Review of the effect of soil compaction on potato growth and its removal by cultivation
Potatoes are very sensitive to compaction at all stages of growth from emergence to harvest but particularly in the first 3-4 weeks after emergence when growth rates of roots are most rapid in loose soil (typically 1.5-2 cm/day). Shallow compaction immediately below the seed piece is therefore more damaging than deeper compaction since it is encountered when roots are growing most rapidly and impeding them at this stage can have serious repercussions on later growth.
Compaction is frequently caused by working soil when it is at, or above, its plastic limit. Soil then shears by compressive rather than brittle failure leading to a smeared profile at the cultivation depth. Earlier planting increases the chances of operations being carried out in conditions where compaction is likely to occur. Growers therefore have to balance the advantages of planting early to establish early canopy cover with the disadvantages of compacting the soil which will considerably reduce canopy expansion and subsequent yield. The effects of compaction will persist throughout the season and, once created, are almost impossible to remove completely in the growing crop.
Symptoms of compaction include delayed and uneven emergence; slow, incomplete and curtailed ground cover development; premature or rapid senescence; wilting of leaves on hot days even in wet soils; chlorotic or conversely dark green foliage owing to impaired nutrient or water uptake; severely reduced yield; increased outgrades from misshapen, bruised or green tubers.
The Review summarised the information on compaction available at the time (2005), including the impact of compaction on yields. Of 16 experiments where potatoes were grown in artificially compacted soil (c.f. loose soil), 13 showed a significant yield decreases owing to compaction. Some of the differences in yield between compacted and uncompacted soil were large (25- 38 t/ha) but were 18 t/ha on average (with a mean yield of 54 t/ha in the absence of compaction). In contrast, published yield responses to subsoiling in potatoes showed that only 28 experiments out of 83 had a significant yield increase in response to subsoiling, with three experiments showing a significantly reduced yield. Many of these experiments measured a significant decrease in soil resistance or strength as a consequence of the subsoiling operation but the effects on yield were often small (e.g. 5 t/ha) or not significant. The average yields in these experiments were lower (c. 42 t/ha) than in the compaction experiments possibly indicating that some factor other than soil conditions was reducing yields.