Lemon Orchard Mulch and Cover Crop Trial
Progress Report

Ben Faber

UC Cooperative Extension, Ventura-Santa Barbara Counties

 

The purpose of this study is to evaluate the impacts of cover crops and yardwaste mulch on soil erosion and air temperatures in lemon orchards. The area is prone to erosion due to the steepness of the plantings, however many growers have avoided planting covers for fear of the increased frost potential, as well as the increased management required. Concern about importing weeds has also been expressed by growers in the context of mulch use.

To this end, 11 sites were developed where either mulch was applied in a 6-foot wide strip in the tree row or a winter annual cover crop was broadcast. The minimum size of both the cover and mulch treatments was an acre and an equivalent amount of land at each site was left untreated as a control. The lemon trees aged from 3 to 18 years, depending on the site. Irrigation systems differed from grove to grove and consisted of drip irrigation, drag lines and microsprinklers.

The steepness of slope limited the mulch applications to sites with less than 10% slope. Ability to use machines for application forced this restraint. The yardwaste mulch was applied by a commercial operator to a depth of 6 inches, although this varied significantly from as little as 3 inches in some spots to more than 9 inches. Hand application could have been performed, but the cost would have been unrealistic from a grower standpoint. The mulch was spread in the winter of 1997. Earlier attempts to spread the mulch were limited by the significant rain that occurred the previous fall and prevented the use of machinery in the orchards.

In half of the cover sites a 'Zorro' fescue/bur medic mixture was used. At the other cover sites a barley cover was used because of greater tolerance to historic pre-emergent herbicide use. It is intended to replant the barley sites to the 'Zorro'/bur medic mix once the herbicide has been depleted. The covers were planted in the fall of 1996 and again in fall 1997.

There were actually 3 attempts at planting the covers in fall '96/winter '97. The first plantings in September came up with the first rains, but died when 6 weeks of no rain followed. All the rain washed out the second planting made in November and the third planting in January failed for lack of moisture that winter. The fall planting of '97 was delayed until November and was successful. Much of the seed from the previous January plantings germinated along with the new seed applied. At one barley site, though, it was necessary to seed three times because wildlife ate the seed.

Sheet erosion was estimated by burying a 3-gallon container at the bottom of the slope and measuring the amount of sedimentation after each rainfall event. Rainfall was measured at each site. Air temperatures were measured at each treated and untreated site with temperature data loggers. Decomposition rates of the mulch were also monitored and the different weeds that grew in the mulch and in the covers were also monitored. Tree growth was also monitored in the sites and tissue analysis made.

Weed Study

Since April 1997, weed species and quantity of weeds have been monitored. Identification and counts were taken along a line approximately three to four feet from the tree trunks, depending on the size of the trees, to determine the variety of plants growing at the edge of the tree canopies. Fifty-five species of weeds were accounted for in a one-year period, most occurring in five plant families. These are recorded in descending order.

The greatest numbers of individuals belonged to the Euphorbiaceae - spurge, turkey mullein and castor bean. These were most numerous where no other cover grew and were most active in the warm time of the year. Plants of the sunflower family - sowthistle, horseweed, prickly lettuce, and common groundsel - occurred from early spring through the summer in mainly plots with no mulch/cover crop. Brassicaceae - mustard and radish - were not very numerous and were more often seen in bare soil areas. The most common Malvacaea was cheeseweed and it occurred primarily in a poorly established cover of fescue and medic. Grass weeds (8 species) were not common other than in one mulched plot. The only weed that was consistently associated with mulch was palm.

Both the mulch and the cover crop were effective at reducing weeds at all sites. The variable nature of yardwaste (source and age), however, means that there is a potential to import weeds, such as grasses and palm amongst other weedy species that have been found in other mulch plots in the County.

Tree Growth, Leaf Analysis and Soil Analysis

Tree size was measured three times. Canopy size and trunk diameter were measured on 20 trees from each plot. During the time of this trial there has been no treatment effect.

With no discernable tree growth effects, it was not surprising the slight differences in tree nutrition since the materials had been in place such a short time (Table 1). There were some interesting effects on soil elements, however. The most pronounced effects were in the mulches where salts, sodium and chloride were lower in the mulched soils. This undoubtedly is due to the higher moisture content in the soils and the improved leaching occurring with irrigations. A disturbing trend, though, is the higher boron levels in the mulched sites. These higher soil levels are not reflected in the tissue analysis, but could be a problem in the long run.

Mulch Decomposition Rate

Mulch decomposition and settling were measured along a transect at each mulch site. Twenty-five depth measurements were made at each mulch site. This was done on a bi-monthly basis. There is a considerable amount of variation in the change of mulch depth from point to point in a mulch strip. This is due to the inherent variation in the material, some points with more wood than others, the initial depth of the applied material and the impact of the wind. At one site, a creek overflowed its banks and moved large amounts of mulch around. On average there was a 40% decline in the depth of the mulch after one year.

This loss is a little higher than rates that have been found in other mulch test plots, but the quality of the material has changed in the last few years. The material used in this test had greater amounts of leafy versus woody matter than in earlier tests. With woodier materials a 15-30% depth loss is more common in this area.

