Term 2 – Week 6 – Soils & Fruit Tree Pruning

This week was split between soil science and fruit tree pruning.

Soil science delved further into soil creation (pedogenesis), looking at the influence of parent material, topography, climate, soil organisms and time. I won’t go too much into that except to say they all play a role in how a soil forms.

We also learnt more about cation exchange capacity of a soil and its water and nutrient holding capacity.

Clay particles and humus are negatively charged, attracting positively charged nutrients in the soil. They will be held in the soil and not leached out. The acidity of the soil (concentration of hydrogen ions) controls which nutrients will be forced off, back into solution in the soil.

Clay will sometimes have a positive charge but phosphate will snag them all leaving sulphate and nitrate anions to be lost by leaching if not taken up by the plant right away.

In sandy soils the macro pores are too big to retain much water. It pretty much drains straight through taking nutrients with it. Clay and humus can improve the structure of the soil, providing more micro pores that hold onto water.

This is where the water holding capacity of the soil comes in. You only want to irrigate to field capacity – the point where the soil is holding as much water as it can without leaching.

Nitrogen is the nutrient that is lost most easily from the soil. Aside from traditional fertilisers, nitrogen fixing plants can be helpful to add nitrogen to the soil. Plant roots and bacteria in the soil form a symbiotic relationship where the plant provides energy for the bacteria and the bacteria provide nitrogen to the plant. When that plant dies and breaks down in the soil other plants can then take up the nitrogen.

We’ve come full circle now and are back to pruning – this week we’ve started pruning the fruit trees at Bannockburn Road. Pruning when trees are dormant means there will be a flush of new growth in spring. If you want to make a cut to a tree without encouraging growth you need to do it in summer.

Apricot trees are trained to grow in a vase shape, cutting back last seasons growth 2/3rds to an outside bud. This lets as much light into the centre of the tree as possible. Also taking out any diseased wood, branches growing in or crossed over.

Pruning apricot trees – Karen and Praneel

Last week’s weather:

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