The two principal nutrients - nitrogen and phosphorus - are a mixed blessing. Plants cannot live without them and without plants, where would animals be?  Nutrients in excess, though, represent a pendulum swung far out to the extreme, and their broad-ranging impacts can overwhelm life and stability in natural systems.

Nutrients appear to have been much less abundant in the 16th and early 17th century Chesapeake that the Native Americans knew. It was also a very different Bay back then. Conditions in that Bay are illustrated rather well when today's scientists examine cores that penetrate deep into the Bay floor - into the Bay's ancient past.

Scientists read this history partly by the "frustules" of microscopic plankton cells called diatoms. Frustules are delicately ornamented silica shells that house the living cell material of these minute plants, and they're absolutely unique for every one in thousands of species.

Cores taken around the Bay by researcher Sherri Rumer-Cooper, formerly at  Johns Hopkins University, document that many of these pre-colonial cells weren't plankton at all. They grew on the Bay bottom or on the blades of submerged Bay grasses across hundreds of thousands of acres, where they were literally suspended in sunlight and the tides bathed them continually with the Bay water's dilute nutrient solution. These diatoms thrived under conditions with very low nutrients.

Surrounding the Bay and her rivers, our vast basin was largely covered by millions of acres of forest, into which pre-colonial native Americans had made only modest inroads. Along the tributary rivers, they conducted slash and burn agriculture to plant their annual crops. Corn was set in May, and beans interplanted, so they could use the cornstalks as trellises. The corn benefited from nitrogen fertilizer, and bacteria living with the beans naturally "fix" nitrogen from the atmosphere, helping to make the crop sustainable. Squash and gourds were planted between these, and in addition to being another food resource, they helped to serve as ground cover, reducing soil moisture loss and suppressing weed growth.

The forest might not have been as continuous as we once thought because Native Americans also burned the forest to drive game. Along the Bay's shorelines, these fires mostly cleared understory and shrubs, making an open, parklike stand of huge and largely uninjured trees. But, farther back in the basin, repeated burnings seem to have perpetuated some pretty extensive prairie or grassland areas in the Shenandoah, and around Wyoming Valley on the Susquehanna. There may also have been only scrub forestacross areas in Western Maryland, or on the poor, barren mineral soils like Soldier's Delight above Baltimore.

But there was mostly forest. Thousands and thousands of square miles of it, and mostly old growth, and like ancient forests today, it was very stingy about letting go of nutrients, leaking very little nitrogen and phosphorus into streams and rivers. Thomas Heriot, in the late 1500s, described "Virginias Chrystall Rivers" and they must indeed have been burdened with very little sediment. More than 60 years after the first colonists came to Maryland, Anglican Cleric Hugh Jones wrote: "All the low land is verry woody like one continued Forest, no part clear but what is cleared by the English."

But this was not to last. Hugh Jones continues:  "And tho we are pretty closely seated (ie: living near each other) we cannot see our neighbours house for trees. Indeed in a few years we may expect it otherwise for the tobacco trade destroyes abundance of timber, both for makeing of hogsheads & building of tobacco houses, besides cleareing of ground yearly for planting."

The forest was inexorably removed, parcel by parcel, for agriculture. Tobacco culture followed a cycle, with each man clearing relatively few acres by girdling and killing trees with his axe and subsequently tending the soil with a hoe. Virtually no fertilizers or pesticides were used, the tobacco worms being picked off each plant by hand. Within a few years, tobacco yields began to fall. Native American-style crops were planted at the site for a few more years until their yields also fell. The field was then abandoned and left fallow.

While a new parcel was attacked, the abandoned "old field" went through a succession of plant species, becoming a young forest within 20 years. The process was repeated again and again, literally for generations. A man or group of men swinging a hoe in the hot Chesapeake summer sun was a relatively gentle process that allowed only modest amounts of nutrients to enter adjacent streams. Some biologists think that the small leakages of nutrients that came from the repeated low grade disruption of the forest helped to "feed" a Bay that was relatively nutrient poor and might have actually made fish and shellfish more abundant.

Then things seemed to change within this possibly "sustainable" economy. At the end of the 17th and beginning of the 18th centuries, there was a series of collapses in the tobacco market, which had funded lifestyles in the Virginia and Maryland colonies. The economic dislocation and loss of purchasing power forced more cropland to be devoted to grain crops for food, a vastly different kind of farming. Draft animals, often powerful oxen, were needed to grow these crops, which were seeded into soils with straight plowed furrows and deep tillage.

