Compost May Not Improve Orchard Performance

Roger Duncan, UC Cooperative Extension, Stanislaus County

Many studies have shown that soil applications of composted green waste or manure can increase the diversity and activity of soil microorganisms, soil water holding capacity, nutrients such as potassium and nitrogen, humic acid, organic matter and carbon sequestration. Although most compost studies focus on improvements in “soil health”, few if any trials have tested if compost actually enhances orchard performance or increases profits.

Current costs for purchase, delivery and application of composted green waste in the Modesto area is approximately $27 - $30 per ton. Common application rates range between five & ten tons of compost per acre, representing a significant investment to the grower.   It  is important to determine if almond growers can improve tree performance enough to recover such a substantial input cost.

Field Trials with Compost. In a Stanislaus County trial, we worked with a local nursery to determine if applying composted green waste prior to planting is beneficial when establishing an almond orchard, and if so, which rate might be best. The field was a sandy  loam  soil where a vineyard had been removed the previous year  and the soil was fumigated.  Prior  to  planting,  composted green waste was applied at 0, 5, 10, 20, or 30 tons to the acre in a concentrated, 4-foot wide  band  down the future tree rows (Fig. 1). The  compost  was then incorporated into the soil and potted trees were planted in May 2017. The orchard was fertilized and irrigated according to the grower’s normal farming practices. The trees were monitored for two years, recording tree growth, leaf nutrients and stem water potential (using a pressure bomb to  monitor  water stress).

Fig. 1: 30 tons of compost per acre applied before planting.

At no time during the two years of the trial did we see or measure any differences in tree performance. Even 30 tons of composted green waste, at a cost of about $800 per acre, had no effect on tree  establishment,  growth, leaf nutrient content or tree water status. If the rows weren’t labeled, you would never know which rows had compost and which did not. So, one application of compost at planting did not affect orchard establishment  or  longer-term tree performance. What about a compost program         where compost is incorporated at planting time AND applied to the soil surface every year for several years? To test this, two replicated  field trials were established in 2015 to document  the  effects of compost and examined the differences between using composted green waste and composted cow manure. One orchard is in Riverbank on a  Hanford  sandy loam soil that has not been  previously  farmed. The test variety is Nonpareil on Nemaguard rootstock  and is irrigated with full coverage sprinklers.  The  second orchard is in North Modesto on  very sandy soil  in a fumigated replant site following an almond orchard removed the previous fall. This orchard is irrigated with microsprinklers. The variety is Independence on Nemaguard rootstock.

In both locations, 5.2 tons of compost per acre were applied in a concentrated band about 4 feet wide and  then incorporated into the soil during planting of the bareroot trees. An additional 0.5 tons / acre was applied to the base of the new trees after one month of growth. Each subsequent spring (2016 –  2019),  approximately 10 tons of composted green waste or manure has been applied to the soil surface in a concentrated band approximately eight feet wide (Fig. 2). Trees are periodically monitored for stem water potential (water stress), and annually for leaf nutrients,  nematodes, growth and yield.

Fig. 2: 10 tons of composted green waste applied annually

Results and Conclusions: Composted green waste and manure both slightly increased July leaf levels  of chloride and nitrogen. Leaf calcium was significantly lower in the compost treatments. Potassium  was increased in the composted manure treatment but not the green waste treatment. No other significant changes in leaf nutrients occurred. After five years of study, the application of composted green waste or manure has not increased growth or yield of almond trees  whether  grown in excellent, first generation orchard soil or very sandy, second generation orchard soil. Stem water potential measurements with a pressure bomb have shown that trees in compost areas have not had less  water stress than without compost. Compost treatments have not reduced pathogenic nematode numbers.

Costs for purchase, delivery and application of composts at 10 tons / acre were approximately $265 annually, or $1,325   per   acre   over   the  five-year  period.     Many growers  only  apply  five  tons  so  their  costs  would  be lower.

Why have we not seen improvements in tree performance? Maybe for several reasons. Although 5-10 tons of compost per acre sounds like a lot, when you consider that an acre of soil weighs well over 5000 tons, it is easy to understand how these rates of compost would not likely substantially influence soil physical properties, especially considering 35-50% of compost is water. Part of the lack of response may be that just placing compost on the soil surface may not significantly change soil physical or chemical properties below the top few inches in a no till orchard system. Preliminary soil samples of a local orchard indicated no difference in soil organic matter, cation exchange capacity or saturation percentage below four inches, even though compost had been applied annually for seven years. Perhaps with today’s high input farming practices (more precise irrigation, liberal use of commercial fertilizers through fertigation, etc.), any benefits of compost would go unnoticed. Composts may be more beneficial in orchards deprived of commercial fertilizers although at a substantially higher cost. Based on leaf analyses from these field trials, manure compost may replace the need for potash (potassium). Even though 10 tons of composted green waste may contain 250 – 300 lb of nitrogen, most of it is in an organic form and only a small percentage is available to the trees during the first year. In these trials, leaf analyses indicated at most an increase of 0.2% nitrogen in July sampled leaves (2.7% N in compost-treated trees, 2.5% without compost), even after several years of application. In some years, there was no difference. It is possible that benefits to orchard systems may be very long term and not observable in just five years. There may also be special cases, like organic orchards or soils with surface water penetration issues where compost may have benefits.

Yield of Almond Trees With and Without Annual Compost Applications
	Yield (kernel pounds per acre)
	3rd Leaf	4th Leaf	5th Leaf	Cumulative Yield
	Orchard A. Nonpareil on Nemaguard; 1rst generation orchard, Hanford sandy loam soil
Untreated	568 a	2148 a	3154 a	5870 a
Green Waste Compost	559 a	1992 a	2898 a	5449 a
Manure Compost	602 a	1977 a	2909 a	5488 a
	Orchard B. Independence on Nemaguard; replant site with sandy soil
Untreated	-	1987 a	1779 a	3766 a
Green Waste	-	2256 a	1788 a	4044 a
Compost
Manure Compost	-	1990 a	1859 a	3849 a
*Data followed by the same letter indicates they are not significantly different (P<0.05)