How Much is Too Much?

By Tim Nerbas, PAg

Conservation Agrologist

As with most things in life, too much of a good thing can do more harm than good. Whether it is chocolate, beer or soil fertility, when we push the toleration limits we should expect the unexpected.

This spring many producers are considering putting all the fertilizer down in one pass. The SSCA promotes one pass seeding for a variety of reasons including soil conservation, reduced fuel usage, and improved nutrient use efficiency. But the key to success lies with successful crop establishment. Sometimes this isn't possible with a single pass for a variety of reasons. Therefore, it is crucial to adhere to some important one-pass guidelines.

Can your existing equipment do the job? If not, depending on your circumstances, you may want to consider some of the retrofits that have proved very successful. The key is a seeding tool that maintains good depth control across the entire machine. Good depth control ensures even emergence throughout the field.

Does your equipment provide proper packing or can you add on-row packing? Packing the seed-row ensures good seed to soil contact. But be aware: research completed by PAMI shows that only minimal packing is necessary to optimize crop establishment (Table 1). Packing also encourages weed development (Photo 1). On-row packing is an important tool to get quick and even crop establishment. Leaving the area between the seed rows undisturbed and unpacked reduces weed growth. Finally, packing is most beneficial under dry conditions and over-packing can occur under wet soil conditions.

Table 1: Emergence and yield results with various opener/packer/force combinations. (PAMI Research Update 749)

Moisture affects not only packing, but also fertilizer requirements. The amount of fertilizer required depends on how much moisture is there in your soil this spring and how much rain you expect during the growing season. A soil test will provide you a snap shot of your fertility levels. Soil tests following the drought of 2002 show some fields require little to no nitrogen for crop production in 2003. Without a soil test, you won't know if you need no nitrogen or 30 or 60 lbs/ac to provide an adequate level of crop nutrition. Perhaps you need to balance the macronutrients N, P, K, and S. Without knowing the level of fertility, it is difficult to make an educated guess, particularly after a drought.

Too much fertilizer N can be toxic to seedlings. Seed placing too much N with the seed can cause reduced plant emergence. This can also cause delays in crop maturity and reduce the overall yield (Figures 1 and 2). It is always a gamble when high rates of N are placed with the seed. Saskatchewan Agriculture has developed guidelines for safe rates of fertilizer applied with the seed. It is based on seedbed utilization (SBU). SBU is the amount of seedbed over which the fertilizer has been spread. Increasing the area over which the fertilizer has been spread reduces the overall concentration of the fertilizer (Table 2).

If your system can provide good seed to fertilizer separation, the risk of seed injury is greatly reduced. If not, top-dressing additional N is an alternative to meeting N requirements. Remember broadcasting urea in the spring is not recommended. Ammonium nitrate may be a practical alternative if broadcasting is warranted. However, there may be difficulty in sourcing this form of N.

Table 2: Approximate safe rates of urea (46-0-0) N applications with the seed of cereal grains if seedbed moisture is good to excellent (soil moisture at or near field capacity). All rates are in pounds actual N per acre. Table courtesy of SAFRR.

If you still require more N than can be placed safely with the seed, don't lose hope. There is still one alternative. There is a urease inhibitor called n-(n-butyl) thiophosphoric tramide (NBPT or AgrotainÒ ). NBPT slows the conversion of urea to ammonium and ammonia over a 14-day period. The result is less seedling damage and lower ammonia losses making more N available for the crop. Dr. Cynthia Grant at the Brandon research center has performed field studies that showed NBPT put on seed-placed urea increased both seedling emergence and grain yield of barley. NBPT also improves the N uptake from top-dressed urea.

The key of course is not to overdo - the nitrogen, the packing, or the capabilities of your existing equipment. Know when to draw the line and don't overindulge. Like all those other good things in life, too much of a good thing can cost you in the long run. All the best this growing season.