Applying Urea as a foliar spray rather than in the granular

form

By Bill Fulkerson, Central Field officer, Norco cooperative Ltd, PO Box 486,

Lismore, NSW 2477.

Conclusion

The results of the present study found that Urea applied as a foliar spray

gave a similar pasture growth response at 40% of the rate of granular Urea, under

the conditions of the study and in a subtropical region. This has implications for

both dairy farm profitability and for the impact on the environment.

From a farm profitability point of view, on a typical dairy farm in this region

milking 150 cows on 80 ha and applying the recommended rate of granular Urea

of 100kg Urea/ha per month equivalent with 8 applications annually, the cost of

Urea would be about $41, 600. This could be reduced to $17,056 by using Urea as

a foliar spray.

From an environmental point of view, the N escaping into the environment,

as ammonia, leached beyond the plant’s root zone or as the green house gas,

Nitrous oxide, would be largely eliminated because the Urea should not enter the

soil but is directly absorbed by the leaves.

Background

There has been growing interest in applying N – based (and other) fertilizers as

foliar sprays, based on the premise that it improves N use efficiency by the

pasture plant. With foliar application the Urea goes directly through the leaves

dermis to the plant rather than through the soil where most of the N losses occur.

Application of fertilizers as a foliar spray has been used in horticulture for many

years, primarily for trace, but also, macro minerals. The arguments against

applying fertilizers, such as Urea, to pastures as a foliar spray was that insufficient

could be absorbed by the leaves and that Urea had to be converted to Nitrate as

the major form in which it is used by the plant.

In the soil, the pathway of N, in granular Urea, to the plant and associated losses

to the environment, is as shown in the diagram below:

Urea is converted to Ammonium by the soil enzyme Urease, immediately it comes

into contact with water. Most of the Ammonium is attached to soil particles and is

therefore stable but some may be converted to Ammonia and volatilized into the

air (4 to 25%). The Ammonium is then converted to Nitrate which is the form of N

most commonly used by the plant, although it can use Ammonium. However, the

Nitrate is not bound and is therefore vulnerable to leaching below the root zone

(nil to 100% under sandy soils and high rainfall), or it can be denitrified to the

green house gas, Nitrous dioxide, under water logged conditions. These pathways

illustrate the potential loss of N in fertilizer as it moves through the soil to the

roots of the plant.

Field studies by Zaman and Blennerhassett (2009) showed that pasture plants

could take up Urea and NH4+ through the leaves as a foliar spray Franke in 1967,

found that the application of Urea to the leaf actually increased the permeability

of the cuticle and hence improved its diffusion into the leaf. Subsequent glass

house studies by Dawar et al (2012) showed that dissolved Urea was about 50%

more efficient in promoting ryegrass growth than Granular Urea. This is in line

with the results of earlier studies by Middleton and Smith (1979) and Castle et al

(2006) who found that less energy is required to synthesise protein from NH4+

when it is directly absorbed from the leaf rather than needing to be converted to

NO3- and then absorbed by the roots.

A recent study in New Zealand by Schofield et al (2012) confirmed the results of

Dawar et al . (2012). They found that the rate of application of Urea could be

reduced by 60% when applied as a foliar spray compared to the granular form.

In the subtropical dairy region, dairy farmers commonly have a summer grass

pasture which provides feed from late spring to early autumn and this is over

sown in early autumn with short rotation ryegrass (Lolium multiflorum) to provide

winter/spring feed. The recommended rate of application of Nitrogen to such

pastures is 46kg N/ha per month as Urea if other soil nutrients are adequate.

The object of this study was to compare the efficacy, under field conditions, of

granular and dissolved Urea for pasture growth in a subtropical dairy

environment.

Methods and Results

The study, comprising 2 experiments, was undertaken on a dairy farm near

Lismore on the far north coast of NSW (lat.28o and Long.153o, elev. 15m).

The long term mean annual rainfall is 1180 mm and the climate is subtropical with

wet summer/ autumns and relatively dry winter/ springs (see Figure 1).

Figure 1.The long term mean monthly rainfall for Lismore and the rainfall in

2015-16 .

Experiment 1

Experiment 1 was located on a kikuyu-based pasture over sown with Short term

ryegrass (Lolium multiflorum, cv. Annual tetraploid ryegrass) in autumn to provide

pasture in late autumn through to mid-spring. The kikuyu-based pasture also

contained some paspalum and couch grass.

The Experiment commenced on the 18 August 2015 and concluded on 30 March

2016.

The experimental design comprised 2 treatments: Urea dissolved in water at a

ratio of 1:3, with 7 replicates (each 9m boom spray width by 55m long) or,

:Granular Urea with 3

replicates (each 18m fertiliser spreader width by 55m long).The pasture mass

0

20

40

60

80

100

120

140

160

Aug Sep Oct Nov Dec Jan Feb Mar

Rai

nfa

ll (m

m)

Month

mean

2015-16

measurements and any pasture sampling was taken in a same transect along the

middle of each plot to minimize edging effects.

