Regarding my advocacy for a ban on the land application of animal waste in the Chesapeake Bay watershed in "Animal waste causes too great a share of Bay's pollution to ignore," (March 2010), Kenneth Carter sheds light in "It's time to study ability of no-till soil, cover crops to trap nutrients," (May 2010), on two processes he claims are "critical in trapping nutrients and do negate many of the (nutrient) losses implied."

Cover crops do sequester some, but by no means all, of the excess nutrients commonly applied to fields to ensure high crop yields. But cover crops do not "quickly take up any remaining nutrients in the soil profile."

Just as is true of any crop, nutrient uptake can never be 100 percent efficient. Even if nutrient losses to erosion and runoff are minimized, nitrogen will be lost to the groundwater and the atmosphere. More widespread use of cover crops is very desirable, but if cover crops are used in Virginia on about 100,000 acres as Carter asserts, and there are about 2.5 million acres of cropland, the role of cover crops in reducing pollution is minimal.

Why are cover crops not used more commonly? It is always about the bottom line. The cost of seeds, fuel, equipment maintenance and time exceeds the benefit to the farmer. If the extra costs are not subsidized, few farmers seriously consider the option.

It would be better if we never applied excess nitrogen in the first place rather than trying to sop it up later with cover crops. The development of controlled-release fertilizers where the cost of "packaging" is offset by the need to use less fertilizer is a much more attractive solution to the problem of agricultural pollution.

Increasing soil organic material by conservation cropping certainly improves soil tilth, especially by retaining moisture. But there is no evidence that soils containing more organic material improve fertilization efficiency.

Organic material does not "bind to free nutrients" and just release the nutrients for plant growth. Every time it rains, whether or not a crop is actively growing, nitrate is leached out of the soil and enters the groundwater.

Most important, chemical fertilizer application must be reduced, based on soil analyses, in the years after the land application of animal waste or buildup of organic material by conservation cropping practices. If this is not done, the excess nutrients will exacerbate pollution, whether the nutrients are derived from the organic material or from the chemical fertilizer.

There is certainly need for more research into the positives and negatives (more use of herbicides) of conservation cropping practices. But agronomists must focus on improving fertilization efficiency and not just on maximizing crop yields. Desirable as more research is, the massive inefficiency of animal waste to grow crops compared with chemical fertilizers remains an uncontestable fact and more organic material in soils from conservation cropping practices does not change that fact. Our goal should be to increase the efficiency of chemical fertilization, minimizing pollution without negatively impacting farm productivity.