Soil-test K Research Articles

Determining Economic Optimum Soil Sampling Density for Potassium Fertilizer Management in Soybean: A Case Study in the U.S. Mid-South

Determining the number of samples to collect in a field to develop soil-test K (STK) maps that are sufficiently accurate for profit-maximizing fertilizer rate prescription maps is complex. The decision also hinges on the application method—variable rate or uniform rate (VRT vs. URT). Using a 400 m2 fishnet grid on a 26.3-ha irrigated soybean field, the authors compared sampling densities ranging from 5 to 60 samples or 5.3 ha/sample to 0.40 ha/sample. Subsequently, the authors simulated yields based on STK maps generated with that range of samples taken to generate i) associated profit-maximizing fertilizer-K rates (K*) that varied by grid with VRT, or ii) a single fertilizer rate based on field-average STK with URT, to compare revenue less fertilizer cost (NR) across VRT, URT, and sampling strategy. With more information, NR increased at a diminishing rate as crop needs could be better matched to fertilizer needs with greater detail in STK maps with VRT. Also, fertilizer use with URT was higher than VRT given the field-specific distribution of STK. Regardless of the sampling strategy, NR was higher for VRT than URT, however, that benefit was smaller than the upcharges for VRT equipment. Marginal benefits from added soil sampling were smaller than their marginal cost leading to an optimal least-cost, 5-sample strategy and URT. Changing one of the 5 sampling locations, however, revealed unreliable field average STK estimates. Since soil samples inform about several macronutrients, splitting soil sampling charges across K and P profitably justified sampling near every 1.5 ha with URT.

Comparison of Bray‐1 and Mehlich‐3 extraction of P and K in Wisconsin silt loam soils

AbstractPhosphorous (P) and potassium (K) fertilizer rate recommendations are based on soil nutrient concentrations, which can be measured using a variety of lab methods. In Wisconsin, Bray‐1 extractant is used to measure soil test P (STP) and soil test K (STK), while most neighboring states measure STP and STK using Mehlich‐3 soil test extractant. This research aims to quantify the relationship between Bray‐1 and Mehlich‐3 extraction methods for both STP and STK for silt‐loam soils in southern Wisconsin. Soil samples were collected from three fields in November 2021. STK and soil test phosphorus STP were measured using both Bray‐1 and Mehlich‐3 extraction methods. The relationship between Bray‐1 and Mehlich‐3 extractants was quantified using linear regression. Soil test phosphorus as measured using Bray‐1 extractant was linearly related to STP as measured using Mehlich‐3 extractant (p < 0.001; Adj. R2 = 0.97), and the relationship between STK results from Bray‐1 extraction and STK results from Mehlich‐3 extraction was also linear (p < 0.001; Adj. R2 = 0.91). For the same soil samples, Mehlich‐3 extractant removed slightly more P and K as compared to Bray‐1 extractant. Due to the strong linear relationship between Mehlich‐3 and Bray‐1 extractants, simple regressions can be used to estimate Bray‐1 STP or STK from a soil test report that was generated using Mehlich‐3 lab methods, and vice versa, on WI silt‐loam soils.

Potassium Fertilizer Rate Recommendations: Does Accounting for Soil Stock of Potassium Matter?

AbstractProfitability, yield, and fertilizer use are compared across three different potassium (K) fertilizer rate recommendation ideologies. Existing agronomic, “build and maintain” rate recommendations (KE) are compared to profit-maximizing rates with and without taking long-run soil-test K (STK) implications into account. Regardless of starting STK, K use equilibrated over the course of 3 years irrespective of ideology. Since taking long-run STK into account did not alter ending STK and only led to a miniscule yield effect, we encourage producers to use annual profit-maximizing K rates that were 3–11% lower than KE rates and generated more profit with minimal yield loss.

Soybean is relatively nonresponsive to K fertilizer rate or placement in Manitoba soils

There has been little comprehensive potassium (K) fertility research for soybeans in Manitoba despite recent, rapid, expansion of soybean production in the province. Our main objective was to assess the efficacy of K fertilizer rate and placement combinations to increase K uptake and seed yield of soybeans grown on low K soils. Even though the seven sites had low concentrations of ammonium acetate-extractable soil test K (STK), midseason tissue K concentration increases with K fertilization, and, at several sites, visual deficiency symptoms in or near control plots, soybean seed yield did not respond to K fertilization, regardless of K fertilizer placement and rate. In a complementary field trial, barley, a crop known historically to respond well to K fertilization in Manitoba, had substantial (>20%) increases in yield with K fertilization where soybean did not respond. Ammonium acetate STK and the current 100 mg kg−1 threshold for recommending K fertilization for soybean and barley predicted barley yield response to K fertilization in our study, but did not predict soybean yield response.

