Archive for the ‘Soil organic matter’ Category

A pasture puzzle partly solved?

Sunday, April 7th, 2019

Its easy to get tied up with the day to day chores of a farm and it seems to be inevitable more so if you’re running a dairy farm! But sometimes its good to take a walk out on the farm without any particular job in mind just to see how things are going. This is when I noticed some problems with a forage crop.

In late autumn we over-sowed some forage oats into the pasture in a couple of paddocks. But in July I noticed that the growth of oats was patchy. Some of the oats were tall and green but in other areas they were short and pale.


Pasture with forage oats poor growth

These forage oats were sown into pasture in Autumn but in winter growth was still very poor.

I decided to do some simple tests to find if there was any difference that might explain the variable growth. 5 samples were taken in areas of good to poor growth. Test were carried out for pH, water capacity, fresh organic matter and nitrate. For each site the growth was ranked – you can see this in the table below. Fresh organic matter represents the fraction that will break down easily to release nutrients. The method used can be seen at A simple test for reactive soil organic matter. Nitrate was extracted using water and measured using the Cadmium reduction method.

Fresh organic Nitrate-N
Site Oats growth pH * % water matter ppm mg/L **
1 Tall / good 5.3 40.7 1584 23
2 Low 5.3 25 873 41.5
3 Poor 5.3 25 1273 20.3
4 Poor 5.3-5.6 28 1660 0
5 Tall / good 5.3 37.5 2640 56
* pH paper ** actual concentration in the soil
solution at each site

pH was low and that didn’t seem to affect oats growth. Some of the higher values for fresh organic matter are reasonably good for dairy farm soils. Fresh organic matter seems slightly higher at sites with good growth.

Nitrate is about 4.3 times higher than Nitrate-N so overall, some of nitrate levels in the soil solution are high. This is specially true for sites where soil water % is also high as it means overall higher nitrate levels. Site 2 doesn’t fit the pattern very well as nitrate levels are fairly high but growth is poor. perhaps there is some other factor important here.

Over-sown oats growing well at Site 5. Nitrate levels in the soil solution seem higher at sites with better growth.

Over-sown oats growing well at Site 5. Nitrate levels in the soil solution generally tested higher at sites with better growth.

 This study provides some base line figures for the farm. This is useful for comparison between farms and for tracking pasture improvement. It doesn’t provide definite answers about the factors affecting pasture growth but can provide some insights and can suggest further study.

Nitrate in soil comes from a process that starts with organic matter breakdown. Microorganisms break down the organic matter to produce plant available nitrogen in the form of ammonium.  Plants take up and utilize ammonium easily. It’s like saying that plants prefer ammonium.

In natural soil systems most nitrogen from organic matter stays as ammonium because oxygen levels are fairly low. But on many farms there is more oxygen in the soil because of factors like tillage, disturbance  and loss of a permanent litter layer. Under these conditions  there are microorganisms that will grab the opportunity to convert the ammonium to nitrate. Why? Because they make use of the energy released when ammonium is converted to nitrate when there is oxygen present. Bad luck for the pasture plants.

Does it matter? Yes, for two reasons. For the plant it takes energy to break down the nitrate to a usable form because plants don’t utilize nitrate directly. Importantly there is an enzyme in the plant required to do this, and it requires molybdenum. So if the trace element molybdenum is in short supply plants may not be able to utilize all the nitrate. Second, whilst ammonium is held fairly tightly by soil, nitrate dissolves easily in water and therefor if the soil is wet nitrate can be carried away down sloping ground and down deeper into the ground.

Whilst nitrate levels in soil can indicate the amount of nitrogen released from organic matter they only provide a partial understanding of soil fertility and plant growth.