Soil Acidity and Fertiliser Management

in Conservation Agriculture

Guy Thibaud

KwaZulu-Natal Department of Agriculture and Rural Development

In this presentation (Main focus on no-till)

● Principles of, and reasons for adopting,

conservation agriculture.

● Look at frequently expressed concerns.

■ Soil acidification and its management

■ Nitrogen requirement and management.

■ Phosphorus and potassium placement and

stratification.

● Take home message.

Principles of Conservation Agriculture

1. Minimum soil disturbance (<25%)

Principles of Conservation Agriculture

2. Abundant and permanent residue covering

the soil. At least 30 % after planting.

Principles of Conservation Agriculture

3. Diverse crop rotations (at least 3 crops).

No-till Conv. tillage

Why conservation agriculture ?

Reduce soil loss caused by wind and

water erosion

Benefits of CA

Moisture conservation

Reduced run-off

Reduced evaporative losses

Benefits of CA

Reduced costs

Fuel

Labour

Machinery

Time

What are some of the concerns with

reduced tillage and no-till systems ?

● Soil acidification and how to manage it without the

use of a plough.

● Acidification of soil surface in no-till cereal

production is a serious problem.

■ Negative effect on herbicide activity- e.g. atrazine.

■ Adverse effect on surface root development.

■ Detrimental effect on the earthworm population.

Soil Acidification

► Use of Nitrogen Fertilisers – The major cause

of soil acidification in cropping systems.

Ammonium Nitrate → Nitric Acid

NH4NO

3 + 2 O

2→ 2 HNO

3+ H

2O

► Al – containing soil minerals dissolve at low

pH to release Al3+.

► Al3+ replaces Ca2+ and Mg2+ on exchange

sites, which leach out of soil profile with

nitrate (NO3-).

► Increase in exchangeable acidity (Al +H)

and acid saturation.

No-till experiment near Howick, KZN (2001)

Nitrogen rates: (0, 60, 120, 180 kg ha-1)

Annual surface-applied lime: (0, 0.75 1.5 Mg ha-1)

Nitrogen source: (LAN & urea)

No lime applied since 2001

Topsoil (0-5 cm) soil acidification

No lime applied (11 years): No-till

2001

Can surface liming prevent soil acidification

in no-till ?

Surface-applied lime: 0.75 Mg ha-1 per year

2001

Surface-applied lime: 1.50 Mg ha-1 per year

2001

Season Annual surface-applied lime (Mg ha-1)LSD (0.05)0.75 1.50

Maize grain yield benefit (Mg ha-1)

2001/2002 0.11 0.19 NS

2002/2003 0.11 0.32 NS

2003/2004 Destroyed by hail in January -

2004/2005 0.16 0.23 NS

2005/2006 0.95 0.84 0.38

2006/2007 0.37 0.43 0.16

2007/2008 0.53 0.67 0.26

2008/2009 0.66 0.64 0.29

2009/2010 1.50 1.85 0.52

2010/2011 0.66 0.99 0.32

2011/2012 1.66 1.94 0.45

2012/2013 1.73 1.93 0.33a

2013/2014 1.66 1.68 0.70

Cumulative yield benefit 10.10 11.71

aSignificant Lime x Nitrogen rate interaction

Nitrogen management

Ploughing

● Loosens and aerates the soil, exposes fresh OM

previously protected within soil aggregates to

microbial attack.

● Extensive and rapid mineralisation of organic N

Tillage effects on N requirement

Winterton experiment, KZN (started in 2003)

● Variable N rates:

0 - 160 kg N from 2003 to 2007

0 - 200 kg N in 2008 and 2009

0 - 240 kg N from 2010 onwards.

● Tillage:

No-till and conventional tillage

● N source

LAN & urea

11 Year average from a tillage experiment at Winterton

Nitrogen

► Greater potential for N loss under no-till

than under conventional tillage due to:

■ Immobilisation of fertiliser N by surface

residues.

