I think it is not surprise - one of the most negative effects of acidity on crops is generation of P deficiencies. Applications of N (if not in the form of sodium or calcium nitrate) increase soil acidity and thus aluminum toxicity. Additionally, N fertilizers with ammonium ion may have additional toxic effect to crop. P and K fertilizers, have rather small effect on acidity (generally, because phosphate rock reduces soil acidity, other forms may slightly increase soil acidity). On strongly acid soil PK treatment may be less harmful to crop, than NP or NK.

Clearly, these are theorethical divagations - to consider your issue it is necessary to know the doses and forms of fertilizers, and if soil pH was measured in water or other extract (KCl?).

You can see also my PhD thesis available here in Researchgate. It is in Polish, but I added English titles of chapters, subchapters and tables, so you can see table 10 on the page 47, with the yields of plots with different treatments applied since about 1922 or 1923 and with or without farmyard manure (since 1992 or 1993). The names of crops in all the tables are: v2000 - ziemniaki - year 2000, potatoes;  2001 - pszenżyto jare - rear 2001, spring triticale;  2002 - żyto - year 2001 (but year of sowing - 2001, autumn), rye (winter rye). If you need some explanations more, please ask me. I will do everything I can to help you.

Thank you very much for giving me explanation and also indicated me to read your thesis. I will do, please.

First of all let me tell you some of the characteristics of one of the soils where I did the experiment:

pH (water): 4.7, P (Olsen): 3.2 mg/kg,  exchangeable K: 0.2 meq/100g of soil, exch. Al : 9me/100g

the treatments of course were untreated control, NP, NK, PK and NPK. Urea, TSP and KCL were used as a source of N, P and K respectively. These treatments were compared under lime and no lime application.

Please, I kindly referee you to read on of m proceeding paper entitled “MITIGATION OF SOIL ACIDITY AND FERTILITY DECLINE CHALLENGES FOR SUSTAINABLE LIVELIHOOD IMPROVEMENT: RESEARCH FINDINGS FROM SOUTHER REGION OF ETHIOPIA AND ITS POLICY IMPLICATIONS” on Researchgate web site for the detail.  

With This background let me come to the point that I agree with your explanation. But still the surprising aspect is that there was no significant difference in the tuber yield of potato or barley or wheat treated with PK + 0 t/ha and PK + 3.5 t/ha.  Again let added up on what I have indicated on my original question that still there was little difference in the yields of these crops treated with NPK + 0/ha lime and NPK + 3.5 t/ha lime. What I have observed in two divergent locations of southern Ethiopia with soils of strongly acidic reaction, if K is applied in along with P or NP, dramatic increases in the yield of the test crops were obtained and under such condition lime application had little or no effect.  But I should bring to your attention that NPK produced significantly higher crop yield than PK treatment in both location.

This was the reality I found on the ground for years, on three crops in three divergent locations with common characteristic of course. Thus, should I conclude that application of K with P or NP able to mitigate soil acidity in addition to its importance as essential plant nutrient in these soils?

Dr.Wassie, very interesting discussion on crop responses to nutrients and added lime and farmyard manure in acid soils. I have also seen your interesting presentation available in your profile on researchgate. In India we have four long term experiments on acid soils at  different parts of the country.The crop response and lime and manure affects in your soils are similar to one of our experiments at Bangalore.In the experiments compared to lime effect,the potassium deficiency and responses to K in finger millet and maize crops  are more spectacular.We know the negative effects of nitrogen in acid soils but even there was no response to phosphorus in the absence of K application.Lime did not influence crop response but manure showed positive effect  on crop and also did not allow pH reduction.In Indian experiments lime increased soil pH,though the response not much.The role of potassium in amelioration of acid soil is questionable.Your experimental soil has very high exchangeable Al, 9 me/100 and exchangeable K is 0.2 me/100.Even with addition of recommended level of K application,it may not be possible to replace Ex.Al with K.In acid soils even H is held stronger than K ion. Potassium is more mobile and subjected to leaching losses in acid soils. In all our long term experiments on acid soils there was lot of build up of available  P in soil  in different treatments possibly because of less utilization by crops.

From your description I asume that the main problem in your soils is Al toxicity to the root. NH4 has in soil acidic rection due to the nitrification proces or when plant roots consume ammonium directly that result in further acidifing the root -soil interface. If P is limiting plant growth in your soil P application might solve your problem,

Dear Professor Subba Rao,

Thank you very much for appreciating the discussion. I fully agree with  detailed explanation you have given and of course as a soil scientist how the different nutrient behave and interact among each other. I also know very well that K doesn't have acid mitigating effect. But still in acid soils of Chencha and HagereSelam, I found that If you apply P + K and NP + K you don't need to apply lime to increase yield. You told me that you seen my paper on Researcgate and that PK + o lime and PK + 3.5 t/ha lime produced similar yield. in fact NPK + 3.5 t/ha lime gave yield slightly higher yield than NPK + 0 lime. But the increased yield was not enough to offset the lime cost. so I found it better to apply NPK alone. This was the reality in the ground. 

