Three growth phases
Phase 1: crop establishment
Good growth in the first two to four weeks of crop life is critical for achieving high marketable yields. Maximising growth in this phase has a major impact on final yield and reduces the number of days from planting until harvest. The benefits of maximising early growth cannot be made up in later growth phases.
To maximise crop nutrition and growth in the establishment phase it is important to keep fertilisers in the top 10–15cm of soil — where young roots are growing — for as long as possible.
During crop establishment on these sandy soils, fertiliser containing low rates of nitrogen and potassium should be applied twice weekly by spraying or broadcasting. Both methods place enough fertiliser within the reach of roots to maintain a steady nutrient supply without excessive loss to the environment, as long as irrigation practices are good and rainfall is not excessive (see Figure 2).
Applying granular NPK fertiliser to the soil surface on the day of planting is critically important for achieving maximum yield, whether spraying or broadcasting is used for following applications.
It takes about four weeks in summer and five weeks in winter for the root zone of a celery crop to become extensive enough to intercept nutrients placed everywhere on the soil surface. By maturity, the roots can access water and nutrients from the top 20–30cm of soil.
Phase 2: rapid growth
Once the root system is established, fertilisers are most effectively applied weekly by banding (Figure 3) until row closure.
Fertigation or broadcasting can also be used in this phase. Applying the required rates of fertiliser with these methods is difficult without unnecessary water use or the risk of damaged foliage and heads from fertiliser granules lodging among the stalks.
Banding with compound granular fertilisers is most suitable as it minimises labour costs for application compared to single nutrient fertilisers.
Phase 3: maturation
After row closure, the head and frame of plants fill out, drawing accumulated nutrients from the frame developed in earlier stages, as well as from the soil. Weekly application of nitrogen through fertigation is required in the four to eight weeks between row closure and harvest.
This can involve some unavoidable losses from overspray on adjacent crops, younger plantings or uncropped ground, but other methods used in earlier phases are less safe for the crop or practical in this phase.