effects of phosphorus and nitrogen on growth of pasture plants and vam fungi in se australian soils...

Effects of phosphorus and nitrogen on growth of pasture plants and

VAM fungi in SE Australian soils with contrasting fertiliser

histories (conventional and biodynamic)

Megan Ryan1,*, Julian Ash

Division of Botany and Zoology, Australian National University, Canberra 0200, Australia

Received 8 July 1998; accepted 17 December 1998

Abstract

The soil biological community has been reported to differ between conventional and alternative (organic and biodynamic)

farming systems. However, few studies have investigated whether this results in substantial differences in the biological

pathways controlling major ecosystem processes, such as plant nutrient uptake. This paper describes a glasshouse experiment

conducted using a red-brown earth (Natrixeralf) soil sampled from three conventional and three biodynamic irrigated dairy

pastures located in the Goulburn River Valley, Victoria, Australia. The biodynamic soils had not had organic or inorganic

fertilisers applied for, on average, 17 years, while the conventional soils had received regular inputs of fertilisers containing

soluble phosphorus (P) and nitrogen (N). The experiment examined whether the contrasting fertiliser histories had resulted in

different pathways of plant nutrient uptake through assessing the response of white clover (Trifolium repens L.), perennial rye

grass (Lolium perenne L.) and the indigenous vesicular-arbuscular mycorrhizal (VAM) fungi to addition of four levels of

soluble P and N. The response to added P and N did not differ between the conventional and biodynamic soils, although, plants

in the biodynamic soils had a slower growth rate and a higher level of colonisation by VAM fungi due to lower initial soil P

and N concentrations. Overall, there was no indication that the biodynamic and conventional soils had developed substantially

different processes to enhance plant nutrient uptake or that the indigenous VAM fungi differed in their tolerance to

applications of soluble nutrients. # 1999 Published by Elsevier Science B.V. All rights reserved.

Keywords: P; N; Clover; Rye grass; Vesicular-arbuscular mycorrhizal fungi

1. Introduction

Biodynamic agriculture is practiced as an alterna-

tive to conventional agriculture in many areas around

the world (Kirchmann, 1994; Lytton-Hitchins et al.,

1994; Reganold, 1995; Murata and Goh, 1997; Ver-

eijken et al., 1997; van Mansvelt et al., 1998). Bio-

dynamic farm management practices re¯ect a desire to

improve the healthiness of produce, which is believed

to occur through harnessing biological processes and

eliminating use of pesticides, herbicides, synthetic

veterinary medicines and readily soluble fertilisers

(Kirchmann, 1994).

Agriculture, Ecosystems and Environment 73 (1999) 51±62

*Corresponding author. Tel.: +61-02-6246-5387; fax: +61-02-

6246-5399; e-mail: [email protected] address: CSIRO Plant Industry, GPO Box 1600,

Canberra, ACT, Australia, 2601.

0167-8809/99/$ ± see front matter # 1999 Published by Elsevier Science B.V. All rights reserved.

P I I : S 0 1 6 7 - 8 8 0 9 ( 9 9 ) 0 0 0 1 4 - 6

As biodynamic farmers do not apply soluble ferti-

lisers, it is possible that the soil biological community

may adapt to play a greater role in plant nutrition (Ritz

et al., 1997). Hence, the relationships between soil

nutrient concentrations and plant growth may differ

between biodynamic farms and conventional farms

and this could be re¯ected in their response to addi-

tions of soluble nutrients. Australian soils are parti-

cularly suitable to test such ideas, as both total and

extractable P concentrations are generally low. Extrac-

table P concentrations are generally 10±400 mg gÿ1,

and sometimes as low as 1 mg gÿ1 (Lindsay, 1985).

While conventional farmers overcome this problem

through applying fertilisers containing readily soluble

P, plant growth on organic or biodynamic farms, where

these fertilisers are not applied, is likely to be limited

by P (Dann et al., 1996).