The ryegrass pasture was grazed 3 times (18 August to the 15 October) by

Friesian milking cows and the kikuyu pasture 6 times (25 November to the 20

March). The grazing interval was 14 to 22 days and 50kg granular Urea/ha was

applied immediately after each grazing or 21kg dissolved Urea/ha was sprayed on

about 4 days after grazing, to allow adequate leaf regrowth to receive the

dissolved Urea. Thirty seven ml of non ionic wetting agent was also mixed with

the dissolved Urea to maximize retention by the grass leaf. The granular Urea was

applied with a fertiliser spreader and the dissolved Urea with a boom spray.

The dry matter (DM) mass of pasture pre- and post-grazing was measured using an electronic rising plate pasture meter (Farmworks, NZ). From this data, the growth rate (kg DM/ha per day) and the amount eaten by grazing milking cows (kg DM/ha) at each grazing (see Figure 2) and the mean growth rate and amount eaten by grazing milking cows, over the total experimental period is shown in Table 1. Grass growth rate (kg DM/ha)

0

20

40

60

80

100

120

140

160

1 2 3 4 5 6 7 8 9

spray

granular

Grazing number

Figure 2. A comparison of the growth rate of Urea applied as a foliar spray or in granular form

Table 1. The mean (9 grazings) +/- se growth rate of grass (kg DM/ha per day)

and the amount eaten (kg DM/ha) by milking cows in Experiment 1.

Treatment

Grass growth Grass eaten (kg DM/ha/day) (kg DM/ha)

Dissolved Urea

51 +/-10 618 +/-61

Granular Urea

45 +/-10 548 +/-73

Although the growth rate of grass and the amount eaten on the dissolved Urea

was higher, this was not significant.

Ryegrass samples were taken pre-grazing to simulated grazing height on 3

September 2015 and the kikuyu samples on 23 March 2016. These samples were

analysed for Total N (%), Nitrate-N and Ammonium –N (mg/kg) at the

Environmental Analysis Laboratory (NATA accredited) at Southern Cross

University, Lismore, NSW (See Table 2).

.

Table 2. The content of total N, Urea-N and Ammonium-N in ryegrass leaf, taken

on 3 September and kikuyu leaf taken at the conclusion of the experiment on 29

March.

Analysis

Date Treatment

Total N Nitrate-N Ammonium-N

(%DM) (mg/kg) (mg/kg)

3/09/2015 Dissolved Urea 4.4 1047 439

Granular Urea

4.6 1903 502

29/03/2016 Dissolved Urea 3.19 24 354

Granular Urea

3.50 23 397

The Total N, Nitrate-N and Ammonium –N were higher in the leaves of both

treatments at the start of the study than at the end and this was probably due to

the residual effects of the previous Urea application (see table 3 for comparable

soil data). There was no significant difference between treatments in any of the

parameters measured.

Thirty soil cores, to 10cm depth, were taken across the whole experimental site at

the commencement of the experiment on 18 August 2015 and again at the

completion of the study over each treatment, pooled then sent to the CSBP

laboratory (NATA accredited), Bibra Lake, WA, for analysis of total N, Nitrate-N

and Ammonium-N (see Table 3).

Table 3. Soil Nutrient levels in samples taken over the whole site at the

commencement of the experiment on 18 August 2015 and for each treatment on

30 March 2016.

Date

Soil Nutrient (mg/kg soil) 18-Aug-15 30-Mar-16

Dissolved Granular

Colwell Phosphorus 171 114 167

Colwell Potassium 309 247 309

Sulphur

48 28 35

Organic Carbon

5.1 4.8 5

pH

5.1 5.0 4.9

Nitrate-N

75 22 60

Ammonium-N

9 11 11

The Nitrate-N concentration in the soil on the Spray plots has dropped markedly

since the commencement of the study whereas the granular Urea plots have

remained similar. Although this is expected in order to provide the environmental

benefits it probably necessitates the need to apply foliar N after every grazing as

there is far less N stored in the soil that the plant can access.

Experiment 2

Experiment 2 was undertaken on a monoculture pasture of Short rotation

ryegrass (cv. Feast 2), established on a prepared seed bed in April 2015.

The experimental design comprised 3 treatments: Granular Urea broadcast with a

fertilizer spreader, Urea dissolved in water at a ratio of 2:1 or Urea plus Lime,

both applied with Multi 1000 Tow and Fert spray unit spray (see below) with plot

size for the granular Urea plots being 18m by 85m and for the dissolved Urea 30m

by 85m and treatments were replicated 3 times.

The plots were grazed by milking cows on 18 August and 21 September and 50kg

Granular Urea/ha applied immediately after each grazing or 25kg dissolved

Urea/ha or 25kg dissolved Urea plus 30kg Agricultural lime/ha, about 4 days after

grazing.

