Review of research on pasture yield responses to fine particle application of fertiliser in New Zealand

ABSTRACT

There have been several claims that because of more efficient uptake and reduction of losses, nitrogen (N) and phosphorus (P) in a fine particle application (FPA) form results in improved pasture yield responses compared with application in the standard granular form. A total of 22 structured trials comparing pasture and wheat yield responses from FPA and dissolved liquid with granular forms were reviewed as part of this study. Of eight statistically analysed trials investigating FPA, only in the two small-scale trials was a significant pasture yield to FPA over a granular product reported. Not one of the nine trials comparing fully dissolved liquid and granular forms of N and P showed an advantage to the liquid form. It was concluded that currently there was insufficient experimental evidence to recommend the use of FPA fertilisers over the standard granular form of application.

KEYWORDS:

• Fine particle application
• granular form
• nitrogen fertiliser
• pasture yield response
• urea

Introduction and review brief

Granular fertilisers containing nitrogen (N) have been applied to New Zealand pastures and crops for decades. The immediate recovery of N by pasture plants from granular fertilisers is estimated at 30%–40% of the N applied based on a typical efficiency of N use by pasture of 10 kg DM/kg N and a pasture N content of 3%–4%. There are many soil flora and fauna which require and consume N (effectively in competition with the pasture plant), resulting in immobilisation of N. Some additional pathways which result in N losses include volatilisation and leaching. It has been suggested that improved recovery of fertiliser N by pasture plants may occur if the pasture roots can access and take-up N before it is lost to these other soil processes and pathways.

Fine grinding of the granular fertilisers and mixing with water immediately prior to application is mainly referred to as fine particle application (FPA). FPA has been claimed to achieve a 40%–50% higher conversion of applied N to pasture dry matter (DM) than granular urea (Quin and Findlay 2009). Several reasons have been given for this higher conversion of fertiliser N from FPA (Quin et al. 2006, 2015, 2017). These include a more even distribution of nutrients by FPA compared to the ‘spotty’ fertiliser distribution in the application of 4–5 mm granules. The possibility that a significant proportion of the N in FPA fertiliser is absorbed directly through the foliage and hence is not as subject to the soil losses associated with the application of solid granular fertilisers is another. For FPA urea, it is also claimed that the urea is absorbed directly into the leaf and this is more efficiently utilised by the plant than mineral N taken up by the roots (Quin et al. 2006, Dawar et al. 2011, Quin et al. 2017).

In New Zealand, there have been several trials comparing the agronomic effectiveness of FPA and granular forms of urea. There has also been research undertaken to compare granular fertilisers with liquid fertilisers where granular fertilisers in their received state have been fully dissolved in water. FPA of nutrients are not as common internationally, and most of the relevant research compares the agronomic effectiveness of granular fertilisers with suspension fertilisers where a stabilising agent is added to the mixture of fertiliser and water to slow down the ‘settling out’ of the solid fertiliser particles.

This review was undertaken to assess the efficacy of nutrient uptake from fertilisers in the FPA form compared with the granular form, both by critically examining the experimental evidence comparing the two forms and also by reviewing the available literature on the various suggested mechanisms that are likely to influence the relative performance of FPA and granular fertilisers. It should be noted that the assumption is made that the only potential benefit is from the N and/or P in the FPA fertiliser and not from any additive such as the urease inhibitor Agrotain or plant growth hormones. Only trials where the effect of the form of fertiliser in one of the categories described in the next section, was measured without the confounding effect of urease inhibitors, are included in the review.

Terminology

This section is derived from ‘Definitions of Fertiliser Forms – Fluid & Suspension Fertilisers Giving Way to Wetted Prills’ (group.one.nz/quininformation-2/fertiliser/fluid-suspension-fertiliser).

Granular fertilisers

Granular fertilisers are made by finely grinding the ingredients before passing the material through a granulation drum or over a pan granulator to produce roughly spherical granules, generally of 4–5 mm diameter, before drying and hardening. The most commonly applied granular N fertilisers in New Zealand are urea, di-ammonium phosphate (DAP) and sulphate of ammonia.

FPA fertilisers

FPA fertilisers are made by fine grinding solid granular fertilisers (e.g. Urea, DAP) to typically between 100 and 200 μm particle size and adding water (30%–40% by weight) to produce a fluid mixture of dissolved fertiliser and fine fertiliser particles that can be boom-sprayed in either aerial or ground application. FPA urea is also referred to as fluidised urea (e.g. Quin and Findlay 2009).

