Posts Tagged: phosphite
P a Simple Little P
It is such a simple little letter, P. It stands for the element phosphorus. It is often misspelled as phosphorous which is an adjective, but even in technical literature it is misspelled. But that's not the end. Phosphorus is an element that takes many forms called oxidation states. When it is in the form of phosphate or phosphoric acid, it is a fertilizer – H3PO4. But when it is in the form of H3PO3 or phosphonate or phosphonite or phosphite or phosphorous acid, it does not perform like a fertilizer. It acts more like a stimulant for a plant to fight off Phytophthora or Pythium. And it works well for avocado and citrus root rots, as well as citrus brown rot.
But a grower recently told me that there is no end of confusion about these two very different forms of P.
A recent article helps to clear up some of this confusion
http://www.spectrumanalytic.com/support/library/pdf/Phosphites_and_Phosphates_When_distributors_and_growers_alike_could_get_confused.pdf
and more if you are still interested
https://edis.ifas.ufl.edu/hs254
Sick avocado on the left and healthy on the right
root rot vs healthy
Implications of foliar phosphite applications on pistachio rootstock
California nut growers have benefited from the use of phosphite products, marketed and sold both as...
Pistachio leaf morphology
The Deal with Phosphite Fertilizers
Here is another interesting aspect of plant nutrition drawn from the October 23 Soil Fertility Short Course at UC Davis.
Phosphorous (P), an essential element for plants, does not occur alone in nature and rather it combines with oxygen and hydrogen. Bonded with four oxygen atoms P makes phosphate, and when bonded with three atoms of oxygen and one atom of hydrogen, P forms phosphite.
The fully oxidized phosphate (the one with four oxygen atoms) is the most stable form of P in the environment, and is preferentially taken up by microbes and plant roots. Then again, phosphate is adsorbed to soil particles more than phosphite, meaning it is less available. However, it does not necessarily follow that this enhanced availability of P via phosphite results in more uptake by plants, but even so, fertilizer formulations of phosphite plus calcium, magnesium or potassium have been formulated with the intent of taking advantage of this greater solubility in the soil.
Several studies examining a greater availability of P through phosphite have found that when phosphite is applied at an equivalent rate of P to phosphate fertilizer, it consistently underperformed when measured in terms of crop productivity, especially in the first year of cropping. In the way of explanation, the slower oxidation process of phosphite to the plant-root preferred phosphate, may be part of the equation of why the phosphite is not as effective as phosphate, in spite of having greater mobility.
In lay terms we can say, yes, phosphite might more available to plants in the soil, but they don’t want it as much as they want phosphate.
On the other hand, foliar applications of phosphite have shown that, while it can be converted to phosphate on the leaves by microbes living there, it is more readily absorbed into the foliage of some crop plants like citrus and avocados. Phosphite application (usually once at pre-bloom and perhaps a second time later in the season) to the foliage of avocados or oranges has resulted in more flowers, greater fruit yield and size, total soluble solids and anthocyanin concentrations.
For further reading, see the paper below, which among other things implies that the positive results experienced in oranges and avocados could be replicated in berries…
http://www.spectrumanalytic.com/support/library/pdf/Phosphite_Fertilizers_What%20are%20they.pdf
/span>Are All Phosphorous Products the Same
South African plant pathologists were the first to show that root rot in avocado could be controlled by trunk injection with both phosphorous acid and the patented material Aliette®. Aliette was briefly registered in California in the late 1980’s, but theregistrant soon lost interest in pursuing a full pesticide registration when it became apparent that other researchers believed phosphorous acid could be registered as a fertilizer - a process much less costly and simpler than a pesticide registration. The company continued to hold on to the patents for the product and the breakdown products that were useful in root rot control. By holding onto the patent, this effectively stopped other companies from pursuing a pesticide registration for phosphorous acid. In 1990, a publication reported that phosphite could be used as a source of phosphorus fertilizer and this became the basis for the registration of phosphite as a fertilizer. Subsequently, when the original patent expired, at least two materials have been registered as fungicides containing phosphite – Fosphite® and Agri-fos®. There are, however, numerous phosphite materials that have been registered as fertilizers (for some brands see Brunings et. al., 2005, http://edis.ifas.ufl.edu/HS254), and every day seems to bring more brands onto the scene each making claims of having the best efficacy.
We wanted to see if we could detect an efficacy difference between Aliette, another registered phosphite fungicide and four different materials registered as fertilizers, for a total of six materials. In a greenhouse, three-month old ‘Topa Topa’ seedling avocados with cotyledons removed were planted into a Phytophthora cinnamomi -inoculated organic potting mix. A control was also planted without the inoculum, as well as an inoculated control. One of six different materials was then applied as a soil drench until draining from the bottom of the liner. The materials were applied at the equivalent phosphorous acid concentration. There were 20 replicates for each of the controls and treatments. The experiment was repeated twice.
At harvest, root fresh and dry weights were highest for the non-inoculated trees and lowest for the untreated, inoculated controls, in both trials. All treatments’ associated weights intermediate between these two were statistically the same. Even a repeat application of one of the treatment materials in trial II didn’t result in greater root weights than single application treatments. Shoot weight, both dry and fresh, was much less affected by root rot and treatments. There were no differences in fresh shoot weight in the second trial, not even between the inoculated and noninoculated controls. The root and shoot weights of all the treatments in the second trial were higher than in the first trial, indicating that either the inoculum was not as effective or that the trial was not continued long enough to produce as much damage.
Root rot studies often have dramatic effects on root weights while shoot weights may remain little affected. It is clear from our data that phosphites reduced the severity of root rot in this study, but that there was no benefit of a single source of phosphite relative to any other source.
Below: Healthy and decaying avocado roots.
avocado RR picture