• Plant Nutrition
• Soils For Horticulture
• Estimating Nutrient Requirements
• Fertiliser Materials
• Calibrating Application Equipment
• Monitoring Fertility
• Further Reading

While fertiliser costs are usually a relatively small fraction of the total production costs in horticulture, incorrect decisions as to the quantity or nutrient composition of the fertiliser applied can have a disproportionate effect on the profitability of the venture. There are many recorded examples in New Zealand where considerable losses of production and in some cases, additional effects on postharvest storage of the crop, have resulted from nutritional disorders arising from such decisions.

This bulletin brings together information on fertiliser requirements of horticultural plants commonly grown in New Zealand. It has been written primarily for advisors and growers with the aim of answering the question; what types and rates of fertiliser are needed to maximise production of my crop? The information is largely confined to conventional solid fertilisers (the basis of most blends), but comments are included on alternative forms such as foliar fertiliser where reliable results are available.

Information on special features, nutrient composition of leaves, fertiliser requirements, and symptoms of common nutritional disorders have been included for each crop where it is available. Additional sections have also been included on soils for horticulture, estimating nutrient requirements, monitoring the nutrient status of the crop with soil and plant analysis, and fertiliser materials and methods of application.

The reader should be aware, however, that for some crops, there are large gaps in our knowledge about their nutrient requirements. In these cases, the recommendations given are very tentative. Sometimes the only information available is from overseas sources. It is intended to update the information when new knowledge becomes available.

PLANT NUTRITION

Essential elements

For an element to be considered an essential plant nutrient, three criteria must be met. These are:

1. A deficiency of the element prevents the plant from completing its life cycle.
2. The deficiency is specific for the element in question.
3. The element is directly involved in the nutrition of the plant, as for example a constituent of an essential metabolite or required for the activity of an enzyme system.

The following chemical elements are known to be essential for higher plants:

Carbon	C	Boron	B
Hydrogen	H	Chlorine	Cl
Oxygen	O	Copper	Cu
Nitrogen	N	Iron	Fe
Phosphorus	P	Manganese	Mg
Sulphur	S	Molybdenum	Mo
Potassium	K	Zinc	Zn
Calcium	Ca
Magnesium	Mg

Plant nutrients may be divided into macronutrients and micronutrients. Macronutrients (C, H, O, N, P, S, K, Ca, Mg) are needed in plants in greater amounts than micronutrients (B, Cl, Cu, Fe, Mn, Mo, Zn). Plants may also contain considerable concentrations of non-essential elements such as aluminium which, in some cases, may be toxic to them.

SOILS FOR HORTICULTURE

The suitability of a particular area of land to grow any crop depends primarily on the climatic conditions at the site and on the physical properties of the soil. The inherent chemical fertility of the soil is less important since nutrient deficiencies can normally be corrected with fertilisers. An exception to this general statement is the presence of calcareous horizons which can induce deficiencies of Fe and Mn and preclude the cultivation of ericaceous species. Soil salinity is another chemical property which restricts the suitability of land for cropping.

While some aspects of climate can be modified by, for example wind breaks or frost control procedures, the specific climate requirements of most commercial crops normally controls their geographical distribution.

Crop plants are usually less specific in their soil physical requirements, so that we can describe properties which make a soil suitable, or potentially suitable for crop production in general.

Soil physical properties and plant requirements

Plants depend on the soil for anchorage, water and nutrients while the roots of most plants require an external supply of oxygen. Space in which roots can grow, the removal of excess water, the retention of adequate supplies of available water and the rapid exchange of air between the root surface and the soil surface all depend on a network of interconnecting soil pores with a wide range of sizes. While this pore system must be reasonably stable, pores must not be so rigid as to prevent penetration and deformation by roots. Texturally, the most suitable soils will vary from sandy to clay loams, will have a friable consistency and a stable, granular structure. The presence of compacted or cemented horizons within the normal rooting depth of the crop to be grown will restrict the movement of air, water and roots and effectively limit the depths of soil which can be exploited by plants. Such layers are still undesirable even if well below the normal range of roots. This is because they cause perched water tables to develop in wet seasons which also decrease the effective depth of available soil.

