fertILIzer GrAde: oxIde vs. eLeMentAL forMs?
When talking or reading about fertilizer P or K, the oxide form is usual y assumed. This is used in all recommendations and when you buy fertilizer. The terms “phosphate” (P2O5) and “potash” (K2O) have been used for so long to refer to phosphorus and potassium in fertilizers, it is likely that they will be with us indefinitely—even if they are confusing. When you apply 100 pounds of potash per acre, you actual y apply 100 pounds of K2O—the equivalent of 83 pounds of elemental potassium. Of course, you’re real y using not K2O but rather something like muriate of potash (KCl). A similar thing is true of phosphate—100 pounds of P2O5
per acre is the same as 44 pounds of P—and you’re real y using fertilizers like concentrated superphosphate (that contains a form of calcium phosphate) or ammonium phosphate. However, in your day-to-day dealing with fertilizers you need to think in terms of nitrogen, phosphate, and potash and don’t worry about the actual amount of elemental P or K you purchase or apply.
if the weather is too wet to apply the fertilizer (and you
commonly used solid N fertilizer, urea, is left on the soil
haven’t put on enough preplant or as starter) or too dry
surface. Also, nutrients remaining on the surface after
following an application for the fertilizer to come into
application are much more likely to be lost in runoff
contact with roots. Then the fertilizer stays on the surface
during rain events. Although the amount of runoff is
instead of washing into the root zone.
usually lower with reduced tillage systems than with
Once the soil nutrient status is optimal, try to bal-
conventional tillage, the concentration of nutrients in
ance farm nutrient inflows and outflows. When nutrient
the runoff may be quite a bit higher.
levels, especially P, are in the high or very high range,
If you are thinking about changing from conven-
stop application and try to maintain or “draw down”
tional tillage to no-till or other forms of reduced tillage,
soil test levels. It usually takes years of cropping without
you might consider incorporating needed lime, phos-
adding P to lower soil test P appreciably.
phate, and potash, as well as manures and other organic
residues, before making the switch. It’s the last chance to
Tillage and Fertility Management: To Incorporate or Not?
easily change the fertility of the top 8 or 9 inches of soil.
With systems that provide some tillage, such as mold-
board plow and harrow, disk harrow alone, chisel plow,
SOURcES
zone-till, and ridge-till, it is possible to incorporate fer-
Mikkelsen, R., and T.K. Hartz. 2008. Nitrogen sources for organic
crop production. Better Crops 92(4): 16–19.
tilizers and amendments. However, when using no-till
OMAFRA (Ontario Ministry of Agriculture, Food, and Rural
production systems, it is not possible to mix materials
Affairs). 1997. Nutrient Management. Best Management
into the soil to uniformly raise the fertility level in that
Practices Series. Available from the Ontario Federation of
Agriculture, Toronto, Ontario, Canada.
portion of the soil where roots are especially active.
Parnes, R. 1990. Fertile Soil: A Grower’s Guide to Organic and
The advantages of incorporating fertilizers and
Inorganic Fertilizers. Davis, CA: agAccess.
amendments are numerous. Significant quantities of
Torstensson, G., H. Aronsson, and L. Bergstrom. 2006. Nutrient
ammonia may be lost by volatilization when the most
use efficiencies and leaching of organic and conventional crop-
ping systems in Sweden. Agronomy Journal 98: 603–615.
van Es, H.M., K.J. Czymmek, and Q.M. Ketterings. 2002. Manage-
ment effects on N leaching and guidelines for an N leaching
soIL tests
index in New York. Journal of Soil and Water Conservation
57(6): 499–504.
Soil tests, one of the key nutrient management tools,
are discussed in detail in chapter 21.
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Building SoilS for Better CropS: SuStainaBle Soil ManageMent