Profit‐maximizing potassium fertilizer recommendations for soybean

AbstractPotassium (K) fertilizer has important yield and cost ramifications in soybean [Glycine max (L.) Merr.] production. Rate recommendations are often based on expected yield response as predicted by a soil test. To that end, soybean response to K application rate studies were analyzed using 86 site‐years from 2004 to 2019. We estimated a generic yield response curve across soybean cultivar and soil texture to allow calculation of profit‐maximizing K rates for producers in the mid‐southern United States that also consider crop value and fertilizer cost. Further, we compared profit‐maximizing fertilizer‐K rates with those currently recommended. Using a spreadsheet‐based decision aid, soybean prices and yields, fertilizer‐K cost, and a range of initial soil‐test K (STK) values, as observed over the last 10 yr, we find that current uniform fertilizer‐K rate recommendations were greater than the predicted profit‐maximizing rates. Profit‐maximizing rates added profit ranging from US$2.32 ha−1 at initial Mehlich‐3 K availability values of 110 mg K kg−1 to US$29.35 ha−1 at 60 mg K kg−1 on average. The corresponding fertilizer‐K rate reductions were 11 and 48 kg K ha−1, respectively, resulting in attendant yield penalties of only 28 and 52 kg ha−1. Furthermore, K fertilization was not economically justifiable beyond STK of 128 mg K kg−1 on average. Hence, performing soil tests and calculating profit‐maximizing fertilizer‐K rates showed promising returns to producers at lesser fertilizer‐K use by sacrificing a minimal amount of yield. Also, variable‐rate applied K fertilizer in comparison to uniform rate application was rarely cost effective within the tested assumptions.

Utilizing soil phosphorus and potassium reserves for soybean production on a claypan soil

AbstractUnder temporary economic stress, producers may elect to grow crops without fertilization. However, information is lacking regarding crop and soil response to growing multiple crops without phosphorus (P) and potassium (K) fertilization on claypan soils in the eastern Great Plains. The objective of this study was to determine the effect of utilizing soil‐test P (STP) and soil‐test K (STK) reserves on yield of 5‐yr continuous soybean [Glycine max (L.) Merr.] grown on a claypan soil and on soil‐test values. In the first two low‐yielding years, soybean yield and yield components were unaffected by STP and STK concentrations. In the subsequent three average‐yielding years, soybean yields were up to 0.35 Mg ha−1 less when STP was initially 5 mg kg−1 or STK was initially 57 mg kg−1 than at greater STP or STK. Greater STP increased pods plant−1, whereas greater STK increased pods plant−1, seed pod−1, and seed weight. Even though P and K uptake at R2 (full bloom) did not correlate directly with yields, greater P uptake at R2 in average‐yielding years increased pods plant−1 and greater K uptake at R2 increased seed pod−1. Both STP and STK declined at more than 3 mg kg−1 yr−1 for larger initial STP and 15 mg kg−1 yr−1 for larger initial STK. While using residual soil P and K of varying concentrations in this claypan soil to grow soybeans marginally affected yields, the 5‐yr decline in STP and STK values may greatly affect subsequent sensitive crops and require high rates of fertilization.

Validation of Soil‐Test‐Based Phosphorus and Potassium Fertilizer Recommendations for Flood‐Irrigated Rice