Soil temperature, which influences microbial activity, root growth and the length of the growing season, is influenced by climate, latitude, soil colour and drainage. Light coloured or poorly drained soils will warm more slowly in spring than those with darker colours and better drainage.

Soil properties and management

Topography influences the suitability of soils for cultivation with slopes greater than 12° causing problems with wheeled vehicles. While sloping land has the advantage of shedding cold air and decreasing frost hazards, it also increases the risk of erosion. Water infiltration rates and the stability of surface structure also influence erosion. The possible frequency, intensity and duration of flooding must be assessed from climate data and the internal and surface drainage characteristics of the soil.

Recontouring land to decrease slopes must be done with great caution to avoid exposing undesirable subsoil materials or producing artificial profiles with poor physical properties.

Ease of cultivation is influenced by soil compaction, the stability of aggregation, stoniness, wetness and the bearing capacity of the soil.

The need for drainage and the suitability for drainage can be assessed from soil properties already described. Similarly, the need for irrigation and the method, frequency and intensity of irrigation are influenced by soil porosity through its control of water infiltration, drainage and retention.

Assessment of soils in the field

When assessing the suitability of soils for horticultural use, the principal physical properties which should be considered are slope, rooting depth, porosity, aeration, texture and the presence of compacted or cemented horizons. Aeration can be assessed by soil colours: warm brown colours imply good aeration, cold greys imply poor aeration and rusty mottles indicate alternate periods of anaerobic and aerobic conditions.

Systems for assessing, quantifying and classifying soil properties have been devised. Limited investigations of soil properties can be undertaken by Landcare Research NZ Ltd.

Distribution of soils for horticulture

Leamy (1974) discussed the classification of New Zealand soils for horticulture and defined those with high actual or potential value for food production. The distinction between actual and potential value is based on the amount of amelioration required. Bollard (1981) lists the areas of soils in these categories in the main horticultural areas of the country. While the total area of such soils comprises 10% of the land area of New Zealand, the area actually used for fruit or vegetable production (in 1980) varied from 0.2 percent of the possible area in Southland, to 30% in Marlborough and averaged 3.4 percent for the whole country. The total area in horticulture (nursery, vegetables and fruit is 101,429 hectares (1994 - Statistics NZ). This has not changed markedly from ten years ago (95,272 ha in 1983 - Statistics NZ). Fruit makes up 45,862 ha of the total; vegetables 55,111 ha and indoor production 456 ha (1994 - Statistics NZ) - this includes fruit/ vegetables, nursery and flowers.

ESTIMATING NUTRIENT REQUIREMENTS

The total requirement of each nutrient element for any crop depends on the yield and the average concentration of that element in the tissues of the plant needed to secure that yield. In some instances detailed information from fertiliser trials on the quantity of fertiliser needed to maintain maximum production is currently lacking, particularly for the diverse soil types and climates which exist in this country. In the absence of such information an alternative approach has been adopted which involves estimating the quantity of nutrients removed in the marketable yield. Nutrients lost in this manner represent the minimum amounts needed to be replaced from soil reserves or fertiliser additions each year if yields are to be maintained. Allowances also need to be made for nutrients removed from the root zone by leaching and for the efficiency with which crops can absorb nutrients applied in fertilisers. The extent to which nutrients are transported down the soil profile varies considerably between soils. Freely drained soils are very prone to nutrient removal by leaching. Of the major plant nutrients, phosphates are leached at the slowest rates while nitrates are readily leached. Fertiliser nutrients are also rendered at least temporarily unavailable to plants if they are strongly adsorbed by soil particles or are incorporated into organic materials in soils.

FERTILISER MATERIALS

Having determined the amount of each nutrient element required to maintain maximum production, recommendations must be converted into rates of commercially available fertiliser materials.

