Agriculture Reference
In-Depth Information
farms (Table 1) with two general groups based on primary organic matter
amendment (manure or composted green waste). Several farms also used
a vetch winter cover crop alone or in conjunction with other amendments
and some applied other nutrient sources (e.g. seabird guano, Chilean ni-
trate, fi sh emulsion) as a sidedressing or through drip irrigation. Tillage
was used on all fi elds and was of similar intensity. Soil series identifi ed
from the SSURGO database are all considered highly productive (Table 1;
Soil Survey Staff, Natural Resources Conservation Service, 2011) and had
similar mineralogy (Schafer and Singer, 1976).
13.2.2 SOIL SAMPLING AND ANALYSES
Surface soil samples were collected in June 2011. Sampling was timed to
coincide with tomato anthesis and early fruit development, a critical pheno-
logical and agronomic period in which tomato nutrient demand is high and
growers often add supplemental nutrients. Fields were all sampled within
two weeks of one another, an average of 68 days after transplanting. In each
field, six plots were established at random locations within a 0.25 ha area to
monitor soil and plants over the course of the season. An intact soil core (15
cm in diameter, 0-15 cm deep) was removed from each plot in between two
tomato plants, situated 15 cm from the centerline of the planting row.
Soil samples were kept on ice until processing within 4 h for different
analyses. After thoroughly mixing the soil sample, fi eld-moist soil was used
in determination of microbial biomass C (MBC) and N (MBN) within 24 h
of sampling (see below). Inorganic N was extracted from moist soils with
2 M KCl and analyzed colorimetrically for ammonium (NH
4
+
) and nitrate
(NO
3
−
; Foster, 1995 and Miranda et al., 2001). Olsen P was determined us-
ing the methods outlined by Olsen and Sommers (1982) at the University of
California Agriculture and Natural Resources (ANR) Analytical Laborato-
ry. Soil pH was determined on air-dried samples using a 1:2.5 soil/water ra-
tio. Gravimetric water content (GWC) was determined by drying at 105 °C
for 48 h. Air dried soil samples were sieved to 2 mm, ground, and analyzed
for total C and N at the UC Davis Stable Isotope Facility. Particle size was
determined by the laser diffraction method according to Eshel et al. (2004).
Additionally, a ~50 g subsample was immediately frozen at −80 °C for
