Spectral Reflectance-based Nitrogen Management for Subsurface Drip

Irrigated Cotton

Raj Yabaji, Kevin Bronson, Cary Green, Eduardo Segarra, and Adi Malapati

Texas A & M Univ. – Texas Agric. Exp. Stn, and Texas Tech Univ, Lubbock, TX

Introduction

• The Southern High Plains of Texas average 3000-4000 ft in elevation, 18 in. rainfall/yr and 80 in. open pan evaporation/yr.

• The densest concentration of cotton production in the world is here. 3 to 4 million ac of cotton are planted each yr. Cotton is mono-cropped.

• Half of the cotton acreage is dryland, half is irrigated from the declining Ogallala aquifer. % 70 of the irrigation is center pivot, 25 % is furrow-irrigated, & 5 % is subsurface drip.

In-season sensing of N status for Cotton

Nitrogen management for subsurface drip irrigated cotton

Objectives

• Test spectral reflectance as an N management guide for SDI cotton

• Refine the window of N fertilizer injection in SDI cotton

• Compare 32-0-0 with 28-0-05S as N fertilizer sources in SDI cotton

• Measure denitrification in SDI cotton as affected by N management

Materials & Methods

Station 1

Station 2

Station 3

Station 4

Station 5

Station 6

Station 7

Station 8

Station 9

Station 10

Station 11

Station 12

Station 13

Station 14

Station 15

Station 16

GS 28MB 32MB 32EB 28EB 32EB GS 32MB Zero 28MB 28EB 28EB 28MB GS 32EB 32MB

Rep 1 Rep 1 Rep 1 Rep 1 Rep 1 Rep 2 Rep 2 Rep2 Rep 2 Rep 2 Rep 2 Rep3 Rep 3 Rep 3 Rep 3 Rep 3

101 105 109 113 117 201 205 209 001 213 217 301 305 309 313 317

102 106 110 114 118 202 206 210 002 214 218 302 306 310 314 318

103 107 111 115 119 203 207 211 003 215 219 303 307 311 315 319

104 108 112 116 120 204 208 212 004 216 220 304 308 312 316 320

Plot plan

600 ft

8, 40-in rows

Treatments • 32-0-0 or 28-0-0 5S injected early bloom to mid bloom (5 weeks)

• 32-0-0 or 28-0-0 5S injected early bloom to peak bloom (8 weeks)

• Zero-N (1 plot/station/rep)

• Reflectance-based mgt: 0.5 X N early to peak bloom, N injection rate adjusted upwards by 100 % when

• GVI (Green vegetative index)reflectance/GVI soil test < 0.95 or statistical significance.

• Amount of N fertilizer = 150 lb N/ac (for 2.5 bale/ac) – 0-24 in soil test NO3-N – irrigation water NO3

Nitrogen requirements for high-yielding cotton

Yield goal Nitrogen requirement1

bales/ac lb N/ac

1.5 90

2.0 120

2.5 150

3.0 180

3.5 210

1Nitrogen fertilizer plus 0-24 inch NO3-N

Cropscan MSR 16

• Passive sensor (natural light)

• 16 wavebands (up & down)

• 48 inches above canopy• Within 2 hrs solar noon• Percent reflectance =

Refltd irrad @ λ /

Incoming irrad @ λ

Vegetative indices (Bausch and Duke, 1996)

• Green vegetative index = R820/R550

• Red vegetative index = R820/R650

R = percent reflectance at λ (nm)

Results

Green vegetative index for SDI cotton Lubbock, 2005

**

0

1

2

3

4

5

6

7

8

9

10

165 175 185 195 205 215 225

Julian Days

GV

I

Soil test based

Reflectance-based

Zero-N

Green vegetative index for SDI cotton Lubbock, 2006

0

1

2

3

4

5

6

7

8

9

165 175 185 195 205 215 225

Julian Days

GV

I

Soil test based

Reflectance-based

Zero-N

**

*

*

Nitrogen fertilizer injections Lubbock, 2005

0

10

20

30

40

50

60

70

80

90

100

165 175 185 195 205 215 225

Julian Days

Cu

mu

lativ

e N

Fe

rtili

zer

( lb

ac-1

)

