Illinois, 17 to 23 ppm P and 120 to 150 ppm K for soil in Figures 1, 3, and 4), the probability of response to additional fertilizer becomes low, and management should shift from a build-up approach to a maintenance approach (Mallarino et al., 2023; Kaiser et al., 2025). Under these conditions, fertilizer applications based on crop nutrient removal can provide a practical estimate of maintenance requirements and help sustain soil fertility over time.

In contrast, when soil test values are below the critical concentration, fertilizer needs often exceed crop removal because nutrients are required not only to replace those removed by the harvested grain, but also to gradually rebuild soil fertility. Applying nutrients beyond the optimum range can increase costs without improving yield and may increase the risk of nutrient losses from the system.

For these reasons, fertilizer guidelines that integrate soil test values with crop nutrient removal are essential. Crop removal values provide a useful benchmark for estimating nutrient replacement, but soil test information remains necessary to determine whether fertilizer applications should focus on building, maintaining, or drawing down soil nutrient levels (Figure 5). 

As a general guide for rebuilding, data from the University of Minnesota indicate that roughly 16 to 18 lb P₂O₅ per acre is needed to raise soil test P by 1 ppm, and roughly 7–10 lb K₂O are required to increase ammonium acetate K tests by 1 ppm (Kaiser, 2024). 

Optimizing P and K management in a challenging market 

When margins tighten, P and K are often among the first inputs growers consider cutting. Unlike nitrogen, P and K that are not removed largely stay in the soil, so soil test levels act as a nutrient bank that can be managed across years rather than fully replenished every season. The goal in a high-cost year is to use soil test values and crop removal together to decide where each fertilizer dollar is most likely to pay. In most cases, a tight year is not the time to build soil test levels above the optimum range; the better mindset is maintenance, letting crop removal, rather than a goal of raising soil test levels, guide how much P and K to apply. In the language of the 4Rs, this sharpens the right rate and right time, directing P and K to the fields and zones where soil tests and removal show they will pay and easing off where they will not. 

On fields or zones already testing above the optimum range, the probability of a yield response to additional P or K is low (Mallarino et al., 2023; Kaiser et al., 2025). These acres are good candidates for reducing P and K and letting crop removal draw soil test levels down toward the optimum range (Figure 5). This is a recommended 4R practice to reduce overapplication and loss of nutrient to environment; it captures the value of nutrients already in the soil and frees cash without sacrificing yield, as long as levels are not allowed to fall below the critical concentration.

Below-critical fields are the opposite case. There, skipping P and K risks a yield loss that usually costs more than the fertilizer saved, and rebuilding later is expensive. Minnesota guidelines suggest 16 to 18 lb P₂O₅/ac is needed to raise soil test P by 1 ppm and roughly 7 to 10 lb K₂O is needed to raise ammonium acetate soil test K by 1 ppm (Kaiser, 2024), though it is important to remember these values can vary from 10 to 35 lb P₂O₅ per ppm for P and 6 to 20 lb K₂O per ppm for K (Mallarino et al., 2023). In fact, research in Illinois shows how build coefficients are relative to the initial starting soil test values as shown in Figure 6 (Bardeggia &Jones, 2026). When the budget forces cuts, protect the responsive, low-testing acres first and take the reductions on high-testing ground, rather than making a 20% cut across all ground.