Important plant nutrients are often categorized into macronutrients and micronutrients, so called because of the rough amounts of them that are
required for healthy growth. The macronutrients are carbon (C), hydrogen (H), oxygen (O), N, P, sulphur (S), K, Ca and Mg. The first three come from air and water and will not be discussed further. Micronutrients (just as important, but not used in high concentrations) are iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), boron (B) and chlorine (Cl). A brief review of each of these follows.
With reference to pictorial symptoms of deficiency or excess, readers are referred to the excellent resources of Pearson and Goheen (1988) and Nicholas (2004). Many times the term chlorosis will occur in describing symptoms: care should be taken in interpreting chlorosis, as it can be caused by shade, ozone damage, sun scald, drought, wet soil, Eutypa, winter injury and some pesticides, as well as by nutrient deficiencies (Jordan et al., 1981). Always take into account as much information as you can obtain about a problem before diagnosis.
Macronutrients
Nitrogen
This element is a key part of many plant cell functions because it is used in making the amino acids used to build proteins, which are used to make the enzymes responsible for much of the work done in plants as well as making structural components, etc. (e.g. RuBiSCO and chlorophyll in photosynthesis).
If it is lacking, vine leaves will turn completely yellow (a form of chlorosis), lower their rates of photosynthesis and cease to grow (Ryle and Hesketh, 1969;
Keller et al., 1998; Chen and Cheng, 2003). The root to shoot ratio will also change, with lower N levels resulting in more roots produced for the same amount of shoot (Marschner, 1986). Excess N leads to overly vigorous growth (significant lateral development, long internodes and thick canes) and large and dark green leaf blades. This can have both indirect and negative effects on vine fruitfulness (Winkler et al., 1974).
Nitrogen is a mobile nutrient in the plant, so can be scavenged from one organ and transported to another that needs it more. For example, labelled N fed to vines at the beginning of growth was found in varying amounts between the various vine organs as the season progressed (see Fig. 7.1) (Conradie, 1990).
To correct deficiency, N should be applied before budbreak and possibly a second time at bloom, as these are the times when grapevines take up most N (Conradie, 1980; Schreiner et al., 2006). However, the forms of N that are available to plant roots (nitrate, NO3⫺and ammonium, NH3+) are mobile in the soil, so if excessive water falls or is applied it can be washed (leached) from the soil profile (Bergström and Brink, 1986). Therefore, in areas with springtime rainfall, N is best applied in multiple smaller amounts of fertilizer rather than as one large dose, and starting in mid- to late season rather than in early season, to maximize vine uptake (Peacock et al., 1989).
Phosphorus
This major plant nutrient is a constituent of phospholipids and nucleic acids used in cell building and is therefore abundant in meristematic regions such as actively growing shoot and root tips. It is also involved in sugar metabolism, respiration and photosynthesis. Vines are rarely deficient in this nutrient, possibly due to efficient capture from the soil and its remobilization within the plant (Jackson, 2000). Deficiency symptoms appear as yellowing of interveinal areas of older leaves, which can turn red if the deficiency is severe (Nicholas, 2004). Unlike N, P is not mobile in the soil, so there must be exploration of new soil to secure it. Fortunately soil is generally able to supply large quantities of P, which roots take up in the form of orthophosphate (PO4–3). In those soils lower in P there are often mycorrhizal associations with the roots that assist in its uptake (Possingham and Groot Obbink, 1971). Phosphorus is frequently applied to maintain vineyard cover crops, but application of P fertilizer can result in lower K levels in the vine (Conradie and Saayman, 1989) and so should not be applied without consideration of the available K supply (Haeseler et al., 1980).
Spring-applied N Total N
Spring-applied N Total N
Spring-applied N Total N
7%
44%
44%
32%
19% 41%
45%
12%
3%
5%
36%
13%
55%
3%
22%
6%
45%
24%
25%
50%
30%
17%
3%
20%
(5853) (1132)
(9582) (1076)
(7683) (1140)
Permanent wood
Leaves &
shoots Bunches
Roots
(a) (b)
(c)
Fig. 7.1. Remobilization of spring-applied nitrogen in sand culture-grown grapevines. The charts indicate the relative proportioning of nitrogen between permanent wood, roots, shoots and leaves and fruit (when present) at harvest (A), start of leaf-fall (B) and end of leaf-fall (C). The numbers in parentheses indicate the total amount of nitrogen/vine (mg) (adapted from and reproduced with permission, Conradie, 1990).
