Soils, Plant Nutrients and Plant Nutrition Are All Important Topics For Gardeners. Understanding What Nutrients Plants Need And Where They Come From Is Vital If We Are To Grow Good Crops.

I’m often asked .. Why are plant nutrients important? What do plant nutrients do?

Like us plants need nutrition.

Plants obtain most of their nutrition from the soil. So ensuring we have good soil or compost for them to grow in is absolutely essential.

Humans need good quantities of essential nutrients, such as proteins fats and carbohydrates plus numerous other elements and minerals in smaller quantities.  Plants are similar to us, they also need essential nutrients. 

Plants require good quantities of N, P and K. That’s nitrogen, phosphorus and potassium.  They also need moderate quantities of a range of other materials such as Calcium and Magnesium plus they require what are often called trace elements in very small quantities. These include Copper, Iron, Boron, Manganese, Zinc, Molybdenum and Chlorine. 

We rarely need to add trace elements because the soil already contains good quantities of most trace elements. Compost, organic manures and manufactured fertilisers also contain enough trace elements to satisfy the needs of our plants. 

What Do The Major, Minor and Trace Elements Do? 

Nitrogen (N)

Nitrogen is needed by plants for growth, especially of leaves. Nitrogen is used to build the proteins in plants and when it is in short supply the plant’s production of chlorophyll slows down and the leaves start to go yellow. Leaf production will slow down and the plant will be stunted and sickly.

Where plants get excess nitrogen they tend to grow loads of leaves and get big and bushy. But this is often at the expense of flowers and fruit or root growth. Too much nitrogen therefore gives big plants but lower yields of fruit such as tomatoes or root/tuber crops such as potatoes.

The aim should always to produce a balanced plant where growth is adequate but flowering isn’t inhibited. This takes experience and knowing when a plant looks balanced is a skill it takes time to acquire.

Interestingly, people often mistake balanced growth for a disorder or problem. For example balanced growth in tomatoes often results in curled leaves that people often think is a problem!

Nitrogen occurs in the soil as nitrates and ammonium. Plants can take up nitrogen in either of the forms. It also occurs in organic materials but is unavailable to plants until the organic material decomposes and releases the nitrogen. Breakdown of organic material is faster in damp warm soils which is good as they are the conditions we often get in spring, just when the plants want to use nitrogen to grow.

Nitrogen is easily leached (washed out) of the soil so if it is released in winter it wil be washed away by rain. Leached nitrogen in various forms is a major pollution source from farmland and is the reason it is advised not to spread slurry and farm yard manure in winter, especially when wet or relatively warm weather is expected.

Nitrogen Sources For Gardeners

We can provide our plants with nitrogen in various ways.

Artificial or Manufactured Fertilisers

“Chemical” fertilisers were developed to enable growers to deliver concentrated nitrogen to crops as and when they were needed. Products such as urea, Ammonium Nitrate (used in the manufacture of IEDs during the “Irish Troubles”), NitroChalk, and other forms have all been used over the recent decades. They are quickly available to plants but also liable to leaching in wet weather.

Liquid Feeds

These can include urea and various plant teas that are rich in nitrogen eg nettle or comfrey tea. They are best used to water plants rather than as a foliar feed.

Green Manure

Green manures extract soil nutrients to produce plants. Some species are shallow rooted and others are very deep rotted and reach nutrients that are deep in the soil. When the plant dies the nutrients in the leaves and roots go back into the soil where they are available to the plants we sow and plant.

Where legumes are grown as green manures their nitrogen goes into the soil and is available to the next crop. Leafy crops do very well after legumes have been grown, provided the roots of the green manure are left in the soil.

The great thing about green manures is that they release their nutrients over time and give the successive crop access over weeks or months.

Garden Compost

When we compost garden waste we are storing nutrients that can then be made available to our soils and plants at a later stage. As well as green organic matter we can add other organic materials such as paper (wood) to the compost. Again the release of nutrients for the soil and plants will be relatively slow, which is ideal as plants don’t want all of it at once.

Farmyard Manure (FYM)

FYM is usually a mix of animal faeces and bedding material such as straw of sawdust. Though years ago the bedding might have been dried bracken or other harvested natural products.

The FYM also contains urine which is a good source of nitrogen.

I frequently see discussion on whether the best manures are cow, pig, alpaca, chicken etc. The reality is that their isn’t a lot of difference. In some cases poultry manure will be more nitrogen rich and could “burn” plants, but that is partly because it tends to have a lower bedding content to dilute the nitrogen.

The important thing is that FYM doesn’t feed the soil and plants until it decomposes or “breaks down”. It is the decomposition process that releases nutrients. The decomposition process can be when it is stacked in a heap and becomes “well rotted” or it can happen in, or on, the soil. The plants will not know the difference!

