Abigail A. Maynard
BioCycle May 2004, Vol. 45, No. 5, p. 48

Objective of Connecticut research was to find what combination of leaf compost and inorganic fertilizer produced greatest butternut squash yields.
With some 92 composting sites for yard trimmings in the state, leaf compost is now the most abundant type of compost in Connecticut. As there are no restrictions on the use of leaf compost in Connecticut, growers appear more likely to consider using it as a soil amendment compared to other solid waste composts. One potential use for large quantities of compost is in commercial vegetable production.
Applications of leaf compost have long been known to improve the physical conditions of many soils. In a 1994 BioCycle report on the impact of composts on vegetable yields, we found that the bulk density of compost-amended soil decreased from 1.21 g/cc to 0.91 g/cc after seven years of annual additions. These additions also increased the organic matter content from 7.5 to 12.6 percent, promoted aggregation of fine soil particles, and reduced crusting following summer rains. Most importantly, higher organic matter content increased the water-holding capacity of the soil from 1.3 to 1.9 inches in an eight-inch plow layer.
Virtually no research has been done utilizing compost on the production of winter squash. Even though encouraging data exists on the benefits of using compost in many other crops, most squash growers require data that is specific to winter squash.
At the Connecticut Agricultural Experiment Station, we conducted a three-year experiment examining leaf compost as a nutrient source for field production of butternut squash. The objectives of this study were to determine whether leaf compost could be substituted for inorganic fertilizer in butternut squash production and what combination of leaf compost and fertilizer produces the greatest butternut squash yields.
RESULTS OF EARLIER EXPERIMENTS
Experiments were conducted at the Valley Laboratory, Windsor, Connecticut on Merrimac sandy loam, a sandy terrace soil with somewhat limited moisture holding capacity; and at Lockwood Farm, Mt. Carmel, Connecticut on Cheshire fine sandy loam (Typic Dystrochrept), a loamy upland soil with moderate moisture holding capacity. Three-foot aisles separated each 15 by 10 foot plot and each treatment was replicated four times in a Latin square design.
Unscreened leaf compost was applied to plots at both sites in April 2001, 2002, and 2003 at the rate of 50 T/A (dry weight basis) (one-inch on the surface) and rototilled into the soil to a depth of six-inches. The compost was produced in a passive pile turned four or five times yearly for two years. Organic matter was determined by loss on ignition, total nitrogen content by the Kjehldahl method, pH by glass electrode, and soluble salts by the electrical conductivity of a saturated paste extract. The pH (1:1 suspension) was 6.5 and the soluble salts were 0.6 mmhos/cm. The nutrient values were 3 ppm NO3-N, 80 ppm NH4-N, 100 ppm P, 250 ppm K, 1200 ppm Ca, and 125 ppm Mg. Organic matter content was 22.4 percent and total nitrogen was 0.8 percent for a C:N ratio of 28. Solvita maturity index was 7. Three sets of the compost-amended plots were fertilized with commercial grade 10-10-10 (N-P2O5-K2O) at three rates: 0, 650, and 1300 lb/A. Yields from these plots were compared to unamended control plots fertilized with 10-10-10 at a rate of 1300 lb/A (full rate). The full rate was predetermined from analysis of soil from both sites before the experiment, which indicated low soil fertility at both sites.
Each year, bush butternut squash was seeded in a greenhouse on May 7-8. The seedlings were grown in peat pots measuring 2-1/4 inches by 2-1/4 inches tapering to 1-1/4 by 1-1/4 by 3 inches (volume 9.2 cubic inches). The seedlings were fertilized with water soluble 20-20-20 (N-P2O5-K2O) (0.5 oz/gal) four weeks after germination. Two weeks before transplanting, the seedlings were transferred to a cold frame for hardening. The seedlings were transplanted on June 13-15 at both sites. Seedlings were spaced one foot apart in rows three feet apart with a population of 10 plants per plot. Weeds were controlled by cultivation. Overhead irrigation was used as needed. Following harvest, the vines were removed from all plots and the plots left fallow over winter.
Soil samples were collected at the end of each growing season. Available soil nutrients were measured using the Morgan soil test. Organic carbon was determined by loss on ignition, pH by glass electrode, and soluble salts by the electrical conductivity of a saturated paste extract.
YIELD COMPARISONS
The greatest yields at Mt. Carmel in 2001 and 2002 were from the unamended full-fertilized control plots (Table 1). Compared to plots amended with compost and the full rate of fertilizer, however, the results were not statistically significant. In 2003, plots amended with compost and the full rate of fertilizer had yields averaging 11 percent greater than the yields from unamended fertilized control plots, a significant difference (p
At Windsor, the greatest yields in 2001 were from the unamended control plots. Again, these yields were not significantly different than plots amended with compost and the full rate of fertilizer (Table 2). In 2002, yields from the control plots and plots amended with compost and the full rate of fertilizer had statistically greater yields than other treatments. In 2003, plots amended with compost and the full rate of fertilizer had yields that were 16 percent greater than the yields from control plots, a significant difference. Similar to Mt. Carmel, the high yields from plots amended with compost and the full rate of fertilizer were also due to a greater number of fruit per plant (Table 2).
