I think it is not so clear.

During my studies in Poland I was taught, that ammonium salts is better source of N than nitrates, because ammonium does not require to be reduced with considerable cost of energy. It is simply cheaper from energetic point of view.

However, during my Erasmus studies in Portugal I was taught, that nitrate is better than ammonium, which is toxic for plants. The explanation was of physiological nature. Both ammonium ions and ammonia easily diffuse through the cellular membranes and this may have negative effect on photosinthesis. These process requires proton gradient on opposite sides of the membranes. The pH within thylakoids should be of about 4-5 (acid), while in stroma - of about 8 (alkaline). The ammonium ions NH4+ in stroma are transformed into ammonia NH3, and, due to different concetration gradient passe through the membrane to inner space of thylakoids. In thylakoids this ammonia transforms in ammonium ions (takes protons), and, again, due to gradient of concentration, it goes from thylakoids to stroma. And this situation may repeat many times. As a consequence, the protons are being transported from thylakoids to stroma. This changes pH - in thylakoids it increases, in stroma - decreases. And this is toxic to plants. I don't know if I explained it clearly.

However, we should be aware, that both opinions - "Polish" and "Portuguese" may be truth in particular conditions of both countries. In Poland there is less light, so, the solar energy may be limiting factor.

An excellent explanation Michal followed by another good  response from Dr Anil Kumar . The question is, which plant is better equipped to accept which form of nitrogen , ammonium or nitrate ions ..?And , which plant root  wall cells have greater density of  nitrate or ammonium transporters to be really decisive in choosing , which form of nitrogen shall be preferentially absorbed..? 

Calcium nitrate compound dissolves quickly in water and can be absorbed rapidly by plants in all soils and climates. The nitrogen in calcium nitrate also helps a plant absorb other nutrients in the soil, while the calcium provides the necessary nutrition to promote cell growth in a plant.

(1). Plants absorb nitrogen from the soil both in the form of nitrate (NO3−) and ammonium (NH4+). Then, I agree with Dr. Srivastava's remark that preference of the plants for absorption of nitrate and ammonium might be decided by the density of NO3 or NH4 ions at the root wall cells.

(2) In aerobic soils where nitrification can occur, NO3 is usually the predominant form of available nitrogen that is absorbed. However NH4 form can predominate in grasslands, strongly acidic soils and in flooded, anaerobic soils.

(3) Plant roots themselves can affect the abundance of various forms of nitrogen by changing the pH and secreting organic compounds or oxygen. NH4 ions are absorbed by the plant via ammonia transporters. NO3 is taken up by several nitrate transporters that use a proton gradient to power the transport. Nitrogen is transported from the root to the shoot via the xylem in the form of NO3, dissolved NH4 and amino acids. Usually most of the NO3 reduction is carried out in the shoots while the roots reduce only a small fraction of the absorbed nitrate to ammonia.

(4) NH4 (both absorbed and synthesized) is incorporated into amino acids via the glutamine synthetase-glutamate synthase pathway. While nearly all the NH4 in the root is usually incorporated into amino acids at the root itself, plants may transport significant amounts of NH4 ions in the xylem to be fixed in the shoots. This may help avoid the transport of organic compounds down to the roots just to carry the nitrogen back as amino acids.

Nitrate is the readily available form of nitrogen to plants. Otherwise, if it is not taken by plants it can be lost in different forms, e.g. through leaching, denitrification, etc.

However, plants vary in their uptake depending on soil condition and their preference. For instance, blueberries have very low nitrate reductase activity and thus are almost completely dependent on NH4-N as the N source. In contrast, the monocots particularly grasses seem to be comfy in utilizing nitrate.

Studies showed that  the rate of nitrate  uptake is usually high and is favored under low-pH conditions while NH4- uptake is best under neutral pH condition and is depressed by increasing acidity. Other studies indicate that higher growth rates are achieved with a mixed supply of both NO3-N and NH4-N. When both forms of N are applied, regulation of intracellular pH becomes easier for plants.  