Soil test-based to peak bloom

Reflectance-based

Soil test-based to early bloom

Nitrogen fertilizer injections Lubbock, 2006

0

20

40

60

80

100

120

165 175 185 195 205 215 225

Julian Days

Cu

mu

lativ

e N

Fe

rtili

zer

( lb

ac-1

)

Soil test-based to peak bloom

Reflectance-based

Soil test based to early bloom

Spring soil nitrate, N fertilizer amounts injected, well water nitrate, and total N supply, Lubbock, TX, 2005

N source N timing Spring

soil NO3

1

Starter N fertilizer

N fertilizer injected

Well water-NO3

Total N supply

--------------------------------- lb N/ac -------------------------------

28-0-0-5 Early bloom2 8 22 90 20 140

28-0-0-5 Peak bloom3 7 22 90 20 139

32-0-0 Early bloom2 8 22 90 20 140

32-0-0 Peak bloom3 8 22 90 20 140

32-0-0 Reflectance-based3

7 22 65 20 115

Zero-N N/A 7 22 0 20 49

Spring soil nitrate, N fertilizer amounts injected, well water nitrate, and total N supply, Lubbock, TX, 2006

N source N timing Spring

soil NO3

1

Starter N fertilizer

N fertilizer injected

Well water-NO3

Total N supply

--------------------------------- lb N/ac -------------------------------

28-0-0-5 Early bloom2 35 0 100 28 163

28-0-0-5 Peak bloom3 30 0 100 28 158

32-0-0 Early bloom2 35 0 100 28 158

32-0-0 Peak bloom3 27 0 100 28 155

32-0-0 Reflectance-based3

24 0 85 28 137

Zero-N N/A 16 0 0 28 44

Correlations of Green vegetative index (GVI), chlorophyll meter (SPAD) readings, leaf N, biomass and lint yield,

SDI cotton Lubbock 2005-6 GVI-early bloom SPAD-early bloom GVI-mid bloom SPAD-mid bloom

2005

Leaf N-early bloom 0.45 - -

Biomass-early bloom 0.37 0.44 - -

Leaf N-mid bloom - - 0.70 0.25

Biomass-mid bloom - - 0.31

Lint yield 0.36

2006

Leaf N-early bloom 0.70 0.59 - -

Biomass-early bloom 0.58 0.44 - -

Leaf N-mid bloom - - 0.85 0.54

Biomass-mid bloom - - 0.47 0.51

Lint yield 0.53 0.28 0.58 0.33

Leaf N SPAD NDVI_cropscan NDVI_greenskr 550nm

Biomass -0.33 -0.21 0.48 0.58

Leaf N 0.37 -0.28 -0.46 -0.47

SPAD -0.32 -0.38 -0.26

NDVI_crops 0.75 -0.20

NDVI_grnsk 0.20

Cropscn_550

Correlations with chlorophyll meter (SPAD) and spectral reflectance at early bloom Lamesa, 2003

Means: Biomass – 704 kg/ha, Leaf N – 3.9%, NDVI_cropscn – 0.43, NDVI_grnskr – 0.52

Leaf N SPAD NDVI_cropscan NDVI_greenskr 550nm

Biomass 0.30 0.34 -0.29

Leaf N 0.53 0.52 0.43

SPAD -0.30

NDVI_crops 0.47 -0.31

NDVI_grnsk

Cropscn_550

Correlations with chlorophyll meter (SPAD) and spectral reflectance at early bloom Lamesa, 2004

Means: Biomass – 1954 kg/ha, Leaf N – 3.7%, NDVI_cropscn – 0.73, NDVI_grnskr – 0.86

0.21

-0.32

N source N timing N

fertilizer injected

Green veg. index

Chlor. meter

Leaf N Biomass

% lb/ac

28-0-0-5S Early bloom1 90 8.7 a 42.8 a 3.5 b 4548 a

28-0-0-5S Peak bloom2 90 8.5 a 41.5 ab 3.6 ab 5810 a

32-0-0 Early bloom1 90 8.5 a 42.6 a 3.5 b 6204 a

32-0-0 Peak bloom2 90 8.8 a 41.0 b 3.7 a 6609 a

32-0-0 Reflectance-

based2 65 8.0 b 40.4 b 3.3 c 5695 a

Zero-N N/A 0 6.4 c 38.9 c 2.9 d 5786 a 1 Injected from 20 June to 22 July 2 Injected from 20 June to 12 Aug

