Safe Rates of Seed-Placed Phosphorus for Manitoba - Narrow Row and Row Crops

Prepared by John Heard, MB Ag Soil Fertility Specialist (FPE)

The most critical nutrient for early season growth is phosphorus and for quick accessibility it is best placed with or close to the seed. In some crops, seeding configurations and soil conditions, all the phosphorus a crop needs may be seed-placed. In other cases unacceptable seedling burn and germination damage may occur.

Seed-placed fertilizer rates are restricted by a number of factors:

Fertilizer type – and salt index and ammonia toxicity
Crop sensitivity
Fertilizer –seed placement
Soil conditions – texture (affecting cation exchange capacity or CEC) and moisture

The salt properties of fertilizers can draw moisture out of germinating seeds. Crops vary in their tolerance to fertilizer salts with cereals being most tolerant, followed by pulses and lastly by oilseeds. The other hazard is ammonia (NH3) toxicity. High seedling zone concentrations of ammonia are toxic to seedling roots, impairing water and nutrient uptake. The portion of free ammonia in the soil is increased with high soil pH, high levels of free lime or carbonates, low CEC and dry conditions.

Now most growers are well versed with the safe rates of phosphorus fertilizer with traditional Prairie narrow row width crops like cereals, flax and canola. These are printed in most Prairie crop production guidelines. For example the following guidelines in Table 1 are for Manitoba¹ and Saskatchewan².

Table 1. Maximum safe rates of actual seed-placed phosphate (P₂O₅) fertilizer as mono ammonium phosphate (MAP 11-52-0 or 12-51-0) for Manitoba and Saskatchewan.

Crop	Actual P₂O₅ lb/ac
	Manitoba*	Saskatchewan**
Cereals	50	50
Canola	20	25
Peas, Flax	20	15
Faba bean	20	40
Dry bean	10	30 Pinto Beans
Soybean	10	-

*Rates are based on disk or knife openers with a 1 in. spread, 6 to 7 in. row spacing and good to excellent soil moisture.

**Rates are based on knife openers with a one-inch spread, 9-inch row spacing and good to excellent soil moisture.

Traditionally the higher salt content associated with potassium fertilizers was considered as toxic as ammonia releasing nitrogen fertilizers. More recent Saskatchewan studies have shown damage from potassium to be less; hence their guidelines indicate that pounds of P₂O₅ and K₂O should not exceed the maximum rate of phosphate stated in Table 1.

There is a lack of Prairie data on seed-placed fertilizer guidelines for row crops like corn, sunflowers, soybeans and dry beans. Tolerance to seed injury is less since:

Seeding in wide rows means a greater concentration of fertilizer with the seed
Productivity is more dependent upon full stands (less able to tiller to compensate)
Seed is expensive, and
Some are quite sensitive to fertilizer injury

Ontario³ limits seed-placed phosphate levels for corn based on the amount of N and K₂O in the fertilizer. Table 2 contains their N and N & K₂O limit for corn in 30” rows with the corresponding application rates of 3 sample granular and liquid phosphate-based fertilizers. I have taken some liberty with some of the values in Table 2 to account for metric conversions and since most liquid fertilizer equipment is sold and calibrated in US gallons per acre (US gpa). These rates are presumed to limit injury to less than 10% of the time. Injury effects would be reduced or germination delayed or growth retarded. OMAF does not consider ANY seed-placed fertilizer to be safe for soybeans, dry beans or peas and does not have guidelines for sunflowers.

The lower part of Table 2 indicates the amount of corn stand thinning that might be expected under differing soil conditions, based on South Dakota State University’s (SDSU) “Seed-Placed Fertilizer Decision Aid” developed by Professor Ron Gelderman⁴. It provides growers with a measure of potential stand thinning that results from seed-placed fertilizer rates for different crops, fertilizer sources, seeder configuration and soil conditions. The relationships were developed under lab conditions and verified with field studies. This spreadsheet is available here Seed-Placed Fertilizer Decision Aid.

Table 2. Safe limits of seed-placed phosphate fertilizer with 30” row corn based on Ontario guidelines (Ontario Ministry of Agriculture and Food, OMAF3) and expected stand thinning based on the SDSU "Seed-placed Fertilizer Decision Guide".

Row width	Ontario Safe Limit	Granular mono ammonium phosphate (MAP) 11-52-0	Ammonium polyphosphate (APP) solution 10-34-0	Liquid 9-18-9
	N or N & K₂O lb/ac	Corresponding lb P₂O₅/ac (US gpa)
30" rows	10	45	35 (8.5)	9 (4.7)
Soil texture/moisture		Predicted stand thinning % based on SDSU spreadsheet
Fine-medium	Moist	15	4	8
Fine-medium	Dry	30	8	17
Coarse	Moist	22	6	13
Coarse	Dry	37	9	21

Based on the amount of anticipated stand thinning in Table 2, one would be very wary about using the higher rates of MAP or 9-18-9 based on Ontario guidelines. Liquid fertilizers with high K₂O content increase potential of salt injury. Stand injury increases with soil dryness and coarser texture.

The SDSU Calculator, (Table 3) shows those rates of seed-placed P fertilizers that would be expected to cause 5% stand thinning in corn and soybeans. That is probably all the thinning one would want to sacrifice with high priced seed. This example includes different row widths and P fertilizers under moist, fine textured soil conditions. If fine textured soil is dry, rates should be adjusted in half. For coarse textured soil, rates should be reduced about 33% for moist soil and 60% for dry soil.

Table 3. Application rates of different seed-placed fertilizers causing an estimated 5% stand thinning on fine-textured, moist soils.

Crop	Fertilizer	Lb P₂O₅/ac (US gpa)
		30" rows	15" rows
Corn	MAP 11-55-0	17	34
	APP 10-34-0	46 (11.6)	92 (23.3)
	Liquid 9-18-9	6 (3.0)	12 (5.9)
Soybeans	MAP 11-55-0	2.5	5
	APP 10-34-0	3.6 (0.9)	7 (1.8)
	Liquid 9-18-9	1.3 (0.7)	2.6 (1.3)
Sunflowers	MAP 11-55-0	7.5	15
	APP 10-34-0	7.0 (1.8)	14 (3.5)
	Liquid 9-18-9	2.7 (1.4)	5.5 (2.8)

In summary, the risk of seed-placed fertilizer injury exists for high value row crops. Some fertilizers can safely meet the starter and maintenance phosphorus needs of the crop, whereas others will require other fertilization strategies, like side-banding, preplant banding, multi-year applications, etc.

I suggest trying the SDSU calculator for your client’s seeding configuration and soil conditions while setting a level of risk to their liking. As confirmation, it is important to plug a fertilizer run for 50 – 100 feet of row. Mark that row and follow-up with a stand count of the seedlings and root inspection of any missing or damaged plants.

References
¹ Manitoba Soil Fertility Guide. 2007. Manitoba Agriculture, Food and Rural Initiatives. P. 17 or http://www.gov.mb.ca/agriculture/crops/soil-fertility/soil-fertility-guide/phosphorus.html

² Guidelines for safe rates of fertilizer placed with the seed. 2009. Saskatchewan Ministry of Agriculture.

³Agronomy Guide for Field Crops. Publication 811. Ontario Ministry of Agriculture, Food and Rural Affairs. Table 9-21

⁴Gelderman, R. 2007. Fertilizer placement with seed: A decision guide. Proc. of North-Central Soil Fertility Conference Vol 23, P. 46-54. http://extension.agron.iastate.edu/nce/ncepdfs/2007/nc2007-p46-gelderman.pdf