Foliar uptake of nitrogen


Although solid urea dominates the New Zealand nitrogen fertiliser market, there are a number of situations in which liquid nitrogen products are used. As a general rule, these are foliar applications of liquid urea, with or without other growth-supporting substances. In this article, we take a look at some of the scientific research that has investigated aspects of the foliar uptake of nitrogen.

Foliar application and absorption of nitrogen (N) has been explored since the 1950s, with many experiments showing that various plant species can rapidly absorb N (and other elements) through the leaf. Compared to soil N application, foliar delivery of N has the potential to more rapidly overcome N stress in plants, particularly when soils are wet or dry, or when root function is impaired, as well as offering potential to reduce losses associated with soil N application.

Mode of uptake

Absorption of nitrogen through the plant foliage is thought to occur through microscopic pores in the leaf cuticle, or through the stomata (Havlin et al., 2005). Stomatal density is generally greater on the underside of leaves, and absorption of N applied to the underside of leaves has often been shown to be more rapid than absorption of N applied to the upper surface of leaves (e.g. Tan et al., 1999). With regard to N absorption through pores in the leaf cuticle, Schonherr (1976) reported that these cuticular pores have a fixed negative charge. This negative charge is likely to repel nitrate (NO32-) because of the common negative charge, in addition to slowing the absorption of ammonium (NH4+) through electrostatic attraction. Urea (CO(NH2)2) has no charge and has generally been shown to be more rapidly absorbed than ammonium or nitrate (e.g. Wittwer et al. as cited by Bowman & Paul, 1992; Tan et al., 1999).

Following deposition on plant foliage, absorption of N by the plant is rapid. Bowman & Paul (1989, 1990a, 1992) in various studies on different turf grass species (Poa pratensis, tall fescue and creeping bentgrass, perennial ryegrass, respectively) found between 35 and 55% of the nitrogen applied to the foliage as urea was absorbed within 2-3 days. This was further supported by pot experiments in a New Zealand study (Dawar et al., 2012) on perennial ryegrass, which showed approximately 30-40% N recovery from foliar-applied urea within a few hours of application. Variation in absorption of foliar-applied N may be explained by many factors, including the rate of N applied, and the amount of water used to carry the dissolved nutrients (affecting retention on the leaf), the plant species, and the climatic conditions at the time of application.

Plant damage

Urea is commonly used as a liquid N fertiliser for foliar application since it is generally less likely to injure the leaves of the crop (‘leaf scorch’) than other forms of N fertiliser. For example, Bowman & Paul (1992) found that sulphate of ammonia and potassium nitrate both caused desiccation of perennial ryegrass leaves within 12 hours of application, whereas urea only caused light damage to the leaf tips from 24 hours onwards. Similarly, Tan et al. (1999) found that damage to the leaves of tomatoes was consistently lower on plants that had received foliar-N as urea, compared to those treated with ammonium or nitrate.

Leaf scorch is thought to result from one of two causes:

  • desiccation of plant cells by osmotic stress, as water moves out from the cell in an attempt to equilibrate intra- and extra-cellular concentrations of ions
  • phytotoxic effects resulting from the build-up of high urea and/or aqueous ammonia concentrations in the plant tissue (Gooding & Davies, 1992).

While foliar application of urea carries a lower risk of plant damage than other nitrogen forms, it is not risk free. The risk of tissue damage grows with increasing rate of N application, as well as higher concentration urea solutions (Gooding & Davies, 1992; Tan et al., 1999). Havlin et al. (2005) noted that the nutrient concentration in liquid fertiliser products used for foliar application should be no more than 1-2% to minimise the risk of scorch. This was supported by the work of Tan et al. (1999), although this may also be influenced by the rate of N application. Furthermore, foliar application when the leaves are damp (e.g. in the early morning or late afternoon) or where atmospheric humidity is high, can also increase the risk of leaf scorch, since these conditions help keep the urea in solution on the leaf surface for longer, promoting more rapid and greater absorption, which can place osmotic stress on plant cells (Gooding & Davies, 1992, citing various publications).

N losses

It has been proposed that foliar uptake of nitrogen could be a more efficient means of supplying nitrogen to the plant, resulting in greater plant nitrogen response efficiencies. The basis behind this is that direct foliar uptake of N minimises risk of ‘unproductive losses’ that are associated with soil N application, e.g. through microbial immobilisation, ammonia volatilisation, denitrification and leaching. However, applied under good agricultural practice, direct fertiliser-N leaching and denitrification losses are likely to be low (Ledgard et al., 1996) and while immobilisation may temporarily remove nitrogen from the ‘plant available’ mineral N pool, the N will later be made available through the process of mineralisation.

