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.
Many Manitoba soils started off dry this spring. Growers should assess seedbed moisture conditions and be prepared to make changes – especially if winds continue to dry seedbeds. Following is a list of guidelines for seed placed phosphorus from a variety of sources:
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. guidelines.*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.
Table 2. Safe limits of seed-placed phosphate fertilizer with 30” row corn based on Ontario guidelines (Ontario Ministry of Agriculture and Food, OMAF3) .
South Dakota State University developed a calculator that takes into account the amount of stand thinning that may occur with different crops, fertilizers, sols, moisture and seeding equipment. It is available online at: http://www.ipni.net/article/IPNI-3268
As confirmation, it is important to plug a fertilizer run for 50 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.Submitted by: John Heard, Crop Nutrition Specialist, MAFRD References 1 Manitoba Soil Fertility Guide. 2007. Manitoba Agriculture, Food and Rural Development. 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. http://publications.gov.sk.ca/documents/20/84100-e42316e3-15ea-4249-ac0e-369212b23131.pdf 3 Agronomy Guide for Field Crops. Publication 811.Ontario Ministry of Agriculture, Food and Rural Affairs. Table 9-21 http://www.omafra.gov.on.ca/english/crops/pub811/9materials.htm#table9-21