Nutritional effects on fruit yield

Extensive surveys of the major kiwifruit growing areas of New Zealand have shown that nutritional disorders can result in serious losses of fruit production and in some cases affect the post-harvest storage quality of the fruit^{4,12,37,38,42,43}. A summary of the effects of various nutritional disorders on fruit yield and post-harvest storage of the fruit is given in Table 1.

Production losses resulted mainly from a reduction in fruit numbers. The average weight of individual fruit from affected and unaffected vines was not greatly different except for fruit from potassium deficient vines which weighed less than those from healthy vines. Apart from vines affected by excess boron, no nutritional disorder examined has had any serious effect on the storage qualities of the fruit. After ten weeks in cool storage brix and penetrometer measurements of fruit from affected and unaffected vines were not markedly different (Table 1). In the case of excess boron, however, there was a marked reduction in firmness of the fruit after only a short period in cool storage. Abnormally low concentrations of calcium in the boron affected fruit may be in part responsible for this premature softening.

Quantitive nutrient requirements of kiwifruit

The total requirements of each nutrient element for any crop depends on two factors: (1) the yield, and (2) the average concentration of the element in the plant tissue needed to secure that yield²⁸.

Precise information from fertiliser trials on the quantity of fertiliser needed to correct specific nutrient disorders and maintain maximum production is currently lacking for kiwifruit grown in New Zealand. In the absence of such information an alternative approach was adopted which involved calculating the approximate quantities of nutrients removed in fruit and prunings from a 10 year old orchard with a yield of 16.5 t/ha (3,900 trays/ha at 85% packout) and relating these losses to fertiliser recommendations¹⁶.

In Table 2 we have extended this approach to include sulphur (S), and the micronutrients iron (Fe), boron (B), manganese (Mn), Zinc (Zn, Copper (Cu), and molybdenum (Mo). Typical values for the mineral composition of the fruit were used in the calculations, as well as a more realistic figure for fruit yield of 25 t/ha (5,900 trays/ha at 85% packout).

Table 2: Estimated annual loss of macro- and micronutrients in fruit from a mature kiwifruit orchard. Fertiliser recommendations are those of Sale^31.

In most cases, fertiliser applications have been more than adequate to compensate for losses of macronutrients in the fruit (Table 2). However, in the case of potassium past recommendations have been barely adequate for consistently high yielding orchards, particularly when other losses such as leaching are taken into account.

Again, as has been shown¹⁶, the summer and winter prunings contain considerable amounts of potassium and other mineral nutrients. Hence removal of prunings from the orchard would further increase the likelihood of potassium deficiency.

The micronutrients in superphosphate should provide all of the iron, zinc and molybdenum, and most of the copper removed in the fruit (Table 2). The amounts of boron and manganese are appreciable also, but less than the likely annual removal in the fruit.

However, it should be appreciated that soils often contain large reserves of micro-nutrients which may render additions via fertilisers unnecessary for many years. For example, the top 20 cm of a typical Kaharoa ash soil contains enough zinc, copper, and iron in plant-available form to meet the estimated losses of these elements in the fruit (Table 2) for about 100 years, and the maganese requirement for about 60 years.

Although the reserve of boron is equal to only about nine years, this does not necessarily mean that boron deficiency is more likely to be encountered than deficiencies of other elements, since relatively large amounts of boron from airborne sea spray are deposited on the land by wind and rain, particularly in coastal areas (Table 3).

Table 3: Quantities of boron in airborne sea spray deposited on parts of New Zealand by rain and wind.

Irrigation water can also add substantial amounts of boron to the soil (Table 4). For example, at 300 l of irrigation water per vine per week a boron concentration of as little as 0.1 mg/l would supply the total boron requirement for fruit production (Table 2) in six to seven weeks (Table 4). The use of bore water naturally high in boron (>0.8 mg/l) has lead to boron toxicity (see section dealing with boron toxicity).

