Phosphorus cycling in grazed, steep hill country : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University

Copyright is owned by the Author of the thesis. Permission is given for

a copy to be downloaded by an individual for the purpose of research and

private study only. The thesis may not be reproduced elsewhere without

the permission of the Author.

PHOSPHORUS CYCLING IN

GRAZED , STEEP HILL COUNTRY

A thesis presented in par tial fulfilment of the requirements for the degree o f Doc tor o f P hilosophy in So il Science

a t Massey University

Allan Gilber t Gillingham 1 9 78

( i )

MS TRACT

Measurements of P uptake by pas ture and P return in pas ture l i tt er and dung \\rere made on two intensively gra zed , nor th- and south-facing

paddocks in steep hil l country with soils of moderate to high P s ta tus . Both P uptake and mor e particularly the return of P in dung by grazing sheep , was high on the rela tively level campsite areas but decreased

sharply as ground surface slope increased . A high graz ing pressure ens ur ed that P return via l itter was low relative to plant up take .

A net P balance d erived for d iffering paddock s trata showed a large net annual gain of P on c amps ites ( 50 . 1 and 1 1 9 . 8 kg ha -1 on north and south aspec ts , respec tively) but a cons id erab l e net P lo ss from bo th 25 s lopes 0

( 19 . 5 and 10 . 0 kg ha- 1on nor th and south aspec ts , respec tively) a nd 4 5°

- 1

slopes ( 1 5 . 3 and 1 3 . 8 kg ha o n nor th and south aspec ts , r espec tively) . Dif f erences bet\veen aspects in the net P balanc e could b e expla ined by the overall differenc e in the topography of the two paddocks as it a f f ec ted relative ca mping and graz ing pressure on each s tratum .

Sub s equ ent simula tion s tudies \vere conduc ted using a ma thema tical mod el based on f ield da ta f rom the nor th-aspec t paddock and val ida ted agains t resul ts from the south-aspec t paddock . Resul ts o btained f rom the mod el indica ted that the quantity of P transf er f rom slopes inc r ea s ed at a

grea ter than direc tly propor t ionate rate a s s tocking rate increas ed and was a:ls o d irec tly related to pas ture P content .

Determina tion o f relative roo t ac tivity using 32 P sho wed tha t

approximately 90% o f P up take by pas ture in spring occurred from wi thin 7cm of the soil surface . The greater proportion of this o ccurred within the 0-3cm s oil dep th . No s ignificant P uptake occurred from depths grea t er than 30cm . Al though the extent of P uptake from 0-3cm depth soil was

(ii)

similar both upslope and dmvnslope from. a P sourc e , the d ir ec tion o f

predominant root activity a t grea ter dep ths was affec ted by the s teepness of s lope, tending to b e a t an angle between ver tical and that normal to the soil surface .

A technique was developed to charac terize shor t term plant-available P ,

. 32 33

using bo th P and P . Resul ts indicated tha t the 32P!33P ra tio o f the wat er-extrac table P f raction more closely res embl ed tha t in the plant than was the case for the O l s en P extrac t . Both ryegrass (Lolium perenne) and white clover ( Trifoliu� repens) apparently u tilized P from the same soi l pool , the measured higher'P content of ryegrass in this s tudy being due only to a more extensive· and rapidly developing roo t sys tem than that o f c lover .

The addition to soil of P extracted by wat er from l i tter , dung , and superphosphate sources showed that all forms had similar effec ts in

inc r easing the wa ter-ex trac table and Olsen P l evels in the soil . Thus i t could b e expec ted that P from these three sour c es would have ^a s imilar ava i lability to plants . The results of these and also P desorpt ion experiments were qualitatively similar to tho s e derived f rom a s imp l e

Langmuir model , sugges t ing that sorption and d esorption o f P in the s o il from the f ield area o c curred a t s ites on the solid phas e with predominantly uniform sorption charac t eris tic s .

