Assignment 3 (5 points) due by the start of class April 23.

A question was asked that if we greatly increase crop yields by genetic enhancement/breeding and/or management practices won’t we take too much from the ground and deplete mother earth? Please provide an answer to this question including whether you think it will deplete the ground or not and why you think as you state. For an example let’s say we increase the harvested dry matter yield of a biomass crop from 20,000 kg/ha to 50,000 kg/ha and this harvested material is converted into energy.

Let’s look at this quantitatively considering the elemental composition of plants and how the major components are made. From the typical composition of plants such as corn one can calculate how much is removed considering the relative mass of the elements:

Element	Zea mays	%	relative	mass	kg/50,000kg
	(corn)	Elements	mass	%
Hydrogen	1,705.0	48.5	4.0	6.3	3,163.0
Carbon	1,000.0	28.5	28.5	44.5	22,261.6
Oxygen	765.0	21.8	29.0	45.4	22,706.8
Nitrogen	29.0	0.83	1.0	1.5	753.2
Potassium	6.5	0.18	0.6	0.9	470.3
Calcium	1.6	0.05	0.2	0.2	118.7
Phosphorus	1.8	0.05	0.1	0.2	103.5
Magnesium	2.0	0.06	0.1	0.2	90.2
Sulfur	1.5	0.04	0.1	0.2	89.0
Chlorine	1.1	0.03	0.1	0.1	72.4
Iron	0.4	0.01	0.2	0.3	142.6
other	1.0	0.03	0.1	0.2	92.8
total	3,514.9	100.0	63.9	99.9	49,971.4

You can see how these were calculated from the linked Excel file. As you can see 96.3% of the total mass harvested is C+H+O and 97.8% C+H+O+N. Thus only ~ 2% of the mass harvested is other than C+H+O+N. Most of the mass is carbon and oxygen due to the small relative mass of hydrogen.

Where does the carbon and oxygen in plant tissue come from? From atmospheric CO₂!

Can this deplete earth’s atmosphere appreciably?

What are the ecological consequences of lowering atmospheric CO₂ levels?

The hydrogen comes from water and we all know that it takes significant amounts of water for plant growth with large amounts of water being bound up in the hydrocarbon of plants. However vastly more CO₂ is needed than water as was illustrated in Assignment 1.

Where does the nitrogen in plant tissue come from? From atmospheric N₂.

The remaining of the material harvested, mainly potassium, calcium, phosphorus, magnesium, sulfur, chlorine and iron is ~ 11 hundred kg or ~ 2% of the 50,000 kg harvested. These ultimately come from the soil and rock (or earth we stand on). Most soils are deficient in potassium and phosphorus (P & K) in the root zone and these need to be added to soils in one form or another for optimal plant growth.

All of the nitrogen, whether natural or synthetic, used for plant growth is “fixed” from atmospheric N₂. When organisms decay the nitrogen is returned to the atmosphere in the form of N₂. Similarly the hydrocarbon is catabolized back to CO₂ and H₂O with the remaining minerals such as P, K, Ca, etc. returning to the earth when plants or animals decay. Thus the elemental composition of our planet doesn’t change and even the forms of the elements in the end are not appreciably changed by enhanced plant growth. However animals and particularly humans can redistribute minerals. Dramatic examples of this are when birds and bats roost in very large colonies gathering P, K, Ca, etc. from large areas and depositing them in small areas. Similarly such minerals are redistributed from farmland to cities and it is ideal for the minerals derived from sewage treatment to be redistributed on farmland. When plant biomass is used for energy or transportation fuels such mineral concentration also occurs and should be returned to farmland. This redistribution occurs regardless of whether low yields are harvested from large areas or high yields from small areas.

The question as to whether the soil or land would significantly subside with such redistribution of minerals can also be simply addressed. The main elements in soil and rock on earth are oxygen, silicon and aluminum. Oxygen indeed is the most abundant component of plants by mass but as we know it is derived from atmospheric CO₂ not from the land. Although next in abundance of the chemical composition of “land”, silicon and aluminum are not appreciably taken up by plants. Thus there is no appreciable impact on land mass with increased crop yield.