Introduction Objectives Materials Conclusions Results ...

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(manufactured by Foremost Holdings Ltd). n Commercially available Gafsa reactive phosphate rock. n Ten DCPs prepared in laboratory using superphosphate ...
Evaluation of nutrient release rates from phosphate fertilisers in a laboratory leaching technique

Scientist

Agresearch Ruakura Research Centre Hamilton New Zealand [email protected]

Introduction

n Water soluble P was highest for superphosphate and for DCP declined with reducing super content to about 75% super. n Citric and formic soluble P in DCP was approximately constant with changes in super content. n Water soluble Ca in DCP declined with reducing super content to about 55% super.

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100

20

80

n Commercially available DCP (manufactured by Foremost Holdings Ltd). n Commercially available Gafsa reactive phosphate rock.

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Water Ca%

20

n Cumulative release of P from DCPs at short times (10 days) was only slightly less than for super.

n Cumulative release of Ca from DCPs was less than from super at all times.

Super L/S 70 DP 75

Cumulative Ca released %

100 80

0

5 40 35 30 25 20 15 10 05 er 4 / 0/ 5/ 0/ 5/ 0/ 5/ 0/ 5/ p 5 u 5 6 7 8 9 6 7 8 9 S

85/15 60

DP 65 DP65 (Fact) 70/30 50/50

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Effect of fertiliser composition on pH value and composition.

pH

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5.9

4

20 0 0

5

10

15

20

25

30

35

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n Super and fresh super/lime mixture had similar P release rates throughout the experiment. These had the highest P release rates up to 0.5 days.

4 3.7

3

2.9

3.1

2.5

er /05 /10 /15 /20 /25 /30 /45 /40 /45 /50 p 5 90 85 80 75 70 65 60 55 50 Su 9

n RPR had a very low P release rates but after 15 days this were greater than for the other fertilisers. n All the reacted super/lime materials had higher P release rates than either RPR or superphosphate between about 1 day and about 8 days.

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XRD analysis %

RPR Guano

4.9

60

Sul of Ca Calcite Anhydrite

40 20

Brushite

CaHP hydrate r 05 10 15 20 25 30 45 40 45 50 e p 5/ 0/ 5/ 0/ 5/ 0/ 5/ 0/ 5/ 0/ u 9 9 8 8 7 7 6 6 5 5 S

0

Leaching of P and Ca at various times as a function of fertiliser composition. 100

60

30 days 16 days 10 days 3 days

40

1 day

80

20

1.5 hr 0

Rate of P leaching at different times for the different fertilisers. 0.01 days

DP65 (Fact) Super L/S 70 50/50 70/30 85/15 DP65 RPR

4.94 days 1.94 days

0

40

DP65 (Fact) Super L/S 70 50/50 70/30 85/15 DP65 RPR

80

0

120

0.1

0.2

0.3

13.44 days 8.44 days 0.04 days

0

100 80 60

30 days 40 20 0

16 days 10 days 3 days 1 day 1.5 hr

S

n Ten DCPs prepared in laboratory using superphosphate and limestone (0 - 50%) and ‘cooking’ for 3 days.

Guano 0

2

P and Ca released from fertilisers

RPR

20

5.7 5.8 5.8 5.7

n Dicalcium phosphate increased with declining super content to a maximum at about 70% super.

DP 75 DP65 (Fact)

40

Citric Ca%

60

70/30 50/50 DP 65

85/15

p e 95 r /0 90 5 /1 85 0 /1 80 5 /2 75 0 /2 70 5 /3 65 0 /3 60 5 /4 55 0 /4 50 5 /5 0

Materials

Formic Ca%

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n Monocalcium phosphate (MCP) declined with decreasing super content to zero at 70% super.

Super L/S 70

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n Citric and formic soluble Ca increased gradually with decreasing super content.

n Fertiliser pH was lowest for super and increased with decreasing super content of the DCP to about 5.7 at 70% super.

Cumulative P released (%)

2

4

6

8

0

0.04

0.08

0.12

0.16

S u

n Compare nutrient release rates of DCP with Superphosphate and Reactive Rock phosphate.

Water P%

100

% sample P released

n Examine P and Ca release rates of DCP produced by using different proportions of lime and super.

Formic P%

Cumulative leaching of P and Ca from a range of P fertilisers including DCP.

0

% sample Ca released

Objectives

n P and Ca release rate was in the order Super > DCP > RPR.

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n DCP is of low solubility but its P is plant available. n Therefore, DCP should be an environmentally friendly fertiliser.

Comparison of DCP with other phosphate fertilisers

Citric P%

80

50

Ken Perrott

100

50 /

P.O. Box 12 Te Awamutu New Zealand [email protected]

Percentage of P and Ca present in fertiliser material as percentage of Total P and Ca.

u p 95 er / 90 05 /1 85 0 /1 80 5 /2 75 0 /2 70 5 / 65 30 /3 60 5 /4 55 0 /4 50 5 /5 L 0 im e

Foremost Holdings Ltd

Fertiliser composition

%P

Research and Development Manager

Results and discussion

% Ca

Chitralekha Ramakrishnan

DP65 (Fact) Super L/S 70 50/50 70/30 85/15 DP65 RPR

27.44 days 20.44 days

0.50 days

0

0.4

0.8

1.2

0

0.01

0.02

0.03

Rate of P leaching (% fertiliser P/day)

Methods n Fertiliser samples leached with distilled H2O in the laboratory.

(Perrott K, Kear MJ (2000). Laboratory comparison of nutrient release rates from fertilisers. Communications in Soil Science and Plant Analysis 31, 11-14)

n Leached P and Ca captured by ion exchange resins. n Leaching of P and Ca followed for up to 38 days.

Acknowledgements We wish to acknowledge NZTech and Foremost Holdings Ltd for providing funding for this project. We also thank Agresearch for valuable assistance in laboratory supervision.

n These results confirm that DCP will be a slow-release fertiliser because it will release P more slowly and over a longer time than will MCP fertilisers such as superphosphate. n These laboratory studies did not involve soil. Therefore, field studies are necessary to confirm similar nutrient availability and release patterns for DCP on soils with various P retention properties.

Conclusions n We can therefore consider DCP-type materials as slow-release P fertilisers, but still more rapid release than for RPRs. n Agronomically, they should occupy an intermediate position between superphosphate and RPR and could have advantages (over superphosphate) where P runoff or P leaching (where the P retention is low) are possible. n We suggest some further research which could help to justify use of DCP-type materials in these and similar situations.