Marked and largely unexplained varia tions. in s everal soil parameters monitored over a year obscured the eff ec ts of P addition as a maintenance

fer til izer applica tion and also the net P transfer by grazing animals . In this and related s i tua tions, so il P analysis may no t provide a s ensitive measure of P s tatus, excep t in the longer term . A mor e detailed examina tion

is r equired to assess the usefulness of 'routine soil P analysis of hill

(iii)

so ils for advisory purpo s es . The signif icant net P traps fer from

slopes by graz1ng animals suggests tha t the complementary roles of gra zing manag ement and fer tilizer r equirements in hill country should also b e

examined further .

ACKNOWLEDGEMENTS

I wish to express my gra ti tude to the following people :

Professor J . K . Syers for guidance, direction� and s timula tion throughout all s tages of this s tudy .

Dr . P . E . H . Gregg for many helpful discussions and comments .

Mr . R .W . Tillman for the d evelopment of computer programmes and for many valuable d iscus sions .

The several s taff from the Hill Country Sec t ion , S o il and Field Research Division, Ministry of Agriculture and Fisher ies and Wha tawhata Hill Country Research·S ta tion who assis ted with the f i eld work .

(iv)

Jan , (my wifeL for her encouragement, und ers tanding , and assistance d uring the term of this s tudy .

D ianne Syers for the excell ent and patient typing o f this thesis . F inally , I w ish to express my thanks to the New Zealand Minis try o f Agricul ture and Fisheries vTho granted m e leave to und er take this study .

(v)

TABLE OF CONTENTS

Page ABSTRACT ^{• . . .}

AC KNOWLEDGEMENTS iv

TABL E OF CONTENTS _V

LIST OF FIGURES xi

LIST OF PLATES xiii

LIST O F TABLES

S ECTION 1 INTRODUCTION

S ECTION 2 REVIill� OF LITE��TURE

2 . 1

2 . 2

Soil P

2 . 1 . 1 Forms of soil P . 2 . 1 . 2 The P sorption proc ess 2 . 1 . 3 Exchangeab le P

2 . 1 . 4 Assessment of plant-availabl e P . 2 . 1 . 4 . 1 Iso tope methods

2 . 1 . 4 . 2 Non-iso tope methods Plant ^p . ^. . . .

2 . 2 . 1 Fac tors affecting plant intake of p

2 . 2 . 2 Fac tors affec t ing the rate of p uptake ---

2 . 3 2 . 4

2 . 2 . 3 Plant P content . . . .

2 . 2 .lf Interspec ies competitibn for Animal P Intake

P Return to Soil

2 . 4 . 1 P return via animal dung

. . ^{. . .} p

.

. .

. . .

2 . 4 . 1 . 1 Dung d is tribu tion by grazing stock 2 . 4 . 1 . 2 Dung excr eted and area covered

. . . . ^.

xiv

1

4 4 7 1 1 1 3 14 17 1 9 1 9 2 1 22 23 25

28 29 29

�excreti�� ^{. •} . ^• . ^• . ^{. . • . •} . ^{. •} 3 1

2 . 5

2 . 6

2 , 7

2 . 4 . 1 . 3 Dung P content ^{. . • .} 2 . 4 . 1 . 4 Rel eas e of P from dung 2 . 4 . 2 P r eturn via pas ture litter

2 . 4 . 2 . 1 Types o f lit ter 2 . 4 . 2 . 2 Plant s enesc ence

2 . 4 . 2 . 3 Release of P from l i tter Availab ility of Returned and Applied P ^{. •}

2 . 5 . 1 P f rom organic residues 2 . 5 . 2 P from fertil izer

Above Ground P Loss es

2 . 6 . 1 p los ses in animal produc ts 2 . 6 . 2 p losses by animal transfer 2 . 6 . 3 p losses in surface runoff General Conclusions . . . .

SECTION 3 .

\vater . . . .

PHOSPHORUS UPTAKE ^�� RETURN IN GRAZED , STEEP HILL PASTURES 3 . 1

3 . 2

Introduction ^{. • . . .} Ma terials and Methods 3 . 2 . 1 Trial site 3 . 2 . 2

3 . 2 . 3 3 . 2 . 4

3 . 2 . 5

Gra z ing management Sampling procedure Phosphorus up take .

