June 2002
 

Sensitivity of Alfalfa and Hulless Barley to Seed-Placed Phosphate Fertilizer

Several producers and agronomists have questioned the safe seed placed fertilizer rates of phosphorus that are used in Manitoba recommendations. Seed toxicity from seed-placed phosphate is primarily due to the ammonium portion of the ammonium phosphate, rather than to salt effect. No seed placed phosphate is considered safe for alfalfa, and experience is lacking regarding the safe rates of seed-placed phosphate fertilizer for hulless barley.

To observe the effect of seed placed phosphate on crop stands, similar treatments were applied to unreplicated, demonstration plots at 5 sites across Manitoba (Diagnostic schools at Carman and Melita and Checkstrip sites at Rosebank, Baldur and Crystal City). Mono ammonium phosphate (MAP 11-52-0) fertilizer was applied during the seeding operation. Stand counts were taken in all plots after emergence and compared to the plots receiving no seed-placed fertilizer. No effort or intention was made to determine final yield.

Demonstration sites, seeding implements used and assigned relative risk of seed toxicity.

* Relative risk of seed toxicity: L=low, M=moderate, H = high, VH = very high
** SBU = seedbed utilization = (width of fertilizer &seed spread) x 100 / row spacing

As expected there was considerable variability in stand reduction among sites since soil conditions (texture, moisture status) and time until rainfall varied. In all instances the seed bed utilization (SBU) was low, indicating that fertilizer was concentrated in the row with the seed. This would represent "worst case" scenarios regarding the seeding implement. In general the stand reductions were associated with the relative risks assigned to soil texture, soil moisture at seeding and seeding implement.

For cereals, stand reductions of 15% are generally considered acceptable since the crop compensates for yield through tillering. The data tends to indicate that hulless barley is more sensitive to seed placed fertilizer than hulled barley on light textured soils. Serious stand thinning was not observed until rates exceed 30 lb P₂O₅/acre. Traditionally Manitoba barley crops would be fertilized at 25-40 lb P₂O₅/ac, and at these rates both hulled and hulless barley established well.

Alfalfa stands were severely reduced with seed-placed phosphate at Carman, but tolerated 30 lb P₂O₅/ac well at the other 2 sites where soil moisture was greater and SBU greater. This risk of reduced forage stand may not be worth the convenience of seed placing phosphate. This confirms the sensitivity of alfalfa, supporting the current recommendation that no phosphate should be placed with the seed. Safer placement options are side-banding, pre-plant banding or preplant broadcasting. Seedplaced phosphate may be acceptable when using a seeder with a high SBU (less concentration of seed with fertilizer) under favourable moisture conditions.

A low rate of seed placed potash was also applied at the Carman site (15 lb K₂O/ac), and tended to cause slightly greater stand reductions of barley and alfalfa than that of phosphate fertilizer at similar rates.

In conclusion, safe rates of seedplaced phosphate follows a similar risk pattern as seed-placed urea. Critical factors observed in these demonstrations were soil texture, soil moisture at seeding, time until rainfall and seeding implement.