Soil testing is a widely accepted practice for evaluating soil‐P and soil‐K availability but can be inconsistent for making accurate fertilizer‐P and fertilizer‐K recommendations. Our research objectives were to assess the accuracy of established soil‐ and tissue‐P and ‐K concentration interpretations for predicting flood‐irrigated rice (Oryza sativa L.) yield response to fertilization at three levels of significance (p ≤ 0.05, 0.10, and 0.25). Six treatments combining two fertilizer‐P rates (0 or 32 kg P ha−1) and four fertilizer‐K rates (0, 56, 84, 112 kg K ha−1) were applied at 24 sites. Soil‐test P (STP) interpretations were 40% accurate, regardless of the significance level, and whole‐plant‐P concentrations at the V6‐V7 stage were 50% accurate. Soil‐test K (STK) interpretations were 40 (p ≤ 0.05 and 0.10) and 36% (p ≤ 0.25) accurate, and whole‐plant‐K concentrations at the R2‐R3 stage were 61% (p ≤ 0.25) or 62% (p ≤ 0.05 and 0.10) accurate in predicting the yield response to K fertilization. Nearly all of the error in both soil‐test‐ and whole‐plant‐P and ‐K concentration interpretations occurred in the suboptimal categories. The accuracy (p ≤ 0.05) of yield response predictions for levels where no fertilizer was recommended was 100% for STP and 86% for STK and 82 to 100% for whole‐plant‐P and ‐K concentrations. The false‐positive error was the most common soil‐test recommendation inaccuracy and suggests that fertilizer recommendations are skewed to reduce the risk of yield loss from insufficient fertilization.Core Ideas Tissue analysis is more accurate at predicting flood‐irrigated rice response to P and K fertilization than Mehlich‐3 extractable P and K. False positive errors are the most common soil‐test‐based interpretation errors for flood‐irrigated rice. Mehlich‐3 extractable P is not a good predictor of rice yield response to P fertilization. The most accurate fertilizer P and K recommendations were interpreted at the 0.05 and 0.10 significance levels.

Cropping System, Landscape Position, and Topsoil Depth Affect Soil Fertility and Nutrient Buffering

Core Ideas Depth to claypan, landscape position, and cropping system influence soil fertility and nutrient buffering. Accounting for landscape‐variable depth to claypan could improve P and K management. Cover crops can help increase soil organic matter content. Cover crops may help reduce P requirements on claypan soils. Precise nutrient management across claypan soil landscapes requires an understanding of how diversity in management practices impacts soil properties and nutrient buffering. Therefore, a study was performed at the Central Mississippi River Basin site of the USDA Long‐Term Agroecosystem Network from 2010 to 2016 to determine how depth to claypan (DTC), cropping system (CS), and landscape position (LP) affect soil properties, and whether accounting for these factors could improve fertility management. Treatments consisted of five CS {MTCS, mulch till corn (Zea mays L.)–soybean [Glycine max (L.) Merr.]; NTCS, no‐till corn–soybean; NTCSW, no‐till corn–soybean–wheat (Triticum aestivum L.)–cover crop; CCRP, cool season conservation reserve program; and HAY, cool and warm‐season hay} and three LP (summit, backslope, and footslope), each with a distinct DTC. Soil test P (STP) was 9 kg P ha–1 greater on footslopes than summits. Soil test K (STK) and soil organic matter content (SOM) were greatest on backslopes and averaged 384 kg K ha–1 and 20 g kg–1, respectively, Soil test P was ≥49% and STK was ≥10% lower in NTCSW than in MTCS or NTCS. However, crop diversity in NTCSW maintained STP and increased SOM 2 g kg–1 over MTCS. Soil test P, pHs, and SOM were similar between CCRP and HAY, while STK in CCRP was greater at all LP. Lastly, deeper DTC caused greater P buffering and less K buffering than shallower DTC. These results indicate that eroded sideslopes with shallow DTC likely need more or more frequent P and less K than other LP.

Is air-drying of soil samples an appropriate step in determining plant available potassium for corn?

ABSTRACTPotassium (K) fertilizer recommendations are mainly based on air -dried soil samples which can lead to over- or under-estimation of plant available soil K. Three on-farm trials were conducted in North Dakota and Minnesota to determine the variation of soil test-K between air-dried (KDry) and field moist (KMoist) soil samples. The differences between KDry and KMoist decreased exponentially as soil K increased, but increased linearly with increasing soil moisture. Soil drying influenced the plant available soil K-test value, producing higher K values compared to the moist soil K. It is unclear based on these initial experiments which method might produce a more predictable K critical value to aid in directing K application for corn in this region.