All fertilisers are registered and sold in New Zealand under the element rating scheme which is expressed as the percentage of NPK in the material. In some countries however, nutrient content is expressed in terms of the oxide concentrations; P₂O₅ for P and K₂O for K. Nitrogen is expressed internationally as elemental N. These differences can lead to confusion as the P₂O₅ content is more than twice the P content. Conversion factors are as follows:

Fertiliser types

The choice of fertiliser to be used depends on:

1. Nutrient content: Tables 4-9 list fertiliser materials and show their nutrient contents. In addition to these materials, there is a range of proprietory mixtures available which contain various combinations of the plant nutrients.

2. Nutrient concentrations: These are also listed in Tables 4-9. These figures are needed to calculate fertiliser rates from requirements given in elemental terms (see below), and to calculate the relative value of fertiliser materials in nutrient terms.

3. Cost per kg of nutrient: This is calculated by dividing the cost per tonne of fertiliser by its nutrient concentration multiplied by 10.

   For example:

   Urea 46% N, $502/tonne

   Cost per Kg N = 502 / (46 x 10) = $1.09

   Calcium Ammonium Nitrate (CAN), 28% N, $434/tonne

   Cost per Kg N = 434 / (28 x 10) = $1.55

   In this example, urea is about two thirds the cost per unit of N than CAN.

4. Nutrient availability: This refers to both the proportion of nutrients applied in fertilisers which is available to plants, and the rate at which they become available. Water soluble nutrients (such as the K in all K fertilisers) are immediately available for either uptake by plant roots, to react with soil materials, or to be washed out of the root zone. Some water insoluble nutrients such as N in organic fertiliser must be released by microbial action before they are available to plants. Other water insoluble nutrients dissolve so slowly that they are of little value to plants, while in others the rate of release of nutrients can satisfy the plants demand. Variable nutrient availability is most important when selecting sulphur and phosphate fertilisers.

   Sulphate-S (superphosphate, ammonium sulphate) should be used when an immediately available supply is required and when leaching losses are not likely to be serious. Elemental S should be used when a more sustained supply is needed, especially on free draining soils in high rainfall areas. Sulphur fortified superphosphate contains both forms.

   Phosphorus applied in superphosphate is rapidly available to plants. The slower release of plant available P from reactive rock phosphates such as those from North Carolina or Sechura, in Peru, may provide less initial benefit to plants but will have an effect for a longer period.

   All forms of N fertiliser generally give similar responses per unit of material.

   Magnesium fertiliser sources include highly soluble materials such as Kieserite and Epsom salts which are best used for correcting existing deficiencies, and slow release materials such as calcined magnesite and dolomite which may be more suitable for building up or maintaining soil reserves.

5. Physical form: Water insoluble materials like elemental S and some rock phosphates must be finely ground if they are to release their nutrients fast enough for plant requirements. However, fine grinding increases handling problems and in the case of S, the risk of explosions. Compromises in which finely ground rock phosphates are formed into granules which decompose in soil, or in which molten elemental S is used to fortify superphosphate are desirable and it cannot be overemphasised that coarse particles of S or phosphate rocks are of little value as fertilisers, especially in dry, cool conditions; slightly coarser S can be used in warm humid climates.

6. Organic fertilisers: Poultry, sheep, cow and stable manure all have a valued contribution and as they have varying degrees of nutrient content it is not easy to give any precise figures. Typical NPK rating of these materials are in Table 1.

   Table 1: Approximate composition of organic materials.

