Title: Lecture 16 Soil Water and Nutrients Speaker: Teresa Koenig Created by: Teresa Koenig, Kim Kidwell. online.wsu.edu

Transcription

1 Title: Lecture 16 Soil Water and Nutrients Speaker: Teresa Koenig Created by: Teresa Koenig, Kim Kidwell online.wsu.edu

2 Photos courtesy of Rich Koenig Photos courtesy of USDA NRCS Soil Water and Nutrients

3 Soil texture has two effects on soil moisture availability: 1. Size of soil pores: water is lost from the largest pores (sand) first. 2. The total amount of surface area per unit mass associated with the particles (clay has more surface area per gram).

4 Surface forces that account for the retention of water by soil particles: 1. Adhesion: the attraction of water molecules to soil particle surfaces. 2. Cohesion: the attraction of water molecules for one another.

5 How does surface tension affect water movement in soils: Example: clay vs. sand?

6 The size of the particles (texture) determines the amount of water a soil can hold. Smaller particles hold water more tightly due to smaller pore size and greater surface area.

7 Example: Water bonds stronger to clay particles than to sand particles due to the smaller pores and higher surface area in clay. Water leaches farther in: sandy loams > loams > clay loams

8 Forms of soil water 1. Gravitational (free) water: Remains in the soil only temporarily and moves down through the soil due to the force of gravity. Runs through the soil so quickly that plants do not have an opportunity to absorb it.

9 2. Available (capillary) water: Water held in small pores (spaces) between soil particles and present as films (layers) around the soil particles. Can be absorbed by plant roots.

10 3. Hygroscopic (bound, unavailable) water: Is held so tightly by the soil particles, plant roots cannot extract it.

11 The strength with which water adheres to soil particles depends on the soil texture and structure.

12 1. The more sand present in the soil, the more easily water can pass through it. 2. The higher the clay percentage, the smaller the particles and pore spaces.

13 Water is held more tightly to clay than sand so less water runs through a clay type soil than a sandy soil.

14 We are most concerned about water that is available to the plant: 1. Field capacity (FC): all the gravitational water has moved through the soil and the pore space is filled with only the water the soil can hold against the force of gravity.

15 Field capacity represents water present in the soil after gravitational water is removed

16 2. Permanent Wilting point (PWP): Plant can no longer extract water from the soil and it begins to wilt.

17 Clay has a higher PWP because it has more hygroscopic water than the other soil types. Smaller particles hold water more tightly than larger particles, which makes it more difficult for the plant roots to absorb it.

18 3. Available Water (AW) = Field Capacity - Permanent Wilting Point

19 4. Saturation capacity: All pores are completely filled with water. The saturation capacity of sand is much lower than clay. i.e. clay soils have much more water in them than sandy soils.

20 Plants don t grow well in clay soils because the high water level decreases the amount of oxygen available. Plants (roots) need oxygen to grow

21 Salinity (Soluble Salts) A bulk measure of soluble inorganic elements such as sodium, calcium, magnesium, chloride, sulfate and bicarbonate. Photo courtesy of USDA NRCS Salty soils are a problem in arid regions or in poorly drained soils

22 Salts originate from: Mineral weathering Inorganic fertilizers and manures Irrigation waters Other sources

23 Salts cause chemical drought which is similar to water stress Or, may be caused by specific ion toxicities (sodium, chloride) Photo courtesy of Rich Koenig

24 Salt problems can be solved by: - Selecting salt tolerant vegetation - Leaching salts out of soil with clean water

25 Plants vary in salt tolerance mg/liter sodium chloride Berries Beans-- Carrots Alfalfa--- Corn---- Beets Bluegrass-- Tall fescue Cottonwood---- Soil salinity (millimhos/cm) Barley Salt tolerant plants have developed mechanisms to tolerate or exclude salts Illustration courtesy of Rich Koenig

26 Soil fertility a. Organic matter (OM): Plants obtain some of their mineral nutrient requirements and much of their nitrogen from the decomposition of organic materials such as roots, stems, leaves, and animal manure.

27 Decaying OM also supports bacteria and fungi which aid in: a. bringing insoluble soil minerals into solution b. improving the physical condition of the soil

28 b. Mineral Nutrition: Essential elements: Plants require these to grow and develop properly. Photo courtesy of Rich Koenig

29 Types of Nutrients: 1. Macronutrients: Required in largest quantities Photo courtesy of Rich Koenig If adequate amounts are not present, the plant will show dramatic deficiency symptoms. Ex. leaf chlorosis or necrosis, yellowing around leaf edges or between veins

30 a. Primary Macronutrients: N, P and K 1. Referred to as primary because they are most frequently found to be deficient 2. Primary components of most fertilizers

31 b. Secondary Macronutrients: Ca, Mg and S. May also be required in quantities nearly as great as N, P, and K; Soils are not found to be deficient in these elements as often as they are for the primary nutrients.

32 2. Micronutrients: Photo courtesy of Rich Koenig Required in small quantities If adequate amounts are not present, the plant will show deficiency symptoms. Ex. leaf chlorosis or necrosis, yellowing around leaf edges, or between veins

33 a. List of micronutrients: Molybdenum (Mo) Boron (B) Copper (Cu) Iron (Fe) Manganese (Mn) Zinc (Zn) Chlorine (Cl)

34 Sources of Nutrients Inorganic fertilizers Manures, composts, and other organic materials/fertilizers Green manures (legumes and others) Peas, vetch, rye, oats, wheat, barley

35 Types of Fertilizers Chemical fertilizers Organic fertilizers (bone meal, compost, manure, etc.) Green manures

36 Fertilizer label and grade 3 numbers always appear on the label to describe the fertilizer content The numbers refer to: % Nitrogen (N) % Phosphorus (P 2 O 5 ) % Potassium (K 2 O)

37 How Much Fertilizer do I Need to Apply? Estimate the amount of fertilizer needed based on soil test results, crop needs, and area to receive fertilizer. Most fertilizer recommendations are in pounds per 1000 square feet, or pounds per acre.

38 Organic Nutrient Sources Much lower concentrations of nutrients Example: for composts Good sources of organic matter Much higher rates of application needed than inorganic fertilizers (10 to 50 times higher) Slow release materials (and unpredictable) May need to supplement with inorganic nitrogen fertilizer