Worldwide, mother nature is not making new phosphate deposits to meet agricultural demand.  The reserves in premium rock phosphate deposits are decreasing.  Thus, it might require the processing of, for example, 125 tons of rock phosphate raw material to obtain the same amount of P derived from 100 tons previously.

Phosphorus is the second most limiting nutrient in crop production.  But its behavior in soils limits its availability and can make adequate crop P nutrition an essential management issue.

Fertilizer for monoammonium phosphate (MAP, or 11-52-0) is currently near $500/ton.  I expect that phosphorus will gradually become more expensive.  In time, I anticipate research will focus on P acquisition and P use efficiency in plants.  This could include crop species that alter the root environment more to procure P, more efficient crop root P uptake mechanisms, and crops that simply require less P thus are more efficient.  Within crop species (corn, grain sorghum, wheat, etc.) studies may also identify a crop’s varieties or hybrids that retain yield potential with limited soil available P.

Phosphorus nutrition to crops is a greater dilemma versus nitrogen (N) which is readily available in the atmosphere.  Air is 82% nitrogen as the molecule N₂.  It requires a lot of energy to break molecular N into a form you can use like ammonium or with further conversion to nitrate.

Adequate phosphorus nutrition in Texas crops presents special challenges for several reasons.

1. The necessary P requirement for numerous crops is often substantial, especially for higher yields. This can approach 100 to 175 lbs. per acre of P₂O₅ equivalent (†see end note) in some cases.

2. Perennial crops like alfalfa and especially long-term bermudagrass and bahiagrass rarely permit incorporation of applied fertilizer in the soil for more ready access by roots.

3. Phosphate fertilizer materials are relatively insoluble. They don’t dissolve easily when placed in the soil or are rained on or irrigated if laying on the soil surface.  Where you place the P fertilizer product is largely where it stays.

4. Soil pH has a great effect on P availability. P application is a challenge as typical broadcast surface-applied phosphate fertilizers are relatively insoluble.  P availability in soils with pH > 7.2 is low, becoming more pronounced at pH 8 and may be largely unavailable to some plants.  This is due to tie-up by calcium.  In acid pH soils (generally pH < 6) phosphate interacts with iron and aluminum to produce relatively insoluble oxide compounds.  This becomes worse the lower the pH, as root development slows or stops due to the acidity.  (For example, acid soils in east Texas may have pH < 5.0).  A pH of about 6.5 is best for P availability in soil.

Fig. 1.  Purple P deficiency symptoms in corn.  Plant stunting is also a common P deficiency symptom. (Photo Credit:  A. Magenot, www.soils.org)

5. Banding of phosphorus fertilizer products, where possible, generally maintains more available P over time for root uptake. This is a further improvement for P availability even compared to tilling in broadcast fertilizer P, which is common in row crops.  Your crop’s annual nutrient requirement may be met with a smaller application of P if applied in a band (granular P with a planter or air seeder; a knife rig if liquid P).

6. Soil compaction coupled with dry soil surfaces limits the ability of the plant to recovery both fertilizer applied P and P throughout the surface- and shallow sub-soil. Evaluating compaction from the soil surface down to 12 to 18” deep of the rooting zone is key to better ensure that soil P is actually plant available.  P is only taken up by the plant through soil root contact.

An Early Look at P Banding in Texas Wheat

Studies conducted by Dr. Travis Miller, former state Extension wheat specialist, evaluated three methods of P application on Rolling Plains dryland wheat.  See the wheat section of http://publications.tamu.edu/ for the discussion.  These methods were A) no applied P, B) broadcast phosphate then tillage into the root zone, and C) incorporation of P using a knife rig (though the spacing was just 10”).

Broadcast incorporated P led to increased dryland wheat forage production in these Rolling Plains trials.  But when P was knifed at 8” depth (10” centers, narrower than any available commercial rig), wheat yields improved up to 47% more forage vs. the original stand without P.  There are two experimental factors at work here.  First, the banding increased P use efficiency, but also (second) in this dryland wheat region, applied P fertilizer using a knife rig placed the P deep enough (8”) where soil likely retains some moisture even during drought.  All nutrient uptake by roots requires at least some moisture else no root nutrient uptake will occur (roots in a dry top layer).

If this research resulted in commercial use, I would suggest knife rig spacing of 30” using GPS (for liquid 10-34-0), and the next time you apply P, move over half the distance.  Twenty inch spacing would be better.  Also, an air seeder using 11-52-0 on 6” spacing was similarly effective in this Rolling Plains research.

Managing Your P Fertilizer Applications

First, ensure you understand the current P status of a field.  Soil test.  You have many lab choices for soil testing.  I assure you that Texas A&M AgriLife Soil Testing Lab analyses are appropriate for your soil type.  See http://soiltesting.tamu.edu  And the lab’s recommendations are based on decades of specific crop research assessing P growth response curves.

