How To: Determine Your Soil Texture

We are very excited to publish our first ‘How To’ information sheet:

How to – Determine Your Soil Texture

Knowing your soil texture will better inform your watering and fertilising regimes, helping your plants and helping you save water and fertiliser at the same time.

Soil texture is the foundation of soil structure – the interaction of the different soil particles you will measure when determining your soil texture. While soil texture cannot be altered, soil structure can be improved. Improving your soil structure will reduce the negatives of sand or clay textured soils.

Our next information sheet will explain the interaction of soil particles to form soil structure and in turn explain how to improve your soil structure.

Happy reading and happy science-based gardening!

Note: The PDF file appears as expected in browser-based and open source PDF viewers. In at least one version of Adobe Acrobat the logo and text are distorted (but still easily readable) and appears as expected when printed.

To Till or not to Till?

No-till farming aims to improve soil structure, soil aggregate stability and biological activity (see this previous article summary), reduce soil erosion and retain greater amounts of nutrients. It has also shown improved carbon storage ability to help mitigate climate change compared to conventional farming. These factors have led to a number of studies looking to quantify these benefits as well as any associated negatives. One negative associated with no-till farming are reports of reduced crop yields. Balancing the pros and cons to decide whether it is better than conventional farming on balance, given the number of variables that could affect results, is a difficult but important and ongoing area of research.

No-till farming avoids disrupting the soil surface and leaving 30% or more of the crop residues on the surface of the land. This contrasts to conventional farming where, usually, machinery is used to dig in crop residues and loosen the soil in preparation for growing new crops.

A recent review in the Journal of Agricultural Science by researchers at the University of Nottingham examined 49 data sets from other research articles comparing soil organic matter content between no-till and conventional till farming, and another 61 sets of data from research comparing crop yield between the two farming methods. The purpose was to compare the asserted benefits on reducing greenhouse gases of no-till farming with the losses in yield.

Greenhouse gas emissions

In general, it was found that the carbon content of soils under no-till farming was greater than that of conventional farming. This resulted from carbon being retained in soil micro-aggregates which were in turn protected by macro-aggregates that, under conventional tilling, would have been broken up and the carbon more easily converted into carbon dioxide. The carbon was mostly retained in the upper layers but the longer the land was not tilled, the more long-lived was the sequestration.

Improvements in soil structure, aggregate stability and biological activity was confirmed but, in the case of the size and frequency of pores in the soil, the improvements came after tilling had ceased for a number of years.

However, the nitrification of soil nitrogen (turned from mineralised or stable soil nitrogen to nitrogen based greenhouse gases) increased under no-till farming. This is considered to be as a result of the increased anaerobic conditions (as the soil isn’t aerated by tilling). This is concerning as nitrous oxides have a significantly greater warming effect than carbon dioxide to the point where the gains in carbon sequestration could be less than the increase in nitrous oxides.

Longer term studies demonstrated that the longer the no-till farming endured, the ability of the biological and soil aggregation improvements to improve porosity and aeration of the soil resulted in lower nitrous oxide emission.

When comparing different greenhouses gases and their warming potential, a standard is used to allow direct comparison. This standard is the carbon dioxide equivalent, which can be calculated per unit of a particular greenhouse gas. A 30 year simulation showed an improvement of 0.56 tonnes of carbon dioxide equivalent per hectare per year under no-till farming, and a 43 year trial found a 1.03 tonne per hectare per year improvement, being a 52 per cent reduction of greenhouse gas emission compared to conventional farming.

Yield effects of no-till farming

The effect on yields is difficult to predict. Just over half of the studies the researchers looked at showed a decreased yield, just under half showed the opposite. Differences were noted between the effect on different crops, but a range of reductions between none and 30% were reported, with an average yield loss of 4.5% compared to conventional farming in the negative studies.

The major factors causing yield loss noted by the researchers were the increased susceptibility to weeds competing with crops, particularly seedlings, for nutrients. The shorter term anaerobic effects also increase water logging and reduced efficacy of fertilisers and pesticides (being unable to penetrate into the root zone).

Longer term studies showed increases in soybean yield over 10 years and comparable yields in wheat over 15 years, whilst another study showed that yield advantages under no-till were lost if the crop stubble was removed.

The difficulties of the effects on yield given different crops, different climates, different soil types and differing lengths of time under no-till mean that drawing a meaningful conclusion is difficult. However, it was noted in the article that there is the potential for yield increase when the benefits of no-till over longer periods on soil begin to show and if weed controls can be adequately implemented.

Conclusion

The article concludes with the observation that, as yet, we do not know when the net effect on greenhouse gas emissions moves from negative (increased nitrous oxide warming potential outweighs improve carbon storage) to positive. It appears that it takes some years for this to occur. Additionally, the improvements to soil structure also take some time to result in either slightly lower, comparable or better crop yields.

The hole in our current knowledge points to the need for a long-term study that notes the, say yearly for example, changes in carbon storage and nitrous oxide emissions to find the point in time where net difference on greenhouse gas emission is zero. At the same time, the improvements in soil structure, particularly the time required for anaerobic conditions in the soil to lessen and the associated issues that can reduce crop yield ease, needs to be ascertained. We can then be better placed to have some surety about what the long-term impacts of no-till farming will be on both issues and better inform ourselves on whether substantial improvements on either, or both, issues can be achieved and when.

Soil Science: Organic Matter Increases with Crop Diversity

A recent study published in Ecology Letters tested whether rotating an increasing number of crops in a plot without adjusting any other soil management practices would effect soil structure, microbial community structure and activity, and soil organic matter.