   	N %	P %	K %	Organic Matter %
   Cow manure	0.7	0.2	0.5	30
   Horse manure	0.7	0.15	0.4	60
   Pig manure	1.0	0.3	0.7	30
   Sheep manure	2.0	0.5	2.3	60
   Poultry manure	1.6	0.6	1.6	50
   Seaweed (kelp)	0.2	0.05	0.5	80
   Grain straw	0.6	0.10	1.05	80
   Fish meal	5-10	1	-
   Wood ashes	-	1	5
   Sewage sludge	5	2	0.5
   Seaweed (kelp)	0.6	-	1
   Garden compost	3	3	1
   Mushroom compost	0.5	0.2	0.9

7. Clear liquid fertilisers: There are a variety of brands on the market which are sold for foliar application and in some cases for inclusion in irrigation systems. They are expensive for using as base nutrient fertilisers but can be valuable as carriers for micronutrients and as foliar feeds to correct nutrient deficiencies. Some materials suitable for clear liquid fertilisers are in Table 2.

   Table 2: Materials suitable for preparing clear liquid fertilisers.

   	N %	P %	K %
   Potassium phosphate	0	21	25
   Diammonium phosphate (technical grade)	21	23	0
   Phosphoric acid 52% P2O5	0	22	0
   85 % P2O5	0	37	0
   Ammonium nitrate crystals	35	0	0
   Calcium ammonium nitrate crystals	28	0	0
   Potassium nitrate	14	0	39
   Urea	46	0	0
   Potassium sulphate	0	0	40

8. Suspension fertilisers: Suspensions compete on a nutrient price basis with solid NPKS fertilisers. They are very useful for accurate row crop application and placement especially under adverse weather conditions when high analysis NPK fertiliser becomes wet and sticky. They are formulated on an ammonium phosphate base with muriate of potash used for the K source. Extra N is added as urea.

9. Controlled and slow release fertilisers: These are used for high value crops. A wide range of controlled and slow release fertiliser materials is available, varying in release rates from a few months to more than a year. Use of these materials is primarily for one or more of the following reasons:
   (i) Availability of nutrient during the entire growing season from just one application
   (ii) Reduced capital and labour outlay
   (iii) Reduced nutrient loss from leaching
   (iv) Reduced leaf burn from heavy rates of fertiliser applications
   (v) Reduced seed and seedling damage from high local concentrations of salts
   (vi) Reduction of nitrogen losses through ammonia volatilisation and denitrification.

   The availability of fertiliser material is controlled either by using compounds with limited water solubility or by coating soluble materials with polymers or S. The release rate of controlled release fertilisers is affected by temperature. For example the release of N from Nutricote (270 days) was 16% at 20°C but 45% at 32°C. Release times for Osmocote and Nutricote are based on average soil temperatures of 21°C. The release rate of nutrients from slow release fertilisers is affected by a number of factors including temperature, moisture content, pH and microbial activity. Detailed release rates for various commercial compounds are given in Tables 3a and 3b.

   Table 3a: Release rates of nutrients from controlled release fertilisers* at 21°C.

   Product name	Release time (months)	% N	% P	% K	Other nutrients added
   Nutricote total Nutricote total Nutricote total (Low N) Nutricote Microsize Nutricote Mix 100 Nutricote 360 day Nutricote 270 day blend Nutricote 360 day superblend Osmocote Plus Osmocote Plus "High K" Osmocote Plus Osmocote Plus "Low P" Osmocote Plus Osmocote Mini Osmocote Nursery Mix Agroblen Agroblen Agroblen Agroblen Agroblen Agroblen Agroblen	3/4, 5/6, 8/9, 12/14 5/6, 8/9, 12/14 3/4, 5/6 2/3, 4/5 3/4 12/14 8/9 8/9 3/4, 5/6 5/6, 8/9 8/9 8/9 12/14, 16/18 2/3, 5/6 8/9, 3/4 3/4 5/6 8/9 8/9 Strawberry 8/9 Fruit 12/14 12/14 Strawberry	13 18 8 12 16 16 18 19 15 10 16 17 15 18 19 13 15 16 21 19 15 19	5.7 2.6 5 5.2 4.4 4.4 4.4 4.4 4.8/4.4 4.8 3.5 1.6 3.9 2.6 2.6/3.0 5.7 3.9 4.4 3.0 3.0 3.9 2.1	10.8 6.6 15 10.1 8.3 8.3 8.3 8.3 10.8/10 15 10 8.7 9.1/8.2 10/9.1 8.3/8.5 10.8 9.1 7.5 8.0 9.0 7.5 8.3	1.2% Mg + TE 1.2% Mg + TE 1.5% Mg + TE 1.2% Mg + TE - - - - 1.2% Mg + TE 0.9% Mg + TE 1.2% Mg + TE 0.6% Mg + TE 1.2% Mg + TE 25kg/10kg Fe 1.8% Mg 1.8% Mg 3.0% Mg 1.8% Mg 1.8% Mg 1.8% Mg 1.8% Mg
   * Controlled release fertilisers, Osmocote and Nutricote, are temperature controlled and release rate is based on an average soil temperature of 21°C under New Zealand conditions. Tabulated and specialist controlled release fertilisers supplying one nutrient are also available but not listed. TE = trace elements