Liquid fertilisers

In the New Zealand context, liquid fertilisers are made by dissolving unground solid granular fertilisers (e.g. Urea) in water through agitation of the solution or heating the water to aid dissolution.

Suspension fertilisers

In most overseas countries, this term is used to describe a fluid made by mixing high-quality fertiliser ingredients which are soluble in water and previously ground to below 100 μm with 40%–60% water by weight. A saturated solution is formed with the rest of the ingredients remaining in suspension as fine particles. The addition of about 10% by weight of a gel or clay such as bentonite keeps the fertiliser particles in a suspended state for some time, although agitation immediately prior to application is usually required to overcome settling. In many situations, suspension fertilisers are injected into the soil in cropping operations.

Review of trials comparing FPA and granular fertilisers

There have been several trials conducted in New Zealand comparing FPA and granular fertilisers. Despite a comprehensive literature search, the authors of this review have not found any published experimental studies overseas comparing an application of a ‘slurry’ or suspension of finely ground urea in water (FPA) with the application of conventional granular (4–5 mm diameter) urea.

This apparent absence of overseas trials is consistent with the observation that none of the published research in New Zealand makes reference to any similar trials overseas.

The following sections therefore are focussed on the experimental results reported for trials conducted in New Zealand.

Peer-reviewed journal studies evaluating FPA

A summary of the trials published in peer-reviewed journals is shown in Table 1.

Table 1. Summary of trial results published in peer-reviewed journals.

Reference	Trial technique	Treatments	Results
Korte et al. 1996	Small plot mowing at three Central Hawkes Bay sites	DAP slurry vs. granular DAP at 2–5 kg N/ha	No significant difference in pasture production between two forms
Dawar et al. 2011	Field lysimeter/mini plots (30 cm diameter)	FPA vs. granular urea at 100 kg N/ha	Pasture production from FPA urea > granular urea (P < .05)
Dawar et al. 2012	Glasshouse	FPA vs. granular urea at 25 kg N/ha	Pasture production from FPA urea > granular urea (P < .05)

In their field trials, Korte et al. (1996) compared DAP slurry at recommended commercial rates (2–5 kg N/ha) with the equivalent rate of granular DAP. The combined herbage production results averaged over the three sites for statistical analysis showed that there was no significant difference (P < .05) between granular DAP and DAP slurry (equivalent to FPA) at the low nutrient application rates used in these trials and neither treatment yielded significantly more than the control.

Dawar et al. (2011) measured pasture DM yield responses to urea in the granular and FPA forms at a high N rate of 100 kg/ha in a field lysimeter/mini plot experiment and reported a significant increase from FPA urea compared to granular urea from one cut. There was a significant increase in pasture yield from both treatments compared to control. They attributed this increase from FPA relative to granular urea to a combination of factors including more uniform application, easier N uptake through both roots and leaves, and more efficient N metabolism.

At the lower rate of 25 kg N/ha in a glasshouse trial, Dawar et al. (2012) also reported a significant increase in pasture yield from FPA compared with granular urea, with both treatments significantly (P < .05) outyielding the control.

Unpublished results evaluating FPA that include statistical analysis

The results from trials reported whose results were statistically analysed but not published in a peer-reviewed journal are shown in Table 2. The FPA urea in the research reported by Muir et al. (2006) was ground to 75 μm sieve size and mixed with 2 l of water for each plot and applied as a slurry using a motorised mist blower. No detailed methodology was provided by Hawke (2007) or Trainor (2007) for their small plot mowing trials. For the liquid urea treatment, Watson (2014) applied urea at 2 l per plot, and for the FPA treatment, a fine mist of water was sprayed over the plots prior to FPA application to allow the fine particles to adhere to the grass leaves. None of these studies reported any significant differences in pasture yield between products.

Table 2. Summary of statistically analysed trial results from unpublished research.