Potassium Uptake by Corn and Soybean, Recycling to Soil, and Impact on Soil Test Potassium

Research is needed to quantify better plant nutrients recycling to the soil. This study investigated K recycling by corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] from physiological maturity (PM) to spring. Aboveground plant samples were collected from 19 Iowa K field response trials (33 sites years for corn and 14 for soybean). Dry matter (DM) yield and K accumulation were measured in vegetative tissue at PM, in all tissues at grain harvest, and in residue four times until spring. Soil test K (STK) was measured at grain harvest each year and in the last 2 yr the following spring. On average, K accumulation in grain or residue at harvest was 68 and 34 kg K ha⁻¹ in soybean and 29 and 52 kg K ha⁻¹ in corn. The K accumulation increase from fertilization was more frequent and greater in residue than in grain (60 and 9% for soybean and 57 and 7% for corn). Potassium accumulated in vegetative tissue at PM that remained in residue 2 mo after harvest was 50% in corn and 19% in soybean and decreased to 31 and 12% by April. The K loss decreased as precipitation increased (R² 0.64 for soybean and 0.38 for corn) and affected more the K loss in soybean than corn. Soil test K increased from fall to spring, and the increase was related to the K lost from residue (r² 0.56 in soybean and 0.16 in corn). Crop type and precipitation strongly influenced K recycling and affected STK temporal variation.

Soil potassium—crop response calibration relationships and criteria for field crops grown in Australia

The Better Fertiliser Decision for Crops (BFDC) National Database holds historic data for 356 potassium (K) fertiliser rate experiments (431 treatment series) for different rain-fed grain crops and soil types across Australia. Bicarbonate-extractable K (Colwell soil-test K) is the most extensively used soil test reported in the database. Data are available for several crop species grown on a range of soil types from all states except Tasmania. Species represented and number of treatment series in the database are: wheat (Triticum aestivum L.), 254; barley (Hordeum vulgare L.), 5; canola (Brassica napus L.), 130; lupin (Lupinus angustifolius L.), 32; sunflower (Helianthus annuus L.), 10; sorghum (Sorghum bicolor L.), 5; and faba bean (Vicia faba L.), 2. About 77% of the available soil-test K (STK) data on wheat, canola, and lupin are from Western Australia. The usual sampling depth of 0–10 cm is recorded for all treatment series within the database, while 68% of experiments have STK information from other soil horizons down the profile, usually in 10-cm increments. The BFDC Interrogator, a comprehensive data search and calibration support tool developed for use with the BFDC National Database, was used to examine STK–yield relationships for each crop across Australia, with more detailed analysis by state/region and then by soil type if data were available. The BFDC Interrogator was used to determine a critical STK concentration to achieve 90% of the maximum relative yield (90%RY) for each crop species, with a critical range (determined by the 70% confidence limit for the 90%RY) also reported. The STK for 90%RY for wheat was 40–41 mg/kg on Tenosols and Chromosols, ~49 mg/kg on Kandosols, and ~64 mg/kg on Brown Ferrosols. There was some evidence of critical values increasing with increasing crop yield and on soils with no acidity constraints to root growth, with effects presumably driven by increased crop K demand. The STK for 90%RY for canola, grown mainly on Tenosols, was similar to that for wheat, ranging from 43 to 46 mg K/kg, but for lupin, also grown mainly on Tenosols, the STK for 90%RY was a relatively low ~25 mg K/kg. Data for sunflower were limited and the STK for 90%RY was poorly defined. A comparison of critical STK concentrations for different crops grown on Tenosols suggested that critical ranges for 90%RY of lupin (22–27 mg K/kg) were significantly lower than that for wheat (32–52 mg K/kg) and canola (44–49 mg K/kg). Critical STK values were not determined for sorghum and faba bean.

Assessment of Potassium Supply for Corn by Analysis of Plant Parts

Tissue testing of corn (Zea mays L.) ear leaves often is used to diagnose K deficiencies. However, little attention has been given to study how testing of different corn tissues evaluates K supply especially in the optimum to excess range. We evaluated tissue tests for corn based on the K concentration of young plants (V5 to V6), ear leaves at silking, lower stalks at maturity (sections cut from 15 to 35 cm aboveground), and grain. Twenty‐eight field trials were conducted in Iowa during 2 yr with fertilizer rates of 0, 56, 112, and 168 kg K ha−1 Initial soil‐test K (STK) by the ammonium acetate extractant (15‐cm sampling depth, air‐dried samples) was 80 to 266 mg K kg−1 across sites. Fertilizer K increased grain yield at one site testing 84 mg K kg−1; and increased the K concentration of plants, leaves, stalks, and grain at 11, 13, 21, and 5 sites, respectively. Published critical K concentrations for corn plants and leaves would have incorrectly indicated deficient soil K supply for yield in many sites. There were upper limits in plants, leaves, and grain K concentrations response to K supply that varied among sites. The limits occurred at STK values of 123, 140, and 148 mg kg−1, respectively, which are near STK values optimum for grain yield. The lack of an upper limit in corn stalks shows promise for using this tissue to assess K availability from deficiency to luxury uptake or excess. However, the large site‐dependent variability in the stalk K concentrations should be recognized as a limitation.

