Orchard Floor Management Affects Soil Structure

Most orchards in Ontario maintain permanent sod between the tree rows.  This practice makes good sense to protect the soil from wheel traffic, erosion and loss of organic matter. 

The area directly under the tree is where the roots are, so it is the critical zone.  Orchard trees do not compete well with grass for water and nutrients so it is often kept bare with herbicides.  Management choices for this soil area affect tree and root growth, soil water and nutrient availability, fruit yield and quality.

So what would we like from the ideal orchard floor?  It should absorb water to prevent runoff and hold this water against gravity until the roots need it.  Then, as the roots take up moisture, water from the soil should move freely in to replace it and carry nutrients along with it.  After all, pretty much everything that makes an apple tree comes into the roots with water except carbon and oxygen.  You can see how beneficial it could be to maintain optimum soil structure under the trees.

Different management methods that make a small change each year can add up to big changes over the lifespan of an orchard. A long-term experiment was done by Dr. Ian Merwin and his research group at an orchard maintained with four different management systems for eight years in Ithaca, New York. Ithaca soil is a Grey-Brown Luvisol derived from limestone like we have in most of Ontario.  Directly under the trees, some plots were kept completely bare with a spring application of residual herbicides.  Others received Roundup™ in May and July each year which allowed re-growth of vegetation later in the season.  The third treatment was turfgrass, which was mowed every two weeks.  The final treatment maintained a 6-inch layer of hardwood mulch under the trees.

After eight years, they measured how well the soil absorbed water, how much water it could hold and how easily water could move through the soil.  They also measured bulk density, an indicator of compaction. The measurement and treatment areas were the zone directly under the trees as opposed to the area between rows.

The clear winner in this study was mulch, with the herbicide treatments finishing in the middle.  Sod, normally associated with improving soil structure, finished last.  The researchers think the mowing caused compaction.  Of the herbicide treatments, Roundup tended to fare better than the residual herbicide treatment, which was continuously bare.  Not taken into account in this study but worth mentioning is that later season re-growth can take up excess nitrogen and potentially improve crop quality, cold tolerance, and groundwater quality.

Herbicide treatments that allow late-season re-growth are not a bad choice, but this study showed that optimum root function is most likely to occur under mulch.  Perennially bare soil is at risk for leaching and organic matter loss.  Mulch increased water infiltration and movement rates, and hence nutrient transport within the soil.  It decreased soil bulk density.  Other research has shown it also increases soil organic matter content and root activity near the soil surface while providing insulation to protect the roots from temperature extremes.

It may be worth penciling out the cost of mulch versus herbicides for your orchard.

Figure 1 - Mulched rows of high density apples can enhance root activity by moderating soil temperatures, improving organic matter, and increasing water infiltration.

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