The present invention relates to a liquid fertiliser composition and a method of stabilising urea based fertilisers for foliar application.

There are two distinct approaches which can be used to introduce nitrogen to plants.

Nitrogen can either be uptaken by the absorption of nitrogen as nitrate at the plants roots or it can be absorbed as urea on the leaf surfaces.

These differences give rise to either soil applications or foliar applications of nitrogen based fertilisers.

For soil application the nitrogen source can be added as urea, ammonium compounds, nitrites or nitrates since in the soil the nitrogen containing compounds are broken along the path. $${\text{Urea → NH}}_{\text{3}}{\text{→ NO}}_{\text{2}}{\text{→ NO}}_{\text{3}}$$

The choice of fertiliser will depend on a variety of factors.

Since urea in the most concentrated form of artificial nitrogen fertiliser available (46%N) it is commonly used for both foliar and soil application.

In both cases the urea is broken down by urease to ammonia. For soil applications this is essential since the nitrogen source can only be uptaken as nitrate but for foliar applications the breakdown results in the leaf surfaces being scorched, which can result in the plant being killed. Consequently dose levels are limited to 10kg Urea-N per hectar in foliar applications for any one application.

In soil whilst it is necessary to have the urea converted to nitrate it is desirably to slow down the enzymic hydrolysis of urea to ammonia since when volatilisation occurs, some volatilised ammonia is lost and does not follow the path to nitrate.

Manufacturers of soil based fertilisers have attempted to tackle this problem and the problem of nitrate leaching by either:

• 1. Dusting prills of urea with sulphur to slow bacterial induced hydrolysis of urea, or
• 2. Adding direct nitrifying inhibitor such as dicyandiamide and 2-chloro-6-(trichloromethyl)pyridine to fertilisers to slow the reaction in the soil of $${\text{NH}}_{\text{3}}{\text{→NO}}_{\text{2}}{\text{→NO}}_{\text{3}}\text{.}$$

Furthermore, Fenn and Co. Workers in EP 55493 and US 4500335 have suggested adding solutions of calcium chloride, magnesium chloride or magnesium sulphate to soil to reduce the volatilisation of NH₃ from the soil.

These salts are claimed to alter the reactivity of the soil solution by an inter-reacting series of effects which include:-

• a) Enhancing the ammonia absorbing power of the crop in the presence of enhanced levels of soluble calcium;
• b) Acidifying the soil solution by interchanging H⁺ and Ca⁺⁺ in the base exchange clay complex, releasing H⁺ short term into the soil solution thus reducing NH₃ volatilisation effects; and
• c) Integrating Ammonium Carbonate, formed by reaction of gaseous ammonia with Carbon Dioxide, and Calcium Chloride to produce Calcium Carbonate and Ammonium Chloride, again reducing volatilisation by fixing the ammonium ion as a salt;

All these effects are described as direct action on the NH₄ ion in the soil, derived either from natural organic sources or added nitrogen fertiliser. Such effects are also described in US-A-4559076.

When used for foliar application the urea is dissolved in water and sprayed. The amount of urea applied is however limited by the fact that the urea is broken down on the plant leaf to volatile ammonia which can scortch thereby damaging or killing the plant or is lost by volatilisation.

Urea solutions are known and GB 1561136 concerns itself with maximizing the amount of urea which can be dissolved in water. The patent discloses adding calcium chloride to urea and water to increase the proportion of urea which can be dissolved in a given amount of water. This specification discloses a liquid fertiliser which comprises a Urea-N:Ca molar ratio of up to 5.6:1 when the urea and calcium chloride are mixed with 58% by weight water, and no claim is made to such a solution.

Other fertiliser solutions include solutions of urea with added magnesium salts such as disclosed in GB 1359884.

US-A-3930832 discloses a foliar spray on the basis of NH₄ NO₃/Zn (NO₃)₂ and urea. These solutions are applied to pecan trees to correct zinc deficiencies.

US-A-235736 discloses adding urea to calcium containing fertiliser for foliar application. Urea is added to the fertiliser since is has a favourable effect on the stability of calcium containing solutions at temperatures below the freezing point of water.

There is however a need for a nitrogen fertiliser which can be sprayed onto plants in a form whereby immediate breakdown to ammonia at the leaf surface is avoided. In this way volatilisation and plant scorch is avoided and the nitrogen uptake levels will be increased, since the urea will remain on the leaf surface in which form it can be readily uptaken.

