1. CROP PRODUCTION Unit 6: Soil Water

2. Water and Life (133) 70% of earth’s surface: 97% salt water, 3% fresh water Of the 3% freshwater: 2.5% in glaciers, ice caps, and atmosphere; 0.5% available in rivers, lakes and aquifers Of the 0.5% available: 0.497% is polluted or too difficult to extract 0.003% of the earth’s water is fresh, available, and usable

3. Water in the Soil (133) Primary source of water for crop growth Carries nutrients Influences soil aeration and temperature In excess, it can lead to erosion Influenced by soil texture and structure

4. Influence of Soil Structure (133) Granulated = more macropores = proper aeration Compacted = more micropores = limit drainage, reduce aeration, and less water for plants

5. Influence of Soil Texture (134) (Complete the table using p. 134) Pore size and total soil particle area most important Macropores – aka non-capillary pores Surface forces –Adhesion – attraction of water molecules to soil particles –Cohesion – attraction of water molecules to other water molecules

6. Forms of soil water (135) Gravitational water – in excess of retentive power of soil particles Capillary water – held by surface forces of soil particles and water molecules (adhesion & cohesion) Hygroscopic water – held as thin films on particle surfaces

7. Soil Moisture and Plants (136) Saturated soil – all pores are filled by water Field capacity – moisture content after thorough wetter and draining of gravitational water Wilting range – range from wilting of first leaves to wilting of the entire plant

8. More Soil Moisture and Plants Terms (136) Moisture stress – total moisture stress on plants (soil moisture suction & osmotic potential) Moisture tension – energy w/which the soil holds moisture Permanent wilting point – moisture content at which plants can no longer absorb water Plant available water – soil water that can be used by plants in growth

9. Energy of soil moisture and its units (136) –Suction – bar (10 6 dynes/cm 2 ) – most commonly used –Tension – atmosphere (1 atm will support a 1033 cm-column of water) – less common –Field capacity usually 1/3 bar –Wilting point usually 15 bars

10. Availability of soil moisture to plants (137) Plant transpires and loses water Water suction in plant cells Cell to cell, all the way to the roots Roots collect water when suction in plant is greater than suction of the soil

11. Availability of soil moisture to plants (137) Soil factors: –Soil texture & structure –Soil moisture suction (tensiometer) Plant factors: –Leaf and stomatal activity – water loss and uptake –Rooting characteristics – system’s ability to collect water Atmospheric factors: –Humidity –Wind, air/soil temps

12. Soil moisture retention and soil texture (138) Soil moisture is from rain or stored moisture Roots absorb water from top 1.5 meters of soil profile Subsoil moisture affects planting rates and fertilizer rates as well as markets (futures) Plant available water in sandy loam (10% of dry weight of soil) Plant available water in clay loam (30% of dry weight) Large surface area of clay particles increases adhesive suction forces on water

13. Moisture & Crop Management (138) Crop selection –Drought avoiding crops – mature before severe drought occurs –Drought resistant crops – able to extract more water than other crops Plant populations and evapotranspiration – water use efficiency is greater with narrow row spacing and high populations

14. Moisture & Crop Management (139) Other management factors – decrease evaporation, runoff, and increase infiltration by: –Mulch –Conservation tillage –Weed control –Contour farming/terracing –**Over 90% of Iowa’s cropland is farmed using some type of conservation practices

15. Irrigation (140) General Considerations: –Artificial application of water to soil to enhance crop growth –“Dryland farming” – crop production in arid or semi-arid regions –Irrigation is used in arid regions and also soils with low WHC when evapotranspiration exceeds precipitation –Most of the irrigated land in the U.S. is west of the Mississippi River = 45 million acres

16. Advantages of Irrigation (140) Improve crop yields and quality Control timing, location, and quantity of water application Water application rates can be adjusted according to runoff, erosion, infiltration and drainage characteristics of different soils Improve food production potential of arid/semi-arid regions

17. Disadvantages of Irrigation (140) Expense; economic risk Increase erosion and sediment deposition with some methods In arid lands, salt accumulation can cause problems, including salinization; soil becomes too salty for plants to survive Worldwide, nearly 75% of freshwater withdrawn is used for irrigation; lowers water table; can lead to groundwater contamination and saltwater intrusion

18. Methods of Irrigation (141) Complete notes using p. 141 in textbook

19. Salt Accumulation (142) Problem in arid and semi-arid regions – not enough precipitation to wash salts through soil Compounds formed from acid/base reactions Soluble salts brought to surface by evapotranspiration loss Salts increase osmotic concentration and decrease/reverse normal water flow into root Increase the amount of water held in the soil at wilting point Get rid of salts by flood and drain, neutralizing soil, more careful use of irrigation and selecting salt-tolerant crops

20. Is it salt-affected? (143) White alkali –High level of salt, pH < 8.5 = saline soil –High level of salt with high sodium = saline-sodic soil Black alkali –Low salt, high sodium, pH > 8.5 = sodic soil

21. Soil Drainage (143) Removal of excess water By draining water, favorable conditions are produced: –Increased granulation –Deeper rooting zone –Warmer soil –Better seedbed preparation –Better weed control –Improved aeration – promotes O 2 diffusion, removal of CO 2, aerobic microorganism activity and availability of soil nutrients – greatest benefit –Reduced iron and manganese toxicity –Reduced heave and thaw damage

22. Soil Drainage (144) However, as wetlands have been drained and farmed, historically, many valuable services are lost – Wetlands provide: –Increased filtration of runoff water –Filter environmental contaminants –Wildlife habitats, esp. fowl –Blueberry, cranberry, and rice production

23. Utilization & Conservation (144) Management goals should focus on conserving soil moisture, minimizing runoff/erosion loss and supplying crop water needs Management programs vary in different climates; long-term productivity depends on water management Balance conservation and water use

24. Revised Universal Soil Loss Equation (RUSLE) A = R. K. L. S. C. P A = predicted soil loss (Ton/ac) R = rainfall factor – erosive potential K = erodibility potential of soil series L = length of slope (run) (combined with S) S = steepness of slope (rise) (combined w/ L) C = crop rotation P = conservation practices applied to soil

25. Website resources: –http://www.ext.colostate.edu/pubs/garden/07754.ht mlhttp://www.ext.colostate.edu/pubs/garden/07754.ht ml –http://geog.ucsb.edu/classes/Geog3/lecture6.htmhttp://geog.ucsb.edu/classes/Geog3/lecture6.htm –http://ga.water.usgs.gov/edu/watercycle.htmlhttp://ga.water.usgs.gov/edu/watercycle.html –http://jdappert.home.mchsi.com/http://jdappert.home.mchsi.com/ –http://www.epa.gov/owow/wetlands/http://www.epa.gov/owow/wetlands/ Chapter 6 –Review Questions (p. 146-147) –Thinker