At both sites, in all years, plots amended with compost and either half the rate of fertilizer or no fertilizer had significantly lower yields compared to the controls. The decreased yields remained relatively consistent over the three-year period at both sites. At Mt. Carmel, yields were reduced 11 to 23 percent from plots amended with compost and half the rate of fertilizer compared to the full-fertilized control and plots amended only with compost had a 21 to 28 percent reduction in yields compared to the control. The reduction in yield was mostly due to a smaller average number of fruit per plant (Table 1). At Windsor, plots amended with compost and half the rate of fertilizer yielded 16 to 18 percent less than the control while plots amended with compost and no fertilizer had 32 to 51 percent lower yields. The reduction in yield was mostly due to a lower average fruit weight (Table 2).
Compared to plots amended with compost and the full rate of fertilizer, plots amended with compost and half the rate of fertilizer had 8 to 25 percent lower yields at Mt. Carmel and 12 to 27 percent lower at Windsor. These lower yields were all statistically significant except for the 2002 yields at Mt. Carmel (-8 percent). Plots amended with compost and no fertilizer when compared to plots amended with compost and the full rate of fertilizer had yields reduced 16 to 35 percent at Mt. Carmel and 36 to 48 percent at Windsor, all significant differences.
When comparing the two compost treatments with reduced fertilizer applications (half and zero), the two treatments had statistically equivalent yields at Mt. Carmel in all three years. At. Windsor, however, yields from compost-amended plots with no fertilizer were significantly lower than plots amended with compost and half the rate of fertilizer. Yields from the compost-amended no fertilizer plots had yields 17 to 40 percent lower than compost-amended plots receiving half the rate of fertilizer. In 2001, the lower yield was due to a lower average weight of fruit (Table 2). In 2002 and 2003, the yields were lower because of a smaller average number of fruit per plant.
LOWER YIELDS WITHOUT INORGANIC FERTILIZER
This study demonstrated that compost could not be used as a substitute for inorganic fertilizer in butternut squash production. Even after three years of compost additions at both sites, a reduction in inorganic fertilizer in compost-amended plots led to a reduction in yields compared to unamended plots receiving the full rate of fertilizer. The fact that optimum yields were attained with compost and the full rate of fertilizer showed that the use of compost did not lead to reduced yields. Soil nutrients measured by the Morgan method ranged from “medium” to “high” on plots receiving the full rate of fertilizer and from “low” to “medium” on plots receiving a reduced amount of fertilizer. It appears then that nutrients were limiting on the reduced fertilizer plots, which led to reduced yields.
These results differ from several other studies where utilization of leaf compost as a soil amendment has been shown to reduce the need for commercial inorganic fertilizers. In an unreplicated study, D.E. Hill found that in 1984 annual amendments of leaf compost sustained higher yields of most vegetables when compared to unamended soil and reduced fertilizer needs by one-third and two-thirds the normal rate. In a 3-year study on tomatoes, I found that, on both loamy and sandy soils, one-inch of leaf compost incorporated into the soil annually can be substituted for inorganic 10-10-10 fertilizer and equivalent tomato yields can be expected in the first year of compost application. For the greatest yields, it appears that a combination of compost and 10-10-10 fertilizer is optimum but the full rate is not always necessary. Another three-year study on cut flowers showed that, for most types of flowers, leaf compost could replace inorganic fertilizer. Optimum yields of cut flowers were attained in most years with leaf compost alone.
This study also demonstrated that the full rate of inorganic fertilizer does not always produce optimum yields and that greater yields can be attained when the full rate of inorganic fertilizer is used with compost. After three years of compost additions, plots at both sites amended with compost and the full rate of fertilizer had significantly greater yields than the unamended control plots fertilized with the full rate of fertilizer.
Increased yields were due to the cumulative effect of the compost after three years of additions. Amending soil with compost for three consecutive years affected the physical and chemical characteristics of the soil at both Mt. Carmel and Windsor. The pH of the soil amended with compost for three years ranged from 6.4 to 6.6 compared to 5.6 in the unamended controls at Mt. Carmel and 5.9 at Windsor. Thus, the compost had a liming effect and raised the pH into the range considered optimal for both microbial activity and nutrient availability. The addition of compost for three years increased the organic matter at Mt. Carmel from 4.7 percent on the unamended control to an average of 6.5 percent on the compost-amended plots. At Windsor, organic matter content increased from 3.5 percent to 5.0 percent after three years. Increased organic matter content of soils has been shown to improve its water holding capacity.
Other studies have shown similar results when using compost and the full rate of fertilizer. In two three-year studies, where the full rate of fertilizer was used, this research found the greatest eggplant, pepper, and tomato yields were from plots amended with leaf compost compared to yields from plots amended with undecomposed leaves and unamended control plots. In another study using the full rate of fertilizer on compost amended plots, we found that leaf compost reduced year-to-year variability in onion yields, increased the yields of most onion cultivars, and, in two of three years, produced a higher percentage of marketable colossal and jumbo sized onions.
While compost does not reduce the inorganic fertilizer requirement in butternut squash production as it does in other crops, it does increase yields significantly when used with the full rate of fertilizer compared to unamended plots. The increased yield occurred after three years of compost amendments and was effective in both loamy upland soils and sandy terrace soils. This study also illustrated the importance of studying the effect of compost on specific crops.