Please check the reference:   

http://www.kno3.org/en/product-features-a-benefits/nitrate-no3-versus-ammonium-nh4

Some excellent points  Dr Kundu and Dr Getachew ,  both of you have added to the quality discussion..How are plants able to distinguish the preferential absorption of ammonium over nitrate or vice-versa..?

Nitrates are the preferred nitrogen source because

1. Nitrate is Non-volatile: unlike ammonium, nitrate is non-volatile, so there is no need to incorporate it in the soil when applied by top- or side dressing, which makes it a convenient source for application.

2. It is mobile in the soil - direct uptake by the plant, highest efficiency.

3. Nitrates synergistically promote the uptake of cations, such as K, Ca and Mg, while ammonium competes for the uptake with these cations.

4. Nitrates can be readily absorbed by the plant and do not need to undergo any further conversion, as is the case with urea and ammonium, before plant uptake.

No acidification of the soil if all the nitrogen is applied as nitrate-nitrogen.

5. Nitrates limit the uptake of harmful elements, such as chloride, into large quantities.

6. The conversion of nitrates to amino acids occurs in the leaf. This process is fuelled by solar energy, which makes it an energy-efficient process. Ammonium has to be converted into organic N compounds in the roots. This process is fuelled by carbohydrates, which are at the expense of other plant life processes, such as plant growth and fruit fill.

Nitrate is not the preferred form of nitrogen from the plants, in fact nitrate must be converted in nitrite, then in ammonium and then in organic nitrogen by use of specific enzyme and consumption of energy (ATP). Usually plant absorb nitrate because in the water soil solution nitrate is more easily present. Has been shown by several researchers that L-amino acid, if present in water solution can be absorbed and used unaltered or transformed in other amino acids. Under a practical point of view, we have noticed that  crops (i.e. apple, pear)  fertilized  with ammonium nitrate need of 3-4 times of nitrogen respect  to the same crops fertilized with  amino acid based fertilizers

One of the reasons , could also be the fact , that plants have to exert much lesser energy while absorbing nitrates than ammonium ions..

Optimum soil water content is very important as a transport medium for NO3-N and NH4-N. In addition, soil water content influences the rate of oxygen supply and thereby controls whether aerobic processes (nitrification) or anaerobic processes (denitrification) dominate within the soil. Low or excess soil moisture can be the major cause of N as N2O from the soil as emission or as NO3 through leaching.

Well said , and i agree with you , deficit or excess of moisture in soil will turn soil towards aerobic or anaerobic , thereby , preference of NO3 over  NH4 or vice versa wil be governed , thereby , energy consumption b the plants as well... 

One of the reasons, could be the fact that,

Some species of plant prefer NO3 then NH4 because plant easily metabolize/ convert NO3 to Protein and Amino acid then NH4. 

Nitrate is readily absorbed by plant root and not need any further conversion while Ammonium -Nitrogen need.

which also depend upon the temperature and moisture of the soil...

I Agree with Sushanta Kumar Naik andAnoop Kumar about reasons that made NO3-1 the preferred form of nitrogen added to the plant.

How does soil moisture vis-a-vis plant moisture dictate the preferential uptake of nitrates over ammonium ions or vice -versa...?

Dear colleagues,

Most, though not all, aspects have been touched upon, but allow me to add some further comments that may clarify some of the apparently contradictory reports.

First of all, we should distinguish clearly between

1. Preferred for actual uptake by the plant at the root surface

2. Preferred for growth and yield formation

3. Preferred by the agronomist / farmer (crops don’t think about costs)

Add 1: Almost all plants take up ammonium more rapidly when both forms are available at the root surface (hydroponics), and this applies even more so to conventional, standard crops. A recent example https://www.researchgate.net/publication/292148832_Effect_of_Small_Quantities_of_Ammonium_on_the_Total_Nitrogen_Nutrition_of_Maize. In this regard, I don’t agree with A K Singh. Crops like blueberries (comment by G Agegnehu) and tea (https://www.researchgate.net/publication/6595696_Effect_of_Nitrogen_Form_and_Root-zone_pH_on_Growth_and_Nitrogen_Uptake_of_Tea_Camellia_sinensis_Plants) are well adapted to acidic soil conditions and show good uptake of and growth with ammonium. At the electrophysiological level the preferential uptake of ammonium is explained by the fact that nitrate uptake is upward the electrical gradient, requiring either direct ATP input or proton cotransport mechanisms. This makes the nitrate uptake process inefficient, in addition to the fact that the negatively charged functional groups in the cell wall repel nitrate, while they attract and concentration ammonium near the root cell membrane, the first metabolic barrier for uptake. The last comments, however, apply to anions vs cations in general and are not specific to N forms.

