Five Top Tips to Make Your Soil Healthy!
By Victoria Burton (Permaculture Association of Britain)
1. Test your soil
Over the last few months GROW Observatory citizen scientists have been taking part in the Great GROW Experiment of Polycultures v. Monocultures and comparing three crops (green beans, spinach and radish) grown together in a polyculture with those same three crops grown separately as monocultures.
In the last couple of weeks Experimenters have been receiving their free Luster Leaf Rapitest Soil Test Kit to measure nutrient levels of nitrogen (N), phosphorus (P), potassium (K), and how acidic or alkaline (pH level) the soil is in the polyculture and monoculture plots (see pic below). This test will be carried out twice, once now and a second time at the end of the Experiment to allow us to control for differences in crop yield due to different levels of soil nutrients and any differences related to the growing methods.
The soil test categorises the soil sample nutrient levels from surplus to depleted — this is fine for normal use but for the GROW Experiment we would like to know the actual amount of each nutrient in the soil sample. The GROW team has done some laboratory tests using the Luster Leaf kit to allow us to convert the kit’s categories into the concentration of each nutrient in the soil. We would like to share this with you: using the graph below, click the tab for the nutrient you would like to convert to see your own results.
2. Feed your soil a balanced diet
More is not always merrier
The Luster Leaf Soil Test Kit gives recommendations for the amount of fertiliser to use depending on your result and what you are planning to grow, but adding fertiliser which is rich in macronutrients (N, P, or K, for example) may not improve soil fertility from a plant perspective if other micronutrients are missing. Even when all other elements needed for growth are plentiful, a deficit of a single nutrient can mean the plant is unable to grow well. This is known as “Liebig’s law of the minimum” and is often conceptualised as a barrel of water, where the water represents growth, and each stave is an element required for growth. Growth is limited by whichever factor is lowest in relation to how much of it the plant needs. So, if, for example, manganese (Mn) is not available in sufficient amounts, plants won’t grow well even if you add fertiliser with the main NPK nutrients.
Nutrients can also limit plant growth when they are overabundant. Some nutrients are toxic at high concentrations and can kill plants if there’s too much. Different plants have different minimum and maximum tolerances of each nutrient, as they do for other factors like temperature, light and moisture. Within the middle of their range they will grow well, but towards either end, growth will be limited. Nutrient availability to plants is also influenced by the activity of soil microbes (for example, nitrogen-fixing, nitrifying and denitrifying bacteria which can increase or decrease nitrogen availability depending on soil conditions), and by soil chemistry, particularly pH.
Using organic fertilisers such as compost, mulches and farmyard manure will add macro- and micronutrients while encouraging soil life such as earthworms which improve soil structure. If you have low phosphorus levels, bone meal (use with care, as ever follow the provided instructions) is for a fast-acting solution and will only take a few months to break down and become available to your plants. A slower option is rock phosphate, which will become available to your plants after a year. If you want to increase potassium in your soil, try sulfate of potash, kelp or seaweed meal are fast-acting. There are also slow release options, such as greensand, granite dust and buried banana peels.
3. Know your soil pH
pH is a measure of how acidic or alkaline a soil is, specifically pH is the concentration of hydrogen ions (H+). The pH scale runs from 0 (acid) through 7 (neutral) to 14 (alkaline) and is logarithmic, so pH6 is ten times more acidic than pH7 (and has ten times more hydrogen ions). Soil pH is normally between 3 and 10, soils outside of this range are rare. Acidic soils have a pH below 7, and alkaline soils have a pH above 7. Plants differ in the minimum, optimum and maximum pH they require for growth, although for most plants this is between pH 5.5 and pH 7.5. The pH ranges of the crops in the GROW Experiment are shown below. Here radish has the most restricted optimum pH tolerances and beans the widest range of tolerance.
