Phosphorous and Potassium Chapter 14

The only thing special about these phosphate species is that they are the dominant ones in the typical range of soil pH, with the singly ionized phosphoric acid dominant at lower pHs and the doubly ionized one at higher pHs.

Conditions to left are for the natural state. The saving grace is that the various forms of P in the soil tend to be strongly retained (at least when the total level of P in the soil is low) and not leach out of the rooting zone. Also, nutrient cycling is important in keeping P in the upper part of the soil. However, with the advent of P fertilizers and the perhaps over-use of organic fertilizers (manure and like materials), high levels of P have built-up in some places. This is thought to pose a risk to water quality, as you shall see.

While the reference text says [P] in the soil solution may be up to 1 ppm, this is likely way higher than the natural state. The 1 ppb number is more likely. Low solution concentration is due to precipitation and adsorption reactions that phosphate undergoes. See figure.

Like in the above figure, at low pH (acidic), P is precipitated by soluble Al and Fe, giving minerals of very low solubility, and at high pH, P is precipitated as Ca phosphates that are even less soluble. Throughout the pH range, P is adsorbed onto soil mineral surfaces and this form is strongly held, i.e., little released.

P in acid soils Reaction with Al 3+ or Fe 3+ Al 3+ + H 2 PO H 2 O → Al(OH) 2 H 2 PO 4 + 2H + Very low solubility This is an example reaction. There are various such. What makes it work is the abundance and solubility of the acidic metals, Al and Fe, at low pH.

P in acid to alkaline soils Adsorption on oxides and silicate clays Anion exchange █Cl - + H 2 PO 4 -  █H 2 PO 4 + Cl - P slowly available This, too, is an example (anion exchange) reaction. The phosphate is strongly held by the + sites on soil mineral surfaces, more strongly say than is Cl -, even SO 4 2- held but it is released back into solution to some extent.

Displacement of bound OH - or H 2 O Al-OH + H 2 PO 4 -  Al-H 2 PO 4 + OH - Al-OH H 2 PO 4 -  Al-H 2 PO 4 + H 2 O P release very slow OH │ - O― P―OH ║ O These are types of surface adsorption reactions that entail covalent bonding. In some instances, the phosphate is bonded to two adjacent Als (or Fes) forming an especially strongly bond form that is not susceptible to release back into solution.

P in alkaline soils Insoluble Ca phosphates form 2H 2 PO Ca CaCO 3 → Ca 3 (PO 4 ) 2 + 2CO 2 + 2H 2 O There are several of these Ca phosphates and the tendency is for a moderately insoluble form to first precipitate but this form slowly be converted to more and more insoluble forms.

Taking all this into consider- ation, it turns out that the solubility of P is greatest in this pH range, interesting.

This figure is supposed to show how mycorrhizal fungi aid P (and certain micro- nutrients) nutrition. P from the granule diffuses to the root but its concentration is greatly reduced along the way by the afore fixation reactions. Fungal hyphae effectively shorten the diffusion distance. P in the fungal hyphae is then translocated to the root.

Photo is inaccurately overly dramatic and probably has nothing to do with P, however, in principle, it could. It would have to do with an ecological change in the water body brought about by added P.

Continuing, we want the upper scenario, not the lower. While there is evident growth of some aquatic vegetation in the upper, the lower has much more, including a lot of algae. More aquatic and algal production means more organic matter in the water body, and this means more decomposition of organic matter and the potential for reduced dissolved O 2 associated with such decomposition. Not good, especially when hot, which is a doubly whammy with respect to dissolved O 2 (you know, lower gas solubility with increased temperature). The algae themselves may also impair water quality directly. If P limits algal and aquatic plant productivity, want to limit P in non-point source pollution.

This is long-term build-up of soil P by continual fertilization. Those fixation reactions reduce P availability so to compensate a bit extra P has been recommended. In some places, it has added up. The downside to manure is relatively high content of P / N compared to plant needs. So, basing application rate on N content leads to soil P build-up. Do it for a few decades and you may have a problem. There is also concern about P lost directly from the manure in runoff.

True and true. First, there is little soil P without fertilization. Second, a tilled soil is mostly bare so there is more runoff and erosion, carrying more P away. However, most of it is bound to eroded particles. Some studies indicate that while the second is true, there is more dissolved P lost from no-tilled soils. This general form is more bioavailable, i.e., more effective in inducing eutropic conditions in downstream water bodies. Note: most believe freshwater is more susceptible to eutrophication by P than by N, and visa versa for salt water.

Soils differ in their capacity to precipitate and adsorb P, the combined processes called fixation or sorption. Here, B more than A, clearly.

More clay, more surface area, more adsorption. General rule, no? However, the types of surfaces, i.e., types of clay minerals, affect the extent of adsorption. Further, if there are a lot of Al and Fe minerals, particularly at low pHs, not only is there adsorption but also precipitation of P as Al and Fe phos- phates. Carbonate minerals can adsorb P and where there are carbon- ates, there is abundant Ca in solution so that Ca phosphates precipitate. The aluminosilicate minerals have a lot of surface area for adsorption of P.

The first option is not economically nor environmentally sound. The second is illustrated to the right. A strip of P fertilizer is applied along a planting row. High concen- tration of P in the vicinity of the strip saturates the fixation capacity there and a portion of the root system near the strip can supply the whole plant. You can also adjust pH to maximize P solubility.

This explains the K paradox –a lot of it in soil but still a need to use K fertilizer. The overwhelming amount of K in soil is in the structure of soil minerals and so is unavailable. The plant takes up K from the soil solution and exchangeable K replenishes the soil solution. You add K as KCl, giving K + which is mostly adsorbed onto – sites.

This is another factor contributing to the need for K fertilization.

Old study from NY. Land given to Experiment Station. Worn out soil. Soil test says way too little K. Agronomists install plot study with trees, some fertilized with K, some not. Pre-1940 data backwards extrapolated. Soil sampled for available K, i.e., exchangeable. Found to go up in surface soil where no K added. Maybe weather- ing of minerals. Uptake by deep roots and litter fall certainly at work here.