October 15, 2021

Comparing methods for the quantification of plant-available nitrogen

Written by Ellie Barbrook:

As the first year of my PhD is coming to a close I am rounding up my first experiment. Comparing methods for quantifying mineralizable/ plant available nitrogen (N) in soils. My PhD is focused on finding ways of retaining N in soils, in both plant available and non-available forms. This experiment was designed to compare the known methods of quantifying mineralizable N in soils, to asses their ease of use along with their effectiveness at demonstrating the most holistic picture of the mineralizable N in soil.

With the easing of lockdown rules in April I began collecting soil samples, starting in Reading at the University’s research farm, then I went down to Dorset to see how the babyleaf Spinach is grown at The Watercress Company, my industry partner. I met with Stuart Carless (my industry supervisor) who showed me round the fields they grow their salad crops on, introducing me to the methods of growing babyleaf Spinach on an industrial scale. Seen in the first image is one of the fields in which Spinach is grown, long beds are created onto which the Spinach is sown. The beds are the same width as the harvesting equipment, which is a long arm of cutters attached to a tractor-drawn container where the Spinach is packed into crates via a conveyer belt.

Image 1 The watercress Company (Dorset) Field of Spinach being harvested, sown on 1st March 2021. 

They use different varieties of Spinach throughout the growing season of March to October which are grouped into three sections depending on their growth rate. The quickest growing Spinach only takes 26 days from sowing to harvest, however the slowest growing will take around 56 days.

Most recently I visited RHS Wisley to take soil samples from one of their field trials, which was headed by Dr Marc Redmile- Gordon (Senior scientist for soil and climate change for the Royal Horticultural Society). This trial is looking at how ‘Living fertilisers’ (White Clover) can help with soil retention of N. In this trial, 1m2 plots were treated with different peat- free compost additives, half of the plots also had clover grown as a cover crop. Seen below in the image is the difference in Clover leaf size between the mushroom (A) and bark (B) compost treatments that I took samples from. Bark is high in Carbon and low in N, whereas Mushroom compost is high in N and low in C. I found there was also a difference in water content/ holding capacity between these two treatments. A combination of both these factors has led to the dramatic difference in Clover leaf size. The effect that the clover had as a cover crop will be seen once I have collated all of my data. I except it to have had a positive impact on the amount of plant available N in the soil. This is because White Clover is a forage legume, so it forms a symbiotic relationship with N-fixing bacteria (Rhizobia) that take N2 gas from the atmosphere and turn it into a form which plants can use in the soil. This is done using the enzyme nitrogenase which catalyses this reaction. White Clover could therefore hold the key to developing a more sustainable way for keeping plant available N in the soil whilst growing crops like Spinach.

Image 2 RHS Wisley living fertiliser field trial, coordinated by Marc Redmile- Gordon. Clover leaf size difference with mushroom compost (high levels of N) and B bark compost (low N levels). A trowel is used as a comparison guide for the size of leaves. 

After the soil samples had been collected, I grew Spinach in 15cm diameter pots for ~four weeks in a glasshouse. Watering them gravimetrically every other day. Six seeds were sown per pot then thinned out to three plants after all the plants in each pot had germinated. All weeds were removed as soon as they emerged as some pots did not have any weeds I wanted to make that equal.  After one week (Image B) I inserted Plant Root Simulator probes (PRS® Probes) into the soil, one pair per pot. One is an anion membrane (Orange) which adsorbs NO3 and the other is a cation membrane (Purple) which adsorbs NH4+. The benefit of using these probes is that they are left in the soil for two weeks so there is a temporal aspect to their quantification of soil N. This was one of seven different methods I have used to quantify the amount of plant available N in the samples.

Image 3 Spinach being grown in the glasshouse on The University of Reading campus. A day of sowing. B 1 week after sowing, PRS probes have been inserted. C 2 weeks post sowing. D 3 weeks post sowing, 1 week prior to harvesting.  

Currently, I have been going through the data I have collected, and I am amazed at the variation between the soils. Before starting the analysis I expected some difference in the soil characteristics, but not to this extent. An example of this can be seen in Image 3 where the first three rows of Spinach are growing well, however in the fifth and sixth rows the Spinach is a lot smaller. This is primarily through the reduced water holding capacity for these soils (in rows 5 and 6), which will be interesting to see if it is linked with N availability. This highlights just how important it is to take care of our soil if we want to achieve a sustainable production of food across the globe. Primarily through choosing the right additives to help keep our soils healthy, which will also aid plant available N to stay in the soil.