It won’t be much longer and many of us will be lingering over stacks of garden and lawn fertilizer bags trying to figure out which one we need where.

Veteran green thumb garden shoppers know about the “Big Three” fertilizers from the list of 16 elements necessary for plant growth. Ten of the other 13 are needed in amounts so small that plants get enough from the soil as is. The other three are needed in large quantities but they are everywhere: air, water, soil. So we don’t add carbon, hydrogen and oxygen as fertilizers.

The three we do add for plant growth are nitrogen, phosphorus and potassium. There is some level of phosphorus and potassium in all of our local soil types. A soil test lets us know if there is enough. Nitrogen itself is abundant and free because air contains more nitrogen than all other gases, but plants can’t utilize straight atmospheric nitrogen. Nitrogen left behind in soil as rotted plants and animals works for plants, as does manure.

Legume plants do the nitrogen-from-air routine, but beans, peas, clover and soybeans are not enough to sustain us. So we add nitrogen to most plants, more so than the other two fertilizers.

To be all organic, compost along with farm animal manures are nitrogen choices. The old farm method from way back of growing nitrogen-producing legumes and plowing them under well before planting time would still work.

For the rest of us, most nitrogen fertilizer is manufactured by combining air and natural gas under pressure. Ammonia nitrate and its derivatives are made this way. Products containing 16 percent, 34 percent and 45 percent Nitrogen are available via this man-made fertilizer.

Bone meal and fish meal are sources of organic phosphorus fertilizers. Animal manures contain both phosphorus and potassium in addition to nitrogen, but the levels of P&K are very small. A dab of cow or horse fertilizer is not going to help. Haul in a load and use repeated wheelbarrow trips to make a difference.

Ground rock phosphate from Florida comes in small bags for organic gardening. Huge quantities of that same rock phosphate mined twenty-four hours a day are treated with acid to reduce the impurities which increase the percent of actual phosphorus per boxcar load. That’s what we buy in 50-pound bags or by the ton.

Potash got its name from “pot of ashes” back when that was the way to make soap and leftover potassium fertilizer. We should never ever add wood ashes to plant root zones without a positive diagnosis the soil needs lime. Ashes work like lime to raise soil pH.

Greensand is always listed as a potash organic fertilizer but I’ve only seen it online. It occurs in numerous spots around the world, deposited by long gone seas.

The processed potash fertilizer used today comes from underground potash mines, ours mostly from Canada. Like the phosphate rock, a chemical process reduces impurities to increase the actual potassium in each shipped ton or pound.

 

Terry Rector writes for the Warren County Soil and Water Conservation District.