26 BULLETIN 499, U. S. DEPARTMENT OF AGRICULTURE. 
HUMUS CONTENT OF THE BASINS AND THE CHECK SOILS. 
SUNNY MOUNTAIN GROVE. 
Determinations of the humus content of the soil in the basined 
area and in the untreated area to a depth of 3 feet are given in Table 
X. Determinations of humus are necessarily somewhat inaccurate, 
owing not only to the variation in sampling but also to the fact that 
humus, so-called, is an indefinite mixture of organic compounds.* 
The Sunny Mountain grove, so far as known, had been treated uni- 
formly previous to the installation of the mulched basins. Composite 
samples collected in 1914 indicated that the check area contained 
slightly more humus than the basined area. In the spring of 1915, 
however, the surface foot of the basined area was higher in humus, 
as shown in Table XIII. The humus content of the surface ‘foot of 
soil below the mulch also increased gradually throughout the sum- 
mer, while the check area, which received no organic material save 
that contained in commercial fertilizers, showed practically no 
change. The increase in humus in the basins is confined practically 
to the first foot of soil. At lower depths the humus content of the 
two tracts differs but slightly, decreasing gradually to a depth of 6 
feet, as shown by the mean values in Table X. If the humus con- 
tent of the soil is limited, it is advantageous to have it localized 
largely in the upper 3 feet of soil, which in most California soils 
includes the principal feeding roots of citrus trees. 
TABLE X.—Percentage of humus in the first and in the second 3 feet of soil in 
the Sunny Mountain grove, California, in 1915. 
[ 
| 
: é Furrow system Furrow system 
B ar upper | Bas ar lower : ’ 
Se eet ae nee She Avy- | near upper end of | near lower end of | Av- 
erage, tract. tract. erage, 
pep and bows bets 
date. ae = - eae : ends/, | = = : ends 
salBelEsl Se eelEeEel 65 | of (Re BelEs| $5 ee EeEsl 4. | of 
HZ\S3\a8| > 2 |=8/SSias| > w | Tt A 8 S3ia3| > Sl s/s eiae) > & | zack 
& wz a _ = MR = < = aR Be j< = MN = < 
First 3 feet: | | | 
Apr. 9...|0.52/0.180.17, 0. 29/0. 53:0. 340.21) 0.36) 0. 33/0. S210: re : ce ai: 270.14] 0.26] 0.26 
June 15 .| . 63] .22| .13] . 33] . 59) .29) .13) 34]. 33) . ; 42] .35| .17| 31). 27 
July 10.-.| . 72) . 26] . 10) 36 BOs 2p) 2d 6le oss) 35 Sole 38 “0 os - 29) - 42) .22 -ol 2t 
Aug. 6. .| . 67] . 28) 14| - 36) . 50} . 23) . 13) 29 33)/-527/ 124 -17| =. 26) . 50) . 44! .18 -3f| 32 
Sept. 21.| .84] .16| .08) 36] .67| .36) 10) .37/ . 37] . 36 .20] 11) 22) .37) .32| .26] 27). 27 
Aver- | fees, | M | | | | | | | | Best: | 
58] . 29] .15) .34 sal a 393} eg ga -30] .28 
age..| .68) .22) .12) .34) . 
1The humus was determined by removing the calcium with 1 per cent hydrochloric 
acid, extracting with sodium hydroxid, and measuring the intensity of the humus color 
against a standard humus solution in a colorimeter. The standard solution was freshly 
made up by weight from uniform dry stock material obtained by extraction with 4 per 
eent ammonia after treating with 1 per cent hydrochloric acid and washing free from 
chlorids. The ammonia extract was filtered through a Pasteur-Chamberland filter before 
evaporating to dryness. 
