Improving potato water use efficiency through understanding soil and plant water balance

Efficiency of water and irrigation usage can be improved by better understanding of how plants balance their water status in relation to evaporative demand under different soil conditions. Previous work had shown that the water balance between leaves and potato tubers is very dynamic and affected by soil water status, evaporative demand and stage of canopy growth. The objectives of this Water Use Efficiency project were to:

• Determine a more quantitative measure of soil conditions in a potato seedbed
• Determine the optimum soil water content for cultivation so that seedbed compaction is avoided and the tilth created is clod-free but structurally stable
• Determine how plants balance their water status in relation to evaporative demand under different soil conditions
• Establish a possible control mechanism for variation in tuber dry matter concentration by examining the relationship between plant and soil water status under varying evaporative demand 

Results

Where soil was cultivated substantially wetter than field capacity, soil bulk density was increased and porosity decreased at cultivation depth compared with soil cultivated in a drier state. Penetration resistance was increased in soils cultivated whilst wet and differences increased as the soil dried, with resistances at 25 cm exceeding the limit for root penetration (3 MPa) in most seasons where the soil dried out through lack of rainfall and irrigation. Clod size distribution within the ridge was generally unaffected by most cultivation treatments, although allowing ploughed or cultivated soil to dry for several days prior to final roto-tilling produced cloddier ridges. Mean clod size decreased slightly during the season as the soil weathered but the rate of degradation was slow. The quantity of available soil water was generally unaffected by cultivation regime but rooting density was reduced in soils cultivated whilst wet by c. 15 %, with roots proliferating in the region above the cultivation interface as a consequence of being unable to penetrate the areas of greater soil resistance.

Cultivating soil whilst wet also increased leaf and tuber water potential and stomatal resistance. Measured soil moisture deficits at 25 cm depth in soil cultivated wet were greater than when cultivated whilst dry, indicating a shallower zone of water uptake by crops grown in compacted soil. Applying nitrogen fertilizer increased the rate of canopy development and persistence and radiation absorption potential more than applying irrigation but cultivating the soil whilst wet had relatively little overall effect on ground cover development and duration over the 3 years of the project. Averaged over the 3 years, tuber fresh weight yields in Maris Piper were increased by similar amounts by nitrogen and irrigation application (c. 12 t/ha) and by a much smaller amount by cultivating soil whilst dry compared with wet (c. 1.6 t/ha). The overall effects of soil compaction on yield were less than in a similar experiment in 2006 (10 t/ha) and only in one year out of three was the difference significant (c. 5 t/ha in 2008), so the effects of cultivation on yield appear to be variable despite similar practices at planting.

A survey of 47 commercial fields showed that within an individual field, the wetter a soil was when cultivating at planting, the greater the bulk density measured at cultivation depth. There were differences in the slopes of the relationship between bulk density and soil water content between fields and these slopes were negatively correlated with both the proportion of clay and silt and organic matter in the soil. However, for fields with similar soil texture and within fields with similar texture, there were large differences in the water content of the soil at 30 cm at planting, so the target of providing an optimum window for cultivation in terms of soil water content needs further work. Nevertheless, it is ultimately hoped that this information can help provide a useful set of recommendations for growers when cultivating fields in spring.

In irrigation experiments, maintaining the soil moisture deficit close to Field Capacity increased tuber yield as long as early over-watering was avoided. Any temporary restriction to irrigation was detrimental to yield. There was no evidence of differential drought tolerance or irrigation yield response amongst the cultivars selected. In the experiments in the UK, there were only slight differences between varieties in how stomatal resistance responded under contrasting irrigation treatments and these were not consistently large enough to point to real differences in stomatal function that may have affected water use efficiency.

Aim: To improve the efficiency of water use in GB production by understanding the interactions between soil and plant water status and evaporative demand.