It would be interesing to know organic carbon content in the soils of these locations and at least approximated soil type, order (Soil Taxonomy) or Soil Reference Group according to WRB. It is also interesting, if the lime is applied every year, together with NPK fertilizers or in one dose per 3-4 years? ANd, when the soil samples were taken. Especially, I am interested, if soil samples were taken before establishing experiment from the whole area of experiment, ofr from each plot separately. Were they taken also during the course of experiment?

In my county lime is used not every year, but every 3-4 years, depending on soil and crop.

Strongly acidic soil has poor nutrients, but organic matter is a vital component for crop growth

As the soil is strongly acidic and exchangeable Al is about 9 me/100 g, there is every possibility for precipitation of added fertilizer P as aluminum phosphate.So addition of well decomposed(humified ) organic manure is needed every year to complex with Al and prevent P fixation. Humus also helps to prevent further lowering of soil pH through buffering action.Thus the availability of P is ensured.Of course manure also supplements other secondary and micronutrients along with improved biological health.

There is nothing unusual to see the better effectiveness of PK  than NK or NP in acid soils. Polymeric aluminium- induced phosphorous fixation in such acid soils is a common feature , besides exchange of K  in replacement to Al ions , allowing both the nutrients to suboptimum level .  Under such conditions, with  N maintained at optimum level , PK will be a more responsive  treatment than NK( P maintained within optimum) or NP ( K maintained within optimum) , since in latter two treatments ,  most of the carbon from  organic matter remains in  humified form , thereby inhibiting the release of N despite high organic matter content . And , very often , there is a response of N application in high organic matter containing soils. Whether or not,  these scenario will operate under pre-limed versus unlimed soils ? 

Dear Dr. Srivastava,

Thank you very much for your excellent explanation. In fact  I  am also grateful  to all contributes to this discussion and their explanation are excellent.  However, I am more attracted and impressed by your justification for what I have observed in my experiment in Acid soils of Ethiopia. As you said  in PK treated soil, there was a possibility that  Al was replaced by K in exchange site  and entered to soil solution where by it reacted with P formed Aluminum phosphate.  As a result the toxic effect of Al was abetted.  

By the way my research question could have been simply resolved just by taking soil samples where I have done my experiment and incubate them with amendments of N, P, NP, NK, PK and NPK. Then make changes in chemical properties of treated soils periodically. However, because of limitation of some facilities I am unable to do it.

Finally, I kindly request you indicate me literature verifying your argument above.  Because, if your justification is not based on research, it will be speculative similar to the speculation I have.

Dear Wassie Haile,

The acid soil you have will have negative impacts from Ammoniated Nitrogen. The Ammoinium absorption with maintain electroneutrality by shedding a Hydrogen proton accentuating Aluminum toxicity as Dr. Srivastava has alluded to.

Phosphate is a anion and the plant will shed a hydroxyl ion to mantain electroneutrality. This will have a beneficial effect in reducing the acid infertility. When Nitrogen is supplied as Nitrate it avoids to Nitrogen promotion of Aluminum toxicity because of anion electroneutrality effect. 

Besides the use of Phosphorus and Potassium the use of compost might be recommended as ti reducees Aluminum toxicity and adds to water and air needs of the plants. Adding the Phosphorus and Potassium to your compost would be ideal for your acidic environment. 

Hope you find this useful for your continuing research and developmental work. 

Dear Dr.Wassie,how long this experiment was conducted and what  were the changes in soil pH over the years?Are you measuring changes in available K and P also ?If P is getting precipitated as Aluminum phosphate, in long run P availability to crops will be a problem.Irrespective of  current  economics, if you want yield sustainability you have to apply lime  once in 4-5 years or alternatively manure need to be applied every year to take of pH reduction over time.

Dear Rao and Happerly,

Thank you very much for your response. Regarding the answers to your questions, the experiment was conducted for 3 years (2007-09). The intial pH (H20)  of Chencha location was 4.8, pH (KCl) : 3.9. At HagerSelam pH (H20) was 5.2, pH (KCl) was 4.3 . Three lime rates (Lo, L1 (half dose) and L2 (full dose were) applied in 2007 only) based on exchangeable Al content. The fertilizers were 0, NP, NK, PK and NPK in a factorial combination laid out in RCB design. The result showed that lime application has increased PH (H2O) for example in Change from 4.8 -5.6, HgaerSelam from 5.2 to 6. Exch. Were also decreased due to liming. But in all case, the pH of soils receiving fertilizer treatment only changed little compared to the initial pH of the respective soils. Even there was a decrease in pH from initial one in some plots that receive N as urea as N source.

However, when we see the effect of treatments on yield of the test crops, NPK and PK treatments produced the highest yield in that order irrespective of lime application. Even there was no significant interaction effect of fertilizer and lime treatments. Indeed this was the result I couldn’t find explanation as to why this happened.

Otherwise, I fully agree with you and most other discussants that application manure on acid soils can greatly mitigate acidity. What I said P has been precipitated is my speculation only.