   
   

   Table 3b: Release rates of nutrients from slow release fertilisers* at 21°C.

   Product name	Release time (weeks)	% N	% P	% K	Other nutrients added
   Progrow Tribon Floranid Permanent	8-10 8-10 8-10	24 16 15	3 3 4	6.6 10 13	+ TE +TE 5% S +TE
   * Slow release fertilisers' release rate is affected by a number of factors including pH, moisture, temperature and microbial activity. TE = trace elements

   
   

10. Solid Fertilisers

    Many fertilisers contain more than one nutrient. Generally, they have been grouped in tables under the nutrient of highest percentage content.

    Table 4: Nitrogen fertilisers

    	N%	P%	K%	S%	Equivalent Acidity
    Ammonium sulphate Ammonium nitrate (prills) Ammonium nitrate (cryst.) Ammoniated super Blood and bone Bone dust Calcium ammonium nitrate (CAN) Crop Fertiliser Diammonium phosphate (DAP) Dried blood Isobutylidene di-urea (IBDU) Liquid nitrogen (urea solution) Monoammonium phosphate (MAP) Potassium nitrate Sodium nitrate Sulphur coated urea Urea Ureaform Nitrophoska (12-10-10) Nitrophoska Blue TE (12-5-14)	21 33 35 6-7 5-8* 3-5* 26-28 5 18 13-15* 32* 20 12 14 16 32 46 38** 12 12	- - - 6 5-8 9-11 - 4 20 - - - 21 - - - - - 10 5	- - - - - - 5 - - - - - - 39 - - - - 10 14	24 - - 15 - - - 12 2 - - - 2 - - 27 - - 1 4	110 60 60 88 - -20 - - 74 23 - 40 55 -23 -29 - 79 60 - -
    * Insoluble in water ** Partially soluble in water

    
    

    Table 5: Phosphate fertilisers

    	Total P%	Plant Available P%	Water Soluble P%	Total S%
    Diammonium phosphate - DAP Superphosphate Super Plus Magphos (5% Mg) Monoammonium phosphate (MOP) Sulphur super Sulphur super extra** Serpentine super Triple superphosphate Reactive phosphate rock (RPR) Egyptian Gafsa Sechura	20 9 - 9.5 15 8 21 7 7 - 7.5 7 - 7.5 20 - 21 13 13 13 - 14	20 8 - 8.5 14 9 21 7 7 5 - 6 16 - 19 * * *	18 - 20 7 - 8.5 14 4 19 - 20 7 6.5 1 - 2 16 - 19 Nil Nil Nil	2 10 - 12 7 11 2 30 28 9 1 - 2 1 Neg 1
    Neg = negligible * % availability in first year will depend on particle size and soil pH. ** Note most fertiliser companies will provide a number of blends of sulphur super fertiliser. The sulphur and phosphate contents of these will vary. The two listed above are examples. Note: Citric solubility: The citric soluble P in superphosphate-based materials (other than those containing reactive rock phosphate) is the best estimate of agronomic effectiveness currently available. However, it may be changed in future if research develops better indicators of phosphate value. Note: The value of P in rock phosphates depends on the reactivity and the particle size and the soil pH.

