Loading manure into a spreader on the Kimberley farm in Jasper County Iowa September 1939.

Throughout the history of farming animal manure has been recognized as a source of fertility.  Plants grown in soils amended with manure not only benefit from the nutrients contained in the manure, but also from improvements in soil physical properties.

Traditionally, manure application rates have been based on the crops need for nitrogen which often results in the over application of other nutrients.  This is because manures generally contain nearly as much phosphorus as nitrogen for example, but crops only require one-fifth to one-half as much phosphorus.  The result is often a gradual buildup of phosphorus and potassium in the soil.

What is wrong with that you might ask?  Soils with excessive nutrients, phosphorus in particular, can increase nutrients contamination of surface waters.  Nutrient rich surface waters result in algal blooms that subsequently deplete oxygen for plant and animal life.  This serious reduction in water quality has adversely affected fisheries in many areas.  Excess potassium can result in soil nutrient imbalances.  Magnesium deficiencies in plants are common, even when adequate magnesium is present, in soils with excessive potassium. 

Does that mean you should stop using manure?  No, properly managed manure can make up a substantial part of a sound fertility management plan.  Care should be taken not to over apply manure.  Excessive manure will not increase yields and will speed the accumulation of phosphorus and potassium.  When soil tests reveal that these nutrients have reached very high levels, reduce manure applications and fertilize with a high nitrogen fertilizer such as McGeary 6-0-4 or McGeary 8-1-1.