To examine whether long-term biodynamic man-

agement had changed the ability of the soil biological

community to in¯uence plant nutrient uptake, a glass-

house experiment was conducted which involved

additions of fertilisers containing soluble P and N

to soil sampled from three paired adjacent conven-

tional and biodynamic farms. Growth of two pasture

species was used as a bioassay of the ability of the soil

to provide nutrients to plants. The level of colonisation

by vesicular-arbuscular mycorrhizal (VAM) fungi

indigenous to each soil was also examined as they

are a prominent part of the soil community, are

involved in plant uptake of nutrients particularly P

(Bolan and Robson, 1983), and have been reported to

adapt in abundance and function in response to nutri-

ent inputs (Johnson, 1993).

2. Methods and materials

2.1. Farm management

At three locations in the Goulburn River Valley near

Shepparton, Victoria, Australia (1458100E, 368200S), a

biodynamic dairy farm was matched with a neighbour

with similar soil type, herd breed, farm area and a

history of consistent conventional management. These

are referred to as Farm Pairs A, B and C.

The farms consisted primarily of permanent sum-

mer-irrigated perennial pastures used to support free-

ranging dairy cattle. These pastures had not been

cultivated for, on average, 35 years and consisted

mainly of white clover (Trifolium repens L.), perennial

rye grass (Lolium perenne L.) and paspalum (Paspa-

lum dilatatum Poir.). Soils on the farms were red-

brown earths (Stace et al., 1968), Natrixeralfs (USDA

classi®cation), with pH of 6.0 in a 1 : 5 water suspen-

sion and organic carbon of 26 g kgÿ1. These soils are

described by Cockroft and Martin (1981) as having

few nutritional problems, with the main de®ciencies

being P, N, and sulfur (S). Use of superphosphate on

conventional farms is generally believed to overcome

the P and S limitations and while the legume compo-

nent of the pasture provides N through biological

®xation, this is not usually suf®cient to meet growth

requirements and nitrogenous fertilisers are generally

also applied.

The largest difference between the conventional and

biodynamic farms was in the area of nutrient inputs.

The conventional farms had at least a 15 year history

of regular applications of the readily soluble fertilisers

superphosphate, diammonium phosphate and urea,

averaging 27 kg haÿ1 per year of P and 17 kg haÿ1

per year of N. In contrast, the biodynamic farmers had

not applied signi®cant amounts of inorganic fertilisers

for, on average, 17 years, instead applying to pasture

a biodynamic preparation made from composted

cow manure, BD500, 1±2 times each year at a rate

of 1±2 g haÿ1 (see Kirchmann (1994) for further

explanation). As cattle were not housed and neither

pasture or crops harvested, the farmers did not produce

and spread compost on their paddocks. Lime was

applied to all farms at 40 kg haÿ1 per year. Cattle

numbers were slightly higher on the conventional

farms (134 compared with 117) and cereal supple-

ments were also higher on the conventional farms

(550 compared with 320 kg per cow per year). The

interval between summer irrigations was slightly

longer on the biodynamic farms. There were no other

major management differences between the farming

systems.

2.2. Glasshouse experiment

Soil to 10 cm depth was collected in October 1994

from 20 sites across one ®eld on each farm, bulked,

passed through a 10 mm sieve and thoroughly mixed.

Extractable P (Olsen et al., 1954) and total N were

assessed on a sub sample of each bulked soil. Previous

52 M. Ryan, J. Ash / Agriculture, Ecosystems and Environment 73 (1999) 51±62

sampling had shown that little variation remained in

soil treated in this manner, with standard deviations

for extractable P and total N of <1.5 mg gÿ1 and

<0.3 mg gÿ1, respectively.

A glasshouse experiment using white clover (cv.

Kopo) and perennial rye grass (cv. Yatsynl) was con-

ducted using soil from the six farms. Four levels of P

(0, 10, 50 and 200 mg kgÿ1 of soil as a solution of

NaH2PO4.2H2O) and four levels of N (0, 9, 45 and

180 mg kgÿ1 of soil as a solution of NH4NO3) were

applied. The experiment was a fully factorial cross,

except that the highest rate of P was applied only to

plants which did not receive N, and the highest rate

of N was applied only to plants which did not receive

P. The ratio of N to P was chosen to be similar to that

of diammonium phosphate.

