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So many variables, so little time

When soil tests indicated a sulphur deficiency in two of his fields, John thought he was doing the right thing by increasing the amount of seed-placed fertilizer blend he applied. But instead of correcting the deficiency, he had unknowingly created another serious issue.

John now had a problem with seedling emergence in those two fields. Meanwhile, the other 1,200 acres of canola he’d planted were doing fine, he told me.

I visited John’s 3,000 acre farm, located northwest of Saskatoon, during the first week of June. John produces wheat, canola and peas. The first canola fields he showed me were completely healthy and developing normally.

The other two fields had noticeably poor crop stands. The plant populations were much sparser in these fields than in his other healthy fields, with only six plants per square foot. The plant density increased in low-lying areas, and it seemed thinner on the slopes and tops of hills.

The plants had been seeded into sandy loam with low soil moisture content. It had not rained in the three weeks since the crop had been sown. John had applied a seed-placed fertilizer blend at planting. He’d increased the rate of fertilizer blend on these two fields to compensate for the sulphur deficiency. What John didn’t realize was at the increased rate the fertilizer had caused ammonia toxicity and salt damage to the seed!

High application rates of fertilizer in the seed row can result in ammonia toxicity (which is caused by most nitrogen fertilizers), salt damage from the dissolving fertilizer, and the absorption of moisture by the fertilizer, causing the seeds to dry out.

Not only had the increased fertilizer rate caused damage, but the conditions present in John’s fields had combined to create the ultimate environment for seedling injury. The soil moisture content at seeding, the lack of precipitation, the type of crop, soil texture and seeding tool settings — all of these factors affect the tolerance of seed to seed-placed fertilizer. For example, the higher the soil moisture content at seeding, the more tolerant the seed is to seed-placed fertilizer. Precipitation after seeding will help reduce damage caused by fertilizer injury. In John’s case, a timely rain may have diluted the fertilizer salts and free ammonia — produced from the urea — and the problem may have been avoided.

The type of soil texture can also increase the risk of salt and ammonia toxicity. Crops seeded into coarse soils such as sandy loam will be more affected by fertilizer toxicity than those seeded into fine-textured soils such as clays. Crops also vary in their sensitivities to seed-placed fertilizer; for example, canola, as an oilseed, is one of the least tolerant crops, whereas cereal crops such as oats, wheat and barley are more tolerant than oilseeds.

It is important to understand the many variables involved when applying fertilizer in the seed row, and to make adjustments accordingly. With the 2012 crop year just ahead, I made some recommendations to John to reduce his risk of seed injury, such as lowering the amount of seed-placed fertilizer when seeding into coarse soils, and using products with a lower salt index. It is also essential to understand the rate of application and capabilities of the seeding tool — John could have used a wider opener. Being knowledgeable and aware of the seed bed utilization of the seeding tool, soil type and conditions at time of seeding also reduce the chances of crop injury. An awareness of these many factors will increase your success when using seed-placed fertilizer. †



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