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Compost for orchard soil management

Using compost in your orchard allows you to:

• turn waste into a resource
• save money on fertiliser
• grow more crop with less irrigation
• get better quality with less disease.

Sustainable soil management

In a natural ecosystem, the litter layer supports organisms that efficiently recycle nutrients back into the plants. When this layer is removed under industrial agriculture practices, nutrients are leached away and lost from the denuded soil, in addition to those removed in the crop. Manufactured fertilisers then become necessary.

In industrial agriculture we have become dependent on manufactured fertilisers. But the price of fertilisers is going up with the price of the fossil fuels used to make them. So even aside from the ecosystem benefits, using compost to retain and recycle nutrients makes economic sense.

It is possible to use compost to totally replace the need for manufactured mineral fertilisers, if the nature of the compost is fully understood. With a bit of work, you can restore the soil’s organic matter and reap many benefits, including improved nutrient- and water-holding capacity, improved soil structure, higher biological activity, better root growth and less disease.

Benefits of compost

• Erosion prevention: Lost soil and organic matter means lost nutrients.
• Moisture retention: Mulching with compost enhances infiltration of rainfall and reduces evaporation from the soil surface, meaning less irrigation requirement.
• Recycling of nutrients: The nutrients present in crop residues and prunings are returned to the soil. (However, the nutrients removed in the crop must be replaced.)
• Improved CEC: Organic matter greatly increases the soil’s cation exchange capacity, which is responsible for the retention of nutrients in the soil.

Composts ain’t composts!

Compost can replace fertiliser, but it depends heavily on the compost. A nutrient-rich compost made from food scraps, crop residues and fowl manure can provide all the nutrients required. But a nutrient-poor one made just from sawdust will create problems.

The key point is that the ingredients that go into the compost determine the nutrient composition of the final product. The following table lists the nutrient properties of various organic wastes that you can use.

Table 1. Examples of common organic wastes.

C/N ratio

The C/N ratio (right-hand column in Table 1) is the ratio of carbon to nitrogen. This is a critical characteristic of a compost. Too high (>20:1) and bacteria will deplete soil N reserves as they break down the material. The soil will then become deficient in N. Too low (<15:1) and the soil will be high in N but not necessarily any other nutrients. In fresh leaf material the C/N ratio is about 10:1. In sawdust it can be as high as 400:1, and in poultry manure as low as 3:1.

As you can see from the table, it is possible to create a suitable compost by blending materials with different characteristics to give a good balance of C, N and other nutrients.

Example blend: 20% sawdust, 30% council green waste, 20% layer manure, 10% dairy manure, 20% fruit peel from local cannery. Result: a very well balanced compost likely to replace all mineral fertiliser needs if applied at a sufficient rate, and that is unlikely to cause long-term nutrient imbalances or acidification. Any final nutrient imbalances can be adjusted with a little fertiliser.

Compost maturity and application method

Materials with very low C/N ratio (= a high N content) will lose a lot of N to the air as ammonium if they are surface-applied. So poultry manure is best ploughed in. But when composted, this ammonium is converted to slow-release organic forms of N and nitrate, which are not lost to the air.

If composted for around 6 weeks with sufficient moisture and the odd turn, compost will be mature enough for use without high loss of N to the air. Incorporation is not necessary, as worms and other soil animals will incorporate it, but scarifying can be appropriate if there is no risk of root damage. The best time to incorporate compost is spring, to coincide with the first foliage flush. An irrigation or anticipated rainfall will improve incorporation.

Compost application rate

First have your compost tested for nutrient content. (This is a standard test offered at SESL.) Then you can calculate how much to apply.

Let us assume that the example compost blend above contains the nutrients listed in Table 2. We can then calculate the amount of compost we need to supply the nutrients required.

Table 2. Calculating nutrient inputs

Having selected the quantity to apply (in this example, 5 t/ha would be a good compromise), you can balance the nutrient content with mineral fertilisers. Or if you have the time and desire, you can fine-tune the compost blend and so avoid the need to use mineral fertiliser supplements.

Costs and benefits

A compost fertiliser program requires a lot of effort in blending, composting, analysing, applying and incorporating in addition to all the usual crop work. This comes at a cost.

The benefits will vary too: Run-down, neglected orchards or areas of poor soil and low organic matter are likely to benefit most. But in other situations the costs will not be justified by the increased yield and quality, and a sensible green manure program integrated into the annual cycle will be sufficient to maintain soil organic matter at healthy levels.

Some benefits are surprising. In a trial on beetroot, the compost we applied gave the same level of control of root fungal disease as the conventional chemical fungicide applied at twice the recommended rate!

Friends and clients of ours back in the 1980s planted the first avocado orchard in Kulnura (a long way south for avocado). Using only compost produced from poultry manure, hay, sawdust, shredded green waste and basalt dust, they produced the biggest and best Hass avocados on the gourmet market and sold straight to restaurants. What’s more, they had no incidence of Phytophthora at all. So it’s possible.

Further information

Experimentation will obviously be required to tailor a program to suit your needs. SESL staff are experienced in the uses of compost, and will be happy to advise you at any or all steps in the process.

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