Phosphorus (P)

Phosphorus has several roles in plant nutrition. It is a component of cells membranes and important in root growth. Plus it is essential for the manufacture of energy in plants.

Plants that are deficient in phosphorus often show signs of “leathery” leaves and a purple tinge to the leaf. Don’t mistake this colouration with the blue / purple colouration on tomato leaves that are feeling the cold! Inexperienced growers sometimes confuse the two.

Phosphorus in the Soil

Various forms of phosphorus are found in the soil

Soluble phosphorus is easily absorbed by plants but can also be leached (washed out) of the soil. Though the quantity leached out is normally lowish.

Organic phosphorus is found in the organic material in the soil. Things like decomposing organic matter, plant residues and plant debris.

Adsorbed phosphorus (not absorbed) is the phosphorus that is bound to the soil particles. Adsorbed phosphorus is tightly bound to soil particles and is not accessible to the plants. For it to be accessible the chemical bonds holding it in place need to be broken.

Most adsorbed phosphorus will be bound to clay and silts though some can bind to larger particles. The reason it is more common bound to clays is that phosphorus is held on soil particle surfaces and clay has a much bigger surface area than the same volume of larger particles.

Unlike nitrogen and carbon, phosphorus can not be lost to the soil as a gas.

Losses are via three primary routes, surface runoff, leaching down through the soil profile and subsurface percolation into water courses.

Small amounts of phosphorus in waterways are polluting in that algae responds to small amounts of phosphorous causing eutrophication .. ie algal blooms or growth. Some algae then produce toxins that degrade water and can it dangerous to drink by livestock or humans.

Sources of Phosphorus For Gardening

Bone meal

Bonemeal is a good organic source of phosphorous.

Phosphate Rich Fertilisers

Many fertilisers contain phosphates. It’s the second item in the NPK plant food ratio that is mentioned on the box or sack.

Rock Phosphate

Rock phosphate is allowed in some organic systems though it is becoming harder to find as its a finite resource.

Potassium (K)

Potassium (potash) is important for flowers and fruit and regulates the water balance in the plant. It is present in solution in all plant cells where it regulates the turgidity of the cells.

Pot and container grown plants that are producing volumes of fruit can “run out of steam” due to a lack of potash.  They therefore benefit from additional potash feeds as they grow. Indeed when I grew tomatoes in the border (soil) I fed them a potassium nitrate solution EVERY day. Admittedly this was on a light greensand based soil but my friends on clay soils did the same!
It is worth noting that clay soils do however tend to have higher levels of potash than sandy soils.
 

Potash Deficiency Symptoms

 
Potash shortage in plants show symptomatic yellowing of leaf edges which then turn brown. Plants grown in cold wet weather often show symptoms that are improved in later new growth, once its warmer and drier.
 
Potatoes suffering from potassium deficiency tend to have dark green foliage which later turns reddish purple. Inter-veinal necrosis is a quite common symptom of potash deficiency in some crops, especially beet.
 

Sources of Potash for Gardening

Wood Ash

Wood ash contains a good quantity of potash and can be mixed with compost to dilute it. This can be done once the compost is made or, as I often do, added to the compost as the organic matter decomposes.

Home made compost is a slow release source of potash and wil be available for fairly long periods.

Fertilisers

Proprietary compound fertilisers contain the stated percentage of potash on the label. Eg. Vitax Q4 contains 10% potash with a plant food ratio of 5.3:7.5:10.

Fertilisers such as Q4 also contain levels of minor and trace elements as will home made composts.

Below are details of some. of the more important elements ts that plants need in varying quantities.

Magnesium (Mg)

Magnesium is essential for chlorophyll production, plants can’t survive without it. Deficiencies result in dark inter veinal patches. However this doesn’t necessarily mean the soil is short of Mg. Often the problem is mobilising the Mg, due to insufficient water being present. If symptoms are bad this is one case where foliar feeds can work. Use a weak solution of Epson Salts .. a fistful in five gallons of water .. and spray onto affected leaves until it runs off.

Calcium (Ca)

Calcium is not very mobile in plants and deficiencies are shown in tomatoes and related fruit as blossom end rot. Eggshells around the plant has no effect because this isnt a soil deficiency issue but a mobility issue. Regular watering is far more beneficial where BEM is a problem as most soils and composts aren’t deficient. Very acid soils also aggravate the issue.

Iron (Fe)

Iron is needed for chlorophyll and the plant processes. Deficiency results in pale leaves with browning at the leaf margins. Mobility is again the issue, especially in line rich soils.

Manganese (Mn)

Another trace element that has multiple uses in the plant. In this case defienencies result in the leaf veins showing brown spots. Again this is a deficiency where foliar feeds sometimes help.

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