May 1994 SCSB# 380

RESEARCH-BASED SOIL TESTING INFORMATION
AND FERTILIZER RECOMMENDATIONS
FOR PEANUTS ON COASTAL PLAIN SOILS


Chapter 5
Phosphorus and Potassium


C. C. Mitchell and J. F. Adams

Current Calibration

The critical value is that which separates the “medium” from the “high” rating and fits the definition given in Chapter 4. Critical soil-test P concentrations for peanuts among laboratories using the Mehlich-1 extraction range from 17.5 mg P kg-1 to 30.mg P kg-1 with a mean of 27 (54 lb P acre-1 assuming 2 x l06 lb acre-1 furrow slice). Three states use 30 mg P kg-1 (Table 1). No yield response to additional application of a particular nutrient is expected above the critical value. Only Virginia recommends fertilizer P for peanuts if the soil tests greater than this critical amount (Table 2).

Critical Mehlich-1 extractable K values range from 40 mg K kg-1 for Alabama soils with a cation exchange capacity less than 4.6 meq l00g-1 to 87 for Virginia soils with a mean of 69 (138 lb K acre-1). Considerable differences in soil test K critical levels exist among neighboring states with similar soils (e.g. low CEC Alabama soils and Georgia soils). Only North Carolina recommends K fertilization for peanuts if a soil tests greater than medium.

Differences in the critical values for P and K may be due to the method of interpretation as explained by Cox in Chapter 4. A quadratic plateau or an exponential function technique of interpreting research data may result in a higher critical value whereas a linear plateau (as used by Cox) results in a relatively lower value. The methods of interpretation by each laboratory are rarely stated in the references cited in Tables 1 and 2. The critical values selected by each laboratory (Tables 1 and 2) may have relied on the individual preferences of those conducting the original research.

A 1991 USDA survey in Georgia and in North Carolina-Virginia indicated that peanut growers in these two areas applied phosphate fertilizers to over 72% of the peanut acreage (USDA-NASS/ERS 1992). The average rate per crop was 45 lb P2O5 acre-1. Potassium fertilizers were applied to 78% of the peanut acreage. The average rate was 83 lb K2O acre-1.

Phosphorus

Research Review

Peanuts seldom respond to fertilizer P applications. Soil test P levels that are adequate for peanuts are often lower than those required for most other crops (Cope et al. 1984). Data from Alabama, Georgia, India, China, and Australia suggest very low critical levels of approximately 5 to 10 mg P kg-1 (10 to 20 pp2m).

Alabama research on farmers’ fields has shown no correlation between Mehlich-1 extractable P and yield or grade increases from fertilization in 39 experimental sites where soil test P ranged from 1 to 45 mg P kg-1 (2 to 90 pp2m) (Hartzog and Adams 1988a, 1988b) (Figure1). Using the current Alabama calibration for peanuts, 20 of the sites would be rated “low” or “very low” in soil test P. Present soil test calibration for P in Alabama clearly does not adequately predict the yield response to applied P. Hartzog and Adams (1988b) suggest that “. . . adjustments in soil-test ratings are needed.” In addition to the above studies, long-term (60+ years) fertility experiments on a Dothan sandy loam (fine-loamy, siliceous, thermic Plinthic Kandiudults) in Alabama have never shown a peanut yield response to P fertilization (Cope 1984, Cope et al. 1984).

  Figure 1. Relative yield of Florunner peanuts vs. soil-test P levels of unfertilized plots.

One experiment begun in 1954 had Mehlich-1 extractable P of 30 mg kg-1. This would be rated “high” using current Alabama calibration (Table 1).Where no P has been applied in 30 years, the soil test level declined to 11 mg P kg-1 which would be rated “low,” yet no yield response to P fertilization has been measured.

On the other hand, some early research in Georgia showed positive yield responses to P fertilization. Carter (1951) reported spanish-type peanut yield increases from 927 to 1,613 lb acre-1 with the addition of P fertilizer on a soil testing 11 mg P kg-1 (extractant unknown). Futral (1952) reported very high yield increases from P fertilization on some soils that were “low in P.” A three-year study on a Troup fine sand (loamy, siliceous, thermic Grossarenic Kandiudults)with an initial Mehlich-1 extractable P of 22 mg kg-1 showed no apparent response to P fertilization of spanish-type peanuts (Walker et al. 1974). There was an apparent yield increase to drilled applications of P fertilizer on runner-type peanuts. Phosphorus fertilization had no effect on the P concentration in the peanuts nor on peanut quality (percent sound mature kernels). Following an extensive review of earlier fertility research with peanuts on Coastal Plain soils, Walker et al. (1974) concluded that “. . . the lack of response (of peanuts) to rates of P and K on this soil would certainly raise some questions about medium to high rates of these elements on the better peanut soil.”

