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.

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)

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.

Average Wheat and Rice yields in China, 221 BC - 1999 Fertilizer use in China, 1950-1999 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

NPK consumption, kg/ha IFA, 2001

Key Soil Fertility Concepts SWES 316 Section B

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”

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

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.

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

Soil Fertility Can also be called: Nutrient Management

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

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?

BROCCOLI

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

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

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

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

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)

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

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?

“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

The Barrel

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.