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LSAT Question for May 21st

As part of an international effort to address environmental problems resulting from agricultural overproduction, hundreds of thousands of acres of surplus farmland throughout Europe will be taken out of production in coming years. Restoring a natural balance of flora to this land will be difficult, however, because the nutrients in soil that has been in constant agricultural use are depleted. Moreover, much of this land has been heavily fertilized, and when such land is left unplanted, problem weeds like thistles often proliferate, preventing many native plants from establishing themselves. While the quickest way to restore heavily fertilized land is to remove and replace the topsoil, this is impractical on a large scale such as that of the European effort. And while it is generally believed that damaged ecological systems will restore themselves very gradually over time, a study underway in the Netherlands is investigating the possibility of artificially accelerating the processes through which nature slowly reestablishes plant diversity on previously farmed land.

In the study, a former cornfield was raked to get rid of cornstalks and weeds, then divided into 20 plots of roughly equal size. Control plots were replanted with corn or sown with nothing at all. The remaining plots were divided into two groups: plots in one group were sown with a mixture of native grasses and herbs; those in the other group received the same mixture of grasses and herbs together with clover and toadflax. After three years, thistles have been forced out of the plots where the broadest variety of species was sown and have also disappeared from mats of grass in the plots sown with fewer seed varieties. On the control plots that were left untouched, thistles have become dominant.

On some of the plots sown with seeds of native plant species, soil from nearby land that had been taken out of production 20 years earlier was scattered to see what effect introducing nematodes, fungi, and other beneficial microorganisms associated with later stages of natural soil development might have on the process of native plant repopulation. The seeds sown on these enriched plots have fared better than seeds sown on the unenriched plots, but still not as well as those growing naturally on the nearby land. Researchers have concluded that this is because fields farmed for many years are overrun with aggressive disease organisms, while, for example, beneficial mycorrhiza—fungi that live symbiotically on plant roots and strengthen them against the effects of disease organisms—are lacking. These preliminary results suggest that restoring natural plant diversity to overfarmed land hinges on restoring a natural balance of microorganisms in the soil. In other words, diversity underground fosters diversity aboveground. Researchers now believe that both kinds of diversity can be restored more quickly to damaged land if beneficial microorganisms are "sown" systematically into the soil along with a wide variety of native plant seeds.

The passage offers which one of the following as an explanation for why native plant varieties grew better when sown on land that had been out of production for 20 years than when sown on the plots enriched with soil taken from that land?

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