Add 2. Plants are NOT smart! Although most plants take up ammonium at higher rates, ultimately most of them do better with nitrate. So we need to be careful when discussing ‘preferred’ forms of N. Numerous physiological and biophysical processes have been discussed for decades to understand the differences between both N forms, usually associated with impaired growth when grown with ammonium (e.g. https://www.researchgate.net/publication/230140043_Physiological_and_Biochemical_Processes_Related_to_Ammonium_Toxicity_in_Higher_Plants). In this regard I’m not aware of any indication that under reasonable experimental conditions the free ammonium concentration in leaves reached levels that could induce of uncoupling of electron transport from ATP synthesis (compare comment of M Stepien). Such hypothesis is based on studies carried out under extreme conditions (‘how to kill a plant in the shortest possible time’), and adds nothing to our understanding of plant nutrition under field conditions.

The energy argument (higher energy requirement with nitrate nutrition) is rather weak. As Raven (1985, New Phytol. 101, 25-77) savings are 8-12% when nitrate assimilation takes place in the shoot, otherwise it’s only 3-6%, and even lower when assimilation is less than 100%. In fact this tremendous advantage of nitrate has not been mentioned yet: It can be transported and stored safely within the plant, at high concentrations (e.g. when light conditions are low). This gives nitrate-grown plants enormous flexibility. Contrary, plants tend to assimilate all ammonium in the root, and should they not be adapted to it, face serious problems (pH regulation, shortage of carbohydrates and C skeletons). In my view, most reports on substantial ammonium concentrations in the xylem are most likely exaggerated because of analytical errors and in this regard I may not agree with D K Kundu. But I fully agree with his statement that ammonium is assimilated by the GS-GOGAT pathway, and not by glutamate dehydrogenase as some researchers still suggest.

Most plants tend to prefer a mixed supply of nitrate and ammonium, with an ammonium contribution of 5-25% for best performance. Acidification with NH4 is much stronger than alkalinisation with NO3, because set aside N uptake plants take up more cations than anions, resulting in acidification. In this respect I agree with S K Naik that nitrate helps the uptake of cations for charge reasons. Incidentally, this often corresponds to the pH-neutral ratio (ratio that doesn’t change rhizosphere pH too much, reducing the stress on charge balance and pH regulation (as already mentioned by G Agegnehu).

Plants well adapted to acidic conditions like tea and blueberry not only tend to absorb ammonium at high rates, but also show better performance with ammonium (see above).

Add 3: The pH argument is worth discussing. Ammonium tends to strongly acidify the rhizosphere, and in the long run ultimately the bulk soil. Whether this is advantageous or disadvantageous for crop growth depends on the crop and the ideal pH (as a function of crop and soil). In calcareous and saline soils nutrient deficiencies due to high soil pH are widespread and an acidification is to be seen as a positive move (e.g. better availability of P, Fe, Mn, Zn), while in acidic tropical (acrisols) and temperate soils (podzols) acidification is to be avoided.

It is certainly correct that nitrate is acting faster than ammonium and urea when applied to the surface of well-aerated soils. That’s indeed to do with the much lower absorption of nitrate to the soil colloids and the ammonification and nitrification of urea and ammonium (see A K Singh’s earlier comment). And indeed most crops get only to see nitrate anyway, irrespective of the N form applied. However, is it appropriate to say that because of that nitrate is to be preferred? In most systems a fast N action is neither required, nor appreciated. In fact, ammonium-containing N fertilisers are amended with urease and nitrification inhibitors to delay that conversion, allowing for a more continues N supply to the crop and better N efficiency. To some extent nitrification inhibitors may increase the delivery of ammonium to the root surface, improving the availability of some nutrients as mentioned already. Unfortunately, very little is known to what extent these measures alter the form of N ultimately taken up by the plant. Ultimately, a one-fits-all approach won’t work.

