The Effects of Cover Crops and Compost on Grapevine Nutrition and Growth

D. Hirschfelt, Viticulture Farm Advisor, Fresno Count
W. Peacock, Viticulture Farm Advisor, Tulare County
P. Christensen, Extension Viticulturist, UCD, Kearney Ag Center

Introduction

Recent research has indicated that nitrogen (N) should be applied during the growing season, or immediately after harvest when uptake is rapid. Rates may be reduced since uptake is more efficient. It was also demonstrated that N rates should not exceed vine requirements, or fruit quality may e reduced. The N contribution from alternative sources such as compost, N-fixing legumes, and green manure cover crops are not as well understood since the kinetics of N release and soil mineralization are more complex. While cover crops are widely used in vineyards, their influence on vine N uptake is not well documented.

This is the second year of a four year study to investigate the interactions between vineyard floor management, alternative sources of N, soil mineralization, and vine N uptake on raisin and table grapes. Cover crop, fertilizer and compost treatments were established in a raisin vineyard, and Thompson Seedless table grape vineyard in fall 1990, and data collected through the growing season. Winter cover crops were planted again this fall, and the research will continue. Our objectives in this experiment are to quantify the effects of vineyard floor management and alternative N sources on vine nutrition, soil N mineralization and availability, vine growth, fruit production and quality. We hope to further refine the "best management practices" for N fertilization in San Joaquin Valley vineyards; to optimize efficiency, minimize leaching, and best utilize or manage existing sources of N.

Procedures

In November 1990, trials were established in a Thompson Seedless raisin grape block at Kearney Agricultural Center (KAC) and a block of commercially produced table Thompson Seedless in Tulare County (TC).

The floor management system at KAC was designed to increase nitrogen utilization by the vines. The experiment is in a uniform, healthy block of young vines. The plot design is a randomized complete block replicated 5 times, using 10 vine plots. Guard rows and vines separate treatment areas. Treatments are as follows:

No cover crop / 50 lbs. nitrogen applied at berry set
No cover crop / compost (rate equal to 50 lbs. N per year) applied in spring
Winter lana vetch as green manure
Winter barley as green manure
Barrel medic reseed
Control - no cover crop, no nitrogen fertilizer

The treatments in Tulare County are designed to optimize uptake of nitrogen and provide a beneficial floor management system for table grape production. This vineyard tends to be excessively vigorous, and treatments are managed to provide adequate but not excessive N to the vines. Plot design is a randomized complete block replicated 5 times. Plots consist of single eighth mile rows with adjacent middles treated. Guard rows are present between data rows. All treatments received identical cultural practices and irrigation quantities.

Treatments are as follows:

Winter barley as green manure
Winter blando brome
Companion® perennial grass mix
Summer sudan grass
Winter lana vetch, mowed for frost protection, allowed to reseed
Winter lana vetch as green manure
Control - no cover crop of N fertilizer
N fertilizer postharvest

Barley, lana vetch, barrel medic, blando brome, and Companion were seeded in fall. Compost was spread and incorporated adjacent to berms in April of both years. Compost samples were collected and analyzed for nutrient content. Summer sudan grass was planted in May. Ammonium nitrate fertilizer was applied in the KAC experiment in May (berry set). Calcium nitrate fertilizer was applied in the TC experiment in October 1991 (postharvest).

Cover crop biomass samples were collected one to two days prior to incorporation, with fresh weight, dry weight, and N content determined. Petiole samples were collected at bloom and collected in one foot increments to a depth of four feet. One set of samples was collected adjacent to the berm and a second set was collected in the center of vine middles. Samples were dried, extracted and analyzed. Soil moisture was monitored with tensiometers at KAC. Irrigation records were maintained and water samples were collected to determine N content.

Grape yield and quality was evaluated in both locations, and leafhopper populations were monitored weekly. Raisin grapes yield and quality was also determined.

Results and Conclusions

Cover crop biomass varied widely depending on stand establishment. Treatments at TC had lower biomass resulting from mowing for frost protection. The blando brome and lana vetch (reseed) treatments did an excellent job at reseeding in 1992. The Companion displayed signs of water stress throughout the summer, but continued to be competitive. The lana vetch (reseed) plants had a tendency to creep up the berms and interfere with cultural practices.

In the KAC raisin vineyard, there were no differences in petiole nitrogen at bloom in 1991. N levels were low in plant tissue, which was expected since the block had not been fertilized in six years. Total inorganic N was higher in the N fertilization and lana vetch treatments by veraison 1991. Data from 1992 is presented in Table 1. Lana vetch had significantly higher nitrate-N in the bloom petioles. The N fertilizer treatment at set significantly raised the nitrogen status by veraison, with the lana vetch treatment intermediate in its effect. It is not understood why the petiole tissue nitrate levels are so low.

In the TC table grape vineyard differences were apparent in 1991, with lana vetch reseed and green manure treatments both having significantly higher N tissue levels at bloom and veraison. Greater differences are seen in the second year of the experiment (Table 2). Bloom petiole N levels were highest in the N fertilizer treatment, followed by the two lana vetch treatments. As the cover crops continued to grow during the summer, their effect on N status became apparent. By veraison the sudan grass, barely, blando brome and Companion significantly reduced N in vines.