Table 4: Quantity of boron applied in irrigation water (g/ha/week)

While it is apparent that generalised maintenance fertiliser recommendations can be broadly based on the quantities of nutrients lost in fruit, they may not be sufficiently accurate to prevent nutrient disorders arising in every situation. For example, in young orchards, additional nutrients will be required for extension of the vine’s framework (roots, stem, leader and canes). Results from a recent experiment⁹ where young vines of varying ages were removed from the ground and the various parts of the plant’s framework (including leaves and fruit) were analyses, show that large quantities of some elements such as nitrogen and potassium are taken up even at an early age (Table 5).

Table 5: Total macronutrient uptake by young kiwifruit vines⁹.

However, it should be noted that these uptake figures in Table 5 represent the minimum quantities required by the vines, as no allowance was made for nutrient interaction with the soil, inefficiency of uptake by the roots, or for losses by leaching. Thus, it is likely that even greater quantities of nutrient than those given in Table 5 will be required to obtain maximum growth.

Timing of fertiliser additions

For kiwifruit it has been found⁴¹ that the period of greatest accumulation of potassium, nitrogen, zinc and copper occurs during the early part of the season with over 80 per cent of the maximum quantity of these elements being accumulated in the leaves on the fruiting laterals by fruit set; slightly lesser amounts of potassium and nitrogen, but not zinc or copper are accumulated by the leaves from the non fruiting shoots over the same period of growth (Table 6).

Table 6: Quantity of macro and micronutrient in leaves of kiwifruit at different stages of growth⁴¹.

By harvest, however, substantial amounts of potassium, and slightly smaller amounts of nitrogen, are lost from the leaves. These losses are greater from the leaves on the fruiting laterals than from those on the non fruiting shoots (Table 6). For zinc and copper, losses from both leaf types by fruit harvest are very small.

Although a large fraction of the phosphorus and sulphur in the leaves is also accumulated during early growth, the proportion of the maximum quantity accumulated in the leaves by fruit set is less than that of potassium, nitrogen, zinc and copper (Table 6). In addition, the losses of phosphorus and sulphur from the leaves by fruit harvest are also comparatively small.

Magnesium calcium, iron, boron and manganese differ from the other nutrients in that the rate of accumulation is similar throughout the entire season. Thus, a comparatively small proportion of the maximum quantity of these elements in the leaves is accumulated by fruit set (Table 6). The losses of these elements from the leaves by fruit harvest are also very small.

The developing fruit are undoubtedly responsible for many of the seasonal changes in the distribution of mineral nutrients in kiwifruit. Generally it has been found⁴¹ that leaves close to the fruit are the main contributors to early growth. As fruit development proceeds, a greater number of leaves provide carbohydrates and mineral nutrients for fruit filling. Consequently the substantial losses of potassium and nitrogen from the leaves, particularly from those on fruiting laterals, reflect the large demand of the developing fruit for these two elements; whereas the much smaller losses of phosphorus, sulphur, magnesium, and most of the micronutrients reflect their lower mobility in the plant and the smaller demand of the developing fruit for these elements.

It follows from the results in Table 6 that fertilisers containing potassium and nitrogen, as well as zinc, and copper, need to be applied before fruit set if deficiencies in the plant are to be avoided. This is because most of the potassium, nitrogen, zinc, and copper will have been taken up by the leaves by this stage of growth. A similar conclusion can also be drawn for phosphorus and sulphur. While it is less important for magnesium and calcium to be applied to the soil before fruit set, any inhibitory effect on the relative growth rate at an early stage of growth can have a sustained effect on the over-all growth rate and yield. Correction of iron and manganese deficiencies in New Zealand should not generally require the addition of fertilisers containing these elements, but rather the addition of compounds which will acidify the soil thereby releasing iron and manganese previously unavailable to the plant¹¹. These measures should be carried out prior to bud break to be fully effective in that season.