Diagnosis of nutritional disorders

Accurate diagnosis is essential if nutrient disorders are to be dealt with effectively. While this booklet is concerned mainly with mainly visual symptoms of nutrient deficiency and toxicity, confirmation of any diagnosis needs to be supported with evidence from soil and plant analysis.

Soil testing

To be effective, soil tests need to be carefully calibrated for the soils and crops of a particular region. The results of soil analysis should also be regarded more in terms of a qualitative guide to soil fertility rather than as a quantitative measure. This is because it is very difficult to find chemical extractants which will simulate the action of plant roots, especially since plant species differ widely in their ability to absorb nutrients from the soil.

Moreover, in many cases samples for analysis are taken from a restricted depth, usually the top 15 cm, which may not reflect the availability of nutrients from the entire root zone. However, in spite of these limitations soil tests can provide valuable information about plant-available nutrients and chemical conditions in the soil, particularly before a crop is planted.

In the case of kiwifruit, there is no definitive information at present on the optimum nutrient levels in soils for any soil test currently being used in New Zealand. Some information is available in particular districts, based on the experience of consultants concerning the local interpretation of analytical results as they relate to kiwifruit nutrition.

In the absence of well defined target levels for each nutrient in the wide range of soil types in which kiwifruit are grown, it would seem that the presence of healthy high-yielding vines ought to be the ultimate arbiter as to whether or not soil conditions are optimal for growth. Annual soil testing should be conducted therefore, with the aim of monitoring and correcting trends in nutrient levels rather than in the pursuit of attaining particular soil values.

The observed trends represent the balance between nutrients removed for vine growth and fruit production, plus losses by leaching, and nutrients added in fertiliser, irrigation water, and rainfall (airborne sea spray).

A summary of analytical data accumulated in the Soil Testing Laboratory at the Ruakura Research Centre from 1981 to 1984 makes it clear that kiwifruit tolerate a wide range of macronutrient concentrations in the soil (Table 7), but inconsistencies related to soil type do occur.

Table 7: Mean* MAF Quicktest levels^† for soils from cropping kiwifruit orchards and those being brought into production. Values in brackets represent the range.

For example, kiwifruit have been observed growing vigorously at pH values as low as 4.5 on peat soils and as high as 6.8 on calcareous alluvial soils at Gisbourne and Hastings, yet a pH of 5.2 in Ohaupo silt loam has resulted in manganese toxicity.

Seedlings on Patumahoe clay loam have shown vegetative growth responses to addition of potassic fertiliser up to a MAF Quicktest level of 20, while for other soils, values of 25 may still be inadequate to overcome potassium deficiency in mature vines because of antagonistic effects from naturally high levels of calcium and magnesium in the soil.

The ability of kiwifruit to tolerate very high levels of phosphorus in the soil (Olsen values as high as 500) while maintaining low concentrations of phosphorus in the leaves (0.20 per cent dry matter) is a further example of problems associated with calibrating soil tests against plant growth for this crop.

Although no preferred time has been found to sample soils in kiwifruit orchards for purposes of comparison it is recommended that samples be taken at the same time each year.

 Plant analysis

Plant analysis has distinct advantages over soil analysis as a diagnostic aid for a deep rooted plant like kiwifruit. Not only must the elements present in the tissues of the plant have originally been available in the soil, they also reflect the availability from the entire root zone. An additional advantage of plant analysis is that all nutrient elements essential for plant growth can be determined by this technique.

Plant analysis can be used in two important ways.

First, as a diagnostic aid for identifying possible causes of poor plant growth and for confirming visible symptoms of nutritional disorders. Sampling kiwifruit leaves for this purpose is largely independent of the time during the growing season. Leaves (blades plus petioles) showing distinctive symptoms should be collected as soon as they appear on the affected vines⁴⁰. At the same time a second sample of leaves should also be collected from an identical position on healthy non affected plants nearby. By taking an affected and an unaffected sample the results can be compared directly and possible disorders identified without having to rely upon standard values.

The second way plant analysis can be used is to monitor the nutrient status of the vine on an annual basis⁴⁰. By repeatedly sampling the vine at the same time each year possible trends in the nutrient status or the early onset of deficiencies or toxicities can be identified, allowing the fertiliser programme to be adjusted before substantial losses in yield occur.

From the results given in Figure 2 for leaves taken from high producing orchards with no obvious nutritional disorders it is possible to assess the nutrient status of leaves from fruiting and non fruiting canes at any time of the year. However, results indicate that for most nutrients, deficiencies and toxicities can best be identified early in the growing season, ie. before fruit set (one exception being magnesium deficiency which usually develops mid season). Early identification of a deficiency allows remedial action to be taken in the current season rather than in the following season as would be the case if leaves were sampled mid season.