Sulphur
Sulphur is taken up by roots in the form of sulphate (SO4–), though it can also be taken up by leaves when it is in its gaseous form (De Kok, 1990), which is the active form of S that prevents powdery mildew infection. Many plant proteins contain S: for example, cystine is a major S-containing amino acid. Sulphur- containing compounds called thiols have been found to be important determinants of white wine aroma, lending box tree, citrus or cooked leek characteristics to the mix (Tominaga et al., 2000). However, S is rarely deficient in grapevines as it is frequently applied in a spray for control of powdery mildew.
Potassium
Potassium is a very important element in protein and fat synthesis, enzyme activation and as an osmotic charge balancer. Its uptake form is the cation, K+, and is used in large quantities by fruit crops, especially. The quantity removed with a moderate crop of grapes is frequently greater than that of N (Winkler et al., 1974). Xylem exudate, sap that bleeds when vines are cut close to budbreak, is high in K (Glad et al., 1992), which is evidence that the roots are actively absorbing soil nutrients.
Vines with K deficiency usually develop interveinal or marginal chlorosis, or both. At more severe deficiencies, marginal necrosis and leaf cupping develops (Nicholas, 2004). False K deficiency (also known as spring fever) is a temporary appearance of symptoms on basal leaves prior to bloom and is associated with cool and cloudy weather conditions, which cannot be corrected through the application of K fertilizers (Christensen et al., 1990). It is thought that N reduction is lowered under these conditions and an accumulation of ammonium ions results, which interferes with potassium’s role in protein synthesis (Maynard et al., 1968). In sand culture, excess of K results in interveinal chlorosis of basal leaves, slightly reduced shoot growth but unaffected root development (Li, 2004). There are also problems with high juice pH associated with excess K in grapevines (see Fig. 7.2; Mattick et al., 1972), so K should be kept within moderate levels in the soil.
Potassium deficiency symptoms are known to be corrected by potash (which is any of a number of K salts) application (Shaulis, 1961), and any deficiency should be corrected as soon as symptoms become visible. Potassium chloride (muriate of potash) is usually the most economical source of K, but in growing areas with high salt concentrations it can result in chloride toxicity, so potassium sulphate is the recommended alternative in this case. Deficiencies of K in low-pH soils may be caused by excess application of Mg (Bates et al., 2002), as these two cations compete for the same uptake channels in root tissues.
Calcium
Known as the cement of plant structure, Ca is a component of the middle lamella that holds plant cells together and maintains cell integrity (Epstein,
1961). It also participates in both carbohydrate translocation and N utilization (Gossettet al., 1977). Calcium is infrequently deficient in vineyards (Nicholas, 2004), but applications of calcium sulphate (gypsum) are used to increase soil pH and to treat high-sodium soils (Winkler et al., 1974).
Magnesium
The central mineral element in the chlorophyll pigment is Mg, which assures the importance of this nutrient in plant growth. Magnesium also functions in chlorophyll synthesis, enzyme activation and can play a similar role to Ca in membrane stability. It also is important in the partitioning of carbohydrates from the leaves to the roots (Cakmak et al., 1994). Deficiency is most severe on basal parts of the shoots due to its remobilization to newer parts of the vine. Symptoms of moderate deficiency are chlorosis between the large veins, which remain green, as does the tissue at the leaf margins (Pearson and Goheen, 1988;
Nicholas, 2004). Severe deficiency results in chlorotic leaf margins and red coloration in red grape varieties (Nicholas, 2004). Magnesium deficiency is most common on very low-pH soils and is increased by excessive K fertilization (Bates et al., 2002). Regardless of pH, however, high soil K interferes with the uptake of Mg (Wolf et al., 1983) and should be avoided. To correct deficiencies, soil pH can be raised (particularly with dolomite, also known as calcium magnesium carbonate ⫺CaMg(CO3)2) or magnesium sulphate ripped into the soil.