Standard 10 cm diameter pots were ®lled with

200 g of soil. Seeds were germinated in a 1 : 1 ver-

miculite : perlite mix and two seedlings transplanted

into each pot at the two leaf stage in early summer

(December) 1994. Glasshouse temperatures ranged

from 15±358C and pots were weeded and watered

by hand each day. The P and N treatments were added

to the soil surface at planting and again after three

weeks. No basal nutrients were applied. Light levels

were around 70% of daylight (up to 1500 mmol

mÿ2 sÿ1).

Treatments were replicated seven times, with each

set of replicates forming a randomised block in the

glasshouse and each block consisting of three ran-

domly allocated sub-blocks, one for each farm pair.

Within each sub-block, pots receiving the same treat-

ment, one containing biodynamic soil and one con-

taining conventional soil, were randomly allocated to

adjacent positions.

The experiment was harvested after 5 weeks.

Shoots of both species and clover roots were dried

at 708C for 48 h and weighed. Roots were sub-

sampled and stained to distinguish VAM fungi (Grace

and Stribley, 1991). The percentage of root length

colonised by VAM fungi, VAM (%), was calculated

using the line-intersect method (Giovannetti and

Mosse, 1980). Shoot P and N concentrations were

determined for clover and rye grass from four nutrient

treatments from Farm Pair A (no added nutrients,

50 mg kgÿ1 of P, 45 mg kgÿ1 of N, and 50 mg kgÿ1

of P � 45 mg kgÿ1 of N) using automated spectro-

photometry (Heffernan, 1985).

Analysis of variance was used to examine clover

and rye grass dry weights and VAM colonisation levels

for (i) the effects of P addition when no N was added,

(ii) the effects of N addition when no P was added, and

(iii) the interactions between P and N, excluding the

highest P and N treatments. In each case, block (1±7),

location (Farm Pair A, B, or C), nutrient (four levels of

P or N) and farming system (conventional or biody-

namic) were included, along with a `nutrient x farming

system' interaction term to assess whether the

response to nutrients differed between the conven-

tional and biodynamic soils.

3. Results

3.1. Comparisons between soils

Table 1 presents the soil extractable P and total N

concentrations in the six soils used in the experiment,

after they had been bulked and sieved. Extractable P

concentrations were 2±3 times higher and total N

concentrations 1.3±2 times higher in the conventional

soils.

Fig. 1 presents the VAM colonisation levels for

plants grown in each soil without added nutrients.

For clover, the VAM colonisation level was signi®-

cantly higher in the biodynamic soils and did not differ

signi®cantly within the three conventional or three

biodynamic soils. VAM colonisation of rye grass was

lower than for clover and was similar in all the soils,

except in the biodynamic soil in Pair A, which pro-

duced signi®cantly higher colonisation. Fig. 2 pre-

sents the shoot dry weights for plants grown in

each soil without added nutrients. Clover growth

was lowest in the biodynamic soil in Pair A and

similar in all other soils. Rye grass growth did not

Table 1

Extractable P and total N in bulked soil sampled from the top

10 cm of irrigated permanent pasture on three conventional (Con.)

and biodynamic (BD) farms

Pair A Pair B Pair C

Con. BD Con. BD Con. BD

Extractable P (mg gÿ1) 27 8 72 32 62 26

Total N (mg gÿ1) 5.7 4.4 6.8 4.0 10.7 5.4

M. Ryan, J. Ash / Agriculture, Ecosystems and Environment 73 (1999) 51±62 53

vary consistently between the conventional and bio-

dynamic soils.

3.2. Comparisons between farming systems

Table 2 presents the outcomes of ANOVAs asses-

sing whether clover and VAM fungi grown in con-

ventional and biodynamic soils responded to P

addition in the same manner. The results are presented

in Fig. 3 as interactions between P addition and farm-

ing system. Table 3 and Fig. 4 present the equivalent

results for clover and N addition.