Spatial Variability of Soil Test Phosphorus, Potassium, and pH of Ontario Soils

Grid soil sampling is typically used for establishing management zones for site‐specific application of nutrients. The geostatistical procedures used to estimate values between sample locations require samples to be taken close enough to together that they are correlated to one another. An evaluation of the scale of variability of soil test P (STP), soil test K (STK), and soil pH for Ontario soils was conducted using autocorrelation analysis of 23 Ontario farm fields, which were grid‐sampled using a 30‐m spacing. The results of the autocorrelation analysis indicated that 13 of the 23 farm fields would require a grid spacing of less than 30 m to adequately assess their spatial variability. For only one site was the commonly used 100‐m grid spacing adequate for the assessment of the spatial patterns of STP and STK. Further analysis using F tests compared the residuals from three gridding procedures (kriging, inverse distance, and nearest neighbor) using 60‐ and 90‐m grid data to that of the residuals using the field mean soil test value. In most cases, soil test variation maps based on 60‐ or 90‐m grid soil samples did not result in an increased ability to predict the soil test level at a given location in the field. It was concluded that a grid spacing of 30 m or less would be required to adequately assess the spatial variation of STP, STK, and soil pH. Sampling at this intensity would require approximately 11 times as many soil samples as the commonly used 100‐m grid, which is likely to make the cost prohibitive.

Corn Grain Yield, Early Growth, and Early Nutrient Uptake as Affected by Broadcast and In‐Furrow Starter Fertilization

In‐furrow starter fertilization for corn (Zea mays L.) is being preferred to alternative starter application methods by farmers in the U.S. Corn Belt. This study assessed corn grain yield, early growth (V5 to V6), and early P and K uptake responses to in‐furrow P–K starter fertilization with or without broadcast P–K fertilization for 2‐yr corn–soybean [Glycine Max (L.) Merr.] rotations. Sixteen trials were conducted on Iowa fields managed with no‐till or chisel‐plow tillage. Soil‐test P (STP) was 5 to 77 mg P kg−1 (Bray‐P1), and soil‐test K (STK) was 88 to 237 mg K kg−1 (ammonium acetate). Treatments replicated four times were a control, 3–8–15 (N–P–K) liquid starter at 5 to 7 kg P ha−1 and 10 to 14 kg K ha−1, granulated P–K fertilizer broadcast at 49 to 66 kg P ha−1 and 112 to 140 kg K ha−1, and broadcast plus starter. Nitrogen was applied across all treatments. Fertilization increased grain yield at nine sites (0.80 to 2.11 Mg ha−1). Starter fertilization produced less yield than broadcast fertilization at five sites (0.30 to 1.48 Mg ha−1 less), four of which tested low in STP (≤15 mg P kg−1). Starter fertilization in addition to broadcast fertilization did not increase yield further at any site. Starter fertilization increased corn early growth and P and K uptake more than broadcast fertilization did at most sites. In‐furrow starter P–K fertilization for corn is not an effective practice when applied in addition to 2‐yr broadcast P–K fertilization rates for corn–soybean rotations.