Reference	Site	Trial technique	Treatments	Results
Muir et al. 2006	Central Hawkes Bay	Small plot (5 × 1.8 m) mowing	FPA vs. granular urea at 30 and 60 kg N/ha	No significant difference in pasture production between two forms
Hawke 2007	South Waikato	Small plot (3 × 1.5 m) mowing	FPA vs. granular urea at 30 and 60 kg N/ha	No significant difference in pasture production between two forms
Hawke 2007	Central Waikato	Small plot (3 × 1.5 m) mowing	FPA vs. granular urea at 30 and 60 kg N/ha	No significant difference in pasture production between two forms
Trainor 2007	South Otago	Small plot mowing	FPA vs. granular urea at 30 and 60 kg N/ha	No significant difference in pasture production between two forms
Watson 2014	Hillsborough, Northern Ireland	Small plot (16 m^2) mowing	FPA vs. liquid vs. granular urea at 25 kg N/ha	No significant difference in pasture production between three forms

FPA evaluations with no statistical analysis

There were several pieces of research carried out and reported but without statistical analysis of the results (Table 3). This work should be recognised but must be given less weight than the statistically analysed results. In the series of trials reported by Quin et al. (2006), the FPA form contained urea both in solution and as particles of less than 10–150 μm. Of these studies, four reported an increase in pasture yield from FPA compared with granular urea (Quin et al. 2006, Zamaan and Blennerhassett 2009) and the other study reported no difference in pasture yield between the two forms (Mackay 1996).

Table 3. Summary of published research with no statistical analysis of the results.

Reference	Site	Trial technique	Treatments	Result
Quin et al. 2006	Whangarei	Small plot mowing	FPA vs. granular urea at 30 kg N/ha	Increase in pasture production from FPA cf. granular form
Quin et al. 2006	Taranaki	Small plot mowing	FPA vs. granular urea at 30 kg N/ha	Increase in pasture production from FPA cf. granular form
Quin et al. 2006	Southland	Small plot mowing	FPA vs. granular urea at 25 kg N/ha	Increase in pasture production from FPA cf. granular form
Zaman and Blennerhassett 2009	Mid-Canterbury	Large plot mowing. FPA applied by helicopter	FPA vs. granular urea at 30 kg/ha	Increase in pasture production from FPA cf. granular form
Mackay 1996	Wairarapa	Small plot mowing	FPA vs. granular DAP/S at 6 kg N/ha	No difference in pasture production between two forms

The form of fertiliser had no significant effect on pasture yield in 7 of the 13 studies (Tables 1–3). The other 6 trials showed an advantage to FPA, but in 4 of these trials there is no reported statistical analysis of the results. In no trials was the performance of FPA worse than that of granular urea.

Despite much of this trial data not being published in a peer-reviewed journal and some of the data presented not being subjected to statistical analysis, some trends are apparent.

The majority of the studies were carried out using small plots and generally with rates of 50 kg N/ha or less. The two trials where FPA significantly outperformed granular fertilisers occurred using mini-plots or glasshouse pots with a much smaller surface area than the plots used in the other trials (Dawar et al. 2011). In addition, the mini-plots had a relatively high rate of 100 kg N/ha applied. It is possible that the foliage was covered with FPA urea to a greater extent than in the studies using the much larger field plots, and this combination of factors could have contributed to the greater pasture production response from FPA than granular urea through increased foliar N uptake.

Studies evaluating the performance of liquid fertilisers

Each of these potential mechanisms would also be relevant to liquid urea, so as part of this review, we evaluated several trials in New Zealand that have compared the performance of nutrients in solid and dissolved fertilisers to gain further insights into the performance of FPA.

Trials conducted in New Zealand comparing liquid and granular forms of fertiliser are summarised in Table 4. All of these studies except one (larger harvested plot wheat trial) were small plot pasture mowing trials with very low rates of P or standard rates of 20–40 kg N/ha (Table 4). Carey (2009) compared 12 applications of liquid urea at 2.5 kg/ha over six weeks with 30 kg N/ha applied as granular urea.

Table 4. Summary of results from trials comparing liquid and granular forms of fertiliser.

