Soil tests and experience should guide fertilizer practices. Nutrient levels in the soil vary greatly among regions, with soil types, cropping history and fertilizer use. Provincial fertilizer bulletins should be referred to for general recommendations regarding fertilizer requirements, rates and placement. It is generally believed that relative to other crops, flax will not perform as well on low fertility soils, even when adequate amounts of inorganic fertilizers are used.
Flax is very sensitive to seed-placed fertilizer with even low rates sometimes causing seedling injury. Some provinces recommend a low rate of phosphate– not more than 20 kg/ha (18 lb./ac.) of P2O5 – may be seed-placed, while others recommend that no fertilizer be placed with the seed of flax. Considerable research evidence has now shown that placement of phosphate 25 mm (1 in.) to the side and 25 mm (1 in.) below the seed is an effective method to improve phosphorus nutrition in the flax plant. Nitrogen (N) should not be placed directly with the seed.
As discussed in the next section, with the advent of direct seeding using a one-pass seeding and fertilizing system, recent work has shown that adding nitrogen to the phosphorus when placed to the side and below the seed does not change the benefits of that phosphorus placement.
Other Placement Methods
Pre-plant deep-banding of phosphorus (P), which is often a good placement method for other crops, is not effective for flax. Broadcasting and incorporation of P fertilizer for flax is relatively ineffective, with up to a fourfold increase in application rates required to achieve the same yield result as sideband placement. The performance of P placed with the seed in the wider bands behind hoe-type row openers is being evaluated.
With the rapid conversion to direct seeding, other placement methods are becoming commonplace. As a rule, with direct seeding, all the nutrients are applied during the seeding operation. With egards to nitrogen management, the practice of mid-row banding and side-banding is commonplace. The question of interest is whether or not adding all the nutrients, i.e. N-P-K-S together in a single band can have a negative effect on the response of those individual nutrients. Recent work has shown that adding the nutrients P-K-S together in a single band did not negatively influence the response to nitrogen, indicating the feasibility of applying all nutrients together in a single band.
Flax often responds well to nitrogen (N) fertilizer application when available soil N is low. For flax seeded on stubble, 35 to 80 kg/ha (31 to 71 lb./ac.) of N should be applied unless more specific information, such as a valid soil test, indicates otherwise.
Select a rate from within the suggested range based on moisture conditions. Use the lower end of the recommended range when soil moisture is low and yield is expected to be limited by drought. Use the higher end of the recommended range when soil moisture is to be maintained near optimum. For flax seeded on fallowed land, the response to N fertilization is generally lower. The relative effectiveness of the various N sources, forms, placements and application times is the same for flax as for other annual crops. N should not be placed directly with the seed.
Response of flax to phosphorus (P) fertilizers is less pronounced or consistent than for most other annual crops. The flax plant appears to prefer high soil P levels from P fertilization of preceding crops, as compared to the application of a high rate of P on the flax crop itself. Rates of up to approximately 35 kg/ha (31 lb./ac.) of P2O5 are recommended for flax on soils with low levels of available P, as long as the fertilizer is not placed with the seed.
Potassium and Sulphur
Deficiencies of potassium (K) and sulphur (S) can limit production of all crops. However, these deficiencies are much more limited in extent than those of N and P, and are usually associated with specific soil types.
Sandier soils of the Dark Grey/Grey soil zones, as well as organic soils, are most frequently deficient in K. Sulphur deficiency is slightly more extensive, and can occur on sandy to loam soils of the Black through Grey soil zones. On irrigated land there is normally enough sulphur in the irrigation water to meet crop requirements. Approximately 34 kg/ha (30 lb./ac.) of sulphur is added to the soil with each 30 cm (12 in.) of irrigation water.
Iron and Zinc
Flax is more sensitive to low levels of iron (Fe) and zinc (Zn) in the soil than are most other Western Canadian field crops. In wet soil conditions, temporary iron deficiency can cause chlorosis yellowing of the leaves) in irregular patterns in the field. However, field tests have seldom shown any flax yield increase attributable to application of these micro-nutrients.
If a micro-nutrient deficiency is suspected, it should be confirmed through plant tissue testing. Following confirmation, measurement of response through small trial application should be undertaken before moving to full field scale application. Confirmation of a micro-nutrient deficiency is essential because research work has shown that applying micro-nutrients can depress flax yields if the nutrients were not truly deficient.