one cannot ignore the importance of foliar fertilization especially in marginal lands/problem soils. When we talk about this, literature is full of foliar fertilization of N, K and micronutrients. Why P is not preferred for this?
Majority of the soil test reports indicates high levels of phosphorus in the soil, but the percentage recovery by plants is only meager. The phosphate availability is mainly microbe dependent. Micro organisms like Bacillus megatherium and Bacillus polymyxa plays a major role in converting these fixed insoluble form to soluble forms, which will be easily available to the plants. The soil microbes and the carbonic exudates of plant roots, helps to dissolve the fixed phosphorus and make them available to the plants. The faster the uptake of phosphate, the more quickly the root system develops and the plant will get greater the amount of energy to maintain growth and development. However when pH is between 7.5- 8.5. Many local soils have pH values within this range. This raised pH can actually be due to excessive amounts of potassium in the soil which has an influence on pH, twice that of calcium (Sodium 4 times and magnesium 1.4 times that of calcium). In this situation, it is better to give foliar applications of potassium to maximise its uptake, particularly in the fruit-filling stages. A soil pH range of 6.2 to 6.8 is generally considered as ideal. Soil pH becomes self adjusting when the important cations - calcium, magnesium, potassium and sodium are in proper equilibrium. Thus it would be better to say that pH is the result, not the cause, of nutrient interactions and levels in the soil.
Evidences are there that foliar application of phosphate fertilizer increase yield. You may please refer one publication as mentioned.
Australian Journal of Experimental Agriculture 43(10) 1263 - 1268
Dear Shashi, by nature and science the macro elements including soluble N,P, K, absorb by root and plant transport them to different parts. Depending on the requirement these nutrients are converted into one or other form. So best application is to give to root!!!! However still some publication show the readily benefit of foliar application of same.
I can think of two main reasons for P not being applied to the leaves. P is needed for root formation, among other things., thus its needed mostly early in the season, when leaves have not formed yet, or just starting. The other reason is that it's needed in large amounts, hence easier and cheaper to apply to the soil. I hope this helps.
P fertilizers can also apply through foliar spray. In may cases these solutions will be acidic ( like phosphoric acid ) and cause scotching effect. But in neutral conditions it will precipitate. To induce flowering in Pomegranate after pruning 2% DAP will be sprayed by the farmers. But there is no recommendation. Mainly solubility is important and as Kuldeep Patel wrote P is mainly required for root & early stages of growth.
thanx to all...:
Spray solution will be acidic in case of P otherwise it will precipitate. This reason could be the possible reason because It is mentioned in the literature that for successful foliar fertilization, the medium should be neutral to slightly alkaline.
Other answer is.......
P needed for root growth in early season, hence, applied to roots directly, this reason seem week supporting evidences because, if applied to leaves, P can very well translocated to different parts due to its mobile nature within the plant system. Secondly, unlike N and K, requirement for P remained almost same through the season in order to provide energy for different processes.
Again thanx to all
Simply because, aerial sprays are not advised for P fertilizer, because P is highly reactive and it will react with air to form other complex.
dear vimarsh, if we apply something like chelating agent along with P.....then?
Fine chemicals always tend to react. It's better not to clash with environment..
Some really good answer , quite appreciating ...Probably for phosphorous nutrition , plant is more equipped to be absorbed through roots by mass flow as well diffusion ,all along the transpiration pull , thereby , distributed more or less uniformly as per sink strength. Moreover , P has a better utility in seed formation and root proliferation , besides the fact that P is mobile inside the plant while immobile in the soil, where the nutrient intercepted by the roots ...There are better presence of nutrient transporters for P in the plasmalema of root cells than cuticles of leaves, this is also one of the reasons...
Please find enclosed few relevant literature for further reading with emphasis on P-nutrition , excellent reading material indeed...
Sir,
P fertilizers are applied as basal doses only. On P deficiency, foliar spray is recommended during initial growth stages.
P foliar nutrition can be enhanced by addition of small quantity of urea to the spray solution.
After formation of flower primordia, the P is not much taken in, hence, no foliar application.
Dear Shashi,
To date, P has not been used in sprays as much as nitrogen for several reasons. First, many P compounds have low solubility. Second, the greatest response from P applied to the soil is often early in the season when leaf area is small. Perhaps the most important reason is the fact that no P compound has been found which can be applied to the leaves in high enough quantities to contribute significantly to the total P needs of a crop without causing damage to the leaves.
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