Phosphorus Fertilizer for Alfalfa

Michael J. Ottman & Ayman M. Mostafa

The University of Arizona, College of Agriculture & Life Sciences, Cooperative Extension

Phosphorus is the primary mineral nutrient likely to be required by an alfalfa crop in Arizona. A single application of phosphorus fertilizer at planting time may be sufficient for the life of the stand in certain situations. However, on soils with a high phosphorus fixing capacity, a single application of phosphorus fertilizer at planting time may not be sufficient. On these soils, phosphorus fertilizer may be necessary on an annual basis, or even several times each growing season. The availability of phosphorus fertilizer in certain soils decreases over time to such an extent the soil may be deficient in phosphorus a few months after fertilizer application, thus warranting multiple applications in a season.

Phosphorus deficiency in alfalfa is characterized by reduced plant height and size of the leaves (Fig. 1). The leaflets may tilt upwards.   The crop may also have a dark blue-green appearance. Alfalfa usually does not exhibit purpling of the leaves as occurs with phosphorus deficiency in many other crops. Phosphorus deficiency in alfalfa is difficult to detect since the reduction in growth that occurs is not apparent unless the crop can be compared to alfalfa that has adequate phosphorus.

P deficiency

Phosphorus can be supplied to alfalfa on soils with a high phosphorus fixing capacity by 1) applying high rates of fertilizer, 2) using multiple applications of fertilizer, 3) banding the fertilizer, 4) applying manure as a phosphorus source, 5) using sources of phosphorus fertilizer that are complexed with humic, fulvic, or other organic acids, and 6) lowering the soil pH. Applying high rates of phosphorus fertilizer may be inefficient since phosphorus fertilizer can be rapidly fixed by the soil, but there will be a brief time of high phosphorus availability after the application. Multiple applications of phosphorus fertilizer, such as every few months, may be needed each season, particularly in the spring, to maintain soil test levels in the adequate zone. Banding fertilizer is not a common practice in alfalfa, but applying a liquid formulation in a narrow band on top of the soil may be a way to reduce phosphorus fertilizer fixation by limiting the amount of soil the fertilizer comes in contact with. The phosphorus in manure is tied up in organic compounds and does not have as much chance to become unavailable in the soil as inorganic fertilizer. Several fertilizer products are on the market in which the phosphorus is complexed with various organic acids, thus making the phosphorus less likely to be fixed in the soil, similar to manure. Lowering the soil pH can make soil phosphorus more available, particularly on soils with a pH between 8.0 and 8.5. Table 1 contains major phosphorus fertilizers used in Arizona.

P formula

Currently, we do not have strong research data to show differences among fertilizers for supplying phosphorus to alfalfa. Therefore, our recommendation is to apply phosphorus fertilizer based on cost per pound of phosphorus or based on convenience of applying fertilizer in the irrigation water in the case of liquid fertilizers.

Additional Resources:

Ottman, M.J., J. Rovey, A.M. Mostafa & W. Burayu. 2015. Phosphorus Fertilizer Rate Effect on Alfalfa Yield and Soil Test P, Buckeye, 2014. University of Arizona Cooperative Extension AZ1672.
https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1672-2015.pdf

Ottman, M. 2012. Alfalfa Nutrient Requirements. http://cals.arizona.edu/crop/presentations/2012/MikeOttman.pdf

Ottman, M.J., T.L. Thompson, M.T. Rogers, & S.A. White. 2000. Alfalfa Response to Forms of Phosphorus Fertilizer. Forage and Grain Report: College of Agriculture and Life Sciences, University of Arizona. http://arizona.openrepository.com/arizona/bitstream/10150/204090/1/az11852a-2000.pdf

Aphids in Sorghum

Ayman Mostafa

After last year’s invasion of sugarcane aphid (SCA), Melanaphis sacchari in sorghum, PCA’s and growers in Arizona should be on the look out for this insect this season. The SCA can be confused with other aphids that are occasionally found on sorghum.  One is the corn leaf aphid, Rhopalosiphum maidis, that is small with bluish green body color with darker strips on the abdomen. The head, legs, cornicles, and antennae are black in color. The corn leaf aphid is usually found deep in the whorl of the middle leaf of preboot silage sorghum, but may also be on the underside of leaves, stems, or in panicles. Sorghum plants generally can tolerate many corn leaf aphids without being significantly damaged and control seldom is justified.

Another similar aphid is the greenbug, Schizaphis graminum. The greenbug is yellow to green in color with a dark green stripe down the middle ofits back. The greenbug can be found occasionally on sorghum and other grain crops, and there is no record of it causing significant damage in Arizona.

The following picture has the three aphids, SCA, corn leaf aphid and greenbug.

SCA_GB_CLA1

Section 18 Emergency Exemption of Transform® WG for the Control of Sugarcane Aphid in Sorghum

******For IMMEDIATE DISTRIBUTION******

Attention SORGHUM producers and PCAs! Through the Arizona Department of Agriculture, the state of Arizona has sought and received a Section 18 Emergency Exemption that permits the use of Transform WG insecticide within Arizona for the control of sugarcane aphid in sorghum. This new pest to Arizona is known to be highly invasive and difficult to control in sorghum with many older chemistries. Because of the imminent economic harm that this pest represents to our sorghum producers and the dairies that they serve, you may now apply for a permit from ADA and use Transform WG immediately to control sugarcane aphid. It is important that you follow and comply with all instructions provided by ADA and on the Section 18 label for Transform WG.

Steps

  1. Obtain the ADA permit form online at: https://agriculture.az.gov/sites/default/files/Transform%20Use%20Application.pdf
  2. Fill out and return the form to ADA licensing by email to: licensing@azda.gov
  3. ADA will provide you a Section 18 label that you must maintain in your possession when using Transform WG under this exemption
  4. Bring your approved permit to your local retailer and purchase Transform WG
  5. Report all applications of Transform WG to sorghum on ADA’s Form 1080 and submit to ADA
  6. Call ADA if you have any questions about the permitting, reporting or Section 18 label for Transform WG: 602-542-3578, anyone there can help you, including Carlos who is lead on this.

Our Thanks & Gratitude

We are grateful for the ADA’s work to facilitate the approval of this important Section 18 Emergency Exemption! On behalf of all stakeholders, we also thank:

  • Director Mark W. Killian for requesting this Section 18 through his letter of request to US-EPA
  • Mike Buben, Independent PCA, for supplying a letter of support and other economic risk information for the submission
  • Ken Narramore, Independent PCA, and the Arizona Crop Protection Association for supplying a letter of support, economic data and other insights for the submission
  • Greg Green, Sorghum Grower and PCA, for his letter of support
  • KC Gingg, Sorghum Grower and Dairyman, for his letter of support
  • Ana M. Kennedy Otto, Government Relations Manager, and the staff of Arizona Farm Bureau for supporting the submission package with a letter of support
  • Brian L. Bret, State Regulatory Manager & Dow AgroSciences for supporting this request and moving materials into distribution swiftly for use by sorghum producers right away
  • Drs. Ayman Mostafa and Peter Ellsworth, as “qualified experts”, and to the staff of University of Arizona’s Arizona Pest Management Center for the Section 18 submission by ADA and for helping to document the need required by this process.

Here are some links with local information about sugarcane aphid in sorghum.

Any products, services or organizations that are mentioned, shown, or indirectly implied do not imply endorsement by the University of Arizona.