This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
All growers, whether amateur or professional, are aware of the difficulty of estimating just what form of plant food is deficient, and usually it is only by more or less costly experiments that some basis can be obtained. Of course many men simply depend on certain quantities, adding such to the soil whether the plants require it or not, until soil sickness occurs, and, in consequence, many pests and plant diseases.
To obviate the above disadvantage, and to obtain a useful knowledge of the composition of the soil, analysis, both chemical and physical, is resorted to, and it is proposed to deal with this in as simple a way as possible. It must of course be thoroughly understood from the beginning that to arrive at an accurate analysis requires much expensive apparatus as well as a long course of practice and study, but enough information will be given to enable the studious grower to obtain many useful data, to save him from wasting his time over unnecessary experiments, and also to open a new field for research and profit.
In the older methods of analysis the soil after preparation was digested by strong acids, hydrochloric being the chief. This extracted all the food that might or might not be available at some future period, but gave no idea of what was immediately available for the plant's use. In consequence of this the results were so misleading that soil analysis was spoken of with scorn. Often enough one ingredient was said with truth to be in large quantity, but the growing crop would show symptoms of starving for want of that very constituent. As an instance, clays and clay loams are usually designated as being rich in potash, yet the addition of potash salts will often give high results on such soils, showing therefore a deficiency in available potash, an entirely different thing from total potash.
Many of the food salts in the soils are so tightly held or combined as to resist even long years of cultivation and weathering.
Nowadays it is the rule to dissolve out the available salts which can be immediately rendered available to the growing crops, by using for this purpose a 1-per-cent solution of citric acid; this strength having been estimated to correspond with the solvent action of natural forces at work.
These forces are the weak organic acids obtained from humus, the work done by the beneficent and other bacteria, as well as by enzymes and certain fungoid moulds, and, lastly, by the solvent action of soil water impregnated with carbonic acid gas.
It is not at all necessary for anyone to attempt a complete analysis. To obtain such would mean probably months of work, and the data obtained would be of very little value to the grower. What is usually done is to give the quantities of lime carbonate (chalk), magnesia, nitrogen, phosphates, potash, the total contents, organic matter and humus.
About 40 to 80 per cent of soil matter is inert and does not enter into the fertilizing, but its composition determines the physical condition of a soil and thus has a great bearing on cultivation.
In reading an analysis care should be taken to note that the magnesia, usually magnesium oxide (MgO), is not in excess of the lime contents, or disease and various other troubles will quickly follow. It will not matter how much lime carbonate is present if the magnesium oxide is multiplied by 2 and then compared with lime carbonate. Sufficient of the latter must be added to give a proportion equal to four of the former (magnesium carbonate) to seven of the latter (lime carbonate).
A sample of soil may be taken as follows: Cut a square about 9 in. with a spade to a depth of about 1 ft.; take the whole of the soil from this block and place in a box. Remove to a shed or laboratory for examination.
If the soil is rather wet, allow it to air-dry somewhat, and then work through a fine sieve to remove all stones. Take a portion of the soil, say 1/4 to 1/2 lb., and place in a beaker, tube, or even an ordinary jug. Fill the vessel containing the soil three parts full with water; stir to a paste, thus separating clay from sand. Allow to settle somewhat, pour off, and repeat this until the added water becomes no longer turbid. Remove the sand from the bottom, air-dry, and weigh, and the result will show you into what class your soil fits, whether it be sand, sandy loam, loam, clayey loam, or clay, the physical constituents of which are given at p. 92.