Limiting Factors

A chain is only as strong as it's weakest link. A limiting factor is rather like that link. It is whatever is holding back our crops from fulfilling an even greater yield. It may vary from year to year or even day by day but at any given moment there is always something that is in shortest supply. If we can understand just whatever is in deficit when and increase it's availability then our harvests will become larger.

Those who once studied horticulture will immediately be thinking of N, P & K. (nitrogen, phosphorus and potassium) These are often cited as being the most crucial necessities for improving plant growth, and indeed they may be the limiting factors. In theory but in reality far more important in many ways are three other inputs; air, light and water.

Water is often the most obviously critical factor. When water becomes in short supply the plants are held back in several different ways. Firstly water is the biggest constituent of almost every part of a plant; held together with carbon water is the major building block from which carbohydrates, sugars and fats are created. Secondly water is the essential fluid within and around the cells in and by which the very processes of the plant take place. Thirdly by the intake and transpiration of water the plant receives and transports inputs from the soil. Fourthly water is the essential solvent in the soil around the plant enabling other living organisms to flourish and create fertility from even the bare rock particles themselves.

Indeed as almost every gardener instinctively understands, water is the key component and even the slightest shortage severely restricts growth and reduces yields. There are few chores so crucial, so simple and yet so rewarding as watering. In particular for the quick growing salad and vegetable crops which we wish to be succulent and sweet. If they get short of water then they crop less, are tougher, bitterer and in the worst case fail entirely. If in doubt add more water; it is hard to overdo it in most gardens during the growing season. In particular many crops respond extremely well to extra water just when they flower as more at this time helps determine their final harvest.

The next most important factor is light. No plant grows if there is no light and in many gardens shade slowly increases until the plants are receiving only a small proportion of their potential insolation. Trees get larger, hedges grow taller and surrounding crops can cut off much of the sun's life giving rays. Much of the problem of weed competition is that they are faster growing than crops and will shade them out given half a chance -that is after all their aim! And don't crowd a crop making them rob each other. As our eyes automatically adjust to lower light levels we often fail to notice how dim it has become inside a dirty greenhouse or under ageing plastic. Likewise we often fail to notice the passing of the years and how our once sunny exposed vegetable plot has become a dappled glade. If your crops, especially potatoes, are growing tall and leggy then it may be time to get the saw and loppers out!

The third most important critical factor is air; the carbon dioxide is only a minute proportion of air so plants need enormous volumes of air passing over their leaves in order to obtain enough. A garden that has become overgrown is also a stagnant garden where the plants are pining for fresh air. And not only does a restricted airflow restrict growth but it also makes many crops more prone to moulds and mildews. Time to get the saw and loppers out again! Under cover things can be much worse especially during a cold spring when we keep the greenhouse tightly shut up to keep the warmth in. One way to increase indoor carbon dioxide availability is to encourage more small animals of any form to live with our plants -so even pests are doing some good. Another and safer way is to put a couple of gallons of fermenting home brew in the greenhouse which will give off ample carbon dioxide as long as the fermentation continues.

But there is another need for air. Although leaves breathe in air taking in the carbon dioxide and giving off oxygen the roots do the reverse. Plant roots behave like animals and need oxygen or they die. This is why water-logging kills so many plants so easily; they are literally drowning as the water starves them of oxygen, only later do they rot. Thus a good soil texture is essential to enable air to filter in and out, and so very thick mulches can be counterproductive. Our best allies for increasing the air penetration of our soils are the earthworms and the rain, which work even better together.

The earthworm tunnels provide a wonderful network of channels for air to enter deep down to the roots and as they move up and down in their tunnels they work like little pumps pushing stale air out and sucking clean air in behind them. In a similar way as heavy rain percolates down it expels the old stale air as it fills the tunnels and pore spaces. Then as the rain sinks lower down it sucks in fresh air behind it. Added to which is the air dissolved in the rain which makes it far more valuable than the poor chemical ridden stuff they sell us from the tap.