. . . . .

3 . 2 . 4 . 1 Pas ture dry matter production 3 . 2 . 4 . 2

3 . 2 . 4 . 3

Assessment of P l imita tion to .E_?s ture dry mat ter produc tion

Pas ture P conc entration Phosphorus return .

3 . 2 . 5 . 1 To tal dung dry weight 3 . 2 . 5 . 2

3 . 2 . 5 . 3 3 . 2 . 5 . 4

Field trial dung for P analysis Dung P concentration

Total lit ter d ry weigh t 3 . 2 . 5. 5 Litter P concentra tion 3 . 2 . 6 S o il phosphorus

Page 32 34 35 36 3 7 38

42 4 3 4 6

4 9 50 5 1 5 1 5 3

55

57 57 6 1 6 1 6 1 6 1

6 2 6 3 63 64 64 65 66 6 6 6 8

(vi)

3 . 3 Results and D iscussion ^{• . � .} . . . . ^. . .

3 . 4

3 . 3 . 1 Phosphorus �p�ak

;

.

3 . 3 . 1 . 1 Pas ture dry matter produc tion =---

3 . 3 . 1 . 2 As s essment of P l imi ta tion to pas ture growth . ^{. . . . . •}

3 . 3 . 1 . 3 Pas ture species compo s ition 3 . 3 . 1 . 4 Pas ture P conc entrat ion 3 . 3� 1 . 5 To tal P uptake by pas ture 3 . 3 . 2 Phosphorus return via dung

3 . 3 . 3

3 . 3 . 2 . 1 To tal dung dry �oJe:lgh t

3 . 3 . 2 . 2 Estimated dung dis tribution pattern 3 . 3 . 2 . 3 Dung P content ^{. . . . •}

3 . 3 . 2 . 4 To tal P return in dung Phosphorus return via pasture litter 3 . 3 . 3 . 1 To tal l i t ter dry weight 3 . 3 . 3 . 2 Litter P content

3 . 3 . 3 . 3 Total P return in l i tter 3 . 3 . 3 . 4 Es tima ted pas ture u tilization

by sheep ^{. . . . • .} 3 . 3 . 4 The phosphorus cycle

3 . 3 . 4 . 1 Annual uptake and r eturn of P 3 . 3 . 4 . 2 Seasonal uptake and return o f P 3 . 3 . 4. 3

3 . 3 . 4 . 4

Dung P and litter P rela tionships Dung P return and net P balance rela tionships . ^{. . . • . . . •} 3 . 3 . 5 Soil phosphorus ^{. • • .}

3 . 3 . 5 . 1 To tal phosphorus 3 . 3 . 5 . 2 Soil DIP

3 . 3 . 5 . 3 Soil DOP .

3 . 3 . 5 . 4 Bicarbona te-extrac table P General Discussion . ^• . ^{. • •} . ^• . ^{• .} .

SECTION 4

UPTAKE ZONES FOR PHOSPHORUS BY PASTURE ON DIFFERING S TRATA WITHIN A HILL PADDOCK

4 . 1 Introduc tion . . ^. . .

. . .

Page 68 68 68

7 0 7 1 7 4 7 8 7 8 78 82 8 5 94 9 6 9 6 9 8 1 0 2

104 107 107 1 10 1 1 3

1 1 6 1 2 1 1 2 1 1 2 7 1 30 1 3 0 1 34

(vii )

4 . 2

4 . 3

4 . 4

Mater ials and Methods 4 . 2 . 1 Treatmen ts . . . . 4 . 2 . 2 Isotope preparation 4 . 2 . 3 Isotope placement 4 . 2 . 4

4 . 2 . 5 4 . 2 . 6 4 . 2 . 7

Soil measurements Pas ture harves ting

3 2

Measurement of P activity Processing of results

Resul ts and Discussion

4 . 3 . 1 Sample prepara tion comparison 4 . 3 . 2 Field resul ts

4 . 3 . 2 . 1 P uptake a fter 2 da�

4 . 3 . 2 . 2 P uptake af ter 29 days General Discussion . .

