356 MISCELLANEOUS FARM SUBJECTS 



give to the soil water these materials in solution if they are to be 

 absorbed by the plant roots. This means that there exists in the 

 soil first, what is termed unavailable plant food, or that existing in 

 these mineral grains and in the humus in more or less insoluble 

 form; and, second, available plant food or that which is gradu- 

 ally dissolved from this rather insoluble supply. The larger part 

 of the mineral plant foods in all soils exists in this unavailable form 

 and since the dissolving action goes on very slowly there is in most 

 soils only a very small part of the total supply that is soluble or 

 available at any one time. The problem of the farmer is therefore 

 to keep within the soil sufficient soluble nitrogen, phosphorus and 

 potassium for maximum yields. 



There are certain agencies which aid in making available the 

 insoluble plant food of a soil, the most important being the action 

 of decaying humus. When humus decays, certain organic acids 

 are set free, which, as they are taken up by the soil moisture, en- 

 a'ble it to dissolve this mineral plant food more rapidly. Other 

 agencies have a place in this action but it is to the presence of con- 

 siderable quantities of decaying humus that the larger part of this 

 dissolving action is due. Considerable quanities of plant food are 

 also fixed within the humus itself and as it decays these are made 

 soluble. These two factors are in large measure responsible for the 

 great productiveness of virgin soils containing large amounts of 

 humus, as well as for the very beneficial effects of adding humus 

 to a worn soil. 



The fact that the supply of available plant food in a soil is 

 largely determined by the quantity of humus present gives to this 

 constituent of soils a very great importance. Moreover, there are 

 various other ways in which humus acts beneficially,, such as in 

 preserving a friable condition of the soil, in conserving moisture 

 and in preventing washing. Humus is almost invariably asso- 

 ciated with fertility. It is to the presence of humus that the dark 

 color in soils is usually due and a dark color is almost always indica- 

 tive of a fertile soil. 



The fertility of the soil is dependent upon four principal facts, 

 viz : Plants must breathe, must drink, must feed, and have a proper 

 sanitary environment. 



All understand that the breathing of the plant is mainly 

 through its leaves; but the soil also may be a very important fac- 

 tor in the breathing of plants, as it is necessary to have a supply of 

 oxygen around the roots. Physiologists differ as to the office the 

 roots have in regard to the absorption of oxygen. Whether it is a 

 true breathing, the taking of oxygen for the plant economy through 

 the roots as through the leaves, has never been decided ; but it is un- 

 questionably a fact that roots of cultivated plants require oxygen 

 around them for their healthy growth. We know that cultivation 

 of the soil is important or necessary for the best development of 

 many crops, and say that it is in order to introduce oxygen and 

 make possible the introduction of more water into the soil. It 

 seems necessary not only to introduce air into the soil, but by stir- 