3.50 3.45 3.40 3.35 3.30 3.25 3.20 3.15 3.10 3.05 3.00
Juice pH
0.100 0.120 0.140 0.160 0.180 0.200 0.220 0.240 0.260 0.280 Potassium (% by weight)
Fig. 7.2. Relationship between grape potassium and juice pH for ‘Concord’ grapes taken from vines of differing potassium status. As potassium concentration in the fruit rose, so did pH, in a linear fashion (adapted and used with permission, Mattick et al., 1972).
Micronutrients
Iron
Although not present in the chlorophyll molecule, Fe is required for chlorophyll synthesis and is also involved with electron transport in respiration. Iron deficiency is observed first in early-season chlorosis in the youngest leaves, which causes reduced photosynthesis (Smith and Cheng, 2007). Iron shows very low mobility within the plant, so symptoms appear in developing plant tissues. Iron chlorosis is often ephemeral, appearing when growth is rapid and soils are cold and more likely to be wet, which limits the ability of the vine to take it up (Tagliavini and Rombolà, 2001). As the soils dry out and warm up, supply from the soil improves and chlorosis no longer occurs in the newly forming leaves. Chlorosis can occur even when the soils have adequate Fe, for different reasons, however (Mengel et al., 1984; Davenport and Stevens, 2006). Chlorotic symptoms are greatly increased in high pH (alkaline)-soils, so plant nutrient analysis under these conditions will detect low concentrations of Fe. Iron chlorosis-resistant rootstocks are available for grapevines, but often result in excessive growth and reduced grape yield (Tagliavini and Rombolà, 2001).
Therefore making sure soil pH allows for Fe uptake by the rootstock desired is important. Vesicular-arbuscular mycorrhizae can ameliorate the soil immediately surrounding the roots (the rhizosphere), altering its pH and increasing nutrient availability (Bates et al., 2002), and thus providing another reason to encourage their presence. In most cases Fe deficiencies are more easily corrected by iron salt or iron chelate sprays rather than by soil amendments.
Manganese
Manganese is essential in the synthesis of chlorophyll and plays a role in enzymatic reduction⫺oxidation reactions that power most functions in the plant tissues. Manganese (and Fe) deficiencies are usually found on high pH-soils, with symptoms of Mn deficiency identified by interveinal chlorosis on shaded and mature leaves, as this mineral can be remobilized within the plant. Manganese sulphate applied to the soil under the vine will correct Mn deficiency, although plant analyses should be conducted in following seasons to track changes in plant concentration. The use of mancozeb for disease control can maintain Mn levels in plants (Deckers et al., 1997), as mancozeb is a coordinated product of Zn (1.9%), Mn (15%) and ethylene bisdithiocarbamate ion.
Copper
Copper is a non-protein component of several oxidizing enzymes such as ascorbate oxidase and tyrosinase, and forms an important part of the electron- transport chain in photosynthesis. Deficiency symptoms for Cu in grapevines are similar to those of Fe-induced chlorosis and, like Fe, it is not moved from one place to another within the plant. Copper is generally very toxic to many organisms, and one reason that it is not frequently found to be deficient in
vineyards is that Bordeaux mixture (copper sulphate and hydrated lime) is very often applied to vines in the control of downy mildew. It is possible for high Cu levels in the soil to reduce root growth and development (Wainwright and Woolhouse, 1977), but analysis of above-ground parts of plants does not seem to indicate high soil and root Cu status (Brun et al., 2001). Therefore, if high Cu levels in soils are suspected, the nutritional status of the root tissues should be determined.
Zinc
Zinc is important in the synthesis of the growth regulator indole acetic acid, is a component of many different enzymes and plays an important role in pollen and fruit formation (Marschner, 1986). Zinc is not very mobile within the plant, so if the nutrient becomes deficient part-way through a season, younger rather than older plant tissues will show symptoms. Zinc deficiency is characterized by abnormal development of internodes (‘zig-zag’ growth pattern of shoots), interveinal chlorosis in early summer, small leaves (also known as little leaf) and a widened petiolar sinus (see Fig. 7.3). Straggly clusters with undeveloped shot berries and generally poor fruit set are also common, and can occur without the appearance of leaf or shoot deficiency
Fig. 7.3. Shoot symptoms of zinc deficiency. Note the small, misshapen leaves that have an open sinus, short internodes and jagged leaf margins.