Soil pH is determined by the underlying parent rock material, vegetation type and land management. Soils become alkaline through weathering of limestone and other rocks which release positive ions of sodium (Na), potassium (K), calcium (Ca) or magnesium (Mg). Soils tend to become more acidic over time as these products of weathering are leached away. Acidity is also added by rainwater (which is slightly acidic), plant growth which releases CO2, and decomposition which releases acidic compounds. Decomposition, which can make soils more acidic, is more rapid when temperatures are warmer so soils can be slightly more acidic in summer than they are in winter.
Soil chemistry includes complex interactions between different nutrients and with the soil pH. These interactions can mean that even when there are nutrients in the soil, plants are unable to make use of them. Soil is a dynamic system where a change in pH or the concentration of one nutrient can influence other nutrients by determining the chemical form the nutrient is in and the chemical reactions they undergo. For example, in low pH soils (below about pH 5.5) nitrogen occurs more in the form of ammonium (NH+4) and in high pH soils more as nitrate (NO3-). Nitrate is a form of nitrogen that plants are most easily able to take-up and it can also help some plants take up other cation nutrients like potassium (K), calcium (Ca) and magnesium (Mg). Ammonium usually needs to be converted into other forms by soil bacteria before it is available to plants adapted to non-stagnant soils with pH 5.5 and above.
4. Be aware of the bigger picture
Nutrient depletion in soils and food
Globally, about 95% of our food is directly or indirectly produced on soils so human nutrition is dependent on healthy soils with sufficient nutrients to grow crops. On the GROW Medium blog Stephanie Reiter (Global Soil Partnership Secretariat) and Lucrezia Caon (UN Food and Agriculture Organization) discuss how people, plants and soil can have a healthier diet. This topic also generated a lot of discussion on GROW Future Learn courses, you can sign up for future ones here.
How healthy are our soils? Studies around the world have consistently found agricultural soils are becoming depleted in soil nutrients, particularly micronutrients. For example, the UK Countryside survey (a repeated nationwide survey soil soils in the UK) showed significant loss of phosphorus (P), nitrogen (N), nickel (Ni) and zinc (Zn) between 1998–2007. The declines were too great to explain by a reduction in fertiliser use alone and the report concludes that nutrient declines may be occurring through deep ploughing, erosion or increased decomposition rates. In summary, more nutrients are being taken out of our soils then are being put in.
On the other hand, the wider countryside is becoming richer in nitrogen and phosphorus due to runoff of fertilisers and animal waste from agricultural land. On land this has led to changes, and often reductions in the diversity of plants. In in rivers, lakes and the sea excess fertiliser is causing algal blooms and mass death of sea life. Nitrogen and phosphorus flows are one of the earth processes which have thought to have exceeded the safe limits for life on earth (also called planetary boundaries, see below for 2015 version)
Quantity over quality?
What is less clear is if the reduction in soil nutrients is also causing a reduction in the nutrients in our food. A classic study in the USA compared nutritional data from fruit and vegetables between 1950 and 1999 and found apparent declines in protein, calcium, phosphorus, iron, vitamin B2 (riboflavin) and vitamin C (ascorbic acid). A similar study using food nutritional data from the UK found similar reductions, while concerns have been raised that climate change may make some foods — like rice — less nutritious.
However a recent review by Robin J. Marles, argued that there was not a clear cause and effect between soil nutrient depletion and nutrient depletion in foods and that many other factors could be contributing, including:
- Changes in how nutrients are measured
- Greater use of imported and “out of season” produce
- Different storage and ripening systems
- Prioritisation of cosmetic appeal over nutrient content
- Changes in varieties bred for higher yield
- Response to modern methods of agriculture
- Post-harvest handling changes
Research is inconclusive, but there is evidence that high yielding crops have reduced levels of nutrients. This has been termed the ‘Breeder’s Dilemma’ — crop breeding has historically been oriented toward high yield, easy and consistent processing, and disease and pest resistance which may have unwittingly led to crops with reduced nutritional quality.
