Mineral Phosphorus Fertilizers and Their Use in Utah Soils

In my last post I discussed the sources and processing of mineral Phosphorus (P) for the many fertilizers available for this critically important plant nutrient. In this post I want to discuss readily available P fertilizers. As with my post on nitrogen (N) fertilizers (see September 2018 post) I’ll include the most common solid and liquid formulations and some guidance on selection criteria.

General Notes on Fertilizer P Management

Phosphorus is one of the more difficult nutrients to manage in alkaline, calcareous soils due to two competing reactions, 1) the high solubility of mineral fertilizer sources of P, and 2) rapid precipitation of soluble phosphate back into the numerous insoluble rock phosphate forms (begins within minutes of P application to alkaline soils). The most common mineral precipitate is Ca-phosphate, or Apatite, due to the high free calcium content of most Utah and Intermountain West soils. The amount of time soluble P is available in the soil for these precipitation reactions, versus the opportunity time for plant uptake, is a key factor in P fertilizer uptake efficiency.

Calcium-phosphate precipitation, as well as surface adsorption of phosphate to organic matter and soil surfaces, render P fairly immobile in all soils, but particularly in calcareous soils in the western US. Movement of P from a fertilizer prill is generally only a fraction of an inch from placement, hence, fertilizer placement is generally much more critical for good P fertilizer management than for N or Potassium (K).

Knowing these properties, it is then easy to understand that efforts to keep P plant-available consist primarily of strategies and technologies designed to control the solubility of the fertilizer, or application strategies that minimize exposure to bulk precipitation reactions in soils. Examples would be banding rather than broadcast applications, timing applications to better coincide with plant need and uptake (i.e., avoiding fall application), etc. These strategies and technologies are common to all P fertilizers discussed, regardless of formulation.

Solid Fertilizers

Triple Super Phosphate (TSP, 0-45-0; plus 15 Ca)

This is the base product of initial treatment of rock phosphate (RP) with green phosphoric acid. Triple Super Phosphate was, until the last decade, the most common commercial P fertilizer product as well. It has been largely replaced by the ammoniated phosphate fertilizers that will be discussed below, but can still be found in many places where the mining of RP occurs, especially outside the US.

Growers may find this fertilizer difficult to find commercially in Utah due to the ammoniated phosphates being higher in P content, and often less expensive to produce. Fertilizer dealers also prefer to market products containing several plant nutrients. If commercially available in your locale, TSP offers the advantage of containing only P and no other macro-nutrients (N or K). This is desirable if nothing other than P is needed, or if one wishes to mix nutrient sources for custom nutrient blends tailored to specific soil conditions. This fertilizer does contain 15 percent soluble calcium (Ca) which may be a desirable additional plant nutrient in some cases, or as a soluble Ca source for conditioning sodic soils (a certain future post).

Mono-ammonium Phosphate (MAP, 11-52-0) and Di-ammonium Phosphate (DAP, 18-46-0)

These are the two most common ammoniated solid P fertilizers, and the most popular commercial P fertilizers in general. Each contains both P and N (as ammonium). Hence, the management issues associated with potential ammonia volatilization are in effect with these fertilizers as they are with the ammonium-based N fertilizers (see September 2018 post).

Both MAP and DAP are high in P content, and have high fertilizer value over many other mixed nutrient sources. Care should be taken to not add too much N when applying these materials directly with the seed. As ammonia develops from ammonium reactions in calcareous soils, this may desiccate seeds or burn seedlings. Moreover, where very little N is needed, the application of excess N is a wasted expense, or worse, counter-productive to the growth of the crop. This is most common in establishing new crop alfalfa, where excess N application can suppress the desired development of rhizobium bacterial symbiosis in the young legume crop. In those cases, side dressing or banding the fertilizer is a more reasonable approach.

While on the subject of banding, let me state that studies more often than not show improved P uptake and response when P fertilizer is placed in a band to the side and below the seed on calcareous soils. This creates a zone of P enrichment that the plant roots can naturally grow near or into to encounter needed P nutrition. In a broadcasted fertilizer condition (as is typical with mobile N fertilizers) the probability of P reaching a root surface is far less certain given its relative immobility.