The applicants have found that by dissolving urea in water with a sufficient amount of a divalent cationic monovalent anionic salt, the urea is made resistant to breakdown by urease.

The invention utilises a liquid fertiliser comprising urea, a divalent cationic monovalent anionic salt and water wherein the divalent cationic monovalent anionic salt is present in an amount sufficient to make the urea substantially resistant to breakdown by urease.

According to one aspect of the invention there is provided a method of providing a crop with urea in a form which has an increased stability to breakdown by urease characterised in that the urea is applied to the crop in a foliar application as a solution containing urea and a divalent cationic monovalent anionic salt selected from calcium chloride, calcium nitrate, magnesium chloride, magnesium nitrate and zinc chloride.

According to another aspect of the invention there is provided a method of providing a crop with urea in a form which has an increased stability to breakdown by urease characterised in that the urea is applied to the crop in a foliar application as a solution containing urea and a divalent cationic monovalent anionic salt, the molar ratio of urea-N to divalent cation in solution being from 8:1 to 3:1.

According to another aspect of the invention there is provided the use of a divalent cationic monovalent anionic salt for making a urea containing liquid fertiliser for foliar application more resistant to breakdown by urease.

Substantial urease resistance has been found to be achieved as the molar ratio of urea-N to the divalent cationic metal -Me of the divalent cationic monovalent anionic salt approaches 4:1.

It is preferred that the Urea-N to Me ratio is from 8:1 to 4:1, more preferably 6:1 to 4:1 and more preferably still 5.5:1 to 4:1, at which point urease resistance is maximised.

It will be apparent that increasing the N:Me ratio in favour of Me, i.e. going from 4:1 to 3:1 will not alter the fact that urease resistance has been maximised, and consequently ratios in which the N:Me ratio is increased in favour of Me beyond 4:1 will not alter the fact that urease resistance is achieved. Consequently such ratios should be included as being within the scope of the claims.

The divalent cationic monovalent anionic salts may be selected individually or in combination, to give the desired ratio, from Calcium Chloride, Calcium Nitrate, Magnesium Chloride, Magnesium Nitrate, Zinc Chloride and Zinc Nitrate.

The fertiliser may further comprise a potassium salt for example potassium chloride or potassium sulphate, or a sulphur product for example ammonium sulphate or ammonium thiosulphate.

Furthermore the liquid fertiliser should exclude trace elements selected from the group copper, iron, colbalt and nickel or phosphate.

The applicants have found that when urea is mixed with a divalent cationic monovalent anionic salt in effective proportions the hydrolysis of urea by urease, which is the main reaction on the plant leaf surface involving the formation of volatile ammonia is inhibited. Whilst not wishing to be bound by theory this phenomena is believed to be caused by the salt causing the urea to assume its Tautomer III configuration, defined in the classical organic literature. It is believed that a complex with the divalent cationic monovalent anionic salt is then formed.

Structure II above is favoured by many as the likely structure of normal crystalline urea, as it explains the monoacidic properties of urea and the ready elimination of ammonia, both by chemical hydrolysis and particularly by enzymatic urease hydrolysis. Urease is endemic in most agricultural situations being particularly rich in decaying organic matter.

Thus it is believed that a divalent cation (X) associated with a monovalent anion (Y), gives a complex with urea of the formula:-

Solutions believed to contain complexes of this type have been shown to be stable to decomposition by urease, thus slowing down dramatically the evolution of ammonia, and giving a longer period for the urea per se to be able to penetrate either the leaves or the hypercotyl direct, rather than via the route Urea - NH₃ - NO₂ - NO₃ for root absorption, with the consequent losses of NH₃ by atmospheric evolution or nitrate leaching. This provides many advantages in normal agromomic practice.

Preferably the effective amount of the divalent cationic monovalent anionic salt will be an amount giving rise to a ratio of 4 urea nitrogens to 1 divalent cation.

In such a molar ratio, a complex is believed to be formed throughout the solution. However since ratios which approach 4:1 have been shown to be substantially urease resistant it is believed but when ratios of the urea-N to salt do not reach this theoretical ratio a complex may still be formed in part.

