Green Manuring

Over the next few posts, I will slowly describe the Organic principles that I have always believed in and the approach that we have taken to start on the quest for Organic Certification.

Organic farming can be described as farming sustainably without the addition of artificial chemicals, farming in a manner that respects the environment, and having a whole farm approach that treats it as a living and growing organism.

At the heart of this process is the soil;

From the outset we have used green manuring to improve the soil, and to prepare the soil for the plantings, given our soil was a little acidic, and had low organic material I set out to improve the ground using cover crops and re-mineralisation

Below is a brief description of these processes

Green Manure / Cover Crop

Green manure crops play a valuable role in agriculture, and especially organic farming practices because of the many benefits they provide to the soil, the ease of addition and to the cropping system.

Green manures are crops grown specifically to improve soil condition and nutrition. They can be used instead of heavy chemical fertilisers and inorganic farming practices, chemical fertilisers supply nutrients but no organic matter. Suppression of weeds and soil-borne diseases are additional benefits of particular green manures.

Instead of being harvested, green manure crops are incorporated back into the soil, usually while they are still lush and green and before they go to seed. At this stage of growth the plants have a relatively high nitrogen and moisture content, and provide an ideal food source for soil microbes and other organisms like earthworms. Under favorable conditions these organisms decompose the green manure reasonably quickly. During this process, organic matter and nutrients are released into the soil where they become available for use by other organisms including crop plants.

Some benefits of green manures

Organic materials:
Organic matter is provided by root growth and mechanical mulching. Soil organisms then decompose this organic matter into humus and other organic compounds. Improving the organic matter level and biological activity of soil is one of the fundamental objectives of organic agriculture, and green manure cropping helps growers achieve this

Weed control and Erosion
Weeds are suppressed by the competition that dense, green manure crops exert for water, light and nutrients. Soil improvement is also achieved by decreased erosion from heavy rains. Soil compaction can also be prevented with the use of a green cover crop.

Nitrogen Fixing
Soil nitrogen levels are increased by leguminous green manures through their association with nitrogen-fixing Rhizobium bacteria. The Rhizobia can take nitrogen (N2) from the air and convert it to the form plants normally obtain from the soil. This process is called nitrogen fixation. When leguminous plants decompose, the nitrogen is released for use by other crops.

Biological tillage
Biological tillage (cultivation) is the term used to describe plants loosening the soil as their roots grow and as they dry out the soil and it cracks. This can help in soil improvement by breaking down the hardpan in heavily compacted soils.

Below are some images of Oats and Dunn Peas that are a winter cover crops that we have used.

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Soil Test Results

The following table presents results of recent soil testing:

Soil Measures
# Measure Unit 2006-June-29 2007-July-12
1 EC Ds/m 0.10 0.19
2 pH (CaCl2) pH 4.6 6.0
3 Bray Phosphorus mg/kg 1.6 8.4
4 Total Organic Carbon % 1.5 1.7
5 KCI extractable nitrate mg/kg 12
6 KCI extractable ammonium mg/kg 12

Exchangeable Cations
# Measure Unit 2006-June-29 2007-July-12
7 Aluminium cmol(+)/kg 0.63 0.07
8 Calcium cmol(+)/kg 2.2 7.4
9 Potassium cmol(+)/kg 0.56 0.54
10 Magensium cmol(+)/kg 8.4 8.2
11 Sodium cmol(+)/kg 0.87 0.62
12 CEC cmol(+)/kg 12 17

 

[1] Ds/m
Preferred level: below 0.15 dS/m (EC1:5)

Electrical conductivity is a measure of salts in the soil. A productive soil’s conductivity should be below 0.15 dS/m (decisiemens per metre).

Plants vary in their reaction to salt stress, from sensitive to tolerant, and the degree of reaction is less in clay soils than in sandy soils. For this reason, soils affected by salt should also have a saturation conductivity test (ECse). However, these results should not be compared with EC1:5 figures.

Salinity problems can be caused by too much fertiliser, salty irrigation water or saline ground water. Salts can be leached out with rainfall or low salinity irrigation water without affecting soil pH. Because of its high rainfall, the North Coast generally does not have a great problem with soil salinity except in some low, poorly draining soils close to tidal rivers.

