9 2 



possible, it should be put into the 

 ground to a depth of 15 to 24 inches 

 to encourage deeper root growth. 



When fertilizer is spread on the sur- 

 face over a period of several years, the 

 tree tends to develop roots at the sur- 

 face of the soil, which interferes with 

 mowing the lawn, and during drought 

 periods cannot obtain sufficient water. 



Maples and elms tend to produce 

 many roots near the surface even under 

 the best conditions, and fertilizer on 

 the soil encourages the habit. 



If a chemical fertilizer is used, it 

 should be thoroughly soaked deep into 

 the soil; otherwise it might cause some 

 burning of plants, and the roots, which 

 take up nutrients only in solution, will 

 be able to get it slowly or not at all. 



The practice of raking and burning 

 the fallen leaves each autumn and re- 

 moving all grass clippings eventually 

 reduces the fertility of the soil to such 

 a low point that trees may not get 

 enough of the mineral elements they 

 need. In the forest the decaying fallen 

 leaves provide a protective mulch that 

 conserves natural moisture and tem- 

 pers summer's heat and winter's cold. 

 They return to the soil the mineral ele- 

 ments necessary for tree growth. Grass 

 under trees may rob them of needed 

 minerals. Top dressing the lawn does 

 not meet the requirements of the tree, 

 but a heavy application of well-rotted 

 manure over the root area of the tree 

 is usually beneficial. 



A SECOND FACTOR that sometimes 

 affects the growth of shade trees is the 

 lack of aeration of the soil. Clay soils 

 that bake hard in summer and that are 

 heavy and sticky when wet are not 

 favorable for good tree-root growth. 

 Such soils can be made looser by the 

 addition of sand or some other suitable 

 material and organic substances, such 

 as manure, peat, compost, thoroughly 

 rotten sawdust, decayed leaves, and so 

 on. The materials can be worked into 

 the top 4 to 6 inches of soil. Ordinarily 

 for that purpose it is better to use a 

 fork than a shovel, because a fork in- 

 jures the roots less. A surface mulch of 



Yearbook of Agriculture 1949 



organic material is often helpful; it 

 helps to conserve water and provides 

 insulation against overheating. 



For the decomposition of newly 

 fallen leaves and sawdust, additional 

 nitrogen is required. Unless nitrogen 

 is added, the addition to the soil of 

 fresh leaves or sawdust may actually 

 reduce the amount of the essential 

 nitrogen available to the tree roots. 

 Moreover, fresh manure, newly fallen 

 leaves, and similar materials mixed 

 into the soil or buried in it may release, 

 during decomposition, injurious sub- 

 stances that are poisonous to the trees. 

 However, leaves and sawdust can be 

 used as surface mulches. 



Trees often get too little water: 

 Many street trees grow in places where 

 the area of soil exposed to rainfall is 

 small; lawn trees have to compete for 

 water with grass and other plants. 

 Moreover, the drain pipes that honey- 

 comb the soil in cities remove from it, 

 every day, thousands of gallons of 

 water and might lower the soil water 

 table so much that established trees 

 cannot get enough water. 



Heavy watering will prevent damage 

 from this cause. Light sprinkling merely 

 moistens the ground for only a few 

 inches and does not benefit the trees. A 

 thorough soaking of the ground for 

 several hours once a week is much bet- 

 ter than sprinkling each evening. 



A COMMON CAUSE of the decline of 

 established trees in residential areas is 

 damage done to them when the houses 

 were built and streets laid out. 



Soil piled on the ground over the 

 root area of a tree can kill it within a 

 few weeks or after a score of years, de- 

 pending upon the depth of the fill, the 

 sensitivity of the species, and other 

 factors. A few inches of earth fill over 

 its roots can kill the American beech. 

 The American elm will often with- 

 stand several feet of earth fill. 



If the earth fill happens to be gravel 

 or has a large amount of decomposed 

 organic matter in it, the effects may 

 not be evident until 15 to 30 years 

 later. Gradual compaction of the 



