The Effects of Too Much Fertilizer

I was planning our food plot strategy with a client when he stated he was going to go ahead and put some triple 16 fertilizer on the plot prior to planting our soybeans. Yikes!

I said whoa, hold on there a second! You spread a bunch of fertilizer last year without a soil test, now this year, you will do the same? Wait until I get the soil tested and we will see what we need, if anything. I made the long trip up to the property and took some samples of mud while it snowed sideways (in April.) Sure enough, the test showed a pH of 6.6, great for most food plot plants. This land is in New York where there are more recently glaciated soils, hence not as old and acid as PA soil. Anyway, the phosphorous was excessive and the potassium was off the charts excessive. Adding more fertilizer would not only been a waste of money but may have done more harm than good to the plants.

First of all, planting soybeans does not require Nitrogen fertilizer since, if inoculated, will produce their own if it is needed.

Phosphate is a pollutant as is Nitrogen. If not taken up by plants, it will leach into the groundwater and streams. This is why we are spending millions paying farmers to provide stream buffers in the Chesapeake Bay watershed. The Bay is polluted by farm runoff.  Also, excessive N fertilizer is said to cause increased acidity in soil over time.

Potash or K, can be toxic to plants and to the animals that feed on it. This is caused by an imbalance of Magnesium and Potassium in the organisms.

Another consideration is that too much mineral and cation concentration in soil can cause osmotic pressure problems, making it difficult for plants to take up water. The water wants to move to the material with higher concentration of mineral, which can be out of the plant, not in.

The amount of fertilizer necessary for maximum crop production depends on what you are growing and what is taken off the site. For instance, a crop of alfalfa that is mowed three times a year for hay takes 230 lbs of K per acre off site, corn grain uses only 35 lbs and corn silage uses 160 lbs per acre. If the crop is not taken off the field, nutrients generally stay on site. As in deer food plots, we don’t cut hay and take it away to feed stock but rather, deer eat it on site and put manure back and we mow sometimes and leave the clippings. In this case, we don’t need to fertilize much at all once the food plot is established.  I find that once a plot is squared away in terms of nutrient and pH levels, you don’t have to do much more soil amendment work.

Also, the type of soil structure and its pH makes a huge difference in the availability of nutrients to plants. A relative number, the Cation Exchange Capacity is the ability of soil particles, mostly clay sized particles to move loosely bonded positively charged nutrient molecules (cations) through the soil to the plant roots. An acidic environment in the soil causes hydrogen molecules to take up available sites and hold tightly, not allowing movement of nutrients.  In a more close to neutral environment, more negetively charged sites are available for cations N, P, K and micronutrients that are loosely held and can move freely.  The CEC measures the ability for nutrient molecules to exchange with others, allowing them to move into the plant for photosynthesis and building plant tissue.

Another thing to keep in mind is that these nutrients work together in photosynthesis and tissue -building. If they are far out of balance, they can hinder plant growth.

As you can see, the chemistry of soil is a very complex thing. I don’t know as much as would like to on the subject even though I studied soils in college. The point of this article is that you don’t really have to know much. If you get a soil test to the lab, and tell them what you want to grow, they make the recommendation of necessary nutrients for your crop. Leave it up to the lab to decide what you need to put on. If this valuable service is available for ten bucks per field, why would you not take advantage of it?

Much better to spend you money on good herbicide treatments and proper planting and maintenance of your plots rather than waste it on unnecessary fertilizer.