[2] pH
Preferred level: pH (CaCl2): 5.5-6.5

Soil acidity is measured on a pH scale from 0 (most acid) to 14 (most alkaline), with 7 as neutral, that is, neither acid nor alkaline. The scale is logarithmic, that is, going down the scale from pH 7 (neutral), each number is 10 times more acid than the one before it. For example:

  • soil with a pH of 6 is ten times more acid than soil with a pH of 7 (neutral);
  • soil with a pH of 5 is one hundred times more acid than soil with a pH of 7.

The term CaCl2 after the pH figure signifies that the pH was measured in a solution of calcium chloride, a test preferred by most soil scientists. When soil pH is measured in a solution of CaCl2, the pH is 0.5-0.8 lower than if measured in water.

[3] Phosphorus
There are two different tests for phosphorus in NSW: Bray and Colwell. Since they give very different results, it is important to know which one is used in your report.
Bray phosphorus levels vary with land use:

  • 15-20 mg/kg for dry land pastures
  • 25-30 mg/kg for irrigated and improved pastures
  • 30-50 mg/kg for tree crops
  • 50+ mg/kg for vegetables

Note: mg/kg is the same as parts per million (ppm).

[4] Organic Carbon
Preferred level: above 2%

Organic carbon is a measure of the organic matter in the soil. It includes undecomposed plant litter, soil organisms and humus. Soil organic carbon stores important nutrients, stabilises soil structure and feeds soil microbes. If soil organic carbon is declining over time, then consider practices such as green manure crops, minimum tillage, mulching or strategic grazing.

Exchangeable Cations
The major cations are calcium, magnesium, potassium, sodium and aluminium. These are held in the soil by organic matter and clay. The percentage of each cation as a proportion of CEC is more important than the quantity of the cation. The preferred percentages and the suggested quantities of exchangeable cations in the soil are given in Table 1.

The following table presents preferred percentages of exchangeable cations as a proportion of CEC, and suggested quantity values:

Cation Preferred percentage (%) Suggested quantity (meq/100 g)
Calcium 65-80 >5
Magnesium 10-15 >1.6
Potassium 2-6 >0.5
Sodium 0-1 >1.0
Aluminium 0 0

Note: meq/100 g or meq% is the same as cmol/kg.

[7] Aluminium
Cation Preferred percentage (%): 0%
Suggested quantity (meq/100 g): 0

[8] Calcium
Cation Preferred percentage (%): 65-80%
Suggested quantity (meq/100 g): >5

[9] Potassium
Cation Preferred percentage (%): 2-6%
Suggested quantity (meq/100 g): >0.5

[10] Magnesium
Cation Preferred percentage (%): 10-15%
Suggested quantity (meq/100 g): >1.6

[11] Sodium
Cation Preferred percentage (%): 0-1%
Suggested quantity (meq/100 g): <1.0

[12] CEC – Cation exchange capacity
Preferred level: above 10
This is a measure of the ability of the soil to hold the nutrients calcium, magnesium and potassium. Good fertile soils with high clay content and moderate to high organic matter levels usually have a cation exchange capacity of 10 or higher. (Note: a cation is a positively charged ion).

Soil pH

Soil PH

The pH level of soil is a determination of the alkalinity or acidity of the soil. The soil pH scale ranges from 0 to 14, with 7 being neutral. Any reading that is greater than 7.0 is alkaline while any reading below 7.0 is acidic.

The Hunter Valley soil is by and large acidic and in the natural range of 4.5 to 7.


Why soil pH is important

Soil pH can significantly affect the nutrient uptake by plants in the soil-water system, as a result many plants growing in strongly acid or strongly alkaline soils may suffer nutrient deficiencies or toxicities, which in turn can negatively affect their productivity, appearance and in some cases kill the plant.

As a general rule, soil PH should be in the 5 to 8 area, depending on the plant and that plants requirements.

Plant nutrients are most readily available for plant uptake and toxicities are less likely to occur in soils with a pH near neutral (7.0). However, while most plants grow well in near neutral soils, some plant species require a particular pH level for optimum growth.


How to change soil pH

Heavy soils or soils with a high clay content require more alkalising or acidifying material to change their pH than sandy soils.