■ Denitrification (NT soils often cooler, wetter

and less well aerated than CT soils).

■ Leaching (higher water infiltration rates).

■ Ammonia volatilisation from urea (High

urease activity, & surface pH from lime).

Ammonia Volatilisation

CO(NH2)2 → (NH4)2CO3 (pH 9.1 - 9.2)urease

urea

(NH4)2CO3 → H2O + CO2 + NH3 (gas)

Ammonium carbonate

NH4+ NH3 (aq) NH3 (gas)

NH4

+

NH3 (aq)

Ammonia Volatilisation

AFFECTED BY: Soil pH and pH buffer capacity.

Soil moisture and temperature.

Rainfall after application.

Urease activity.

Rate of urea application.

(Fox et al., 1986; Fox & Piekielek,1993)

Time after

application

(days)

Site 1 ( pHw= 5.6

CEC = 6.7 cmolc kg-1)

Site 2 (pHw = 5.5

CEC = 12.3 cmolc kg-1)

--------------- Nitrogen loss (kg ha-1) ---------------

Day of

application 4.2 2.5

1 32.3 8.0

2 9.0 5.0

3 1.5 2.5

4 1.9 Terminated due to 58 mm

of rainfall and flooding5 1.4

Total 50.9 18.0

N (urea) broadcast uniformly on top of a moist

soil/residue surface at 168 kg N ha-1

(Keller & Mengel, 1986)

Nitrogen Nitrogen applied (kg ha-1)

source 56 112 168 224

Maize grain yield (Mg ha-1)

UAN 2.94 4.27 5.88 6.80

Urea 2.71 4.24 5.48 6.45

Amm. Nitrate 2.71 4.92 7.05 7.72

LSD (0.05) 0.72

N Source effects under NT (3-yr average)

Howard and Tyler, 1989

Season Nitrogen source N source comparisonLSD (0.05)

Significant interactive effects involving N source

LAN Urea

Yield increase above control (Mg ha-1)

2003/2004a 0.17 0.26 NS NS

2004/2005 0.81 0.82 NS NS

2005/2006 1.77 1.69 NS NS

2006/2007 2.41 2.36 NS NS

2007/2008 6.49 6.35 NS NS

2008/2009 6.00 5.96 NS NS

2009/2010 3.88 3.55 0.27 Tillage

2010/2011 5.02 4.42 0.56 Tillage, N Rate

2011/2012 3.95 3.98 NS NS

2012/2013 3.48 3.09 0.34 NS

2013/2014 7.06 6.82 NS NS

a Non significant yield response to N fertilizer

Winterton Experiment (Tillage x N rate x N source)

LSD (0.05)

Conv. Till

No-Till

Tillage x Nitrogen experiment at Winterton

Increased potential for NH3 volatilisation at high

rates of urea application e.g.

240 kg N ha-1 90 kg N ha

-1

N treatmentMemphis

(6-yr average)

Collins

(2-yr average)

------- Maize yield (t/ha) -------

- lime + lime - lime + lime

Urea (168 kg N).. 7.14a 5.98b 9.58a 8.90b

Urea split (84 + 84 kg N) 7.45ac 7.83c 9.33a 9.36a

Ammonium nitrate (168 N). 8.64d 8.50d 11.31c 10.91c

(Howard & Essington 1998)

Effect of surface-applied lime and N source on maize yield

with no-till (All N broadcast). Lime applied at 1.2 t/ha.