The apparent (because they don’t use the same transporter) antagonism between nitrate and chloride is indeed worth mentioning (as S K Naik already pointed out). In some work this is used to reduce the nitrate concentration in salad before harvest, under saline conditions it’s used to reduce the chloride uptake of sensitive crops like avocado. I suggest checking Uzi Kafkafi’s work at RG.

Last but not least allow me to add that plants can easily take up urea as well, provided it’s available for uptake at the cell membrane. This is normally only the case in hydroponics and with foliar application (e.g. my publications on RG on Ni as an essential nutrient). Indeed, because urea is an uncharged molecule, much higher N concentrations can be administered and higher N amounts can be delivered than with the other N forms.

Well, that's not all on the subject, but enough for now ...

Conference Paper Effect of Small Quantities of Ammonium on the Total Nitrogen...

Article Effect of Nitrogen Form and Root-zone pH on Growth and Nitro...

Article Physiological and Biochemical Processes Related to Ammonium ...

Some very interesting points Dr Joska..appreciate your excellent response..

The form of nitrogen taken up (NO3− vs. NH4+) has the most prominent influence on rhizosphere pH which may increase in plants that are fed NO3− or decrease in those that are fed NH4+ as a consequence of differences in cation–anion uptake ratio.The higher nitrogen uptake efficiency with nitrate, compared to ammonium fertilisation, was clearly demonstrated by Legaz et al (1996). They found that the highest efficiency in N absorption (labelled isotope N-15) in citrus trees in function of the type of fertiliser (KNO3, ammonium sulphate), applied to a sandy and a loamy soil, and measured over a six month period, was obtained with nitrates. Differences in N-uptake were greatest in the sandy soil with 60 % N-uptake efficiency when N applied as potassium nitrate, and only 40 % N-uptake efficiency, when N was applied as ammonium sulphate.

Another point for consideration could be viewed in : Nitrogen deficiency is characterized by chlorosis of the older leaves, while younger leaves may remain green. Leaf chlorosis increases and growth decreases in Vaccinium species supplied with nitrogen in the form of nitrate as opposed to ammonium (Korcak, 1988). These symptoms appear to be related to decreased uptake and assimilation rates of nitrate compared with ammonium (Spiers, 1978; Merhaut and Darnell, 1995). Assimilation of nitrate is regulated by the activity of nitrate reductase (NR) (Crawford, 1995). This enzyme is found primarily in the cytoplasm of root epidermal and cortical cells and shoot mesophyll cells (Crawford, 1995; Berczi and Moller, 2000), although a small percentage of activity has been found associated with the plasma membrane (Berczi and Moller, 2000). Nitrate reductase catalyses the reduction of nitrate to nitrite as the first step in nitrate assimilation.

Some thoughts ....

Dear A K Srivastava, many thanks for supporting the statement that crops like Vaccinium do well on ammonium by several references.

However, regarding the 15N tracer studies supposedly showing that nitrate is more efficient, we should be more cautious. One of the shortcomings of using 15N in fertiliser efficiency studies is that it - normally - does not consider isotope exchange. The 15N-ammonium is subject to absorption/cation exchange and nitrification just as the soil-born 14N-ammonium. Assuming that the total soil NH4 pool is substantially changed in the long run, the unlabelled soil-borne 14N-ammonium is diluting the 15N-ammonium, but only because fertiliser-borne 15N-ammonium is replacing the soil-borne 14N-ammonium. So, ultimately, some soil-borne (14N-)ammonium became available only because fertiliser ammonium was added, so the credit goes to the fertiliser N. To the contrary, 15N-nitrate hardly undergoes any conversion, is weakly interacting with the soil matrix, and is taken up by the crop as it is.