Unfortunately access to the article requires payment. A previous meta-analysis by the same researchers on this topic and which formed the platform from which this study launched is freely available here.

In the meta-analysis the researchers found 122 studies that compared the effects of diversified agriculture with that of a monoculture for at least a three year period. Those studies reported on the affect of the rotation of one or a combination of the following soil characteristics:

  • Total soil Carbon or Nitrogen; and
  • Microbial biomass Carbon or Nitrogen.

Using the data from the studies they found that there was a general increase in carbon and nitrogen pools and a substantially large increase in microbial carbon and nitrogen when one or more crops were added in rotation to a monoculture. The effect was reduced after adding more than 3 crops in rotation and the most significant effect on soil organic matter was where a cover crop (being a crop not grown for cultivation and sale but manage soil) was used as one of the rotations.

Although the findings supported the positive effects of crop diversity on soil organic matter the researchers pointed out that it was impossible to separate the effects of cover crops from the effects of the diversification of crops grown. They also conceded that the studies did not control for changes in other management practices at the same time as the diversification of crops used, introducing a variable that could have significant effect on soil properties aside from the rotation of crops.

This question of the effect of rotation on soil properties devoid of the adjustment of other variables was the basis for the study published in Ecology Letters.

The Experiment

The researchers performed a 12 year experiment with the hypothesis that an increase in crop diversity through rotation = alteration in microbial community = positive effects on microbial activity = enhanced soil aggregation and stability of aggregates = increased soil organic matter accumulation = increases in soil fertility. The Biodiversity Gradient experiment has a website.

In soil science aggregates of clay particles and organic matter are crucial to soil fertility. Smaller (micro) aggregates usually contain more processed and harder to release organic carbon and nitrogen which remain in the soil longer, while larger (macro and mega) aggregates contain more readily available matter but can degrade quickly, therefore allowing the organic matter to be, for example, washed away. Greater stability of aggregates usually means greater fertility of the soil and increased microbial activity has been linked to increase stability of aggregates. A history of the research into the link between aggregates, microbes and organic matter is well set out in this paper.

The 12 year experiment compared a corn monoculture, to a

  • Corn and soy rotation;
  • Corn and cover crop (red clover);
  • Corn and soy and wheat;
  • Corn and soy and wheat and red clover; and
  • Corn and soy and wheat and red clover and rye cover.

The figure below is reproduced from the article and shows how the rotations were organised through the study.

rotational crops

My first question of the methods used in the study is around the timing of the sample. In the paper they suggested that all the samples were taken while the plots were under the same treatment (corn), however the figure above suggests this isn’t the case. If the figure is correct there may be some treatment bias. My second question is whether there is any usefulness sampling the soil at the start of the trial. The researchers mention that there was some greater input in some plots prior to the trial starting, and therefore my question is whether this affected the result. It may be that the plot preparation and the length of time of the study means that any difference at the beginning is likely to be of nil effect at the end of the study.

Results

Some main results taken from the study were:

  1. More water stable mega-aggregates were found in the high diversity rotations, the greatest in the corn and soy and wheat and red clover plot;
  2. Mega-aggregate stability correlated significantly with diversity and the soil organic carbon, total nitrogen and fungal abundance found in those aggregates;
  3. The concentration of soil organic carbon and total nitrogen in all aggregates (when corrected for sand content in those aggregates) increased with increasing rotations; and
  4. Statistically significant positive correlations existed between increasing diversity and labile (readily available/liable to change) carbon, nitrogen and microbial enzymes.

Discussion

In discussing their findings the researchers stated their concern that the use of legume cover crops may have exaggerated the effects of diversity given the nitrogen inputs created by legume crops. However they were able to show that there were diversity effects beyond just the effects of the legume crops, particularly the linear positive relationship between the diversity of crops and microbial communities the found. This may have played an important role in the finding that as crop diversity increased so did the stability of mega-aggregates, soil organic carbon concentration and total nitrogen concentration. The figure below from the paper represents these findings:

aggreages

Trajectory of aggregate and SOM formation and stabilization
under a high diversity rotation (e.g. SWC2) versus monoculture crop (e.g.
Cm). (a) Greater quantity and quality of residues entering soils in high
diversity rotations enhances microbial activity with positive impacts on
rates and extent of (b) mega-aggregate formation and stabilization. This
also leads to (c) enhanced microbial activity in high diversity microaggregates and concomitant increases in microbial by-products that
accelerates micro-aggregate formation, resulting in (d) increasing stocks of
stable SOC and TN.

Conclusion

It appears likely that diversifying crops leads to greater organic matter content in soils. If you were to take one point home to your veggie garden, market garden, community garden or farm it would be that rotating your crop with at least one legume cover crop will have a definite positive effect on your soil health; rotating in two or three food crops with a legume cover crop will be probably be even better.

Welcome to the Legume Laboratory!

Welcome to the Legume Laboratory!

The purpose of the Legume Laboratory is focused on doing three things, and doing them well:

  1. Providing reliable updates and critique on new research findings in the fields of plant biotechnology, farming practices and soil science;
  2. Providing information to help growers and beginners learn, understand and use the scientific knowledge to enthuse and assist their own food production;
  3. Consider and provide prompts leading to discussion of what the future of food production may look like.

The overarching purpose tying these points of focus together is to develop a desire in readers to understand the accumulated knowledge in food production including current issues and developments as well prompting the consideration of how food production may look in the future.

Please feel free to share, comment or suggest recent studies that you think warrant a closer look.