    
    

    Table 6: Potassium fertilisers

    	N%	P%	K%	S%
    Potassium chloride (Muriate of potash) Potassium nitrate (Fertiliser grade) Potassium sulphate	- 13 -	- - -	50 37 - 38 40 - 42	- - 17 - 18
    NB: Fertiliser mixes containing potassium and phosphate are commonly used; eg, 30% potash superphosphate. These will usually contain sulphur as well as phosphate and potassium, and may have magnesium added too.

    
    

    Table 7: Sulphur fertilisers

    	N%	P%	K%	S%
    Ammonium sulphate nitrate Gypsum (Calcium sulphate) Magnesium sulphate Sulphur super Superphosphate Super/lime 50/50 Durasul* Maxi sulphur super Sulphur bentonite	26 - - - - - - - -	- - - 7 9 - 9.5 5 - 5 -	- - - - - - - - -	14 18 13 30 10-12 6 100 50 90
    NB: All fertiliser mixtures containing superphosphates, potassium sulphate or ammonium sulphate also contain sulphur. * These products contain elemental Sulphur and the release rate of elemental Sulphur will vary depending on the Sulphur fineness and location.

    
    

    Table 8: Magnesium fertilisers

    	Mg%
    Epsom salts (Magnesium sulphate) Dolomite Kieserite Calcined magnesite (Magnox, Calmag) Granmag Magnesium chelate (EDTA)	10 11.5 15 50 30 6

    Table 9: Trace element materials

    Element	Material	Elemental content%
    Boron Cobalt Copper Iron Manganese Molybdenum Zinc	Borate -64 Borax pentahydrate Cobalt sulphate Cobalt sulphate prills Solubor, Fbor, Timbor Copper oxide Copper sulphate Copper chelate (EDTA) Iron chelate (EDTA) Ferrous sulphate Manganese sulphate Manganese chelate (EDTA) Sodium molybdate Zinc sulphate hepta Zinc sulphate mono Zinc chelate (EDTA)	13 14.5 21 10 20 25 23.5 15 13 20 24 13 39 22 35 15

Application Rates

Having chosen the type of fertiliser to use, calculate the rate to apply from the estimate of the elemental requirement and the concentration of the available nutrient in the fertiliser:

For the N, K, S and Mg fertilisers use the total concentration of the nutrient in this calculation. For example, a kiwifruit crop requiring 100 kg N/ha; how much urea (46% N) should be applied?

The situation is less simple for phosphate fertilisers. For superphosphate and rock phosphate based materials (Tables 5 and 6) fertiliser requirements should be calculated by multiplying the kg/ha of P required by a factor based on the concentration of P in the material that is plant available rather than the total P content. Estimates of the concentration of plant available P in phosphate materials are given in Tables 5 and 6.

For example, a citrus crop requiring 68 kg P/ha; how much 15% Potassic Magphos (Plant available P% = 7.7) should be applied?

Equivalent acidity is the number of parts by weight of calcium carbonate required to neutralise the acidity caused by using 100 parts of the fertiliser. Negative values indicate the liming effect of the fertiliser.

CALIBRATING APPLICATION EQUIPMENT

Updated by John Maber, Lincoln Ventures Ltd

The evenness of fertiliser application is most important, especially to crops, to ensure a maximum yield.

The flow rate of fertiliser from spreaders is usually controlled by a ground driven wheel so that the faster you go the greater the flow, and a constant application rate per hectare is obtained. However, to measure evenness of spreading accurately, metre square boxes or trays with baffles to stop the fertiliser bouncing out can be laid on the ground and the machine driven past them. The baffles can be made from any 'honeycomb' like material. Some hollow plywood household doors have a spacing material inside them which is ideal for this purpose. These tests will give a good idea of the evenness of spread across the swath. However, to get precise data, the tests should be done in still air (inside a large building), and the tray weight data used to calculate a coefficient of variation for the spread pattern at different track spacings.