Reference	Site	Trial technique	Treatments	Result
Karlovsky 1954	Ruakura, Waikato	Small plot mowing	Liquid vs. granular application of various forms of P (including DAP and MCP)	No significant difference in pasture production between forms
Karlovsky et al. 1978	Rotomanu, Westland	Small plot mowing	Liquid vs. granular P at 1 kg/ha	No significant difference in pasture production between two forms
	Ruakura, Waikato	Small plot mowing	Liquid vs. granular P at 2 kg/ha	No significant difference in pasture production between two forms
	Waikato	Small plot mowing	Liquid vs. granular P at 1 kg/ha	No significant difference in pasture production between two forms
	Canterbury	Small plot mowing	Liquid vs. granular P at 2 kg/ha	No significant difference in pasture production between two forms
	Southland	Small plot mowing	Liquid vs. granular P at 2 kg P/ha	No significant difference in pasture production between two forms
McCloy 2009a	Burnham, Central Canterbury	Small plot mowing	Liquid vs. granular urea at 20 and 40 kg N/ha	No significant difference in pasture production between two forms
McCloy 2009b	Greendale, Central Canterbury	Larger plot harvested (wheat)	Liquid vs. granular urea at 20 and 40 kg N/ha	No significant difference in yield between two forms
Carey 2009	Lincoln, Central Canterbury	Small plot (5 × 2.5 m) mowing	Liquid vs. granular urea at 30 kg N/ha	No significant difference in pasture production between two forms

There were no significant differences in pasture DM yield between liquid and granular forms of nutrients. This is consistent with the statement by Quin et al. (2017) that dissolved urea ‘is on average no more efficient than granular urea on pasture’.

Trials overseas have also shown little or no difference in agronomic performance of urea applied in solid and liquid forms to pasture (e.g. Low and Armitage 1954; Nowakowski 1961; Widdowson and Penny 1964; Mundy 1966). Care must be taken in relating the overseas trials directly to the New Zealand situation (in which the solid urea is applied as 4–5 mm granules) because the physical form of the solid urea applied in the overseas trials was either not specified (e.g. Nowakowski 1961; Widdowson and Penny 1964; Mundy 1966) or are fine crystals (Low and Armitage 1954).

Nevertheless, the authors of this review have been unable to find any published research showing an agronomic advantage of liquid over solid urea when applied to pasture.

Overall Discussion

Quin et al. (2006, 2015, 2017) attributed the apparent improvement in N use efficiency from FPA in some of the trials reported above to the following factors:

• Better ‘coverage’ with FPA., i.e. 350–450 prills/m² compared with an application density of 35–45 urea granules/m² from granular urea from the application of a common N rate of 30 kg N/ha. This better ‘coverage’ would allow more even distribution of fertiliser N in the soil root zone contributing to greater plant assimilation efficiency.

• Less energy required by the plant for conversion of N to amino acids, meaning more energy for leaf growth and nutrient uptake by the plant roots.

• Improved ability to absorb the required nutrients from the soil, due to the additional energy available.

• Wetting the prills assists direct uptake of N as urea through the leaves reducing energy requirements further and avoiding the influence of cold temperatures in the root environment. Urea adsorbed through the leaves and cuticles is more efficiently converted to protein by the plant than is nitrate taken up through the roots.

• Reduced ammonia volatilisation meaning more N for uptake. The high ‘coverage’ of small prills on the soil surface prevents the formation of the very high pH levels (>8) around each granule of granular urea.

• Reduced nitrate leaching, meaning more N for uptake by plants.

Ground coverage

In this review, it is estimated that an application rate of 30 kg N/ha, applied as 4–5 mm granules, results in an application density of approximately 35–45 granules/m².

It is asserted (e.g. Quin et al. 2006; Quin et al. 2017) that this application density is low relative to the density of pasture plants, and as a result many of the plants may not receive any direct input of fertiliser N, simply because they do not have roots in the vicinity of a granule. In addition, the more even spread achieved by FPA may enable more rapid N uptake by plants and thus forestall N losses through volatilisation, immobilisation and leaching.

The authors of this review have not been able to locate any research demonstrating that recovery of fertiliser N by pasture is adversely affected by the spatial distribution or particle density resulting from the use of 4–5 mm granules of urea in New Zealand or overseas. When it was first proposed to manufacture urea in Taranaki, New Zealand, a New Zealand Institute of Agricultural Sciences Working Party produced a report in 1980 on ‘the agricultural implications of the decision to manufacture fertilizer nitrogen in Taranaki, New Zealand’ (NZIAS Working Party 1980). This report noted that the manufacturing plant would produce granular urea with 90% of the granules having a diameter of 3–5 mm. The focus at that time appeared to be to produce a free-flowing product. Concerns about the granule size influencing agronomic effectiveness of urea were not raised.