The isotopic exchange described for ammonium leads to a systematic underestimation of fertiliser efficiency with the 15N approach, as compared to the more conventional difference method. This could be adjusted only by considering long-term, residual effect over the following few years, which often is not done. Also, some researchers combined both approaches to get a better control (Peschke, Berlin, unfortunately mostly published in German language only). Somehow, the 15N approach should be considered as what it actually is - a tracer experiment. It allows you to decipher were the applied N ends up, indicating options for improving N efficiency.

Well said Joska , thoroughly well supplemented , I do agree  with your excellent gaps that you have picked up in our conventional methods of computing NUE. Plants preferring nitrate -N and studies carried out through 15N carrying nitrate form is more realistic than those ones preferring ammonium-N and applied isotopic ammonium -N form with regard to NUE.... 

 Sir,

Is there any difference about the, nitrate uptake by plants in alkaline soils?

Well, theoretically, and as demonstrated in solution culture experiments, nitrate is more readily taken up at low pH (uptake mediated by proton co-transport), while the opposite holds true for ammonium (it's easier for the plasma membrane ATPase to energise the membrane by pumping protons out). On top of that, at very high pH free ammonia may penetrate the plasma membrane.

When it comes the applied side, that depends, as always: in an alkaline, saline soil nitrate uptake will reduce uptake of chloride, which is certainly beneficial. In an alkaline, calcareous soil uptake of ammonium will acidify the rhizosphere, with positive consequences on availability and uptake of several nutrients (P, Fe, Mn, Zn).

Did that at least in part answer your question?

The uptake of nitrate-N is more due to its solubility in water, secondly it have negative charge having less attachment with soil which makes it more available to plants.

Nitrogen based on ammonium form will acidify the soil. This can be useful in alkaline soils but a problematic in neutral and acidic soils as acid soil toxicities can develop.

Nitrate is not acidifying and even can alkalinize the soil medium as such nitrates are to be favored in an acid soil environment and can be problematic in a neutral to alkaline soil condition.

Ammonium has the ability to attach to clay and is not readily leached into water. When nitrates are used the repelling with negatively charge clay will cause great loss to the water system.

For both nitrate and ammonium losses are greatest in sandy soils and those low in soil organic matter.

Nitrate nitrogen can be more expensive per unit than ammonium. Concentrated ammonia injected into the soil when a maize crop is knee high can be a very cost effective usage of Nitrogen applied only at the need of the immediate crop.

For farmers using ammoniated fertilizers the acidification of the soil needs to taken into consideration and liming practices to prevent soil degradation from low soil pH.

Dear Dr.Hepperly ,I wish to further comment on your second paragraph. Nitrate behaviour in either acid , neutral and alkaline environments depends on the associated cations in the fertilizers (Nitrate is acid radical). Sodium nitrate, ammonium nitrate and calcium ammonium nitrate sources will behave differently in soil.Calcium ammonium nitrate is a popular fertilizer in neutral and alkaline soils.It can also be used in acid soils .The old/classic sodium nitrate can be preferred in acid soils.Ammonium nitrate may have near neutral reaction in soil can safely be used in neutral and alkaline soils.

Dear Doctor Rao Point well taken. Sodium nitrate as Chilean nitrate is a mined source which can be used in some certified organic agricultural systems as you aptly point out it will be best used in the acid environment. In areas of sodicity calcium without the carbonate can be helpful as sodium resources can contribute to the sodium issues. In sodic areas the dispersion of sodium is based on mixing gypsum to interchange the sodium for calcium without increasing soil pH. Where sodium and alkalinity are issues together the use of ammoniated fertilizer and sulfur can help drive soil reaction to a non alkaline state desirable for a full potential in crop production. I believe the sodic and alkalinity issues have been increasing with desertification and intensive irrigation. Thank you for your observations, Paul Reed Hepperly

sorry: this is such an old sock: you can find arguments for each form, just as it fits your likings! There are better subjects to have an exciting scientific discussion on!

I think, some valid explanation has been put forth by Dr Joska Gerendas, worth considering ....

