Fertiliser
rates for
tomatoes
Autumn 2022 follow-up study to benchmark current fertiliser practices.
TOMATOES of the same variety, picked at the same colour, could have very different shelf-life.
BY JONATHON KAMMANN DEPARTMENT OF PRIMARY INDUSTRIES AND REGIONAL DEVELOPMENT
In 2016, a benchmarking study was conducted [by Neil Lantzke and Truyen Vo] to assess the fertiliser and irrigation practices of 10 tomato growers in Carnarvon. They found that the rates of both fertiliser and irrigation varied significantly without a corresponding difference in yield.
I conducted a follow-up study in the autumn of 2022 to benchmark current fertiliser practices and to assess for possible changes since 2016. With Giao Nguyen’s help, I interviewed 10 tomato growers in Carnarvon about their fertiliser usage and collected soil and leaf samples from each property at two different stages of crop development. I also assessed the quality and shelf-life of tomatoes from each participating grower.
Results indicate four key findings:
1 Variation in rates and timings: Figure 1 shows the huge difference in the maximum and minimum rates of nitrogen (N), phosphorus (P), and potassium (K) used by participating growers. There was also a significant variation in the timing of applications, with 3 of the 10 growers relying on one bulk application pre-planting followed by semi-regular microbial applications, while the other 7 growers applied fertiliser via fertigation every 1–3 weeks.
Soil and leaf tests showed no relationship between the amount of N, P, and K applied and crop nutrition, suggesting that growers could reduce their fertiliser rates without affecting crop health.
2 Abundance of soil K: Plant-available K ranged between 600 and 1500kg/ha in the top 15cm of soil at each of the grower’s properties*. Such concentrations should be sufficient to meet crop K requirements without additional K input. At a minimum, this will allow for flexibility in the timing of K applications.
FIGURE 1. BREADTH OF NITROGEN, PHOSPHORUS AND POTASSIUM RATES USED BY TOMATO GROWERS IN CARNARVON, 2022.
3 Timing is key for N fertilisation: The soils on the grower’s properties had less than 1.5% organic matter. A test of potentially mineralisable N found that the soils could provide a maximum of 6kg/ha of N across a week from the breakdown of this organic matter. This means that the supply of N to the tomato plants is mainly regulated by the timing and rate of fertilisation. A second soil test at mid-harvest showed that some crops had access to less than 15kg/ha of N, and due to the low mineralisation rate these crops were vulnerable to running low on N between applications.
4 Management affects quality: Shelf-life assessments demonstrated that tomatoes of the same variety, picked at the same colour, could have very different shelf-life characteristics depending on the way they were grown. Some of the produce would likely be considered sub-standard by consumers due to poor colour development, softness, and low sugar and acid contents. The breadth of management practices and varieties used made it difficult to identify the specific factors causing these problems, and for this a replicated trial will be needed.
* Assuming a bulk density of 1g/cm. Continued through article.
MORE INFORMATION
Contact John Kammann, john.kammann@dpird.wa.gov.au
THE benchmarking study found that the rates of both fertiliser and irrigation varied significantly without a corresponding difference in yield.
Growers could reduce their fertiliser rates without affecting crop health.