Plant Nutrition & Lab Testing

plant nutrition and lab testingEver had a tree that seems ill and seemingly won’t respond to fertilisers?  It cound be a plant nutrition problem! Sometimes I am asked to help diagnose specimen issues and it’s rarely a simple matter and almost never involves a single factor.  Sometimes it’s easier and cheaper to involve a laboratory for soil and plant tissues testing.

Disease diagnosis, water and nutrient availability and determining  fertiliser requirements are all common reasons to undertake plant and soil testing with a lab.

Most laboratories offer a basic soil test and a basic plant test, both of which have  additional testing components for specific nutrients as required.  The basic standard soil test will looks at pH (potential hydrogen), Cation Exchange Capacity (CEC), saturation, bulk density, Olsen Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg) and Sodium (Na).  Soil test options will largely depend on the purpose of the testing.   For example, for general fertiliser requirements, you may want to test for Sulphate-Sulphur(SO4) (available sulphur), while for organic farming, organic matter and total carbon might be looked at.

The basic test items for plants are Nitrogen (N), Phosphorus, Potassium, Sulphur, Magnesium, Manganese (Mn), Boron (B), Copper (Cu), Iron (Fe) and Zinc (Zn).  Additional items for mixed pastures include Molybdenum (Mo), Cobalt (Co) and Selenium (Se).  There are others as well but the exact selection will be crop dependant.

Nitrogen, Phosphorus and potassium are considered the primary macro nutrients and are usually the first nutrients to become deficient in soil because plants uptake large amounts of them.  Accordingly most fertilisers focus on these and tell you how much of each is in the fertiliser.  Calcium, Magnesium and sulphur are secondary macro nutrients and are usually adequately provided for by the decomposition of the plants foliage and other organic matter.  Nutrient recycling via this pathway cannot be over emphasised. Other nutrients are considered micro in the quantity up taken by plants. It in no way reflects importance to healthy plant growth.

Some plants species are more susceptible to deficiencies of macro nutrients than others.  Also some plants show signs of a particular deficiency / toxicity more clearly than others.  While it is usually easy enough to see that the plant is experiencing a nutrient issue it is often much harder to identify which nutrient and why, as many of the symptoms overlap, i.e. it reduces chlorophyll production and results in some level of chlorosis.  These physical symptoms can also be caused and or complicated  by micro nutrient levels, herbicide injury, disease, insect damage or environmental conditions. Therefore it is often quicker, cheaper and more  reliable to diagnose such issues by conducting quantitative tests in addition to assessing the plants visual symptoms, particularly if, for example, a heritage / registered specimen is involved.

Nitrogen is involved in almost all proteins, enzymes and metabolic processes, including chlorophyll. Plants with adequate nitrogen often exhibit richer shades of green in their foliage and more vigorous growth of foliage and seed, while symptoms of a deficiency include chlorosis of older (but sometimes younger) leaves, poor growth rates and a generally pale color.  Many plants get nitrogen from sources in the soil and air.  Molybdenum (Mo) helps plants to use nitrogen.

Phosphorus is an important macro nutrient involved in photosynthesis, respiration and formation of oils, sugars and starches.  While Phosphorus is not normally a common deficiency, it is low in many soils in New Zealand, particularly in areas with high rainfall and heavy clay soils.  Olsen Phosphorus tests the amount of phosphorus available for uptake by plants.  Plants unable to uptake sufficient quantities of phosphorous exhibit a blue-purple color in foliage, (older leaves first).

Potassium is also a macro nutrient.  It’s role is to assist enzymes involved in photosynthesis and respiration. Plants deficient in potassium develop dead spots, leaf chlorosis, scorching and curling of leaf tips and poor growth rates, particularly roots, seeds and fruits.  Plants in soils with excessively high potassium experience a reduced capacity to utilise calcium and magnesium. Both nitrogen and potassium are mobile nutrients and so will often affect the older leaves first as the plant moves the nutrient to younger leaves.

Calcium (Ca), Magnesium (Mg), Sodium (Na) are micro nutrients. These are sometimes called cation nutrients because they are positively charged ions when dissolved in soil solutions. Negatively charged irons are referred to as anions.  Calcium is an element used in plant cell wall structure.  Common calcium sources include gypsum and superphosphate. Magnesium is a component of chlorophyll essential for photosynthesis. It is found in soil minerals and organic material.  Other micro nutrients include Iron (Fe) which is involved in the production of chlorophyll. Sulphur  which is essentially used for protein and enzyme production.  It is involved in the formation of chlorophyll and stimulates root growth and seed production.  Boron (B) is used in plants to regulate other nutrients and produce carbohydrate.  Copper (Cu) and Chloride (Cl) assist in the plant metabolism. Manganese (Mn) is used in carbohydrates breakdown and nitrogen metabolism. Zinc  (Zn) is also involved in the transformation of carbohydrates and regulation of sugar consumption.

A plants capacity to uptake sufficient minerals is greatly affected by characteristics of the soil. The soils capacity to attract and exchange cations is quantified by its Cation Exchange Capacity or CEC.  Soils with high clay content and soils with high levels of organic matter typically have a high density of negatively charged ions giving them a high CEC.  As such the CEC also reflects the soils make up and texture.

Soil has varying amounts of sand, silt, clay, and organic matter. This is called its texture and impacts the nutrients and water the soils is able to retain. Clays and organic soils are able to retain nutrients and water better than silts and sands.  Often the chemical bonding within the soil limits the availability of water for plants to uptake and the free flow of water through the soil leaches nutrient away.

Saturation and bulk density measure moisture content and weight per volume respectively. Both tests will reflect pore space within the soil containing air and water. Available water, drainage and soil compaction levels are all critical factors in plant health and a plants tolerance of such will vary from species to species.

PH is the balance of alkalinity and acidity in the soil.  It stands for potential hydrogen and it greatly influences the availability of nutrients to the plant by altering their solubility within the soil.  pH is measured on a scale between 0 (acidic) and 14 (alkaline).  The optimum pH range for plant nutrient availability is 6 to 6.5.  Below this range potassium, sulphur, nitrogen, phosphorous, molybdenum, selenium, calcium and magnesium become significantly less available. Below 5, uptake of copper, boron, zinc and cobalt are also compromised.  Ph levels above this range will also hinder nutrient uptake but  greater shifts are required.

Correction of nutrient deficiency is often easily done by the application of appropriate mulch.  Toxicities and pH are more complex and it would be prudent to seek advice from professional horticulturists.

Soil samples can be taken with auger or spade and posted to the relevant lab.  Labs send out sampling packs with instructions but normally sample depth is approx 3 inch for pastures and 6 inch for  gardens , crops and orchards etc. Sample weight is about half a kilo. Keep soil samples random and representative of the test area and be sure to use clean tools. Areas of localised contamination like compost heaps and drains etc should be avoided.

Plant sample should also be taken from a random representative area within the testing zone.  Again the exact sample method and treatment will be included in the instruction from whichever lab you decide to use.

Hill Laboratories, Quantum Labs and ARL Labs are widely used in New Zealand. Other labs do exist and a quick browse on the Internet will reveal others. Lab testing is not prohibitively expensive and definitely has its place for the domestic green finger.