For July, 1921 



647 



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A Lesson on Soil Improvement 



Being One of a Series of Lessons of a Home Study Course on (jardening Appt-aring Regularly in The Gardeners' Chronicle 



Under the Direction of ARTHUR SMITH 



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A SOIL may be naturally very bad or very good, with all the 

 intermediate stages from one extreme to another, for 

 many reasons, but the worst soil will undoubtedly respond 

 to intelligent efforts for its improvements while the best soil will 

 as surely fail to produce the best results if it is neglected or 

 wrongly treated. 



The first point to be considered is that of drainage. If a soil is 

 not naturally well drained, then some means must be taken to 

 drain it artificially, otherwise all other work in connection with it 

 is more ct less ineffective. Good drainage is essenta! in order to 

 regulate the water supply. An excess of water in the soil is 

 equally, or more, injurious than a scant amount. If the water 

 which falls upon the land is allowed to flow over the surface and 

 can not enter the soil there is not sufficient reserve w'ater for 

 crop growth. After the soil has absorbed all the water it can, 

 drainage is necessary in order that the surplus of unabsorbed 

 water may escape, ijecause stagnant water around plant roots 

 (leaving out of consideration those of a more or less aquatic 

 nature) is poisonous to plant-life. Good drainage stores more 

 water in the soil in an absorbed — not free — condition and pre- 

 vents mere surface accumulation and loss. Good drainage is as- 

 sisted by deep and thorough cultivation; this has to be accom- 

 panied by tile drainage if the subsoil is not sufficiently porous to 

 allow the unabsorbed water to escape. 



Well-drained land is warmer in the Spring, has a larger reserve 

 store of water, therefore withstanding drought better, and is 

 altogether in a better condition for crop grow'th. Many swampy 

 lands are highly productive when properly drained, and in any 

 case a high state of productiveness is impossible w'ithout suitable 

 provision for drainage. 



When the pores of the soil are filled with water, air is excluded 

 and the necessary chemical and bacteriological changes which 

 result in rendering plant-food available, fail to take place. Many 

 of the bacterial diseases to which crops are subject are caused 

 primarily by a diseased condition of the soil. These diseases can 

 often be checked by securing good drainage. Undrained soils are 

 unsanitary; the harmful products of decay of the organic matter 

 accumulate in the soil and produce toxic or poisonous compounds 

 which affect crops. When soils are drained, air is admitted which 

 prevents the formation of these products. 



To improve the physical character of extreme soil conditions, 

 whether of the lightest sand or of the heaviest clay, the same 

 methods are, in several directions, applicable. The terms ''heavy" 

 or "light" as generally applied to soils indicate ease or otherwise 

 in cultivating rather than their actual weight, as a cubic foot of 

 clay soil weighs approximately thirty pounds less than one of 

 fine sand. 



.\fter drainage has been attended to when required, liming i; 

 usually the ne.xt step in soil improvement. The action of lime is 

 physical, chemical and biological, and is more valuable in these 

 indirect ways than as actual plant-food. 



Tlie meclianical action of lime in improving the physical char- 

 acter of a soil may be looked upon as of great importance, and 

 in a general way lime makes heavy soils lighter and light soi's 

 heavier. On 'clay soils the adhesive property and tendency to 

 puddle is lessened by the application of lime; such a soil being 

 thereby made more friable when dry, since lime, unlike clay, does 

 not shrink in. the process of drying and therefore the mixture drys 

 off in smaller particles instead of contracting into the familiar 

 hard lumps. In order to understand more clearly this action of 

 lime upon clay soil it will be well to examine into the cause cf 

 the well-known adhesive and plastic projierties of clay. 



The difference between a heavy and a light soil — using the 

 comparison in its popular sense — as shown by mechanical analysis, 

 is that the former contains a preponderance of very fine particles, 

 some of them so minute as to assume a gelatinous nature when 

 separated from the coarser particles; whereas the latter contains 

 a preponderance of large particles, the line above mentioned gcl.i- 

 tinons substances being practically absent. 