The land could not be abandoned so quickly now because people were streaming in from England's countryside and fertile, unoccupied land along the Bay's rivers was beginning to run out. Colonists began to move upstream and inland from the coastal plain.

On the piedmont, where tidewater ends and coastal streams tumble down from hill country, the soils are thinner and more easily eroded when natural vegetation is disrupted. By about 1760, as farming techniques developed on the flat coastal plain spread to the steep erodible piedmont terrain, hundreds of thousands of tons of sediment were pouring into the Bay's rivers. Communities downstream which had been tobacco shipping ports for many decades - Port Tobacco, Upper Marlboro, JoppaTowne - found their harbors silted in and useless.

At the end of the 18th century, Thomas Jefferson served in France as an envoy of the United States. His personal journal during that time contains a sketch where he proposes a new form of tillage device. He built this "moldboard plow" upon his return to Virginia and found it was easily  pulled by fewer and smaller draft animals. It didn't just break the soil, but turned the sod, helping to kill weeds. Within a few years though, having shared the design with neighbors near Monticello, he wrote:

"Our soil was rapidly running into the rivers. We now plow horizontally following the curvatures of the hills and hollows."  ...  "If your farm is hilly," he exhorted painter Charles Wilson Peale, "let me beseech you to make a trial of this method."

This plow design never earned Tom a nickel and he died in debt, but its use spread rapidly after the first decade of the 19th century. Ignoring Jefferson's warnings, and his example, unwise use of the moldboard plow probably made it the greatest single instrument of soil erosion before the bulldozer.

The massive loss of farm soils was to continue for about two centuries. The frequency and intensity of floods increased as more and more moisture-conserving forest was logged off for timber and charcoal, and cleared for agriculture.

Tremendous quantities of phosphorus came into the Bay along with the incredible sediment load, and it is amazing how the Bay's living resources, from underwater grasses, to its many abundant fisheries, managed to prosper despite this onslaught. The first clue as to why, is that a very large proportion of phosphorus remained bound to the sediment particles on which it was borne. The Bay's waters, even in summer, seem to have had abundant oxygen, and it is low oxygen or anoxia (lack of oxygen) that triggers the release of phosphorus from sediments.

The seeds of destruction came with the first importation of fertilizer. This began in 1824, according to University of Maryland scientist/historian Court Stevenson. Within the next decade, certainly by the late 1830s, Baltimore and other ports were regularly transporting guano from the vast deposits of bird droppings on some islands of the Caribbean and later, off the west coast of South America. This trade continued for about 100 years, and nutrient-rich Chilean nitrate from guano was one of the last cargoes upon which square-rigged sailing ships were able to remain profitable in the age of steam.

Farmers had routinely used the wastes of their draft animals to manure fields (Even John Smith talked about its value!) but the quantities were small. The stimulation of crop yields from guano was quickly recognized and it became an increasingly important farming tool. The proper ratio between nitrogen and phosphorus additions, and the increasing yield per unit fertilizer "side-dressing" added were worked out early in the 20th century by England's Sir Ronald Fisher at the Rothamstead Agricultural Experiment Station. His analytical methods literally formed the basis for modern statistics and farmers worldwide sat up and took notice of the compelling evidence he provided.

The ability to produce fertilizers cheaply came with industrialization. Chemists had quickly learned how to synthesize nitrogen compounds that plants absolutely adored, but it was the liberation of the chemical industry's vast manufacturing capacity after the World War II that turned loose agriculture - and marketing.

Nitrogen loading is arguably the single greatest pollutant challenge we face in restoring the Chesapeake Bay because it echoes throughout the ecosystem, from plankton bloom chlorophyll through submerged Bay grasses, to dissolved oxygen loss in deep water and problems with fisheries. 

Fertilizers and manures from farming are only one group of sources for nutrients reaching the Bay. As the population grew, so did municipal and industrial waste discharges to the water and air. And, as the spread of "modern" sanitation outside municipal service areas encouraged septic tanks over outhouses, another highly significant source of nitrogen introduction to groundwater followed.

Nitrogen, once it is flowing deeply entrenched in our groundwater, moves slowly. Rainwater may percolate into the soil and recharge our aquifers, but it slowly carries nitrogen down and then horizontally, inexorably heading toward our streams and the Bay. In some areas of the coastal plain, nitrogen fertilizer applied by farmers in the 1940s is still on its way to the estuary - a time bomb for half a century.

That is how we got to where we are today, and why some of the problems started with our first colonists will still be with us for a long time to come.