Can extra fertilizer compensate for tight soils?

A fertilizer program should not be the primary focus in restoring soils damaged by last fall’s difficult harvest conditions, but some adjustments to your program will likely be needed

The fall of 2006 has presented some very difficult harvest conditions, leaving many fields with less than ideal soil conditions. We can expect that crops growing in these fields will have restricted root systems, whether from compaction or from poor soil structure, and this will lead to reduced nutrient uptake.

The inevitable question, then, is whether we should apply more fertilizer to make up for a smaller root system. The answer is that increasing fertilizer rates above normal will not pay in most cases, but there may be a response to changing how the fertilizer is placed.

The absorption of nutrients by crop roots is a product of both the total root area, and the concentration of nutrients in the soil around the roots. We have all seen fields that produced tremendous crops when they were first broken out of a long-term sod, even though the soil tests were low. They simply had such large root systems that they were able to get the nutrients they needed despite the low concentration. As the soil structure declined with more years of cropping, the crop yield declined as well, unless nutrients were applied to improve the fertility of the soil.

Nutrient supply from the soil could be increased in the entire soil volume (as with broadcast incorporated fertilizer), or in specific bands or zones (as in banded starter fertilizer).

Broadcast fertilizer is most appropriate for nutrients that may be harmful at too high a concentration due to excess salt or free ammonia, and for nutrients that the crop needs in greatest amounts after the root system is well established.

Banded fertilizer, particularly in a starter band, is most appropriate for nutrients that get bound tightly to the soil, and for nutrients that the crop requires in high concentrations before a large root system is established.

If the crop needs more nutrients than it can get from the bulk soil, roots will respond to the higher concentration of nutrients in the band by branching profusely.

Limits to uptake.
Increased nutrient concentration will compensate for reduced root growth up to a point, but this eventually reaches a limit. Each square centimetre of root surface can only absorb a finite quantity of any nutrient per day, so a crop with a very small root system could run short even if there is ample fertility in the soil.

The soil itself can get in the way of nutrient uptake, since many nutrients are rapidly depleted from the water around the root and must diffuse around soil particles to replenish this supply. In a soil with poor structure, the pathways these nutrients must follow can be extremely twisted and tortuous.

Finally, the concentration of nutrients may be high enough to damage root tissue, as in fertilizer burn, so increasing nutrient concentration actually decreases nutrient uptake.

The practical consequence of these limits is that the nutrient level cannot be increased enough to overcome the restricted root growth in a severely compacted or deformed soil. In moderately affected soils, there may be an increase in nutrient uptake with higher fertilizer rates, but this may or may not create a nutrient response.

Limits to response.
Plants growing with restricted root systems suffer from more than reduced nutrient uptake. The ability of the plant to absorb water from the soil is also reduced, and in severe cases the roots will not be able to anchor the plant. Dry weather conditions will create moisture stress in small-rooted plants much more quickly, and the negative impact on yield will be much more severe than in plants with normal root systems.

Plants that lodge due to small root systems may not be able to fill the grain that was produced, or it may produce grain that cannot be harvested because it is lying on the ground.

Managing nutrients on tight soils.
The fertilizer program should not be the primary focus of bringing soils damaged in last fall’s harvest back into full productivity, but some adjustments can be helpful as part of a complete program.

Nutrient deficiencies will likely hurt the crop more than normal, so you should do soil tests to ensure that the proper amount of fertilizer is applied, if it is needed. Banded starter fertilizer will likely give a greater response than normal.

If equipment is available, you may want to consider banding some potash, as this is the nutrient that has been most affected by compaction in other areas. Finally, side dressing nitrogen into corn may provide an additional benefit by aerating a tight soil and allowing increased root growth in early summer. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs based in Stratford. keith.reid@ontario.ca