Where row cropping is undertaken the fertiliser applied in the row is easily measured when the planter is in the paddock. For 75 cm (30 inch) rows the machine will have to move 13.3 m to equal 1/1000 of a hectare. By removing the tube which delivers the fertiliser from the box to the disc coulter and tying a plastic bag on the end, then driving a measured 13.3 m, the weight of the fertiliser in the bag (kg) represents the rate per hectare when multiplied by 1000.

When it is necessary to determine application rates for small areas, rates given in kg/ha can be converted to g/m² by dividing by 10. Similarly, tractor speeds in km/hr can be converted to m/min by multiplying by 16.67.

MONITORING FERTILITY

While generalised maintenance fertiliser recommendations can be based on the quantities of nutrient lost in the marketable yield, they may not be sufficiently accurate to prevent nutrient disorders arising in every situation. Furthermore, for young plants additional nutrients will be required for development of the plant's framework. Therefore, it is essential to monitor closely the nutrient status of the crop with soil tests 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 a 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. In addition, 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, particularly for deep rooted perennial plants. However, in spite of these limitations, soil tests can provide valuable information about plant available nutrients and chemical conditions in the soil, especially before a crop is planted.

A soil sample for analysis consists of 15-20 cores taken at random. Prior to establishment of deep rooting perennial crops, it is advisable that samples be taken at 3 depths; 0-20, 20-40 and 40-60 cm, rather than just the standard 0-15 cm used for horticultural crop species. Under these conditions the tests are particularly useful for establishing whether or not gross nutrient abnormalities exist and whether there are unusual pH conditions at depth which will affect the uptake of trace elements such as Mn. This provides an opportunity to significantly amend the subsoil prior to planting. Detailed instructions for soil sampling are given in some crop sections.

There is little definitive information available on the optimum soil test values for most horticultural crops grown in New Zealand. In the absence of well defined target values for each nutrient, it would seem that the presence of healthy high yielding plants should 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 values rather than in the pursuit of attaining particular soil test values. One exception to this advice is for field grown vegetable crops. Work at Levin Horticultural Research Station has allowed the definition of target values for soil P and K; these are given in the sections of the bulletin covering these crops. For other crops, particularly perennials, observed trends in soil fertility represent the balance between nutrients removed for growth and production plus losses by leaching or fixation, and nutrients added in fertiliser, irrigation water, and in rainfall. Little work has been done to define the best time during the growing season for taking soil samples for analysis. However, in the absence of generalised information, it is recommended for purposes of comparison, that samples should be taken at the same time each year, and not within three months following fertiliser or lime application. It is wise to test at least one month prior to planned fertiliser application.

Soil test values used here refer to MAF quick test soil units. While some commercial laboratories use MAF soil test methods, others do not, and it is important to determine which methods have been used. Factors for converting MAF quick test units to other units are given in Appendix Table 3.

Plant analysis

Plant analysis has distinct advantages over soil analysis as a diagnostic aid for horticultural plants. not only must the elements present in the plant originally have been avialable 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 should be used in two important ways:

First, as a diagnostic aid for identifying possible causes of poor plant growth and to confirm visible leaf symptoms of suspected nutritional disorders. Sampling leaves for this purpose is independent of time during the growing season. Leaves showing distinctive symptoms should be collected as soon as they appear on the affected plants. 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 unaffected sample the results can be compared directly and possible disorders identifed without having to rely upon standard values. Early identification of a deficiency also allows remedial action to be taken in the current season rather than the following season.

Secondly, plant analysis is valuable as a monitoring aid. An essential part of any fertiliser programme is to monitor the nutrient status of the crop on an annual basis. By repeatedly sampling plants at the same time each year possible trends in 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. The sampling procedure for this purpose differs from that used for diagnostic purposes. To be meaningful, leaf samples should be taken at the same physiological stage of growth each year. This is because of the large seasonal variation in the concentration of macronutrients and micronutrients which generally occur in the leaves of most plants. It is also important that the same area is monitored each year.

