In papers written by (Peng et al, 2007, 2008) and (Kant et al, 2011), they use soil-based system to grow Arabidopsis and found that 10 mM PO43- is sufficient to provide enough phosporus to avoid Pi stress effects. While 0.5 mM induces a severe -Pi stress (Pi deficit).
Meanwhile, (Conn et al, 2013) grew Arabidopsis in a 0.6 mM supplemented hydroponic medium and the plants did not experience any stress symptoms .
Can anyone explain me the difference between the concentrations used in both growth conditions?
What concentration would be the Pi-deficient condition under hydroponic growth fo Arabidopsis?
Hi Carlos
Only 10-30% of the inorganic P applied to soils is available to plants, some times even less, depending on the P fixing capacity of the soil. The rest of it is precipitated, adsorbed, organically-complexed and some part is locked in the microbial biomass.
There is a great difference in the Pi availability between soil-based and hydroponic systems. As it is known, phosphorus in the soil is adsorbed on the soil colloids and is not readily available for plants. Therefore, feeding the plants with less than 0.5mM PO4- results in Pi deficit, whereas the same Pi dose in a hydroponic system is rather adequate, since Pi in hydroponics are not absorbed by soil particles.
Just notice that sometimes an "inert medium" (i.a. gravel) is used in hydroponic systems which might be also a strong sorbent for phosphate however usually to a much lesser extent than a (e.g. iron-rich) soil. However, this phenomenon was responsible that one of our field experiments using "inert gravel" was failing. Later on we did a chemical extraction with 1M hydrochloric acid and found that major part of previously added P was bound by the gravel material (containing iron). However, I cannont state to which extent the "fixed P" is not plant-available anymore. Many plant species are able to use fixed P such as Ca-P or Fe(III)-P species i.a. due to the release organic acids via the rhizosphere or part of fixed P is just released if dissolved P becomes lowered as an equilibrium reaction between the soild and the aqueous phase.
Although i am not expert at fertilization, following may be make it easy to understand the reasons..
1. In hydroponics, most used growing media are perlite, cocopeat, peat and mixes.. So physicochemical properties of the medums are completely different than soil... For example, if you use perlite, you need the supply all the required nutrients via irrigation water (fertigation)
2. Same is true for water holding capacity and available water to plant.. For example, in soil 1 irrigation per day or even 1 irrigation each two or three days can be adequate. But in soilless culture, irrigations can increase even 20-24 times per day!!!
3. Plant roots only grow in small volumes in pots, and generally around 6-8 liter per plant.
4. LEaching is necessary around 15-25 in normal conditions. It should be increased up to %40 in saline conditions.