Only when there is ample air, light and water can there be any point in considering such as N, P & K. However they are then indeed important, especially the nitrogen. With little doubt nitrogen is essential for plant growth and any shortage will slow or stop it down almost entirely. However too much will make the plants grow lushly and become prone to pests and diseases. Ideally nitrogen should be continually but moderately available to the plants without ever becoming excessive. The best way to achieve this is to have a moist soil rich in humus and animal life. Natural slow release fertilisers such as well rotted manure, blood, fish & bone meal and seaweed meal are all ways of ensuring this essential element without making it too available too quickly.

Phosphorus is in some ways an easier element to apply without risking over-feeding although in some soils applying it to excess may then make other elements difficult for the plants to take up. However you would need to apply rather large and expensive amounts to achieve that especially as it gets locked up easily itself! Significant amounts of phosphorus are found in animal manures so well rotted dung is an excellent source. Bone meal is one of the best sources though ground rock phosphate is a good, if slower acting, alternative for the squeamish and those who object to using animal by-products. I pick up bones whenever I see them, roast them over the next bonfire and then they are easily powdered -nothing improves my turnips like a little burnt bone.

Potash, or potassium, is often in short supply as it is so easily washed out of soils by heavy rain. Well rotted manures again supply sufficient for many crops especially if the urine has been retained in the manure. Human urine is a rich source and provides considerable nitrogen as well though in a rather over-active form. Thus urine should be applied well diluted to avoid over-stimulating the crops, and traditionally after six weeks fermentation. Wood ashes are another excellent source of potash and many crops such as gooseberries, cooking apples, beets, onions, potatoes and tomatoes benefit greatly from wood ashes mixed into their soil before sowing or planting.

Similar in many ways to potassium is calcium, or lime, which is needed to keep our soil sweet. Most vegetable crops, and the stone fruits, need large amounts of calcium to thrive thus the old recommendation of adding lime every fourth year of a rotation. (With the beans and peas, or with the brassicas and well before the potatoes who do not like it in excess). Indeed one common mistake is to add manure year after year without adding lime- after a decade or so most crops start to fail for no apparent reason. One good dose of lime and harmony is restored. But as with all the other elements mentioned so far if any excess is applied the laws of chemistry mean that yet other, minor or trace, elements become less available to our plants.

These minor elements as they are called are not minor at all and are equally, if not more importantly, needed by our crops. They are called trace elements as the amounts needed are incredibly small; rather like vitamins are for us. And in a similar manner a shortage or an excess can be damaging. Sulphur is a prime example and brassica crops need considerable amounts (relatively) in order to flourish but an excess makes their soil infertile. Others such as magnesium, iron, molybdenum and copper are needed in remarkably minuscule amounts. Cobalt for example is required in amounts of far less than a gram per acre. Surprisingly the plants themselves have little need for cobalt but it is absolutely essential if the food produced by them is to be good for stock or us to eat.

Some of the commoner trace element deficiencies create obvious symptoms in some plants so these can be looked for and corrected, whole books are dedicated to their descriptions. However as any one of the many trace elements may be a limiting factor without always betraying obvious symptoms they must certainly all be catered for. Most trace elements are available from applications of well rotted manure which has always been recognised as one of the essential inputs for a productive garden. Well made garden compost is another good source, though if made solely from the garden's own products may be deficient in one or another element.

Thus these plant 'vitamins' are best provided by the application of seaweed based products. These contain all the known necessary trace elements in small amounts and indeed contain traces of almost all the elements that exist. Fortunately in seaweed products most of these essential elements are in a form that is available to the plants but not too quickly or dangerously so. For example calcified seaweed is a rich source of lime with all the trace elements together and should be used where ever lime is required. I especially find that a dilute spray of seaweed solution applied once a month to foliage and soil throughout the growing season works wonders. The effect is apparent within a few days, if you do not believe me just try spraying half of each crop and then compare your results. You'll be amazed!