5.15 - Sustainable Agriculture

Learning Objectives

I. Soil Conservation (Erosion Prevention)

• Contour plowing: Plowing across a slope, following its contour lines (lines of equal elevation), instead of straight up and down the hill. Each furrow forms a mini terrace that catches water runoff and stops it from carrying away precious topsoil.

• Terracing: Cutting flat platforms into a steep hillside. These platforms catch water that would otherwise carry topsoil off the slope during heavy rain, conserving both water and topsoil. It also makes agriculture possible in very steep areas.

• Perennial crops: Crops that live year-round. Leaving the plant in place means its root structure keeps anchoring the topsoil and preserving it from erosion. A complex root structure also helps retain soil moisture and prevents runoff.

• Wind breaks: Trees or other plants planted around fields to block the force of the wind so it cannot carry topsoil away. A nice side effect is that they can generate income (firewood or fruit) and provide valuable habitat for pollinators.
• Real example: After the Dust Bowl, the US planted over 200 million trees in windbreaks across the Great Plains between 1934 and 1942.

• No-till agriculture: Not tilling or mixing the fields at the end of the season. Leaving crop residue on the field adds organic matter, acts as a moisturizing layer that traps moisture, and anchors the soil in place. Tilling loosens soil, which dries it out and makes it erode more easily.

• Strip cropping: Planting alternating strips of different crops. Dense crops like wheat or hay get planted between less dense crops like corn. The dense strips catch soil and water that would otherwise erode out of the loosely planted rows, which limits topsoil erosion. Strips are often laid out along the contour.

II. Improving Soil Fertility

• Crop rotation (fertility focus): Replanting the same crop depletes the same nutrients over and over. Rotating crops gives the soil time to recover.
• Nitrogen-hungry crops like corn can be alternated with legumes (peas and beans). Legumes host nitrogen-fixing bacteria in their roots, which return atmospheric nitrogen to the soil. This keeps nitrogen levels stable and reduces the need for synthetic fertilizer.

• Green manure (cover crops): Instead of leaving fields bare after harvest, a quick-growing cover crop like oats or clover gets planted in the offseason.
• The cover crop’s roots stabilize the soil and limit erosion.
• The crop is cut down and the remains are left on the field. This green manure breaks down and adds nutrients to the soil, and it also acts as a moisturizing layer that prevents evaporation loss.

• Crushed limestone addition: Limestone is mostly calcium carbonate ($CaCO_3$). Calcium carbonate is a base, so adding crushed limestone neutralizes acidic soil.
• Acidic soil (high $H^+$ ions) leaches nutrients by displacing positively charged ions. Neutralizing the soil (moving pH closer to 7) increases nutrient retention.
• Acidic soil also makes toxic metals like aluminum more soluble, and aluminum can damage and kill plant roots. Adding limestone keeps aluminum from becoming soluble.
• The calcium released also contributes to soil fertility.

III. Rotational Grazing

• Overgrazing consequences: Overgrazing clips grass down too low to survive, which kills the grass, and it compacts the soil, which leads to erosion.

• Benefits of rotation: Rotation actually promotes more growth than leaving the pasture alone. By continually clipping the grass back to a medium height, the animals keep the grass in its optimal, rapid growth stage. That sustained rapid growth leads to longer, healthier root structures over time.