SECTION 5

(viii)

Page 144 144 1 4 5 1 4 7 148 148 150 1 5 1 152 1 5 2 ;

1 54 1 56 1 56 1 6 9

THE ASS ES SMENT OF SOME P SORPTION CHARACTERISTICS OF SOILS FROM THE FIELD TRIAL AREA

5 . 1 5 . 2

5 . 3

Introduc tion

Ma terials and Methods ^{. . . . •} 5 . 2 . 1 Labora tory experiments .

5 . 2 . 2

5 . 2 . 1 . 1 Measurements of P desorption

5 . 2 . 1 . 2 Evaluation of P exchange reac tions 5 . 2 . 1 . 3 Measurements of P sorption

The Langmuir model . ^• . .

5 . 2 . 2 . 1 Measurements of P desorption (Langmuir

173 1 7 5 1 7 5 1 7 5 1 7 7 1 7 8 1 7 9 model) . ^• . ^{. • •} . ^{. . . . . . •} 180 5 . 2 . 2 . 2 Fa te o f an appli ed ' label ' (Langrnuir model) 180 5 . 2 . 2 . 3 Measurements of P sorption .

Results and Dicussion . ^{. . . • .} 5 . 3 . 1 Measurements of P desorption

5 . 3 . 1 . 1 Effec t of wa ter : soiL ratio and extrac tion period on desorption of P . ^{• . •} . . 5;3 . 1 . 2 Effect o f success ive extrac tions with

wa ter on desorption of P

18 1 1 8 1 18 1

1 8 1

184

5 . 4

5 . 3 . 1 . 3 Effec t o f antecedent .soil mois ture content on desorption of P

5 . 3 . 2 Evaluation o f P exchange reac tions

5 . 3 . 2 . 1 Fa te of appli ed carrier-fr ee 32P . 5 . 3.2 . 2 Effect o f soil P s tatus on net

sorption of appl ied carrier-free 3 2p from soil solution

5 . 3 . 3 Effec ts o f the addition to soil of P from various sources . . .

3 1

5 . 3 . 3 . 1 Changes in extract P content 5 3 3 2 eh . . . anges 1n extrac t . . 32p content General Discussion . . ^. .

SECTION 6

CHARACTERIZATION OF SHORT-TERM, PLANT-AVAILABLE P USING A DOUBLE LABELLING TECHNIQUE

6 . 3

6 . 4

Introduc tion ^{. • •}

Materials and Methods

6 . 2 . 1 Prepara tion of plants 6 . 2 . 2 Preparation of soil ^{• .}

6 . 2 . 3 Sampling and analysis proc edures 6 . 2 . 3.. 1

6 . 2 . 3 . 2

Soil analysis

Plant sampling and analysis Results and Discuss ion

6 . 3 . 1 6 . 3 . 2 6 . 3 . 3

3 1

Soil P l evels . . Plant 3 1P levels

32 33 .

So il and plant P : P rat1os ^{• .} General Discus s ion . ^{. . • • . . • •}

SECTION 7

SIMULATION OF "ABOVE-GROUND" COMPONENTS OF THE P CYCLE IN STEEP , GRAZED PASTURE

7 . 1 I ntroduction . ^{• .}

Pag e

1 8 8 19 1 1 9 1

1 9 5

200 200 206 208

2 13 214

216 216 217 218 218 2 2 0 2 2 0 2 2 3

2 2 7 (ix)

7 . 2

7 . 3

7.4

Materials and Method s . ^•

7 . 2 . 1 Effec t of s tocking ra te on the net P balance on each s tratum

7 . 2 . 2 Effec t of overall pas ture utilization level on the net P balance on each s tra tum . 7 . 2 . 3 Effec t of pas ture produc tion l evel on

the net P balance on each stratum . 7 . 2 . 4 Effec t of pas ture P content on the net P

balance on each stra tum Results and Discus s ion .