Broadcast and Deep‐Band Placement of Phosphorus and Potassium for Soybean Managed with Ridge Tillage

The efficiency of broadcast and band P and K placement methods for ridge tilled soybean [Glycine Max (L.) Merr.] has not been extensively researched. This study evaluated grain yield, early plant growth, and nutrient uptake (V5–V6 stages), and soil‐test responses to broadcast and deep‐band (15–20 cm below the ridge surface) placements in 14 trials. Fertilizer rates were 0, 14, and 56 kg P ha−1 and 0, 33, and 132 kg K ha−1 A randomized complete‐block design was used. Soil‐test P (STP) was 7 to 61 mg kg−1 (Bray‐P1 test, 15‐cm depth) and soil‐test K (STK) was 100 to 197 mg kg−1 (ammonium acetate test). Phosphorus increased yield (P ≤ 0.1) at four sites. Band P was better at one site (30 kg ha−1) and broadcast P was better (40 kg ha−1) at another site. Both P placements increased dry weight (DW) at two sites. Yield and DW responses to P were observed when STP was <19 mg kg−1 Phosphorus increased plant P uptake at eight sites, and band P was better than broadcast P at three sites. Potassium increased yield at two sites, and only band K increased yield at one of these sites. Potassium increased DW at four sites, and increases were always larger for band K. Band K increased K uptake at 10 sites and broadcast K increased K uptake only at two sites. Soil‐test P in ridges predicted P sufficiency better than mean STP from ridges and valleys at two sites. Although K fertilization seldom increased yield in this study, increased plant K uptake with banding compared with broadcasting suggests that banding may be more efficient for grain production in other conditions.

Grain Yield, Early Growth, and Nutrient Uptake of No‐Till Soybean as Affected by Phosphorus and Potassium Placement

More information is needed about P and K placement for no‐till soybean [Glycine max (L.) Merr.]. This study evaluated plant responses to P and K fertilization and placement in 10 long‐term trials and 11 short‐term trials in Iowa from 1994 to 1997. Treatments were various P and K rates broadcast, banded with the planter, and deep banded (at a 15‐ to 20‐cm depth). Measurements were plant weight, P uptake, and K uptake at the V5 stage and grain yield. Phosphorus fertilization increased yield when soil‐test P (STP) was less than 9 mg P kg−1 (Bray‐P1) at a 0‐ to 15‐cm depth or 12 mg P kg−1 at a 0‐ to 7.5‐cm depth. The P placement did not influence yield. The band K placements produced slightly higher yield than the broadcast placement. Responses to K were not related to soil‐test K (STK) levels, which varied from 90 to 262 mg K kg−1 (ammonium acetate), or stratification. The P or K placement had little influence on early growth but influenced early P and K uptake. Banding with the planter was more effective than broadcasting for P uptake, and the two band placements were more effective for K uptake. Only the responses of K uptake and grain yield to banded K were correlated across sites. A shallow sampling depth will improve only slightly the prediction of response to P. The observed small no‐till soybean yield response to banded K would seldom offset increased application costs in similar soils.

Long‐Term P and K Applications: II. Effect on Corn and Soybean Yields and Plant P and K Concentrations

Increasing emphasis on environmentally sound nutrient management requires an improved understanding of fertilizer P and K needs for profitable crop production. A 20-yr study was conducted on two glacial till soils to determine: (i) the effect of various P and K rates on corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields and profitability during a 12-yr application phase and an 8-yr residual phase, and (ii) whether triennial applications of P and K produce crop yields equal to those from annual applications. Initial Bray P 1 and exchangeable K soil tests were 22 and 150 ppm, respectively, for the Webster clay loam (fine-loamy, mixed, superactive, mesic Typic Endoaquolls) and 10 and 228 ppm, respectively, for the Aastad clay loam (fine-loamy, mixed Pachic Udic Haploboroll). Phosphorus and K were applied annually for 12 yr at rates of 0, 50, and 100 lb P 2 O 5 /acre and 0, 50, and 100 lb K 2 O/acre while rates of 150 lb P 2 O 5 /acre and 150 lb K 2 O/acre were applied every third year. Yields were improved by the 50-lb P 2 O 5 rate in 6 of 12 yr when the soil test P (STP) was <22 ppm for the Webster soil and in 8 of 12 yr when the STP was ≤10 ppm for the Aastad soil. Yields were not improved further by the 100-lb P 2 O 5 rate on either soil. Yields from triennial P application were similar to those from annual P application in 23 of 24 site-yr. Annual application of fertilizer K increased yield only in 3 of 24 site-yr. During the 8-yr residual period, yields were increased above the eontrol in all 15 site-yr by carryover from the 50-lb P 2 O 5 rate. Corn yields were optimized at leaf K concentrations ranging from 1.14 to 2.29%, indicating the importance of knowing nutrient uptake characteristics of hybrids when assessing the K status of corn. Economic return was generally highest with the 150-lb P 2 O 5 rate applied every third year. Results from this study indicate that most profitable long-term returns can be obtained with broadcast applications of 150 lb P 2 O 5 /acre on a triennial basis and by monitoring STP and soil test K (STK) to take advantage of residual availability.