Foliar uptake

Quin et al. (2006) note that ‘urea can be rapidly absorbed and assimilated by leaves of many species following foliar application (Harper 1984; Turley and Ching 1986)’, and Dawar et al. (2011) suggested that because FPA application resulted in a uniform distribution of urea particles over each plant, a significant proportion (estimated at approximately 70%) was observed on pasture leaves 12 hours after application. It was suggested that these deposited urea particles may enable the plants to absorb urea directly through their leaves/cuticles (Watson et al. 1990), and this would in turn facilitate efficient conversion of urea into plant protein. The plants would then be left with extra energy (Middleton and Smith 1979) which could be used for extra growth.

The capacity of plants to absorb nutrients through aerial tissues has long been recognised (Wittwer and Teubner 1959; Fernandez and Eichert 2009), and foliar nutrition has a long history of use in a wide variety of crops including perennial fruits, cereals and grasslands (Low and Armitage 1954; Swietlik and Faust 1984; Gooding and Davies 1992). It is claimed that foliar nutrient application has significant advantages in terms of nutrient use efficiency with higher proportions of the applied nutrient being recovered by the crop compared to soil-applied nutrients (Weinbaum et al. 2002).

Whereas plant nutrient uptake from the soil involves both passive and active mechanisms, foliar uptake is entirely passive (Fallahi and Eichert 2013). The principal barrier to the absorption of foliar-applied nutrients is the leaf cuticle and particularly the associated wax layers (Weinbaum 1988). Passage through the cuticle is driven by diffusion and the electrochemical gradient formed by a negative charge increase across the cuticular membrane, with the rate of absorption being much influenced by temperature and the concentration gradient (Swietlik and Faust 1984; Wojcik 2004). The properties of the cuticle and therefore the uptake of foliar-applied nutrients vary greatly between species, with leaf age, and with environmental factors, such as nutrition, light, humidity and temperature (Weinbaum 1988).

Uptake often parallels stomatal density indicating direct stomatal uptake or increased absorption by guard and subsidiary cells (Haynes and Goh 1977). There is evidence for continuous liquid water connections between the leaf apoplast and the leaf surface via the stomata, which would facilitate the stomatal uptake of foliar-applied nutrients (Peuke et al. 1998; Burkhardt 2010). Uptake can also be through trichomes, especially in the basal parts of glandular trichomes that have an abundance of ectodesmata and less cuticular development (Haynes and Goh 1977). Spray retention and, presumably uptake, has a positive correlation with leaf hair density (Hall et al. 1997).

The effectiveness of foliar nutrition also depends on application techniques such as spray volume, droplet size and dispersal pattern, and droplet spread. High-volume spraying can lead to runoff and decreased retention of applied nutrients, while insufficient carrier volumes can reduce coverage. Surfactants can promote uptake by improving coverage, increasing cuticle permeability and enhancing stomatal uptake (Haynes and Goh 1977; Fernandez and Eichert 2009). But surfactants can have the opposite effect if they result in increased runoff of the spray (Leece and Dirou 1979). While uptake is correlated to coverage and spread of spray droplets, after evaporation of an aqueous carrier, the remaining fertiliser residue is often restricted to a much smaller area than the original droplet spread. This can be 20%–30% of the original wetted area, and this measure corresponds better to final uptake (Kraemer et al. 2009). It is unclear how the distribution of FPA urea deposits on leaves differs from low concentration urea foliar sprays and whether any such differences could enhance the foliar uptake of FPA urea.

In the context of this current review, the authors note that urea is a common foliar treatment in perennial fruit and arable crops and is the most effectively absorbed form of N for foliar application (Furuya and Umemiya 2002; Mengel 2002). Urea is particularly suitable for foliar application because it is water-soluble, and the molecule is electrically neutral and therefore less restricted in its movement through the epicuticular wax and cutin layer. The cuticular membrane is 10–20 times more permeable to urea than to inorganic ions (Swietlik and Faust 1984), suggesting its initial uptake is not by simple diffusion but rather by ‘facilitated diffusion’ (Kannan 1980). In this respect, there is evidence for urea being involved in a chemical interaction with the membrane that loosens its bonds (Yamada et al. 1965; Wojlik 2004). This might explain the observation that urea can promote the uptake of other compounds applied with it (Swietlik & Faust 1984; Weinbaum 1988). Because foliar urea has inherent surfactant properties, it is often used without adjuvants (Swietlik and Faust 1984). Urea may also be able to penetrate the plasma membrane directly without dependence on active transport channels (Mengel 2002). Rapid metabolic assimilation would maintain a gradient for further absorption of urea from the leaf or fruit surface (Mengel 2002). Urea might also move out of the recipient tissues before reduction or assimilation since the molecule is phloem mobile (Swietlik and Faust 1984).

