For an ah hah moment. It is very easy to figure. If, you buy a 10-10-10 fertilizer it is 10%
nitrogen (N) 10% phosphate(P) and 10% potassium (K). Thinking as may make more sense to the original poster. If, the bag weighs 100 lbs you bought ten pounds of nitrogen, ten pounds of phosphate, 10 pounds of potassium. Simple math-a 50lb bag is 5 pounds of each. They suggest not more than 1 pound nitrogen per 1000 sq ft. Reason to prevent lawn burning AND lessen fertilizer run off into the sewer system.
100 lbs of 5-5-5 equals 50 lbs of 10-10-10. If, they are priced the same the 100 lbs simply has more filler. It is of course heavier. It might spread easier-without bands etc due to missed spots.
You want at least 25% slow release nitrogen-less likely to burn your lawn and will feed longer. It will also cost more. You can apply less each time but apply more often.
They no longer suggest you apply phosphates regularly to your lawn. Since, the mowing guys leave the cuttings on the lawn it simply recycles. Our soil is high in phosphates anyway and it is a major cause of pollution in the lakes etc,
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