NITROGEN

Much of the nitrogen on earth is found in rocks (contrary to the intuitive guess) and not in the atmosphere, primarily in basal granite rocks. Minerals that develop from these rocks include ammonium, known as "fixed ammonium" or "crystallized ammonium". In sandy soils, with little clay  content or in shallow soils - the content of fixed ammonium is of course smaller.

In nature nitrogen is found with all possible degrees of oxidation as follows (the degree of oxidation appears in brackets):

nitrate (+5) - NO₃^-
nitrite (+3) - NO₂^-
monoxide (+2) NO (gas)
nitrous oxide (+1) N₂O
atmospheric nitrogen (0) N₂ (gas)
ammonia (-3) NH₃   (ammonia in association with organic carbon as amide, R-NH₂)
ammonium (-3) NH₄⁺.

Nitrogen in fertilizers  appears mainly as : ammonium, ammonia (as high-pressure liquid), nitrate and as amide in urea.

In fact in the soil nitrogen transform  from ammonia to nitrate by microbial oxidization. The nitrate in soil is facing 3 alternative ways: uptake by plants, leaching below the root zone, or reduction by microorganisms. It is reduced in stages through NO₂, NO, N₂O to N₂ so that it is hard to predict and to know which form of nitrogen will be found and where in a given soil system even at a relatively short time after it was added to it.

In the "nitrogen cycling" in nature (Fig. 3.01) the relatively stable Organic N is placed in the center. Nitrogen in the air might enter to the nitrogen pool in the soil as a result of electrical discharges and small quantities of nitrogen are introduced to the soil as ammonium or as nitrate, by rain. These processes is referred to as "Atmospheric Fixation".

Fertilizer nitrogen is applied to the soil and relatively quickly turns into nitrate, or turns into organic nitrogen in the bodies of microbes and plants. The mineralization process of organic material yields ammonium. the NH₄⁺,  is oxidized in the soil in two stages : first to nitrite (NO₂^-) and then to nitrate  (NO₃^-). Nitrite is usually unstable and lasts only temporarily. The more stable nitrate is the final result of the biological nitrification process. The nitrate can leave the root zone in a number of ways : absorption by plants and leaching by rain or irrigation water, or by reduction (denitrification) to N₂O or N₂.  The denitrification process is responsible for " N losses" which are not less and sometime more than the amount of N taken up by  plants.

The denitrification process occurs primarily when the oxygen supply in the soil is suddenly depleted due to flooding or intensive irrigation during plant growth. The soil microbes that use O₂ as an electrons acceptor in their respiration process turn to nitrate, the compound with the highest oxidation potential available in the soil when insufficient oxygen in the soil air and soil water. In irrigated summer crops ( when soil temperature is high and so is the total activity of the microbial population), and primarily in clay soil, denitrification might become the main process of  nitrogen loss from the soil.

Nitrite (NO₂) exists only in a high pH such as after hydrolysis of urea, when oxidization is incomplete. During denitrification ,nitrite nitrogen it is not found in significant concentrations in the soil.

The intermediate stages in the nitrification and denitrification produce N₂O which is released as a gas to the atmosphere and can be measured. This process exists in all soils in the world and even in uncultivated soil. It should be mentioned that in the stomach of ruminants a process of breaking down proteins takes place through micro-organisms, and in their breathing nitrogen is expelled as ammonia into the air.

There are evidences of measurement of the direct loss of nitrogen as ammonia from the leaves of plants and particularly when they grow old. In drip irrigation there is also a loss of nitrogen through denitrification from the saturation zone. The saturated area contains fewer roots, because of the low aeration, and most of the ammonium  is  sorbed to the soil near to where it is given in the trickle source.

Mineral nitrogen is formed in the soil continuously from the soil organic nitrogen pool. It is, therefore, worthwhile knowing in each case the quantity and rate of nitrate production in every typical soil.  The amount of N - fertilizer  addition  should take into account the amount of nitrogen available in the soil in the growing period from the organic pool (Hadas et al ., 1988, Fig. 3.02).

On  Eden soil  in the Bet Shean valley (highly calcareous soil), Dr. Aviva Hadas measured the nitrate nitrogen in incubation (nitrification capacity) in the soil at various depths. It was found that the 0-20 cm layer released as nitrate in the 32 weeks about 350 kg N/ha (a calculation that is close to the values of Fig 3.02 (on the assumption that 4 ppm N(mg/kg) in a layer of soil at a depth of 20 cm with bulk density of 1.25  is equal to 10 kg nitrogen per hectare). The 20-40 cm layer about 190 and 40-60 cm released a further 60 kg/ha nitrogen as nitrate. The total for 0-60 cm was 600 kg N/ha.

