Hi Peter,

Phosphorus is likely to be a limiting nutrient in many acidic soils.  When urea is applied to soil it is converted to Ammonium and then undergoes nitrification.  As nitrification is an acidifying process, this may lead to further decline in soil pH (particularly in soils with low organic matter), thus exacerbating the problem, unless P fertilizer is applied along with Urea. I guess this may be one of the contributing factors.

-SJ 

yes susantha your are correct. During nitrification H+ ion will be released into the soil. This hydrogen ion will reduce the pH of the soil further. Hence, available P will be fixed as Fe and Al phosphate which will not be easily available to the plants. Calcium Ammonium Nitrate (CAN) is the best fertilizer for acidic soil than urea. Application CAN fertilizer along with P fertilizer can solve this problem 

the reasons would be more authentic if u proovide the data regarding pH of soil and the name of  P fertilizer added. 

Urea is an acidifying fertilizer which makes the soil more acidic. High levels hydrogen ions due to urea application are toxic to roots of plants. Hydrogen ions influence the permeability of plant root membrane which interferes with the absorption of cations. These will lead to unhealthy plant growth resulting to low crop yields. Liming such soils can help solve the problem

Urea acidifies the soil. Phosphorus (P) is fixed in form of Al and Fe compounds in acidic soils. This means that when high quantities of urea is applied, the soils become acidic and thus some of the available P is fixed. To offset this and have some available P for the plants, a higher application rate of  P is needed, thus why when you apply a low rate of P, all the available P is fixed  making it unavailable. Application of higher rate of P ensures that after some is fixed, the overflow is available for crop. Try liming the soils (2t/ha lime) or  apply farmyard manure ( 5t/ha). Others sources of N like Calcium Ammonium Nitrate (CAN) fertilizer can be more beneficial.

Dear pal

I also agree with your scientific interpretation of above subject. Urea has the capacity to reduce 0.1 unit /100kg urea. So in order to enhance the P use efficiency in acid soil go for  liming depends on the pH of your soil and try improve your organic matter by adding organic manures or biochar @5 t/ha because biochar has more carbon as well as liming potential, which has more P sorption and desorption potential  

In addition to all the answers given above, I wish to add that you add the urea and phosphorus fertilizer the same time

As Susantha briefly and appropriately explained, P is one of the most limiting macro nutrients for plant growth and development. As it is very immobile element  in the soil it is easily fixed by heavy metals, such as Al and Fe  as Al- and Fe-phosphates in acid soils, mostly below 5.5 pH,  and by Ca as Ca-phosphate in calcareous soils, mostly when a pH of soil is over 8.0. 

Generally, soils low in organic matter, highly weathered and Fe-rich acid soils contain P in forms less available to plants. Fe interferes with the absorption, translocation, and assimilation of P by forming Fe phosphates.  According to several research works conducted on P,  a 10:1 ratio of N:P is considered optimum for many crops. Based on research findings,  ammonium based fertilizers decrease P availability via their acidifying effects, conversely they increase P availability in calcareous soils.

Though application of calcium containing fertilizers like calcium nitrate (CN) and calcium ammonium nitrate (CAN) may improve the availability of P, they may not be economically profitable for food crops production. They may be profitable for high value crops. Rather, improving the organic matter content of soils along with liming for acid soils would be a sustainable solution for balanced nutrient availability and agricultural productivity in acid, ferralitic soils of the humid tropics.

Hi Peter,

The solution to this problem is to lime the soil. This will reduce the acidity of the soil and make available already fixed P and enhance the utilization of added P.

While I have no timely response to maize, we have several trials established in the forestry sector in which we find the same answer and try to bring to your question. Our soils are Ultisols with pH 4.5 to 5.5. Available soil phosphorus is 0.5% of the total phosphorus. Similar to the results found by you, when we apply urea, planting grows less than the unfertilized. Subsequent soil sampling indicated that the availability of phosphorus in the soil is less. While we have not yet quantified soil microorganisms, we hypothesize that the increased presence of nitrogen in the soil increase the number of microorganisms, which are available soil phosphorus that could use plants consume. Therefore, slower growth of plants is due to the smaller amount of available P, the which was consumed by microorganisms.

RM

Urea is an amide fertilizer as u r aware provided works well under slightly acidic to neutral conditions even alkaline for optimum yields.  

Ur soils may be acidic, since u hv used urea the pH might hv been worsened little more as the concept is ammonia to nitrification n raise in acidity @ 0.1 units per 100 kg hardly. The better option is liming for few seasons n further addition of organic matter for buffering.  

Shifting to CAN or neutral fertilizers are not so economical for field crops. It is additional burden to ur crop balance sheet. 

Application of P on a layer of organic manures works out after urea application. 

Hi, Prof Peter Opala Thanks for an interesting question.

Your observation is correct and can be justified scientifically.

Application of urea without P activates ammonification which leads to nitrification. This lowers soil pH by releasing extra H+ ions into the soil solution. Increased H+ irons in the soil lead to Aluminium and Fe+ toxicity which in turn affects plant yield.

Then why must we add P together with Urea?

Back to soil chemistry. Urea releases H+ ions which are positively charged.TSP or any other source of phosphorus releases phosphate which is negatively charged. What this means is that phosphorus diffuses to fix the H+ ions from urea that would otherwise reduce soil pH. (Electrostatic law, unlike charges( H+ and p04-3), attract each other.

This should equally explain why a reverse option should be made, thus, application of urea in alkaline soils would lead to an increase in maize yields.

Check this Article Interaction of urea with triple superphosphate in a simulate...