“When I speak of clay’s horrid habit of tight squeezing, I always have to stop and talk about the two great needs of all soils.  One is the need for water; the other, for air.  A soil cannot exist without these two things any more than we can.  Without these, or poorly supplied with them, a soil is as if it were half-starved.

“That trouble always comes from a lack of one or the other is quite sufficient to prove to us that these are essential.  Just see how sand lacks water, as does lime soil too!  But there is plenty of air space, unless these soils are too finely powdered.  Now look at clay! plenty of water, but how about the air?  When clay begins its packing, then air is excluded.

“So one of the questions to be asked in soil improvement concerns the water and air problem.  We must have air spaces, and we must have water-holding capacity.

“Before we go home I must just speak of soil and subsoil.  When you strike your spade down into the earth and lay bare a section of the soil this is what you see:  on top is the plant growth, the soil beneath this, dark in texture and about our locality of a depth of from six to eight inches.  This layer is called the topsoil.  In sections of the West it is several feet in depth.  Now below the topsoil is a lighter coloured, less fertile, more rocky layer, the subsoil.  Beneath comes a layer of rock.

“And finally you may be a bit confused by the word loam.  It is often given as one of the classes of soils.  By loam we mean clay, sand and humus.  You will often hear people speaking of a sandy loam or a clayey loam according as there is a greater percentage of sand or clay in the soil.

“Next Friday I shall talk about soil fertility.  So trot home lively now!”

II

PLANT FOOD

A soil, as I have said before to the boys, may contain all the food necessary for plant growth and still not support any good growth at all.  That means then we ought to be able in some way or other so to understand the soil that it will be possible to unlock these good things for the plants to live on.

“I see a question in Josephine’s and Miriam’s faces.  I guess that this question is concerning what the plant food is in soils.  That is right, is it not?

“Well, I’ll take that up first, then;—­different ways of improving and increasing the goodness of the soil.

“The foods that are necessary and essential to plants and most likely to be lacking in the soil are nitrogen, potash and phosphorus.  Now by no means must you think that these are the only chemicals which are foods, for there are something like thirteen, all of which do a share in the food supply.  Oxygen and carbon are very necessary indeed.  Oxygen is both in the air and in water.  Carbon plants take entirely from the air.  I might go on and tell you of iron, of sulphur, of silicon and all the others.  But you would only get confused, so I am going to make you acquainted with these three entirely necessary ones.  They are capricious; often missing, and when not missing hard to make into available food for plants.