Soil temperature

The temperature  of a soil is important as it affects how fast plants can grow. Soil temperature  also affects how quickly plants take up water and nutrients. Clay soils are  cold, wet soils. Germination and seedling growth is usually slow.

Because sandy  soils don't contain much water but lots of air, they warm up quickly. They are useful  for growing early crops.

Soil  temperature affects the speed of chemical reactions. Warm temperatures speed up  reactions and colder ones slow them down. Soil temperature affects the  breakdown of parent material and how fast micro-organisms work. Both are  important in adding and returning nutrients to the soil. Soil temperature is  influenced by the climate of the area and the season of the year.

Climate and season

In a warm  climate or during summer, the soil is full of chemical and physical activity.

Micro-organisms  will slow down when it is too hot (above 35°C). Micro-organism also slows down  in the middle of winter as the soil is too cold. They become inactive below  6°C. In winter it takes a long time for organic matter to break down in soils  in the cooler parts of New Zealand.

Plants are also affected by soil  temperature. When the temperature is right for a particular plant its roots  will keep growing. If the soil gets too hot or too cold then the uptake of  water and the growth of roots will slow up.

Aspect

The slope of  the land and the direction that it faces directly affects the temperature of a  soil. Sun will fall on north-facing land during the day in both summer and  winter.

During the  winter south-facing slopes will get less sun during the day. The soil on these  slopes cools down quickly in the autumn and warms up slowly in the spring. In  the middle of summer these areas will have better growth because the soil is  not as hot and dry as the soil on a north-facing slope.

Plant cover

Soil covered in  plants is protected (insulated) from fast heating or cooling of the soil (temperature  fluctuations). In a recently cultivated paddock, soil will heat up quickly  during the day, but it will also lose heat quickly once the sun goes down.

Cultivated soil

Paddock  covered in vegetation

Soil colour

A dark coloured  soil with a lot of organic matter in it heats quickly as it absorbs more heat  energy.

Soil water

Wet soil will  be cooler because it takes a lot of energy to heat water. Wet soils take longer  to heat up in spring than soils that are well drained.

Soil depth

The deeper you go down in a soil profile the less the soil temperature will fluctuate. Soil is a good insulator. It can take a while for the soil at the bottom of a profile to heat up, but it will also take a longer time for it to lose the heat that is stored there.

Teacher-marked activity

Complete Activity 7A, the teacher-marked activity in your workbook when you have completed the  following topics:

• chemical properties
• biological properties
• physical  properties.

What's next?

1. Complete the self-assessment in the back of your workbook.
2. Return your workbook to your teacher.
3. Continue with HT1033 - Soils 3.