Optimum plant analysis ranges over the growing season are available for a range of crops - kiwifruit, nashi, persimmons, tamarillo, nectarine/peach, apricot, avocado, asparagus and apple (royal gala). Contact the laboratory to check recommended sampling times.

After collection, leaf samples should be sent as quickly as possible to a laboratory that specialises in plant analysis. The samples should be sent in strong paper bags in preference to sealed plastic bags where they are more likely to decay.

Interpretation of the results of plant analysis is usually based on the concept of critical levels. This assumes that when the nutrient concentration in the plant tissues is very low, the yield is also low. As nutrient availability increases, both yield and nutrient concentration in the tissues increases until a point is reached where further improvement of nutrient supply no longer stimulates yield. However, the concentration of nutrient in the tissues continues to increase. At extremely high rates of nutrient supply toxic concentrations may occur in the tissues and yield will be reduced. In much of the literature, the critical concentration in the leaf for deficiency of an element is defined as the concentration range (associated with 90-100 percent 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 the concentration above which a yield reduction is to be expected. There are very few crops however for which these concentrations have been determined. Consequently, the tabulated standards for each of the crops presented are for leaf concentrations associated with deficient, optimum, and excess levels. These terms are defined as follows:

• Deficient: Concentration is too low for active growth; deficiency symptoms are likely to be present.
• Optimum: Concentrations should be adequate for near maximum growth; no visual symptoms.
• Excess: Concentrations too high for maximum growth; toxicity symptoms may or may not be present.

Used correctly, plant analysis provides the grower with valuable information about the nutrient status of his crop and the fertiliser programme being used.

Visual symptoms

Visual leaf symptoms can play an important part in diagnosing nutrient disorders. However, clearly recognisable symptoms associated with a specific disorder usually appear only after metabolic processes in the plant have been seriously disrupted and losses of yield have already been sustained. 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 plant.

Under conditions of deficiency, elements such as N, P, K and Mg are generally 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. Thus, the most obvious symptoms of deficiency of phloem-mobile elements are on the older leaves.

Elements such as Ca, B, Fe and Cu, 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 the 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 nutrients absorbed by the plant are distributed in a pattern which closely follows that of water loss due to transpiration. The fully expanded leaves tend to receive a greater share of the water and nutrients entering the shoots than do fruit or immature leaves, because they present a large evaporating surface relative to their volume. Hence, 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 for the 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 symtpom 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 that a visual diagnosis is confirmed with plant analysis. Although there may be some differences in symptoms shown by different plant species and even varieties of the same species for a particular disorder, the basic symptoms are similar for most plants.

A brief guide to deficiency symptoms of macronutrients and micronutrients is presented in the following pages:
Nutrient deficiency symptoms where older leaves are affected first
Nutrient deficiency symptoms where young leaves are affected first

FURTHER READING

Bollard, E.G. (1981). Prospects for Horticulture: A Research Viewpoint. New Zealand DSIR Discussion paper No. 6, Wellington. pp 212.

Bould, C., Hewitt, E.J. and Needham, P. (1983). Diagnosis of Mineral Disorders in Plants. Vol. 1. Principles. (Robinson, J.B.D., Ed), HMSO, London. pp 170.

Cornforth, I.S. (1980). Soils and Fertilisers. Soil Analysis. Ministry of Agriculture and Fisheries AgLink, AST 8.

Leamy, M.L. (1974). Resources of highly productive land. New Zealand Agricultural Science 8: 179-191.

Mengel, K. and Kirby, E.A. (1982). Principles of Plant Nutrition. International Potash Institute, Switzerland. pp 655.

Smith, G.S., Asher, C.J. and Clark, C.J. (1985). Kiwifruit Nutrition - diagnosis of nutritional disorders.

Winsor, G., Adams, P., Fiske, P. and Smith, A. (1987). Diagnosis of Mineral Disorders in Plants. Vol. 3. Glasshouse Crops.