Rock and Roll Agronomy

Soil Organic Matter: Example Where Agronomics Impact Economics

By Jason Hanson
Certified Crop Advisor
jlhanson@agriliance.com

If there is one difference in most of the off-season farm talk so far this winter, it seems that there is less focus on agronomics and more on economics. That’s to be expected, with natural gas, plant food and energy costs starting to consume more of the crop budget. 

However, in our concern about managing the economic side of our crops, let’s not overlook managing the agronomic side. In fact, I can tell you that every farm out there in the Northern Plains has already accomplished (or ignored) a fundamental agronomic practice last fall that will have an economic impact on the crops planted this spring – a basic soil test.

With the highest plant food costs we have probably seen to date, there is still way too many acres that are not getting soil tested. Why on earth would we not want to at least find out how much is available after last year going into record high fertilizer costs? The main concern is nitrogen, but also phosphorous and potassium. I would include zinc, chloride, sulfur, copper and maybe boron in some instances. Some nutrients are more important to some crops than others (review NDSU crop fertility publications online at www.ext.nodak.edu/extpubs/soilfert.htm ).

Soil organic matter is another key component to crop productivity, yet few understand it – most of us comprehend what $500+/ton anhydrous means, but I’d venture to guess there’s quite a few of us who would not be sure whether 3.2% organic matter is high or low, good or bad.

Importance of organic matter
Soil organic matter is composed of mainly incorporated crop residue, plant and animal residues in various stages of decomposition (such as plant roots, bacteria and fungi) and substances secreted by these organisms. Organic matter is important to many aspects of soil chemistry, which relates back to soil fertility. The higher the soils organic matter, generally it is said that a soil has better potential for producing larger yields. Here are some of the soil factors that organic matter plays a part in:

• Cation exchange capacity (soil’s ability to supply positively charged elements/plant nutrients, such as calcium, magnesium, potassium, and sodium)
• Source of sulfur and phosphate
• Source of nitrogen
• Energy for microbes
• Soil structure
• Water holding capacity

All of these factors will affect the soil’s ability to produce a crop, and affect your plans for managing input costs. A sandy soil generally has a low OM level, also having a lower cation exchange and water holding capacity. Clay is on the opposite end of the spectrum and generally has better soil fertility with better water holding capacity. The darker the soil, the generally better it is felt to be, and those dark soils are higher in organic matter than lighter colored or sandy ground.

But enough with the science lecture, how does this pertain to me and my crop in 2006? Well, have you ever fertilized a crop for say 120 bushel corn and it came back at a 180 bushel yield? How did that happen? And why doesn’t it happen every year? See, all the factors of soil organic matter interact to improve tilth of the soil. The larger the quantities of residue returned to the soil and the ability to decay them help improve soil productivity, which in turn affects yield and your ability to profit.  Getting the point on how good agronomics can impact good economics?

The best way to increase residue in the soil is to raise crops that yield well and return that residue (properly distributed after harvest). Large crops are made by using the right amounts of crop nutrients. It is not just residue, but also practicing reduced tillage and avoiding fallowing that also help increase soil organic matter.

Physical movement of soil by erosion is really bad for your soil organic matter content. Another factor is the nitrogen content of crops that are returned back to the soil as residue. A well-fertilized corn crop may contain 0.75-1% nitrogen in the stover, while under-fertilized will contain 0.5%. A well-fertilized corn crop will also develop a more extensive root system that can distribute organic matter deeper in the soil profile after the harvest and the decomposition can begin.

So to keep tabs on long term soil trends in relation to our fertility plans and rotation – for instance, to see if factors such as no-till, CRP, wheat on wheat or beans on beans is changing our soil structure – we need to include organic matter in our soil test program. I would suggest including an analysis of OM in your soil test every few years. The University of Minnesota has information on managing organic matter online at www.extension.umn .edu/distribution/cropsystems/DC7402.html.  Your local agronomist/certified crop advisor is also an excellent resource for advice on OM analysis and management.

Fertility costs are getting crazy, but so is not taking care of your soil by maintaining good soil organic matter. That starts with tracking it on a soil testing program, fertilizing for good yields, returning that residue to the soil and reducing the amount of tillage where possible. Costs are real, but when those right years come along and you have excess yield for the amount of fertilizer used, those expenses are returned back as profits. Nothing pays the bills more than yield, so plan on getting it by fine tuning your fertility program, which includes knowing and managing your soil organic matter. 

Hanson is a certified crop advisor near Devils Lake, N.D.