7 . 3 . 1 Mod el valida tion 7 . 3 . 2 Model manipulations ^•

7 . 3 . 2 . 1 Eff ec t o f s tocking ra te on the net P balance on each s tra tum .

7 . 3 . 2 . 2 Eff ec t of overall pas ture utiliza tion l evel on the net P balance on each stra tum .

7 . 3 . 2 . 3 Eff ec t o f pa s ture produc tion and P concentration level on the net P balanc e on each stra tum

General Discussion . . .

SU�1ARY AND CONCLUSIONS ^• B I BLIOGRAPHY .

APPENDICES . .

Page

2 2 8

2 2 9 2 30 230

23 1 2 3 1 23 1 2 3 3 233

2 3 5

2 37 237 243

2 8 1 (x)

FIGURE A

2 . 1 3 . 1 3 . 2

3 . 3

3 . 4

3.5

3.6 3 . 7

3 . 8 3 . 9 3 . 10 4 . 1 4 . 2

LIST OF FIGURES .

"

Slope : area distribution curves for the total area , for campsites and for tracks in paddocks of b o th north- and south-facing aspects ^{. . •}

Components of the P cycle in grazed , fertilized pasture ^{. . . • . . . . •} Relationship b e tween the concentration of DIP and TP in pas ture ^{• . • .} . . ^. Annual total P uptake by pas ture and total P return in l i t ter and dung , and net total P balance (kg ha- 1) including S tandard

Errors of es timate for each value ^{• • . • •} Rela tionship b e tween tota l P return in litter and to tal P return in dung in each s ea son

(as % of P uptake by pas ture) ^{. . • o • •} Rela tionship b ett.:reen net total P balance and total P return in dung in each s eason

(kg ha- 1) ^{• . . . o . • • • • • • • • • o •} Relationship b e tween net total P balance and to tal P return in dung in each season

(as % of total P up take by pas ture) ^{• • .} - 1

Soil total P content (� g g ) on campsites

' 1 1 p ( - 1 ) 2 5° ¹

So1 tota content � g g on s opes . Soil total P content (�g g-l) on 4 5° s lopes ^• Soil DIP ( as % o f TP)

Ols en P (as % o f TP) ^•

Effec t of sample s ize on measured 32 P at

Page

285

5

7 7

108

1 1 5

1 1 7

1 20 1 2 2 123 1 2 4

• 1 28 1 3 3

three levels o f a ctivity ^{• •} . ^• . ^{. • • •} . . . 1 53 Lateral and vertical dis tribution o f

relative roo t a c t ivity ( % ) (Correc ted for

soil P sta tus at each d epth) ^{• • • •} 1 6 1 4.3 Lateral and vertical dis tribution of relative

root activity ( %) adj acent to s tock tracks

(Corrected for soil P s ta tus a t each dep th) ^{• . • • •} 1 6 3

(xi)

4 . 4 Relative root ac tivity at each depth for the north campsit e , nor th 25° s lopes , and north 45° slopes , and estima ted integrated relative roo t activity for each soil dep th range on nor th camps ite , north 25° slope and north 4 5°

4 . 5

5 . 1

5 . 2 5 . 3

5 . 4

5 . 5 5 . 6

5 . 7

5 . 8

5 . 9

slope ^{. . . . . . . • .}

Calcula ted mid point of la teral roo t activity distribution at each sampling depth and

derivation of the d irec tion of predominant

root ac ti vi ty on each s tra turn . ^{. . . • • • • . . • •} Effect of '"a ter : soil ratio ·and extraction

period on desorp tion of P from north ₀ 25 slope soil ^{• • •}

Relationship bet\veen DIP release per

extraction and cumulative total DIP releas e ^• Rela tionship b etween P release per

