We have been farming at Project Spectra for close to a year. The rooftop conditions are especially challenging with the heat and wind. Only certain plants that can adapt to these conditions do better at our garden, like beans and legumes.

During the school holidays, we decided to do some soil amendment by adding biochar. Terra preta is man-made soil created by an ancient farming method in the Amazon Basin between 450 BC to AD 950. It literally means “black earth” because of the high charcoal and humus content in the soil. The ancient farmers have digging charcoal into the soil sometimes up to a depth of 2 meters. There are a few reasons for doing so:

1. Biochar reduces nutrient leaching which is a major problem in places with very high rainfall, like the Amazon Rainforest and Singapore. Much of the plant nutrients are water-soluble and they get washed away during a heavy rain into the rivers (or in the case of SIngapore, the drains). Charcoal is extremely porous so they have a very high surface area to volume ratio and these pores hold on to the nutrients.
2. The high porosity characteristic of charcoal also encourages microbial activities in the soil. They provide spaces for bacterial and fungi that help with the ecosystem of the soil.
3. Biochar is slightly alkaline and helps to balance the pH of tropical soil, which is typically acidic.
4. Biochar helps to moderate the wet and dry cycles in the soil. The numerous pores absorb moisture when the soil is wet and release them when the soil is dry. The roots of most plants enjoy a condition where it is moist enough but not too wet that they suffocate without air.
5. Lastly, biochar will last a very long time in the soil. Unlike compost or ash, it doesn’t break down. This is why we still find Terra Preta in the Amazon after thousands of years. Talk about permanent agriculture!

We ordered a jumbo bag of biochar from a manufacturer of charcoal in Singapore. They can only sell the bigger pieces of charcoal and are left with the small bits and powder. We mixed the biochar into the soil with compost. In the process, we got very black ourselves!

Small bits of biochar and biochar powder.

Before mixing. Compost in the centre and biochar on the sides.

Biochar and compost.

After mixing.

I read that it takes some time for the biochar to get “charged”. When they are mixed into the soil and uncharged, they will absorb lots of nutrients from the surrounding. This might rob nutrients from the plants in the short term. However the benefits start to show after they have absorbed enough nutrients to be charged.

We planted some lettuce and malabar spinach seedlings very soon after adding the biochar. They are doing ok but I wouldn’t say fantastic. Only time would tell whether we have created Terra Preta!

Leaves are a little yellow

Growing lettuce hasn’t been very easy on the hot rooftop.

Using Urine as Fertilizer

Something else that we have been trying is using urine as fertilizer. Compared to manure,

1. Urine can be used instantly after dilution.
2. It is higher in nitrogen and other nutrients.
3. It is already in liquid form and the water-soluble nutrients will be directly available for the plants to absorb.
4. It is sterile and doesn’t contain the pathogens in manure. Traditionally urine has been used to wash wounds to protect against infection.

The important thing to remember is that without dilution, urine can be so rich that it burns the young plants. Don’t leave it for too long as well because it starts to smell after a while.

We did a small experiment on two batches of eggplants that we transplanted from pots. The results are below with the urine fertilized ones on the left. They are significantly bigger and look healthier. However I have to admit that I was probably biased which undermined the scientific validity of the experiment! The urine fertilized batch were transplanted a few days earlier and probably a little bigger when transplanted…