The sampling procedure for monitoring purposes differs from that used for diagnostic purposes⁴⁰. Because of the large seasonal variation in the concentrations of macronutrients and micronutrients in the leaves of kiwifruit (Figure 2), it is important for purposes of comparison that leaf samples should be taken at the same physiological stage of growth each year. That is, the time of sampling should be measured in terms of weeks after bud break rather than on a strict calendar basis.

Leaves (the youngest fully expanded leaves on current season canes) should be collected prior to fruit set. However, if samples are to be collected after fruit set when it is likely that nutrient disorders will be diagnosed, then the second leaf past the final fruit cluster on a fruiting lateral should be taken.

In both cases leaves from at least 20 vines (two to three leaves per vine) should be collected within the area of the orchard to be monitored. It is important that the same area is monitored each year.

Interpretation of the results of plant analysis is usually based on the concept of the critical levels⁴⁹. This concept assumes that when the mineral nutrient concentration in the plant tissues is very low, the yield will also be low.

As the nutrient availability increases both yield and nutrient concentration in the tissues increase until a point is reached where further improvement of nutrient supply no longer stimulates yield. However, the concentration of nutrient in the tissues will continue to increase. At extremely high levels of nutrient supply, toxic concentrations may accumulate in the tissues and yield will be reduced. These effects are summarised in Figure 1.

In much of the literature, the critical level in the leaf for a deficiency of an element is defined as the concentration range (90 to 100 per cent of maximum yield) below which the application of that element will generally result in a yield increase, and above which no such increase is to be expected. Similarly, the critical concentration in the leaf for toxicity is that concentration above which a yield reduction is to be expected.

To date no precise information has been published on critical nutrient concentrations in kiwifruit. Meanwhile, some indication of the concentrations of individual elements required in the leaves can be gauged from the results given in this publication of concentrations found in the leaves of kiwifruit vines showing clearly recognisable symptoms of deficiency or toxicity.

 Visual symptoms

Experience with other crops has shown that visual symptoms can play an important part in diagnosing nutrient deficiencies and toxicities in the field^3,10,45. Clearly recognisable leaf symptoms associated with a specific disorder usually appear only after metabolic processes in the plant have been seriously disrupted and losses of yield already sustained (Figure 1). Hence, the presence of visible symptoms usually indicates that a serious problem exists.

Because of differences in mobility of elements within the plant, symptoms of nutritional disorders tend to occur in particular positions on the plant³.

Under conditions of deficiency, elements such as nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) are withdrawn from the older leaves and transported to younger actively growing parts of the plant. Since the redistribution of these elements is by way of the phloem, such elements are classified as phloem-mobile elements. In general, the most obvious symptoms of deficiency of phloem-mobile elements are on the older leaves.

Elements such as boron, iron, and copper which are not redistributed to any great extent in the plant under deficient conditions are described as phloem-immobile elements.

Plants must have a continuous external supply of the phloem-immobile elements to maintain healthy growth. Any interruption of this supply will cause deficiency symptoms to appear on young actively growing parts of the plant including the root tips. The remaining essential elements are of intermediate phloem mobility, but usually show symptoms of deficiency mainly on the younger growth.

Symptoms of nutrient toxicity, on the other hand, usually appear first and most prominently on the older leaves. This is because the mineral nutrients absorbed by the plant are distributed in a pattern which closely follows that of water loss due to transpiration²⁸.

The fully expanded leaves tend to receive a greater share of the water and mineral elements entering the shoots than do fruit or immature leaves, because they present a large evaporating surface relative to their volume. Because of this, the highest concentrations of the element in excess will be found in the older leaves since it is in these leaves that accumulation has been going on for longest period of time.

In addition to having a direct effect on the plant, an excess of one element may reduce the uptake of a second element or interfere with its utilisation in the plant. Under these conditions the main symptom is likely to be that of a deficiency of the second element. The symptoms, therefore, may or may not be on the older leaves.

Because leaf symptoms can also be produced or modified by non-nutritional factors such as water-stress, temperature, light, herbicides, pests, and diseases, it is important from a diagnostic point of view to distinguish these symptoms from those caused by nutritional problems. for this reason, deficiencies and toxicities of each nutrient element were induced deliberately in kiwifruit seedlings grown in a glasshouse using hydroponic techniques². Results from these studies are included in this publication to supplement and confirm the observations made in the field.

The order in which the descriptions of the individual nutrient disorders appear in the following sections is based primarily on the leaf positions where symptoms first appear.

1. Older leaves
2. Recently matured leaves
3. Younger leaves.

Within each of these groupings, the disorders have been further ranked according to the frequency with which they were seen in the field. (See: Table of Contents)