VAM colonisation of clover differed signi®cantly

between soil from the three locations and was sig-

ni®cantly higher in the biodynamic soils. The addition

of P reduced colonisation, an effect that was signi®-

cantly more marked in the biodynamic soils

(Fig. 3(a)). The lowest level of N caused a small

increase in colonisation, but there was no effect on

colonisation at the higher two levels (Fig. 4(a)).

Clover shoot dry weight was higher in the conven-

tional soils and was not signi®cantly affected by P

addition; although the lowest level tended to increase

shoot weight, this effect decreased as the level of P

increased (Fig. 3(b)). Addition of N tended to increase

shoot weight, although not signi®cantly, particularly

in soils from the conventional farms (Fig. 4(b)).

Clover root dry weight tended to be higher in the

conventional soils and was not signi®cantly affected

by P or N addition, although the lowest level of P did

tend to increase root weights, particularly in the

biodynamic soils. Overall, the variation in root

weights was not well described by the models which

were not signi®cant (Tables 2 and 3). The clover root±

shoot ratio differed signi®cantly between plants grown

in soils from the three locations and was signi®cantly

higher in the biodynamic soils. Neither addition of P

or N had a consistent effect on the root±shoot ratio.

However, in the biodynamic soils, the root±shoot ratio

was signi®cantly lowered by the two higher levels of P

addition (Fig. 3(d)).

Tables 4 and 5 and Figs. 5 and 6 present the out-

comes from ANOVAs of rye grass shoot growth and

VAM colonisation. VAM colonisation differed signif-

icantly between locations and was signi®cantly higher

in the biodynamic soils. Addition of P did not con-

sistently in¯uence VAM colonisation, although it did

cause a decrease in colonisation in the biodynamic

Fig. 1. Percentage of root length colonised by VAM fungi in (a)

clover and (b) rye grass grown in soil with no added P or N from

three conventional/biodynamic farm pairs; estimated means and

LSD at p � 0.05.

Fig. 2. Shoot dry weight of (a) clover and (b) rye grass grown in

soil with no added P or N from three conventional/biodynamic

farm pairs; estimated means and LSD at p � 0.05.


soils (Fig. 5(a)). Addition of N did not signi®cantly

in¯uence VAM colonisation. Shoot dry weight dif-

fered signi®cantly between locations, but was not

signi®cantly in¯uenced by P. Addition of N signi®-

cantly increased shoot growth, an effect which was

signi®cantly greater in the conventional soils

(Fig. 6(b)).

There were no signi®cant interactions between P

and N and these analyses are not presented. Thus, the

only instances of a strong, consistent and signi®cant

interaction between nutrient addition and the farming

system from which the soil originated were the effect

of P addition on clover VAM colonisation and the

effect of N on rye grass growth.

The concentrations of P and N in the shoots of the

plants in Farm Pair A which received no nutrient

additions are presented in Table 6. Phosphorus

concentrations were signi®cantly higher in plants

grown in the conventional soil, while N concentrations

were similar for all plants. When these were graphed

against VAM colonisation, along with results from

three other nutrient treatments from each soil in

Farm Pair A, there was a strong negative correla-

tion, with the relationship being stronger for clover

(Fig. 7).

4. Discussion

4.1. VAM fungi

Addition of P markedly reduced VAM colonisation

in clover (Fig. 3(a)) and shoot P concentration had a

strong negative linear correlation with VAM colonisa-

tion (Fig. 7). Shoot and root P concentrations have

often been reported to exhibit this relationship with

Table 2

Results from ANOVAs of the effects of four levels of P addition on VAM colonisation and growth of clover in soil from three conventional/

biodynamic farm pairs in a glasshouse trial

Dependent variable Predictor variable F-ratio P R2 n

VAM (%) Full model 18.6 <0.0001 0.62 164

Block 1.3 0.3

Location 27.9 <0.0001

P 35.6 <0.0001

Farming system 77.8 <0.0001

P � farming system 10.2 <0.0001

Shoot dry weight Full model 3.6 0.0008 0.14 164

Block 1.2 0.3

Location 5.9 0.004

P 1.1 0.3


P � farming system 0.01 1.0

Root dry weight Full model 1.5 0.1 0.06 163

Block 1.8 0.1

Location 0.1 0.9

P 2.1 0.1

Farming system 4.3 0.04


Root±shoot ratio Full model 7.9 <0.0001 0.39 163

Block 8.0 <0.0001

Location 16.1 <0.0001

P 1.9 0.5



Parameters included are block (1±7), location (Farm Pair A, B, C), P addition (0, 10, 50, 200 mg kgÿ1 of soil) and farming system

(conventional, biodynamic). There were no other significant interaction terms.


