Transcript Phosphorus and Potassium - Soil 4213 Precision Agriculture

Phosphorus and Potassium
How is P managed?
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Key to managing soil and fertilizer P: Knowledge of whether or not the level of soil
solution P is adequate (about 0.05 ppm) to meet the needs for plant growth.
When the level of solution P is not adequate, it is important to know how much P
fertilizer should be added, and/or how much yield loss will occur if the P deficiency
is not corrected. Phosphorus soil tests have been developed to help provide this
information.
The concentration of plant available soil-P is extremely low and does not represent
the total amount that may become available during a growing season.
Effective soil tests extract P that is immediately available (intensity factor) and a
representative portion of the P that will become available during the growing
season.
The latter fraction represents aluminum and iron phosphates in acid soils and
calcium phosphates in near neutral and basic soils. Because the tests do not
exactly simulate plant root extraction of P from the soil, relationships must be
developed (correlation) between what the soil test extracts and what plants
extract.
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P soil tests
• In the early period of soil test development, many chemical
solutions and extraction procedures were used.
• Over time, similarities have been recognized that allow
reliable extraction and analysis to be made using only one
procedure, with consideration for soil pH.
• A common P soil test for acid soils is the Bray P1 procedure,
developed by Bray and Kurtz at the University of Illinois.
• The procedure is designed to dissolve Al-phosphates by
precipitating Al with fluoride (F).
0.025 M HCl + 0.03 M NH4F + soil AlPO4 = NH4H2PO4 + AlF3
Olsen
• For neutral and basic soils a bicarbonate
solution developed by Olsen at Colorado State
University, has proven effective in dissolving
Ca phosphates by precipitating Ca with
carbonate.
0.5 M NaHCO3 (at pH 8.5) + soil Ca3(PO4)2  CaCO3 + CaHPO4
Mehlich
• A more recently developed procedure (1980’s)
developed by Adolph Mehlich, working at the North
Carolina Department of Agriculture lab uses a
solution of acetic acid, ammonium nitrate,
ammonium fluoride, and EDTA to extract a portion of
plant available P from either acid or basic soils.
• This procedure, identified as the Mehlich-3, is
becoming widely used and is replacing regionally
specific procedures like the Bray P1 and Olsen’s
bicarbonate.
Correlation
• For any P soil test procedure to be beneficial, the extracted P
must relate to crop response or growth and development in
the field.
• The extent to which this relationship is found can be identified
by a statistical procedure called correlation
• When there is a good general relationship between the soil
test extraction values (usually expressed in ppm-P or lb/acreP) and the percentage of maximum yield obtained (%
sufficiency), then the procedure has promise as an effective
tool to help manage fertilizer-P inputs.
100
% Max
Yld
Generalized
correlation of soil testP and crop response
0
10
Soil Test P
65
(Bray P1 or Mehlich-3)
Calibration
• Calibration is a process that involves continuation of the
research to identify the amount of fertilizer-P that must be
added by a conventional method (usually preplant
incorporated) to correct an existing deficiency.
• An important aspect of the calibration process is to identify
the “critical level”, or soil test level that corresponds to a soil-P
fertility conditions above which plant response does not occur
when fertilizer-P is added (this may also have been identified
in the correlation process) .
• For the Mehlich-3 procedure this corresponds to about 33
ppm P (65 lb/acre). (see next slide)
• Calibration
Calibration of soil test –P values to percentage sufficiency and fertilizer-P requirement
for wheat.
P Soil Test Index*
% Sufficiency
Fertilizer Required (lb P2O5/acre)
Bray-P1
Olsen
0
0
25
80
10
6
45
60
20
12
80
40
40
18
90
20
+
65
24
100
00
*Values are in pp2m (lb/acre), Olsen is the bicarbonate extraction for neutral and
calcareous soils.
ppm * 2 = pp2m
ppm * 2 = lb/acre
2,000,000 lbs /afs (0-6”)
P Fertilization
• Sufficiency: Fertilize the Crop
• Maintenance: Fertilize the Soil
• Maintenance
– Replace what the crop removes
– Often used with Build-up model.
– Build up the soil then maintain
P Buildup
• Since soluble fertilizer forms of P react with the soil
to form less soluble compounds soon after they are
added, plant uptake efficiency, or fertilizer recovery,
for soil incorporated fertilizer is usually only about
15 percent for most growing seasons (crops)
• As a result of this, about 85 percent of the fertilizerP remains in the surface soil in forms that are only
slightly soluble, but which do contribute a small
amount of plant available-P.
P Build-UP
Soil Test P (M-3)
250
200
150
y = 0.064x + 119.785
2
R = 0.953
100
50
0
-600
-400
-200
0
200
400
600
800
1000 1200 1400 1600 1800
Net P2O5 Input
Soil test-P associated with net P2O5 input. (Lahoma502, 1971-1997).
Build-UP
• With continued annual fertilization a gradual buildup of P results in developing a soil-P condition that
will provide adequate P to meet crop needs.
• This development can be monitored by annual soil
testing, and while it varies depending on the soil and
the soil test procedure used, for the Bray P1 and the
Mehlich-3, the build up is about 1 soil test unit (lb
P/acre or pp2m) for every 15 lb P2O5 fertilizer P
added in excess of crop removal.
P Build Up
• Build up of soil-P (soil test-P) that will become available to
plants during a growing season can also be envisioned using
the reservoir diagram
• The small reservoir represents soil test-P and the large
reservoir to which it is connected represents the amount of
slowly available soil-P.
• When fertilizer additions exceed crop removal the large
reservoir eventually “fills up” to the point where the soil test
reaches 65 and fertilizer may be unnecessary for several
years.
Soil Test-P
- 65
- 40
- 20
- 0
Soil test-P in relationship to soil capacity to adsorb and
precipitate P
Correcting P Deficiencies
• Although the relationship varies somewhat for different soils, one can use
the relationship of 15 lb P2O5/ STP (unit of soil test-P) to estimate the
amount of fertilizer, and cost, required to correct a deficient soil to a
fertile soil.
• A soil that tested 15 would require about 750 lb P2O5 in excess of
harvested removal to raise the soil test to 65 (65 STP-15 STP = 50 STP; 15
lbs P2O5/STP x 50 STP = 750 lb P2O5).
• At $0.40/ lb P2O5 (a realistic price) it would cost about $300/acre to build
the soil test from 15 to 65. Estimates such as this are useful in comparing
the relative value of lands that have widely differing P fertility levels.
• Calculating the amount of P2O5 required to change a deficient soil to a
fertile soil is also useful when it is desirable to make a “long-term”
adjustment prior to starting a small-scale perennial crop or planting that
will not be cultivated.
Correcting P Deficiencies
• In a home landscape, trees and bushes may be grown more
successfully if a single large application of lb P2O5 is
incorporated into the intended rooting area prior to planting
(calculations must convert lb/acre rates to lb/1000 ft2 basis
or smaller).
• When possible, straight phosphate fertilizer (0-46-0) should
be used instead of ammonium phosphates to avoid excess N
applications, and the fertilizer should be applied a few weeks
or months before planting to allow some “aging” (water
soluble P reacting to form insoluble P) to avoid exposing the
new plants to abnormally high levels of plant available P
(H2PO4- and HPO42-).
• KSU Fertility Rec.