Season Nitrogen source N source comparisonLSD (0.05)

Interactive effectsbLAN Urea

Yield increase above control (Mg ha-1)

2001/2002a -0.03 0.15 NS NS

2002/2003 0.59 0.54 NS NS

2003/2004 Experiment destroyed by hail - -

2004/2005 1.11 1.13 NS NS

2005/2006 1.42 1.19 NS NS

2006/2007 1.96 2.00 NS NS

2007/2008 3.19 3.29 NS NS

2008/2009 2.93 2.80 NS NS

2009/2010 2.58 2.28 NS NS

2010/2011 2.01 1.71 0.29 NS

2011/2012 3.57 3.53 NS NS

2012/2013 4.47 4.02 0.38 NS

2013/2014 0.92 0.75 NS NS

a Non significant yield response to N fertiliser.b N Source by N rate, N source by Lime and N Source by lime by N rate

Surface (0-5 cm) pHKCl

Urease Inhibitors

● Rate of urea hydrolysis promoted by high urease

activity, and moist, warm soil conditions.

● May cause a sharp rise in pH around the urea granule and

promote N loss through ammonia volatilisation.

● Urease inhibitors decrease rate of hydrolysis.

● Many compounds tested. NBPT (N-(n butyl)

thiophosphoric acid triamide) consistently the most

effective.

Nitrogen Nitrogen source used

applied

(kg ha-1)

Ammonium

nitrate

Urea Urea-NBPT

---------- Maize grain yield (Mg ha-1) ----------

Control (0) 7.7

80 8.7 7.8 8.5

160 9.5 8.6 9.4

240 9.9 9.2 9.9

LSD (0.05) 0.40

Urease Inhibitors (NBPT)

Report by Hendrickson (1992) on maize

response to NBPT

● 78 field trials conducted over 5 yrs (1984-1989).

● 17 states across U.S.A.

● 45% of trials were under no-till, 45% under

reduced tillage & 10% on conventional tillage.

Av. yield increase

= 0.27 Mg ha-1

Negative response

in 34% of cases

Phosphorus and Potassium

► Potassium (K) and especially Phosphorus

(P) strongly adsorbed by soil clay minerals.

► Minimal leaching of P and K.

► In no-till, accumulation of P and K at soil

surface due to surface-application of

fertilizer. Accentuated through deposition,

and non-incorporation of surface residues.

Tillage Soil

depth

P soil test K soil test

In row Between row In row Between row

mm ------------------------------ mg kg-1 ---------------------------------

Plough 0 -75 35 30 94 102

75 -2 75 28 28 106 106

No-till 0 - 75 117 50 226 176

75 - 275 11 10 70 59

(Mackay et al., 1987)

Phosphorus and Potassium Stratification in No-Till

► Most of P and K taken up by plants reaches

the root surface by diffusion. Therefore,

uptake highly dependent on soil moisture.

► During periods of low or erratic rainfall, P

and K may become “positionally”

unavailable.

► P and K uptake reduced by low temperature,

excessive moisture and compaction.

Mulch rate and Distance from row

Percentage cover In the row 19 cm 38 cm

------- Root density (cm root cm3 of soil) -------

Zero (<5%) 3.90 0.79 0.51

Normal (60%) 4.64 1.61 1.44

Double (>90%) 5.25 1.59 2.50

LSD (0.05) 0.49

Yibirin et al, 1993

Effect of soil cover on surface root

development (0-5 cm)

No-Till

● Many studies showing that surface-applied P and

K is highly effective.

● Subsurface placement ( 5 cm to side and below

seed) has proved superior in some instances.

● A few reports showing deep subsurface placement

of K (15 to 20 cm) better than surface or shallow-

banding. Yield benefits small, economics

questionable.

Surface Residue

CRITICAL

Take home message

► Without intervention (liming), soil acidification is

inevitable where maximum yields are targeted

(high N use).

► In no-till, prevention of acid build-up in surface

layers is critical if the system is to be sustainable.

► Must not allow acidity to escape i.e. regular

surface liming.

► Must start with low acidity (e.g. Acid Sat < 10 %),

and keep further acid inputs under control.

► Extra N (30-70 kg ha-1 ) is required under no-till in

the early stages (< 12 yrs).

► NBPT has potential to reduce N loss through

ammonia volatilization from urea.

► Maintain as much surface cover as possible.

► Band placement of P and K. Total N + K in band

should not exceed 70kg/ha.

Thank you