VAM colonisation (Lu et al., 1994; Ryan, 1998). This

is consistent with the impact of P being mediated

through the effects of phospholipid content of root

cells on root cell membrane permeability and exuda-

tion of the carbohydrates on which the fungi depend

for energy (Graham et al., 1981).

VAM colonisation of rye grass did not show a

signi®cant response to P and colonisation levels were

Fig. 3. Interactions between P addition and the farming system

from which the soil originated for clover (a) percentage of root

length colonised by VAM fungi, (b) shoot dry weight, (c) root dry

weight and (e) the root±shoot ratio; estimated means and LSD at

p � 0.05.

Fig. 4. Interactions between N addition and the farming system

from which the soil originated for clover (a) percentage of root

length colonised by VAM fungi, (b) shoot dry weight, (c) root dry

weight and (e) the root±shoot ratio; estimated means and LSD at

p � 0.05.


Table 3

Results from ANOVAs of the effects of four levels of N addition on VAM colonisation and growth of clover in soil from three conventional/



VAM (%) Full model 18.1 <0.0001 0.60 166

Block 0.8 0.5

Location 28.9 <0.0001

N 2.7 0.05


N � farming system 0.1 0.9

Shoot dry weight Full model 3.8 <0.0001 0.20 166

Block 3.1 0.007

Location 3.4 0.04

N 2.2 0.09



Root dry weight Full model 1.3 0.2 0.04 166

Block 1.9 0.08

Location 1.8 0.2

N 0.5 0.7



Root±shoot ratio Full model 8.3 <0.0001 0.40 165

Block 6.2 <0.0001

Location 18.8 <0.0001

N 0.8 0.4



Parameters included are block (1±7), location (Farm Pair A, B, C), N addition (0, 9, 45, 180 mg kgÿ1 of soil) and farming system


Table 4

Results from ANOVAs of the effects of four levels of P addition on VAM colonisation and growth of rye grass in soil from three conventional/



VAM (%) Full model 7.0 <0.0001 0.31 165

Block 5.5 <0.0001

Location 12.3 <0.0001

P 0.9 0.5



Shoot dry weight Full model 3.2 0.0002 0.17 164

Block 2.9 0.01

Location 10.0 0.0001

P 0.9 0.5



Parameters included are block (1±7), location (Farm Pair A, B, C), P addition (0, 10, 50, 200 mg kgÿ1 of soil) and farming system



much lower than for clover. Rye grass is less depen-

dent on VAM fungi for provision of P as its ®ner, more

branched root system, and many root hairs, allow

better access to P (Schweiger et al., 1995). Nitrogen

addition did not signi®cantly affect VAM colonisation

(Tables 3 and 5), an effect consistent with other

studies (Ryan et al., 1994).

Table 5

Results from ANOVAs of the effects of four levels of N addition on VAM colonisation and growth of rye grass in soil from three conventional/



VAM (%) Full model 9.5 <0.0001 0.43 168

Block 1.4 0.2

Location 19.6 <0.0001

N 0.3 0.9



Shoot dry weight Full model 12.0 <0.0001 0.50 166

Block 8.2 <0.0001

Location 23.0 <0.0001

N 22.3 <0.0001



Parameters included are block (1±7), location (Farm Pair A, B, C), N addition (0, 9, 45, 180 mg kgÿ1 of soil) and farming system


Fig. 5. Interactions between P addition and the farming system

from which the soil originated for rye grass (a) percentage of root

length colonised by VAM fungi and (b) shoot dry weight; estimated

means and LSD at p � 0.05.