Is the preference of nitrates over ammonium ions and vice-versa by plant roots linked to the density of nutrient transporters embedded in plasmalemma of a given crop , if the issue is to be elucidated the cellular or subcelular level..??

Please also have a look at this useful RG link.

https://www.researchgate.net/post/Which_form_of_N_is_most_readily_available_for_plant_uptake_Is_NO3-N_or_NH4-N_Is_there_preferential_difference_in_crops_for_these_nutrients

Very good question I fellow it.

Lets think above and beyond that, this discussion is really old now.

Hi Maher Souguir ..

The three main sources of nitrogen, used in agriculture are urea, ammonium and nitrate. The biological oxidation of ammonia to nitrate is known as nitrification. This process consists of various steps, and is mediated by autotrophic, obligately aerobic bacteria. In waterlogged soils the oxidation of NH4+ is thus restricted. Urea is decomposed by the enzyme urease or chemically hydrolyzed into ammonia and CO2. In the ammonification step, ammonia is converted by ammonium-oxidizing bacteria into ammonium. In a next step, ammonium is converted by nitrifying bacteria into nitrate (nitrification).

The nitrogen conversion rate depends on the conditions, present in the soil for nitrifying bacteria. Nitrification of NH4+ to NO3- preferably occurs under the following conditions:

• In the presence of nitrifying bacteria.
• Soil temperature > 20 °C.
• Soil pH 5,5 - 7,5.
• Sufficiently available soil moisture and oxygen.

Ammonium may accumulate in the soil, when this nitrogen conversion is limited or completely stopped if one or more of the following soil conditions are present (Mengel and Kirkby, 1987):

Of the soil nitrogen reserve only 1 to 3% is mineralized in any year. When soils are low in organic matter the mineralization will be at low end and when soils are optimized in soil organic matter the mineralization rate will be greater as it governed by microbial activity of the soil. As soil organic matter is increased the rate of mineralization as well as the total amount of soil nitrogen both are better. I believe when we optimize soil organic matter the "need" for an input approach is much less important. When the emphasis is solely on the nitrogen input in most cases this will result in management which depletes low soil organic matter situations. In the low soil organic matter situation rain fed agriculture will experience much greater issues with periodic drought. Soil organic matter can be an effective method to drought proof agricultural systems. There is great wisdom in feeding soil rather than looking to directly feed the plants. Humic materials have enormous ability to stimulate plants well beyond the source of its nutrient effect. This resides in growth regulating activity of humic materials and soil microflora which is beneficial for most part.

I agree with Dilip Kumar Kundu

it depends on system of cultivation

- in soil cultivation systems ammonium is much better than nitrate from environmental, economical and health points of view,

- In hydroponic nitrate is better when considering the sole, however a 10-20% N-ammonium along side of nitrate works well

Nitrates will solubilize the caton complexing the bound phosphates in soil, thereby will release phosphates too, while ammonium will be competing cation

Nitrates are the preferred nitrogen source:

• Non-volatile: unlike ammonium, nitrate is non-volatile, so there is no need to incorporate it in the soil when applied by top- or side dressing, which makes it a convenient source for application.
• Mobile in the soil - direct uptake by the plant, highest efficiency.
• Nitrates synergistically promote the uptake of cations, such as K, Ca and Mg, while ammonium competes for the uptake with these cations.
• Nitrates can be readily absorbed by the plant and do not need to undergo any further conversion, as is the case with urea and ammonium, before plant uptake.
• No acidification of the soil if all the nitrogen is applied as nitrate-nitrogen.
• Nitrates limit the uptake of harmful elements, such as chloride, into large quantities.
• The conversion of nitrates to amino acids occurs in the leaf. This process is fuelled by solar energy, which makes it an energy-efficient process. Ammonium has to be converted into organic N compounds in the roots. This process is fuelled by carbohydrates, which are at the expense of other plant life processes, such as plant growth and fruit fill.

The absorption of nitrate involves the release into the soil of a hydroxyl ion which causes a local alkalinization. This can be very positive in an acid soil in need of this pH moderation. However, in neutral and alkaline soils the nitrate can work to immobilize keep micronutrients such as iron and zinc.