Wlien lime is added to a clay soil it collects together these 

 fine, or "colloid" substances as thev are technically termed, form- 

 ing them into loosely-bound particles ; that is to say. it coagulates 

 the liner particles into coarser grains, and automatically frees the 

 larger particles from the adhesive action of the colloids. This 

 action is known as flocculation. .\ simple illustration of it is 



easily afforded by the addition of a little lime to a glass vessel 

 containing muddy water from a clay soil. It will be seen that 

 the lime immediately flocculates the fine particles held in solution 

 and precipitates them to the bottom of the vessel, leaving a 

 clear fluid. The practical result of this flocculation of the line 

 particles of the clay is to give the effect of a coarser-grained soil 

 Under these circumstances not only is the soil more friable wheu 

 dry, but it is also less retentive of moisture when wet ; it there- 

 fore becomes permanently drier and warmer, permitting of earlier 

 cultivation in the Spring, which is a matter of the utmost impor- 

 tance with heavy soils. The disintegrating effect of lime upon 

 clay soils is also due to the chemical change of the oxide of cal- 

 cium (CaO) to the carbonate (CaCOj). 



Upon light, sandy soils lime acts as a binding agent and enables 

 such soils to obtain more moisture from the subsoil by capillary 

 attraction, at the same time increasing their retentive powers — in 

 short, making a sandy soil more like a sponge and less like a sieve. 

 This increase of retentive power also increases their ability to hold 

 plant-food. 



Lime also improves the crop-producing capacity of soils by its 

 chemical action. Lime is a powerful alkali, and as such com- 

 bines with and neutralizes any acid with which it may come into 

 contact. .Vcids are continually being formed in the soil by the 

 decay of organic matter in the form of plant-residues and cover- 

 crops turned under; they are also produced after the application 

 of stable or other organic manures. Other and stronger acids 

 are introduced into the soil by such fertilizers as acid phosphate 

 and sulphate of ammonia. 



Lime has a further and more complicated action in rendering 

 the three chief ingredients of plant-food in the soil (nitrogen, 

 phosphates and potash), available for plant nutrition, and re- 

 taining them in this form. Its work in connection with phos- 

 phates and potash is practically purely chemical, but as regards 

 nitrogen it is almost entirely biological, inasmuch as nitrogen is 

 only brought into an available condition by being changed by 

 bacteria into nitrate, these bacteria cannot carry on their w-ork in 

 an acid soil nor complete it in a soil which does not contain lime 

 in a free state. .\s we have previously in these columns gone 

 somewhat fully into the various reactions which are chemically 

 brought aliout by lime, and also shown the several ways in which 

 lime acts biologically in connection with the various species of 

 the population inhabiting the soil both as to its good effects^ in 

 connection with beneficial bacteria, and its good work in checking 

 or preventing the growth of those other micro-organisms which 

 are harmful to the soil and to higher forms of plant-life, it does 

 not appear necessary to do more now than merely restate that 

 lime has a great many attributes of usefulness in soil improve- 

 ment. 



As there are several forms in which lime may be used it ap- 

 pears worth while to briefly consider them. 



Lime exists in Nature as calcium carbonate (CaCO.), forming 

 whole mountain chains of limestone, chalk, marble, etc., and 

 comprising approximately one-sixth of the earth's crust. The 

 preparation of lime consists in the first place of burning the na- 

 tural stone in a kiln; as the result of this process carbonic acid 

 gas is driven off and caustic, quick, or oxide of lime (CaO) is 

 produced. This is put on the market just as it comes from the 

 kiln, as well as after being ground. .-Vnother form which is now 

 used perhaps more than anv other for soil improvement is cal- 

 cium hydrate or slaked lime (CalLO,), the slaking bringing it 

 into the liiiest possible state of subdivision. Of late years lime 

 has been much used in its natural state of calcium carbonate by 

 grinding up the rock and sold under the name of ground lirne- 

 stone. .\ distinct form of lime uncoinected geologically with 

 limestone rock is calcium sulphate (CaSO*), otherwise known as 

 gypsum or land plaster, this being found in various parts of the 

 world in more or less thick deposits. 



While, so far as improving a soil by increasing its lime content 

 is concerned, anv of the above forms of lime may be used, there 

 are various special connections which render one form more suit- 

 able and desirable than others. 



Quicklime is the most active form, but it should only be used 



upon heavy clay, swampy, or muck soils. Its oxidizing action 



upon the humu.s' content of soils is so rapid that it is liable to do 



harm upon those which do not contain humus to excess. The 



(Continued on fagc 65L) 