In Bet Dagan control, plot that has not been fertilized for 30 years (the permanent fertilization experimental plots at Bet Dagan)  still provides  80-100 kg nitrogen per hectare.  This value explains the ability of these plots to produce a reasonable yield, (even if not maximal.) without the addition of nitrogen.

In a loesial soil Gilat,  the rate of release was almost linear over time. It released during the experiment (32 weeks) 220 kg N from the 0-20 cm layer and 180 kg N/ha from the 20-40 cm. Already during the first  8 weeks the soil provided a very substantial amount of nitrogen by nitrification from the organic pool.

In all soils the amount of organic nitrogen in deep soil layers is lower  and also more stable against decomposition than that from the top soil. It is clear that if the soil is not wet and aerated, the nitrification rate is slowed and there is not full exploitation of its "nitrogen potential" for growth. When drip irrigation wets only a small layer of soil in the top soil, the soil volume that contribute mineral N from the organic pool is minimal and therefore in trickle irrigation addition of fertilizers is always needed. The "soil nitrogen potential" is defined as the asymptote of the parabolic lines in Fig. 3.02

The laboratory testing method for the determination of "nitrification potential" use  5 grams of wet and aerated soil placed in incubator at 35 ˚C the maximal best conditions for nitrification. The extrapolated calculation per volume (or weight) of soil per hectare at a particular depth, of the nitrogen available to plants from a certain soil  might introduce a huge error value.

 Another  approach to estimate the available N in the soil was developed by Dr. J. Amir and is known in Israel as "the Gilat method". In this method a 25 diameter auger is used to sample down to 60 cm. The soil is mixed and  a 10 kg is put in a bucket of 15 L and corn is grown for a full month. The nitrogen extracted by the crop during 1 month is taken as the amount of available N in the field. This method is successfully used by farmers to predict N demand for dry land wheat in the southern part of Israel when rain is usually the limiting growth factor.

Working with labeled organic material (N-15 and C-14) ( Fig. 3.03) it was found in Rothemstead , England, that about 70% of the carbon disappeared from the soil within the first 4 - 5 weeks (the experiment lasted for 210 weeks). On the other hand the rate of nitrogen loss from the organic phase was more slowly. In this way the C:N ratio  of the fresh organic material in the soil changed during the organic material decomposition.

Ya'akubovich, Levin and Kafkafi examined the rate of nitrification of 500 ppm (mg N per kg) nitrogen as ammonium sulfate in a clay soil at Kibbutz Huliot, incubated at a constant temperature of 20˚C ( Fig. 3.05) at 3 moisture levels: wilting, half field capacity and field capacity.  This is  a high average concentration, but can be found near a localized fertilization band.  At  wilting point moisture content even after 24 days still 350 ppm N remained as ammonium. At half field capacity - half the nitrogen turned into nitrate within 18 days and after 24 days about 230 ppm N still  remained as ammonium . At "field capacity" moisture, after 14 days the ammonia and nitrate concentrations were equal (50% nitrification) and after 24 days  about 120 ppm N as ammonium were still available.

The soil moisture, fertilizer concentration (which induce salinity) and temperatures - influence the rate of the nitrification process. In spite of this, even relative dryness (wilting point) does not completely stop the nitrification process (Ya'akubovich, Levin and Kafkafi).

In the following illustration (Fig. 3.06) the process is presented with the same soil and fertilization data, but this time the temperature is 10˚C and the ammonium was preserved throughout the experiment almost without any production of nitrate.

The temperature, therefore, has a great influence on the rate of nitrification which, as aforesaid, is a biological process.

Adding urea instead of ammonium sulfate ( Fig. 3.07), also at a concentration of 500 ppm nitrogen per soil weight, it was found that at 10˚C after 50 hours 90% and more of the urea became ammonium at a moisture content of both "field capacity "and "half field capacity" and was almost the same at "wilting point". In other words: the moisture influences hydrolysis of urea to ammonium very little. Even at 10˚C the process lasts only for hours and not for days.

With the same disposition, at a temperature of 30˚C, the hydrolysis of urea to ammonium reached 50% within 10 hours, and was totally completed within 24 hours ( Fig. 3.08).