Nitrate is more expensive than ammonium sources.

While some plants prefer nitrates some prefer the ammoniated sources of Nitrogen.

In high rainfall environments the leaching of nitrates is common as well in over saturated water environments denitrification will cause sizable losses environmental and health issues.

In soils with significant clay the cationic ammonium can be conserved on the clay.

It can be argued that for cost and the environment legume nitrogen is the best nitrogen source. It is much cheaper and contributes a legacy resource to the soil. The cereal crops in rotation are favored their need for nitrogen outside the soil much reduced and the yield potential is higher while diseases and pests are reduced by the rotation.

In many environments the soils have been depleted in soil organic matter the provision of nutrition from composts and manures address the ability to make better use of scarce water resources resilency to droughty environments and counteracts the greenhouse gas increase in the atmosphere when soil organic matter can be increased and stabilized in the soil.

When recommending the preferred sources of nutrients local testing and defining the soil conditions is recommendable based on some of these local adaptability issues and the local availability and economic issues among others.

Hello,

I'am using two different electrolytes for dye decoloration (NaCl and NH4Cl) both are fine (NaCl is better than NH4Cl) but if we will consider that nitrate is better for the plants than I have to choose NH4Cl.Am I right? and thank you all for these usuful informations.

I'd like to turn your attention to a paper by Teyker and Hobbs (Agron. J. 84:694-700, 1992): They reported on a study in which they compared hydroponics and slight-alkaline soil fertilized, either with nitrate- or with ammonium-N. The corn responded better to ammonium. Yet, they comment that "differences in pH regimes between the hydroponic and soil-based experiments may account for the contrasting results".

Will the results be different with a different crop?

As nitrate ion carries net negative charge it moves in soil freely when compared to ammoniacal ion. As we know in soils CEC > AEC. So ammonical ion can be fixed by clay which cannot move free as nitrate ion. Due to this reason for many plants nitrate is more available in the vicinity of roots...

In the above-mentioned study (Teyker and Hobbs, 1992), the soil they used was highly sandy, so the fixation/holding back mechanism seems unlikely. Still, you pointed out on a basics problem, that we do not know enough on the behavior of ions at the soil/root interface.

I agree with Dilip Kumar Kundu

Dear Dr. Silberbush

Do you have any information about the differential effect of nitrate and ammonium as N carriers for bearing avocado trees?

Many thanks,

oded

Ammonia is toxic to plants and usually difficult to store. Nitrates are relatively better for storage for later use when the nitrogen demand skyrockets.

Plants can utilize both nitrate and ammonium forms of nitrogen with equal ease. It all depends upon which form is more available in a given environment. For example, in submerged rice it is ammonium which is more available in the soil. Whether nitrates are not formed or lost under anaerobic conditions is a different issue. In upland crops like wheat, nitrate-N are not preferred because it is nitrate-N, but because ammonium-N is readily converted to nitrate-N. I would like to repeat - - plants can use both nitrate- and ammonium-N with equal ease.

Agree with most answers here. Soil type, moisture conditions, and plant availability are important factors.

In low light conditions as in shade houses/greenhouse nitrate-N is more efficiently utilized than the ammonium-N because the latter must be assimilated immediately (cannot be stored in cells because it is toxic) after uptake due to the high demand for carbon skeletons for the synthesis of amino acids that incorporate it.

The plant can absorb nitrate and ammonium, but the plant’s absorption of any of the two ions depends on the extent of the dominance of any ion in the solution, and since the nitrate ion is a negatively charged ion, it is in the form of an ion dissolved in the soil solution and is ready for absorption by the plant

I agree with most your answers here.

I do agree with Njue Mugai answer. But, what could be the best Nitrogen source in sandy loam soils as they are prone to leach out or washed out provided without amending the top layers of soil profile for quick growing crops?

Some farmers apply nutrients on the tree leaves. I am not sure what they would do for herbaceous plants...

Thanks Yu-Hsin. This would make sense.