Plant testing and irrigation
Plant analysis
Use plant analysis to monitor the adequacy of the fertiliser program. It is important to take several tissue samples over the life of the crop and obtain a graph of the crop nutrient needs. The line on the graph of the nutrient analysis over time will slope or trend upwards, downwards or be constant. Compare this with what is expected for high yielding crops at the same stage as an indication of whether the fertiliser program is adequate.
A single, one-off nutrient measurement in a crop will not give an adequate indication of the trend for nutrient use across the life of the crop.
To determine the nutrient needs of a crop through plant testing, petiole sampling should start when the largest or king tuber under two or three plants has reached 10mm in length.
For each planting, collect the youngest mature potato leaf from 20 to 30 plants. The youngest mature leaf is the fourth or fifth from the top of the plant. Repeat every two weeks or as necessary. Submit samples to a recognised plant testing service for analysis. The analysis results can be compared with the range of nutrient standards at each growth stage, at which the maximum yield for potatoes could be expected. More information is available at Petiole analysis for fertiliser management of potatoes on sands.
Irrigation
The suggested fertiliser applications assume the crop is not over or under irrigated and that the irrigation system applies the water uniformly.
Potatoes require water to be applied to replace 100% of pan evaporation from planting to 20 days after emergence. This should be increased to 150% by 40 days after emergence. Beyond 80 days after emergence, irrigation should be reduced gradually to 100% of replacement just prior to harvest.
If using tensiometers — use blue tip sand models — they should be placed mid way between two plants within the row, with the midpoint of the ceramic cup 30cm below the top of the hill. Irrigate so the tension does not drop below about 5-7kPa from planting to the start of senescence. If rainfall is heavy and the water requirements of the crop are exceeded, additional nitrogen and potassium may be needed. Petiole analysis can assist in determining if nutrients are adequate or deficient.
Acknowledgements
Information in this page is based on work carried out in Project Number PT 213 (MA Hegney, IR McPharlin and RC Jeffery) on fertiliser and irrigation management of potatoes, funded by Horticulture Innovation Australia, Department of Agriculture and Food Western Australia and Western Australian ChemCentre.