Conclusion: The hydrolysis process of urea is quick, and lasts only for a period of hours. It is therefore worth working the urea into the soil, close to it being spread either mechanically , by irrigation or to coincides it's application with the rain.

At a more moderate concentration - 120 ppm- the urea disappeared within 10 hours. The nitrification of the ammonium from a concentration of 120 ppm at about 30˚C was completed within 12 - 14 days. However it should be stressed that the first stage of urea hydrolysis involve a very sharp increase in pH in the immediate zone near the fertilizer granule.

Since the "nitrogen cycle"  is controlled by biological activity, high local fertilizer concentration slows the rate of the nitrogen transformation processes .

 

FERTILIZATION WITH AMMONIUM SULFATE  OR UREA?

While urea produces NH₄OH (and result an increase in pH) as the immediate produce of the hydrolysis process in soil,  ammonium sulfate gives NH₄⁺ and a low pH.  The difference is therefore in the pH and, as a result, the ammonium-ammonia equilibrium in the soil system.  Duisberg and Buehrer (1954)  ( Fig 3.09a) described the rate of the processes in the soil as a result of the injection of anhydrous ammonia into the soil. For 4 days  following the injection, no  nitrification was  measured. Four days later nitrite start to appear  and accumulate and after a further four days accumulation of nitrate started. The pH declined continually from the moment of fertilization, from pH 9 at the time of the injection. Only after the pH dropped to 7.8 did the production of nitrite begin and at pH 7.4 nitrate began to accumulate. CO₂ in the soil air is responsible, at the first stage for about 3 days for the titration of the high pH. In this experiment the rate of the decline in the concentration of ammonia was fairly steady for 10 days: about 25 ppm N/ day. Together with the decline in the concentration of ammonia the concentration of nitrite increased. So long as the pH was above 7.5, only nitrite was produced.

The explanation for this is : the nitrosomonss microbes  that oxidize ammonium to nitrite can tolerate relatively high concentration of ammonia (NH₃). The nitrobacter microbes which oxidize nitrite to nitrate are very sensitive to ammonia.. Since the ratio of ammonium:ammonia in the soil solution depends on the pH, decrease in the pH, will decrease the ammonia concentration. When the oxidation of nitrate is delayed - nitrite is accumulated. This also occurs with a high concentration of ammonium and a high pH. With a low concentration of ammonium, the nitrite does not accumulate even if the pH is high since high  concentration of ammonia that is poisonous for nitrobacter could not accumulate due to the initial low ammonium concentration. Urea which is given in a concentrated band can, therefore, when applied as side dressing can lead to the collapse of growth, particularly when it is given next to the roots of very young plants. Side dressing with ammonia can cause a similar situation, if it influence a sufficiently large part of the root volume. In practice, the situation with nature cotton is not causing any damage since the fertilizer is given in a narrow band when the cotton roots already occupy a large volume of soil and the common side dressing of urea or liquid ammonia treatment therefore are successful.

In a classic experiment ( 3.10) urea was placed in the soil and  a sharp increase in pH was observed immediately after the fertilization which then faded over time to a lower pH than in the control.  

The forms of nitrogen which were found in the soil with time ( Fig. 3.11) were firstly ammonium which faded gradually,  nitrite peak was reached after 3 weeks and the nitrate accumulated only after week 4, over the course of the 6-week experiment.

When pots with maize seedlings were "placed" on the treated soil at various dates,  the yield  was high when ammonium was dominant and also when there was much nitrate, but were very badly affected in the period of time that there was a great amount of nitrite in the soil. It should be stressed that the total amount of soil, water and nitrogen was the same in all the treatments.

 

THE IMPORTANCE OF  THE pH:

The elongation of cotton  root seedlings was measured in a nutrient solution as a function of different ammoniac nitrogen concentration  at 4 constant pH values ( Fig. 3.12 ). It was found that at pH 8 and higher the roots were held back, but at pH 7 they grew well.

The ammonium: ammonia ratio explains these findings: at a high pH the concentration of ammonia (NH₃)_aq rises in the solution and its effect is expressed in the delay in root growth.

The pH will determine whether there is nitrite, ammonia or nitrate and ammonium in the soil ( Fig. 3.13).

The equation:  K_w (NH₃)_aq (H⁺) = K_b (NH₄⁺) describes the concentration of ammonia in water, when K_b - the base equilibrium constant, K_w - the water constant. The equation shows that the concentration of the ammonia (NH₃) increases with the decrease in the proton (H⁺), in other words with increase in pH.