extraction at a 40 : 1 wa ter : so il ratio (Langmuir model ) ^{. • • . • . . . •} Effec t of antecedent soil mois ture on desorp tion of DIP ^{• • . . . . • .} . . Fat e o f carrier-free 32P applied to north 25° s lope soil during subs equent incubation Change in iso tope content during incuba tion of water and Olsen P extrac ts from soils of differing P s ta tus ^{. . . . • • . . . . • .} Change in isotope content (Rela tive Specific Ac tivity) during incubation of Hater and O l s en P extracts from soils o f differing P s tatus . Change in 31P content of A wa ter-extrac table P ( 10 : 1 ratio) , B Mater-extrac table P ( 40 : 1 ratio) and C Ols en P a t various t imes following add i t ion to soil of water-extractable P from superphosphate , l i tter , dung and control - nil added P . ^{. . •} . ^{. . . . . . . . . . . . •} Change in 32P content o f A water-extractabl e P

( 10 : 1 ratio ) , B water-extrac table P ( 40 : 1 ra tio) and C Ols en P at vario us times following

addition to soil of 32P-labelled '"ater­

extrac table P from superphosphate , litter, dung and control - carrier. free 32p added ^{• • •}

(xii)

Page

165

167

182

185

1 8 6

1 9 0

1 9 3

1 9 6

1 9 9

20 1

207

LIST OF PLATES

PLATE 3 . 1 S tudy area paddock with a north-facing aspect ^• 3 . 2 S tudy area paddock with a south-facing aspec t ^• 3 . 3 Dung and l itter collection plo t .

Dung was collec ted from the complete ( 0 . 2 m2) plot and l i tter was collected f rom only the 8 subplo ts occupying the c entral and left side longitudinal thirds .

4 . 1 Equipment used for loading gela tin capsules with 3Zp , showing capsule

holder in posi tion on the mobile p la tform o f the modified b eam balance . ^.

4 . 2 Wooden template in posit ion above th e

"ho t l ine" with rods us ed to make

isotope placement holes to required dep th 4 . 3 Pasture on a 25° s lope plot prior to

harvest 29 days following isotope p lacement . Perspex sheet in pos i t ion to shield operator from the "ho t l ine" and ba ttery-powered harves t ing shears ready for use . ^• . ^•

6 . 1 Ryegrass grown in sand within nes ted pots ( lef t side) and shm.,ring root development (centre) prior to placement in contact with isotope-labelled and incubated soil

( right side) ^{• • • • • • • . .} . . . ^.

(xiii)

Page 58 58

6 7

1 4 6

1 4 6

1 4 9

2 1 5

TABLE 3 . 1

3 . 2

3 . 3

LIST OF TABLES

Paddock slope composition (% of to tal area per s tratum)

S easonal and annual pas ture produc tion (kgDM ha- 1 ) from each s tratum ^{. . . • •} S easonal and annual pasture produc tion from topdressed and untopdres s ed s lope s trata (kgDM ha- 1 ) . ^{• • . . . •. •}

3 . 4 Pas ture species composi t ion ( % by wt . )

3 . 5 3 . 6

3 . 7 3 . 8

3 . 9

3 . 10

3 . 1 1

in each season

Amounts of P forms in pas ture in each season S easonal and annual up take of P by

pasture on each s tratum (kg ha- l ) . Seasonal and annual deposi t ion of dung on each s tratum (kgDM ha- l)

Estima ted time required for dung f rom grazing sheep to completely cover various proportions of each s tratum . ^{. . . • • •} . ^{• •} Dry matter , TDP and TP content of who l e , undried sh eep dung from " improved" compared with

"unimproved" pas tu re . ^{. . • • •} . ^{. • .} . Amounts of P forms (f.lg g ) in var iously -1

prepared forms o f dung as determined by two methods o f analysis ^{. . . •} . ^• . ^{• .}

- 1 Amounts o f water-extractab le P (f.lg g )

in variously prepared forms of dung ^{• .} 3 . 1 2 Wat er-extractable P removed from oven-dried ,

crushed dung by both rep ea t ed and prolonged extrac tions ^{• .} . . .

3 . 1 3 Amounts o f P forms in dung collec ted

3 . 14 3 . 1 5

at each s tratum . . .