These findings are of great importance since it enables urea to be applied to crops in greater amounts by foliar application as opposed to soil applications. The reason for this is that in appropriate ratios the breakdown of urea to ammonium is inhibited. Solutions having effective ratios have been shown to slow the action of urease on the urea depending on the divalent cation/nitrogen ratios. This slowing down in the conversion of urea to ammonium means that the urea can be absorbed by the plants leaves. Without an effective ratio the urea is rapidly degraded to ammonia which can not be uptaken by the plant leaves and which furthermore can cause scorching of the plant and loss by volatilisation.

It is believed for example that when a urea/calcium chloride solution having an effective ratio is applied to a leaf, the calcium will be absorbed against a concentration gradient. The gradient is created by calcium accumulating in the leaves, some being built into cell wall structure, as pectate, and excess precipitated as oxalate.

Calcium uptake would be slow and the stable urea complex maintained. Urea would be absorbed by the leaf as urea rather than as ammonium-N from the stable structure and will be controlled probably by the rate of calcium absorbtion.

It is known that the absorbtion of nitrogen by the leaf is more efficient if absorbtion takes place as urea-N rather than ammonium-N. The presence of calcium chloride promotes this effect.

The socio-economic benefit of these findings is enormous. Nitrogen may be foliar applied and taken up efficiently and sufficiently to grow high quality yields without the problems of nitrate water pollution.

A variety of compositions have been tested and are used by way of example only, to illustrate the invention.

Two qualative tests and one quantative test have been used to test whether urea degradation by urease occurs when the urea was mixed to various cationic/anionic salts in various proportions.

These methods are described below and the results achieved are noted in the tables.

A test model was prepared whereby air, which has previously been scrubbed with boric acid was bubbled over a urea solution containing 3280mg of Urea in a closed container. A standard urease tablet was added, sufficient to decompose 400mg Urea in 3 hours at 35°C and the aspired air was then scrubbed with boric acid and the evolved NH₃ analysed by one of the following three methods.

• 1) The pH at the beginning and end of the reaction was checked. Normal Urea with urease was shown to give a rise of pH from 7-10 whereas stabilised urea remained at pH7, even in the presence of urease. A constant pH therefore indicates stabilising activity. Qualitative.
• 2) NH₃ gas evolution is easily detected by inserting a piece of pH paper into the container and measuring the pH change in the atmosphere - Qualitative.
• 3) The method can be quantified by direct analysis of NH₃ by the Kjeldahl method - Quantitive.

a) Quantitive Test Method:

10 Mls of test solution was warmed to 27° in a 50 ml beaker. One BDH urease tablet was added and crushed with a glass rod. The beaker was placed on a constant temperature tray and maintained at 27°C. It was covered with a bell jar and ammonia-free air was passed across the sample for 6 hours. The air was passed through 100mls of 2% boric acid solution containing indicators. The resulting solution was analysed for ppm ammonia nitrogen (NH₃ - N).

The molar ratios of Urea-N to Metal (Me) can be simply calculated from the weight ratios given.

For example a 14:0:0:10 solution N:P:K:Me where the divalent cationic monovalent anionic salt is calcium chloride has a N:Me/molar ratio of 4:1$$\text{No moles =}\frac{\text{wt}}{\text{rmm}}$$$$\text{N =}\frac{\text{14}}{\text{14}}\text{: Ca =}\frac{\text{10}}{\text{40}}$$$$\text{1 : 0.25}$$$$\text{4 : 1}$$

In practice a 14:0:0:10 will be made from$$\text{14 x}\frac{\text{60}}{\text{28}}\text{urea,}$$ and$$\text{10 x}\frac{\text{111}}{\text{40}}\text{calcium chloride,}$$ i.e.

   30 g of Urea and

   27.75g Calcium chloride,

The mix being made up to 100 g with water.

Table 1 shows the stabilising effect that different divalent cationic monovalent anionic salts have on urea.

That monovalent cationic and divalent anionic salts do not show a stabilizing effect on urea can be seen from table 2.

Salts such as MgSO₄ and KC1 give virtually no stabilisation nor do the sulphates and phosphates of urea.

Metal salts of variable valency state (i.e. Fe, Ni, Mn, Co, Cu) were found to decompose in the presence of urea to complex oxides and hydroxides.

There are clearly defined ratios within which the stabilisation effect occurs which have been determined by quantitative studies with regard to the chlorides as given in Table 3 and confirmed with a series of qualitative studies for chlorides and nitrates as given in Tables 4 to 9 inclusive.