The Mehlich-3 extracting solution extracts between 1.5 and 2.0 times as much soil P as does the Mehlich-1 (Gascho et al. 1990). North Carolina data from a long-term study with virginia-type peanuts on a Goldsboro soil suggest an M3 critical P value of 17 to 25 mg kg-1 (mean of 21). (See Chapter 4).

Recommendations

A factual interpretation of current research information regarding soil test P calibration for peanuts would mean dramatic changes in current “critical values” for all states testing soil for runner-type peanut production. This would also result in little or no P fertilizers recommended for peanuts on most Coastal Plain soils. The subcommittee agreed that an acceptable and realistic “critical value” for Mehlich-1 extractable P would be 10 mg kg-1 (20pp2m or lb acre-1).

Figure 2, modified from a figure presented by Gary Gascho (GA), interprets soil test P and fertilizer P2O5, recommendations (in pounds per acre) for application to land to be planted in peanuts. Crop rotation is an essential, highly recommended practice for peanut production. Fertilization of any crop grown in rotation with peanuts (corn, cotton, small grains, temporary winter grazing, bahiagrass, bermudagrass, etc.) according to established recommendations based on soil tests will eliminate the need to apply additional P to the peanut crop.

 Figure 2. Phosphorus calibration and interpretation for peanuts on Coastal Plain soils

Potassium

Research Review

In general, there are contradictions and poor correlations between plow-layer soil-test K and peanut yield response to K fertilizers (Cox et al. 1982). In personal correspondence, Fred Cox, Professor of Soils at North Carolina State University, explained the contradictions:

The soil test for K is not too indicative of K availability for peanuts on our low CEC soils. Much of the K utilized by the crop is from the subsoil and that is not currently measured. Topsoil K cannot be built appreciably by fertilization and it will not decrease much below about 0.1 cmol L-1. So, all in all, the soil test for K is not too reliable for indicating K requirements, and I am sure our recommendations are on the high side because of this uncertainty. Fertilization of the previous crop and even the returning of corn stalks normally add enough K for peanuts.

A 1974 literature review found little justification for direct K fertilization of peanuts (Walker et al. 1974). In some reported cases of yield increases from K fertilization on “low” K soils, the increase in yield was not sufficient to pay for the additional fertilizer materials. However, in their three-year study on a Troup sandy loam with an initial Mehlich-1 soil test level of 13 mg K kg-1, both spanish-type and runner-type peanuts produced a positive yield response to K fertilization. In a separate three-year study, Walker et al. (1989) reported a positive yield response to K fertilization on a Lakeland sand (therrnic, coated Typic Quartzipsamments) with an initial Mehlich-1 soil test of 10 mg K kg-1 but found no yield response to K fertilization on a nearby Fuquay loamy sand (siliceous, thermic, arenic Plinthic Paleudults) with a soil test of 24 mg K kg-1.

In early Alabama research, Scarsbrook and Cope (1956) reported an average yield increase to K fertilization of 170 kg ha-1 of peanuts in 13 cooperative experiments where soil test K was rated “low” for other crops. No positive yield response was observed in five tests where soil test K was “high.”

After 34 on-farm tests with no yield response to K fertilization, Hartzog and Adams (1973) concluded that adding fertilizer directly to peanuts was not a good practice but that fertilizer should be added to crops rotated with peanuts. Multiple crops on long-term fertility plots indicate that the relative response to soil test and fertilizer K levels was cotton > grain > sorghum > corn> soybeans > wheat and peanuts (Cope et al. 1984). In one study, with a Mehlich-1 soil test of 38 mg K kg-1, peanuts showed yield increases from the application of up to 20 lb K2O5 acre-1 from 1973-75. No K response was observed in 1981-83. In another study on the same soil, Dothan sandy loam (fine-loamy, siliceous. thermic Plinthic Kandiudults), with an original soil test of 45 mg kg-1, no peanut response to K fertilization was observed.

Plow-layer, soil-test K levels have decreased very little in over 60 years of cropping in one Alabama study. However, K in subsurface horizons decreased with depth at all K fertilization rates (Cope et al. 1984) observed that “. . although the amounts below the plowed layer were less than in the surface soil, they represent substantial reserves above that of the untreated plots. This helps explain why such soils can produce maximum yields of peanuts or other low K requiring crops for several years without K application after ‘high’ soil test levels are attained by fertilization.”