Fig. 6. Interactions between N addition and the farming system

from which the soil originated for rye grass (a) percentage of root

length colonised by VAM fungi and (b) shoot dry weight; estimated

means and LSD at p � 0.05.


For VAM colonisation of clover there was a sig-

ni®cant interaction between the effects of P and farm-

ing system (Table 2), with colonisation in the

biodynamic soils being more markedly reduced by

P than colonisation in the conventional soils. This

could indicate that the VAM fungi in the conventional

soils, after >15 years of soluble P fertiliser applica-

tions, had adapted to be more tolerant of P. However,

while soil extractable P and plant P concentrations

initially increase proportionally, eventually, plant P

concentrations will plateau while soil extractable P

continues to increase (Bolan and Robson, 1983; Ryan,

1998). Hence, the addition of the lowest level of P

presumably had less effect on plant P concentrations

and therefore on VAM colonisation levels, in the

conventional soils, due to initially higher shoot P

concentrations. In addition, the similar relationship

between VAM colonisation and shoot P concentra-

tions for clover in conventional and biodynamic soil

(Fig. 7) also suggests that the VAM fungi in conven-

tional soils were not more tolerant of P.

This conclusion contrasts with Jasper et al. (1979)

who examined VAM colonisation and shoot P con-

centrations in rye grass grown in soil from virgin land

and in soil from adjacent cultivated land which had

received regular P applications for 30 years, in Wes-

tern Australia. VAM colonisation was consistently

higher at any given shoot P concentration for plants

in the cultivated soil and it was concluded that the

VAM fungi in the cultivated soil were less sensitive to

P. However, as only spores of Acaulospora laevis were

found in the virgin soil and only spores of Glomus

monosporus in the cultivated soil, the different colo-

nisation response to shoot P could have resulted from

colonisation rates and characteristics differing

between the two species (Edathil et al., 1996). Exam-

ination of spore populations on the farms sampled in

this study did not indicate differences in VAM com-

munity structure (Ryan, 1998).

4.2. Plant growth

Plants grown in the biodynamic soil from Farm Pair

A were consistently smaller than plants in the other

soils, re¯ecting a lower soil extractable P concentra-

tion. Shoot P concentrations in clover in Pair A grown

with no nutrient additions were <0.25%, suggesting

growth was limited by P (Weir and Cresswell, 1994).

For all soils, mean clover shoot dry weight increased

by 15% in response to the lowest level of P addition

(Fig. 3(b) and Fig. 4(b)). However, growth was

decreased by the highest level of P addition, perhaps

indicating that P concentrations were becoming toxic.

A similar trend was evident for clover roots. In both

soils in Farm Pair A, the concentration of P in rye grass

was suf®cient for normal growth (Weir and Cresswell,

Table 6

Concentrations of P and N in shoots of clover and rye grass grown,

with no nutrient additions, in soil from the conventional and

biodynamic farms in Farm Pair A

Phosphorus Nitrogen

Clover Conventional 0.36 (0.03) 3.1 (0.1)

Biodynamic 0.24 (0.02) 2.9 (0.1)

Rye grass Conventional 0.33 (0.01) 2.1 (0.1)

Biodynamic 0.22 (0.02) 2.1 (0.1)

Mean and standard error of the mean in brackets.

Fig. 7. Simple regressions between the percentage of root length

colonised by VAM fungi and shoot P concentration for (a) clover

(r2 � 0.97) and (b) rye grass (r2 � 0.61); means from four nutrient

treatments applied to the conventional and biodynamic soils from

Farm Pair A.


1994) and P had no effect on growth of the rye grass in

any of the soils.

Nitrogen increased growth of clover and rye grass,

particularly in the conventional soils. Regarding rye

grass shoot dry weights, the signi®cant interaction

between N and farming system probably resulted from

plants in the biodynamic soils becoming limited by P

at the highest level of N addition. While no signi®cant

interactions were found between P and N, the highest

levels of P and N were not included in the analysis, as

they were not applied together. Thus, unexpectedly, N

was the major limiting nutrient in all soils. Indeed, in

Pair A when no nutrients were applied, clover shoot N

concentrations were low in the conventional soil and

de®cient in the biodynamic soil, while rye grass shoot

N concentrations were well below that necessary for

normal growth (>3.5%; Weir and Cresswell, 1994).