June 1992 soil N status is presented in Figures 1 through 4. Samples adjacent to the berm were collected separately from samples collected in the middle of the row. This was done to detect any differences in treatment incorporation, since N fertilizer and compost were banded close to the berm, while cover crops were incorporated down the middle of rows. At KAC, lana vetch had significantly higher N in the top 4 feet of soil in both berm and middle samples. It was not significantly different from the N fertilizer in the zone next to the berm between 12 and 36 inches in depth. There were no differences between compost, barrel, medic, barely and the control.

The variability of soils in the TC table grape vineyard was expected due to the larger plot size. Significant differences were detected adjacent to the berm, with lana vetch green manure consistently higher than most treatments. The Companion usually had the lowest N levels but generally was not different from the untreated control. No differences were detected in row middle samples, except at the three foot depth. In this zone the Companion, barley, lana vetch reseed treatments had the lowest N levels. We expect greater differences in soil N as the experiment progresses.

Table grape yield and quality data is presented in Table 3. No yield or quality differences were detected. Raisin yield and quality data is presented in Table 4. Treatments that contributed the most to soil and vine N (N fertilizer, lana vetch and barely) reduced raisin quality.

Leafhopper populations were monitored weekly from June through August. Evaluation of cover crop and compost affects on leafhoppers was not a primary objective of the experiment since plot size and geometry will limit conclusions, however interesting observations were made. There were no significant differences in variegated or grape leafhopper nymph between cover crop treatment, compost, or ammonium nitrate at any sample data in any year. There were no differences in accumulated nymphal days for either pest.

It is evident that some legume cover crops such as lana vetch, can be used as the primary source in N in vineyard systems. The costs for a cover crop program, such as additional cultivation, frost concerns, cost of seed, and additional water consumption, must be weighed against the benefits of improved soil tilth, and steady N contribution to soils. The influence of fruit yield and quality depends on vine N uptake. It appears that competitive cover crops can be used as a tool to manage excessive vigor in vines, but the results are gradual and may take several seasons. The N response to compost appears low in the initial season and may require higher rates under the conditions of this study. Vine response was most rapid with applications of N fertilizer.

Table 1. Influence of floor management on vine nitrogen status in KAC Raisin Block 1992 (Year 2 of experiment). ^1/
Treatment	Bloom Petiole Nitrate-N	Veraison Petiole Nitrate-N
Lana Vetch - green manure	233 a	263 b
N Fertilizer - set (50 lb) ^2/	50 b	428 a
Barley	19 b	25 a
Barrel Medic	14 b	88 c
Control	13 b	24 c
Compost	12 b	18 c
p=	.0007	.0000

1/ Treatment means within a column, followed by different letters are significant at the 5% level (DMRT).
2/ Fertilizer was applied after bloom petioles were collected.

Table 2. Influence of floor management on vine nitrogen status in Tulare County Table Thompson Seedless Block 1992 (Year 2 of experiment). ^1/
Treatment	Bloom Petiole Nitrate-N	Veraison Petiole Nitrate-N
N Fertilizer - postharvest^2/	1742 a	1109 b
Lana Vetch - green manure	1190 b	2048 a
Lana Vetch - re-seed	1167 b	1334 b
Sudan Grass	1032 b c	436 c
Control	955 b c d	466 c
Barley	722 c d e	574 c
Blando Brome	668 d e	655 c
Companion®	481 e	356 c
p=	.0000	.0000

1/ Treatment means within a column, followed by different letters are significant at the 5% level (DMRT).
2/ Fertilizer was applied postharvest in 1991.

Table 3. Influence of floor management on yield and quality in Thompson Seedless Table Grapes, Tulare County - 1992.^1/
Treatment	Clusters/Vine	Wt/Berry (g)	Pickable Yield (lb/vine)	ºBrix	Rot No./vine
Barley	21.0	5.76	36.3	19.4	3.9
Blando Brome	24.2	5.56	35.6	19.4	4.6
Companion®	18.4	5.46	19.7	19.4	4.8
Sudan Grass	17.7	5.77	23.0	18.1	5.2
Lana Vetch-reseed	26.3	5.36	37.5	18.9	11.0
Lana Vetch-green manure	23.7	5.55	34.3	18.9	8.7
Control	23.0	5.98	43.9	19.2	8.2
N Fertilizer	21.9	5.73	28.7	19.1	9.4
	n.s.	n.s.	n.s.	n.s.	n.s.

1/ Treatment means within a column, followed by different letters are significant at the 5% level (DMRT).

Table 4. Influence of floor management on yield and quality in Thompson Seedless Raisin Grapes, KAC - 1992. ^1/
Treatment	Clusters/Vine	Wt/Berry (g)	Raisin Yield (lb/vine)	ºBrix	% B or Better	Pruning Wt (lbs)
N Fertilizer (set)	60.6	1.9	15.1	20.8	77.4 c	8.27
Compost	53.6	1.8	13.1	22.1	91.1 a	7.25
Lana Vetch	59.0	1.8	14.7	21.1	81.3 b c	8.06
Barley	67.4	1.8	16.7	20.6	79.0 c	7.25
Barrel Medic	59.6	1.9	13.6	21.3	86.6 a b	6.95
Control	53.2	1.8	14.3	22.1	88.9 a	6.58
	n.s.	n.s.	n.s.	n.s	.009	n.s.

1/ Treatment means within a column, followed by different letters are significant at the 5% level (DMRT).

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