1. Trends in World Food Supply
Technology has decreased acreage needed per person. With modern agriculture, about 0.1 ha/person.
Number of people experiencing famine has decreased since 1950.
Rapid increases in developing world population mean that greater per-acre yields will be necessary.

2. Trends in U.S. Agriculture
Cropland area has remained stable.
Fewer farms and farmers, larger farms.
Yield increases 1950 – 1992:
Corn 340%
Wheat 290%
Soybeans 170%
Alfalfa 170%
Why?
Improved technology (genetics, fertilizers, pest control, irrigation)

3. Role of Nutrients/Fertilizers
Since 1960, fertilizer use in the U.S. has increased:
N fertilizers by 400%
P fertilizers by 50%
K fertilizers by 150%
Estimates are that about 40% of yield increases since 1930 are due to increased fertilizer use.

5. Average Wheat and Rice yields in China, 221 BC - 1999
Fertilizer use in China,
Beginning of the 80’s marks a large growth spurt in for cereal yields….coincides with large increase in N (beginning in the 1950s), then P (in the late 1950s and early 1960s), and lastly K (largely popularized in the mid to late 1980s) fertilizer.
PPIC, FAO 2002

6. NPK consumption, kg/ha
IFA, 2001

7. Key Soil Fertility Concepts
SWES 316
Section B

8. Outline for First Section Today
Defining soil fertility
What makes soils fertile?
Define growth curves
Define response curves
Evaluating outcomes of nutrient management
The “law of the minimum”

9. Definition of a Soil
A natural occurring body, covering the earth in a thin layer, synthesized in profile form, from weathering and weathered rocks and minerals, decomposed and decomposing organic matter, and when properly supplied with water and nutrients provides sustenance for plant growth

10. Soil Fertility Defined
Brady: The quality of a soil that enables it to provide essential chemical elements in quantities and proportions for the growth of plants.
Anonymous: … an aspect of science concerned with the sources and availability of essential elements for plant growth with respect both to quantity and quality of the crop produced.

11. What makes soils fertile?
Lack of weathering
Parent material (mineralogy)
High CEC
Organic matter
Adequate moisture
Neutral pH
Lack of excess salts

12. Soil Fertility
Can also be called: Nutrient Management

13. Role of the Soil in Plant Growth:
Anchorage
Storage/Supply of Water
Storage/Supply of O2
Storage/Supply of Nutrients

15. Crop Growth Curve
The generalized crop growth curve shown applies to annual crops.
Nutrient uptake follows the same general pattern, but is slightly earlier in time than growth.
Growth and nutrient uptake in perennial crops is more complicated.
How can understanding
a crop growth curve
be useful?

17. BROCCOLI

18. Response Curves
Represents plant response to additions of any rate-limiting input.
Important points:
Decreasing slope
Diminishing returns
MC = MR
Upper limit =
Other factors can intervene
How can understanding
a crop response curve
be useful?
Genetic potential

19. Model Response Curve Crop Response Maximum Yield Amount of Input
Profit
Crop Response
Maximum
Yield
Amount of Input

20. Important Points on the Yield (Response) Curve
Maximum Yield
Determined by either genetic potential of the crop or the presence of a limiting factor
Maximum Profit
Point where marginal cost = marginal return (MC=MR)
Depends on two things:
Cost of the input
Value of the crop

21. B: The input was needed
A: The input was not needed
C: Another factor limited
growth

22. Nutrient Management
Soil Fertility is often concerned with the problem of managing nutrients to be applied to plants.
Three general considerations:
Agronomic (how much is needed)
Economic (how can profit be maximized)
Environmental (minimize pollution)

23. Response Factors Crop Yield Crop Quality
The most important consideration in nutrient management for most crops
Crop Quality
Maximum yield and optimum quality are not always compatible. Example:
Malting barley yield vs. protein
Fruit Quality

24. Theory vs. Reality
In theory, any time we are practicing nutrient management we should consider agronomic, economic, and environmental ramifications.
However, in reality some of these factors are often ignored:
Economics of adding nutrients may be unimportant in some high-value crops.
Environmental outcome is often ignored—difficult to see or measure, who’s checking anyway?

25. “Law of the Minimum”
Crop yield is limited by the most limiting factor. When that factor is removed, yield is limited by the 2nd most limiting factor, and so on.
Barrel metaphor

26. The Barrel

27. Liebig’s Law of the Minimum Justus von Liebig – 1803-1873
He stated that the nutrient present in least relative amount is the limiting nutrient.
soil contained enough N to produce 50 bu/ac
soil contained enough K to produce 70 bu/ac
soil contained enough P to produce 60 bu/ac
N would be the limiting nutrient
The crop uses up all of the deficient nutrient in the soil making the yield directly proportional to the amount of the deficient nutrient.
More description of Liebig’s Law of the Minimum.
Review these points.