EXERCISE 19 



THE SOIL MOISTURE WHICH PLANTS CAN USE 



Statement. The amount of soil water which enables the plant to grow most rapidly is called the 

 optimum moisture content. The least amount of moisture in the soil which will enable the plant to 



live in that soil is called 

 the critical moisture con- 

 tent. Plants die from lack 

 of moisture before it is all 

 removed from the soil. 



Object. To determine 

 whether or not all soils 

 give their water to growing 

 plants equally freely and 

 fully, and whether the 

 critical moisture content 

 is the same for different 

 types of soils. 



Materials. Three tin 

 cans or flowerpots ; lettuce, 

 tomato, or cabbage plants ; 

 corn kernels or lima beans ; 

 balances ; garden soil ; 

 evaporating dishes. 



Directions. Fill one 

 flowerpot with each of the 

 following: clay, loam, and 

 sandy soils. Transplant 

 some tomato, lettuce, or 

 cabbage plants into each 

 of the three pots of soil. 

 Plant corn kernels or lima beans in the pots and grow them for three weeks before beginning the 

 test. Keep the plants under conditions favorable to growth until they attain a good size. Water freely. 

 Remove a ten-gram sample of soil from each pot at a depth of an inch below the surface. Weigh 

 each, and after heating for two hours at the temperature of boiling water, reweigh. Compute the per- 

 centage of water present in each soil. 



Cease watering and observe the plants daily for evidence that they are suffering for lack of moisture. 

 As soon as the plants in any soil have wilted, remove a ten-gram sample of soil from this pot and deter- 

 mine moisture content. Compute the percentage of moisture present in the soil. By subtraction 

 determine the percentage of moisture given up by the soil before the plant shows signs of suffering. 

 When the plants in the other soils are v. 'Ited as badly as they were in the first soil, remove a sample and 

 test and compute as before. 



Compare the soils as to the percentage of water present in them when they no longer support 

 plant life. 



Questions. Which type of soil gave its moisture most freely and fully to the plant ? What is the 

 structure of a soil which gives up its water generously ? In which type of soil will plants best with- 

 stand drought? What is the relation between the tilth of a soil and its capacity to carry plants safely 



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24. Optimum and critical moisture content 



The picture at the left shows a plant growing in a soil with an optimum moisture content ; the one at 



the right shows a plant growing in a soil in which the moisture content has reached the critical stage. 



The soil in which the plant is wilted contains as much as 4 per cent of moisture . 