Seasonal and annual re turn o f P in dung to each s tratum (kgDM ha-1) . ^{• •} . ^{• •}

S ea sonal and annual pas ture lit ter accumulation on each s tratum (kgDM ha- 1 ) ^{. • . . . • • . • •}

(xiv)

Page

6 0 6 9

7 2

7 3 7 5

7 9 8 0

8 4

8 6

8 7 8 9

9 0 9 3 9 5

9 7

TABLE 3 . 1 6

3 . 1 7

3 . 1 8

3 . 19

3 . 20 3 . 2 1

3 . 22 3 . 23

4 . 1

4 . 2

4 . 3

5 . 1

Amounts of P forms in l i t ter collec ted at each season ^{. •}

Eff ec t o f sample condition on amounts of wa ter-extrac tabl e inorganic (DIP ) and organic P (DOP ) in autumn l i tt er . ^{. . •} Seasonal and annual return o f total P in litter to each s tratum (kg ha- 1 )

Es timated seasonal and annual utilization of pas ture on each s tratum by grazing sheep Seasonal to tal P net balance on each stra tum Seasonal to tal P net balance on each stratum assuming a 3-month delay in . availability of P from litter and dung (kg ha- 1 ) ^{• .} Organic ma tter content of so il on each stra tum at four sampling dates ( % by wt . ) Water-extractable organic P content of

soil on each s tratum (as % of TP) .

Mean 32P levels ( cpm) measured in \..:rhole , undried pas ture two days following iso tope placement in soil at various dep ths ^{• • . . • • •}

Relative root activity ( %) a t va rious soil depths on each stra tum es timated both with and without correc t ion for soil Olsen P values Soil charac teristics on each s tratum at trial corr�encement ( 1 3 . 9 . 76) and also soil mois ture content at f inal harves t - Nor th aspec t . . .

Effect of solution soil ra tio on extraction of P (Langmuir model) ^{• • • . • . . •} 5 . 2 To tal water extrac table P ( 1TIP) from air-dried

soil from each s tratum of the field trial area and moisture content of each soil during

5 . 3

sub sequent incuba tion .

Effect of soil P status on fate o f applied label and time required to rea ch equilibrium

(Langmuir model) . ^{• • . • • • • • • • •} . ^•

(xv) Page

9 9

1 0 1

1 03

1 0 5 1 1 1

1 1 2

1 26

1 3 1

1 57

1 58

1 59

1 8 3

1 8 9

1 9 7

TABLE 5 . 4

5 . 5

6 . 1

6 . 2

7 . 1

7 . 2

7 . 3

7 . 4

Effect of soil P sta tus on fate of applied P ( Langmuir model) ^{• . . •}

S ta tus o f severa l soil P fra c t ions after 53 days incuba tion both with and withou t addition of wa ter-extrac tab le P f rom pasture li tter , dung and superphospha te sources ( �g p g- 1 ) ^{. . • • . . • • .}

Soil P levels before plant P up take and so il and plant P levels af ter plant P uptake ( � g g- 1 ) . . ^{. .} . ^. .

32p 33p : ra tlos ln sol ^{. . ' 1} b f e ore p.₁ ant . _p uptake and in soil and plant ma terial after plant uptake ( 32Pcpm : 33pcpm)

S imulated P uptake and return in paddocks on north- and south-facing aspec t s

(Sec t ion 3 ) incorpo ra ting paramet er values measured in the field . ^. . .

S imulated P uptake and return in paddocks of differing slope composi tion a t both full and half maximum stocking ra tes . . Simula ted P uptake and return a t maximum pasture grazing utilization l evels 65 , 70 and 55% o f pasture growth on campsites , 25° slopes and 45° slopes resp ec t ively in paddocks o f differing average slope

S imulated P uptake and return at

(i) pasture produc tion levels equivalent to 80% o f those set in Sec tion 7 . 2 . 1 and (ii) at a pasture P content o f 3500�g g- 1

using the same pasture production levels set in Section 7 . 2 . 1 ^{. • • . • . • . . • •}

(xvi)

Page

203

205

2 1 9

222

232

2 34

236

2 38