The clover root±shoot ratio, an indicator of plant

response to soil nutrient de®ciencies (Wilson, 1988),

was not affected by P addition in the conventional soil,

but was signi®cantly reduced at the highest two levels

of P addition in the biodynamic soils (Fig. 3(d)). It

was also consistently higher in the biodynamic soils.

This could indicate that P was limiting clover growth

in the biodynamic soils, making the small growth

response to P unexpected. The root±shoot ratio also

tended to decrease as N was applied (Fig. 4(d)),

although the ability of clover to biologically ®x N

through the Rhizobium symbiosis may have resulted in

this trend not being more marked. All clover plants

were well nodulated.

4.3. Do conventional and biodynamic soils respond

to nutrient additions in the same manner?

Various studies have compared the microbial bio-

mass or occurrence of certain groups of soil organisms

between conventional farms and either biodynamic or

organic farms (Sivapalan et al., 1993; Penfold et al.,

1995; Wander et al., 1995; Werner, 1997). Such

studies often report biodynamic and organic farms

to have a greater abundance of certain soil organisms,

however, few studies have then attempted to compare

the functioning of the soil ecosystems between the

different farming systems.

In the present experiment, there were only two

instances of a signi®cant interaction between farming

system and nutrient addition. The greater negative

effect of P on clover VAM colonisation in the biody-

namic soils, which appeared to be due to lower initial

concentrations of soil extractable P, and the greater

response to N of rye grass in conventional soils, which

was probably due to P limiting growth at the highest

levels of N addition in the biodynamic soils. Thus,

after an average of 17 years of contrasting fertiliser

regimes, the response to soluble P and N fertilisers of

VAM fungi and pasture plants growing in soil from

conventional and biodynamic farms did not greatly

differ. While VAM colonisation levels and plant nutri-

ent concentrations did differ, these were predictable

responses to quanti®able factors such as concentra-

tions of soil extractable nutrients.

In another Australian study, fertiliser trials were

conducted on a conventional and an organic mixed

cereal-livestock farm in the southern Australian

wheatbelt (Dann et al., 1996). The organic farm had

ceased applying soluble P fertilisers 30 years pre-

viously, instead applying poorly soluble rock phos-

phate. However, there was no evidence that the

organic soil ecosystem had adapted to make the rock

phosphate more available to wheat on the organic farm

and maximum yields were obtained on both farms

when soluble P was applied. Hence, while changing to

organic or biodynamic farm management may result

in enhanced levels of some soil organisms, such as

VAM fungi (Ryan et al., 1994), this will not necessa-

rily compensate for any decreases in yield due to non-

addition of readily available fertilisers.

However, the response of the soil biological com-

munity to contrasting farm management systems may

depend on the types of processes affected. For

instance, large additions of organic matter on biody-

namic and organic farms have been found to alter the

composition of the soil community, resulting in a

greater ability to decompose cellulose in organic soil

(Penfold et al., 1995) and an increase in organisms

antagonistic to pathogens (Sivapalan et al., 1993;

Knudsen et al., 1995).

5. Conclusions

The response of plants and VAM fungi growing in

soils from conventional and biodynamic farms to

additions of soluble P and N did not differ, in spite

of the biodynamic soils not having received soluble P


and N fertilisers for 17 years. While the biomass and

community structure of the soil ecosystem may vary in

response to farming system, it cannot be assumed that

fundamentally different processes will be governing

important pathways, such as plant nutrient uptake, or

that the soil ecosystem will compensate, in terms of

yield, for lack of inputs such as readily soluble fer-

tilisers.

Acknowledgements

Doug Small coordinated the research project on the

biodynamic and conventional dairy farms and gener-

ously provided data on these farms. Tony Willis

assisted with the glasshouse experiment. M. Ryan

was funded by a Grains Research and Development

Corporation (GRDC) Junior Research Scholarship.

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