Tables 4 to 9 show that significant amount of Urea-N can be maintained on the leaf surface at ratios of Urea-N:Me which require only limited amounts of Me. For example at 10:1 N:Ca little or no smell was noted after 120 hours, indicating a large proportion of the nitrogen remains as urea-N. (table 4) For example at 15:1 N:Ca only a slight smell of ammonia was noted, indicating significant Urea-N remains even after 168 hours. (table 5). From these qualative tests it can be seen that even at ratios of 23:1 N:Zn urea resistance can prove significant.

It is clear from the tables that more significant inhibition commences at 14-0-0-5 Ca for CaCl₂, 19-0-0-7 Ca for Ca(NO₃)₂, 14-0-0-4 Mg for MgCl₂, and 19.6-0-0-4.8 Mg for Mg(NO₃)₂.

In the case of Zn, inhibition of hydrolysis also occurs but as Zn can easily poison the enzyme and nutritional Zn requirements of plants are of a low order, Zn is only ever considered as an additive to either Ca, Mg or mixed Ca/Mg products for practical use in agriculture. However with 8.5 to 9%N in a solution a minimum of 4% Zn is necessary to effect stability in its own right (9-0-0- 4% Zn).

It has been further found that the stabilising effect on urea can be achieved by mixing the divalent cationic monovalent anionic salt so that the molar ratio approaches 4:1.

Field Test Work

Seven field trials have been carried out on four crops - Winter Wheat, Spring Wheat, Spring Barley, and Potatoes. The broad conclusions of this work can be summarised as follows:

• (a) Phytotoxicity on Cereals
  
  

  The phytotoxicity of urea to cereal crops caused by rapid evolution of ammonia under high temperature conditions is dramatically reduced by the presence of CaC1₂ at optimum ratios with an almost identical effect from Ca(NO₃)₂ and MgC1₂. See Table 10 below.

• (b) Phytotoxicity on Potatoes
  
  

  In one trial on potatoes phytotoxicity was not prevented by CaC1₂ with urea. It is thought that the presence of C1 compromised the results. C1 is known to be highly toxic to potatoes and Ca(NO₃)₂ is being checked as an alternative.

• (c) Nutritional Responses
  
  

  From the nutritional stand point the following trends can be noted:

  • i. Urea/CaC1₂ or Urea/MgC1₂ are, in general, equal or superior in nutrient uptake response when compared to urea alone. This variability would be expected as it is only when significant loss of NH₃ occurs from a urea alone spray that the inhibition effects of CaC1₂ or MgC1₂ can express themselves as a superior nutritional N response.
  • ii. Slight to marked effects show on the improvement of the uptake of secondary nutrients such as calcium and magnesium when either Urea/CaC1₂ or Urea/MgC1₂ is applied. See Table 11 below.
  • iii. Urea/CaC1₂ and Urea/MgC1₂ as foliar sprays at lower N rates approaches the nutritional value (measured as yield, specific weight, 1000 grain weight) of traditional soil applied granular fertilisers at standard rates, in some cases, but never exceeds them. There is a trend for Urea/MgC1₂ to be slightly better than Urea/CaC1₂ but this may be related directly to the Mg status of individual soils. See Table 12 above.
  • iv. There are strong indications that in the series of trials carried out that Urea/CaC1₂ and Urea/MgC1₂ have a greater effect on quality than on yield. This would be expected, however, from the spray timings involved. See Table 13.

  
  
  

  Trials were carried out on cereals mainly as late season foliar sprays at a time when nutritional responses from nitrogen would be expected to be in crop quality rather than yield. This is the current traditional market for foliar applied urea.

• (d) Summary of all Trials
  
  

  Table 14 summarises the field trials. Results which were positive for Urea/CaC1₂ or Urea/MgC1₂ are indicated +, results which where positive for Urea alone are indicated -, results where no difference was observed or measured are indicated =, and where the characteristic was not tested is indicated x, except for Part 3 see below.

The conclusion to be drawn from these trials are that foliar treatments with Urea or their stabilised forms of Urea/CaC1₂ and Urea/MgC1₂ can act as an alternative to soil applications when it comes to maintain crop quality and yields. Furthermore they have the benefits of far less risk of leaching because of the property of direct absorbtion into the leaf.