Objective:

Develop and incorporate soil specific phosphorus (P) recommendations into the Missouri soil test recommendation system.

• Evaluate the Missouri P buildup algorithm by testing the effect of added P on soil test P for 20 soils selected from the five major soil regions of Missouri.
• Correlate differences among the 20 soils in the amount of P needed to reach an optimum soil test P level with soil characteristics currently available or easily obtained by Missouri soil testing laboratories.
• Evaluate the effect of initial soil test P level on rate of P reaction with the soil and the change in soil test P after P addition.

Importance of research area:

Missouri has soils with a wide range of mineralogical and chemical properties. The Ozark soil region has ancient soils dominated by kaolinitic clays and that have high aluminum and iron oxide contents, traits associated with high P buffering capacity. The other four soil regions all have younger soils but with a diversity of parent material ranging from loess to sediments. It is likely that P buffer capacities range widely among Missouri soils although there is little data to quantify these differences.

U.S. soils required as little as 5 to as much as much as 11 lbs/acre of phosphate to raise soil test P one unit. Missouri P recommendations assume that all soils have the same buffer capacity; there is no mechanism to predict which soils are likely to require more or less P to raise soil test. It is likely that the current recommendation system recommends too little P on some soils and too much on others for building P levels in soil.

Missouri phosphorus recommendations also assume that it takes more P to raise soil test P at low soil test P levels than high soil test levels. A literature review of 20 field studies indicated that the same amount of added P was required to raise soil test P one unit at all these P levels for a given soil. The relationship between added P and the increase in soil test P is predominantly a linear relationship except in very low testing soils.

This project is will determine if Missouri should convert to a linear algorithm as suggested by the literature review of soils in other states and countries. We will also attempt to identify soil characteristics that can be used to predict differences in the amount of added P needed to raise soil test P among Missouri soils.

Methods:

Table 1. Soils collected for the P study.

Region of state	County	Soil type	Initial soil test P1
			lb/acre
Bootheel	Mississippi	Commerce silty clay loam	64
Bootheel	New Madrid	Sharkey clay	29
Bootheel	Stoddard	Lilbourn fine sandy loam	59
Bootheel	Stoddard	Loring silt loam	15
Clay pan	Boone	Mexico silt loam, eroded	38
Clay pan	Knox	Putnam silt loam	41
Clay pan	Monroe	Putnam silt loam	13
Clay pan	Monroe	Mexico silt loam	17
Loess/Drift	Gentry	Grundy Silt Loam	54
Loess/Drift	Lafayette	Higginsville silt loam, eroded-Combo	80
Loess/Drift	Linn	Loganda silt loam	33
Loess/Drift	Ray	Sharpsburg silt loam-9% slope, eroded	26
Osage plain	Vernon	Barco loam	27
Osage plain	Vernon	Osage silty clay	14
Osage plain	Vernon	Barden silt Loam	27
Osage plain	Vernon	Barden silt Loam	54
Ozarks	Christian	Clarksville very cherty silt loam	21
Ozarks	Laclede	Viraton silt loam	11
Ozarks	Lawrence	Creldon silt loam	10
Ozarks	Polk	Goss gravelly silt loam	37
River bottom	Saline	Haynie silt loam	47

¹ Bray-I P.

Table 2. Location of field experiments.

Region of state	County	Soil type
Clay pan	Boone	Mexico silt loam, eroded
Loess/Drift	Linn	Loganda silt loam
Ozarks	Lawrence	Creldon silt loam

Laboratory study

• Soils were identified and collected from each of the 5 major soil regions of the state with the aid of local Natural Resource Conservation Service and University Extension personnel (Table 1).
• Soils were analyzed for soil test P, potassium, calcium, magnesium; percent organic matter; pH, and percent water holding capacity. Soils will be analyzed for phosphorus sorption capacity, Mehlich-III soil test P, aluminum (Al) and iron (Fe) concentration in Mehlich-III and Bray-I extracts, oxalate extractable Al, and oxalate extractable Fe.
• Seven P treatments (3 replicates) were added to 150 g of soil as diammonium phosphate. The treatments were 0, 0.01, 0.03, 0.05, 0.08, and 0.4 mg P/g soil.
• A single manure treatment was included on all soils. Poultry litter was added at the rate of 0.05 mg P/g soil. The 7 P treatments were duplicated with manure on the 3 soils included in both the laboratory and field experiments.
• Treated soils were adjusted to field capacity based on water holding capacity determined for the soil. Soils were incubated in the dark at 20 oC in covered plastic cups with an air exchange hole. There were a total of 513 incubation cups.
• Cups were mixed and had water content corrected for moisture loss based on sample weight every 10 days during the first month. All samples will be mixed every 2 weeks thereafter and have moisture loss corrections every month.
• All cups were sampled 32 days after P addition. Samples will be sampled again 90 and 180 days after P addition. Samples are dried at 30 oC to constant weight.
• All samples will be analyzed for Bray-I P. Linear and nonlinear regression will be used to model the effect of added P and soil type on soil test P. Linear regression will be used to correlate soil properties with differences among soils in their response to added P.

Field study

• Three field locations were located at University of Missouri South farm, Forage Systems Research Center and Southwest Research Center (Table 2). Soils from these locations were also included in the laboratory experiment (Table 1).
• Seven fertilizer treatments and 1 manure treatment (3 replicates) will be surface applied to established fescue in September 2001.
• Plots will be 6 ft. X 6 ft. A buffer area will be established of at least 40 feet downslope between plots. Plots will be clipped periodically and the forage will returned to the plots.
• Plots will be sampled 1, 3 6 and 12 months after P application. Samples will be dried at 30 oC and analyzed for Bray-I P.
• Linear regression will be used to compare the response of soil test P to added P in the laboratory and field experiments.

Project status, summer 2001:

Notification of funding in 2000 came too late to initiate the experiment in 2000. Soils and experimental sites were located, soils were collected and experimental protocol and laboratory logistics were designed in winter 2001 through spring 2001. Soils were analyzed for basic soil test parameters and water holding capacity in spring 2001. Treatments were added to the soils in the laboratory study in July 2001. Field sites were identified in spring 2001 and the field treatments will be applied in September 2001. We anticipate completing all aspects of the project in 2002.

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