Special Purpose P Fertilizers

There are many solid fertilizer formulations containing P for specialty applications. One group of these, which are commonly used in production horticulture settings, are known as the “complete” fertilizers (16-16-16, 12-12-12, 10-10-10, 8-10-10, 8-8-8, etc.) which contain relatively equal proportions of N, P and K. These are particularly useful in greenhouse media (potting soil, peat, etc.) that contain naturally low levels of these primary plant nutrients, or in any setting where all three nutrients may be regularly needed (for example, tree fruit production on coarse sandy, or gravelly soils in Utah County).

There is a group of newer solid fertilizers containing high P content that are being heavily marketed over MAP and DAP. A number of fertilizer dealers in Utah have discussed with me the use of 12-40-10-1 (contains 1% Zinc (Zn) and 6 to 10 % Sulfur (S), depending on the manufacturer). One of the interesting aspects of this fertilizer is that it is not blended (i.e., made by mixing several fertilizers of different elements together). Rather, it is purported to be an all-in-one prill, meaning that it contains all the stated nutrients, in the stated proportions, in each prill. This may offer some advantages in nutrient distribution uniformity over custom blends for broadcast applications. That said, if Zn is needed (which is not entirely uncommon in Utah soils) banded application of this material to the side and below the seed should be considered over broadcasting to improve the opportunity for plant uptake.

With all the above-mentioned multi-element fertilizers, know that you are paying for each nutrient delivered. There is no free lunch, so to speak. Multi-nutrient fertilizers offer some one-step application options where several nutrients may be needed, but they are often not cost-efficient if you need just one or two of the nutrients. Make sure to base fertilizer choice primarily on the cost per unit nutrient needed.

Liquid Fertilizers

Ammonium Polyphosphates (APP, 10-34-0 or 11-37-0)

Phosphate polymerization was discussed in my last post, but as a review, as phosphoric acid dehydrates, individual (or ortho-phosphate) ions combine into polymer chains forming the basis for these very popular liquid fertilizers (additional reaction with ammonia produces the final product containing N as ammonium).

About half of the phosphate in these liquid fertilizers is in the “ortho” or single ion form, and hence, immediately available for plant uptake as well as the surface complexation and chemical precipitation reactions previously mentioned. Within a week or two in moist, warm soils, the remaining phosphate polymers are chemically and biologically broken down into the “ortho” form.

While this breakdown would at first seem to be a mechanism to prolong P solubility and plant availability, particularly in calcareous soils, it does not present a significant agronomic advantage. Therefore, choice between solid or liquid formulations of P fertilizers is generally based on a combination of cost per unit P, and a grower’s fertilizer handling and application method preferences.

There are some that cite improved crop performance using liquid P fertilizers, but this has been shown to be more an effect of improved placement of P, rather than a response to formulation (for example, wash-off of liquid P fertilizer within proximity of active roots near the plant crown, etc.).

As with all ammoniated fertilizers, care should be taken to minimize the potential for N loss as ammonia. In that regard, liquid APP fertilizers have a handling advantage in that they can be applied with irrigation water, allowing incorporation of soluble mobile N deep enough to prevent ammonia volatilization.

Final Comments

Lastly, a couple final comments on P management in soils. I often receive questions on high testing P soils through our USU soil testing lab (see soiltest.usu.edu). The concern with high test P in soils is never a plant toxicity issue. Two things are of potential concern, however, as follows.

The first, and foremost, is the potential for erosion of soil high in plant-available P content, into surface waters. These eroded sediments will release P into the water and contribute significantly to algae blooms, resulting in water eutrophication. Some of these algae species can be toxic to humans and livestock—simply put, “don’t P in the water!”

Care should always be taken to both prevent soil erosion, and minimize P application to soils already high in plant-available P content. In fact, Utah guidelines specify that soils between 50 and 100 ppm Ammonium-Bicarbonate-extractable P (Olsen Method) can only receive P according to the annual need for P of the crop being grown. On soils that test above 100 ppm, no application of P is allowed.

Secondly, high plant-available P in soils can cause competition at the root surface for the uptake of some of the secondary and micro-nutrients, particularly Ca and Zn. Watch for deficiencies in these nutrients in visual symptoms or tissue tests of your crop, and if found, adjust application of P and the deficient nutrients accordingly.