## Do I have enough soil moisture to make a crop?

The dry conditions of the 2007 growing season to date have many wondering if they have enough soil moisture to finish out their corn and soybean crops. A few simple calculations can give us a pretty good idea whether or not we have a chance of making it to the finish line. For starters, we know that a 150 to 200 bushel corn crop or 50 to 60 bushel soybean crop require approximately 20 to 24 inches of water. By adding up our seasonal rainfall and estimating the amount of water stored in our soil profile, we can get a pretty good idea if we have adequate moisture available to reach our yield goal.

For example, my field consists of a Blount loam soil. By checking a USDA Soil Survey or by looking up the USDA, Natural Resource Conservation Service web-based Soil Survey at: http://websoilsurvey.nrcs.usda.gov and following the prompts, I can get a good estimate of my soils water holding capacity. In my particular case with a Blount loam soil, the water holding capacity is listed as: 0.17 cm per cm in the top 36 inches of soil. The cm per cm unit is actually a straight percentage so: 36 inches x 0.17 percent = 6.12 inches of available soil water in the upper 3 feet of soil assuming I start at field capacity in the spring.

The next step is to add in the amount of rainfall received since planting. If you don’t record this information for yourself, you can use precipitation data recorded at the “Michigan Automated Weather Network” (MAWN) station nearest your field which can be accessed at: http://enviroweather.msu.edu/home.asp. Checking the MAWN website, I find that I have received only 6 inches of rainfall in my vicinity since planting. Assuming that I began the season with my soil moisture at or near field capacity, I calculate: 6 inches of rainfall received + 6.12 inches stored soil water = 12.12 total inches of available water.

This leaves me a water deficit of 8 to 12 inches short of the precipitation levels needed to meet my yield goal. What are the chances I’ll get another 8 to 12 inches of rain while my crops can still benefit from it? By using 30-year average rainfall data I can get a pretty good idea. Thirty-year average precipitation levels are available at: http://climate.geo.msu.edu/. The 30-year average August rainfall in my area is about 3.8 inches. If August precipitation levels are near average, my total crop available precipitation would then be just short of 16 inches (12.12 inches +3.8 inches) which is still considerably short of the 20 to 24 inches desired. The 30-year average data also show that I could expect another 3.8 inches in September, but most of that would likely be too late for the corn plant to benefit in terms of grain yield. Therefore, it appears that 2007 precipitation levels will likely limit my corn and soybean grain yields below my targeted yield goals.

Of course these calculations should be considered “ballpark estimates” as actual soils vary considerably in terms of water holding capacity. Additionally, crop rooting depth will vary based on soil structure, crop genetics, nutrient status, and early season water levels.