Efficiency of uptake of foliar-applied urea has been reported as being between 48% and 69%, which is 40% higher than recovery of soil-applied N (Weinbaum et al. 2002). Toselli et al. (2002) reported that in apples, maximum uptake of urea occurred in the first hour after application and least uptake between 48 and 120 hours. Spray carrier volume did not affect final uptake but higher volume and low concentration had faster initial uptake in the first 48 hours.

Uptake of urea and corresponding increases of leaf N appear to be less dependent on the existing N status of the plant than is the case for other foliar-applied nutrients such as P and K. It was suggested that this might be because urea and ammonium are not naturally present in the leaf and therefore do not have to move against a diffusion gradient. Bondada et al. (2001) also found that N-sufficient citrus leaves absorbed more foliar-applied urea than leaves from N-deficient trees. Apart from the absence of a concentration gradient for urea and ammonium, decreased uptake of foliar-applied N in trees of low N status might be due to thicker cuticles and wax layers in such trees (Bondada et al. 2001).

The metabolism of foliar-applied N once absorbed into the leaf is not different from that taken up by the roots (Swietlik and Faust 1984; Wojcik 2004). Following uptake through the cuticle, there may be active or passive transport through the cell wall and plasmalemma into the cytoplast where the foliar-applied N is assimilated into phloem-mobile forms or moved into vacuoles for storage (Swietlik and Faust 1984; Wojcik 2004). The assimilation of foliar-applied urea and NO₃⁻ differs mainly in the initial stages, although neither nitrate reductase or urease are likely to be rate-limiting (Swietlik and Faust 1984). Following hydrolysis or reduction, NH₄/NH₃ compounds are incorporated into amino acids (Guo et al. 2007).

Foliar application of nutrients can cause phytotoxicity, the most common symptoms of which are ‘burning’ or ‘scorching’ of the leaves, i.e. leaf necrosis (Krogmeier 1989; Burkhardt 2010). Urea is generally less phytotoxic than other N forms used for foliar N sprays (Weinbaum 1988; Klein and Weinbaum 1984; Swietlik and Faust 1984), although tolerance varies between different species. In fruit crops, concentrations above about 1.2% w/v are generally only suitable for post-harvest or pre-leaf fall applications because of the risk of leaf damage (Weinbaum 1988). Quin et al. (2017) state that applications of dissolved urea to pastures at rates greater than 10 kg N/ha are likely to cause leaf scorch and temporarily reduced production.

Typically young leaves are more susceptible to damage from foliar-applied chemicals so that the risk of foliar damage is greater with early season applications (Weinbaum 1988). Foliar damage is usually related to localised desiccation of the leaf tissues due to the hydroscopic properties of the foliar-applied solutes and the relatively high concentrations that remain on the leaf surface (compared to those around the roots in the soil solution) following evaporation of the spray carrier (usually water) (Wojcik 2004; Fernandez and Eichert 2009; Burkhardt 2010). Therefore, application techniques and the use of adjuvants that reduce runoff and allow more uniform distribution of the spray on the leaves could help to minimise phytotoxic effects. Early morning applications when dew is still present on leaves might be more phytotoxic than those made at midday or during the late afternoon, but this probably depends on the prevailing atmospheric conditions (Frageria et al. 2009). For example, ‘leaf tip yellowing’ with low rates of foliar urea (0.5% w/v) applied to citrus during hot weather was attributed to rapid drying and high concentrations of residual urea remaining on the leaf surface (Kiang 1982). Plants with the highest rates of urease activity appear to be the most susceptible to damage from foliar urea sprays and factors or additives that slow hydrolysis can reduce phytotoxic symptoms (Hinsvark et al. 1953). Thus, the inclusion of urease inhibitors or the use of slow release forms of urea such as trizone urea may reduce the risk of phytotoxicity (Clapp and Parham 1991). If FPA urea results in a more uniform deposit on leaves, it might allow higher rates of N to be applied than are normally possible without causing damage to the crop. It is interesting to note the comment of Quin et al. (2017) in relation to FPA urea that ‘if crushing [of the granular urea] was too fine, leaf scorch checked pasture response for a few days’.

