<P> Many crops which are critical for the market, like vegetables, are row crops (that is, grown in tight rows). While often the most profitable for farmers, these crops are more taxing on the soil Row crops typically have low biomass and shallow roots: this means the plant contributes low residue to the surrounding soil and has limited effects on structure . With much of the soil around the plant exposed to disruption by rainfall and traffic, fields with row crops experience faster break down of organic matter by microbes, leaving fewer nutrients for future plants . </P> <P> In short, while these crops may be profitable for the farm, they are nutrient depleting . Crop rotation practices exist to strike a balance between short - term profitability and long - term productivity . </P> <P> A great advantage of crop rotation comes from the interrelationship of nitrogen fixing - crops with nitrogen demanding crops . Legumes, like alfalfa and clover, collect available nitrogen from the soil in nodules on their root structure . When the plant is harvested, the biomass of uncollected roots breaks down, making the stored nitrogen available to future crops . Legumes are also a valued green manure: a crop that collects nutrients and fixes them at soil depths accessible to future crops . </P> <P> In addition, legumes have heavy tap roots that burrow deep into the ground, lifting soil for better tilth and absorption of water . </P>

What crops were discovered that could replenish the nutrients of the soil