The pre and post grazing pasture mass was determined as for Experiment 1 (see

Table 4).

Table 4. The mean +/- se ryegrass growth (kg DM/ha/day) for the 3 treatments

A soil sample, pooled over each of the 3 treatments were taken on 18 August (see

Table 5) and analysed as for Experiment 1.

Table 5. Soil nutrient level for samples taken at the commencement of the study

on 18 August 2015.

Soil Nutrient

(mg/kg)

Colwell Phosphorus 110

Colwell Potassium 227

Sulphur

30

Organic Carbon

pH

5.1

Nitrate-N

14

Ammonium-N

Treatment

Grass growth

(kg DM/ha/day)

Dissolved Urea

44

Dissolved Urea + Lime 51

Granular Urea

38

A pooled pasture sample was taken to simulated grazing height on 3

September (see Table 6) and analysed as for Experiment 2.

Table 6. The content of N, Nitrate-N, Ammonium-N and Calcium in the

leaves of ryegrass in Experiment 2.

Treatment Total N Nitrate-N Ammonium-N Calcium

(%) (mg/kg soil) (mg/kg soil) (%)

Dissolved Urea 3.8 304 490 0.35 Dissolved Urea + Lime 4.1 486 710 0.45

Granular Urea 3.7 555 111 0.36

The interesting issue in Table 6 is the 29% increase in the Calcium content

of the ryegrass sprayed with Lime and this is unlikely to be on the surface as there

was 32 mm of rain fell in the period from application of lime herbage sampling.

Practical implications

In the light of these studies, farmers have started to apply Urea and other N-

based fertilizers as foliar sprays, primarily on cereal crops, but more recently on

dairy pastures. This has led to development of spray units (Tow and Fert,

Metalform Ltd, NZ) which have continual agitation thus allowing lime, as well as

other soil nutrients, herbicides and insecticides and even pasture seed to be

applied at the same time. There are about 200 Tow and Fert machines in use on

NZ dairy farms and quite a few on farms in Victoria and Tasmania.

There are a number of issues to consider when using foliar spray to apply urea to

pastures:

1. The effectiveness of the foliar spray depends on the fluid being applied to

the leaves and not the soil. For this reason, Urea is sprayed on ryegrass

pastures about 4 days after grazing to provide more leaf area. Delaying this

long after grazing is not a big issue as it takes about 5 days for N from

granular Urea to move through the soil to the plant roots anyway. There is

no need for this delay with kikuyu grass or other summer grasses as there

is always enough green leaf/stolon available.

There may also be a need to apply granular Urea on seedling ryegrass

emerging on a worked up seed bed, otherwise too much would be applied

to the soil and its effectiveness diminished.

2. The ratio of Urea to water when applied with the boom spray was about

1:3 but with the continual agitation of the Tow and Fert machines this ratio

can be reduced to 1:2. Although the Tow and Fert Manufacturers

recommend applying the dissolved Urea without any additives, we did

apply non ionic wetting agent at 180 ml/1000L water and this seems to

ensure more spray adheres to the grass leaves but the requirement for this

has not been proven.

3. At temperatures above about 28oC, the foliar application should be made

in late afternoon to prevent burning of the leaves.

4. There may be a need to apply foliar Urea after every grazing, whereas

granular Urea can be applied after every second grazing if the grazing

interval falls below 20 days. This is because with granular Urea, more N as

Nitrate is available in the soil whereas this reserve is not available when

Urea is applied as a foliar spray. Nitrate-N dropped from 75 mg/kg at the

start of Experiment 1 to 22 mg/kg when Urea was applied as a foliar spray

whist with granular Urea it stayed at 60 mg/kg.

5. The N in granular Urea needs soil moisture to be able to move to the root

zone whereas with the foliar spray the N goes directly to the plant,

irrespective of soil moisture and in this case the plant can more quickly

respond to improved soil moisture conditions after a dry spell.

6. A real advantage of the Tow and Fert machines is that they allow other soil

nutrients, lime and insecticides and herbicides to be applied in the one

pass.

Acknowledgements

Thanks to Greg Mcnamara and Micheal Undery for initially kindling my interest in

this topic and to Todd Mcnamara for the excellent cooperation in managing the

plots. The assistance of Haman in the study is also greatly appreciated.

References

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Dawar K el al. (2012). Applying Urea with Urease inhibitor (N-(n-Butyl)

thiophophoric triamide) in fine particle application improves N uptake in ryegrass

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Frank W (1967). Mechanism of foliar penetration of solutions. Ann. Rev.

Plant Physiol., 18, 281-300.

Jones C, A et al. (2014). Extension notes, Montana State University, USA.

Middleton K R and Smith J S (1979). A comparison of ammoniacal and

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Schofield P et al. (2012) Using humic compounds to improve effiecy of

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