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Fluid Journal : Fluid Journal 1993-1995
2 Fluid Journal Fall 1995 Stratification One problem that can develop with conservation tillage is stratification of immobile elements (e.g., P, K, and Zn) in the top few inches of soil. This is observed routinely. Surface applications and nutrient cycling without deep mixing cause nutrients to accumulate on the surface. Note the highest accumulation of P and K in the top three inches as shown in Figure 1. Crop residue decomposition is what mines nutrients from subsurface soils and deposits them on the surface. Although some no-till soils may be moist enough to support root growth closer to the surface, roots ultimately will have to explore deeper soils for water and nutrients as surface soils dry during warm weather. Controllable factors Two controllable factors in reduced-till are timing and placement of nutrients. Nitrogen and sulfur are affected by timing and placement while the non- mobile elements P and K are more affected by placement. Because a lack of regular soil mixing can cause nutrient stratification, as already mentioned, subsurface banding and surface banding have been investigated as alternative remedies. A number of such studies have shown the advantages of banding or surface banding under low fertility or high nutrient fixation conditions. P/K placement. Starter fertilizer is a foundation for proper nutrient management in conservation tillage. Starter application is the strategic placement of supplemental plant nutrients near the seed at planting time. Studies throughout the United States have shown crop response (yield) has increased markedly when starters are used. One such study at Purdue University showed that the probability of a profitable corn yield response to starters in reduced-till increased dramatically at all soil fertility levels (Figure 2). In the no-till system, yield improved in eight of eleven experiments! All sites were high fertility locations where starter would not have been recommended with conventional tillage. In the upper Midwest, particularly Minnesota and Wisconsin, band applications of K are an important part of fertility programs in no-till and ridge- till, even at high K soil test levels. A small amount of K applied at planting has given consistent yield increases in many soils in this area. N placement. Surface residue present in conservation tillage systems serves as a protecting mulch against erosion and increases water infiltration. It also affects a number of other soil properties such as soil temperature, moisture content and aeration. All of these factors have implications on how efficiently N from fertilizers, manures, legumes, and native soil will be used by crops. Increased surface residue increases microbiological activity. Nitrification, denitrification, and mineralization rates increase under reduced-till systems, which suggests N transformations occur more quickly in reduced-till systems. Greater microbial activity increases the likelihood that N will be immobilized or tied up by soil microbes. Significant N loss from volatilization may occur without sufficient rain (quarter inch or below) to incorporate surface-applied urea-containing fertilizers. Volatilization is the transformation of urea to gaseous ammonia and subsequent loss to the atmosphere. Recent research has quantified volatilization from different sources. Figure 3 shows results from very sensitive micrometeorological measurements in fields that did not receive any moisture for six days after application. The smallest loss was from dribbled UAN and the largest from broadcast urea. Ammonia loss from the 120 Ibs/A of N applied ranged from 13 percent for dribbled UAN to 24 percent for broadcast urea. The best assurance to obtaining maximum performance from N fertilizers in reduced-till is to minimize contact with surface residues. Studies have shown the dramatic effect subsurface placement of N can have in reduced-till. Figure 4 contrasts surface broadcast and injected N in three tillage systems. Note placement method had no effect on yield in the plow system. However, injected N was clearly more efficient in the system leaving more residue. Injecting N below residue causes some tillage action and may require relatively high-powered equipment. At the same time, surface banding, new technology (equipment), and fertilizer Figure 2. Effectiveness of starter on corn yield in a reduced-till system, 11 experiments, Mengel, Purdue University 1990. 8765432 10 Conventional No-till Tillage System No. yeild responses 11 10 9876543210 Avg. yeild increase-bu/A Yeild No. of responses Urea Broadcast Uan Spray Uan Dribble lbs/A 30 25 20 15 10 5 0 Figure 3. Total ammonia loss over 16-day period using different sources and methods, Fox et al. , Penn State University, 1993.
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