Profit-Maximizing Critical Values of Soil-Test Potassium for Corn

Identification of critical values for soil tests is a fundamental step in using soil testing to guide fertilization. Critical values usually are selected to attain a desired percentage of maximum yield. Here we demonstrate how critical values can be selected to maximize profits. Twenty-eight response trials with corn (Zeu mays L.) were conducted during 1989 and 1990. Treatments were 0, 50, 100, and 150 lb K/acre. Plant-available K was measured by ammonium acetate, Mehlich-3, K saturation, nitric acid, and rates of release soil tests. Analyses of yield responses involved fitting various models to relationships between relative yields and soil-test K (STK) values. Profitability indices were calculated for multifield scenarios in which alternative STK values were evaluated for their abilities to generate net returns when used to guide fertilization. The results showed several advantages of using net returns instead of yield levels: (i) critical values directly reflect the economic rationale for fertilization; (ii) costs of using inappropriate critical values can be easily evaluated; and (iii) soil tests can be compared easily for their abilities to generate profits when used to guide fertilization. Profit-maximizing critical values identified for the soil tests were 112 ppm for the ammonium acetate, 70 ppm for the Mehlich-3, 831 ppm for the nitric acid, and 0.90 lb/acre/d for the rate of release. The K saturation soil test was not appropriate for the soils in this study. Research Question Soil testing is widely used to guide K fertilization for corn, and critical values of soil tests are used to indicate when fertilization is, or is not, needed. Failure to identify a critical value leaves uncertainty concerning when fertilization is needed. Although fertilizers are applied to increase profits, the methods used to identify critical values have focused more on yield responses to fertilization than on profits from fertilization. Here we show how to select critical values to maximize profits from fertilization. Literature Summary Critical values for soil tests have been most often identified by analysis of relationships between soil-test values and relative yields (a measure of yield response to fertilization). Models usually are fit to these relationships and then solved for soil-test values associated with desired yield levels (expressed as percentages of the highest yields attained by adding fertilizer). Recent studies involving soil tests for P in corn, however, revealed marked disagreements between models commonly used to identify critical values. These disagreements make it difficult to determine which critical values should be used to guide fertilization. Study Description Twenty-eight response trials with corn having treatments of 0, 50, 100, and 150 lb K/acre were conducted in farmers' fields. Plant-available K was evaluated by five soil tests. Yield responses were expressed as relative yields by representing yields of nonfertilized plots as percentages of the mean yields of the two highest-yielding treatments for each site. Net returns to fertilization were estimated by subtracting assumed costs of fertilization from the assumed value of additional grain produced on fertilized plots compared with nonfertilized plots. Critical values of soil-test K (STK) were identified by analysis of yield responses or analysis of critical-value profitability (CVP) indices. Analyses of yield responses involved fitting various models to relationships between relative yields and STK values. The CVP indices were based on net returns to fertilization calculated for scenarios in which alternative STK values were used to guide fertilization over 28 fields corresponding to the 28 trials in the study. The STK values that produced maximum CVP indices were denoted as profit-maximizing critical values. Applied Questions What are the advantages of using net returns to identify critical values? The results show that analysis of net returns has several advantages over analysis, of yield responses. These include (i) critical values directly reflect the economic rationale for fertilization; (ii) the costs of using inappropriate critical values can be easily evaluated; (iii) uncertainty caused by disagreements among models is avoided because relationships between CRP indices and soil-test values show clear maxima; and (iv) soil tests can be easily compared for their ability to generate profits when used to guide fertilization. What were the best soil-test methods and the profit-maximizing critical values? Maximum profitability indices for the ammonium acetate, Mehlich-3, nitric acid, and rate of K release soil-test methods were similar. Profit-maximizing critical values for these methods were 112 ppm for the ammonium acetate, 70 ppm for the Mehlich-3, 831 ppm for the nitric acid, and 0.90 lb/acre/d for the rate of release. The K saturation method was not appropriate for the soils in this study. Recommendation Critical values for soil tests should be identified by considering net returns rather than yield responses to fertilization.