For high pH soils across West Texas the procedure used for P analysis, Mehlich-III, is adequate.  Some prefer the Olsen P test, but that requires a separate lab procedure that measures P only.  It is not suitable for other soil nutrients.

When you can band fertilizer P, do so.  It increases the value of your applied P.  This is especially true for row crops where liquid 10-34-0 is placed in a root zone band near the row of seed.  When feasible, banding is economical in most situations.  In fact, the Texas A&M AgriLife Soil Testing Lab has considered how banding might reduce the recommended fertilizer P application to meet crop P requirements.

P Fertilizer and Perennial Crops

Alfalfa grown 5 years or more presents a special consideration.  It has a high P requirement where 12 to 15 lbs. of P₂O₅ are removed per ton per acre.  Once alfalfa is established, it is impractical to incorporate P fertilizers into the soil.  Thus, producers must rely on common granular fertilizer, primarily 11-52-0, to supply P nutrition to alfalfa.  This surface-applied, relatively immobile P is largely restricted to the very top layer of soil.  This is true even in regions where soil pH is 7 or less.

Texas growers of bermudagrass and bahiagrass could have their fields for decades.  So, the field is rarely if ever in condition to incorporate surface-applied phosphate.  In these cases, we must accept that regular surface-applied P is the best we can do.  We rely on time, rainfall, warmer temperatures, etc. to gradually dissolve P over time to make some P available to the roots at the surface.

Insuring adequate P can also impact the availability and utilization of other soil and fertilizer nutrients.  Multiple field studies have found direct correlation between P availability and uptake of potassium and multiple micronutrients.  These correlations were attributed directly to the enhanced and more prolific root systems that were able to explore and take up greater water and soil nutrients.

How do we address this P issue in perennial forages?  Can farmers soil incorporate higher pre-plant P by broadcast/tillage or knifing in a larger portion of the expected forage P requirement for the next several years?  This is a feasible approach.  The key is P in the root zone where it is needed.  The potential cost of higher P fertilizer rates before seeding may be a deterrent to applying larger amounts of P (>100 lbs. of P₂O₅ per acre) in advance.  There is also concern about high broadcast P applications tying up zinc. This is a well-known potential nutrient issue in many field crops, especially in high pH soils.  Targeted banding of P can reduce the potential for significant zinc tie-up

The timing of surface P applications when this is your only option for perennial forages may be a partial solution.  Typically, in alfalfa or perennial grasses broadcast topdress P is applied in the spring.   Texas A&M Soil Testing Lab director Dr. Tony Provin (t-provin@tamu.edu, (979) 845-4816), notes some observations in central Texas suggest fall applications of broadcast P may be better than spring.  This allows extra time for the P to slowly move into the soil surface (more frequent winter rains in central Texas?).

We recognize the value of potential advance applications of P fertilizers to be incorporated before seeding.  This places the P where it is needed—in the root zone.  In contrast, surface-applied low-soluble fertilizers, especially on higher pH soils, are only slowly available to growing alfalfa.  The slow dissolution and short-distance movement of this P may not be enough to adequately meet immediate P needs of a high demand crop.  The reduced efficiency is a concern with 11-52-0 prices in the NM/west TX region currently in the range of $500/ton.

In Summary

P fertilizer is a special challenge for adequately meeting crop requirements.  Understand how this P, when applied, behaves differently from nitrogen.  Nitrogen when applied is mobile and can move to the root zone.  P is not mobile.  Where you place it is where it stays.

For further questions about phosphate fertilizer on your farm, please call us (Provin, Trostle as well as other soil scientists working for AgriLife Extension agronomy—Dr. Jourdan Bell, Amarillo, Dr. Jake Mowrer, state soil fertility Extension specialist, College Station).

For further reading of a simple summary of the soil’s inherent factors affecting soil phosphorus availability consult this NRCS webpage:

https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_053254.pdf

†Phosphate chemistry and phosphate composition uses an older form of expression (like potash, K₂O; but not nitrogen which is simple N) that expresses the content of phosphate as phosphorus pentoxide rather than P alone.  For example, the common formulation of monoammonium phosphate fertilizer (MAP) gives a % nutrient content of 11-52-0 for N/P₂O₅/K₂O.  For actual P, the content is about 23% P.  Commercially, almost all fertilizer uses the P₂O₅ expression.  Thus, 100 lbs. of MAP give about 52 lbs. of P₂O₅ equivalent, or about 23 lbs. of actual P.  This may be confusing.  It would be better if strictly P was reported rather than its phosphate equivalent.

Dr. Calvin Trostle, Professor & Extension Agronomist, TAMU Dept. of Soil & Crop Sciences, Lubbock, (806) 746-6101, ctrostle@ag.tamu.edu

Dr. Tony Provin, Professor & Extension Soil/Water/Forage Lab Director, TAMU Dept. of Soil & Crop Sciences, College Station, (979) 845-4816, t-provin@tamu.edu