Crop nutrient replacement: calculator for fertiliser requirements

Reviewed: June 2005

Background

In the past, fertiliser practices for many of the fruit crops grown on the North Coast of New South Wales were adopted from research conducted on subtropical fruits in Queensland and on low-chill stone fruits from the high-chill producing areas. This was done without consideration for differences in vegetative growth and fruit growth cycles, geographic locations, soil types, tree management, and varietal and environmental factors. As a result, many fertiliser recommendations and rates applied were not appropriate, and resulted in overfertilising the soil.

Problems caused by overfertilising

Overfertilising is a major concern in fruit production, as it:

  • contributes to the problems of soil acidification, salinity and soil structure;
  • causes nutrient imbalance;
  • increases the levels of nitrate and phosphate contamination in waterways (and groundwater), leading to algal blooms and aquatic weed growth — a concern to all environmentally conscious people;
  • wastes growers’ money.

New strategy: crop nutrient replacement

Sustainable fertiliser management involves:

  • maximising fruit production;
  • preventing on-site soil degradation;
  • minimising off-site movement of nutrients.

For these reasons, crop nutrient replacement provides a sound basis for determining annual fertiliser recommendations.

Recent studies on some subtropical fruit crops at the NSW Centre for Tropical Horticulture in Alstonville has resulted in the development of new recommended fertiliser rates, using a crop nutrient replacement strategy. With crop nutrient replacement:

  • nutrients removed from the soil by the fruit, including the flesh, skin and seed, have been determined;
  • nutrients removed by the leaves, shoots and roots have been established from overseas and local research;
  • losses from leaching, erosion and soil fixation are estimated.

Application rates for fertilisers are based on these calculations, rather than on prescribed amounts.

This fertilising method should prevent underfertilising and overfertilising. On a highly fertile soil, replacing all nutrients that have been removed may not be necessary due to adequate soil reserves. This is why monitoring using soil and leaf analysis is so important.

Software program: spreadsheet

Growers can now calculate the crop nutrient replacement for a range of fruit types by using a relatively simple software program (spreadsheet). The following information is entered into the spreadsheet:

  • plant spacing;
  • yield;
  • soil leaching;
  • phosphorus fixation percentages.

The program then calculates:

  • plant density;
  • yield per hectare;

and constructs graphs of:

  • fruit nutrient removal;
  • total nutrient replacement.

Example of the information in the spreadsheet (for avocados)

Table 1. Crop replacement data

Variety

Hass

Block House
Age (years) 8
Between-row spacing (m) 10.0
Within-row spacing (m) 8.0
Package weight (kg) 6
Yield (packages/plant) 16
Report date 20-Sept-04

Leaching/fixation factor
 
Nitrogen (%) 30
Phosphorus (%) 80
Potassium (%) 20
Calcium (%) 10
Magnesium (%) 20

Calculations
 
Plant density (plants/ha) 125
Yield (tonnes/ha) 12.0

Crop replacement rates

Nutrient requirements Nutrient replacement (kg/ha)
Fruit only Estimated total
Nitrogen (N) 46 84
Phosphorus (P) 8 17
Potassium (K) 68 99
Calcium (Ca) 6 41
Magnesium (Mg) 7 20

A monthly fertiliser schedule (Table 2) can be calculated for the major nutrients — nitrogen, phosphorus, potassium, calcium and magnesium — by inserting a percentage figure in the months during which you want to apply your fertilisers. Growth cycle stages can be used in lieu of the month, if required.

Table 2. Monthly fertiliser schedule

Month/
Growth stage
NitrogenPhosphorusPotassiumCalciumMagnesium
Applic.
(%)
(kg/ha) Applic.
(%)
(kg/ha) Applic.
(%)
(kg/ha) Applic.
(%)
(kg/ha) Applic.
(%)
(kg/ha)
January 30 25   0 25 25   0   0
February     0 25 25   0   0
March 25 21 50 9 25 25 50 20 50 10
April     0 25 25   0   0
May 25 21   0   0   0   0
June 20 17   0   0   0   0
July   0 50 9   0 50 20 50 10
August   0   0   0   0   0
September   0   0   0   0   0
October   0   0   0   0   0
November   0   0   0   0   0
December   0   0   0   0   0
TOTAL1008410017100991004110020

In addition, fertiliser calculators (Table 3) can be used to calculate the rate of fertiliser (straight or N:P:K mixes) required per hectare or per plant to meet crop replacement rates for both ground-applied and fertigation programs.

Table 3. Fertiliser calculator

Using straight fertiliser Nutrient
(%)
Fertigation rate
(%)
Replacement rate
(kg/ha)
Amount of fertiliser
(kg/ha)(kg/plant)
Nitrogen (N) 46 100 84 183 1.5
Phosphorus (P) 9 100 17 192 1.5
Potassium (K) 41 100 99 242 1.9
Calcium (Ca) 20 100 41 205 1.6
Magnesium (Mg) 54 100 20 37 0.3
Using N:P:K fertiliser Nutrient
(%)
Replacement rate (kg/ha) N:P:K
(kg/ha)
Amount of fertiliser
(kg/ha)(kg/plant)
Nitrogen (N) 12 84 84 702 5.6
Phosphorus (P) 3 17 21
Potassium (K) 14 99 98

Individual Excel spreadsheets

Links to Excel spreadsheets are provided for the following fruit crops so that you can calculate crop nutrient replacement:

Further reading

Dirou J & Huett D  2000, ‘Passionfruit nutrient replacement’, in The Passion Vine, Feb. 2000.

Dirou J & Huett D  2001, ‘Crop nutrient replacement for avocado’, in Talking Avocados, Mar. 2001.

Huett DO & Dirou JF  2000,  ‘An evaluation of the rationale for fertiliser management of tropical fruit crops’, in Australian Journal of Experimental Agriculture, 40, 1137–1143.

Slack JM, Huett DO & George A  1996, Fertilising low-chill stonefruit, Technical Bulletin (28 pp.), NSW Department of Primary Industries.

ALWAYS READ THE LABEL

Users of agricultural (or veterinary) chemical products must always read the label and any Permit before using the product, and strictly comply with the directions on the label and the conditions of any Permit. Users are not absolved from compliance with the directions on the label or the conditions of the Permit by reason of any statement made or not made in this publication.

Author: John Slack, John Dirou