How much fertilizer do my crops need?

By Ned M. Birkey, Spartan Ag

Profitable crop production -- including the efficient use of fertilizer and other best management practices -- and environmental protection should go hand in hand.  Soil and plant fertility accounts for about one-third or more of total crop yield.  On many high yielding or highly managed fields, increased production from fertilizer could be as high as 60 percent.

Fertilizer management can affect one or more of several components of farmer profits.

The average field crop farmer is projected to lose an average of $100 per acre for the next two years, according to USDA.  Maximizing the efficiency of all inputs is a goal shared by most farmer-businessmen and businesswomen today.

Michigan Right to Farm Generally Accepted Agricultural and Management Practices (GAAMPS) for Nutrient Utilization essentially means following best management practices based upon sound science.  The January 2016 revision to the Nutrient GAAMPS were developed and reviewed with industry, university and multi-governmental agency input.

Agricultural fertilizer has been perceived as a nonpoint source pollution problem in some watersheds in Michigan as far back as a 1988 MDNR (now MDEQ) report.   Phosphorus (P) loading of Michigan’s lakes and streams has been documented by MDNR in 1985 and was an issue raised by Heidelberg College in a meeting in Monroe County in 1996.  Nitrate contamination of groundwater has been documented in 1984 and since.

Although the use of other fertilizer nutrients has also increased, changes in soil test levels of nutrients such as potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), and micronutrients have been less dramatic.  Currently these nutrients are not causing any known environmental damage, and there are no concerns for their continued use as long as they benefit the farmer agronomically and environmentally.

Managing crops for efficient fertilizer use may mean considering some of the following practices.

1. Do not apply any fertilizer, especially phosphorus, unless a current soil test shows a deficiency situation exists.

2. Soil test on a regular basis before nutrients from fertilizer, manure, biosolids or compost are applied.  Obtain a nutrient analysis of manure, biosolids or compost.  Grid soil sampling is more precise than traditional soil sampling by soil type if the fertilizer dealer can apply a variable rate of the nutrient source.  A current soil test means every four years for farmers in a two crop rotation situation or every three years for farmers in a three crop rotation.

3. Selecting a realistic yield goal for crops is one of the most important steps in obtaining economic and environmentally sound recommendations.  Excessively high yield goals can lead to loss of income and over-fertilization that may threaten water quality. A yield goal that is both realistic and achievable should be based on the soil potential and the level of crop management utilized.  The nutrient GAAMP considers a realistic yield goal to be one that is achievable at least fifty percent of the time.  If a yield goal is seldom achieved, the entire crop management system should be re-evaluated to identify those factors other than soil fertility that are limiting yields.

4. Take nutrient credits for cover crops, legumes, organic matter, manure, previous crops or other biological materials.

5. Good recordkeeping demonstrates good management and will be beneficial for the farmer if their management practices are challenged.

6. Use soil erosion control practices, such as filter strips, to minimize nutrient runoff and soil loss.