Paul D. Stukenholtz, Richard T. Koenig, David J. Hole and Bruce E. Miller
Further analysis partitioned the yield response into nutrient and nonnutrient con-
tributions according to the procedure described earlier (Figure 1; Table 2). The differ-
ence in moisture between the two years had a large affect on the nutrient and nonnu-
trient effects of compost. At the south site (wet year), the ratio of nonnutrient to nutrient
-1
effects at an application rate of 10 Mg ha was 0.25:1, while the ratio at the north site
(dry year) was 2.2:1 (Figure 1). Overall, the nutrient effects were more pronounced than
nonnutrient effects at low to moderate compost rates in the wetter year. In the dry year
the nonnutrient effects were more pronounced than the nutrient effects at all compost
application rates. This is also reflected in the efficiency factor (C) for the nonnutrient re-
sponse, which was higher in the drier year than in the wetter year (Table 2).
The ratio of nonnutrient to nutrient effects of manure reported by others (McIntosh
and Varney 1972; Magdoff and Amadon 1980; Klausner et al. 1994) has ranged from 0
to 0.25:1. In these earlier experiments, however, the available N supplied generally did
not far exceed the N requirement, and the crop (corn) was raised in an environment less
limited by moisture. Neither of these conditions was true for this experiment. Moisture
limitation, and the addition of compost rates that far exceeded the nutrient require-
ments of the crop, could both be expected to amplify the nonnutrient effects over those
observed in previous experiments. Avnimelech and Chen (1986) described nonnutrient
to nutrient benefit ratios as high as 3.3:1 in low moisture, low organic matter soils, but
noted that this effect was extremely variable and depended on soil texture, original soil
nutrient levels, and moisture. Galler et al. (1978) found positive yield responses to com-
-1
post and sawdust applications up to the highest rate measured (448 Mg ha ), and at-
tributed the increase to both increased nutrient availability and a near doubling of field
capacity soil moisture retention at the highest compost rate.
According to the analysis, the ratio of nonnutrient to nutrient effects in the present
study varied with compost rate. At the south (wetter) site, the nonnutrient effect had
-1
less influence on yield than the nutrient effect below 71 Mg ha compost (Figure 1). At
the north (drier) site, the nonnutrient effects of compost application dominated the nu-
trient effects for the entire range of compost application rates. These differences sug-
gest that moisture plays an important role in the nonnutrient effects of compost on
winter wheat, and that the nutrient and nonnutrient benefits of compost in a dryland
system may vary widely from season to season depending on precipitation levels.
The partitioning of nutrient and nonnutrient effects indicates that nutrient effects
-1
were maximized at the south site at approximately 27 Mg ha compost rate, and then
declined above this rate (Figure 1). Mathematically, this can be explained on the basis
that the Mitserlich equation predicted a near linear nonnutrient effect over all compost
rates at the south site (Figure 1). When nonnutrient effects were subtracted from the
yield response, the nutrient effects were predicted to decline. The analysis employed
in this study does not allow us to determine whether the trend in declining nutrient
effects has any mechanistic basis. It may be that the additional nutrients available with
higher compost application rates stimulated early season vegetative growth, thereby
depleting soil moisture and limiting grain filling. An additional explanation could be
that the higher rates of compost contributed significantly to the soil salinity (Table 1),
and this was expressed as a decline in nutrient effects on growth.
Contrary to results obtained from experiments in which moisture is less limiting
(McIntosh and Varney 1972; Magdoff and Amadon 1980), results from this study sug-
gest that nonnutrient effects of compost applications can be significant and perhaps
even exceed nutrient effects when moisture is limiting. The lower yields in a dryland
wheat farming system require less nutrients, and nutrient requirements may be met or
exceeded by low rates of compost application. Higher rates of compost application may,
242 Compost Science & Utilization
Summer 2002