Negative responses to K fertilizer have been reported, especially where soil Ca supply is short (Cope et al. 1984, Whitty et al. 1986).

Recent Alabama data from on-farm, replicated tests have been used to define a critical Mehlich-1 soil-test K value (Hartzog and Adams 1988a, 1988b). Figure 3 indicates a critical Mehlich-1 K level of approximately 13 mg K kg-1 using a quadratic plateau technique. All responsive sites were on soils (Paleudults or Quartzipsamments) with a Bt horizon deeper than 70 cm, while most of the non-responsive sites were on Paleudults with shallower Bt horizons. These results, along with the observations of peanut researchers in other Coastal Plain soils, suggest that subsoil K testing, or at least depth to the argillic (Bt) horizon, should be a consideration in interpreting soil test K results.

Figure 3. Relative yield of Florunner peanuts vs. soil-test K levels of unfertilized plots.


Cox has identified a Mehlich-3 critical extractable K value averaging 0.105 cmol L-1 (41 mg K L-1) for virginia-type peanuts in North Carolina (see Chapter 4). For a soil with an assumed sample density of 1.3 g cm-3, this would be 32 mg K kg-1. The critical range would be 16 to 48 mg K kg-3. However, he also found that prior fertilization and recycling of K into the subsoil has an effect on the critical K level for peanuts. Without subsoil sampling to refine the K recommendation, he suggests including some field history on K to adjust the critical level.

Recommendations

Changes in soil test K interpretations based on research using the Mehlich-1 extract for Coastal Plain soils, like P interpretations, will result in dramatic changes in the traditional approach to soil testing for peanuts. All states producing runner peanuts should re-evaluate the basis for their current calibration and interpretation. Evidence indicates a need to consider depth to argillic horizon and subsoil K levels when interpreting soil test levels. Nevertheless, sufficient research evidence has been presented to warrant recommending a critical Mehlich-1 soil test value of 20 mg K kg-1 for runner-type peanuts on all Coastal Plain soils. This value is a compromise between that identified by Alabama research (13 mg K kg-1) and values currently used (40 to 88 mg K kg-1). This critical value will result in very little direct K fertilization on most Coastal Plain soils—especially the finer-textured soils where the Bt horizon is often near the soil surface. Moderation of direct K fertilization of peanuts should also decrease incidences of Ca:K imbalances which can result in decreased yields and grade and increases in pod rot especially on sandier soils with a low CEC. A critical value for Mehlich-3 extractable K for virginia-type peanuts may be only slightly higher than this based on North Carolina research.

Figure 4, modified from a figure presented by Gary Gascho (GA), interprets soil test K and fertilizer K2O recommendations (in lb acre-1) for direct application to land to be planted into peanuts the current year. Crop rotation is an essential, highly recommended practice for peanut production. Potassium fertilization of crops in rotation with peanuts according to soil test interpretations for those crops will assure adequate K for peanuts the following year.

 Figure 4. Potassium calibration and interpretation for peanuts on Coastal Plain soils.

Nutrient Removal

Peanuts are very efficient at obtaining P and K from the soil due to a deep and extensive root system. At current yields of 4,000+ lb acre-1 of pods, nutrient removal is comparable to other crops traditionally produced on Coastal Plain soils and often grown in rotation with peanuts (Table 3). Failure to replace these nutrients, especially near critical soil test levels of P and K, could have detrimental effects on subsequent crops. However, as previously shown, peanuts have much lower critical levels of P and K than most other crops produced on Coastal Plain soils. Fertilization of the crops in rotation with peanuts according to established soil test interpretation for those crops will assure adequate nutrients for both crops regardless of crop removal.

Conclusions

Modifying soil test calibration, interpretation, and recommendations for P and K on peanuts on Coastal Plain soils will require dramatic changes in existing programs in all peanut-producing states. Programs should emphasize soil testing and proper fertilization for crops in rotation with peanuts rather than direct fertilization of peanuts. This management practice has been encouraged in all states for many years. The “Soil Testing and Peanut Fertility” subcommittee of SERA-IEG-6 suggests the following critical values based upon research conducted over the past 20 years:

 Mehlich-1 extractable P 10 mg kg-1
 Mehlich-1 extractable K 20 mg kg-1

These values represent the level at which direct fertilization of the respective nutrient will not produce a peanut yield increase. Based on reported yields in research and on-farm tests, sites with soil test P or K near the critical value are capable of producing in excess of three tons (6,000 lb) peanuts per acre, provided other soil and crop limiting factors are controlled.

References


Document Prepared by:
Leigh H. Stribling, lstribli@acesag.auburn.edu
Alabama Agricultural Experiment Station
Auburn University

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