No synthetic fertilisers
Managing nutrients is important because synthetic chemical fertilisers are not permitted. Many conventional growers wrongly believe organic systems use no fertilisers at all. In fact a wide (and increasing) range of nutrient inputs is permitted, making it possible to correct any soil imbalance and provide specific supplements as required.
The main difference from conventional systems is that inputs are used to manage soil fertility through biology. Growing plant cover over the orchard floor and mowing to create mulch fuels soil biological activity generating soil fertility and plant available nutrients.
Applying supplementary fertiliser inputs only becomes necessary when an imbalance or deficiency can be demonstrated. The amount of supplementary inputs diminishes over time to maintenance levels.
A number of growers also use the 'Albrecht' method of balancing soil chemistry. Dr William Albrecht was an American soil scientist who established a set of ideal ratios for the main soil cations (Ca, Mg, K, Na). This method stresses the importance of the Ca to Mg ratio as a key driver of soil and plant health. Various other alternative soil management approaches are briefly described in the attachment.
Mangoes are generally known to be sensitive to excess nitrogen during the fruit development and ripening stages. This can result in poor quality and green fruit with poor storage characteristics.
Calcium and potassium levels are also thought to influence fruit quality and storage.
Trace elements like zinc, copper, boron and manganese are important. The role of boron in flowering and internal fruit quality is especially important.
In general, nutrient supplements can be applied to remedy identified soil deficiencies - rather than applying as a routine event. Occasional foliar nutrients are permitted.
Soil and plant tissue tests are important to verify nutritional requirements. The general approach toward correcting any deficiency is via the soil, rather than applying directly to the plant (leaves). Of course, in the early years of conversion, some foliar applications may be required while soil imbalance is corrected. The grower needs to demonstrate that measures are being taken to correct the soil. Care must be taken to use materials that are beneficial to soil biological processes and comply with certification standards.
Compost can be a valuable input to be used in conjunction with an integrated soil fertility management program. However, availability, quality, purpose and cost of compost are important considerations, depending on location. Careful consideration must be given to the timing of organic matter decomposition and nitrogen release to ensure this matches the appropriate stage in the trees’ growth cycle.
Some acceptable organic input nutrients may cost more than conventional products. However, the nutrient quantities applied may be less making the costs similar. The table below lists sources of key nutrients.
| Nutrient | Acceptable products |
|---|---|
| Nitrogen |
|
| Phosphorus |
|
| Potassium |
|
| Calcium |
|
| Magnesium |
|
| Sulphur |
|
| Trace elements |
|
Plant availability of a number of the nutrient sources listed above can differ from highly soluble conventional products. The lead time required from first applying the input until useful quantities are plant–available must be carefully considered – especially in the first few transitional years. Over time residual pools of nutrients held in soil biomass can compensate for this time lag.
The impact of growing orchard floor cover and producing mulch must also be considered in calculating a nutrient management program.
In some instances too much clover and legume growth can lead to excessive nitrogen levels in soil and have a detrimental effect on fruit quality and storage.