Crop Water and Soil Fertility
Relationships
Bob Glodt
Agri-Search, Inc.
Plainview, Texas
“It ain’t what you
don’t know that
gets you into
trouble. It’s what
you know for sure
that just ain’t so.”
Mark Twain
There is no soil test that can
compensate for a poorly
collected sample!
Basics of Soil Sampling
• Soil sampling depth must
be consistent.
• Sub-sample size must be
consistent.
• Samples should be crop
or field specific.
• Use plastic bucket when
collecting samples.
• Keep samples cool and
deliver to the lab
immediately.
• 12 – 15 cores per sample.
Liebeg’s Law of
the Minimum - 1840
Yield is
proportional to the
amount of the
most limiting
nutrient.
N, P, and Zinc
• These are the three most important nutrients
that affect cotton, corn, grain sorghum, and
wheat yields.
• A good fertility program should meet the
crop needs for these nutrients first.
• Growers who own their own land or with
long term lease agreements should consider
build maintenance fertility programs for the
non soluble nutrients (P and Zinc)
Nutrient Management
Based on Water Solubility
Insoluble Nutrients
Soluble Nutrients
• Phosphorus
• Zinc
• Nitrogen
• Sulfur
Sufficiency Guidelines For P in
Corn Production (Kansas State University)
Soil Test
Bray P1
ppm
0-5
5-10
10-15
15-20
20+
Removal
Corn Yield Goal in Bushels Per Acre
60
55
40
25
15
0
20
100
140
180
220
60
45
25
15
0
33
Lb. P205/A
70
50
30
15
0
46
75
55
30
15
0
59
80
60
37
15
0
73
Maintenance Build Phosphorus Rec.
2008 – 2015 Gunter Farm (E ½ Pivot)
82
CORN
+ 53 P
69
CORN
+ 42 P
2010
82
COTTON
+0P
2009
COTTON
+0P
CORN
+ 48 P
5
COTTON
+0P
58
15
10
79
76
CORN
+ 62 P
PPM – Soil Test
25
20
96
93
CORN
+ 45 P
30
2012
2013
2014
2015
0
2008
2011
Maintenance-Build Zinc Rec.
2008 – 2015 Gunter Farm (E ½ Pivot)
2
1.8
1.4
1.2
2008
2009
2010
2011
2012
CORN
+ 0 Zn
CORN
6 LBS. Zn
0
COTTON
+ 4 LBS. Zn
0.2
CORN
+ 3 LBS. Zn
0.4
COTTON
+ 0 Zn
0.6
CORN
+ 6 LBS. Zn
0.8
COTTON
+ 0 Zn
1
CORN
+ 0 Zn
PPM – Soil Test
1.6
2013
2014
2015
Cotton Nutrient Uptake
• Soil nutrients are taken up in direct
proportion to growth and temperature.
• Total nutrient uptake for N, P, and K tracks
cumulative heat units precisely.
• During the spring growing period when heat
units are low, cotton grows slowly and takes
up only limited amounts of nutrients.
• It is during the peak growing months of June,
July, and August when nutrients need to be
available.
Days After Planting
maturity
first open boll
first bloom
first
square
emergence
Inches Per Day
HYPOTHETICAL WATER USE
CURVE FOR COTTON (INCHES PER DAY)
Cow Feeding Program?
Put all the hay
out in the fall
and come back
and check on
the cows in the
spring.
Nitrogen Uptake - Cotton
• Cotton stores N in the leaves during periods of
greater availability for later use during the peak
demand period of boll setting.
• The highly soluble nutrients are swept into the
roots with the soil water that is pulled up
through the plant and out the leaves (via
transpiration).
• Mass flow is greatest when cotton is well
watered.
• The uptake of most nutrients, not just N, is
reduced when crop water use is decreased from
drought, cold weather or wet humid weather.
Prescription For Top Yields
• Determine reasonable yield goals based on your
ability to irrigation at a given percentage of PET.
• Meet N, P, and Zinc demands for the crop based on
accurate soil tests and reasonable yield goals.
• P and Zinc are immobile and therefore, should be
applied and incorporated into the soil prior to
planting.
• Nitrogen demand increases at approximately the
same rate as heat units are accumulated.
• Split nitrogen applications to coincide with demand
and drive nitrogen down to the effective rooting
depth.
Nitrogen Leaching Versus
Denitrification
Denitrification
Leaching
• A major event when
• A physical event.
soils are saturated.
• Loss of NO3
nitrogen as it moves • Saturated soils
remove oxygen from
with the water
the soil.
below the root zone.
• Oxygen is required
for root tips to grow
where N is taken up.
Which Irrigation System Best
Minimizes Nitrogen Loss?
LEPA (ALTERNATE FURROW)
BROADCAST SPRAY
Which Irrigation System Is Best Suited
For In Season Nitrogen Applications?
LEPA (ALTERNATE FURROW)
BROADCAST SPRAY
Which System Is Best Suited For
Meeting Water Crop Demand?
LEPA (ALTERNATE FURROW)
BROADCAST SPRAY
THANK YOU FOR
YOUR ATTENTION!