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Fluid Journal : Fluid Journal 1996-1998
Winter 1997 when residue is converted to organic matter. We know, for instance, that corn residue has a carbon/nitrogen ratio of 50:1, or, for purposes of this illustration, corn residue has a 5 percent protein level. For ruminants, the best protein level is not 5 percent but 10 to 12 percent because they digest stalks best at that level. So, residue with a 50:1 carbon/nitrogen ratio, not unlike cattle, obviously needs extra nitrogen in order to convert cellulose into organic matter. Similarly, if cattle are fed roughage, they will need additional protein (or nitrogen) so that bacteria in the rumen can multiply and do a good job of breaking down roughage. Out the other end, of course, comes manure or organic matter. We want to encourage this same reaction in soil. Think of a field as one giant rumen. We want to incorporate residue, add nitrogen, and digest the residue into organic matter. It is simple if conditions are favorable for this conversion to take place. Adjusting ratio. Why do it and how is this done? In the example above, we know there are 1,000 lbs of carbon in 2,000 lbs of corn but only 20 lbs of nitrogen, giving us the 50:1 ratio. Add 20 pounds of N per ton of residue, the ratio will be lowered to 25:1. This helps bacteria by giving them a balanced diet. They now can proliferate, grow and digest residue. The same quantity of straw takes even more N---30 lbs per ton of straw---to lower a 100:1 carbon/ nitrogen ratio to 25:1. The concept is the same as feeding cattle: the soil is like a giant rumen. Instead of feeding extra urea or protein, nitrogen is added to decay residue into organic matter. Building tilth. Good residue management builds soil tilth. Analyzing Kentucky data, Table 1 shows that in ten years organic matter increased from 2.5 to 5 percent! The amount of organic matter doubled from 17,000 to 34,000 lbs/A. There was also a net gain of 700 lbs/A of nitrogen contained in the organic matter. The gain per year of organic matter in the top two inches was 0.25 percent. Spreading the increase over the top six- inch layer of soil, the increase would amount to 0.15 percent per year. Note also that nitrogen tied up in organic matter amounted to 70 lbs/yr/A. The production of organic matter was not cheap. It cost money in terms of nitrogen. But that nitrogen was not wasted. It went into the organic fraction, available to be taken out in the summer months under proper conditions. In Table 2, the possible addition to the organic matter reserve for 150-bu/A corn is 0.18 percent per year in a six-inch profile. Some may question the extra $20/A to tie up 75 lbs/A of N and increase organic matter. Is it justified? If it means a more productive soil, higher yields, and improved profits, the answer Table 1. Increase in organic matter after ten years in a Kentucky no-till field under good residue management. % organic Organic Nitrogen in Year matter matter organic matter _________________________________________________________ (0-2") lbs/A lbs/A 1970 2.5 17,000 700 1980 5.0 34,000 1,400 Net gain (10 yrs) 2.5 17,000 700 Gain/yr 0.25 1,700 70 "We can help organisms convert residue to organic matter by using proper tillage and providing enough food." Figure 1. Organic matter level can make a difference in the productivity of soils.
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