May 1994 SCSB# 380
Chapter 5 Phosphorus and Potassium C. C. Mitchell and J. F. Adams Current CalibrationThe 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). 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. PhosphorusResearch ReviewPeanuts 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).
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. RecommendationsA 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).
PotassiumResearch ReviewIn 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:
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. 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.
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. RecommendationsChanges 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 soilsespecially 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.
Nutrient RemovalPeanuts 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. ConclusionsModifying 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:
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. ReferencesDocument Prepared by: Leigh H. Stribling, lstribli@acesag.auburn.edu Alabama Agricultural Experiment Station Auburn University |
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