Ammonia volatilisation from urea is a phenomenon well documented; typical losses from solid urea applied to pasture in New Zealand are reported in the range of 10-15% of the N applied (Black et al., 1985). However, the urease enzyme that catalyses the conversion of urea to ammonium (from which ammonia volatilisation can result) is ubiquitous; in addition to being found in the soil it is also present on (and within) plant foliage. Ammonia loss from foliar urea applied to various grasses has been measured within the range of <1% - 16% of the N applied (e.g. Stiegler et al., 2008; Bowman & Paul, 1989; Bowman & Paul, 1990b) and estimated to be as high as 30% (Bowman & Paul, 1992) for rates of N similar to that applied to pasture in New Zealand. However, it must be noted that most of the foliar-urea ammonia volatilisation studies have been carried out on turf-grass species under turf culture management conditions, rather than on ryegrass/white clover pastures.

Yield gains

The efficiency gained by the reduction of loss from foliar urea compared to solid urea is generally relatively small. Furthermore, temporary damage (scorching) to plant tissues may offset some of the benefit from increased N utilisation efficiency, so the reported yield response advantages to foliar N over soil N application are generally extremely variable. For example, Gooding & Davies (1992) reviewed a large number of foliar N trials on cereals and reported many examples that showed either a significant increase, a significant decrease, or no significant difference in grain yield, relative to soil applied N. Recently, in 2009 Ballance funded several trials on pasture, kale and wheat and similarly found no significant differences between solid urea and foliar urea applications., while risk of scorch increased at rates of 40 kg N/ha and above, particularly when the liquid urea was applied as a fine spray (i.e. using herbicide spray nozzles). Based on the literature available, as well as our own experience and research, we recommend that plant N response efficiency to granular and foliar urea should be considered equivalent, provided the same amount of nitrogen is applied in both circumstances.

References

Black, A.S., Sherlock, R.R., Smith, N.P., Cameron, K.C., & Goh, K.M. (1985). Effects of form of nitrogen, season, and urea application rate on ammonia volatilisation from pastures. New Zealand Journal of Agricultural Research, 28, 469-474

Bowman, D.C., & Paul, J.L. (1989). The foliar absorption of urea-N by Kentucky bluegrass turf. Journal of Plant Nutrition, 12, 659-673

Bowman, D.C., & Paul, J.L. (1990a). The foliar absorption of urea-N by tall fescue and creeping bentgrass turf. Journal of Plant Nutrition, 13, 1095-1113

Bowman, D.C., & Paul, J.L. (1990b). Volatilisation and rapid depletion of urea spray-applied to Kentucky bluegrass turf. Journal of Plant Nutrition, 13, 1335-1344

Bowman, D.C., & Paul, J.L. (1992). Foliar absorption of urea, ammonium, and nitrate by perennial ryegrass turf. Journal of American Society of Horticultural Science, 117, 75-79

Dawar, K., Zaman, M., Rowarth, J.S., & Turnbull, M.H. (2012). Applying urea with the urease inhibitor (N-(n-butyl) thiophosphoric triamide) in fine particle application improves nitrogen uptake in ryegrass (Lolium perenne L.). Soil Science and Plant Nutrition, 58, 309-318

Gooding, M.J., & Davies, W.P. (1992). Foliar urea fertilization of cereals: a review. Fertilizer Research, 32, 209-222

Havlin, J.L., Beaton, J.D., Tisdale, S.L., & Nelson, W.L. (2005). Soil fertility and fertilisers. An introduction to nutrient management (7th Ed.). New Jersey, United States of America: Peason Prentice Hall.

Ledgard, S.F., Clark, D.A., Sprosen, M.S., Brier, G.J., & Nemaia, E.K.K. (1996). Nitrogen losses from grazed dairy pasture, as affected by nitrogen fertiliser application. Proceedings of the New Zealand Grassland Association, 57, 21-25

Schonherr, J. (1976). Water permeability of isolated cuticular membranes: the effect of cuticular waxes on diffusion of water. Planta, 131, 159-164

Stiegler, C., Richardson, M., McCalla, J., Landreth, J., & Roberts, T. (2008). Indirect measurement of ammonia volatilisation following foliar applications of urea on a cool- and warm-season putting green turfgrass species. Arkansas Turfgrass Report 2007, Arkansas Agricultural Experimental Research Series, 557, 80-84

Tan, X.W., Ikeda, H. & Oda, M. (1999). Absorption, translocation, and assimilation of foliar applied urea compared with nitrate and ammonium in tomato plants, Soil Science and Plant Nutrition, 45, 609-616

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