ORCHARD TILLAGE, FERTILIZERS AND SHADE CROPS 13 
results: after one hour .16% of the insoluble phosphoric acid had been 
made soluble; in two hours .28%. Magnesium phosphate similarly 
treated yielded 16.33% hour and 22.35% hours of soluble 
phosphoric acid. 
Carbonic acid is, then, an important agent in dissolving rock par¬ 
ticles and with its aid, latent fertility is brought into a condition which 
plants may use. 
Organic matter is often almost entirely consumed under arid con¬ 
ditions very quickly after it is incorporated with the soil. The heat of 
the sun in conjunction with insufficient moisture, produces conditions 
which oxidize the matter, or in other words, burn it much as though 
it had been consumed by fire, hence the burning out of soil, an ex¬ 
pression common among farmers. This change in organic matter is 
no doubt begun by soil organisms while the later stages are probably 
of a chemical nature. 
The decay which results in humus, takes place at moderate tem¬ 
peratures. Organisms do not develop in a low temperature and if it 
is too high, oxidation or burning results. An apparent contradiction 
to this statement is found in the difficulty with which unrotted stable 
manure decays when plowed into land, if moisture is not abundant. 
The lack of moisture probably prevents the development of bacteria in 
sufficient numbers to begin decay, consequently the chemical changes 
which result in burning do not occur. The presence of such material 
is a detriment to growing plants as it keeps the soil so open that an 
undue amount of moisture is permitted to escape. This effect is so 
noticeable that many of the so-called dry farmers make no use of 
stable manure. This wanton waste might easily be prevented by 
composting as there is sufficient moisture in the rain and snow to 
thoroughly rot manure treated in this manner. 
The compound resulting after organic decay is completed is 
humus. Such decay results in a dark colored material, without 
Organic structure, soluble in alkalies after the lime has been removed. 
Humus gives a dark rich color to otherwise light colored soils. It is 
complex in composition and is especially important as being a res¬ 
ervoir of nitrogen. Much of the nitrogen that is brought to the soil 
by leguminous plants, as well as that which is stored in the plant in 
the process of growth, is not made available to growing plants until 
the humus stage is reached. 
Humus usually exists in small quantities in arid soils but often 
i-s entirely absent where clean cultivation without manuring has been 
practiced. It may also be present in small quantities along with par¬ 
tially decayed organic rf^atter. Much of this valuable material is lost 
' by constant clean cultivation and the consequent burning of the organic 
materials, before it reaches the stage where humifaction may take 
place. 
A large part of the loss could be prevented, not only of nitrogen 
but of the many benefits to be derived from decaying organic matter 
and humus as well, by shading the soil. 
