Compost Effect on Radish Growth

You can read the previous blogs:
Blog 1: The Junior Internship at Mud Valley Institute here
Blog 2: My Research Project and Junior Internship Journey here

This experiment tested Raphanus sativus radish growth in eight different soil types- two soils and six soil/compost treatments. For the soils, we would use unamended soil taken from the control area on the farm (labelled AD), and soil from the farm’s market garden (labelled MG). The six treatments consisted of market garden soil combined with one or both out of two kinds of compost- worm castings (also known as vermicompost) produced on the farm, or a thermophilic compost called Nutriverde®, which is made by a local company and already used on the farm. All of the treatments and their labels are listed below:

• • AD: Algarvian soil, unamended and untouched from control area
  • MG: Market garden soil, unamended
  • WC20: 80% market garden soil, 20% worm castings
  • WC40: 60% market garden soil, 40% worm castings
  • AS20: 80% market garden soil, 20% Nutriverde® 
  • AS40: 60% market garden soil, 40% Nutriverde® 
  • WAC: 80% market garden soil, 10% worm castings, 10% Nutriverde® 
  • MAT: 100% market garden soil with a layer of Nutriverde® on top

Radish seeds were seeded in 24 pots- three pots of each soil type used in the experiment- and were placed in Vale da Lama farm’s greenhouse to best facilitate growth. We checked the seedlings every week for 6 weeks to water them and measure the height of their leaves to keep track of their growth. In the sixth week, we took the radishes out of their pots and measured their final weight, leaf height, and the length of their roots. Afterwards, we also conducted a taste test to determine if any difference in taste could be discerned. Our findings regarding the compost effect on radish growth are written below.

On week 6 of the experiment, two types of radish weight were recorded- the weight with roots and the weight without. In the results, only the weight without roots was used, as the difference between the weights was not significant, and because radishes are sold without their roots. The following bar chart shows the average weight of the radishes grown in each type of soil.

The chart shows that WAC– which consisted of a mixture of soil, worm castings, and Nutriverde® – produced radishes with the highest average weight, followed by WC40, and then by AS40. AD performed the worst by far, with MG and MAT also being noticeably worse than the other five treatments. Radishes with higher weight are larger, which means they are of higher quality and are more likely to appeal to the consumers of the market garden’s products. It also results in more profit for the farm, as radish pricing is based on weight.

Leaf height and root length are grouped together in this section so as to compare the ratio between the roots and the shoots of the radish seedlings. The leaf heights recorded here are the ones taken during week 6; for the seedling measurements taken from weeks 1 through 5, check the full project report. Note that the following chart takes the average from all three of the pots per treatment.

The results show that WAC and WC40 had the highest leaf heights, whereas WC40 and WC20 were the treatments with the longest roots. Despite having a higher average leaf height, WAC had the third-shortest roots, with MG and MAT being shorter. AD was the only treatment with longer roots than leaves. Both variables are important in measuring the growth of the radishes, but leaf height may be more valuable, as consumers may judge the quality of a radish by how large and healthy the leaves look, while roots are not present at all at the market stall.   

Our hypothesis did not include anything regarding the taste of the radishes originally- however, we decided that taste was an important factor in determining the quality of our products because the purpose of a radish is to be consumed. Therefore, after the previous measurements were taken, we conducted a taste test of the radishes in order to determine if there was a compost effect on their flavor, texture, or spice level. Eight people participated in this test- us four interns, our two internship advisors, and two members of the farm’s staff. Interestingly, there did seem to be an effect- our notes on the treatments and how they affected the radish’s flavor profile are as follows.

• • WC20 and WC40 radishes were extremely spicy.
  • AS20 and AS40 radishes were mild and juicy.
  • AD radishes were bland.
  • MAT radishes were slightly spicy, and were rather juicy.
  • WAC radishes had notably fibrous taproots and thick skin.
  • MG radishes were rather fibrous, but neutral in taste.
  • Radishes of any type that had gone to flower were bland.
  • There was no notable taste difference between the radishes produced from the same compost type but different concentrations- AS20 versus AS40, and WC20 versus WC40.

Considering all three categories of weight, leaf height, and root length, it was found that WAC, WC40, AS40, and WC20 treatments performed best, while AD, MG, and MAT were consistently in the lower end of rankings. AS20 was ranked in the middle, never appearing in either the top or bottom three for each category. From this, it can be determined that worm castings were more effective than the thermophilic Nutriverde®.

Treatments with a 40% concentration of compost performed better than those with a 20% compost concentration. However, it only performed around 9-11% better, raising the question over whether procuring an extra 20% of compost is worth the additional cost.

The poor performance of the MAT treatment- which consisted of market garden soil with a layer of Nutriverde® on top- is interesting to note. Its initial growth was one of the fastest, but after week 3 of measurements its growth halted significantly. This implies that it is important to mix the compost in with the soil, especially in the case of root vegetables, where the vegetable would grow underneath the area affected by the compost.

From the results of our research, we can conclude the following:

• • Compost has a powerful effect on radish growth- with worm castings and a mixture of both worm castings and thermophilic compost performing especially well.
  • Mixing the soil is important, especially when growing root vegetables
  • Having soil that consists of 40% compost produces larger radishes with more leaves than only having 20% compost- however, we concluded the difference is not significant enough for the cost of the extra compost to be worth it.
  • Compost type does have an effect on the taste of radishes, but the concentration of compost does not.

Regarding which treatment of compost produced the highest quality radish, the WAC and WC40 treatments produced the radishes with the highest weight, root length, and leaf height- however, an important aspect of radish quality is also the taste. Due to taste preferences being subjective, it cannot be measured which treatment created the radish that tastes the best. However, the group of taste-testers used in this experiment preferred the AS20 and AS40 radishes.

The full written report, Compost Effect on Radish Growth, can be found in the resources section of this blog.