In addition, foliar uptake of N from urea in the FPA form might reduce the volatilisation of ammonia from the urea from less being on the soil surface. This claim discounts any potential volatilisation of FPA urea from the leaf surface. Dawar et al. (2011) reported a non-significant difference between the ammonia losses from urea applied in the granular and FPA form which suggests no beneficial effect of FPA on volatilisation losses from this factor.

Citing several other studies of foliar N uptake by cool and warm season turf grasses, Totten et al. (2008) state that foliar absorption can be between 30% and 60% of the N applied.

In summary therefore, there is experimental evidence that plants (including pasture plants) can absorb urea rapidly through their leaves and that this urea N can then be metabolised within the plant in much the same way as nitrogen taken up in different forms (e.g. nitrate and ammonium) through the roots. Foliar uptake of urea may therefore be a credible mechanism by which FPA could outperform conventional granular fertilisers as suggested by Dawar et al. (2011, 2012). But this mechanism apparently did not operate to produce more pasture growth from FPA compared with granular urea in the statistically analysed results from the small plot trials. It also appears likely that the mixture of dissolved and non-dissolved urea in FPA enables a higher rate of N application without scorching of the leaves than would be the case using fully dissolved urea.

Nitrate leaching

Quin et al. (2017) suggest that there are higher nitrate leaching losses from granular than FPA urea, because of the relatively high concentration of nitrate around the solid urea granule. However, although this might be the case at a micro-scale (Dawar et al. 2011), at commonly used annual rates of N (50–200 kg/ha/yr), direct field-scale leaching losses of fertiliser N are usually very low, with Ledgard et al. (1996) only measuring significant leaching losses of 10% of the N in urea at a very high application rate of 400 kg/ha/yr.

Volatilisation

There have been several studies on the effect of granule size on volatilisation of fertiliser N and potentially at least, this may account for some of the observed differences in the agronomic performance of FPA and granular urea. The effect, however, is not straightforward. Watson et al. (1990) report that increasing pellet (granule) size can increase (e.g. Black et al. 1987), decrease (e.g. Nommik, 1973) or have no effect on (e.g. Watkins et al. 1972) volatilisation loss of fertiliser N. The work of Black et al. (1987) is perhaps of most relevance here because it was conducted on pasture under New Zealand conditions.

These authors measured volatilization losses from urea applied as a powder or as granules of 1–2, 3–4, 5–6 or 8 mm diameters. They found that at an application rate of 30 kg N/ha,

there was no significant difference in the total cumulative percentage NH₃−N loss as the granule size increased from powder to 5–6 mm diameter. However, application of the largest granules (8 mm diameter) did result in a significant increase in loss to 32.1% compared with a mean value of 19.2% for the smaller granule sizes and powder.

Therefore, the smaller granule size of urea in New Zealand (4–5 mm, cf. 8 mm diameter) would be expected to have a similar effect on volatilisation as FPA urea with its smaller prill size. This is supported by the work of Dawar et al. (2011) who found no significant difference in N loss by volatilisation between FPA and granular urea. This is in contrast to a study on turf grass (Titko 1987) showing that volatilisation losses from surface applied granular urea may be greater than from liquid urea

Conclusions

1. Several field trials have evaluated the relative agronomic performance of FPA and granular fertilisers with mixed results.

2. In the six small-plot field trials where results were statistically analysed, there was no significant increase in pasture yield from urea or DAP applied in the FPA compared with the granular form.

3. The only published data where a significant increase in pasture yield from FPA compared to granular urea was measured were in two trials where FPA urea was applied at a higher rate (one trial) to a small surface area (both trials).

4. In the only field trials where the raw data support a benefit in pasture yield from FPA, there was no statistical analysis of the results.

5. Moreover, a further nine small plot field trials comparing nutrients applied in fully dissolved liquid and granular forms failed to show the advantage to the liquid form that would have been expected if the FPA form was superior.

6. A literature-based analysis of possible mechanisms to identify reasons why FPA might offer some advantage identified only foliar uptake.

7. Over all the trials reviewed, there is insufficient experimental evidence to show any agronomic advantage of FPA over a granular fertiliser product.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The Fertiliser Association of New Zealand funded the publication of this review.