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Why is corn so uneven this year?

George Rehm, Dept. of Soil, Water, and Climate, University of Minnesota

Minnesota corn growers are accustomed to looking at corn fields where all plants are the same height. The uniformity of height was missing in many Minnesota fields during the early portion of the 2003 growing season. In fact, the uneven corn growth stimulated several telephone calls.

As with many problems there is probably no single easy answer to this one. There are several factors that could cause uneven growth. For many fields, the problem could be attributed to more than one.

In some situations, the uneven growth can be attributed to a deficiency of one or more essential nutrients. In 2003, deficiencies of potassium, sulfur, and zinc have been noted in parts of some fields. Research trials and experience with precision farming have taught us that there can be a wide range in relative levels of plant nutrients in soils. For example, an analysis of a composite soil sample may show that the potassium value is in the medium or high range. Yet, there may be areas having a low soil test for K in the field. These areas having the low test, without the adequate application of K, may be the cause for stunted corn and subsequent uneven growth. The same explanation can apply to zinc. The soil test values for zinc may vary more than soil test values for potassium.

Nutrient deficiencies, do not explain all stunted corn situations. Observations from the fields where stunted corn has been a problem include: 1) high fertility, 2) a nice loose seedbed at planting, 3) one pass with a field cultivator for secondary tillage, 4) no problems in recently manured fields, and 5) taller corn in wheel tracks associated with the secondary tillage operation.

So, why should corn be taller in parts of the field usually associated with some compaction (wheel tracks, for example)? A reasonable, but not proven, explanation combines our knowledge of root growth and nutrient uptake.

Plant nutrients are known to move to plant roots by the three processes of: 1) mass flow, 2) diffusion, and 3) root interception. Nitrogen as nitrate - N (NO3-N) moves to roots by mass flow as water is absorbed from the soil by plant roots. Phosphorus as (H2PO4- or HPO4=) and K+ reach the root by diffusion and root interception. In the diffusion process, the nutrient moves from an area of higher concentration (the soil particle) to an area of lower concentration (the root). Root interception is the process whereby the root comes in contact with the nutrient as it grows. Therefore, N can move over longer distance compared to P and K. The relative distances of movement of these three nutrients in soils is shown in Figure 1.
Figure 1. Relative movement of N, P, and K in soils.

Soils were loose at planting this past spring. As a result, the pore space (the open area between soil particles) had a high percentage of air and less water. In general, there were not heavy rains during the early part of the growing season. There was less air space under wheel tracks or where some other form of compaction existed. As roots grew into the pore spaces in these more compacted areas, there was more space occupied by water. Thus it was easier for young roots and young corn plants to get needed nutrients. As a result, plants in slightly compacted areas showed better earlier growth. A general relationship among soil particles, pore space, plants, and nutrients is shown in Figure 2.

Is there evidence for this explanation? Yes. Some crop advisors have collected corn plants from the compacted and non-compacted areas and found higher concentrations of N, P, and K from the taller plants growing in the slightly compacted areas. Crop advisors have also reported uniform height of corn in field that, for some reason, received 2 or more secondary tillage operations before planting.

Does all of this mean that compaction is good? NO! There are ample research projects which have pointed to the conclusion that compaction has a negative effect on yield. These situations observed in 2003 are the consequence of a unique set of conditions that occurred at planting and for a short period of time after planting.

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