NURSERY PRACTICE FOR PRAIRIE-PLAINS PLANTING 89 



230° F., and its moisture content computed. The total moisture con- 

 tent, represented by the weight of the moisture in the sample divided 

 by the oven-dry weight of the soil, can then be compared with the 

 recorded estimate. After a number of trials, the estimates will be 

 reasonably close. 



A quicker method of determining total moisture content than 

 ovenization, described by Bouyoucos (2), consists in weighing the 

 moist soil in a special cup, then pouring in an igniting alcohol to drive 

 off the water, and reweighing the sample. Moisture content is 

 computed as in the standard method of oven drying. The process 

 takes only 10 to 20 minutes and is reasonably accurate in mineral soils 

 containing less than 1 1 percent organic matter. 



A still simpler device for rough determination of the moisture 

 condition of soil, devised by Livingston and Koketsu (21), is a small 

 hollow porcelain cone about 3 inches long with an absorptive area of 

 almost 2 square inches. The cone, first dried in an electric oven or 

 over some desiccating agent such as calcium chloride or sulfuric acid 

 and weighed, is inserted in the soil to a desired depth, left for an hour, 

 and reweighed to determine the amount of moisture absorbed. In 

 dry soils about 0.1 gram of moisture is absorbed, in moderately moist 

 soils about 1.0 gram, and in very moist soils about 2.0 grams. 



All soils have a certain amount of moisture which is unavailable to 

 plants, and the percent of unavailable moisture (known as the wilting 

 coefficient) must be subtracted from the total moisture content of the 

 sample to obtain the available moisture content. If available soil 

 moisture amounting to at least 4 to 6 percent of moisture in excess of 

 the wilting coefficient is not maintained throughout the active growing 

 season, poor growth and inadequate storage of reserve food in the 

 nursery seedlings will certainly be the result. The quantity of un- 

 available moisture in soil depends on texture; the finer the soil, the 

 greater the amount of water held unavailable. Wilting point for 

 nursery seedlings for several soils are: Sands, 2 to 4 percent ; light sandy 

 loam 4 to 6; fine sandy loam, 6 to 8; loam. 8 to 12; silt loams, 12 to 15. 



A simple but practical method of determining wilting point 15 

 is to grow the seedlings in a series of 10 to 20 pots for a period of 2 

 months and then to discontinue watering. When the leaves show 

 definite signs of wilting, measurement of an average sample of the 

 soil from the zone of root development gives a close value of approxi- 

 mate wilting point. 



Various other instruments are available which measure the water- 

 supplying power of the soil (22), or which record total or available 1 

 moisture on an instrument based on a tensiometer principle (32), but 

 these require considerable care and attention and are not as practicable 

 and simple as the methods listed here. 



ROOT AND TOP PRUNING 



Root pruning may be practiced to some 1 extent on taprooted plants 

 to hold back top development or change rooting habits. Summer root 

 pruning is commonly practiced on walnut to force lateral root develop- 

 ment. It is possible that other taprooted species might be handled 

 advantageously by summer root pruning. A cool, cloudy day should 

 be selected for this operation and it should be followed immediately 



" Wilting points can also be determined by a quick field method based on cohesiveness of the soil (3 or 

 by a more exact method involving use of a centrifugi 



