Soil fertility and the "Green Revolution"

A scene that later Easter Islanders could only dream of!

On Easter Island, one of the key issues (in Diamond's categories: Environmental damage and fragility) was the low replenishment rate of soil nutrients. What can be done to increase nutrition? To increase and improve crop yields? Is soil health more than just nutrition? [YES!]

Plant nutrition

Plants need carbon to build cell walls. They use sunlight to get it from air.

All the other nutrients that plants need come from the soil:
University of Nebraska, Lincoln

  • essential nutrients for plant growth, like nitrogen, phosphate, and potassium.
    Look up the meaning of "NPK" numbers of fertilizers & find pictures of fertilizer labels and interpret.
  • trace minerals like magnesium, zinc, copper,

If the soil doesn't have it, farmers have to add those, if they want healthy crops and high yields.

Organic matter

What distinguishes "topsoil" from other soil?

Healthy topsoil contains a lot of organic matter (humus) from decayed and decaying plant/animal matter. Organic matter is not a plant nutrient! However high organic content provides many benefits to plants:

Sustainable Agriculture Research and Education (SARE) funded by USDA

  • Retains (stores) water / Reduces water runoff:
    • keeps water close to plant roots
    • less soil erosion from water and wind
    • reduced runoff of herbicides and pollutants
  • Supports a healthy ecosystem of worms and micro organisms
    • Worms break up crusted soil
    • some micro organisms break up those pollutants before the make it into the groundwater
    • Diverse micro organism ecosystem protects against diseases and parasites
    • Supports nitrogen-fixing bacteria in soil and on the roots of legumes such as peanuts, beans.
  • Buffers soil in a healthy acid / base range
  • Improved nutrition storage / reduces nutritional runoff.

The organic material is high in carbon--This is unimportant for plant nutrition, but attractive from a climate change perspective, because it represents carbon sequestration: Carbon that is secuestrado ("kidnapped" in Spanish!) from air.

Increased population - greater need for food

Earth's population has been increasing, but the supply of arable farmland is limited and has not been increasing.

Luckily

Crop yields have been increasing...

Mark Biegert plot, using USDA data

What happened after 1940??

The Green Revolution


The Green Revolution refers to a period between 1950 and the late 1960s in which a package of techniques were developed that yielded huge improvements in crop yields. These techniques included:

  • [*] mechanization of farming: Tractors, combines,...
  • [*] agro-chemicals, including pesticides
  • Plant breeders developed high yield varieties (HYV) of staple cereal crops.

    Harold Kauffman, GC '61, was a plant breeder at the International Rice Research Institute in the Phillipines, 1969-1981.

  • irrigation from dams, river water diversion, or [*] pumped groundwater
  • [*] large scale production of chemical fertilizer

* - Some of these require energy / fossil fuel inputs

Green Revolution: Curse or Blessing - IFPRI

  • [IR-8] The rice that changed the world

    • Fertilizers

    One of the main nutrients that plants require is nitrogen. $N_2$ makes up almost 80% of our atmosphere! But most plants are unable to extract it from the air. 😢 😢

    "Fixing" nitrogen in the soil

    The main exception is legumes such as: Beans and soybeans, peas, lentils and peanuts. Also alfalfa and clover. They host (symbiosis) rhizobia bacteria in their root nodules (pictured).

    The legumes trade sugar for ammonia from the rhizobia (diagram).

    By Nefronus - Own work, CC BY-SA 4.0

    Natural fertilizers

    All plants get Nitrogen from the soil not directly from the air. It flows from the roots to all the other parts of plants...

    Crop rotation and crop residues

    • So plant material is one possible source of Nitrogen. Plowing any plants into the soil should help! These can be dead plant remains (See the New Guinea farmers), or compost.
    • Or a farmer can plant a cover crop in the off season (which helps avoid erosion) and then plow that crop under just for the Nitrogen.
    • Nitrogen fixing plants like Casuarina and soybeans actually increase soil nitrogen while they're alive. And then they can be plowed under as well to help once dead.

    Another important natural fertilizer is...

    Manure

    • Manure from all kinds of animals are a good source of N-P-K. Animal manure is a mixture of urine and feces [faeces] (poop!). Here is the nutritional content of human manure:
      Del Porto, et al, by way of Andy Long, Northern Kentucky University
      "ppd" - Per Person per Day

    • Indigenous cultures in the Andes spread bat and seabird guano since about the 4th century on their fields to spur plant growth.
    • Europeans found out about it, and started importing guano from the islands off of Peru and Chile. The U.S. eventually passed the Guano Islands Act of 1856, which allowed any American to claim unoccupied islands with guano deposits on behalf of the United States, to secure a supply of this valuable fertilizer for American farmers.

      Guanay cormorants in Peru (An en Alain/Flickr via Atlas Obscura<

    Chemical fertilizers

    In the early 20th century, Fritz Haber and Carl Bosch developed the first industrial process to generate ammonia, $NH_3$, a nitrogen-containing liquid at room temperature. It depended on these two chemical reactions. This one gives off heat: $$N_2+3 H_2\to 2 NH_3 +\text{heat: 91.8 kJ/mole}$$

    The hydrogen comes from "steam reforming" of natural gas, and it consumes heat input: $$CH_4+2H_2O+\text{heat: 165 kJ/mole }\to CO_2+4H_2.$$

    Multiplying to get the same amount of hydrogen in both equations: $$\begineq &3CH_4+6H_2O+\text{heat: 3*165 kJ }&\to & 3CO_2+12H_2\\ +&4N_2+12 H_2&\to& 8 NH_3 +\text{heat: 4*91.8 kJ}\\ =& 3CH_4+6H_2O+4N_2+127.8\text{ kJ}&\to&3CO_2+8 NH_3 \\ \endeq$$ The heat required is 127.8 kJ / 8 moles of $NH_3$ or 16 kJ / mole of $NH_3$ (Ammonia).

    So far, this energy has come largely from burning fossil fuels! Ammonia production currently involves

    1% to 2% of global energy consumption, 3% of global carbon emissions, and 3 to 5% of natural gas consumption..

    Unfortunately...

    • High yielding varieties displace / reduce crop species variety.
    • Overuse of chemical fertilizers poisons soil micro organisms and results in nitrous oxide - a powerful GHG.
    • Repetitive plowing and displacement of organic fertilizers with chemical fertilizers $\to$ loss of humus $\to$ soil erosion.
      Farms across the Midwest may have lost 1/3 of their topsoil (NPR)

    See drawdown solutions related to:

    Study questions

    What are the top 3 nutrients that growing plants require?

    Where do most plants get these nutrients from? - Soil? Atmosphere? Groundwater?

    What are some examples of food crops which can get Nitrogen directly from the atmosphere (with the help of bacteria!)?

    What were the innovations developed during the Green Revolution that enabled farmers to grow more food on less land?

    How do different crops compare? - Calories / acre for different food crops.

    Image credits

    ProgressiveForage.com, Iowa State University