Raising soil pH

Liming materials. Liming corrects soil acidity. The three main types of liming materials are:

  • agricultural lime — calcium carbonate obtained by crushing limestone rock. It moves slowly in all but very sandy soils and is used mainly for treating strongly acid topsoils. The finest quality lime is preferred;
  • dolomite — about 60% calcium carbonate and 40% magnesium carbonate;
  • magnesite — 100% magnesium carbonate.

Other liming materials include burnt lime (calcium oxide), hydrated lime (calcium hydroxide) and waste materials such as kiln dusts, sewage residues and blast furnace slags.

Lime or a mixture of lime and gypsum can also be used to improve soil structure, but this is not recommended for alkaline soils (pH over 7).

To change the soil pH by one point, 5.0 to 6.0, apply at 120g/square metre for sandy soils and up to 380g/square metre for heavy clay soils.

The soil pH should be checked after 3 months and apply more lime or dolomite if necessary. Repeat this process until the correct PH is attained.

Lowering soil pH

Two materials normally used for lowering the soil pH are aluminum sulfate and sulfur. Aluminum sulfate will transform the soil pH instantly because the aluminum produces the acidity as soon as it dissolves in the soil. Sulfur, however, requires some time for the conversion to sulfuric acid with the aid of soil bacteria. The conversion rate of the sulfur is dependent on the fineness of the sulfur, the amount of soil moisture, soil temperature and the presence of the bacteria. If the conditions are not ideal, the conversion rate of sulpHur may be slow. For this reason, most people use the aluminum sulfate.

As a guide, to reduce the soil pH by one point from 8.0 to 7.0, elemental sulphur (99% sulfur) can be used at an application rate of 30g/square metre for sandy soils and up to 120g/square metre for clay soils. Check the soil pH after 3 months and apply more sulphur if necessary.

Pine needles, pine bark, rotting sawdust and leaf mulch all have an acidifying affect on the soil over time. Some fertilizers including sulphate of ammonia and urea can have an acidifying affect on the soil over time.

References:

Soil Management for Orchards and Vineyards (1993), G O’Conner, J Strawhorn, K Orr, AGMEDIA – Department Of Agriculture, Victoria, ISBN – 0 7306 3018 8

A Brief Blurb

Australia has at least six species of native citrus that can be found from the harsh desert to tropical rainforest. Previously they were classified in two genera (Eremocitrus and Microcitrus). However, recent taxonomic work has lead to their reclassification and they are now included in the genus Citrus (along with oranges, lemons, limes, etc).

Native species are able to hybridise with a range of other citrus species, the CSIRO has lead the way as well as some private cultivators being produced. There has been successful grafting onto conventional citrus root-stocks as well as private growers trying to break into a commercial market with their own combinations of wild varieties, tested root-stocks and various examples of citrus. These abilities, along with attributes such as drought and salinity tolerance and disease resistance, have long attracted the interest of citrus researchers and breeders. Improved selections and hybrids of native citrus also have potential in their own right for commercial production of fruit for both the fresh and high-value processing markets. Chefs from Australia and abroad have long delighted at the beauty, flavour and versatility of Australian Native Citrus.

The following blogs are devoted to the Citrus Australasica commonly known as the Australian Finger Lime, which can be found as an understorey shrub or tree in rainforests in southern Queensland and northern New South Wales. Finger Limes have been found growing wild on ridges and flats, in a wide range of soil types. Some have even been found growing on a sandstone hillside. When established, they are a hardy shrub!

Finger Limes produce finger-shaped fruit, up to 10cm long, with thin green or yellow skin and green-yellow compressed juice vesicles that tend to burst out when the skin is cut, the fruit is acidic and flavuorsome, not dissimilar to a Tahitian Lime, but with a more subtle flavour. A pink to red-fleshed form with red to purple or even black skin also occurs in the wild. Fruit from wild harvest and limited plantings are used by the native food industry, with our own trials and plantings we are hoping to add something to the Native Food Industry.

We have been trialling different varieties, rootstocks and environments for 18 months and will continue to test and learn then plant a small orchard in Spring 2007 or 2008. The following pages are a blog of our quest to achieve these goals!