Air Temperature

Air temperatures within the tree canopy have been monitored in both the cover cropped and mulched sites since winter of 1997. Loggers are placed within 12 rows (240 feet) of each other in the treated and untreated areas, so as to minimize the positional variation found in the sloped canyons. There are significant diurnal variations that exist in these canyons throughout the year. Even in winter, there are regularly 25° F differences in a 24 hour period and as much as 40° F differences in a 10-day period. On any given day there could be, 5-10° F difference in the lows between sites.

Comparing the paired loggers from a site, there was little discernable difference between a cover cropped or a non-cover cropped site (Graphs 2 and 3). The month of January was the coldest in 1998 and low temperatures are shown. One reason for little difference in air temperatures is that the covers did not really start growing until after January when it started to warm up. Therefore they had very little insulating effect. In spite of the low coverage, they still had an effect on erosion (Table 2). Mulches, on the other hand, had a slightly lower air temperature than unmulched areas (Graph 1). Although the mulches were applied in a narrow six-foot strip down the tree row, they did insulate the soil, preventing the soils from warming in the day.

It may be that the areas devoted to this trial are not large enough to develop a temperature difference between treatments. A minimum of one acre was devoted to each treatment block. Making larger treatment areas creates a very real problem of temperature difference simply based on position in the landscape. If these plot sizes are not large enough to distinguish between treatments, it probably will not be possible to determine a treatment effect on air temperature.

Erosion

From November 1997 to March 1998 in this area, there was approximately 32 inches of rainfall (Table 2). In a mulch site, accumulated sediment from overland flow was 0.16 inches, while in the non-mulched site it was 20.7 inches. In a barley site, there was 3.4 inches of sediment and 16 inches in the non-barley cover area. In a fescue/medic cover area there was 7 inches of sediment and 32 .2 inches in the non-cover area. These results are comparable only for a given site. Each site had different slope angles and complexity. The covers were seeded in November 1997, and the barley had made a good cover by December, while the fescue mix only made full coverage towards the end of February. It is surprising how a little cover can make a significant reduction in erosion.

Table 2. Sediment (in inches) Collected in Buckets
Rain (in.)	Month	Rain (in.)	Mulch	None		Barley	None		Fescue	None
2.48	November	2.48							tr	0.68
5.2	December	5.2	0.16	2.48		1.04	0.12		1.04	3.32
3.44	January	3.44	trace	0.68		0.28	tr		0.16	2.6
16.54	February	16.54	trace	14.96		3.3	14.48		4.96	23.2
3.96	March	3.96	trace	2.88		0.48	tr		0.76	2.68
31.62	Totals	31.62	0.16	21		5.1	14.6		6.92	32.48

Table 1. Summary of leaf analysis for Grimes Canyon Trial, 1998
Means followed by * are significantly different at the 95% confidence level.
	Site 1		Site 2		Site 3		Site 4		Site 5		Site 6
Element	barley	no barley	fescue	no fescue	fescue	no fescue	mulch	no mulch	mulch	no mulch	mulch	no mulch
% N	2.82	2.98	2.96	2.9	2.5	2.64	2.66	2.62	2.62	2.72	2.72	2.74
	0.14	0.18*	0.15	0.15	0.13	0.15*	0.14	0.14	0.14	0.13	0.14	0.14
% K	0.65	0.51	0.48	0.57	1.06	1.29	0.6	0.74*	0.55	0.56	1.76	1.98
% Ca	4.18	3.78	4.08	3.96	4.9	5.22	4.66	4.98	5.32	4.44*	4.3	4.9
% Mg	0.3	0.32	0.34	0.3	0.26	0.26	0.29	0.29	0.3	0.31	0.21	0.24
ppm Zn	50	54	52	48	100	150	142	127	95	111	105	89
ppm Mn	46	42	40	46	94	134	117	108	74	86	113	100
ppm Fe	204	194	172	176	172	192	122	110	107	105	184	182
ppm Cu	8	7	8	6	19	16	12	17	14	13	14	9
ppm B	100	99	110	92	72	66	86	86	99	90	77	73
% Na	0.009	0.007	0.009	0.007	0.006	0.008*	0.0104	0.007	0.004	0.006	0.01	0.01
Summary of soil analysis for Grimes Canyon Trial, 1998
Means followed by * are significantly different at the 95% confidence level.
ppm NO3	20.4	16.48	16.48	6.12*	36.2	55.8	52.56	155.2	33.7	25.4	24.28	10.22
ppm P	29	25	39.8	19.6*	48.2	40.6	120	142	72.8	25.4*	48.8	16.8
ppm K	198	128*	182	190	410	254*	564	514	462	120	220	74*
meq/L Na	0.76	1.36*	0.32	0.76*	2.06	1.12*	0.96	2.12	1.52	4.82*	0.82	1.26*
ppm B	0.29	0.23	0.19	0.15	0.27	0.22	0.91	0.21	1.01	0.25*	0.74	0.09*
meq/L Cl	0.86	0.74	1.14	0.34*	1.58	0.46*	0.7	1.38	1.72	6	0.52	32
pH	6.4	6.44	6.62	7.32*	6.5	5.96*	6.84	6.72	7.12	7.76*	7.28	6.90*
Ece	0.85	0.7	0.72	0.93	1.09	1.15	1.45	3.56	1.32	2.2	0.98	0.56*

Graph 1. Low air temperatures in January 1998 in mulched and unmulched plots.

Graph 2. Low air temperatures for fescue/medic cover crop plots versus clean plots in January 1998.

Graph 3. Low air temperatures in barley cover crops versus clean cultivated lemons.

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