Evidence of this comes from field studies comparing old and new varieties grown next to each other. Modern varieties do tend to have higher yields but lower nutrient levels due to an increase in carbohydrates without a proportional increase in other nutrient levels — known as the ‘dilution effect’. For example, a study in 2000 found commerical broccoli hybrids tended to have bigger heads but lower calcium and magnesium than traditional varieties. In 2006, modern high yield wheat varieties were found to have lower concentrations of iron and zinc than older varieties. However in both these studies there were wide variations depending on the soil type, climate and time of year. Marles’ review notes that the benefits of increased yield may outweigh the nutrient dilution effect and that the variability of nutrient levels within crops are likely greater than any decline over time.
The dilution effect may become more pronounced in the future due to increasing CO2 levels. A review of many studies which compared growing crops in current and higher CO2 levels found that most grains and legumes have lower concentrations of zinc and iron when grown in CO2 levels predicted in next 40–60 years. Most crops other than legumes also had less protein, although maize was less affected having only reduced iron levels.
Breeding for yield may even affect the flavour of our food. A study comparing wild tomato ancestors with modern varieties found reduced sugars, organic acids and volatiles in cultivars and suggested that that by focussing on yield, shelf life and disease resistance means we may be missing out on taste!
Generally you can’t tell by looking at your produce what the nutrient levels are but an exception are orange and red coloured vegetables as this colour is caused by beta-carotene — a precursor to vitamin A (retinol). In contrast to other crops, studies have found that carrots, sweet potatoes and tomatoes have actually increased in vitamin A content due to a breeding for a darker colour which consumers prefer.
Shelf-life and freshness
Out of season food supply and longer supply chains — where food has to travel faster from the place where it is produced to reach the consumer and stay fresh on the supermarket shelves for longer — may also have a negative effect on food nutrients. In particular, antioxidant vitamins A, C and E are easily destroyed by long-term storage and processing, although this depends on the crop.
A study of brassicas found that vitamin C was lost rapidly at ambient temp but storing at 4℃ prevented decline for 7 days. Freezing did not reduce nutrients itself but preprocessing such as chopping or cooking did, particularly cooking in water where antioxidants were leached away. Special plastic films are also used to retain nutrients and can extend storage life of fresh produce by 50–400% — those plastic wrapped vegetables may not be as unnecessary as they are appear — and while reducing plastic use is necessary we must take care not to replace one problem with another. Of course buying local or growing your own solves these three problems of packaging, freshness and nutrient levels.
5. YOU are part of the solution
Can our current agricultural systems which prioritise yield be changed to a system where crops with high nutritional quality are valued rather than quantity? There is also the potential to deliberately increase nutrients in crops (biofortification) by regular breeding methods or via genetic engineering (for which biological, nutritional, political and socio-economic pros are the topics of an ongoing debate). Where yield and nutrients are mutually exclusive properties of crops this could be overcome by growing complementary varieties, for example of wheat, which are then milled together.
These solutions only address the impact, not the root of the problem. Mitigation might include: reducing causes of climate change, reducing supply chains and demand for out-of-season foods matured in artificial ways, and using regenerative growing practices. Regenerative food growing practices include using crop residues, manure and other organic matter to return nutrients to the soil more quickly than they are extracted — it’s about going beyond sustainable use of soils and working to actively restore them.
Our GROW citizen scientists are helping us gain a better understanding if, how and where regenerative practices work on the smaller scale, including the comparing polycultures and monocultures in the GROW Experiment. Growing crops together can help reduce depletion of a single nutrient as different crops have different demands. Including nitrogen-fixing crops, like beans in this case, can also help improve soil nutrient status in the longer term.
It’s noticeable that small-scale cultivated areas such as urban allotments have not shown similar declines in soil quality compared to conventional mechanised agriculture — showing clearly that you as a grower can make a difference to soil health.
Further reading
Countryside Survey Soils Report (pdf)
Changes In Usda Food Composition Data For 43 Garden Crops, 1950 To 1999
Dirt Poor: Have Fruits And Vegetables Become Less Nutritious? (pdf)
Rising CO2 Threatens Human Nutrition
