DRIS usage to diagnose tree nutritional status from leaf analysis and production data

Summary

Leaf nutrient concentrations and yield data of various growers from major citrus producing regions in SA were collected and used to do a preliminary investigation into the use of Diagnoses and Recommendation Integrated System (DRIS) to diagnose citrus tree nutrient status. DRIS norms compared well to industry norms.

It was found that calcium norms for the Eastern Cape should be higher than the established norms, irrespective of cultivar, while the norms for phosphorus and potassium in the same region should be slightly lower. This study has also shown that DRIS indices can be very useful to pinpoint specific nutrient limitations, and to validate fertiliser responses, specifically on farm level.

Introduction

Citrus mineral nutrition is widely recognised as a major contributor to fruit yield and quality, and leaf nutrient content of fruit bearing terminal shoots is considered a reliable indicator for tree nutritional status (Raath, 2021). However, tree nutritional status is affected by a variety of factors like time and type of fertiliser application, climate and soil physiochemical properties.

Leaf nutrient content can assist to make fertiliser recommendations, but soil chemical properties and nutrient status, as well as tree vigour, should also be considered. Furthermore, it must be borne in mind that yield and fruit quality are not exclusively affected by mineral nutrition – all management and pest control practices affect fruit yield. For this reason, it is important to develop cultivar and region-specific norms, even up to a farm level, to assist with designing fertiliser programs (Menino, 2012; Hernandes et al., 2014).

Interpretation of leaf mineral nutrient concentrations by comparing it to a set of reference norms is often regarded as too simplistic an approach due to the variety of factors that potentially dictate leaf nutrient concentration. In the past, the DRIS was proposed as a better approach to interpret or utilise leaf analysis results for Valencia oranges in SA (Woods and De Villiers, 1992). It is based on these principles:

1. ratios of nutrient concentrations are often better indicators of a deficiency than individual leaf nutrient concentrations
2. some ratios are more important than others
3. maximal yields are only obtained when ratios are near the ideal (Jones, 1981).

The proposed benefits of DRIS diagnoses are that they reflect nutrient balances, identify the order in which nutrients limit yield, and that they can be used at any phenological stage (Srivastava and Singh, 2008). A DRIS index can be calculated for each nutrient, based on important nutrient ratios/expressions (e.g. for N as N/P, N/K, N/Ca and N/Mg, etc.). The formula is designed to allow the optimum DRIS index to be zero, while negative indices indicate deficiency.

The generated DRIS indices, irrespective of sign, can also be totalled for each sample to determine the level at which nutrition is limiting, i.e. the smaller the value the higher demand exists for that specific nutrient.

In this project the potential value of using DRIS generated norms for citrus grown in the major South African production regions, i.e. Limpopo, Eastern Cape and Western Cape, was investigated and also compared to the currently accepted generic nutrient leaf norms for fruit bearing terminals.

Data used:

Annual (2015 to 2021) leaf nutrient analysis and production data from orchards in full production (i.e. older than five years or production that exceeds 30 tonnes/ha) was obtained from producers in each major region. The leaves were sampled according to standard industry protocol, namely from fruiting terminals in March/April. Leaf mineral analysis was done according to standard protocol followed by the various commercial laboratories – accuracy of results of the laboratories is ensured through the AgriLASA inter-laboratory proficiency scheme.

Generation of DRIS indices:

For each macro nutrient, a DRIS index was created using the method proposed by Jones (1981) and validated by Woods and De Villiers (1992); Srivastava and Singh, 2008; Hernandes et al., 2014) – each data set was split into a high/ideal and low/poor sub-group on account of the mean yield (tonnes/ha) for each production region and cultivar. For each sub-group, each nutrient was then expressed in relation to the other nutrients – DRIS indices (data not shown) was then calculated using these ratios and the method described by Jones (1981).

The variance ratio for each relation between the high and low yielding sub-group was used to identify a significant ratio/expression. Furthermore, DRIS indices equal to and greater than zero for the high yielding sub-group were then used to determine optimum leaf nutrient concentrations, the mean of these concentrations (± the standard deviation) was used to establish the optimum range for each nutrient.

Outcomes:

Only expressions which differed significantly between high and low yielding sub-groups, based on variance ratio, were used in final calculation of DRIS indices (Table 1 is an example of this, as calculated for Valencia oranges in the Western Cape – Citrusdal). From Table 1, for instance, it was found that phosphorous (P) and potassium (K) leaf concentration were most significantly influenced by calcium (Ca) and magnesium (Mg) levels in Valencia trees in Citrusdal. Variance ratios for nitrogen (N) and Ca with other macro-nutrients all differed significantly.

Therefore, all N and Ca expressions were deemed as significant in affecting N and Ca leaf concentrations. From Table 1 means for each expression from the high-yielding population can then be used to determine DRIS indices for Valencia grown in Citrusdal, but ideally these means can be determined for a specific farm.

Norms generated using DRIS:

From all the DRIS indices generated for each region and variety, (data not shown) optimal nutrient ranges could be determined. With a few exceptions, these DRIS ranges corresponded well with norms provided in the CRI handbook for fertilisation of citrus (Raath, 2021) – see Table 2. The DRIS generated norms were found to be very comparable to the established norms. However, the following differences might be worth noting:

• The DRIS generated P norms were slightly higher for Valencias and navels grown in the Eastern Cape
• The DRIS generated K norms were slightly higher for most cultivars and the range was narrower in most cases – a notable exception is for mandarins in the Eastern Cape, where the DRIS generated norms are distinctly lower
• Lower Ca norms were obtained through DRIS for the Western Cape than the established norms, but higher Ca norms were established for the Eastern Cape
• The DRIS generated Mg norm lower limits were slightly higher for Eastern Cape cultivars, except for lemons.

Conclusion:

This study emphasised the value of using DRIS indices to identify nutrients that are limiting tree performance, and also to evaluate possible responses to fertiliser applications. Leaf nutrient norms that were generated using DRIS indices compared well to most of the established norms – where differences occurred, they are worth taking note of. They will be tested as more data is collected and through further data analysis. The use of DRIS can also be very useful to create nutrient norms on farm level, where less variability in management practices and climatic conditions occur, or where it is possible to note differences in orchard performance. The research team at CRI will continue to refine significant relationships/ratios using DRIS, and to further improve the leaf nutrient reference norms for the citrus industry.

Acknowledgements

The authors would like to thank and acknowledge growers from all the mentioned production regions for supplying us with leaf and production data, and their overall assistance.

References

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