In the table ( Fig. 3.14) an experiment is described that was conducted in Denmark. Nitrate, sulfate, ammonium, phosphorous, sodium and magnesium were tested in a sand culture in which barley grew for 52 days. The composition of the solution and the disappearance of the various elements in it were monitored. The nitrogen fertilization was given as calcium nitrate,  ammonium sulfate or urea.  The concentration of nitrate was initially very high, of course, with the calcium nitrate fertilization. On the other hand, at the end of a month - the nitrate  was taken up by the plant and disappeared from the solution. Ammonia sulfate produced  nitrate after a delay, whereas the urea (thanks to the increase of the soil pH which was acidic at the beginning) increased the nitrate concentration before the ammonium sulfate. The ammonium from ammonium sulfate lasted for a longer time than that from the urea.

These observation are due to both the pH and mainly in the higher salt concentration in the  ammonium sulfate treatment. The concentration of potassium in the soil solution was fairly similar to the nitrates: so long as there was an anion in the solution, it also contained potassium and without the nitrate, potassium  disappears.

The same applies to the other cations - in the absence of the anion (nitrate) the cations also

disappear from the solution, as there has to be an electrical balance in the solution.

A test of the total cations and the pH in the solution shows that in the nitrogen fertilization as calcium nitrate, with the plant absorbing nitrate, the solution pH during the experiment

increased from 6.2 to 6.5. Fed with ammonium sulfate - the pH declined from 6.0 to 4.5. When the nitrogen was given as urea - the pH increased to 7.15 initially, and in the course of the experiment dropped to 4.75. The remaining cations in the solution were greater with treatment by ammonium sulfate, as there is little absorption of the sulfate, and as the sulfate remains in the soil solution - cations have to remain in the solution to balance the electrical charge.

 

NITROGEN  LOSSES

Our discussion so far has focused on nitrogen entering the soil system as fertilizer and leaving it through the plant.

But there are also other processes responsible to nitrogen losses from the soil.

In the illustration ( Fig. 3.15) the results of an experiment at Bet Dagan are presented (Bar-Yosef and Kafkafi, 1972). In this experiment the nitrogen in the soil was measured in the 0-20 cm layer at various dates after basal of   fertilization of  ammonium sulfate was given  before sowing maize. Irrigation given: 130 mm immediately after fertilization (basal irrigation), and irrigation after planting which included irrigating to secure plant emergence (40 mm), at the age of 23 days (68 mm), at the age of 40 days,  (60 mm), at the age of 60 days (68 mm), and at the age of 75 days (72 mm). A week after the first irrigation  the ammonium concentration decreased markedly. The nitrate concentration was in accordance with the amount of fertilizer application,.

Immediately after the second irrigation most of the nitrate was lost in all the treatments. The balance in the plants and the soil does not explain the disappearance of the nitrates.  The explanation is : a loss of nitrate through denitrification to N₂. This explanation was encouraged and reinforced by the prevailing circumstances: the existence of an appreciable concentration of nitrate, vigorous bacterial activity (summer growth), lack of oxygen ( due to over dose of irrigation) and an abundant source of carbon. As soon as irrigation leads to the accumulation of a layer of water on the soil surface the rate of oxygen diffusion  into the soil drops to one millionth of  the situation a moment earlier.

The vigorous consumption of oxygen by the micro flora in spite of the interruption of its supply, and the creation of anaerobic conditions brings about immediate denitrification While nitrification is brought about by two specific groups of microorganisms, denitrification is brought about by a large variety of micro-organisms in the soil, which use the nitrate as an electron acceptor  as soon as the oxygen is missing or lacking. Therefore the denitrification is much faster than nitrification and various measurements have found that all the nitrates can disappear from the soil within a few hours.

The oxidation of ammonium in the soil is subject to certain conditions : ( Fig. 3.16) .

1. Osmotic tension (caused by the concentration of fertilizer or salts) of more than 10 bars is sufficient to stop nitrification. This happens, for example, with a concentrated band of fertilizer. A concentration of ammonium of above 3000 ppm meets this condition, and suppresses nitrification.

2. pH 8 or higher - delays nitrification.

3. When the pH is higher than 7.8 there will be an accumulation of nitrite.

4. If the pH is equal to 7.8 or less - the nitrification to nitrate will occur.

Now it is clear that under various conditions of moisture, salinity, nitrogen concentration, pH and fertilizer type, various forms of nitrogen will be present at a certain time in the soil.