Mid-bloom biomass, green vegetative index, chlorophyll meter readings as affected by N management, Lubbock, TX, 2005

Mid-bloom biomass, green vegetative index, chlorophyll meter readings as affected by N management, Lubbock, TX, 2006

N source N timing N

fertilizer injected

Green veg. index

Chlor. Meter

Leaf N Biomass

% lb/ac

28-0-0-5 Early

bloom1 100 7.8 a 48.2 a 3.36 a 5823 a

28-0-0-5 Peak

bloom2 100 7.5 b 48.1 a 3.58 a 5705 a

32-0-0 Early

bloom1 100 7.6 ab 49.4 a 3.40 a 6261 a

32-0-0 Peak

bloom2 100 7.7 ab 49.4 a 3.60 a 5902 a

32-0-0 Reflectance-

based2 85 7.0 c 47.9 a 3.49 a 5430 a

Zero-N N/A 0 5.0 d 39.9 b 2.61 b 3056 b

N source N timing N

fertilizer injected

Total N uptake

Recovery efficiency

Biomass Seed yield

Lint yield

------- lb N/ac ------ % ------------- lb/ac ---------------

28-0-0-5S Early bloom1

90 - -

- 2611 a 1865 a

28-0-0-5S Peak bloom2

90 - -

- 2598 a 1829 a

32-0-0 Early bloom1

90 - -

- 2629 a 1879 a

32-0-0 Peak bloom2

90 160 a 63 a 9647 a 2549 a 1812 a

32-0-0 Reflectance-based2

65 143 a 62 a 9164 a 2511 a 1817 a

Zero-N N/A 0 103 b - 8047 b 2072 b 1620 b 1 Injected from 20 June to 22 July 2 Injected from 20 June to 12 Aug

First open boll biomass, N uptake, seed and lint yields as affected by N management, Lubbock, TX, 2005

First open boll biomass, N uptake, seed and lint yields as affected by N management, Lubbock, TX, 2006

N source N timing N

fertilizer injected

Total N uptake

Recovery efficiency

Biomass Seed yield

Lint yield

------- lb N/ac ------ % ------------- lb/ac ---------------

28-0-0-5 Early bloom1

100 - - - 2002 a 1430 a

28-0-0-5 Peak bloom2

100 - - - 2094 a 1496 a

32-0-0 Early bloom1

100 117 a 71 a 8450 a 2038 a 1456 a

32-0-0 Peak bloom2

100 120 a 75 a 8636 a 2038 a 1456 a

32-0-0 Reflectance-based2

85 107 a 71 a 8048 a 1970 a 1407 a

Zero-N N/A 0 46 b - 4285 b 1324 b 1034 b 1 Injected from 12 June to 17 July 2 Injected from 12 June to 7 Aug

Year Irrigation Lint yield Recovery effcy-diff

Recovery effcy-15N

N application details

lb/ac --------- % ---------

2001 Sub drip 1230 60 31 3 injections of 30 lb/N/ac

2001 LEPA 1321 42 37 3 injections of 30 lb/N/ac

2002 LEPA 1227 40 - 3 injections of 30 lb/N/ac

2005 Sub drip 1678 63 - 30 injections in 8 wks

2006 Sub drip 1407 71 - 31 injections in 8 wks

2006 Furrow 1220 20 - Side-dress at 1st square

Recovery efficiency of 90 lb fertilizer-N/ac in cotton plants

Summary• Green vegetative index related to leaf N and

biomass, and responded to N fertilizer treatments. GVI reflected net N mineralization in zero-N plot.

• We tested spectral reflectance-based N management, i.e. initial N injections of ½ X of soil test recommendations, and adjusting upwards as GVI falls significantly below soil test management for 2 ½ bale/ac yield goal.

• Spectral-reflectance N injection saved 28 and 15 % N compared to soil test-based management for 2005 and 2006, respectively.

Summary cont.

• Recovery efficiency of N fertilizer injected daily in subsurface drip irrigated cotton is high, i.e. 62-75 %

• Denitrification from N fertilizer injections in SDI cotton was not detectable.